Transformer Protection – Over Current and Earth Fault

Transformer Protection:

Transformer Protection – Transformers are static devices, totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitates to provide adequate automatic protection for transformers against possible faults.

Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay is required. However, the probability of faults on power transformer is undoubtedly more and hence automatic protection is absolutely necessary.

Common Transformer Faults:

1. Open Circuits
2. Overheating
3. Winding Faults or Internal Faults
4. Over fluxing
5. Through faults

The principle relays and systems used for transformer protection:

• Buchholz device – Providing protection against all kinds of incipient faults.
• Overcurrent relays – Providing protection mainly against phase-to-phase faults and overloading.
• Earth fault relays – Providing protection against earth faults.
• Differential System – Providing Protection against both earth and phase faults.
• Harmonic Restraint and Harmonic Blocking.

Here the blog covers about the Overcurrent and Earth Fault protection of Transformer.

Over Current Protection:

The inherent time delay of the IDMT element provides back-up for the LV side. High-set instantaneous overcurrent is also recommended on the primary side mainly to give high-speed clearance to HV bushing flashovers. However, to ensure that these elements do not pickup and trip for faults on the LV side as discrimination is important.

51- IDMT Over Current Protection

Pickup Current (I >):

Time Multiplier Settings (T >):  

For Transformer time dial is set above inrush characteristics of the equipment to avoid maloperation during energization and relay operates before transformer damage. TDS should be selected in such way that proper time discrimination is available when it is coordinated with downstream relay or release.

Curve Type:

For Transformer feeder IEC Extremely Inverse (EI) is chosen in order to have better co-ordination with downstream and also below the damage curve of transformer.

When release is available on LV side then the characteristics of Long (L) will be extremely inverse so it is recommended to have Extremely inverse on transformer HV Side.

50 – Instantaneous Over Current Protection

Pickup Current (I>>):  

It should be higher than the inrush current and Through fault current.

At 20ms the difference between Ib symmetrical and Ib asymmetrical is less than 1.2 then 1.3 margin is enough if not the margin to be increased from 1.25 to 2 as per IEEE 242 Standard (Margin It depends on Transformer X/R ratio).

Time Dial:

No intentional time delay.

Addition Points:

We should make sure that the fault on the transformer LV side relay is failed to clear the fault then upstream Transformer HV side relay should clear the fault above 500ms and less than 2000ms including circuit breaker opening time, when executing relay coordination.

Because Transformer has ability to withstand Through fault current for 2 seconds.

50 Instantaneous Over current covers the internal fault that happens on transformer HV Side.

Earth Fault Protection:

51N- IDMT Earth fault Protection

Pickup Current (I >):

Time Multiplier Settings (T >):  

TMS is selected typically as 0.1s if the time dial is reduced less than 0.1s but that may increase the probability of maloperation during energization

Curve Type:

IEC Normal Inverse

50N – Instantaneous Earth Fault Protection

Pickup Current (I>>):  

Time Dial:

No intentional time delay. Minimum time available in the relay. Few Cases there will be some maloperation in that case we can increase to 100ms.

Manual Calculation:

Rated Voltage	:	11 kV
CT Ratio	:	150/1A
Protection Relays	:	ABB RET615
Transformer HV Current	:	105A

Non-directional over current protection 51 (I>) RET615 relay:

Required Current Setting = FLA x 110% / CT Primary Amps

Pickup = 0.77 (115.5A)

Curve = IEC Extremely Inverse

TMS = 0.3 (From ETAP Graph)

Instantaneous Phase protection 50 (I>>) RET615 relay:

Required Current Setting = 130% x Through Fault Current / CT Primary Amps

Pickup                               = 15.15 (2272A)

DT                                                = 0.02

Non-Directional Earth fault protection (51N) RET615 relay:

Required Current Setting             = 10% of Load Current / CT primary amps

Pickup = 0.07(10.5A)

Curve = IEC Normal Inverse

TMS = 0.1

Instantaneous Earth fault protection (50N) RET615 relay:

Required Current Setting             = 100% of Load Current / CT primary amps

Pickup = 1 (150A)

DT             = 0.02

Graph: