REACTIVE POWER COMPENSATION

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Dr. S SUMATHI

PROFESSOR & DEAN( Academics)

Department of EEE

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RNS INSTITUTE OF TECHNOLOGY� BANGALORE

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Objectives of Power System operation

• The System must be able to meet the changing demand in Active and reactive Power, with sufficient spinning reserve.
• Power quality should be met as per standards (with regard to frequency, amplitude and waveshape).
• The energy should be supplied at minimum cost.

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Active Power Balance

• The load is dynamic and changing
• Generation must be scheduled to meet the demand.
• Σ P_GI = Σ P_DI +P_L
• AGCs are used to match the generation with load.
• Mismatch of generation and load+losses causes frequency deviations.
• Frequency increases if generation exceeds demand
• Frequency decreases if demand exceeds generation

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Permissible frequency deviations

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Reactive Power Balance

• Σ Q_GI = Σ Q_DI +Q_L
• Mismatch in reactive power generation and reactive power demand causes voltage deviations.
• Reactive power balance is to be maintained to maintain voltages within permissible range.

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Indian Electricity Rules, 1956

• The permissible voltage variation in India depends on the voltage level, as per the Indian Electricity Rules, 1956:
• Low or medium voltage: Up to 6% variation
• High voltage: Up to 6% variation on the higher side, and up to 9% variation on the lower side
• Extra-high voltage: Up to 10% variation on the higher side, and up to 12.5% variation on the lower side

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Reactive power and Voltage control

For efficient and reliable operation of power systems, the control of voltage and reactive power should satisfy the following objectives:

• Voltages at the terminals of all equipment in the system are within acceptable limits. Both utility equipment and customer equipment are designed to operate at a certain voltage rating.
• System stability is enhanced, to maximize utilization of the transmission system
• The reactive power flow is minimized so as to reduce I²R and I²X losses to a practical minimum.

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BASIC concepts of Reactive power

• Reactive power cannot be transmitted over long distances.
• HENCE voltage control has to be effected by using special devices dispersed throughout the system.
• This is in contrast to the control of frequency which depends on the overall system active power balance.
• The proper selection and coordination of equipment for controlling reactive power and voltage are among the major challenges of power system engineering.

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Production and Absorption of Reactive Power

• Synchronous generators can generate or absorb reactive power depending on the excitation. When overexcited they supply reactive power, and when underexcited they absorb reactive power.
• Overhead lines, depending on the load current, either absorb or supply reactive power. At loads below the natural (surge impedance) load, the lines produce net reactive power; at loads above the natural load, the lines absorb reactive power
• Underground cables, owing to their high capacitance, have high natural loads. They are always loaded below their natural loads, and hence generate reactive power under all operating conditions
• Transformers always absorb reactive power regardless of their loading
• Loads normally absorb reactive power.

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Production and Absorption of Reactive Power

• Synchronous generators can generate or absorb reactive power depending on the excitation. When overexcited they supply reactive power, and when under-excited they absorb reactive power.
• Overhead lines, depending on the load current, either absorb or supply reactive power. At loads below the natural (surge impedance) load, the lines produce net reactive power; at loads above the natural load, the lines absorb reactive power

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Methods of voltage control

• The control of voltage magnitude is accomplished by controlling the production, absorption, and flow of reactive power at all levels in the system
• The generators the automatic voltage regulators control field excitation to maintain a scheduled voltage level at the terminals of the generators.
• Additional means are usually required to control voltage throughout the system.

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Devices used for voltage control

• Synchronous Generator (AVR)
• Tap changing transformers
• Shunt capacitors and reactors
• Series capacitors
• Static Var Compensators
• STATCOM

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Automatic Voltage Regulator

• An Automatic Voltage Regulator (AVR) is used for controlling the output voltage of a generator (alternator) in a power station.
• The terminal voltage of a generator fluctuates due to change in load.
• Modern AVR uses solid-state devices such as thyristors.

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�Working principle of an AVR�

• Basically, an Automatic Voltage Regulator is a feedback control system.
• The output voltage of the Generator is measured using a potential transformer then rectified, filtered and fed to a comparator.
• This comparator compares the obtained output voltage with a set reference voltage.
• The difference between generator output voltage and the reference voltage is called as the error voltage.
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AVR

• This error voltage is amplified and then used to control the excitation of the Generator.
• An AVR controls the terminal voltage using excitation control method, i.e. the output voltage of the terminal can be kept by varying the field current of the alternator
• If the excitation current of an alternator is increased, the output voltage of the alternator will also increase.

