CHAPTER-1

ELECTRICAL POWER SYSTEM

OF

DIGBOI REFINERY

1.1 SOURCES OF POWER:

        Through captive Power generation in 11 KV system (under the CPP & utility department)

CAPTIVE POWER PLANT:

The generation scheme of the Captive power Plant (CPP) is of-co-generation type. there are three Gas turbine Generators, each having a generating capacity of 8.5 MW and one having a capacity of 20 MW. The exhaust of the Gas Turbines (along with additional supplementary firing) is used to run three heat Recovery Steam Generator (HRSG) plants of 40.5 TPH capacities and another of 100 TPH capacity. The Captive power Plant (CPP) has got three Gas turbine Generator unit of frame IV with the Following specifications:

GENERATOR SPECIFICATION FOR GT 1,2,3:

 Gas turbine specifications for gt1,2,3:

GENERATOR DATA FOR GT-4(NEW)

MAIN EXITER DATA FOR GT-4

 Permanent magnet generator data:

CHAPTER-2

                                          GAS TURBINE

• Definition And Working Principle

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustion chamber, where air is mixed with fuel and ignited. Combustion increases the temperature, velocity and volume of the gas flow. This is directed through a nozzle over the turbine’s blades, spinning the turbine and powering the compressor. Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.

The thermodynamic cycle upon which all gas turbines operate is called the Brayton cycle. The Figure below shows the classical pressure-volume (PV) and temperature entropy (PS) diagrams for this cycle.

                                                     FIG 2.1

   Here, path 1 to 2 represents the compression occurring in the compressor, path 2 to 3 represents the constant-pressure addition of heat in the combustion systems, and path 3 to 4 represents the expansion occurring in the turbine. The path from 4 back to 1 on the cycle diagrams is indicative of a constant-cooling process.

           Gas turbines in IOCL, Digboi

IOCL, Digboi has four gas turbines which generates electricity with three of them having a capacity of 8.5 MW and one 20 MW generator, the total capacity being 44.5 MW. Each of the three 8.5 MW unit is accompanied by 3 HRSG (Heat Recovery steam Generator) of capacity 37.5 TPH and the 20 MW unit is accompanied by 1 HRSG of capacity 100 TPH.

When the exhaust from the turbines are recycled with the help of the HRSG unit, the cycle followed by the gas turbine is known as co-gen Cycle and when the exhaust is directly released into the atmosphere, the cycle of the turbine is known as Simple Cycle. The cycle which is used mostly in IOCL, Digboi is the Co-gen Cycle.

• Fuel used:

To run the gas turbines two kinds of fuel are generally used. The primary fuel used is Natural Gas (NG) at a pressure of 12-14 Kg/cm2, and the secondary fuel being HSD (High Speed Diesel_. When the pressure of the gas is not sufficient enough to run the turbine, it is switched over to the mixture mode and when the pressure of the mixture mode is also not sufficient it switches to the liquid fuel mode.

 2.2 GAS TURBINE FUNCTIONAL DESCRIPTION

The gas turbine assembly consists of the following assembly comprising five major sections:

• Diesel Engine
• Air inlet
• Compressor
• Combustion system
• Turbine
• Exhaust

A functional description of each major gas turbine section s air and combustion gases flow through the gas path from inlet to exhaust is presented below.

2.3 SYSTEM DESCRIPTION:

1. Starting System

Before the gas turbine can be fired and staged it must be rotated or cranked by accessory equipment to obtain a sustainable speed. This is accomplished by a diesel engine operating through a torque converter to provide the power required by the turbine for startup. Once it reaches the sustainable  speed the gas turbine is driven through the accessory gear by the diesel engine, torque converter output gear and the starting clutch.

      Thus the starting system components include:

• The diesel engine
• Torque converter with ratchet mechanism
• Starting Jaw clutch
• hydraulic ratchet self-sequencing control value assembly

            In addition, there are several supplementary components required for sequencing and        operation of the turbine starting system.

1.  Inlet Air System

Gas turbine performance and reliability is a function of the quality and cleanliness of the inlet air entering the turbine. Therefore, for most efficient operation, it is necessary to treat the atmospheric air entering the turbine and filter out contaminants. It is the function of the air inlet system its specially designed equipment and ducting to modify the quality of the air under various temperature, humidity and contamination situations and make it more suitable for use in the unit. This system combines the functions of filtering and silencing the inlet air with the function of directing the air into the turbine compressor. This system basically includes self cleaning air filters which including the job of filtering the air cleans itself after a fixed interval of time.

3. Compressor section

In the compressor, air is confined to the space between the rotor and stator where it is compressed in stages by an alternate series of rotating (rotor) and stationary (stator) airfoil shaped blades. Rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters the following rotor stage at the proper angle. The compressed air exits through the compressor discharge casing to the combustion chambers. Air is also extracted from the compressor for turbine cooling and for bearing in the lube oil extracted from the compressor for turbine cooling and for bearing in the lube oil sealing.

4. Combustion Section

The combustion section consists of combustion chambers, fuel nozzles, crossfire tubes and transition pieces. Air for combustion is supplied directly from the axial- flow compressor to the combustion chambers. This arrangement is called a reverse flow system. Each combustion chamber is equipped with a fuel nozzle that introduces fuel into the combustion liner. Gaseous fuel is admitted directly into each chamber through metering holes. When liquid fuel is used, it is atomized in the nozzle swirl chamber by means of high-pressure air.

The combustion chambers are interconnected by means of crossfire tubes. These tubes enable flame from the fired chambers containing spark plugs to propagate to the unfired chambers during startup. Combustion of the fuel and air mixture is initiated by spark plugs with retracting electrodes. The spark plugs are installed in two of the combustion chambers.

        During operation, it is essential that an indication of the presence or absence of flame be transmitted to the control system. For this reason, a flame monitoring system is used

control system. For this reason, a flame a flame monitoring system is used consisting of multiple sensor, which are installed on combustion chamber. The ultraviolet flame sensor consists of a flame sensor containing a gas-filled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation, which is emitted by a hydrocarbon flame.

5. Turbine Section:

The hot gases from the combustion chambers flow through separate transition pieces. The gases then enter the two stage turbine section of the machine. Both stages consist of a row of fixed nozzles followed by a row of rotating turbine buckets. In each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop in the following row of moving buckets: a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor. After passing through the 2nd stage buckets, the gases are directed into the exhaust hood and diffuser, which contain a series of turning

Vanes to turn the gases from an axial direction to a radial direction to minimize exhaust losses. The gases then pass into the exhaust plenum and are introduced to atmosphere through the exhaust stack. Resultant shaft rotation is used either to turn a Generator rotor for electrical power production, or to drive a centrifugal compressor in industrial process application.

6. Exhaust system

Hot exhaust gases produced as result of combustion in the turbine are cooled and attenuated in the exhaust system ducting before being released to atmosphere. These exhaust emissions must meet certain environmental standards of cleanliness and acoustic levels depending on site location. The noise generated during gas turbine operation is attenuated by means of absorptive silencing material and devices built into the inlet and exhaust sections which dissipate or reduce the acoustical energy to an acceptable level.

2.4 WORKING PRINCIPLE:

A schematic diagram for a simple-cycle, single shaft gas turbine which is used in the 20 MW units is shown in the figure below:

                                            Fig 2.2        Single shaft gas turbine

STEPS:

• Air enters the axial flow compressor at point 1. Air entering the compressor at point 1 is compressed to some higher pressure which raises the air temperature so that the air at the discharge of the compressor is at a higher temperature and pressure.
• Upon leaving the compressor, air enters the combustion system at point 2 where fuel is injected and combustion occurs. The combustion process occurs at essentially constant pressure.
• When the combustion mixture leaves the combustion system and enters the turbine at point 3. it is at a mixed average temperature. in the turbine section of the gas turbine, the energy of the hot gases is converted into work. This conversion actually takes place in two steps. In the nozzle section of the turbine the hot gases are expanded and a portion of the thermal energy is converted into kinetic energy. in the subsequent bucket section of the turbine, a portion of the kinetic energy is transferred to the rotating buckets and converted to work. Some of the work developed by the turbine is used to drive the compressor, and the remainder is available for useful work at the output flange of the gas turbine.As shown in figure above, single-shaft gas turbines are configured in one continuous shaft and therefore all stages operate at the same speed. these units are typically used for generator-drive applications where significant speed variation is not required.

A schematic diagram for a simple-cycle, dual shaft gas turbine used in the three 8.5 MW units is shown in the Figure below:

        Fig 2.3    Dual shaft gas turbine

Here, the low-pressure or power turbine rotor is mechanically separate from the high pressure turbine and compressor rotor. This unique feature allows the power turbine to be operated at a range of speeds and makes two-shaft gas turbines ideally suited for variable speed applications. All of the work developed by the power turbine is available to drive the load equipment since the work developed by the high-pressure turbine supplies all the necessary energy to drive the compressor. The starting requirements for the gas turbine load train are reduced because the load equipment is mechanically separate from the high- pressure turbine.

