TRANSFORMER PROTECTION, HOW TO PROTECT ?????

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The Transformer protection in overload, overcurrent, and many short

circuit time, here is more knowledge of transformer protection, and

more extra practical knowledge of Transformer Protection.

POWER TRANSFORMER AND DISTRIBUTION TRANSFORMER

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EXPLORATION OF TRANSFORMER PROTECTION

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➪ Why transformers fail ??

➪ Quick review of protection principles of Transformer

➪ Guide for Power Transformer Protection

➪ Discuss non-electrical protections

➪ Discuss electrical protections

⤷ (1) Overcurrent based

⤷ (2) Through fault protection

⤷ (3) Overexcitation

⤷ (4) Differential

• CT performance issue

• Transformer protection challenges

• Percentage differential characteristic

• Restraints for inrush and overexcitation

➪ Analysis tools to view relay operation

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TRANSFORMER PROTECTION – ENGINEERING TECH

T & D TRANSFORMERS

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TRANSFORMER :- STEP-UP

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FAILURE OF TRANSFORMERS

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WHY DO TRANSFORMERS FAIL ????

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COSTS AND OTHER FACTOR TO BE CONSIDERED

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What fails in Transformer ?????

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⤷ Malfunction of mechanical switching mechanism

⤷ High resistance contacts

⤷ Contamination of insulating oil

⤷ Overheating

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[5] Miscellaneous

– CT Issues

– Oil leakage

– Oil contamination

• Metal particles
  Moisture

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Core Construction

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• Shell construction is lighter than core construction

• 3-leg shell core causes zero sequence coupling

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Guide for Protective Relay Applications for Power Transformers

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Care fully read and follow the rules with safety :- “Guide for Protective Relay

Applications for Power Transformers

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IEEE Act C37.91 – 2008

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IEEE Devices used in Transformer Protection

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 24:   Overexcitation (V/Hz)

 26:   Thermal Device

 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 (Buchholz Relay)

 64G:  Transformer Tank Ground Overcurrent

 81U:  Under frequency

 87H:  Unrestrained Phase Differential

 87T:   Transformer Phase Differential with Restraints

 87GD: Ground Differential (also known as “restricted earth fault”

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TRANSFORMER PROTECTION REVIEW

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Internal short circuit

• Phase faults

• Ground faults

System short circuit ( 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 )

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Types of Protection

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There are two Types of Protection

MECHANICAL AND ELECTRICAL

 Accumulated Gases

Arcing by-products (Buchholz Relay)

 Pressure Relays

[1] Arcing causing pressure waves in oil or gas space

(Sudden Pressure Relay)

 Thermal

[2] Caused by overload, overexcitation, harmonics and

Geo-magnetically induced currents (GIC)

– Hot spot temperature
– Top Oil
– LTC Overheating

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Buchholz Relay

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Sudden Pressure Relay

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 Drawback of using sudden-pressure relays is tendency

to operate on high current through-faults.

• The sudden high current experienced from a close-in through-fault causes windings of the transformer to move.

• This movement causes a pressure wave that is transmitted through the oil.

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 Countermeasures:

Overcurrent relay supervision

• Any high-current condition detected by the instantaneous overcurrent relay blocks the sudden-pressure relay.

• This method limits the sudden-pressure relay to low-current incipient fault detection.

 Place sudden-pressure relays on opposite corners

of the transformer tank.

• Any pressure wave due to through-faults will not be detected by both sudden-pressure
  relays.

• The contacts of the sudden-pressure relay are connected in series so both must
  operate before tripping.

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Sudden Pressure Relay Supervision for transformer protection

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Phase and Ground Overcurrent supervises SPR (63)

SPR (63) employs

• Pickup delay for overcurrent supervision

• Drop out delay to allow SPR (63) to reset

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Causes of Transformer Overheating

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Transformers may overheat due to the following reasons :

 High ambient temperatures

 Failure of cooling system

 External fault not cleared promptly

 Overload

 Abnormal system conditions such as low frequency, high

Voltage, non-sinusoidal load current, or phase-voltage unbalance.

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Transformer Heating

Undesirable results of overheating

 Overheating shortens the life of the transformer insulation

in proportion to the duration of the high temperature and in

proportion to the degree of the high temperature.

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 Severe over temperature may result in an immediate

insulation failure (fault).

 Overheating can generate gases that could result in an

electrical failure (fault) Severe over temperature may result

in the transformer coolant heated above its flash temperature,

with a resultant fire (fault and a bang!).

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Heating and Relative Transformer Temperatures

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 Temperature may be monitored multiple places

– Hot Spot

– Top Oil

– Bottom Oil

– LTC Tank

– Delta of the above

 The “hot spot” is, as then name indicates, the hottest spot.

 Other temperatures are lower

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Transformer Temperature Monitoring

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Types of Transformer Protection

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ELECTRICAL PROTECTION SYSTEM

Fuses

– Small transformers (typ. <10 MVA)

– Short circuit protection only

Overcurrent protection

– High side

[1] Through fault protection

[2] Differential back-up protection for high side faults

– Low side

[1] System back up protection

[2] Unbalanced load protection

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Transformer Protection Functions

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[1] Internal Faults :

 87T = Phase Differential with Restraints

 87H = Unrestrained Phase Differential

 87GD = Three Ground Differential elements (Restricted Earth Fault)

 64G = Tank Ground Overcurrent

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[2] Through Faults :

 50/51 = Phase Overcurrent

 50G/51G = Ground Overcurrent

 50N/51N = Instantaneous Residual Overcurrent

 46 = Negative Sequence Overcurrent

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[3] Abnormal Operating Conditions :

 27 = Under voltage

 24 = Overexcitation (V/Hz)

 49 = Thermal Overload

 81U = Under frequency

 50BF = Breaker Failure

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[4] Asset Management Functions :

 TF Through Fault Monitoring

 BM Breaker Monitoring

 TCM Trip Circuit Monitoring

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High Side Overcurrent

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Instantaneous units that are subject to transient overreach

are set for pickup in the range of 125% to 200%.

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Low side Overcurrent

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ER. SURENDRA KUMAR CHAUHAN [eengineeringtech.in]