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Schematic diagram of AVR

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COMPONENTS OF AVR

• Voltage Sensing: The AVR constantly monitors the terminal voltage of the generator. This voltage sensing is typically done through a potential transformer (PT) or a voltage transformer (VT), which steps down the generator's voltage to a manageable level for the AVR to process.
• Comparison and Error Detection: The AVR compares the sensed voltage to a reference voltage or setpoint. This reference voltage is the desired output voltage that the generator should maintain. Any deviation from this setpoint voltage is considered an error.

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Components of AVR

• Error Amplification: To correct the error, the AVR uses an error amplifier. This amplifier magnifies the error signal to a level suitable for controlling the generator's excitation system.
• Excitation System Control: The excitation system is responsible for providing the necessary field current to the generator's rotor windings. This field current determines the strength of the magnetic field and, consequently, the generator's output voltage. The AVR adjusts the excitation system to either increase or decrease the field current based on the error signal.

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Components of AVR

• Feedback Loop: The entire process is iterative and continuous, creating a feedback loop. As the AVR makes adjustments, it keeps monitoring the voltage, ensuring it stays within the desired range. This continuous correction maintains a stable and regulated output voltage.
• Thyristor Control (Modern AVRs): In modern AVRs, solid-state devices like thyristors are used for precise control of the excitation system. Thyristors are semiconductor devices that allow for rapid and accurate adjustments to the field current. They offer a faster response time compared to older electromechanical systems, enabling tighter voltage control.

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Components of AVR

• Protection Features: AVRs often come with built-in protection features. These features can include over-voltage protection, under-voltage protection, over-excitation protection, and more. They safeguard the generator and connected equipment from voltage-related issues that could cause damage.
• Communication and Monitoring: In many setups, AVRs are integrated into a larger control and monitoring system. This allows operators to remotely monitor the generator's voltage and receive alerts or alarms if any voltage-related issues arise.
• In conclusion, an Automatic Voltage Regulator plays a vital role in maintaining a stable voltage output from a generator. It continuously monitors the voltage, compares it to a setpoint, and adjusts the excitation system to ensure that the generator produces the desired voltage. With the advent of solid-state devices like thyristors, modern AVRs offer precise and efficient voltage control, contributing to the reliability and performance of power systems in various applications.

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Tap changing transformer(OLTC)

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Tap changing transformer

• Provide voltage control by changing turns ratio.
• Taps are normally provided on HV side
• Since the current is lower in HV side
• HV winding is wound outside LV provides easy access.

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Onload Tap changing transformer

• The essential feature of all methods of tap changing under load is that circuit continuity must be maintained throughout the tap stepping operation

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Onload Tap changing transformer

• In this Initial position load current is equally divided into the two half of the reactor.

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• To change the tap TS1 is opened
• Selector switch is moved to tap 2
• TS1 is closed

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• Because of difference in voltage between S1 and S2 circulating current flows in the reactor super imposed on the load current.

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Onload Tap changing transformer

• To change the tap TS2 is opened.
• Selector switch is moved to tap 2.
• TS2 is closed.

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Load drop compensator

• The tap step voltage is applied to the whole of the winding of the inductor and the circulating current is limited by the total impedance.
• The OLTC operation is mechanised and a motor is used to control the switching operations.
• Taps may be provided on both sides of the line
• The voltage change between taps is around 1% to 1.25% of the nominal voltage.

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• For tighter regulation of transmission voltage, line Drop compensator is used.

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Load drop compensator

• Load drop compensator allows the OLTC to see the actual load voltage and corrects for the voltage drop along the feeder.
• The current proportional to the load current is injected through an impedance Zc which models the network impedance.
• The voltage drop across the Zc is combined with voltage to be controlled to obtain the actual load voltage.

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Shunt reactors

• Shunt reactors are used to compensate the effect of line capacitance.
• They are used to limit the rise in voltage during light load conditions
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Shunt Reactors

• Shunt reactors are used with leading power factor circuits
• Shunt reactors are connected to EHV bus to maintain normal voltage under no load and light load conditions.
• Shorter lines do not need permanent reactors. Only switchable reactors are used.

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Shunt capacitors

• Shunt capacitors are used extensively in distribution systems for power-factor correction and feeder voltage control.
• Distribution capacitors are usually switched by automatic means, responding to voltage or current-sensing relays.
• The objective of power-factor correction is to provide reactive power close to the point where it is being consumed, rather than supply it from remote sources.
• Capacitors are either directly connected OR connected through the tertiary winding of a main transformer.
• They boost local voltage and can be used throughout the system.

