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Power transformer protection

The document discusses transformer protection. It describes various failures that can occur in transformers such as winding failures, bushing failures, and tap changer failures. It provides statistics on historical transformer failures. It also discusses different types of protection for transformers including electrical protection methods like differential protection, overcurrent protection, overexcitation protection and thermal protection. Internal short circuits, system short circuits, and abnormal conditions are some of the issues addressed by transformer protection schemes.

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Focus on transformer protection, discussing the key components and themes related to transformers.

Explains operational stresses on transformers like thermal cycling, vibration, and overloading.

Addresses the economic impact of transformer failures, including repair costs and loss of production.

Identifies potential failures in transformer windings due to insulation deterioration and mechanical issues.

Discusses aging and mechanical issues in bushings and core problems leading to transformer failure.

Highlights other failure factors like CT issues and oil leakage that can affect transformer reliability.

Presents data on transformer failures categorized by type, with numerical insights sourced from IEEE.

References guidelines for protective relay applications specific to power transformers.

Lists key IEEE devices used for transformer protection and their respective functions.

Discusses internal and system short circuits as well as abnormal operating conditions for transformers.

Describes the various types of protective measures for transformers, including mechanical and thermal.

Details electrical protection issues, emphasizing different fault types affecting transformer operations.

Outlines detection elements used in transformer protection for various fault scenarios.

Discusses detection elements tailored for abnormal conditions like open circuits and overexcitation.

Analyzes the types of protection applicable to different transformer setups, focusing on fuses.

Categorizes transformers based on their through-fault protection capabilities and requirements.

Explains low-side and high-side overcurrent protection measures and their significance in transformer reliability.

Investigates causes of overexcitation including system errors and operational challenges affecting transformers.

Detailing control issues related to voltage and overexcitation, including the effects of the Ferranti effect.

Illustrates how manufacturers specify V/Hz curves necessary to prevent overexcitation.

Explains the principle of differential protection in transformers and its advantages for fault detection.

Describes phase differential protection schemes and their application in transformer operations.

Details the basic operation of differential relays and their roles in detecting internal and external faults.

Details ground differential functions, highlighting operation without inrush restraints.

Focuses on challenges faced in transformer differential protection, including common mismatches and errors.

Discusses how classical differential compensation works, particularly addressing current transformation issues.

Explains phase angle compensation techniques utilized in digital relays for accurate functioning.

Outlines advantages of using Wye CTs for protection and metering applications in transformers.

Describes methods for detecting inrush currents, emphasizing the importance of maintaining relay functionality.

Discusses the impact of switching transformers at near-zero voltage on inrush currents and harmonics.

Examines overexcitation cases, emphasizing V/Hz levels and associated protective relay strategies.

Details enhanced sensitivity measures for ground faults, discussing differential sensitivity techniques.

Describes protection methods during switch-on scenarios, emphasizing high-speed tripping methods.