                                                                                                                          CHAPTER-3

                 AC GENERATOR AND EXCITATION SYSTEM

The power driving the rotor, from a steam or gas turbine, is transferred to electrical power in the stator winding in the generator section. AC generators or alternators generate electricity by the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

        The armature winding of conventional synchronous machine is on the stator and is usually a three phase winding. The field winding is usually on the rotor. The rotating magnetic field induces an AC voltage in the stator windings.   The three sets of stator windings are physically offset so that the rotating magnetic field produces three phase currents, displaced by 120 degrees with respect to each other in the stator as shown in the figure below.

The speed of the generator under steady state condition is proportional to the frequency of the current in its armature. The magnetic field created by the armature currents rotates at the same speed as that created by the field current. The speed is given by N (in rmp) =120 *f/p where f is the frequency and p is the number of poles in the generator.

The field winding on the rotor is excited by dc current, or permanent magnets. The dc power supply required for excitation usually is supplied through a dc generator known as exciter, which is often mounted on the same shaft as the

3.1 EXCITATION SYSTEM:

CONCEPT OF BRUSHLESS EXCITATION SYSTEM

Supply of high current by means of slip rings involves considerable operational problems and it requires suitable design of slip rings and brush gear. And also the maintenance of brush rings creates problems. This necessitates the use of brushless excitation system. The block diagram is shown below diagram is shown below:

In this type of excitation system the generator is associated with an AC exciter, a PMG (Permanent magnet generator), rotating non controlled rectifiers and AVR/DAVR. In this system generator field, AC exciter’s armature, non controlled rectifiers and MPG’s field are connected to the same rotating shaft.

        The PMG’s field is a permanent magnet. As this permanent magnet also rotates with the generator’s shaft and there is a constant magnetic flux of the permanent magnet, a constant voltage is induced in the 3 phase stationary armature of the PMG. This PMG output voltage will be normally 220 v, 3 phase, 400Hz/150Hz/75Hz. This PMG output is connected to the thyristors located in the AVR panel.

        The controlled DC output from thyristor bridges is connected to non moving field of AC exciter. Inside the rotating shaft the diodes are arranged between the AC exciter armature and generator field. These rectifiers will convert the AC output of the exciter into DC which is fed to the field of the generator.

        There is no flow of current from any non moving part to the moving part or vice versa. Therefore there is no need for brushes. Thus brushes and slips rings are eliminated. The rotating diodes inside the shaft will have greater redundancy for meeting the voltage/current requirement of generator field.

        Compared to static excitation system brush less excitation system is mostly preferred in industries for the absence of brush assembly. A typical brush less excitation system has a DAVR (Digital automatic voltage regulator) system for bringing smooth control over the terminal voltage of the generator.

        In the above scheme, power is delivered to the rotor through the air gap by means of induction which in case of a Static excitation system is achieved by a set of brush assembly. By this way the generator field current is controlled by simply controlling the Thyristor firing angle which is taken care by the intelligent DAVR system.

        The use of PMG in the above scheme can’t be replaced by an usual station auxiliary supply as the rotating device produces perfect sinusoidal voltage signal which is used by the DAVR system for interrupt generation to the microprocessor of the DAVR for various limit checks.

3.2 BRIEF DESCRIPTION OF DAVR.

        The DVR/DAVR (Digital Automatic Voltage Regulator) regulates the terminal voltage and flow of reactive power of a synchronous machine in general by direct control of the main exciter field current using thyristor Converters.

3.2.1PRINCIPLE OF OPERATION

        To regulate the voltage and the reactive power of a synchronous machine, the field voltage must be adjusted quickly to the changes in the operating conditions (with a response time that does not exceed a few ms). In DVAR most of the delay that occurs originates in converter since the firing pulse for changing the rectifier DC  output are only issued periodically every 33ms for generators rated for 50 Hz.

        The DAVR regulates, calculates the control variable from the measured and reference data in very short time intervals. This results outwardly in quasi-continuous behavior with a negligible delay time.

        The calculation are made in binary number system. The set-point and limit values have already been defined in DAVR in binary form.

3.2.2DAVR AND ITS SYSTEM ARCHITECTURE

AUTO AND MANUAL CHANNEL

        The voltage regulation is done by two independent closed loop control systems namely auto and manual. Each has an independent power supply unit, gate control set and thyristor set.

        CHANNEL 1- auto channel controlling the generator voltage.

        CHANNEL 2- manual channel controlling the exciter field current.

During normal operation in AUTO channel the output pulses from manual channel final stage are blocked. Both channels are equipped with tracking feature so that the inactive channel always generates the same control variable as the active channel under steady state operation. This ensures smooth switch over from AUTO to MANUAL and vice versa. To ensure that the MANUAL channel will in case of a switch over initiated by a malfunction in AUTO channel will take over without disturbing the initiated by a malfunction in machine as it was prior to the problem the response of the tracking for the channel is set relatively slow.

        The most important monitoring inputs for the excitation system (If=Field Current, Ug= Generator Voltage, Usync- synchronizing Voltage) are redundant i.e 2 fold in nature. The excitation monitoring system checks for these inputs for discrepancy and initiates an alarm, a changeover to the standby channel if permitted.

        The auto channel is equipped with all the necessary limiting actions for the excitation system before the protection system of the generator comes into action. The various limiters on which the auto channel operates are:

1. Over fluxing limiter
2. Maximum field current limiter
3. Inductive stator current limiter
4. Capacitor current limiter
5. Load angle limiter
6. Power factor limiter
7. MVAR limiter

Unlike AUTO channel, MANUAL channel is a simple current regulator without any additional limiting acting. But MANUAL channel is used during

Faulty condition of the AUTO channel or testing of the excitation system or during the maintenance.

Unlike auto channel, manual channel is a simple current regulator without any additional limiting action. But manual channel is used during faulty condition of the AUTO channel or testing of the excitation system or during the maintenance.

MICRO TERMINAL

DAVR is equipped with a Micro terminal to view parameters and signal values of various processor systems and to change temporarily/permanently stored values in certain address range. It is also provided to enable/disable certain limiter/special function.

                                                                                                                           CHAPTER-4

              ELECTRICAL POWER  DISTRIBUTION SYSTEM

The power generated at 11kv is directly feed to the 11 KV Bus system-Bus A & b at the CPP Switchgear room through generator breakers. This power is then fed to the following substations from Bus A & Bus B from CPP switchgear room (Single line diagrams enclosed as Annexure):

Under the electrical distribution system the following Diesel Generator (DG) sets are available for providing emergency power supply:

4.1SUBSTATION:

Each substation has mainly three sections:

1. Switchgear room
2. cable vault
3. Battery Bank

4.1.1 Switchgear Room:

The Switchgear Room houses the control panels that constitute.

1. power Control Circuit (PCC): This controls the overall power in the substation.
2. Motor control Circuit (MCC): This controls the power to the electrical machines located in the process plants.

The PCC & MCC panels consists of :

• Circuit Breakers: Circuit breakers consists essentially of current carrying contacts called electrodes. Thses are normally engaged but, under predetermined conditions, separate to interrupt the circuit. When the contacts are separated an arc is struck between then. This arc plays an important role in the interruption process as it provides for gradual transitions from the current carrying to the voltage withstanding states of the contacts, but ist is dangerous on account of the energy generated in it in the form of heat that may result in explosive forces. This main problem in a circuit breaker is, therefore, to extinguish the arc shortly after it has stated and before the energy generated by it has reached a dangerous value. For this purpose it requires a arc quenching medium, and based on this circuit breakers are classified as:

• minimum Oil Circuit Breakers (MOCB)
• Bulk oil circuit Breakers (BOCB)
• Vacuum Circuit Breakers (VCB)
• SF6 Circuit Breakers

Of all these VCB’s are used in the substations. For these, the contacts are enclosed in a vacuum interrupter, where the arc produced during closing and tripping of the breaker is quenched.

• Control Switches and indicators: The switches basically control closing and tripping of the breakers, power on & off functions of the electrical machines.
•  The indicators include wattmeter, voltmeter, Ammeter, colour coded lights, which indicates status of various electrical apparatus.
• Relalys: Protective Relays are devices that detect abnormal conditions in electrical circuits by constantly measuring electrical quantities, which are under normal, or fault conditions. Having detected the fault the relay operates to complete the trip circuit which result in the opening of the circuit breaker and therefore in the disconnection of the faulty conditions. Depending on the fault relays are classified Earth fault Relay, under Voltage relay, Over Voltage relay, Over Current relay etc. There are P&B or numeric relays, which are of intelligent type and incorporates several protection in single unit, For motor protection there are special relays called motprorelays.