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Shunt capacitors

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Shunt reactor / shunt capacitor

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Shunt capacitor

• When a capacitor is installed across an inductive load (i.e, parallel to the load), the load starts receiving reactive power from the capacitor and thus capacitor power neutralizes the reactive power requirement of the inductive load.
• Consequently the supply system is relieved from producing reactive power to the load and it can able to deliver greater active power.
• When the power factor improves towards unity, the transmission losses decreases due to reduced current and the system voltage improves.
• A healthy supply enables machines and protection equipment to give optimum output in terms of performance and operational life span.

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Shunt capacitors

Some of the advantages due to installing shunt capacitors in the power system are

• For a particular active power (kW) the resultant demand (kVA) is subsequently reduced. So additional machines can be installed for given sanction of kVA load
• Since voltage drop is minimized, motor torque capability ( Torque α Voltage² ) improves, so starting time and the motor heating gets considerably reduced. Motor current requirement for the same output is lesser
• Because of  less heating, the ageing of the insulation becomes slow and thus the life of the machine and the cables increases
• Switchgear wear and tear is minimized because of lesser arcing energy dissipation (i.e, lesser acing time) at higher power factor
• Reduced losses in the feeders lead to lesser voltage drop, hence greater voltage regulation

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Series Capacitors

• Series capacitors are connected in series with line to compensate the inductive reactance.

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Series Capacitors

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Series capacitors

• Reduces line voltage drop
• Increases steady state power transfer capability.
• Influences power flow in parallel lines.
• For a given power transfer reduces transmission angle.
• Increases system stability.

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Series capacitors

• Series capacitors are subjected to high over voltages , when short circuit current flows through them.
• The capacitor is protected using spark gaps and nonlinear resistors across it.
• The capacitors are bypassed during fault and reinserted after clearing the fault.
• Location of series capacitor is influenced by cost, accessibility, fault level, voltage level and power transfer levels

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Series Capacitors with Protective Device

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Series capacitors

• Mid point compensation is preferred when compensation is less than 50% . But this location is not convenient in terms of access for monitoring and maintenance.
• Sub Synchronous resonance.
• The series capacitor forms a resonant circuit with the transmission line inductance at a frequency below normal power frequency.

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Series capacitors

• In the event of a disturbance sub hormonic frequencies are excited.
• If the sub-hormonic frequency coincides with natural frequency of the of the mechanical system of nearby steam turbine , it may lead to tortional oscillations. This phenomenon is called SSR.
• SSR may cause major turbine shaft failure.

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Comparison between series capacitor and shunt capacitor

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Application to EHV transmission system�

• Because series capacitors permit economical loading of long transmission lines, their application to EHV transmission has grown.
• They have been primarily used to improve system stability and to obtain the desired load division among parallel lines.
• Complete compensation of the line is never considered.

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FACTS Controller

• FACTS controller: A power electronic based system and other static equipment that provide control of one or more AC transmission system parameters.
• FACTS can be classified as

Shunt connected

Series connected

Combined series- series

Combined shunt- series

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FACTS Controller

Depending on the power electronic device used they can be classified as

• Variable Impedance type
• Static VAR Compensators-SVC (Shunt connected)
• Thyristor Controlled Series Capacitor- TCSC (Series connected)

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• Voltage source convertor based
• Static Synchronous Compensator-STATCOM (Shunt connected)
• Static Synchronous Series Compensator-SSSC (Series connected)
• Unified Power Flow Controller-UPFC- (Shunt & Seirs Connected)

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FACTS Controller

• FACTS controllers provide voltage support at critical buses (With shunt connected controllers)

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• Regulate power flow in critical lines ( with series connected controllers)

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• Both voltage and power flow are controlled by the combined series and shunt controller(UPFC)

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Control variable for FACTS controller

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Static VAR Compensator

Static var compensators (SVCs) are shunt-connected static generators and/or absorbers whose outputs are varied so as to control specific parameters of the electric power system.

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Types of SVC

• Saturated reactor (SR)
• Thyristor-controlled reactor (TCR)
• Thyristor-switched capacitor (TSC)
• Thyristor-switched reactor (TSR)
• Thyristor-controlled transformer (TCT)
• Self- or line-commutated converter (SCC/LCC)

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Static VAR Compensator

• Static var systems are capable of controlling individual phase voltages of the buses to which they are connected.
• From the viewpoint of power system operation, an SVS is equivalent to a shunt capacitor and a shunt inductor, both of which can be adjusted to control voltage and reactive power at its terminals.

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Characteristics of SVS

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Static VAR Compensator-TCR

• The basic elements of a TCR are a reactor in series with a bidirectional thyristor switch

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Static VAR Compensator-TCR

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• Firing angles between 0 and 90 0 are not allowed as they produce asymmetrical currents with a dc component

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• Fourier analysis of the current waveform gives the fundamental component:

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• This susceptance control principle is known as phase control. The susceptance is switched into the system for a controllable fraction of every half cycle. The variation in susceptance as well as the TCR current is smooth or continuous.

