INTRODUCTION TO POWER�SYSTEM PROTECTION

TIPS ENGINEER ZONE�www.tipsengineerzone.in

CONGRATULATIONS

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On choosing the field of system protection. It is an exciting, challenging profession.

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System protection has changed considerably in the

past 20 years.

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Many learning and growth “opportunities” will come

your way in the future.

What is System

Protection?

System protection is the art and science of detecting problems with power system components and isolating these components.

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Problems on the power system include:

1. Short circuits
2. Abnormal conditions
3. Equipment failures

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NERC defines the protection system as:

Current Approved Definition:

Protective relays, associated communication systems, voltage and current

sensing devices, station batteries and DC control circuitry.

Purpose of System Protection

• Protect the public
• Improve system stability
• Minimize damage to equipment
• Protect against overloads
• Employ relay techs and engineers

What Components (Equipment) Do We Protect?

What Components (Equipment) Do We Protect?

• Generators

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• Transformers, Reactors

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• Lines

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• Buses

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• Capacitors

What Components (Equipment) Do We Protect?

Some Basics

Protective relays monitor the current and/or voltage of the power system to detect problems with the power system. Currents and voltages to relays are supplied via CT’s and PT’s.

Some Basics

Current Transformer (CT)

A device which transforms the current on the power system from large primary values to safe secondary values. The secondary current will be proportional (as per the ratio) to the primary current.

Some Basics

Potential Transformer (PT)

A device which transforms the voltage on the power system from primary values to safe secondary values, in a ratio proportional to the primary value.

What Components (Equipment) Do We Protect?

I. Generator Protection

A. Construction & Theory of Operation

Three Gorges Dam in China Largest in the world (22,000MW, 26 Generators)

I. Generator Protection

What can go wrong?

A. Stator Winding Problems

1. Winding-winding short
2. Stator ground

I. Generator Protection

What can go wrong?

A. Stator Winding Problems

�Generator Protection

How Do We Protect the Stator?

1. Differential Protection (what goes in must come out)
1. Detects phase-phase faults
2. Stator Ground Protection
• 59N (95% of Stator)
• Third Harmonic Voltage Method (100% of Stator)
• Signal Injection (100% of Stator)

Generator Protection

What can go wrong?

B. Rotor Problems

1. Loss of field
2. Field ground
1. First ground
2. Second ground

=TROUBLE

Generator Protection

Rotor

Generator Protection

How Do We Protect the

Rotor?

1. Loss of Field

a. Impedance

Generator Protection

How Do We Protect the Rotor?

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2. Field ground

a. DC voltage relay (64F)

The field ground relay is connected from the negative side of the field to DC ground. Detects voltage from the field to ground.

Generator Protection

What else can go wrong?

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C. Abnormal Conditions

1. Over/Under Frequency
2. Over Excitation
3. Reverse Power
4. Out of Step
5. Unbalance Current

Generator Protection

Transformer Protection

TRANSFORMER PROTECTION

Power transformers are expensive, and are a long lead-time item

(1 year or longer) so protection must be effective

Transformer Protection

Construction

Construction

Transformer Protection

Construction

Transformer Protection

Transformer Protection

Transformer size and rating

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• MVA: the capacity of the transformer in terms of million volt-amps. Size can range from less than 1 MVA to 500 MVA and higher.

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• Transformer rating (MVA) is determined in part by the amount of cooling employed. MVA rating increases with more cooling. OA, FOA (stage 1), FOA (stage 2)

Transformer Protection

What can go wrong?

