Electrical Power Systems Protection And principle

IntroductionSystem Protection: the equipment use to detect and isolate the faulty section from the system automatically.

IntroductionShort circuit occur when equipment insulation fails due to system overvoltages caused by:Lightning or switching surgesFlashover line-line (caused by wind)Flashover to treeInsulation contamination by dirt/saltMechanical failureCable insulation failureNatural causesTower/pole or conductor fallsObjects fall on conductors

IntroductionShort circuit currents can be several orders of magnitude larger than normal operating currentsIf it is allowed to persist, may cause:Damage to the equipment due to heavy currents, unbalanced current, or low voltage produces by the short circuitFire and explosion effect equipment/peopleDisruption of service in the entire power system area

IntroductionCareful design, operation and maintenance of system protection can minimize the occurrence of short circuit but cannot eliminate them.

Fault Currents and Voltages

Function of System ProtectionCause the prompt removal from service of any elements of power system when it suffers a short circuit, or when it start to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system.Provide indication of the location and type of failure so that the data can be used to assist in expediting repair and analyzing the effectiveness of fault-prevention and mitigation features.

Function of System ProtectionWhy do we need system protection:Detect faultIsolate faulted componentRestore faulted componentAims:Continued supply for rest of systemProtect faulted part from damage

Types of ProtectionA FusesFor LV Systems, Distribution Feeders and Transformers, VTs, Auxiliary SuppliesB - Over current and earth faultWidely used in All Power SystemsNon-Directional Directional

Types of ProtectionC - DifferentialFor Distribution Feeders, Busbars, Transformers, Generators etc

High Impedance Low Impedance Restricted E/F Biased Pilot Wire

Types of ProtectionD - DistanceFor Transmission and Sub-transmission Lines and Distribution Feeders, Also used as back-up protection for transformers and generators without signaling with signaling to provide unit protection e.g.:Time-stepped distance protection Phase comparison for transmission lines Directional comparison for transmission lines

Types of ProtectionE - Miscellaneous:Under and over voltage Under and over frequency A special relay for generators, transformers, motors etc. Control relays: auto-reclose, tap change control, etc. Tripping and auxiliary relays

Design Criteria/CharacteristicsSimplicityEconomySpeedSensitivitySelectivityReliability

Design Criteria/CharacteristicsReliabilityOperate dependably and in healthy operating condition when fault conditions occur, even after remaining idle for months or years.SelectivityClearly discriminate between normal and abnormal system condition to avoid unnecessary, false trips.SensitivityAbility to distinguish the fault condition, although the different between fault and normal condition is small.

Design Criteria/CharacteristicsSpeedFault at any point in the system must be detected and isolated rapidly to minimize fault duration and equipment damage. Any intentional time delays should be precise.EconomyProvide maximum protection at minimum costSimplicityMinimize protection equipment and circuitry

Economic FactorTotal cost should take account of :Relays, schemes and associated panels and panel wiringSetting studiesCommissioningCTs and VTsMaintenance and repairs to relaysDamage repair if protection fails to operateLost revenue if protection operates unnecessarily

Economic FactorThe cost of protection is equivalent to an insurance policy against damage to plant, and loss of supply and customer goodwill.Acceptable cost is based on a balance of economics and technical factors. Cost of protection should be balanced against the cost of potential hazards.There is an economic limit on what can be spent.MINIMUM COST :Must ensure that all faulty equipment is isolated by protection.

Relationship between reliability of supply, its value and cost to the consumer

System Protection ComponentsTransducer / Instrument TransformerRelayCircuit Breaker

System Protection ComponentsFunction:Transducers/Instrument TransformersProvide low current and voltage, standardized levels suitable for the relays operation.RelaysDiscriminate between normal operating and fault conditions.When current exceed a specified value relay will be operated and cause the trip coil of CB to be energized/open their contact.Circuit BreakersOpen the line

System Protection Components

System Protection Components

System Protection Flow Relay Transducer Fault Occur Circuit Breaker Fault Clearvoltage or current rise from normal conditionvoltage/current is reduced to match with relay ratingactivate circuit breaker circuit isolation

Classification of relaysCommon methods used to classify relay are:FunctionInput sourceOperating principlesPerformance characteristics

Protective relayProtective relay function to detect defective lines or equipment, or other power system conditions of abnormal or dangerous nature, and to initiate appropriate control circuit action. It can be used to initiate switching operations or actuate an alarm.

Protective relay (contd)Over current relaysUnder voltage relayDifferential relaysReverse sequence relays etc

Auxiliary relaysAuxiliary relay: provides a specific, or secondary, function to assist another relay or control a device in performing a general function. Typical function performed by auxiliary relay include circuit seal-in, time delay, control signals or lights, and contact multiplications.

