44911023 Transformer Protection

TRANSFORMER PROTECTION

Introduction A Power Transformer is a vital link in a power transmission system and impact of a transformer fault is more serious than a transmission line outage. Following are important. High quality transformer. Operating the transformer within specified limits of temperature and voltage. Proper checking and maintaining OLTC. Providing suitable protective relays and monitoring devices.

Insulation Breakdown Main causes of this are Aging of insulation due to over temperature during long time. Contaminated oil. Corona discharges in the insulation. Transient overvoltages due to thunderstorms or switching in the network. Current forces on the windings due to external faults with high current.Aging of Insulation Aging of insulation is a function of time and temperature. Part of the winding operated at highest temperature undergoes greatest deterioration. Improved cooling of transformer helps avoid accelerated aging of the insulation.

Overheating due to overexcitationOil contamination and leakage Quality of oil should be checked to ensure dielectric strength at site.

Silica get breather helps avoid moisture.

Oil level monitored to avoid breakdown of insulation. The overexcited transformer flux is forced through metal tank and other unlaminated parts of the transformer and result in heating up.

Curve shows IEEE general guide for permissible short time over excitation.

To get correct representation V/Hz relay should be connected to PT measuring voltage of an untapped transformer winding.

Fundamental of differential protectionBasic considerationType of transformer

Vector

Requirement of CT

Type of differential

Fundamental of differential protectionBasic considerationTYPE of transformerGenerator transformer

Sub station transformer

Furnace transformer

Rectifier transformer

Fundamental of differential protectionBasic considerationVectorPhase shift

Fundamental of differential protectionBasic considerationCTRatio

Class

Polarity

Connection

Fundamental of differential protectionTypes of differentialHigh impedance differential::Here a high impedance is added to relay circuit to prevent relay operation due to CT saturation under through fault conditions.This is very sensitive and fast operating for internal faults.Biased differential :Here the operation depends upon differential current exceeding the bias current.The bias characteristics is variable so that it is applicable to a wide variation in transformer design and configuration. This bias slope is set to stabilizeThe protection for small differential currents,which flow due to tap changer variation and CT tolerance under through fault conditions.

Harmonic restraint

Harmonics present in transformer charging in rush current

Reduced Cooling Forced cooling systems should be supervised to get alarm. Oil temperature should be watched and appropriate action taken if transformer gets overheated.

Under Impedance Relay Overcurrent relays are not suitable for system transformer connecting two networks or in networks with a large difference between maximum and minimum short-circuit fault MVA.

Under impedance relay used should be having same reach for two and three phase faults.

Harmonic Restraint Overcurrent Relay Overcurrent relay with second harmonic restraint can be used which will be stable for magnetizing inrush.

Ground Fault Protection Low impedance residual overcurrent relays or harmonic restraint overcurrent relays can be connected according to connection A.

Should be delayed to give chance for other protections in the network to operate.

They act as slow back up for transformer differential relays.

High Impedance Restricted Earth fault Relay Provides sensitive high speed restraint protection.

Vk > 2 Us.

CTs should be dedicated and having identical turns ratio. The combination of relays on the same CT core should be avoided. Due to impedance of REF relay differential relay may not get enough current for operation for a phase-ground fault. Non-linear resistors should be connected in parallel with high impedance relay. This reduces the high peak voltage which can be developed during an internal fault. The interconnected secondary circuit of the CT should be grounded at only one point.

Overexcitation Protection Overexcited transformers become overheated and damaged.

V/Hz overexcitation relay is required for transformers which may be operated at too high voltage or low frequency. Especially GT can be overexcited during acceleration and deceleration of turbine. Ratio should not exceed 1.1 times the ratio of rated voltage and frequency of the transformer.

MonitorsGas Detector Relay During fault, arching occurs releasing gas.

Gas collected in alarm device gives alarm.

Can detect a slowly developing fault before it becomes more serious. Trip devices responds to the high flow of oil which occurs during the sudden occurrence of a serious fault. Monitors are very important devices which detect faults and abnormal service conditions which may develop into fault.

Temperature Monitoring Transformer can stand short time overload upto 1.5 times the rated.

Overcurrent relays cannot be used for overload monitoring as they have to be set above the set short time overload.

Oil temperature and winding temperature therefore provide better monitoring. Static thermal relays with characteristic matching can also be used. Other devices used include Pressure relay for OLTC Oil level monitor Silica gel dehydrating breather.

Fault Currents The reactance decreases rapidly for fault close to neutral. Primary fault current for ground fault between 0-40% from neutral is below 1.5In and therefore O/C relay will not be able to detect this. Primary current is approximately proportional to square of the short circuited fraction of the winding.

Turn-to-turn Faults Turn to turn faults between a few turns is difficult to detect by current measuring relays. Fault current is of the order of rated current when 2 to 4% of the turns are short circuited. The current in the short circuited loop is high (50-100 times In) and causes local damage and release of gas. Therefore rate of rise of pressure relay may detect this fault.

Protective Relays UsedProtective relays limit the damage in case of fault and monitorsto prevent the fault. Therefore fast and reliable protective relaysshould be used.Normal protections used areFor transformers larger than 5 MVA Transformers smaller than 5 MVA Gas detector relay (Buchholz) - Gas detector relay (Buchholz) Overload protection (thermal relays - Overload protection or temperature monitoring relays) - Overcurrent protection Overcurrent protection - Ground fault protection Ground fault protection Differential protection Pressure relay for tap-changer compartment Oil level monitor

Differential Relay The protective zone of a Differential relay includes faults in transformer, faults on Buses or cables between the CT and transformer. Therefore it has a large protective zone than a gas detector relay. A transformer differential relay must be able to cope with the following conditions. 1. Magnetizing inrush current: This is developed when voltage is returning to normal after a line fault and depends on - The size of the power transformer - The source impedance - The magnetic properties of the core material - The remanence of the core - The moment when the transformer is switched inThe magnitude can be 5-10 times the rated current when switchingis done on outer winding of the transformer and 10-20 times ratedcurrent when done on the inner winding.

Damping of inrush current depends on total resistance of source network and lasts for few seconds. Inrush can also develop in an energized transformer when a parallel transformer is switched. (The damping of the combined inrush current will then be less than normal and inrush may last for several minutes) 2nd harmonic restraint prevents unwanted operation of the relay due to inrush is prevented.

Inrush current test

2. Normal service: Differential current flows due to excitation current of transformer, ratio errors in CT and predominantly due to position of tap changer. A setting 15% higher than mismatch is usual.3. Internal Faults: Operating time of typical differential relay(ABB RADSB relay ) for a fault current of 5 times the rated current is 27ms. Unrestrained operation circuit to speed up the operation for a high fault current 8ms at 10 times the set operating current.

Recommended Setting for Unrestrained Operation: Setting of 20*In required when large through fault currents cansaturate the CTs and causes a large differential current for 1 & 1/2CB arrangement.

Power Transformer

Connection (1)

Rated Power

Recommended setting

*In when energizing from

HV Side

LV Side

100 MVA

8

8

Yd

-

13

13

Dy

100 MVA

8

13

4. External Faults: For faults outside the protective zone of the relay a relatively large differential current can occur due to position of the tap changer and differences between the CTs. The differential relay should not operate for this differential current. The differential relays are provided with a through-fault restraint circuit which makes the relay operate for a certain % differential current related to the current through the transformer.

Restraint characteristic

Use of Auxiliary CTs: Aux. CTs of Y are required even for YY transformer to prevent any operation of relay for external ground fault. For Y Power transformer, aux. CTs are required for balancing of currents and for correction of phase angles. Aux CTs are recommended on all sides of the transformer so that same time is taken for saturation for all the inputs. Connection of aux. CTs will depend on the connection of the Power transformer.

Differential Protection for Auto-transformers: Delta winding may or may not be connected to the network. If not connected CTs are not required.The differential relay will protect the main winding as well as the delta winding.High impedance relay can be used by applying CTs in the neutral point of the main winding. The relays protect the main winding but not the delta connected wdg. All CTs should have the same ratio and auxiliary CTs can not be used. Saturation voltage of all the CTs should be at least twice the selected operating voltage.