The TCR requires a control system which determines the firing instants (i.e., firing angle a) measured from -the last zero crossing of the voltage (synchronization of firing angles).

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Thyristor-switched capacitor (TSC)�

• A thyristor-switched capacitor scheme consists of a capacitor bank split up into appropriately sized units, each of which is switched on and off by using thyristor switches. Each single-phase unit consists of a capacitor (C) in series with a bidirectional thyristor switch and a small inductor (L)
• The purpose of the inductor is to limit switching transients, to damp inrush curents, and to prevent resonance with the network

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Thyristor-switched capacitor (TSC)�

• The switching of capacitors excites transients which may be large or small depending on the resonant frequency of the capacitors with the external system. The thyristor firing controls are designed to minimize the switching transients.
• The switching-on instant (h) is chosen so that the bus voltage V is at its maximum and of the same polarity as the capacitor voltage; this ensures a transient-free switching

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Practical static VAR system

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Steady State Characteristics

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SVC

• A static var compensation scheme with any desired control range can be formed by using combinations of the elements .
• Several SVS configurations have been successfully applied to meet differing system requirements.(TCR-FC, TSC, TCR, TSC-TCR)
• The required speed of response, size range, flexibility, losses, and cost are the important considerations in selecting a configuration for any particular application.

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SVC

• Static VAR compensators generate or absorb reactive Power by synchronously switching capacitors and inductor banks ‘in ‘ and ‘out’ of the system.
• The aim of this approach is to produce a variable shunt reactance that can be adjusted.
• They basically use AC capacitor or reactor.

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Voltage source converters

• Controllable reactive power can be generated by all type of DC to AC and AC to AC converters.
• In DC to AC converters, DC terminals are considered as input terminals .
• If the input side of the converter is shunted by a voltage source(capacitor) it is called as VOLTAGE SOURCED CONVERTER.
• If the input side of the converter is shunted by a current source(inductor) it is called as CURRENT SOURCED CONVERTER.

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Installations In India

• In 2018, Siemens commissioned the world's largest STATCOM installation at the Rourkela substation of the Power Grid Corporation of India Limited (PGCIL). The 400 kV STATCOM solution was designed, delivered, and commissioned in 22 months. It automatically regulates transmission variations to provide a stable and reliable power supply to consumers
• In 2019, Gujarat Energy Transmission Corporation Limited (GETCO) became the first state utility in India to implement a STATCOM. The STATCOM solution was installed in the 220 kV substation to mitigate voltage variation.

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STATCOM

• Static Synchronous Compensator (STATCOM)
• A Static synchronous generator operating as a shunt connected VAR compensating device whose capacitive or inductive output current can be controlled independent of the AC system voltage.
• STATCOM or Static Synchronous Compensator is a power electronic device using force commutated devices like IGBT, GTO etc. to control the reactive power flow through a power network and thereby increasing the stability of power network.

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• STATCOM

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Working principle of STATCOM

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STATCOM –Working Principle

• The exchange of reactive power between the converter and the ac system can be controlled by varying the amplitude of the 3-phase output voltage, Es, of the converter,
•  If the amplitude of the output voltage is increased above that of the utility bus voltage, Et, then a current flows through the reactance from the converter to the ac system and the converter generates capacitive-reactive power for the ac system.
•   If the amplitude of the output voltage is decreased below the utility bus voltage, then the current flows from the ac system to the converter and the converter absorbs inductive-reactive power from the ac system.

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• Negative current indicates capacitive operation.
• Positive current indicates inductive operation.
• Vref corresponds to Zero output current

• VI Characteristics

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Multi-pulse converters

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24 pulse converter

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STATCOM with energy source

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Comparison between SVC and STATCOM

• STATCOM at reduced voltage can still inject maximum current, whereas SVC current capability reduces in proportion to voltage.
• As a result, STATCOM has superior dynamic response and for comparison, STATCOM may be rated for 75% of SVC rating for same performance in response to line fault.

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Comparison between SVC and STATCOM

• When the system voltage drops sufficiently due to loss of parallel line to force the STATCOM output current to its ceiling, its maximum reactive output current will not be affected by the voltage magnitude. Therefore, it exhibits constant current characteristic.
• In contrast the SVC's MVA output is proportional to the square of the Voltage variation with SVC . Thus the capacitive reactive power decreases, just when needed. This is major disadvantage of SVC

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Comparison between SVC and STATCOM

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SSSC & UPFC

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Thank you

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