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PowerPower TransformerTransformer ProtectionProtection
The electrical windings and the magnetic core in a transformer are subject to a number of different forces during operation, for example: Expansion and contraction due to thermal cycling Vibration Local heating due to magnetic flux Impact forces due to through-fault current Excessive heating due to overloading or inadequate cooling Why do Transformers Fail? Transformer Protection
Cost of repairing damage Cost of lost production Adverse effects on the balance of the system The spread of damage to adjacent equipment The period of unavailability of the damaged equipment Costs and other factors to be considered Transformer Protection
Windings - Insulation deterioration from: Moisture Overheating Vibration Voltage surges Mechanical Stress from through-faults LTCs - Malfunction of mechanical switching mechanism - High resistance contacts - Overheating - Contamination of insulating oil What Fails in Transformers? Transformer Protection
Bushings - General aging - Contamination - Cracking - Internal moisture Core Problems - Core insulation failure - Open ground strap - Shorted laminations - Core overheating What Fails in Transformers? Transformer Protection
Miscellaneous - CT Issues - Oil leakage - Oil contamination Metal particles Moisture What Fails in Transformers? Transformer Protection
1003891001127100262Total 26101672412Miscellaneous 1422437Core failures 313671719 Terminal board failures 1142101141541Bushing failures 2285212311949 Tap changer failures 371445561551134Winding failures % of TotalNumber% of TotalNumber% of TotalNumber 19881983-19821975-19651955- Failure Statistics of Transformers Source: IEEE C37.91 Failure Statistics of Transformers Transformer Protection
87 T 50 51 51 G High Side Low Side “Guide for Protective Relay Applications for Power Transformers” ANSI / IEEE C37.91 Transformer Protection
24: Overexcitation (V/Hz) 46: Negative Sequence Overcurrent 49: Thermal Overload 50: Instantaneous Phase Overcurrent 50G: Instantaneous Ground Overcurrent 50N: Instantaneous Residual Overcurrent 50BF: Breaker Failure 51G: Ground Inverse Time Overcurrent 51N: Residual Inverse Time Overcurrent 63: Sudden Pressure Relay (Buccholtz Relay) 81U: Underfrequency 87H: Unrestrained Phase Differential 87T: Transformer Phase Differential with Restraints’ 87GD: Ground Differential (also known as “restricted earth fault”) IEEE Devices used in Transformer Protection Transformer Protection
Internal Short Circuits - Phase Faults - Ground Faults System Short Circuits (Back Up Protection) - Buses and Lines Phase Faults Ground Faults Abnormal Conditions - Open Circuits - Overexcitation - Abnormal Frequency - Abnormal Voltage - Breaker Failure - Overload - Geo-magnetically induced current (GIC) Transformer Protection Review Transformer Protection
Accumulated Gases - Arcing by-products (Ex: Buchholz Relay) Pressure Relays - Arcing causing pressure waves in oil or gas space (Sudden Pressure Relay) Thermal - Caused by overload, overexcitation, harmonics and Geo-magnetically induced currents Hot spot temperature Top Oil LTC Overheating Types of Protection Mechanical Transformer Protection
In and Out of Zone Phase Fault Ground Fault Breaker Failure Phase Fault Ground Fault Breaker Failure Overexcitation Undervoltage Underfrequency Open Conductor Overload Transformer Electrical Protection Issues Overexcitation Transformer Protection
Internal Short Circuit - Phase: 87T, 87H - Ground: 87T, 87GD System Short Circuit (Back Up Protection) - Phase and Ground Faults 50, 50G, 50N, 51, 51G, 51N, 46 Detection Elements Transformer Protection
Abnormal Operating Conditions - Open Circuits: 46 - Overexcitation: 24 - Undervoltage: 27 - Abnormal Frequency: 81U - Breaker Failure: 50BF, 50BF-N Detection Elements Transformer Protection
Fuses - Small transformers (typ. <10 MVA) - Short circuit protection only Overcurrent protection - High side Through fault protection Differential back-up protection for high side faults - Low side System back up protection Unbalanced load protection Types of Protection Electrical Transformer Protection
Protection against heavy prolonged through faults Transformer Category -IEEE Std. C57.109-1985 Curves High Side Overcurrent Above 30,000Above 10,000IV 5001-30,0001668-10,000III 501-5000501-1667II 15-5005-500I Three-PhaseSingle-PhaseCategory Minimum nameplate (kVA) Transformer Protection
Through Fault Category 1 Transformer Protection
Through Fault Category 2 Transformer Protection
Cat. 2 & 3 Fault Frequency Zones Transformer Protection
Through Fault Category 3 Transformer Protection
Through Fault Category 4 Transformer Protection
Failed Breaker 51 51 G Provides protection against uncleared faults downstream of the transformer May consist of phase and ground elements Coordinated with line pro- tection off the bus Low Side Overcurrent Transformer Protection
46 Negative sequence overcurrent provides protection against unbalanced loads and open conductors Easy to coordinate with down stream devices Low Side Overcurrent Transformer Protection
Overexcitation - Responds to overfluxing; excessive V/Hz - Constant operational limits ANSI C37.106 & C57.12 - 1.05 loaded, 1.10 unloaded Inverse time curves typically available for values over the allowable maximum Transformer Protection Types of Protection
Generating Plants -Excitation system runaway -Sudden loss of load -Operational issues (reduced frequency) Static starts Pumped hydro starting Rotor warming Transmission Systems -Voltage and Reactive Support Control Failures Capacitor banks ON when they should be OFF Shunt reactors OFF when they should be ON Near-end breaker failures resulting in voltage rise on line (Ferranti effect) Runaway LTCs Causes of Overexcitation Transformer Protection
30-40 MVAR 10-20 MVAR 10-20 MVAR Caps ON When They Should Be Off System Control Issues: Overvoltage and Overexcitation Transformer Protection
30-40 MVAR 10-20 MVAR 10-20 MVAR Reactors OFF When They Should Be On System Control Issues: Overvoltage and Overexcitation Transformer Protection
30-40 MVAR 10-20 MVAR 10-20 MVAR Ferranti Effect System Control Issues: Overvoltage and Overexcitation Transformer Protection
30-40 MVAR 10-20 MVAR 10-20 MVAR Run-Away LTC System Control Issues: Overvoltage and Overexcitation Transformer Protection
This is typically how the apparatus manufacturer specifies the V/Hz curves 0.01 0.1 1.0 10 100 1000 MINUTES v/Hz(%) 90 100 110 120 130 140 150 Overexcitation Curve Transformer Protection
This is how protection engineers enter the v/Hz curve into a protective device Overexcitation Curve Transformer Protection