Overexcitation: For an overvoltage of 20%, the excitation current can increase above the pick-up level of differential relay. An overexcited condition is not a transformer fault and hence the differential relay should not operate. If differential relay operates valuable time will be wasted on the investigation of the transformer. 5th harmonic restraint will prevent the tripping for Overexcitation as overexcited condition results in pronounced 5th harmonic component.

Time Overcurrent Relays: Used on all feeding circuits of a transformer to provide back-up to differential relay and relays on the load side of transformer. An instantaneous highset overcurrent element is normally used to give fast fault clearance to severe faults. Time Overcurrent relay is set to 150% of the rated current and time delay must be set long enough to avoid tripping due to magnetizing inrush. The instantaneous element should be set to about 25% above the maximum through fault current and above the maximum inrush current. With this setting instantaneous tripping is obtained only for severe faults on the feeding side of the transformer.

Relay operates delayed for faults on the remaining parts of the windings and for faults on the load side of the transformer.

TRANSFORMER PROTECTION TERMINALS

electromechanical single functionstatic single functiondigital single functiondigital multifunction relaysnumerical multifunction relaysnumerical multifunction systemsHistorical evolution

protection functions realised with different HWQuantity and types of protection func. fixed and limited HW-extensions difficultNo. of CT's and PT's higherRequirements to primary transformers higherfixed HW prot.functions realised with SWComplete library of func. available Adaptation by SWNo.of CT's and PT's lowerRequirement to primary transformers lower Comparison of technologies conventional numerical

Settings and operation locallyno documentation ( only hand-made)Only binary information Periodical tests necessary

Various spare parts Settings and operation locally or remoteSelf-documentation of all settings and events etc.Numerical information, meas..values, events, etc.Selfsupervision and test functions reduces maintenance. Five different types only Comparison of technologies conventional numerical

integration to control systems difficultonly protection

only protection

fixed solutionintegration to control systems possibleintegrated protection and control possiblemonitoring with available information possibleextension and new developments possible --> open architecture Comparison of technologies conventional numerical

Transformer TerminalGenerator TerminalControl TerminalLine Terminal

SoftwareLibrary

Complete library with functions for bay control, monitoring,protection of generators, transformers.Software and hardware proven and well introduced.Extremely powerful and cost efficient solutions for MV and HV applications.

Selective Protection of: Two or Three winding Transformer Auto Transformers Generator-Transformer unitDetection of Faults: All phase faults Earth faults at solidly or Low impedance grounded systems Inter-turn faults

No interposing CT'sStandard wiring diagramInputs for external functions (Buchholz, temperature sensors) availableProgrammable indication of tripping and signalingIndication of measuring valuesContinuous self-monitoringModular SW protection functions4 serial interfaces: - one front for local communication (PC) - one rear for remote communication - two others (spare)

1) Analog input unit up to 6 transformer3) CPU with serial port4) Binary input/output unit5) Communication PCMCIA6) Mother Board7) Power Supply74444 312 62) Digital/Optical unit 5 1 2 3 Open communication strategyFlexible input and output configurationRBayUnits

74444 315 6Interbay bus8 1

3

4

5

6

7

8Analog input module, up to 9 input transformers for AC voltage and current

CPU

Binary I/O modules (max. 56 binary inputs, max. 32 binary outputs)

Communication interface for the interbay bus (PC-Card)

Connection module

Supply module

Communication interface for the process bus (MVB PC-Card)

RBayUnitsHardware concept

Hardware conceptP C -C ARDabcdDC ACDC+5V+15V-15V+24VPower SupplyA/DDSPSerial ControllerRS 232FLASH EPROMRAMPC-CardLONe.g. LONSPA / IEC870-5-103 (VDEW6)LED'sSCS SMSSerial ControllerRS 232DPMPC-CardProcess busIEC1375(MVB)

abcdDC ACDC+5V+15V-15V+24VPower SupplyA/DDSPSerial ControllerRS 232FLASH EPROMRAMLONe.g. LONSPA / IEC870-5-103 (VDEW6)LED'sSCS SMSSerial ControllerRS 232DPMProcess busIEC1375A/D DSP Tx(MVB)Hardware conceptP C -C ARDPC-CardPC-Card