Auxiliary examplesControl relaysTime delay relaysLockout relaysTrip and close relays

Monitoring A monitoring relay function to verify that system or control circuit conditions conform to prescribed limits

Monitoring relay exampleAlarm relaysFault detector relayNetwork phasing relaysVerification relaysSynchronism relays

Regulating relayA regulating relay responds to normal changes in system operating conditions and functions to control system parameters within specified operating limits. Example of regulating relays are:Transformer tap changersGenerator governors

Programming relaysFunctions to detect or establishes electrical sequences. Example are:Accelerating relaysPhase selector relaysReclosing relaysSynchronising relaysInitiaeting relays

Relay classification by inputsCurrent relaysVoltage relaysPower relaysTemperature relaysPressure relays

Operating principleElectromagnet relaysSolid-state relaysHarmonic restraint relaysElectromechanical relaysPercent differential relaysSudden pressure relaysThermal relaysMicroprocessor or numerical relays

Performance characteristicsHigh speed differential relaysDirectional-over current relaysReverse power relaysMho relaysImpedance relaysOver current relaysUnder voltage relaysPhase balanced relaysReactance relaysFrequency relaysOverload relays

Zones of ProtectionFor fault anyway within the zone, the protection system responsible to isolate everything within the zone from the rest of the system.Isolation done by CBMust isolate only the faulty equipment or section

Zones of ProtectionZones are defined for:GeneratorsTransformersBusesTransmission and distribution linesMotors

Zones of Protection

Zones of ProtectionCharacteristics:Zones are overlapped.Circuit breakers are located in the overlap regions.For a fault anywhere in a zone, all circuit breakers in that zone open to isolate the fault.

Overlapped of ProtectionNo blind spot:Neighboring zones are overlapped to avoid the possibility of unprotected areas Use overlapping CTs: Isolation done by CB. Thus, it must be inserted in each overlap region to identify the boundary of protective zones.

Overlapped of ProtectionOverlap accomplish by having 2 sets of instrument transformers and relays for each CB.Achieved by the arrangement of CT and CB.

Primary & Back-up ProtectionPrimary protection is the protection provided by each zone to its elements.However, some component of a zone protection scheme fail to operate.Back-up protection is provided which take over only in the event of primary protection failure.

ExampleConsider the power system shown below, with the generating source beyond buses 1, 3 and 4. What are the zones of protection in which the system should be divided? Which circuit breakers will open for faults at P1 and P2?

1

2

3

4

P1

B

P2

A

C

Fault at P1 = A, B, CFault at P2 = A, B, C,D, E

1

2

3

4

P1

B

P2

A

C

D

E

ExampleIf three circuits breakers are added at the tap point 2, how would the zones of protection be modified? Which circuit breakers will operate for fault at P1 and P2 under these conditions?

1

2

3

4

P1

B

P2

A

C

Fault at P1 = A, FFault at P2 = C,D,E,G

1

2

3

4

P1

B

P2

A

C

D

E

H

F

G

Zone DiscriminationA system as shown with relays and breakers marked. A single fault has resulted in the operation of breakers B1, B2, B3 and B4.Identify the location of the faultAnswer:Fault in the overlap zone at breaker B2 as shown

Back-up Protection1.Duplicate PrimaryProvide primary protection when the primary-relaying equipment is out of service for maintenance or repairDisconnect when primary relaying operates correctlyOperate with sufficient time delay (coordination time delay) if primary not operateWhen short circuit occur, both primary and back-up start to operate, but if primary is operate, then the back-up will reset.

Back-up Protection2.Remote Back-uplocated outside boundary of Zone of Protection

ExampleC, D, EA, B, F

FaultPrimary Back-upK

ExampleC, D, E, F, G, HA, B, I, J

FaultPrimary Back-upLine E, F

TransducersAlso known as Instrument TransformerUse to reduce abnormal current & voltage levels and transmit input signals to the relays of a protection system.Why do we need transducer:The lower level input to the relays ensures that the physical hardware used to construct the relays will be small & cheapThe personnel who work with the relays will be working in a safe environment.

TransducersCurrent and Voltage TransformersCorrect connection of CTs and VTs to the protection is important directional, distance, phase comparison and differential protections.Earth CT and VT circuits at one point only;

VT and CT Schematic

Voltage TransformersVT is considered to be sufficiently accurate. It is generally modeled as an ideal transformer.VT secondary connected to voltage-sensing device with infinite impedance.

Voltage TransformersTypes of VTs Electromagnetic VT Capacitive VTBusbar VTsSpecial consideration needed when used for line protectionLV application(12 kV or lower)Industry standard transformer with a primary winding at a system voltage and secondary winding at 67 V(line-to-neutral) and 116 V(line-to-line).

Voltage Transformers

Voltage Transformers

Voltage/PotentialTransformer (VT/PT)

Voltage Transformers

Voltage Transformers

Voltage TransformersHV and EHVCapacitor-coupled VT (CVT)C1 & C2 are adjusted, so that a few kVs of voltage is obtains across C2Then, stepped down by TVTs must be fused or protected by MCB.