Differential Protection Transformer Protection • Differential protection provides high speed detection of faults that can reduce damage due to the flow of fault currents • The location of the fault is determined more precisely (i.e within the zone of differential protection) Advantages: Types of Protection
Phase Differential - Applied with variable percentage slopes to accommodate CT saturation and CT ratio errors - Applied with inrush and overexcitation restraints - Pickup/slope setting should consider: magnetizing current, turns ratio errors due to fixed taps and +/-10% variation due to LTC - May not be sensitive enough for all faults (low level, ground faults near neutral) Types of Protection Transformer Protection
TRANSFORMER TAP W-1 TAP W-2 Restraint W-1 Restraint W-2 Operate RELAY Basic Differential Relay Transformer Protection Phase Differential
TRANSFORMER TAP W-1 TAP W-2 Restraint W-1 Restraint W-2 Operate RELAY = 0 Basic Differential Relay - External Fault Transformer Protection
TRANSFORMER TAPW-1 TAPW-2 Restraint W-1 Restraint W-2 Operate RELAY Basic Differential Relay - Internal Fault Transformer Protection
Ground Differential - Sensitive detection of ground faults, including those near the neutral - Does not require inrush or overexcitation restraint - Low impedance grounded systems use directional signal for added stability - Low impedance grounded systems do not require dedicated CTs, same set of CTs can be used for phase differential, phase overcurrent, ground differential and ground overcurrent protection Types of Protection Transformer Protection
What goes into a “unit” comes out of a “unit” Kirchoff’s Law: The sum of the currents entering and leaving a junction is zero Straight forward concept, but not that simple in practice with transformers A host of issues presents itself to decrease security and reliability of transformer differential protection Differential Protection Transformer Protection
UNIT I1 I2 I3 I1 + I2 + I3 = 0 Typical Phase Differential Characteristic Transformer Protection
CT ratio caused current mismatch Transformation ratio caused current mismatch (fixed taps) LTC induced current mismatch Delta-wye transformation of currents - Vector group and current derivation issues Zero-sequence current elimination for external ground faults on wye windings Inrush phenomena and its resultant current mismatch Unique Issues Applying to Transformer Differential Protection Transformer Protection
Harmonic content available during inrush period due to point-on-wave switching (especially with newer transformers with step-lap core construction) Overexcitation phenomena and its resultant current mismatch Internal ground fault sensitivity concerns Switch onto fault concerns CT saturation, remanance and tolerance Unique Issues Applying to Transformer Differential Protection Transformer Protection
CT ratios must be selected to account for: - Transformer ratios - If delta or wye connected CTs are applied - Delta increases ratio by 1.73 Delta CTs must be used to filter zero-sequence current on wye transformer windings Classical Differential Compensation Transformer Protection
Classical Differential Compensation Transformer Protection
Transformer ratio CT ratio Phase angle shift and √ 3 factor due to delta/wye connection Zero-sequence current filtering for wye windings so the differential quantities do not occur from external ground faults Compensation in Digital Relays Transformer Protection
The compensation for the phase shift is achieved as follows: Iabc: Measured CT currents (assuming CTs are connected in Y) I'abc: Compensated three-phase currents. Assuming a two winding transformer with a phase angle difference between the primary and secondary as shown in the following 12 cases. Phase angle shift due to transformer connection in electromechanical and static relays is accomplished using appropriate connection of the cts. The phase angle shift in Numerical Relays can easily be compensated in software for any transformer type. All cts can be connected in Y which allows the same cts to be used for both metering and backup overcurrent functions Phase Angle compensation in Numerical Relays Transformer Protection
ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 100 010 001 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 101 110 011 3 1 ' A C B C B A A C B B C A C B A A B C B A C C A B B A C C B A A C B ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 001 100 010 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 010 001 100 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 100 010 001 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 001 100 010 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 010 001 100 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 011 101 110 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 110 011 101 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 101 110 011 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 011 101 110 3 1 ' 1: 0O 3: 60O 5: 120O 7: 180O 9: 240O 11: 300O B A C ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 110 011 101 3 1 ' 2: 30O 4: 90O 6: 150O 8: 210O 10: 270O 12: 330O y g As an example, if we have a two winding transformer with Y/Delta-AC connection (or YD1) with Y-Y cts. This will be equivalent to case 2 with a 30o phase shift. Transformer Protection
1:1, Y-Y 1:1, 3Y1:1, 3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' IB' = Ib' IC' = Ic' IA', IB', IC' Ia', Ib', Ic' Compensation: Base Model Transformer Protection
1:1, Y-Y 1:1, 3Y4:1, 3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' / 4 IB' = Ib' / 4 IC' = Ic' / 4 IA', IB', IC' Ia', Ib', Ic' Compensation: Change in CT Ratio Transformer Protection
2:1, Y-Y 1:1, 3Y1:1, 3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' / 2 IB' = Ib' / 2 IC' = Ic' / 2 IA', IB', IC' Ia', Ib', Ic' Compensation: Transformer Ratio Transformer Protection
1:1, -Y 1:1, 3Y1:1, 3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' * 1.73 IB' = Ib' * 1.73 IC' = Ic' * 1.73 ANSI standard, high lead low by 30, Current pairs are: IA-IB, IB-IC, IC-IA IA', IB', IC' Ia', Ib', Ic' Compensation: Delta – Wye Transformation Transformer Protection
3I0 = [Ia + Ib + Ic] I0 = 1/3 *[Ia + Ib + Ic] Used where filtering is required (Ex: Y/Y transformer). Compensation: Zero-Sequence elimination Transformer Protection
All wye CTs shown Digital Relay Application Transformer Protection
