Introduction to Power System protection

Md Irshad AhmadIrshad.ahmad@jit.edu.in

Electrical Engineering DepartmentSubject: NEE –602: SWITCHGEAR AND

PROTECTION

Unit I: Introduction to Protection System: Introduction to protection system and its elements, functions of protective relaying, protective

zones, primary and backup protection, desirable qualities of protective relaying, basic terminology.

Relays: Electromagnetic, attracted and induction type relays, thermal relay, gas actuated relay, design

considerations of electromagnetic relay. Unit-II: Relay Application and Characteristics: Amplitude and phase comparators, over current relays, directional relays, distance relays,

differential relay. Static Relays: Comparison with electromagnetic relay, classification and their description, over

current relays, directional relay, distance relays, differential relay. Unit-III Protection of Transmission Line: Over current protection, distance protection, pilot wire protection, carrier current protection,

protection of bus, auto re-closing, Unit-IV: Circuit Breaking: Properties of arc, arc extinction theories, re-striking voltage transient, current chopping,

resistance switching, capacitive current interruption, short line interruption, circuit breaker ratings.

Testing Of Circuit Breaker: Classification, testing station and equipments, testing procedure, direct and indirect testing.

Unit-V Apparatus Protection: Protection of Transformer, generator and motor. Circuit Breaker: Operating modes, selection of circuit breakers, constructional features and

operation of Bulk Oil, Minimum Oil, Air Blast, SF6, Vacuum and d. c. circuit breakers.

Syllabus SWITCHGEAR AND PROTECTION

Introduction to protection system and its elements Functions of protective relaying Protective zones Primary and backup protection Desirable qualities of protective relaying Basic terminology. Relays: Electromagnetic Attracted and induction type relays Thermal relay & gas actuated relay Design considerations of electromagnetic relay

UNIT-1

Switch gear - It's function

SWITCHGEAR : Gear for Switching

Switching DevicesControl & Sensing

DevicesDecision Making

Devices

Circuit breakersIsolatorsSwitches

Current Transformers;

Voltage Transformers;

Reactors;Tripping Units.

Protective RelaysLightning Arresters

Fuses

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

What is System protection?

The power system must maintain acceptable operation 24 hours a day

Voltage and frequency must stay within certain limits Protect the public Improve system stability Minimize damage to equipment Protect against overloads

Need for the protection

Damage to Main Equipment

Reliability: System operate properly Security: Don’t trip when you shouldn’t Dependability: Trip when you should

Selectivity: Trip the minimal amount to clear the fault or abnormal operating condition

Speed: Usually the faster the better in terms of minimizing equipment damage and maintaining system integrity

Economics: Don’t break the bank

Desirable Protection Attributes

• Transformers - to step up or step down voltage level • Breakers - to energize equipment and interrupt fault current to isolate

faulted equipment• Insulators - to insulate equipment from ground and other phases• Isolators (switches) - to create a visible and permanent isolation of

primary equipment for maintenance purposes and route power flow over certain buses.

• Bus - to allow multiple connections (feeders) to the same source of power (transformer).

Primary Equipment & Components

• Grounding - to operate and maintain equipment safely• Arrester - to protect primary equipment of sudden overvoltage (lightning

strike).• Switchgear – integrated components to switch, protect, meter and

control power flow• Reactors - to limit fault current (series) or compensate for charge

current (shunt)• VT and CT - to measure primary current and voltage and supply scaled

down values to P&C, metering, SCADA, etc.• Regulators - voltage, current, VAR, phase angle, etc.

Primary Equipment & Components

Typical Bulk Power SystemGeneration-typically at 4-20kV

Transmission-typically at 230-765kV

Subtransmission-typically at 69-161kV

Receives power from transmission system and transforms into subtransmission level

Receives power from sub transmission system and transforms into primary feeder voltage

Distribution network-typically 2.4-69kV

Low voltage (service)-typically 120-600V

1. Generator or Generator-Transformer Units2. Transformers3. Buses4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)6. Capacitor or reactor (when separately protected)

Protection Zones

Unit Generator-Tx zoneBus zone

Line zoneBus zone

Transformer zone Transformer zone

Bus zone

Generator

~XFMR Bus Line Bus XFMR Bus Moto

r

Motor zone

1. Overlap is accomplished by the locations of CTs, the key source for protective relays.

2. In some cases a fault might involve a CT or a circuit breaker itself, which means it can not be cleared until adjacent breakers (local or remote) are opened.

Zone Overlap

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at both sides of CB-fault between CTs is cleared from both remote sides

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at one side of CB-fault between CTs is sensed by both relays, remote right side operate only.

Thank You