I> U< Z

Typical tripping timeA/IB/OBinaryoutputisolationAlgorithm and LogicprocessorDigitalfilterAmplifierLow passfilterShuntAnaloginputisolation472300 ms0 ms3 ms5 ms12 ms21 ms25 msZ

IetcFUPLAetc9A/DSH

Software Library

Metering (UlfPQ)

Frequency (81)

Overexcitation with Inverse time delay (24)Overexcitation (24)

Instantaneous Overvoltage (27/59)Definite time Over and Under Voltage (27/59)Inverse time Overcurrent (51)

Instantaneous Overcurrent (50)

Definite time Over and Under Current (51DT)Thermal overload (49)

Restricted Earth Fault (64)

Function LibraryTransformer-differential2 or 3 Winding (87T)

4 parameter sets

Counter, Timer

Logic's (OR, AND, RS-FF)

Remote Inputs and Outputs

Additional I/O units

Operating values I, U, P, Q, f

Event recording

Disturbance recorder

Self supervision

Remote communication

Human Machine Interface

MONITORING ANDAUXILIARY FUNCTIONSLocal Display unit

Function Library

RE21604I >51I > >50I 60I 87 LI TH49U > 59U 59Software concept

HMI functionality LED-displays Measurand display Event list Operating instructions Disturbance recorded information Self supervision Acknowledgement functions Optical connector for external HMI

LED indicationsAvailabilityStartOperationMeasurand displayAnalog channels (amplitude, angle, frequency)Functional measurands (e.g. differential current)Binary signals (I/O signals, tripping)Event list (tripping values only, e.g. distance to fault)Operating instructionsHMI functionality

Disturbance recorder informationNumber of recorded events and dateDiagnosticsOperating status of the unitOperating status of the interbay busOperating status of the process busAcknowledgement functionsResetting the LED'sResetting the latching outputsEvent erasingWarm startHMI functionality

Transformer Differential Protection (87T)Features:

Non-linear, current dependent operating characteristic. High stability during through faults and in the presence of CT saturation. Short tripping times. Three phase measurement. Inrush current restraint. using the second harmonic. detection of the highest phase current. detection of the load current to determine whether the transformer is energized or not. Compensation of phase group. Compensation of CT ratio. DC current component filter and harmonic filter.Differential protection of two or three winding power transformers& generator/transformer units.

Analogue Inputs:

Current ( 2 or 3 sets of 3 inputs)

Binary inputs:

Blocking

Binary Outputs:

Tripping R phase trip S phase trip T phase trip

Measurements:

R phase summation current S phase summation current T phase summation current R phase restraining current S phase restraining current T phase restraining current

Inputs & Outputs

AI 1,2,3AI 7,8,9AD DIFFADTransformer Differentialfor 2-windings

Transformer Differentialfor 3-windings

Operating Characteristic: I = | I 1 + I 2 + I 3 |Operating (diff.) currentRestrain currentWhere I1 = greatest of I 1 , I 2 , I 3 I2 = I 1 + I 2 + I 3 - I1

= ( I1 - I2 )

Fault outside protected zoneLow short circuit current 123

412345IHIv=50%I1I2IH = I1 = I2 = ILoad < (1.5...3)*IratedgI < (1.5...3) * Iratedcos = 1Load

Fault outside protected zone High short circuit current 123

412345IHIv=50%I1I2IH = I1 = I2 gI > (1.5....3) * Iratedcos = 1v= infinite

bIsc

Fault inside protected zone 123

412345IHII1I2IH = 0gcos < 0

Thermal Overload Protection (49)Features: 1st order thermal model Alarm and tripping stages Adjustable initial temperature Single or three-phase measurement Maximum value detection for three-phase measurement Temperature rise calculated 40 times for each thermal time constant setting

