Slide 1

Session1-2

Power System Protection-Overview

N. Theivarajan . Head, Power Plant Control Division .Reactor Design GroupIGCAR.

This Material Is Adopted From Many Sources For MAPS/NPCIL Course In March 2015

Typical Power Plant SLD For Coordination Study

Major Power System Objects of Protection

TransformersMotorsGeneratorsSwitchgear Cable Systems/FeedersTransmission Lines/Associated Systems Power Electronic Converters/Inverters Power System Components Capabilities.Broad Performance CapabilitiesVoltage: Rated , Power Fre., Switching, Impulse Withstand

Current:Rated, Short time, Short Circuit withstand,Negative Phase sequence,

Power: Real, Reactive, Apparent

Thermal Capabilities:

Speed : Over speed, Critical Speed,..

Some of The Major Issues With Power System Components InsulationDesign defects or errorsImproper manufacturingImproper installationAging insulationContaminationElectricalLightning surgesSwitching surgesDynamic overvoltagesThermalCoolant failureOvercurrentOvervoltageAmbient temperatureMechanicalOvercurrent forcesEarthquakeForeign object impactSnow or iceDeviation on Performance Characteristics /Capabilities due to:

Three-phase fault (a-b-c, a-b-c-g)Phase-to-phase fault (a-b, b-c, c-a)Two-phase-to-ground fault (a-b-g, b-c-g, c-a-g)Phase-to-ground fault (a-g, b-g, c-g)

Voltage Surges

Exceeding Thermal Limits

GENERAL PROTECTION PERSPECTIVES

PROTECTION DEVICES &CHARACTERISTICES Input Parameters Available For Protection Devices.

CurrentVoltageFrequencyPressureTemperatureFlowVibration

Type of Protection Devices :

Thermal Electromechanical Static Microprocessor/Numerical Performance CharacteristicsOver current- Phase or GroundDirectional Over current Inverse time/Definite time / IDMT Under voltageOvervoltage

Differential- Low , High Impedance, REF, Current Distance- Many Characteristics Phase comparisonDirectional comparison

MCB and MCCBs With Built in Protection FunctionThermal-magnetic MCCB operating characteristic

Typical electronic low-voltage circuit breaker trip characteristic

Magnetic-trip MCCB operating characteristic

HRC Fuses

Thermal Over Load Relays

Protective Relays

Over current Relays

Directional Over current Relays

Earth Fault Relay Connection

Power Relays

Differential Relays

High Impedance Differential Protection

This type of protection is normally used for protection of motors, generators, bus bars as well as for restricted earth fault protection of transformers.

This type of protection is used to protect an area with the same voltage level.Percentage Biased Differential Protection For Transformers

The term percentage differential stems from the fact that the operating threshold increases with the through current.

Simple comparison of the currents in each upstream phase with the currents in the same downstream phases is not suitable fortransformer differential protection.

The upstream and downstream currents of a power transformer do not have the same amplitude or the same phase angle,

When the transformer is energized, its magnetizing current is only seen upstream.

Low Impedance Differential Protection.

This protection is used for bus bar differential protection. It is very costly and space-consuming, as it requires a large number of modules and matching CTs which need one ormore cubicles according to switchboard size

In the case of a double bus bar switchboard, the protection must be continually informed on the position of the transfer switches in order to direct the currents of each feeder and incomer to the relay associated with monitoring of the bus bar on which this feeder or incomer is connected.

The CTs associated with this sophisticated protection may have different ratios. Their secondaries are also defined as class X in most cases. However, as saturation can be tolerated , knee point voltage requirements are less severe than for high impedance differential protection.

Pilot protection schemes -Three categories.

Directional comparison scheme

Distance or Directional relays determine whether each terminal sees the fault as forward or reverse.

By exchanging this information, the fault can be classified as either internal or external to the protected line segment.

Phase comparison- Current only Pilot protection.

Phase comparison protection systems compare the phase angles of currents entering at one terminal of the line and the currents leaving at the other terminal of the line.

If the fault is external, the currents entering and exiting the line should be in phase with each other. Current differential schemes

Send information about the magnitude and angle of the currents entering and exiting the line.

This type of pilot protection requires higher bandwidth tele protection channels, and it is becoming more common with the availability of fiber-optic networks.