Voltage Transformers

Voltage TransformersVT ratios:ratio of the high voltage/secondary voltage1:12:12.5:14:15:120:140:160:180:1100:1200:1300:1400:1600:1800:11000:12000:13000:14500:1

Current TransformersCT is an instrument transformer that is used to supply a reduced value of current to meters, protective relays, and other instruments.The primary winding consist of a single turn which is the power conductor itself.CT secondary is connected to a current-sensing device with zero impedance.

Current TransformersCTs ratio(secondary current rating is 5A)50:5100:5150:5200:5250:5300:5400:5450:5500:5600:5800:5900:51000:51200:5CTs also available with the secondary rating of 1A

Current Transformers

Current Transformers

Reclosers and FusesAutomatic reclosers are commonly used for distribution circuit protection.Recloser: self-controlled device for automatically interrupting and reclosing an AC circuit with preset sequence of openings and reclosuresHave built-in control to clear temporary faults and restores service with momentary outages.Disadvantages: increase hazard when circuit is physically contacted by people.Recloser should be locked out during live-line maintenance.

Reclosers and FusesAn upstream fuse/relay has detected a faultDownstream system isolated by fuse or breakerAutomatic re-closing after delay successful if fault not permanent

RelaysDiscriminate between normal operating and fault conditions.Type of RelaysMagnitude RelayDirectional RelayDistance/Ratio RelayDifferential RelayPilot Relay

Magnitude RelaysAlso called as Overcurrent RelayResponse to the magnitude of input quantities ie. current.Energize CB trip coil when the fault current magnitude exceeds a predetermined value or trips when a current rises above a set point (pick-up current).If it is less than the set point value, the relay remains open, blocking the trip coil.Time-delay Overcurrent Relay also have the same operating method but with an intentional time-delay.

Directional RelaysResponds to fault only in one direction, either to the left or to the right of its locationOperation depends upon the direction (lead or lag) of the fault current with respect to a reference voltage.The directional element of these relays checks the phase angle between the current and voltage of one phase, and allows the overcurrent unit to operate if this phase angle indicates current in the reverse direction.

Ratio RelaysOperate for certain relations between the magnitudes of voltage, current and the phase angle between them.Measures the distance between the relay location and the point of fault, in term of impedance, reactance and admittance.Respond to the ratio of two phasor quantities as example Voltage and Current (Z = V/I)Also called impedance or distance relay

Differential RelaysRespond to the vector difference between two currents within the zone protection determined by the location of CTs.Not suitable for transmission-line protection because the terminals of a line are separated by too great a distance to interconnect the CT secondaries.For the protection of generators, transformers, buses,Most differential-relay applications are of the current-differential type.

Differential RelaysFault occur at XSuppose that current flows through the primary circuit either to a load or to a short circuit located at X. If the two current transformers have the same ratio, and are properly connected, their secondary currents will merely circulate between the two CTs as shown by the arrows, and no current will flow through the differential relay.

Relay

A flow on one side only, or even some current flowing out of one side while a larger current enters the other side, will cause a differential current.In other words, the differential-relay current will be proportional to the vector difference between the currents entering and leaving the protected circuit; and, if the differential current exceeds the relays pickup value, the relayRelayDifferential Relays

Differential RelaysWhen a short circuit develop anywhere between the two CTs.If current flows to the short circuit from both sides as shown, the sum of the CT secondary currents will flow through the differential relay. It is not necessary that short-circuit current flow to the fault from both sides to cause secondary current to flow through the differential relay.Relay

Pilot RelaysThe term pilot means that between the ends of the transmission line there is an interconnecting channel of some sort over which information can be conveyed.Use communicated information from remote sites as input signals.

Pilot RelaysTransmitting fault signals from a remote zone boundary to relays at the terminals of a long TLPilot relaying provides primary protection only; back-up protection must be provided by supplementary relaying.Type : wire pilot, carrier-current pilot and microwave pilot.

Pilot RelaysStation 1 consist of meter for reading voltage, current and power factor.Distance relay, tell the different between fault at A (middle) and B (end) by knowing the impedance characteristic per unit length of the line.ZAZB

1

2

A

B

C

Pilot RelaysCould not possibly distinguish between fault B and C because impedance would be so small- Mistake in tripping CB for fault B or CSolution- indication from station B, when the phase angle of the current at S-B(with respect to current A) is different by approximately 180o from it value for fault in the line section AB.

1

2

A

B

C

Pilot Relays(with respect to current A) is different by approximately 180o from it value for fault in the line section(with respect to current A) is not different in degree from it value for fault in the line section12BCA

Thank you. Nagode. Eseun. Imeela. Masabuke.

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