Thermal overload protection with accuratethermal image of the protected unit49

Analogue Inputs:

Current

Binary inputs:

Blocking

Binary Outputs:

Alarm Tripping Measurements:

Temperature rise Power dissipation Current

Inputs & Outputs

Definite time Over & Under Current (51DT)Features: Single or three-phase measurement 2nd harmonic restraint for high inrush currents Insensitive to DC component Maximum respectively minimum value detection in the three-phase mode May also be used as REF protection with additional hardware General purpose current function forPhase fault protection and Back-up protn.I < >51

Analogue Inputs:

Current

Binary inputs:

Blocking

Binary Outputs:

Pick-up Tripping Measurements:

Current amplitude

Inputs & Outputs

Setting Parameters: Delay: Time between the function picking up and tripping

I-Setting: Pick-up current setting

MaxMin: Over or Under current

NrOfPhases: 1ph or 3ph measurement

CurrentInp: Analog current input channel

BlockInp: Input for blocking the function

Trip signal: Tripping signal

Start signal: Pick-up signal

Instantaneous Over Current (50)Features: Maximum or Minimum function (over & under current) Process instantaneous values and is therefore fast and largely independent of frequency Single or three-phase measurement Stores the peak value following pick-up

Maximum value detection in the three-phase mode Adjustable lower frequency limit fminGeneral current monitoring with instantaneous responseI < >50

Analogue Inputs:

Current

Binary inputs:

Blocking

Binary Outputs:


Current amplitude (only available if function trips)

inputs & Outputs:


I-Setting: Pick-up current setting

f-min: Minimum frequency for which measurement is required


NrOfPhases: 1ph or 3ph measurement





Inverse time Over Current (51)Features: Operating characteristic according to British standard 142 Single or three-phase measurement Detection of the highest phase value in the three-phase mode Wider setting range than specified in B.S.142Overcurrent function with time delay inverselyproportional to the current and definite minimumtripping time (IDMT) I >51

Inputs & Outputs Analogue Inputs:

Current

Binary inputs:

Blocking

Binary Outputs:


Current amplitude

Setting Parameters: c-setting: Select operating char. According to BS142 or RXIDG char.

k1-Setting: Time grading

I-Start : Pick-up current at which the characteristic becomes active


t-min: Definite minimum tripping time

NrOfPhases: Defines the number of phases measured


IB-setting: Base current for taking account of differences of rated current




Definite time Over & Under voltage (27/59)Features: Single or three-phase measurement Maximum value, respectively minimum value, detection for three-phase measurement DC component filter Harmonic filterStandard voltage applications (overvoltage &undervoltage function) U < >59/27


Voltage

Binary inputs:

Blocking

Binary Outputs:


Voltage amplitude


V-Setting: Voltage setting for tripping

MaxMin: Over or Under voltage selection

NrOfPhases: Number of phases included in the measurement

VoltageInp: Analog input channel




Instantaneous Overvoltage (27/59)Features: Processes instantaneous values and is therefore fast and largely independent of frequency Stores the peak value following pick-up Single and three-phase measurement Maximum value detection in the three-phase mode Adjustable lower frequency limit fminGeneral voltage monitoring with instantaneousresponse (over & undervoltage) U > >59/27


Voltage

Binary inputs:

Blocking

Binary Outputs:


Voltage amplitude (only available if function trips)


V-Setting: Pick-up voltage setting

f-min: Minimum frequency for which measurement is required

MaxMin: Over or Under voltage setting

NrOfPhases: Defines whether 1ph or 3ph measurement

VoltageInp: Analog voltage input channel




Overexcitation (24)Features: Evaluation of the voltage/frequency ratio Single phase measurement Definite time delay Determination of frequency from the complex voltage vector Over or Underexcitation mode Insensitive to DC components & harmonicsProtection of generators and power transformers against excessive flux U/ f24


Voltage

Binary inputs:

Blocking

Binary Outputs:


Voltage / frequency frequency

Setting Parameters: Delay: Time delay between the function picking up and tripping