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• Winding-to-winding faults

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• Winding-to-ground faults

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• Bushing faults

Transformer Protection

Transformer Protection

Protection Methods

• Fuse

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• Overcurrent

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• Differential

Transformer Protection

High Side Fuse

Transformer Damage Curve

Transformer Protection

Transformer Protection

Overcurrent Relays

Transformer Protection

Differential Protection:

What goes in must come out….. P-in = P-out

Transformer Protection

Microprocessor Relays

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187T1-T

Wraps transformer

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187T1-B

Wraps transformer and bus

Transformer Protection

187 T1-T

Zone of Protection

Transformer Protection

187 T1-B

Zone of Protection

Transformer Protection

Some terms you will be learning about this week:

Restraint

Operate Slope Inrush

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2^nd Harmonic

LINE PROTECTION

Transmission Line Protection

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• Construction can spacing between the conductors and from the conductors to ground. At the other end of the scale are metal lattice structures with bundled conductors (2 or more conductors per phase) with large spacing between conductors and between conductors and ground.

Transmission lines can vary in length from several hundred feet to several hundred miles, and in voltage (be simple, such as a single wood pole with insulators atop a cross arm, with little e-to-line) from 46KV to 750KV.

Transmission Line Protection

Transmission Line Protection

Transmission Line Protection

What Can Go Wrong?

FAULTS (Short Circuits)

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Some causes of faults:

• Trees
• Lightning
• Animals (birds, squirrels, snakes)
• Weather (wind, snow, ice)
• Natural Disasters (earthquakes,

floods)

• Faulty equipment (switches, insulators, clamps, etc.)

Ice Storm

Transmission Line Protection

Transmission Line Protection

Transmission Line Protection

Faults

“Faults come uninvited and seldom go away voluntarily.” Fault Types:

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• Single line-to-ground
• Line-to-line
• Three Phase
• Line-to-line-to-ground

Transmission Line Protection

How Do We Protect Transmission Lines?

1. Overcurrent
2. Directional Overcurrent
3. Distance (Impedance)
4. Pilot
1. DCB (Directional Comparison Blocking
2. POTT (Permissive Overreaching Transfer Trip)
5. Line Current Differential

Transmission Line Protection

Overcurrent Protection

Non-Directional

Relay responds to overcurrent condition

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Instantaneous (IOC) device #50 No intentional time delay

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Time Overcurrent (TOC) device #51

Various curve types, including inverse, very inverse, extremely inverse

Transmission Line Protection

Overcurrent Line Protection

Transmission Line Protection

AC Schematic

Time Overcurrent Curves

Transmission Line Protection

Directional Overcurrent

Protection

Transmission Line Protection

Relay responds to overcurrent condition in the forward direction only (device #67, 67N, 67NT)

Will not respond to reverse faults Compares the current in the line versus a

known reference that will always be the same (such as a voltage or polarizing current source)

Transmission Line Protection

Transmission Line Protection

Directional Overcurrent Example

Transmission Line Protection

Distance Protection

A distance relay measures the impedance of a line using the voltage applied to the relay and the current applied to the relay.

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When a fault occurs on a line, the current rises significantly and the voltage collapses significantly.

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The distance relay (also known as impedance relay) determines the impedance by Z = V/I. If the impedance is within the reach setting of the relay, it will operate.

Transmission Line Protection

Distance Protection

Electromechanical distance relays use torque to restrain or operate

KD, GCY, etc. Device #21

Microprocessor distance relays use equations to restrain or operate SEL, ABB, GE, Areva, etc. Device #11

Transmission Line Protection

Distance Relay

CT and PT

Connections

Transmission Line Protection

Transmission Line Protection

Distance Protection

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Typical zone reach settings

Transmission Line Protection

Distance Protection

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When a fault occurs on a transmission line, the current increases and the angle of the current with respect to the voltage changes to a lagging angle, usually between 60 to 85 degrees.

Transmission Line Protection

Distance Protection

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The most common characteristic (or protection shape) of distance relays is the mho characteristic, a circular type reach characteristic.

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Distance relays have a settable maximum torque angle (mta), which is the angle of the current compared to the angle of the voltage at which the relay is most sensitive. In the drawing on the right, the mta is approximately 75 degrees.

Introduction to System Protection

Terminology

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Dependability: the certainty that a protection system will operate when it is supposed to

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Security: the certainty that a protection system will not operate when it is not supposed to

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Reliability = Dependability + Security

Transmission Line Protection

Pilot Relaying Scheme

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A protection scheme which employs communications to send a signal from one station to another to allow high speed tripping (permission) or to prevent high speed tripping (blocking).