Phase segregated line currents - Individual line current oscillography - Currents may be easily used for overcurrent protection and metering - Easier to commission and troubleshoot - Zero sequence elimination performed by calculation However, for protection upgrade applications where one wants to keep the existing wiring, it will be necessary for the relay to accept either delta or wye CTs and recalculate the phase currents from delta CTs for overcurrent function (Some digital relays like M-3310/3311 have these features). Benefits of Wye CTs Transformer Protection
Inrush Detection and Restraint - Initial inrush occurs during transformer energizing as the core magnetizes - Sympathy inrush occurs from adjacent transformer(s) energizing, fault removal, allowing the transformer to undergo a low level inrush - Characterized by current into one winding of transformer, and not out of the other winding(s) - This causes a differential element to pickup - Use inrush restraint to block differential element during inrush period Inrush Restraint Transformer Protection
Inrush Detection and Restraint - 2nd harmonic restraint has been employed for years - “Gap” detection has also been employed - As transformers are designed to closer tolerances, the incidence of both 2nd harmonic and low current gaps in waveform have decreased - If 2nd harmonic restraint level is set too low, differential element may be blocked for internal faults with CT saturation (with associated harmonics generated) Classical Inrush Detection Transformer Protection
Inrush Detection and Restraint - 4th harmonic is also generated during inrush - Even harmonics are more prevalent than odd harmonics during inrush Odd harmonics are more prevalent during CT saturation - Use 4th harmonic and 2nd harmonic together Use RMS sum of the 2nd and 4th harmonic as inrush restraint - Result: Improved security while not sacrificing reliability Advanced Inrush Detection Transformer Protection
2nd and 4th Harmonics During Inrush Typical Transformer Inrush Waveform Inrush Oscillograph Transformer Protection
As most circuit breakers are ganged three-pole, one phase will be near voltage zero at the moment of transformer energization When a phase of a transformer is switched on near zero voltage, the inrush is increased and so is the resultant harmonics Low levels of harmonics (especially modern transformers) may not provide inrush restraint for affected phase – security risk! Employ cross-phase averaging to compensate for this issue Point-on-Wave Considerations During Switch On Transformer Protection
Provides security if any phase has low harmonic content during inrush Cross phase averaging uses the sum of harmonics on all three phases as the restraint value Cross Phase Averaging Transformer Protection
Overexcitation occurs when volts per hertz level rises (V/Hz) above the rated value This may occur from: - Load rejection (generator transformers) - Malfunctioning of voltage and reactive support elements - Malfunctioning of breakers and line protection (including transfer trip communication equipment schemes) - Malfunctioning of generator AVRs The voltage rise at nominal frequency causes the V/Hz to rise This causes the transformer core to saturate and thereby increase the magnetizing current. The increased magnetizing current contains 5th harmonic component This magnetizing current causes the differential element to pickup Overexcitation Restraint Transformer Protection
VoltageCurrent Transformer Magnetizing Current Current into Transformer due to Overfluxing Rated Voltage Overvoltage Voltage 24 Elements Tripping Overexcitation Event Transformer Protection
VoltageCurrent 11.57kV (RMS) 19.40kV (RMS 0.0 kA (RMS) 1.42kA (RMS) !!! 24 Elements Tripping Overexcitation Event Transformer Protection
High 5th Harmonic Currents Overexcitation Event Transformer Protection
259V of 5th Harmonic 190 A of 5th Harmonic !!! Overexcitation Event Transformer Protection
Use 5th harmonic level to detect overexcitation Most relays block the differential element from functioning during transformer overexcitation An improved strategy is to raise the pick up level to accommodate the increased difference currents caused by the transformer saturation This allows the differential element to rapidly trip if an internal fault occurs during the overexcitation period Result: Improved reliability while not sacrificing security Overexcitation Restraint Transformer Protection
Trip Characteristic – 87T Transformer Protection 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 87T Pick Up 87T Pick Up with 5th Harmonic Restraint Slope 1 Slope 2 Slope 2 Breakpoint TRIP RESTRAIN
87T element is typically set with 30-40% pickup This is to accommodate Class “C” CT accuracy during a fault plus the effects of LTCs That leaves a portion of the winding not covered for a ground fault Employ a ground differential element to improve sensitivity (87GD) Improved Ground Fault Sensitivity Transformer Protection
Use 87GD IA + IB + IC = 3I0 If fault is internal, opposite polarity If fault is external, same polarity IG IA IB IC . Improved Ground Fault Sensitivity Transformer Protection
IG IA IB IC IG IA IB IC Internal ExternalInternal External • . Improved Ground Fault Sensitivity Transformer Protection
- The directional element is used to provide security against relay misoperation due to ct saturation. With low level faults, the directional element is not necessary as the CTs will not saturate. Improved Ground Fault Sensitivity Transformer Protection
IG IA IB IC 3I0 IG Residual current calculated from individual phase currents. Paralleled CTs shown to illustrate principle. 0 90 180 270 IG 3IO -3Io x IG cos (180) = 3IoIG Trip Characteristic – 87GD Transformer Protection
IG IA IB IC 3I0 IG Residual current calculated from individual phase currents. Paralleled CTs shown to illustrate principle. 0 90 180 270 IG 3IO -3Io x IG cos (0) = -3IoIG Trip Characteristic – 87GD Transformer Protection
Transformer is faulted on energizing Harmonic restraint on unfaulted phases may work against trip decision if cross phase averaging is used. This may delay tripping until the inrush current is reduced - Unfaulted phase may have high harmonics and delay 87T operation 87H and 87GD can be used to provide high speed protection for this condition Switch-onto-Fault Transformer Protection
High set element 87H can provide high speed protection for severe faults as this function is not restrained by harmonics 87H is set above the worst case inrush current If fault is near the bushing end of the winding, the current will be higher than the inrush current Ground differential function can provide fast protection during switching onto ground faults as this element is not restrained using harmonics Switch-onto-Fault Transformer Protection ©2008 Beckwith Electric Co., Inc.

