Today we learn about Protective Relays. Transformers are one of the most important components of an electrical power system and protecting them is an essential requirement.

The Protection Schemes used for a transformer depends upon the rating and application of the transformer. For example, a power transformer having a higher voltage and power rating than a distribution. Transformer will need further means of protection. Such as differential protection, overcurrent protection, and over-fluxing protection. And protection against inter-turn faults while a distribution transformer can be protecte by MCCBs and MV fuses alone.

Power transformers need extra protection because of their high rating and sensitivity to their location in a power system. Power transformers are use where a transmission line usually originates i.e. From a generating station or where the transmission line terminates such as a grid station and then power is distribute.

This means that an undetected fault or inadequate protection. Of a transformer can result in huge losses of power Protective Relays. As well as large costs to cover in case of any damage that occurs. A fault may also result in a blackout of a vast area. For a long period if proper protection has not been implemente. This ultimately reduces the reliability of our power system as a whole. The feeder is affecte due to a fault in an upstream transformer.

Overcurrent Protection using Phase and Ground Protective Relays

Overcurrents in a transformer occur due to single line ground faults and phase to phase faults. These are also known as short circuit faults and are large currents resulting in over-heating, fire, and damage to equipment. Overcurrent protection of a transformer is using phase and ground relays.

Phase relays have a pickup current that is greater than the normal load current and the allowable overload and it should be low enough to detect the smallest amount of fault current in our system.

As for ground relays, the pickup current is kept as low as possible as only unbalanced or zero sequence currents flow through the ground or neutral. However, 3rd order harmonics must also be considered which are caused by the disturbances due to electronic loads.

It should also be noted that such overcurrent protection mainly acts as backup protection for power transformers. However, it can act as the primary protection for transformers having lower KVA ratings.

Overcurrent Protection using Percentage Differential Relays

In case of phase to phase or phase to ground faults in a power transformer, a percentage differential relay, also known as a Merz price relay can be used.

This protection scheme is based on the circulating current principle and relies on the vector difference between the currents entering and leaving the transformer terminals, while the average current flow through the restraining coil.

So, under normal operating conditions, the difference between the incoming and outgoing CT currents across a transformer is almost zero, hence, the relay does not actuate but in the event of a line to ground fault or a phase to phase fault in the transformer, a difference in the current is detected in the relay’s operating Protective Relays coil and it operates, sending a signal to the circuit breaker to trip. We can say that differential protection is usually employed to trace out internal faults while the phase relays for overcurrent protection are used to distinguish external faults.

The Solution:

The best way to avoid the spill path current is that the CTs on the delta side of the transformer should be connected in Wye, while the CTs on the Wye side of the transformer should be in delta, this can be understood from the diagram below.

Figure 1: Transformer CT connections

Interposing CT:

However, there is one more complication that must also be catere to. The delta configured CT secondaries will result in the line current of that CT being √3 times the value of the phase current, while the Wye connected CTs will not be having such a multiplier. This again will result in a currency mismatch.

Therefore, to eliminate these √3, we will connect the delta configured CTs to a CT known as interposing or matching CT having a CT ratio of √3 ∶1. Now the spill path current will be zero and the relay can operate without any error.

Dealing with Inrush Current

Inrush current is the current the transformer at the moment it is energize. Now because a power transformer usually operates around the knee point of saturation, it requires a high magnitude of flux hence a very large amount of magnetizing current upon switching the transformer on.

This current is of a very high magnitude of about 8 to 30 times the rated current has a non-sinusoidal waveform and lasts for only milliseconds or a few seconds in a worst-case scenario.

Harmonic Restraint:

However, the problem is that this inrush current appears as a differential current to the Transformer differential relay coil and might issue a false trip signal therefore a percentage different relay with a Harmonic restraint feature

This relay works on the basis that transformer inrush current comprises mainly of 2nd harmonic i.e around 30 to 70% as compared to other fault currents. The relay will restrain its operation upon detecting current having around more than 15% 2nd harmonic currents hence there won’t be any tripping under an inrush current.

Commonly Used Relays:

Presently, microprocessor-based numerical relays are employed in industries for transformer protection. Apart from protection, such relays are also equipped with a variety of other functions such as control, measurement, monitoring, and data recording. Moreover, these relays are user-friendly and can be easily integrated with control systems such as SCADA.

Protection of Transformers using Time Current Curves (TCCs)

An electric power system has transformers along with its protective devices, and Protective Relays but appropriate coordination of these devices is necessary. We do not want our circuit breakers to trip in a non-faulty condition (also known as nuisance tripping), nor do we want to delay the tripping such that the transformer or its cables start to get damaged.

Therefore, we coordinate our protective devices using Time Current Characteristic Curves (TCCs).

You may check out our blog on employing TCCs to learn about how transformer protection is achieved using TCCs and how to work with TCCs in general.

You may check out our blog on Time Current Characteristic Curves to learn how transformer protection is achieved using TCCs and how to work with TCCs in general.

Summary of Transformer Faults and their Protection

The overall protection of a transformer can be summarized in the table below:

Protective Device and Scheme
Transformer faults Primary Secondary
Over-Load faults Overcurrent relay with thermal characteristics
Phase to phase faults Percentage differential relay Overcurrent relay
Phase to ground faults Percentage differential relay Overcurrent relay
Oil Leakage Buchholz relay
Inter-turn Faults Buchholz relay
Over-fluxing fault Volts/Hertz relay


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