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Pilot protection allows over-reaching zones of protection to ensure full protection of the line as well as high speed tripping.

Transmission Line Protection

Pilot Relaying Scheme

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Directional Comparison Blocking (DCB)

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A communications based protection scheme where high speed over-reaching tripping is allowed unless a block signal is received.

Transmission Line Protection

Pilot Relaying Scheme

Permissive over-reaching transfer trip (POTT)

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A communications based protection scheme where high speed over-reaching tripping is allowed only if a permissive signal is received

Transmission Line Protection

Relay

Relay

STATION “A”

STATION “B”

BLOCKING SCHEME OPERATING PRINCIPLE

Relay

Transmission Line Protection

Relay

BLOCKING SCHEME OPERATING PRINCIPLE

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External Fault

DO NOT TRIP!!!

Transmission Line Protection

BLOCKING SCHEME OPERATING PRINCIPLE

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Internal Fault

No block signal is sent

Relay

Transmission Line Protection

Relay

Relay

STATION “A”

STATION “B”

Permissive Scheme

Transmission Line Protection

Relay

Permissive scheme internal fault

Relay

STATION “B”

STATION “A”

Transmission Line Protection

Relay

Relay

STATION “B”

STATION “A”

Permissive scheme internal fault

Transmission Line Protection

Relay

Relay

You Can Go Ahead and Trip If You Want To!

You Can Go Ahead and Trip If You Want To!

High Speed Tripping Takes Place at Station A and B

STATION “B”

STATION “A”

Permissive scheme internal fault

Transmission Line Protection

Relay

Relay

STATION “B”

STATION “A”

Permissive scheme external fault

Transmission Line Protection

Relay

Relay

STATION “B”

STATION “A”

Permissive scheme external fault

Transmission Line Protection

Relay

Relay

You Can Go Ahead and Trip If You Want To!

No High Speed Tripping Takes Place Because the Fault Is Reverse to the Relay at Station B.

STATION “B”

STATION “A”

Permissive scheme external fault

Transmission Line Protection

BLOCKING VS. PERMISSIVE

• Blocking
• Increased dependability because if the carrier fails, the protection will trip

anyway.

• Decreased security because if the carrier fails, the protection will trip for an out of section fault.
• Permissive
• Increased security because if the communication fails, the protection will not

trip high speed.

• Decreased dependability because if the comm fails, the protection will not trip high speed for an in section fault.

Transmission Line Protection

LINE DIFFERENTIAL

No fault or external fault, current at each end is balanced

The current going into the line is going out at other end

Transmission Line Protection

Line differential

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Internal fault, relay trip is processed

Bus Protection

Bus Differential:

Current into bus must equal current out of bus

Bus Fault

Bus Protection

Single bus with XFMR

Bus Protection

Double bus, breaker-and-a- half

Bus Protection

Bus Protection

Double bus, double breaker

Capacitor Protection

Purpose of capacitors:

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Shunt capacitors raise the voltage on a bus or line to a higher level, thus helping keep the voltage at desired level

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Series capacitors cancel out the inductive reactance of a line, thus making the line appear shorter increasing load flow on the line.

Capacitor Protection

Capacitors connected in parallel add

Capacitor Protection

Capacitors connected in series sum like they are in parallel

Capacitor Protection

Capacitors are connected in series and parallel combination

to obtain the desired total capacitance for the bank

Capacitor Protection

Capacitor Protection

Voltage across cap bank is determined by current flow and impedance

(capacitive reactance) of bank.

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If a capacitor fuse blows or if a capacitor shorts, the voltage drop across the bank changes due to a change in capacitive reactance of the bank.

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A voltage relay detects the higher voltage and trips the breaker

Capacitor Protection

�Introduction to System Protection��THE END���

��TIPS ENGINEER ZONE�

TIPS ENGINEER ZONE

www.tipsengineerzone.in