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Power transformer protection

  • 1.
  • 2.
    The electrical windingsand the magnetic core in a transformer are subject to a number of different forces during operation, for example: Expansion and contraction due to thermal cycling Vibration Local heating due to magnetic flux Impact forces due to through-fault current Excessive heating due to overloading or inadequate cooling Why do Transformers Fail? Transformer Protection
  • 3.
    Cost of repairingdamage Cost of lost production Adverse effects on the balance of the system The spread of damage to adjacent equipment The period of unavailability of the damaged equipment Costs and other factors to be considered Transformer Protection
  • 4.
    Windings - Insulation deteriorationfrom: Moisture Overheating Vibration Voltage surges Mechanical Stress from through-faults LTCs - Malfunction of mechanical switching mechanism - High resistance contacts - Overheating - Contamination of insulating oil What Fails in Transformers? Transformer Protection
  • 5.
    Bushings - General aging -Contamination - Cracking - Internal moisture Core Problems - Core insulation failure - Open ground strap - Shorted laminations - Core overheating What Fails in Transformers? Transformer Protection
  • 6.
    Miscellaneous - CT Issues -Oil leakage - Oil contamination Metal particles Moisture What Fails in Transformers? Transformer Protection
  • 7.
    1003891001127100262Total 26101672412Miscellaneous 1422437Core failures 313671719 Terminal board failures 1142101141541Bushingfailures 2285212311949 Tap changer failures 371445561551134Winding failures % of TotalNumber% of TotalNumber% of TotalNumber 19881983-19821975-19651955- Failure Statistics of Transformers Source: IEEE C37.91 Failure Statistics of Transformers Transformer Protection
  • 8.
    87 T 50 51 51 G High Side LowSide “Guide for Protective Relay Applications for Power Transformers” ANSI / IEEE C37.91 Transformer Protection
  • 9.
    24: Overexcitation (V/Hz) 46:Negative Sequence Overcurrent 49: Thermal Overload 50: Instantaneous Phase Overcurrent 50G: Instantaneous Ground Overcurrent 50N: Instantaneous Residual Overcurrent 50BF: Breaker Failure 51G: Ground Inverse Time Overcurrent 51N: Residual Inverse Time Overcurrent 63: Sudden Pressure Relay (Buccholtz Relay) 81U: Underfrequency 87H: Unrestrained Phase Differential 87T: Transformer Phase Differential with Restraints’ 87GD: Ground Differential (also known as “restricted earth fault”) IEEE Devices used in Transformer Protection Transformer Protection
  • 10.
    Internal Short Circuits -Phase Faults - Ground Faults System Short Circuits (Back Up Protection) - Buses and Lines Phase Faults Ground Faults Abnormal Conditions - Open Circuits - Overexcitation - Abnormal Frequency - Abnormal Voltage - Breaker Failure - Overload - Geo-magnetically induced current (GIC) Transformer Protection Review Transformer Protection
  • 11.
    Accumulated Gases - Arcingby-products (Ex: Buchholz Relay) Pressure Relays - Arcing causing pressure waves in oil or gas space (Sudden Pressure Relay) Thermal - Caused by overload, overexcitation, harmonics and Geo-magnetically induced currents Hot spot temperature Top Oil LTC Overheating Types of Protection Mechanical Transformer Protection
  • 12.
    In and Outof Zone Phase Fault Ground Fault Breaker Failure Phase Fault Ground Fault Breaker Failure Overexcitation Undervoltage Underfrequency Open Conductor Overload Transformer Electrical Protection Issues Overexcitation Transformer Protection
  • 13.
    Internal Short Circuit -Phase: 87T, 87H - Ground: 87T, 87GD System Short Circuit (Back Up Protection) - Phase and Ground Faults 50, 50G, 50N, 51, 51G, 51N, 46 Detection Elements Transformer Protection
  • 14.
    Abnormal Operating Conditions -Open Circuits: 46 - Overexcitation: 24 - Undervoltage: 27 - Abnormal Frequency: 81U - Breaker Failure: 50BF, 50BF-N Detection Elements Transformer Protection
  • 15.
    Fuses - Small transformers(typ. <10 MVA) - Short circuit protection only Overcurrent protection - High side Through fault protection Differential back-up protection for high side faults - Low side System back up protection Unbalanced load protection Types of Protection Electrical Transformer Protection
  • 16.
    Protection against heavyprolonged through faults Transformer Category -IEEE Std. C57.109-1985 Curves High Side Overcurrent Above 30,000Above 10,000IV 5001-30,0001668-10,000III 501-5000501-1667II 15-5005-500I Three-PhaseSingle-PhaseCategory Minimum nameplate (kVA) Transformer Protection
  • 17.
  • 18.
  • 19.
    Cat. 2 &3 Fault Frequency Zones Transformer Protection
  • 20.
  • 21.
  • 22.
    Failed Breaker 51 51 G Provides protectionagainst uncleared faults downstream of the transformer May consist of phase and ground elements Coordinated with line pro- tection off the bus Low Side Overcurrent Transformer Protection
  • 23.
    46 Negative sequence overcurrent providesprotection against unbalanced loads and open conductors Easy to coordinate with down stream devices Low Side Overcurrent Transformer Protection
