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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

TRANSFORMER PROTECTION

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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
  • 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

TRANSFORMER PROTECTION

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TRANSFORMER :- STEP-UP

PROTECTION OF TRANSFORMER

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FAILURE OF TRANSFORMERS

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WHY DO TRANSFORMERS FAIL ????

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The electrical windings and the

magnetic core in a transformer

are subject to a number of different

forces during operation :

  • 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
TRANSFORMER

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COSTS AND OTHER FACTOR TO BE CONSIDERED

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[1] Cost of repairing damage

[2] Cost of lost production

[3] Adverse effects on the balance of the system

[4] The spread of damage to adjacent equipment

[5] The period of unavailability of the damaged equipment.

TRANSFORMER FAILURE PROTECTION

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What fails in Transformer ?????

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[1] Winding

Insulation deterioration from :

  • Moisture
  • Overheating
  • Vibration
  • Voltage Surges
  • Mechanical stress from through- faults

.

[2] LTCs

winding

Malfunction of mechanical switching mechanism

High resistance contacts

Contamination of insulating oil

Overheating

.

[3] Bushings

  • General aging
  • Contamination
  • Cracking
  • Internal moisture

[4] Core Problems

  • Core insulation failure
  • Open ground strap
  • Shorted laminations
  • Core overheating
bushings

[5] Miscellaneous

– CT Issues

– Oil leakage

– Oil contamination

  • Metal particles
    Moisture

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Core Construction

CORE

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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

transformers protect

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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

.

Buchholz Relay

.

Gas accumulator relay

 Applicable to conservator tanks equipped

 Operates for small faults by accumulating the gas

over a period of time

• Typically used for alarming only

 Operates or for large faults that force the oil

through the relay at a high velocity

• Used to trip

• Able to detect a small volume of gas and accordingly can detect arcs of low energy

 Detects

• High-resistance joints

• High eddy currents between laminations

• Low- and high-energy arcing

• Accelerated aging caused by overloading

buchholz relay

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Sudden Pressure Relay

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When high current passes through a

shorted turn, a great deal of heat

is generated

  • Detect large and small faults

 This heat, along with the accompanying

arcing, breaks down the oil into

combustible gases.

.

 Gas generation increases pressure

within the tank

 A sudden increase in gas pressure can

be detected by a sudden-pressure relay

located either in the gas space or under

the oil.

.

 The sudden-pressure can operate before

relays sensing electrical quantities, thus

limiting damage to the transformer.

Sudden Pressure Relay

 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.

.

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.

.

Sudden Pressure Relay Supervision for transformer protection

.

sudden pressure relay

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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.

.

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.

.

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!).

.

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

.

Transformer Temperature Monitoring

.

transformer temperature

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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

.

[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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High side

Back up to differential,

Sudden pressure

 Coordinated with line protection off the bus

• Do not want to trip for low-side external faults.

High side overcurrent for internal faults

Set to pick up at a value higher than the maximum

asymmetrical through-fault current.

• This is usually the fault current through the transformer for

a low side three-phase short circuit.

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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Low side

Provides protection against uncleared faults

downstream of the transformer.

May consist of phase and ground elements

Coordinated with downline protection

off the bus.

low side overcurrent

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