There are Light Distribution Boards (LDB), which consists of switches, and timers that control the power to the lighting system of the refinery. The power to the LDB’s is supplied through 1:1 ratio Dry Type Light transformers. The is done so as to avoid voltage drop from affecting the lighting system during starting of electrical machines.

There is a Remote input output (RIO) Terminal Box Located in very switchgear room. The panels are hardwired to this terminal box. Inside the terminal there are digital &  Analog I/O cards. This terminal is connected to the ECS via a RIO cable (for redundancy there are two cables). The data conveying status of the substation equipments and control signals from the ECS, like closing and tripping of the breakers is serially transmitted as digital signals.

2. Cable Vault: All the cables to and from the substation are brought in and out through the cable vault in the cable trace.

3. Battery Bank: The requirement of 110 V DC control power supply is made is made available through battery charger unit with battery banks as backup at

substation level. The batteries are of Nickel Cadmium type. The power is distributed to the load units through DC distribution boards

                               FIG 3.1 SUBSTATION

Transformers:

The refinery has got the following capacity power and distribution transformers:

The 110 V, Uninterrupted power supply (UPS) system is also available at different capacities to enable to sustain the operation of control systems/ logics during total

Power failure situation. The details of UPS units are listed below:

                                                                                                                   CHAPTER-5

                      DISTRIBUTED CONTROL SYSTEM (DCS)

The class of communication, which in addition to executing the stated control functions also permits transmission of control measurement and operating information to and from also permits transmission of measurement and operating information to and from a single or plurality of user specified locations connected via a communication subsystem is called a Distributed Control system.

The system of modular construction is expandable in future by adding additional modules. On line replacement of any module shall be possible in such a way that removal and addition of module shall be possible with de-energizing the system and whenever redundant modules are provided there should not be any interruption in system while replacing a faulty module. System as minimum shall meet the following requirement:-

1. Control data acquisition and monitoring
2. Alarming
3. Historical data storage
4. Logging and report generation

  5.1COMPONENTS OF DCS:

   The basic components of a DCS are mentioned below (for a CENTIUM –CS-3000 system) are as follows:

• Human interface Station (HIS): The HIS is mainly used for operation and monitoring- it displays process variables, control parameters and alarms necessary for users to quickly grasp the status of the plant. It also incorporates open interfaces so that supervisory computers can access trend data, messages and process data.

1. Console-type HIS: This high-end human interface station has the same functionality as its centum predecessors.
2. Desktop-type HIS: This is based on a generic PC (running Windows NT).

• Field control station (FCS): The FCS controls the entire plant. There are two types to meet different needs,. communication interfaces are also available for connecting programmable Logic controllers (PLC) and data Acquisition Units.

1. Standard-type FCS (LFCS): This FCS has powerful control function, high reliability and remote I/O support. In the standard FCS the Field Contjrol Unit (FCU) is liked by the Remote I/O (RIO) bus to I/O Nodes and I/O modules.
2. Compact-type FCS (SFCS): This is usually installed near the equipment to process its controls, and is ideal for communicating with subsystems. In the compact FCS, the FCU and the I/O modules are connected to the same back plate.

• Engineering PC (ENG): This is the PC with engineering functions used to perform CENTUM CS 3000 system generation and maintenance management. It can be the same type of general purpose PC as the HIS and can even be the same PC as the HIS.
• By having HIS operation and monitoring functions on the same PC one can use the test (control station emulation) functions to provide an efficient and easy-to-use engineering environment.
• Bus converter (BCV): This links the V-net system bus to another CENTUM-CS-3000 domain or to an existing CENTUM or MICRO-XL system.
• Communicator Gateway unit (CWG): This links the V-net control system bus to an Ethernet bus (to a supervisory computer system or personal computer). By CWG wide area communication function, one can also link two CENTUM-CS-3000 V-net different places using a dedicated telephone line.
• RIO Bus (for LFCS only): This communication bus links remote I/O units to FCS CPU. It can be dual redundant.
• Nodes (used only with FCS): Remote I/O units on the RIO bus interface between bus signals to and from an FCS and digital, analog signal.
• V-Net: The V-net real time control system bus links station such as FCS, HIS, BCV and CWG. Dual redundant V-net support is standard.
• Ethernet: Ethernet is used to link HIS, ENG and supervisory systems It is also used for transferring data files supervisory computers and for HIS data equalization.
• Field bus: The foundation field bus in the multi drop digital communication bus for field instruments and is expected to replace the conventional 4-to 20 milli-ampere analog interfaces.
• Unified Operation/Monitoring Station (UHMIS): This provides OPC server functions to enable applications in a supervisory PC access CENTUM-CS-3000 data. It provides a link between control layer and business data processing layer.
• Business Information PCS and Supervisory Computers: These can run MES and ERP integrated business management software. They can access the DCS via UHMIS or CWG.
• LFCS-XL and LFCS-V Control Station for migration to CENTUM-CS-3000:- one can leave the I/O cards and the field wiring of a CENTUM-XL or CENTUM-V system “as is” and replace the CPU nest with an LFCS, which can be connected to the V-net just like CENTUM-CS-3000 system FCS. SI bus is used as be a (dual redundant)j bus connecting existing FCS I/O units to new FCS CPU.

5.2FUNCTIONING OF DCS:

This is the centralized location in the refinery providing access to all signals from the devices present in the field whether they are located in safe or hazardous areas without disrupting operating. In the field, the various transmitters convert their respective physical parameters into a current signal of 4 to 20 mA CS and transmit it through the junction box. The transmitted signal enters the Barriers in the control room and reaches the DCS, where the signal is monitored. The Barriers are assemblies of components for use as safety interfaces between safe and hazardous areas and to limit the amount of energy, which can be transferred from a safe to hazardous areas in the event of faulty conditions occurring in the safe area.

The whole process takes the shapes of a closed loop and ultimately, result are obtained in the computer monitor which act as a Man machine interface. The configuration and controlling of all field instruments is done in the DCS. The operator in the DCS sends the proper current signal to the I/O card, which is then fed to the input converter.  The input converter correspondingly controls the control elements.

5.2.1PLC-5 PROGRAMMABLE LOGIC CONTROLLERS:

PLC-5 processor in a system is designed for centralized or distributed control. the main purpose of programmable controller is to control inputs and outputs of field devices like switches, valves and thermocouples. These must occupy a location in the processor memory so that they can be addressed in the control program. Each terminal on an input and output module that can be wired to field devices occupies a bit within the processor memory. The part of the processor memory that houses I/O addresses in the input image table and output image table. I/O addressing helps connect the physical location of an I/O module terminal to bit location in the process memory. I/O addressing is must a method to segment process memory.

The basic function of a programmable controller system is to:

1. Read the status of various input devices.
2. Make decision using a control program ladder logic based on the status of the status of these devices.
3. Set the status of output devices.

The processor performs two primary operations:

1. Program Scanning – where logic is executed and housekeeping is performed.
2. I/O Scanning – where input is read and output levels are set.

Plant wise Implementation of PLC System:

1. Captive Power Plant incorporates:

• PLC-5/60 for HRSG and BOP
• PLC-5/25 for Burner Management System (BMS)
• SLC-5/03 for Soot Blower system.

2. CDU/ VDU and CRU incorporates the PLC-5/40.
3. NDCU incorporates PLC-5/60
4. WHFU incorporates PLC-5/60
5. DM plant incorporates PLC-620-25 series.

Advanced process Control:

The basic idea of any plant process in the Refinery is to produce a product within certain specifications so that it could be sold off. In order to maintain these specifications certain process parameters namely pressure, temperature, level and flow must be controlled in every stage of the process. Again the specifications of the finished products lie within a certain range, for example a hypothetical product, say X in the market provided its boiling point lies in this range.

In most of the plants, at present, the parameters namely pressure, temperature, flow and level, is maintained through the DCS. This is a set point based system where the operators constantly monitors the process values and provides set values, which the corresponding values follow. In a process sometimes products, which go off the required specifications are produced. The is a set point based system where the operators constantly monitors the process values and provides set values, which the corresponding values follow. in a process sometimes products, which go off the required specifications are produced. The rectification of this takes time, as only a laboratory analysis of the sample would confirm it. In the meantime a lot of off specification products are already manufactured which would bring in losses to the plant.

Advanced process Control (APC) is the generic name given the software package, which eliminates the above mentioned problems. APC basically performs a back calculation of the entire process. it determines the specifications of the product from within the specifi.range.which causes minimization  of product variation. The system constantly monitors the product  specifications in real time and manipulates the set values. The ultimate goal of the APC is a mentioned of product variation and thus economization of the manufacturing process.

CHAPTER -6

CONTROL SYSTEMS

6.1 GAS TURBINE CONTROL SYSTEM:

           The turbine control system is referred to as SPEEDTRONIC control system. The objective of this control is to improve GT reliability, availability application, flexibility and serviceability. The SPEEDTRONIC control system is mainly used to control start up, acceleration, speed, temperature, shut down of the GT. These can be controlled by controlling the FSR (fuel stroke reference).

1. Mark-IV (used to control GTG 1,2,3)
2. Mark –V (used to control GTG 4)

In both mark-IV and mark-V there are three micro controllers and a communicator The micro-controllers are <R>,<S>, <I>. They are identical, independent, have their own power supply and input and output channels. The communicator is represented as <C>. <C> has independent communication channels with <R>,<S>. <I>. Sensors send inputs to the controllers sensing the turbine conditions and each controller sends its value to the communicator. Now the communicator will decide how much the fuel valve has to open thus controlling the FSR. Sensors that are not critical to the operation are brought directly into <C>. This system is based on 2/3 logic. That is output from <RST> section must be rated 2 out of 3. Mark-IV and Mark-V are identical but in case of Mark-V the controllers are also interlinked making the system more reliable.

Below are the block diagrams showing Mark-IV and Mark-V control system:

6.2 ELECTRICAL CONTROL SYSTEM (ECS)

The electrical control system is used for online monitoring and controlling of the CPP and refinery electrical distribution system. The following are the modes of operation to control all electrical parameters.

ECS mode: the basic operational philosophy of this mode is the PLC (programmable logic controller) mode of operation. This mode entails operation from the PLC control desk. PLCs are used for faster response.

EACP mode: in this case the basic operational philosophy is the MIMIC mode of operation. This mode entails operation from the MIMIC panel.All energy management functions like load shedding, synchronization can be done from ECS mode. EACP is a backup for ECS mode.

PCD and the use of ECS are explained elaborately taking the example of synchronization and load shedding.

1. SYNCHRONIZATION:

1. Through ESCL

There are two modes of synchronization:

1. Auto mode
2. Manual mode

Auto mode: once the auto option is selected all the parameters (voltage, frequency, phase) for synchronization will be automatically adjusted by LSM module and the operator does not need to adjust them.

Manual mode: once this mode is selected the operator has to raise/lower speed and raise/ lower voltage as required by clicking the appropriate buton on the screen.

2. Through EACP:

The circuit breaker to be synchronized and the mode of operation are selected in the MIMIC panel. The GT which has to be given commands is selected. The “ Sync ON” button is pressed and the synchroscope will show deflections. Appropriate commands viz. voltage raise, voltage lower, speed raise and speed lower are given to the GT. Once the synchroscope confirms synchronization the operator immediately closes the circuit breaker.

6.3 LOAD SHEDDING:

        The purpose of load shedding is to sustain generation for Protecting Power distribution to various critical Loads of IOCL Digboi Refinery under abnormal conditions/ instability in the Power system Network.

There are two schemes of load shedding:

1. Block load shedding
2. Under frequency load shedding.

1. Block Load shedding: This is a fast acting PLC based load shedding package in the electrical control system.

Whenever load shedding occurs, various data will get recorded in the PLC. When we talk of block load shedding, we come across the terms called priority list I and for priority list 2. Priority lists have been made by keeping in mind the various operating conditions. In the most critical places like the control unit load shedding is disabled. Let us suppose that two generators GTG-2 and GTG-4 are running and suddenly GTG-2 trips. A part of load of GTG-2 will be taken by GTG-4 but the rest has to be shed.

2. Under frequency load shedding;

When there is overload, frequency decreases and this is sensed by a under frequency relay. When frequency reaches below a certain frequency the relay trips and circuit breaker opens to shed load.

3. Generator healthiness

ECS also tells us about the generator healthiness. It indicates various parameters of the generator protection.

4. Voltage, current, consumption and generated loads are indicated by the ECS.

                                                                                                               CHAPTER-7

                                       INSTRUMENTATION SECTION

The instrumentation section of the refinery performs 2 basic operations:

1. Measurement and monitoring of various parameters involved in various processes.
2. Controlling the output of a specific process.

A distributed control system (DCS) is used to control various processes in the refinery like petroleum cracking, sulphur refining and hydrogen generation etc.

A DCS uses custom designed processors as controllers and uses both proprietary interconnections and communications protocol for communication. input and output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (a.k.a. field ) and transmit.

Instructions to the output instruments in the field. Buss connect the processor and modules through multiplexer or demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human-Machine interface (HMI) or control consoles.

DCSs are connected to sensors and actuators and use set point control to control the flow of material through the plant. pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning input/output (I/O) device. When the measured variable reaches a certain point, the controller instructs a valve or  actuation device to open or close until the fluidic flow process reaches the desired set point.

A typical DCS consists of functionally and geographically distributed digital controllers capable of executing  many regulatory control loops in one control box. The input/ output devices are located remotely via a field network. The controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control. Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber optic cable.

CHAPTER-8

REFINERY ELECTRIC

The Refinery Electric section looks after electrical System within the Refinery including the OPG Dispatch Unit, Effluent Treatment Plant & New Tank farm and fire detection & Alarm System. The distribution system consists of 11 KV, 6.6 KV, 3.3 KV, 0.415 KV, 0.23 KV, 0.11 KV grade. For the purpose of power distribution to the consuming end both underground and over ground cabling systems are used.

This section consists of a workshop where various tests of motors are carried out and sent for repair and rewinding if necessary. There is also a Testing Laboratory with state of the art testing equipment. These include a Transformer Oil Tester (to test the breakdown voltage of transformer oil), Primary Current injection set and Secondary Injection set (for testing of CT’s ) Digital Megger and among others a Digital Micrometer (to test the contact resistance of circuit breakers).

7.1 STUDY ON P &B RELAYS:

Supervision series

• MOTOR VISION MV2

Motorvision is for the Protection, Monitoring and control of LV & KV Voltage contactor or Circuit Breaker controlled Motors.

Features.

This unique product range has f fully graphical display that can display the motor starting characteristic. The advanced PCB surface mount technology supports Current and voltage measurement with optional Temperature measurement, disturbance recording, time stamping and smart Card. Motorvision is modular and can have 3 output relays and 5 digital inputs or 4 outputs and 12 inputs. An optional 4-20 mA output can be selected scaled for example for motor Load, Power or Real Power.

• Advanced Motorvision AMV2

Intelligent Prot3ection & Control for the more important Motros.

Characteristics

Advanced Motrovision comes complete in a DIN Standard case. Its Modular concept allows the user to take advantage of all the standard MOTORVISION features and add the following options;

Trip Circuit Supervision.

Back spin Protection

Three Phase VT card

Temperature card with 4 or 8 RTD inputs.

Extra Relay card with an additional 4 output relays.

Extra Digital input card with an additional 12 Digital inputs.

• FEFDERVISION FV2

This intelligent protection & control relay provides three phase O/C & E/F or two phase O/C E/F plus standby Earth Fault. The relay supports various IEC characteristics as well as time Stamping, Disturbance recording and can be supplied with a smart card.

Characteristics

Disturbance recording can be viewed on the relays fully graphical display or through its RsS232 serial output. It provides a range of protection functions including Current, Single or three phase voltage and requency and can be supplied with the following option:-

• Trip Circuit Supervision
• Breaker Fail Protection
• Extra Relay card with an additional 4 Output relays.
• Extra Digital Input card with an additional 12 digital inputs
• .Extended Feedervision EFV 2

    Is the name given to a feeder vision relay card and/ or an extra Digital input card fitted.

• ADVANCED FEEDERVISION AFV2

This product is designed for use as part of an auto changeover scheme.

Characteristics

Four Advanced Feeder vision relays are required to complete the scheme – two to protect and control the incoming feeders and one for the Bus section and one to handle the logic.

It complies with the normal logic required by the Exxon, IOCL, BPCL & Reliance Oil Auto POWER DISTRIBUTION SYSTEM

                                      CHAPTER-9

                   SUBSTATION SWITCHING OPERATIONS

A typical substation normally consist of the following equipment:

• Sets of HT switchgear panels (11KV / 6.6KV).
• Sets of 415 volts LT power control centres (PCC).
• Sets of LT motor control centres and ASS board.
• Battery charger / battery bank / DCDB.
• Annunciation / signaling panel.
• Inverter for instrumentation power supply.
• Air blower for pressurization system.

In case any signals appearing on the annunciation panel, the Shift Engineer / Technician will accept the hooter signals and will note down all the light widow facial which have started flickering. Also, he should inspect all the relays of affected breaker panel and note down relay numbers with description on which flags have appeared. All these observations are to be recorded in the daily logbook.

The schematics and switching operations for a typical switchgear (Jyoti) are elaborated below.  As the schematics/design basis/vendors may be different in various locations, the same can be used as a reference for switching operations in sub-stations.

8.1 DC CONTROL SYSTEM

DC control system is the heart of the control and protection schemes of switchgears, which is required for ensuring reliable relay & switchgear operation. To ensure availability of uninterrupted control supply even in case of total power failure, the control supply for control circuits of the HT switchgears, normally 10 volts DC, which is obtained from the combination of:

1. Set of DC batteries
2. Battery charger panel
3. DC distribution board

Like any normal DC system, in this system also, the normal DC control supply power demand is catered by the charger unit, the bank of DC batteries remaining in floating condition. The DC output is brought in the DC distribution panel from where it is distributed.

The basic typical scheme of DC control supply to switchgear panels are as under:

Normally, two outlet from DCDB are taken to dummy panel in each section of HT switchgear where 2 nos. Primary DC control bus I & II are formed through switch and contactor in each dummy panel. Both these DC buses are normally brought to the bus coupler panel and inter connected though a switch, which is kept ON. Both the contactors are electrically interlocked such that only one contactor is ON at a time. From this DC Primary bus tapping is taken through a MCB in each HT section to energized the DC control bus, which is running through out the HT section. The DC control supply of each panel is tapped from the control bus.

8.2 BUS BAR SECTION PROTECTIONS (Typical scheme)

Normally, both the HT sections are provided with following protections / group tripping.

• Bus bar differential protection (with annunciations).
• Group tripping of motors due to under voltage (with / without annunciation).
• Bus bar earth fault (annunciation only for UE system).
• Group tripping of selected feeders under “LOAD SHEDDING”.

Normally, all the above protections / group trippings of each sections are covered by one common control circuit in the bus PT panel of the respective sections. Therefore, if the DC control switch in the bus PT panel is kept OFF or the control fuse of bus PT panel gets fused, all the above protections will stand bypassed. Therefore, in case “Bus PT Control Supply Failure” signal appearing it is very essential to attend to it and restore the DC control supply of bus PT panel without any loss of time.

8.3 Bus Bar Differential Protection

Separate CTs are provided on all the panels of each section including its incoming and bus coupler. Secondaries of all these CTs are wired along with the coils of relay 87B in such a way that under healthy conditions the currents are balanced and there is no resultant current in the coil of differential protection relay 87B.

When this tripping occurs, it is accompanied by an audible signal as well as “Bus Differential Acted” light signal on the central annunciation panel.

It may be noted that contacts of relays 86B directly gives tripping impulse to the shunt trip coils ‘B’ of all motors and transformer feeders and therefore, no signal in respect of tripping these breakers appears. But in case of the incomer breaker, the contact of relay 86B gives the tripping impulse through its master tripping relay 86B and therefore, “Incomer Breaker Auto Trip” signal also appears along with “Bus Differential Acted” signal.

Action to be taken when bus differential acts

1. Inform production control room to start standby HT motors.

2. Cancel / acknowledge the hooter / bell.

3. Change switch 43BT of bus coupler panel from AUTO to MANUAL position.

4. Do not attempt to close the bus coupler breaker manually.

5. Inform TPS to switch OFF the affected feeder as the case may be and inform them about the differential acting.

6. Switch OFF DC control switch of all the affected breakers of the  section
7. Pull out all the breakers of this section in TEST position.

8. Call maintenance and testing engineer.

8,4 Under Voltage Group Tripping Of Motors

There is a provision of group tripping of all the HT motors of a section in case of failure of its incoming supply (i.e. under voltage). In this group tripping also there can be  further provision of selecting or segregating the motors into two groups viz. “Critical and non-critical”. This is made possible by providing two sets of group tripping control bus bars viz. critical tripping control bus and no-critical tripping control bus, throughout the sections. Any motor can be selected to critical or non-critical tripping made with the help control switch provided in each motor feeder breaker panel. Shift Technicians are NOT ALLOWED to change or alter the position of the switch.

If under voltage occurs on any section, then, within 0.5 to 2 seconds of such under volt occurrence the non-critical tripping control bus throughput the section gets energized through time relay auxiliary relay on PT panel / incomer panel and passing contact of the breakers of all the motors whose switch is put in non-critical made trips within this time (through their auxiliary relay 27X).

However, if the under voltage (or no voltage) continues for 2 to 15 seconds through contacts of another time relay and auxiliary the critical tripping control bus throughout the section also gets energized and trips all the remaining motors of this section (through their auxiliary relay 27X).

Actions to be taken

When the Shift-in-Charge comes to know about section under voltage he shall proceed as follows:

1. Inform production control room to start standby motors.

2. Check, if the incomer breaker is in TRIPPED condition.

3. Check, if the bus coupler breakers has become ‘ON’.

4. Inquire from TPS electrical control room if anything is wrong with the concerned feeder.

5. Check, if the voltmeter on the incomer panel indicates any voltage. (it should not show any voltage).

6. Ensure that protection relay ‘86’ on incoming panel HAS NOT operated.

7. Switch OFF DC control switch of incomer breaker. If till now, voltage is not restored to the section then.

8. Change the bus-tie switch  from AUTO to MANUAL position.

9. Switch ‘ON” the bus coupler breaker with the help of control switch 52CS only. This should be done in presence of Shift Engineer).

10. Observe the voltage on the voltmeter mounted on bus PT panel.

11. Inform production control room that power supply is on one feeder only.

12. Call Electrical Testing Engineer.

8.5 Load Shedding

Facility of load shedding is normally provided in both the HT section. A relay normally mounted in bus PT panel is connected in the control circuits of bus PT panel. This relay will get energized from an actuating load shedding command from TPS. One N.O. contact of this relay energizes auxiliary relays. N.O. contacts of these relays are wired to the shunt trip coils of motor and transformer feeders of the section and shall trip these breakers when operated.

8.6  Bus Bar Earth Fault (Signaling Only For UE Systems)

The bus bar potential transformer has two secondary windings, one is used for metering and protection while the second windings are connected in open DELTA. A relay 64, is connected across the two terminals of this open delta winding. When the 6.6KV side voltage on all the three phase is OK i.e. balanced, then no voltage is appearing across the coil of this relay 64. But, if there is an earth fault on 6.6KV bus bars the voltage across the two terminals of secondary of open delta winding will no more remain zero and some resultant voltage will appear which, in turn, will energize the relay 64.

The gives only annunciation ‘Bus Bar Section Earth Fault”. But no tripping of breaker is associated with it.

Actions to be taken

If the bus bar earth fault signal appears for either of the section then proceed as follows:

1. Enquire from TPS electrical control room regarding any abnormality on that particular feeder.

2. Inform Shift-Engineer.

3. Inform Testing Engineer.

4. Check the voltage in all the three phases on the voltmeter fixed on incomer panel.

If found equal it is OK. If found unequal draw out incomer breaker (following all usual procedure) and check up the HT fuse on line PT.

5. Check voltage in all three phases on the voltmeter fixed on bus PT panel.

If found equal OK. If found unequal draw out the bus PT (following safe procedures) and check the HT side fuses. Replace fused fuse.

8.7 HT INCOMING BREAKERS

The HT incoming feeder breakers are normally  provided with the following protections and annunciation.

• Under voltage tripping with no annunciation in the signaling panel.
• Over current tripping on three individual phases with annunciation “Breaker Auto Trip”.
• “Control Supply failure” annunciation only.
• “Line PT MCB Trips” annunciation only.
• Earth fault tripping.( for earthed system)

The following actions are to be taken in the event of incoming breaker trips or annunciation appears due to the above abnormalities.

8.7.1 Under Voltage Tripping

The incoming breaker will trip on under voltage through relays 27-1A, 27-1B and Timer T-1 when all the following conditions are satisfied.

1. There is sufficient under voltage (40% or below) to drop down the under voltage relays 27-1A and 27-1B both mounted on the incoming panel and under voltage relays 27-2 mounted on bus PT panel of the same section.

2. The other section must be healthy and the U/V relay 27-2 on the bus PT of this healthy section must be in picked-up condition.

3. Bus coupler selector switch in the bus tie panel must be in AUTO position.

4. Lint PT MCB must be in ON condition.

If all the above conditions are satisfied and under voltage on the bus section occurs, the incoming breaker will trip.

In the case of under voltage, the incoming breaker will trip first through relay T-1 and then the bus coupler will close in, if its selector switch is kept in “AUTO” position. If the bus coupler has closed in, its selector switch is to be put into the MANUAL position. Main control room of the TPS is to be informed and if necessary, the incoming feeder breaker of the affected section is to be racked out as per normal procedure.

8.7.2. Over Current Tripping

The over current relay 51-R, 51-Y or 51-B of that particular phase will operate and through the tripping relay 86, the incoming breaker will trip.

The tripping of incoming breaker will be associated with the annunciation “Breaker Auto Trip” in the main annunciation panel.

With the tripping of incoming breaker due to over current the bus coupler breaker will not close automatically even its selector switch is in AUTO position. In this case the following actions are to be taken:

1. Inform production control room to start the standby motors of those motors have tripped.

2. Rack out the incoming feeder breaker and bus coupler as per normal procedures.

3. Inform electrical testing and Electrical Maintenance Engineers.

4. Change the position of the bus coupler selector switch from AUTO to MANUAL position.

5. But do not attempt to switch on the bus coupler manually.

6. The section can NOT be charged till clearance from Electrical Testing Engineer is obtained.

8.7.3 Differential Protection Tripping

When the differential protection acts, its relay 87-B shall trip the incoming breaker along with all the outgoing HT feeders of that section.

When incoming breaker trips due to operation of the differential relay of that section, the annunciations, “Incoming Breaker Auto Trip” and “Bus Differential Acted” will appear in the annunciation panel.

The Shift Technician has to follow similar instruction as mentioned for over current tripping of incoming breaker above article.

8.7.4 Control Supply Failure

Appearance of control supply failure signal is only a warning signal and is not associated with tripping of breaker. This signal will appear, if either of the following conditions takes place.

1. Control fuse blows OFF.

2. No voltage relay (80) drops out.

3. The closing coil circuit is disrupted.

4. The tripping relay circuit is disrupted.

The following actions are to be taken when control supply failure signals appear:

1. Check indication flags of relays 80, 80-2 and 86 X 1-1 mounted on the incoming panel have dropped. If so, it indicates either of the control fuses F-1 or F-2 has blown off. Replace the blown off fuse, reset the indication flags of all relays and signal will disappear.

2. If the indication flag of relay 80 has dropped only, then inform Electrical Testing and Electrical Maintenance to attend the relay 80 and its circuit.

3. If the indication flag of relay 80-2 has dropped only, then there may be discontinuity in the closing coil circuit. Immediately, inform to Electrical Maintenance & Testing Engineers for repairs.

4. If the indication flag of relay 86X1 has dropped down then, there may be discontinuity in the tripping relay circuit, which is of very serious nature. Inform Electrical Testing Engineer to attend “IMMEDIATELY” on “EMERGENCY” basis.

8.7.5 Line PT MCB Trip

This will appear as an annunciation only when line PT MCB.

The significance of this signal is that, in case of under voltage in that section, the incoming breaker will not trip and as a result bus coupler will also not close automatically, if the situation arises.

In this case, if the PT MCB has tripped, the same should be switched on ONCE after physical checking of any short circuit evidence in its circuit inside the panel compartment. If it holds, it is OK. If it does not hold then, it should not be switched on for a second time and Testing Engineer is to be informed immediately.

9.1BUS TIE PANEL

The HT bus tie panel breaker is normally provided with the following protections and annunciation:

• Over current tripping with annunciation “Breaker Auto Trip”.

• Bus differential tripping with annunciations “Breaker Auto Trip” and Bus Differential Acted”.

• “Control Circuit Failure” annunciation only.

• Earth fault protection (for earthed system)

The following actions are to be taken when the bus coupler breaker has tripped and annunciations appeared due to the above abnormalities.

9.1.1Over Current Tripping

If there is an over current in any one of the three phases of bus coupler breaker which was feeding power to either of the two HT sections, then the instantaneous over current relay (50 R/Y/B) will operate and the bus coupler will trip instantaneously or after some time delay through an auxiliary relay.

The following actions are to be taken when the bus coupler trips on over current:

1. Inform production control room engineer to start the standby motors of those, which have tripped.

2. Inform Electrical Testing and Electrical Maintenance Engineers to attend the problem IMMEDIATELY ON EMERGENCY basis.

3. DO NOT ATTEMPT to switch on the bus coupler breaker manually after resetting the relays.

4. Meanwhile, if the incoming feeder breaker of the affected section is available for service the same MUST NOT BE SWITCHED ON till clearance from Testing Engineer is obtained.

5. Rack out the bus coupler breaker to test position as per normal practice.

9.1.2 Bus Differential Tripping

When the bus differential protection acts 87-B, it will trip the bus coupler breaker instantaneously if the same is in ON condition at that time.

Tripping of bus coupler breaker due to above protection will be associated with annunciations “Breaker Auto Trip” and “Bus Differential Acted”.

9.1.3 Control Circuit Failure

This will appear as a warning signal only in the annunciation panel, when control supply of the tie feeder panel fails. As it is a warning signal only, therefore, it does not associate with any tripping of the breaker. The annunciation appears due to the following reasons:

1. Control fuse has blown off.

2. Closing coil circuit of the breaker is not healthy.

3. Tripping relay (86) at circuit of the breaker is not healthy.

10.1 TRANSFORMER FEEDER BREAKER

The HT transformer feeders are provided with the following protection and annunciations:

• Over current tripping with annunciation “Breaker Auto Trip”.

• Buchholz tripping with annunciation “Breaker Auto Trip”.

• Oil temperature tripping with annunciation “Breaker Auto Trip”.

• Winding temperature tripping with annunciation “Breaker Auto Trip”.

• Restricted earth fault tripping with annunciation “Breaker Auto Trip”.

• Transformer HT side earth fault – annunciation only.

• Transformer trouble – annunciation only.

• Control supply failure – annunciation only.

The following actions are to be taken, if the transformer feeder breaker trips or the annunciation appears due to the above abnormalities.

10.1.1 Over Current Tripping

If there is an over current in either of the phases of the outgoing feeder, the over current relay (50/51 R, 50/51 Y, or 50/51B) of that particular phase operates and will give tripping impulse through lockout relay (86) to the feeder breaker. The feeder breaker may trip instantaneously or after some time delay depending upon the magnitude of the over current.

The tripping of the feeder breaker will be associated with annunciation “Breaker Auto Trip”.

The tripping of HT transformer feeder breaker will be followed by tripping of PCC LT incoming breaker of that same transformer because, a normally close (NC) contact of an auxiliary switch of HT breaker has been used for giving tripping impulse for PCC incoming breaker. However, the PCC bus coupler breaker will close automatically (if its selector switch is in AUTO position) if the PCC incoming breaker trips due to the tripping of HT feeder (i.e. relay 86 of PCC incoming breaker has not operated).

The following steps are to be followed when the HT transformer feeder breaker trips due to over current:

1. Check the bus coupler has acted automatically, if its selector switch is kept in AUTO position (Normally, it is to be kept so). If the bus coupler has acted, change the position of the selector switch to MANUAL position.

2. If the PCC bus coupler HAS NOT ACTED AUTOMATICALLY, then ensure the lockout relay 86 on the incoming breaker of the affected PCC section HAS NOT operated. if it is so, change the bus coupler selector switch to MANUAL position and switch the bus coupler breaker ON in presence of Electrical Shift Engineer-in-Charge.

But, if the lockout relay 86 of the PCC incomer has operated, but coupler will not close automatically and the same CAN NOT be made ON MANUALLY also. Inform Electrical Testing and Maintenance Engineers to take actions IMMEDIATELY. Inform production control room also to start the spare motors of those, which have been affected.

3. If the bus coupler has closed automatically then check the load of the incoming breaker in service and ensure that the load is below the full load capacity of the transformer.

4. The HT & LT PCC breakers of the affected transformer feeder is to be racked out to TEST position.

5. Inform Electrical Testing and Maintenance Engineer to take necessary actions.

10.1.2 Buchholz Tripping

The HT transformer feeder breaker will trip through its Buchholz relay when there is a surge of gas generation inside the transformer due to its any internal fault. The PCC side bus coupler will act automatically as described in the case of over current tripping and all the steps, described above.

10.1.3 Oil Temperature Tripping

If the temperature of oil inside the transformer increases due to over load or any other reasons, an alarm “Transformer Trouble” will appear first through relay 74X2. However, if it increases further beyond a specific set value, the HT breaker will trip and signal “Breaker Auto Trip” will appear in the annunciation panel. The PCC side bus coupler will close in automatically as described in the case of over current tripping.

10.1.4 Winding Temperature Tripping

If the temperature in the core winding increases due to over load or any other reasons, an alarm “Transformer Trouble” will appear first through the relay 74X1. If the temperature increases further, the HT transformer feeder breaker will trip and signal  “Breaker Auto Trip” will appear in the annunciation panel. The PCC bus coupler will close in automatically and all steps are to be followed as described in the case of over current tripping above.

10.1.5 Restricted Earth Fault Tripping

When there is an earth fault between transformer secondary windings and PCC incoming breaker the restricted earth fault relay 64R (Mounted on each PCC incoming panels) operates and will give tripping impulse on both the HT outgoing and LT incoming breakers of the transformer. The HT breaker will trip through its lockout relay when the LT PCC incoming breaker will trip through auxiliary relay 52X3. As a result, the PCC bus coupler breaker will be allowed to closed in automatically as described in the case of over current tripping above and the same steps are to be followed.

10.1.6 Transformer HT Side Earth Fault

For unearth system, this will appear in the form of warning signal when there is an earth fault in the HT feeder cable or in the HT winding of a transformer.

A core balance CT is mounted on the outgoing cables of the HT switchgear for this purpose to give impulse to the earth fault relay (50N) of transformer feeder.

When the above signal appears in the annunciation panel, the following actions are to be taken.

Check the voltage on the voltmeter in all the three possible combinations on the bus PT panel of the HT section from which the affected transformer is fed.

Case-A:        If the voltage is unbalanced

If the voltage is not found to be equal in all the three combinations (on voltmeter of PT panel) and wide variations are observed; then, it can be assumed that an earth fault on that feeder has occurred. In that case, the following actions are to be taken.

1. Switch OFF the HT breaker of the affected transformer feeder immediately. With the tripping of HT breaker, it is to be ensured that the PCC incoming breaker for that transformer has also tripped and bus coupler breaker for that PCC section has closed automatically (as its selector switch is kept in AUTO position).

2. Inform electrical Shift-Engineer-in-Charge about the earth fault on the HT transformer feeder.

3. After checking that HT feeder is isolated from both HT & LT side, inform production control room engineer to start the motors that have tripped during the PCC change over period.

4. Inform Electrical Testing and Maintenance Engineers to attend the problem.

5. Rack out both the HT and PCC incoming breakers for the affected transformer feeder to TEST position.

6. Put the PCC bus coupler selector switch to MANUAL position.

Case-B:        If the voltage is balanced

If the voltage is found equal in all the three combinations on the PT panel voltmeter of the HT section then:

1. Check the voltage on the PCC incoming panel for that transformer and ensure that it is equal. If it is so, then put the bus coupler selector switch to MANUAL position and switch ON the bus coupler breaker and switch OFF the affected PCC incoming breaker with a minimum possible paralleling time. Switch OFF the HT side breaker for that transformer and rack both the PCC and HT breakers out to TEST position. Inform to Electrical Testing Engineer to attend.

2. If the voltage on the PCC incoming panel is not equal as described in  (a) above, then it is to be assumed that there is an earth fault and all the steps described for the case of CASE-A above are to be followed without time delay.

10.1.7 Transformer Trouble

This is a warning signal in the form of “Transformer Trouble” only when there is an abnormality in the transformer due to any one of the following reasons:

1. Winding temperature high though relay 74X1.
2. Oil temperature high through relay 74X2.
3. Oil level low through relay 74X3.
4. Gas accumulation in Buchholz relay chamber through relay 74X4.

Therefore, whenever the “Transformer Trouble” signal appears in the annunciation panel, all the above four relays for that particular HT panel is to be checked and the steps are to be taken for operation of the individual relays as described below:

        CHAPTER 10

                                                      MOTOR FEEDER

The HT motor feeder breakers is normally provided with following protections and annunciations:

• Instantaneous short circuit current tripping.
• Instantaneous negative sequence current tripping.
• Thermal over current tripping.
• Lock rotor tripping.
• Earth fault tripping.
• Under voltage tripping
• Bus differential tripping.
• Control circuit failure.

The following actions are to be taken when either of the above abnormalities takes place in the HT motor feeder.with CTMM RELAY protection

11.1 Instantaneous Short Circuit Current Tripping

The HT motor feeder will trip, if element I1 of CTMM relay (96) operates.

The tripping of HT breaker will be associated with annunciation “Breaker Auto Trip.

With the tripping of breaker due to element I1 of relay ‘96’ the following steps are to be followed:

1. Inform control room production engineer to start the spare motor of that which has tripped.

2. Inform Electrical Testing and Maintenance Engineer to attend the problem.

3. Rack out the HT breaker to test position.

4. Meggar the motor by 2.5KV meggar along with the power cable (after discharging for all the possible combinations from the switchgear end.

11.2 Instantaneous Negative Sequence Current Tripping

The HT motor feeder breaker will trip if the element I2 of CTMM relay (96) operates.

The tripping of HT breaker will be associated with annunciation “Breaker Auto Trip”.

The similar actions are to be taken in this case also be indicated for short circuit current.

11.3 Thermal Over Current Tripping

The HT motor feeder breaker will trip, if the element ‘Th’ of the CTMM relay (96) operates.

The tripping of HT motors breaker will be associated with the annunciation “Breaker Auto Trip”.

With the tripping of the HT breaker due to the operation of the thermal element (Th) of relay (96) the following steps are to be followed:

1. Inform control room Production Engineer to start the spare motor of that which has tripped.

2. Rack out the HT breaker of the motor to test position.

3. Discharge the cable and meggar the IR value by 2.5KV meggar for all possible combinations along with the power cable from the switchgear end.

4. Check the rotor freeness.

5. Inform Electrical Testing and Maintenance Engineers to attend the problems.

11.4 Lock Rotor Tripping

The HT motor feeder will trip, if the lock rotor element 50 L/R of CTMM relay (96) operates.

The tripping of HT breaker due to the operation of the 50 L/R element will be associated with annunciation “Breaker Auto Trip”.

The lock rotor element of CTMM relay may operate during the starting time or stalling period of the motor. With the operation of this element of the lock rotor relay the following steps are to be followed:

If the 50 L/R element operates during starting time (i.e. when its standby motor is in service) due to failure of picking up; then the motor freeness is to be checked and Electrical Testing and Maintenance Engineers are to be informed for necessary actions.

If the 50 L/R element operates during running time then following actions are to be taken:

1. Inform the control room Production Engineer to start the spare motor of that which has tripped.
2. Check the rotor freeness and inform the Electrical Testing and Maintenance Engineer to take necessary action.

3. If the spare motor is not available then ensure the rotor is free from any jamness and if the Production Engineer insists for running the CTMM relay ‘96’ can be made reset and motor can be restarted again.

4. If the motor is started due to above condition (c) the relay ‘96’ CAN NOT be made reset for second hot start-up due to tripping of the motor once again and Electrical Testing and Maintenance Engineers are to be informed to attend IMMEDIATELY on EMERGENCY basis.

11.5 Earth Fault Tripping (50N)

The HT motor feeder breaker will trip due to the operation of the earth fault relay (50N).

The tripping of this breaker will be associated with annunciation “Breaker Auto Trip”.

With the tripping of the motor feeder breaker due to operation of the earth fault relay 50N, the following actions are to be taken:

1. Inform Production Control Room Engineer to start the spare motor of that which has tripped.

2. Rack out the HT breaker to test position.

12.1 PCC INCOMING FEEDER

The LT PCC Incoming breaker is normally provided with following protections and annunciations.

• Under voltage tripping with annunciation ‘Incomer-Bkr. Auto Trip’.
• Restricted Earth fault tripping with annunciation ‘Incomer-Bkr. Auto Trip’.
• Tripping of PCC Incoming breaker due to the tripping of HT side breaker of the transformer with annunciation ‘Incomer Bkr. Auto Trip’.
• Overload (Bi-metal) tripping with annunciation ‘Incomer Bkr. Auto Trip’.
• Instantaneous short circuit tripping with annunciation Incomer Bkr. Auto Trip.
• Bus-Bar earth fault tripping with annunciation ‘Incomer Bkr. Auto Trip’.

12.1 Under Voltage Tripping

The PCC incoming feeder will trip due to under voltage, if both the following conditions are satisfied.

1. Both the under voltage relays 27-A and 27-B (or 27-C & 27-D) of the affected sections have dropped down.
2. And, the under voltage relay 27-B or 27-D which one is applicable depending upon the incoming feeder of the adjacent spare bus section must be in picked up condition i.e. adjacent section of the affected one must be in healthy condition.

With the tripping of the PCC incoming breaker ‘Incomer Bkr. Auto Trip’ signal will appear in the annunciation panel.

When PCC Incoming Breaker is tripping on under-voltage the Bus-coupler will close in automatically, if the selector switch ‘CS-2’ is in ‘Auto’ position. If the bus-coupler has closed automatically, then put its selector switch to ‘Manual” position for safety. Check the HT breaker of the affected feeder, and be the same is to be switched ‘Off’ if it remained in ‘on’ position. The HT breaker is to be racked out as per normal procedure, if required.

If the bus-coupler breaker does not close automatically when the incoming breaker has tripped due to under voltage because of malfunctioning of relays & switches (as it happened some times) then ensure that the relay ‘86’ mounted on the affected incoming panel has not operated and, if so, bus-coupler selector switch ‘CS-2’ is to be brought to Manual position and the bus-coupler breaker is to be made on through its switch CS-1 in presence of the electrical shift engineer-in-charge.

12.2 Restricted Earth Fault Tripping

If there is any earth fault between transformer secondary winding and PCC incoming breaker, the restricted earth fault relay (64R) will operate and will give tripping impulse to both the PCC incoming breaker and HT outgoing breaker of that faulty transformer feeder. The tripping of LT incoming breaker will be associated with annunciation breaker auto trip. The LT bus-coupler on the PCC sections will close in automatically if its selector switch is in auto position. The similar steps are to be followed as indicated in the case of under voltage tripping.,

12.3 HT transformer breaker tripping

If the HT Transformer feeder breakers trips due to any one of the reasons the PCC incoming breaker of that feeder will also trip automatically with annunciation ‘Breaker Auto trip’. However, PCC Bus-coupler breaker will close in automatically if its selector switch is kept in auto position. Similar steps are to be followed as indicated in the case of under voltage tripping above .

12.4 Overload tripping

If there is any overload on a particular PCC Section, the LT incoming breaker for that section will trip through its lock out relay 86.

The tripping of PCC incoming breaker on overload will be associated with annunciation ‘Breaker Auto Trip’ in the annunciation panel.

It is to be noted here, that the PCC bus-coupler breaker will not close automatically, when the incoming breaker has tripped on overload, even its selector switch ‘CS-2’ is in ‘Auto’ position. Attempts for closing the bus-coupler breaker manually is not permitted as it may endanger the other healthy sections also. In this case, the following steps are to be followed.

1. Put the bus-coupler selector switch ‘CS-2’ to Manual position. This is very much required here because resetting of thermal O/L & Lock-out relay in tripped incomer panel may allow to close the Bus-coupler breaker automatically which may danger the other cause healthy section also as described above.
2. Inform control room production engineer to start the spare motors of those which have been affected.
3. Inform electrical testing and maintenance engineers to attend the problem immediately on emergency basis.
4. Make the affected HT transformer breaker ‘Off’ electrically from HT Panel.
5. Rack out the LT PCC incoming and bus-coupler to test position.
6. Switch off the MCC and ASS board outgoing feeder breakers off of the affected section as they are dead. However, the outgoing motor feeders of this section will trip automatically due to its inherent under voltage release coil.
7. Affected PCC bus-coupler section can only be charged after getting clearance from the electrical testing engineer.

12.5 Instantaneous short circuit tripping

When there is any short circuit on PCC bus-section the incoming breaker will trip through its inbuilt micro-switch ‘MS’ which will also energise the lock out relay ‘86’ of the incoming breaker. This micro-switch is a mechanical unit and actuates by either of current operated inbuilt short circuit trip coils 52 TR, 52 TY and 52 TB.

The tripping of the incoming breaker due to above condition will be associated with the annunciation ‘breaker auto trip’ in the annunciation panel.

In this case also, the bus-coupler breaker will not close automatically even its selector switch ‘CS-2’ is in Auto position. Attempt for closing the bus coupler breaker is not permitted as this will endanger the other healthy section also.

The similar actions are to be taken here, also; as described in the case of overload tripping.

12.6 Bus Bar Earth Fault Tripping

When there is an earth fault on the PCC Bus-bars, the incoming breaker of this section will trip through its inbuilt micro-switch ‘MS’ which will also energise the lockout relay ‘86’ of the breaker. This micro-switch is a mechanical unit and actuates with the operation of inbuilt earth fault relay (52T E/F).

The tripping of the incoming breaker due to earth fault will be associated with annunciation ‘Breaker Auto Trip’ in the annunciation panel.

In this case also, the bus-coupler breaker will not close automatically, even its selector switch ‘CS-2’ is in Auto position. Attempts for closing the bus coupler breaker ‘Manually’ is not permitted as this will endanger the other healthy section also.

The similar steps are to be followed here also, as described in the case of overload tripping.

                                CONTENTS

                FRONT PAGE

1. ACKNOWLEDGEMENT………………………………………………………………………………………. (i)
2. PREFACE……………………………………………………………………………………………………………(ii)
3. LIST OF CONTENTS……………………………………………………………………………………………..(iii)

S.NO.                                                                       CHAPTER                                                        PAGE NO

1.                                   ELECTRICAL POWER SYSTEM DIGBOI REFINERY………………..1,2,3
2.                                                                GAS TURBINE……………………………………………4

1. DEFINATION AND WORKING……………………….4,5
2. GAS TURBINE FUNCTIONAL DESCRIPTION…….6
3. SYSTEM DESCRIPTION………………………………….6,7,8
4. WORKING PRINCIPLE……………………………………..9,10
5. FIG-2.2…………………………………………………………….9
6. FIG-2.3……………………………………………………………..10

3.                             AC GENERATOR AND EXCITATION SYSTEM……………………………11,12,13,14
4.                              ELETRICAL POWER DISTRIBTION SYSTEM………………………………15,16,17,18          

                  3.1      SUBSTATION…………………………………………………….18

                                                                             FIG-3.1………………………………………………………………19

5.                      DISTRIBUTED CONTROL SYSTEM…………………………………………….20,21,22,23,24

                                                      3.1     COMPONENTS OFDCS……………………………………………20,21,22

                                                     3.2 FUNCTIONS OF DCS………………………………………………..22

                                                     3.3 PROGRAMMABLE LOGICAL UNIT……………………………..23,24

       6.                                    CONTROL SYSTEM…………………………………………………………………………25,26                            

                                                                 6.1 GAS TURBINE CONTROL SYSTEM………………………………25

                                                                  6.2 ELECTRICAL CONTROL SYSTEM………………………………….26,27

      7.                                       INSTRUMENTATION………………………………………………………………………..  28

     8.                                   REFINERY ELECTRIC………………………………………………………………………….29,30

     9.                                    SUBSTATION SWITCHING OPERATION…………………………………………….31-41                                                   

   10.                                   MOTOR FEEDER PROTECTION………………………………………………………42-45

                                                 ACKNOWLEDGEMENT

                          I Yugank Sharma of B-tech (electrical)-IVYear(VI semester), had done my training from INDIAN OIL CORPORATION LIMITED,DIGBOI ,ASSAM. My successful completion of training would not had been possible without the help of the Training Department of IOCL . and the training officer Mrs.Deepali Baruah.  

I would also thanks Mr.R.M.Mohonta(SPUM)(El) and special thanks to Mr.S.Biswas(ELE) for his proper guidance and knowledge and thanks to other staff members.

• Mr. P. K Patowary (CPUM)
• Mr .S.K.Dowara (SM-T&D)
• Mr.Mridul Shyam (SPUM-OPR))
• Mr. P.D.Malakar((PUM-R)
• Mr.O.P.Chetry(DPUM-OPR)
• Mr.J.C DAS(DPUM-R/E)
• Mr.N.Kachari(SPUE-OPR)
• Mr.R.Nayak(SPUE-OPR)
• Mr.S.Biswas(ELE)
• Mr.M.Kumar(ELE)
• Mr.J.K.Baruah(ELE-O/S)
• Mr.P.Borthakur(Sr  operator ,boiler)
• Mr.R.Shethi(Sr operator,boiler)
• Mr.B.Dutta(Sr operator,electrical)
• Mr.A.K Baruah(Sr electrician)
• Mr.R.Bhatachargy(JrEA)
• Mr.J.M.Reddy(JrE                                                                                                  

                                                                          (i) 

                                                            PREFACE

This report has been prepared by Yugank Sharma a student of Yagyavalkya Institute of Technology,sitapura, jaipur pursuing electrical engineering of IV Year(VII SEM).

The report contains all the topics that he has seen ,studied during his traning period. It contains all the topics in detail manner and in a very easy language so that it can be understood by everybody . It contains some detailed diagram of the gas turbine in proper manner, it also has an over view of the companie’s historical detail its total per year production by which the reader can make his/her own idea about the company .The report contains topics only related to his subject

Yugank Sharma is an average student of his class  but has a positive attitude towards life,his aim to become a good electrical engineer to serve his nation proudly.

So Yugank Sharma welcomes you to the world electricity .

                                                                                                              YUGANK SHARMA

(ii)

A

SEMINAR REPORT

ON

POWER AND UTILITY SECTOR

OF IOCL

MATHURA

SUBMITTED AS REQUIREMET OF PRACTICAL FOR

THE DEGREE OF “BACHELOR OF TECHNOLOGY”

IN

ELECTRICAL ENGINEERING

FROM

RAJASTHAN TECHNICAL UNIVERSITY

SESSION 2010-2011

SUBMITTED TO:                                                                                 SUBMITTED BY:

Mr. SUNIL GUPTA(HOD)                                                              YUGANK SHARMA

Mr. NAGENDRA JAT(LECTURER)                                             B.TECH IVTH YEAR

DEPARTMENT OF ELECTRICAL ENGINEERING

YAGYAVALKYA INSTITUTE OF TECHNOLOGY