  • 24.
    Overexcitation - Responds tooverfluxing; excessive V/Hz - Constant operational limits ANSI C37.106 & C57.12 - 1.05 loaded, 1.10 unloaded Inverse time curves typically available for values over the allowable maximum Transformer Protection Types of Protection
  • 25.
    Generating Plants -Excitation systemrunaway -Sudden loss of load -Operational issues (reduced frequency) Static starts Pumped hydro starting Rotor warming Transmission Systems -Voltage and Reactive Support Control Failures Capacitor banks ON when they should be OFF Shunt reactors OFF when they should be ON Near-end breaker failures resulting in voltage rise on line (Ferranti effect) Runaway LTCs Causes of Overexcitation Transformer Protection
  • 26.
    30-40 MVAR 10-20 MVAR 10-20MVAR Caps ON When They Should Be Off System Control Issues: Overvoltage and Overexcitation Transformer Protection
  • 27.
    30-40 MVAR 10-20 MVAR 10-20MVAR Reactors OFF When They Should Be On System Control Issues: Overvoltage and Overexcitation Transformer Protection
  • 28.
    30-40 MVAR 10-20 MVAR 10-20MVAR Ferranti Effect System Control Issues: Overvoltage and Overexcitation Transformer Protection
  • 29.
    30-40 MVAR 10-20 MVAR 10-20MVAR Run-Away LTC System Control Issues: Overvoltage and Overexcitation Transformer Protection
  • 30.
    This is typicallyhow the apparatus manufacturer specifies the V/Hz curves 0.01 0.1 1.0 10 100 1000 MINUTES v/Hz(%) 90 100 110 120 130 140 150 Overexcitation Curve Transformer Protection
  • 31.
    This is howprotection engineers enter the v/Hz curve into a protective device Overexcitation Curve Transformer Protection
  • 32.
    Differential Protection Transformer Protection •Differential protection provides high speed detection of faults that can reduce damage due to the flow of fault currents • The location of the fault is determined more precisely (i.e within the zone of differential protection) Advantages: Types of Protection
  • 33.
    Phase Differential - Appliedwith variable percentage slopes to accommodate CT saturation and CT ratio errors - Applied with inrush and overexcitation restraints - Pickup/slope setting should consider: magnetizing current, turns ratio errors due to fixed taps and +/-10% variation due to LTC - May not be sensitive enough for all faults (low level, ground faults near neutral) Types of Protection Transformer Protection
  • 34.
    TRANSFORMER TAP W-1 TAP W-2 RestraintW-1 Restraint W-2 Operate RELAY Basic Differential Relay Transformer Protection Phase Differential
  • 35.
    TRANSFORMER TAP W-1 TAP W-2 RestraintW-1 Restraint W-2 Operate RELAY = 0 Basic Differential Relay - External Fault Transformer Protection
  • 36.
    TRANSFORMER TAPW-1 TAPW-2 Restraint W-1 RestraintW-2 Operate RELAY Basic Differential Relay - Internal Fault Transformer Protection
  • 37.
    Ground Differential - Sensitivedetection of ground faults, including those near the neutral - Does not require inrush or overexcitation restraint - Low impedance grounded systems use directional signal for added stability - Low impedance grounded systems do not require dedicated CTs, same set of CTs can be used for phase differential, phase overcurrent, ground differential and ground overcurrent protection Types of Protection Transformer Protection
  • 38.
    What goes intoa “unit” comes out of a “unit” Kirchoff’s Law: The sum of the currents entering and leaving a junction is zero Straight forward concept, but not that simple in practice with transformers A host of issues presents itself to decrease security and reliability of transformer differential protection Differential Protection Transformer Protection
  • 39.
    UNIT I1 I2 I3 I1 +I2 + I3 = 0 Typical Phase Differential Characteristic Transformer Protection
  • 40.
    CT ratio causedcurrent mismatch Transformation ratio caused current mismatch (fixed taps) LTC induced current mismatch Delta-wye transformation of currents - Vector group and current derivation issues Zero-sequence current elimination for external ground faults on wye windings Inrush phenomena and its resultant current mismatch Unique Issues Applying to Transformer Differential Protection Transformer Protection
  • 41.
    Harmonic content availableduring inrush period due to point-on-wave switching (especially with newer transformers with step-lap core construction) Overexcitation phenomena and its resultant current mismatch Internal ground fault sensitivity concerns Switch onto fault concerns CT saturation, remanance and tolerance Unique Issues Applying to Transformer Differential Protection Transformer Protection
  • 42.
    CT ratios mustbe selected to account for: - Transformer ratios - If delta or wye connected CTs are applied - Delta increases ratio by 1.73 Delta CTs must be used to filter zero-sequence current on wye transformer windings Classical Differential Compensation Transformer Protection
  • 43.
  • 44.
    Transformer ratio CT ratio Phaseangle shift and √ 3 factor due to delta/wye connection Zero-sequence current filtering for wye windings so the differential quantities do not occur from external ground faults Compensation in Digital Relays Transformer Protection