V/f- setting: Setting of the voltage/frequency ratio for tripping

MaxMin: Over or Under fluxing setting

VoltageInp: Analog voltage input channel




Overexcitation with Inverse time delay (24)Features: Evaluation of the voltage/frequency ratio Single phase measurement Inverse time delay according to U/f ratio Determination of frequency from the complex voltage vector According to IEEE guide C37.91-1985 Insensitive to DC components & harmonicsProtection of generators and power transformersagainst excessive flux, especially in heavily loaded non-laminated metal parts, and the associated excessive heating of the unit. U/ f24


Voltage

Binary inputs:

Blocking

Binary Outputs:


Voltage / frequency frequency

Frequency (81)Features: Measurement of one voltage Frequency calculation based on the complex voltage vector Undervoltage blocking Insensitive to DC components & harmonicsUnder and Overfrequency,Load shedding


Voltage

Binary inputs:

Blocking

Binary Outputs:

Under voltage blocking Start Trip Measurements:

Frequency Voltage

Metering (UIfPQ)Features: Single phase measurement Phase-to-ground or optionally phase-to-phase voltage measurement Suppression of DC components and harmonics in current & voltages

Compensation of phase errors in main and input CTs and VTs Measurement of voltage,current,real &apparent power and frequency.


Voltage Current

Binary inputs:

none

Binary Outputs:

none Measurements:

Voltage (unit UN) Current (unit IN) Real power (unit PN (P)) Apparent power (unit PN (Q)) Frequency (unit Hz)

SMSRemote Substation Monitoring SystemOn-demand informationSCSSubstation Control SystemOn-line informationSMSLocal Substation Monitoring SystemOn-demand informationTERMINAL

SwitchMinute pulseGPS-ClockSPA-LOOPSRIORE.316ModemRemote communication and time synchronisation

Remote communication and diagnostics Brings a terminal to the user - evaluations - disturbance clarifications - diagnostics - change and control of relay setting - etc. TERMINAL

Last specified number of events stored Event-No., Date, Time, Funct.-No...........Selectable informationFunction outputs (Start / Trip and special outputs)Binary inputs Trip-valuesStatus ON/ OFF per eventAbsolute and relative time (after GFC fulfillment)Event recorder

9 analog channels16 binary channels (function outputs, binary inputs)12 function measurements (e.g Idelta, I2, Z )Total record time 5sPre-Event 400ms,Event 3000ms, Post-Event 400msSelectable triggering (GFC, trip or functions and binary inputs)Stop on full or overwrite modeDisturbance recorder

Disturbance Recorder - Recording timest pt pret ft limt prePre-fault time (0.04- 0.40 sec)t fFault timet p1Post-fault time (0.1 - 3.0 sec) t limTime limit for total recording (0.5 - 4.0 sec)

A/D-Conversion3ph-Voltages and-CurrentsExternal and internal Power supplyRead/Write comparison Checksum functionTolerance checkSymmetry checkcontinuous conversion of 2 reference signalsMemoriesP C - C A RDabcdRS232RS232RAMTrip OutputsSign. OutputsBin. InputsI / O PortsLONSPA / IEC 870-5-103SelfsupervisionIEC 1375Serial Data TransferHamming distance 4 to 6by frame format definition,16 bit CRC or check sum+parity bitSerial Cont.

Password protectedTest protection functionssend a numerical value to each functiontest characteristic setting and related outputs Test signaling relaysTest tripping relaysTest LED'sTest function

Advantages Self monitoring Long term stability Event recorder Self documentation Number of CT cores reduced User designed performance Selectable protection functions Facility for communication to SMS/SCS

REFERENCESABB manuals

Alsthom manuals

Easun Reyrolle manuals

Art & science of protective relaying by Russell Mason

The typical tripping time for the distance protection function in the relay is 40 ms for the version with under impedance start and faster with overcurrent start only. Note that in order to issue a trip signal the calculated impedance has to be within the set value for two consecutive calculations.

Documents

44911023 Transformer Protection