  • 45.
    The compensation forthe phase shift is achieved as follows: Iabc: Measured CT currents (assuming CTs are connected in Y) I'abc: Compensated three-phase currents. Assuming a two winding transformer with a phase angle difference between the primary and secondary as shown in the following 12 cases. Phase angle shift due to transformer connection in electromechanical and static relays is accomplished using appropriate connection of the cts. The phase angle shift in Numerical Relays can easily be compensated in software for any transformer type. All cts can be connected in Y which allows the same cts to be used for both metering and backup overcurrent functions Phase Angle compensation in Numerical Relays Transformer Protection
  • 46.
    ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 100 010 001 ' ABCABCII ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 101 110 011 3 1 ' A C B C B A A C B B C A C B A A B C B A C C A B B A C C B A A C B ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 001 100 010 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 010 001 100 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 100 010 001 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ = 001 100 010 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 010 001 100 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 011 101 110 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 110 011 101 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 101 110 011 3 1 ' ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 011 101 110 3 1 ' 1: 0O 3: 60O 5: 120O 7: 180O 9: 240O 11: 300O B A C ABCABC II ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − − − = 110 011 101 3 1 ' 2: 30O 4: 90O 6: 150O 8: 210O 10: 270O 12: 330O y g As an example, if we have a two winding transformer with Y/Delta-AC connection (or YD1) with Y-Y cts. This will be equivalent to case 2 with a 30o phase shift. Transformer Protection
  • 47.
    1:1, Y-Y 1:1, 3Y1:1,3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' IB' = Ib' IC' = Ic' IA', IB', IC' Ia', Ib', Ic' Compensation: Base Model Transformer Protection
  • 48.
    1:1, Y-Y 1:1, 3Y4:1,3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' / 4 IB' = Ib' / 4 IC' = Ic' / 4 IA', IB', IC' Ia', Ib', Ic' Compensation: Change in CT Ratio Transformer Protection
  • 49.
    2:1, Y-Y 1:1, 3Y1:1,3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' / 2 IB' = Ib' / 2 IC' = Ic' / 2 IA', IB', IC' Ia', Ib', Ic' Compensation: Transformer Ratio Transformer Protection
  • 50.
    1:1, -Y 1:1, 3Y1:1,3Y IA, IB, IC Ia, Ib, Ic IA' = Ia' * 1.73 IB' = Ib' * 1.73 IC' = Ic' * 1.73 ANSI standard, high lead low by 30, Current pairs are: IA-IB, IB-IC, IC-IA IA', IB', IC' Ia', Ib', Ic' Compensation: Delta – Wye Transformation Transformer Protection
  • 51.
    3I0 = [Ia+ Ib + Ic] I0 = 1/3 *[Ia + Ib + Ic] Used where filtering is required (Ex: Y/Y transformer). Compensation: Zero-Sequence elimination Transformer Protection
  • 52.
    All wye CTsshown Digital Relay Application Transformer Protection
  • 53.
    Phase segregated linecurrents - Individual line current oscillography - Currents may be easily used for overcurrent protection and metering - Easier to commission and troubleshoot - Zero sequence elimination performed by calculation However, for protection upgrade applications where one wants to keep the existing wiring, it will be necessary for the relay to accept either delta or wye CTs and recalculate the phase currents from delta CTs for overcurrent function (Some digital relays like M-3310/3311 have these features). Benefits of Wye CTs Transformer Protection
  • 54.
    Inrush Detection andRestraint - Initial inrush occurs during transformer energizing as the core magnetizes - Sympathy inrush occurs from adjacent transformer(s) energizing, fault removal, allowing the transformer to undergo a low level inrush - Characterized by current into one winding of transformer, and not out of the other winding(s) - This causes a differential element to pickup - Use inrush restraint to block differential element during inrush period Inrush Restraint Transformer Protection
  • 55.
    Inrush Detection andRestraint - 2nd harmonic restraint has been employed for years - “Gap” detection has also been employed - As transformers are designed to closer tolerances, the incidence of both 2nd harmonic and low current gaps in waveform have decreased - If 2nd harmonic restraint level is set too low, differential element may be blocked for internal faults with CT saturation (with associated harmonics generated) Classical Inrush Detection Transformer Protection
  • 56.
    Inrush Detection andRestraint - 4th harmonic is also generated during inrush - Even harmonics are more prevalent than odd harmonics during inrush Odd harmonics are more prevalent during CT saturation - Use 4th harmonic and 2nd harmonic together Use RMS sum of the 2nd and 4th harmonic as inrush restraint - Result: Improved security while not sacrificing reliability Advanced Inrush Detection Transformer Protection
  • 57.
    2nd and 4th Harmonics During Inrush TypicalTransformer Inrush Waveform Inrush Oscillograph Transformer Protection
  • 58.
    As most circuitbreakers are ganged three-pole, one phase will be near voltage zero at the moment of transformer energization When a phase of a transformer is switched on near zero voltage, the inrush is increased and so is the resultant harmonics Low levels of harmonics (especially modern transformers) may not provide inrush restraint for affected phase – security risk! Employ cross-phase averaging to compensate for this issue Point-on-Wave Considerations During Switch On Transformer Protection
  • 59.
    Provides security ifany phase has low harmonic content during inrush Cross phase averaging uses the sum of harmonics on all three phases as the restraint value Cross Phase Averaging Transformer Protection
  • 60.
    Overexcitation occurs whenvolts per hertz level rises (V/Hz) above the rated value This may occur from: - Load rejection (generator transformers) - Malfunctioning of voltage and reactive support elements - Malfunctioning of breakers and line protection (including transfer trip communication equipment schemes) - Malfunctioning of generator AVRs The voltage rise at nominal frequency causes the V/Hz to rise This causes the transformer core to saturate and thereby increase the magnetizing current. The increased magnetizing current contains 5th harmonic component This magnetizing current causes the differential element to pickup Overexcitation Restraint Transformer Protection
  • 61.
    VoltageCurrent Transformer Magnetizing Current Currentinto Transformer due to Overfluxing Rated Voltage Overvoltage Voltage 24 Elements Tripping Overexcitation Event Transformer Protection
  • 62.
    VoltageCurrent 11.57kV (RMS) 19.40kV (RMS 0.0kA (RMS) 1.42kA (RMS) !!! 24 Elements Tripping Overexcitation Event Transformer Protection
  • 63.
    High 5th HarmonicCurrents Overexcitation Event Transformer Protection
  • 64.
    259V of 5thHarmonic 190 A of 5th Harmonic !!! Overexcitation Event Transformer Protection
  • 65.
    Use 5th harmoniclevel to detect overexcitation Most relays block the differential element from functioning during transformer overexcitation An improved strategy is to raise the pick up level to accommodate the increased difference currents caused by the transformer saturation This allows the differential element to rapidly trip if an internal fault occurs during the overexcitation period Result: Improved reliability while not sacrificing security Overexcitation Restraint Transformer Protection
  • 66.
    Trip Characteristic –87T Transformer Protection 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 87T Pick Up 87T Pick Up with 5th Harmonic Restraint Slope 1 Slope 2 Slope 2 Breakpoint TRIP RESTRAIN
  • 67.
    87T element istypically set with 30-40% pickup This is to accommodate Class “C” CT accuracy during a fault plus the effects of LTCs That leaves a portion of the winding not covered for a ground fault Employ a ground differential element to improve sensitivity (87GD) Improved Ground Fault Sensitivity Transformer Protection
  • 68.
    Use 87GD IA +IB + IC = 3I0 If fault is internal, opposite polarity If fault is external, same polarity IG IA IB IC . Improved Ground Fault Sensitivity Transformer Protection
  • 69.
    IG IA IB IC IG IA IB IC Internal ExternalInternal External •. Improved Ground Fault Sensitivity Transformer Protection
  • 70.
    - The directionalelement is used to provide security against relay misoperation due to ct saturation. With low level faults, the directional element is not necessary as the CTs will not saturate. Improved Ground Fault Sensitivity Transformer Protection
  • 71.
    IG IA IB IC 3I0 IG Residual current calculated from individualphase currents. Paralleled CTs shown to illustrate principle. 0 90 180 270 IG 3IO -3Io x IG cos (180) = 3IoIG Trip Characteristic – 87GD Transformer Protection
  • 72.
    IG IA IB IC 3I0 IG Residual current calculated from individualphase currents. Paralleled CTs shown to illustrate principle. 0 90 180 270 IG 3IO -3Io x IG cos (0) = -3IoIG Trip Characteristic – 87GD Transformer Protection
  • 73.
    Transformer is faultedon energizing Harmonic restraint on unfaulted phases may work against trip decision if cross phase averaging is used. This may delay tripping until the inrush current is reduced - Unfaulted phase may have high harmonics and delay 87T operation 87H and 87GD can be used to provide high speed protection for this condition Switch-onto-Fault Transformer Protection
  • 74.
    High set element87H can provide high speed protection for severe faults as this function is not restrained by harmonics 87H is set above the worst case inrush current If fault is near the bushing end of the winding, the current will be higher than the inrush current Ground differential function can provide fast protection during switching onto ground faults as this element is not restrained using harmonics Switch-onto-Fault Transformer Protection ©2008 Beckwith Electric Co., Inc.