Why we need Protective Relays for Transformer Protection?

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 depend 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, over-fluxing protection and protection against inter-turn faults while a distribution transformer can be protected by MCCBs and MV fuses alone.

Power transformers need extra protection because of their high rating and sensitivity of their location in a power system. Power transformers are used 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 distributed.

This means that an undetected fault, or inadequate protection of a transformer can result in huge losses of power 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 of time if proper protection has not been implemented. This ultimately reduces the reliability of our power system as the whole feeder is affected due to a fault in an upstream transformer.

Transformer Faults and Their Protection

Overcurrent Protection using Phase and Ground 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 accompanied by excessively large currents resulting in over-heating, fire, and damage to equipment. Overcurrent protection of a transformer is therefore accomplished using phase and ground relays.

Phase relays have a pickup current which 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, 3^rd 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 a 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 up on the circulating current principle and relies on the vector difference between the current entering and leaving the transformer terminals, while the average current flows 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 coil and it operates, sending a signal to the circuit breaker to trip. We can say that a differential protection is usually employed to trace out internal faults while the phase relays for overcurrent protection are used to distinguish external faults.

CT Secondary Connections:

It should be noted that the CT secondary connections on each side of a 3 phase transformer are equally important for the relay.

To explain this importance, let’s take a 3-phase delta-wye transformer.

The Problem of Spill Path Current:

Now, if both the CTs on primary and secondary side of the transformer are connected in wye configuration then there will be spill path current in the relay even under normal conditions. This is due to the phase shift of 30 degrees on the delta side of the transformer, hence our vector current difference is not zero, thus the relay will mal-operate.

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.

Interposing CT:

However, there is one more complication which must also be catered to for. The delta configured CT secondaries will result in the line current of that CT to be √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 current mismatch.

Therefore, to eliminate this √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 drawn by the transformer at the moment it is energized. 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 magnetizing current is drawn upon switching the transformer on.

This current is of a very high magnitude of about 8 to 30 times the rated current having 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 differential current to the transformer differential relay coil and might issue a false trip signal therefore a percentage different relay with Harmonic restraint feature is used.

This relay works on the basis that transformer inrush current comprises mainly of 2^nd 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% 2^nd harmonic currents hence there won’t be any tripping under 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.

The Buchholz Relay for Inter-turn Fault Protection:

Inter-turn faults occur between the transformer winding turns which become shorted with each other and can result in large currents to flow through them. However, this current has a small magnitude when seen from the transformer terminals, so the above discussed protection schemes find it difficult to detect these faults.

These faults create areas of extreme hotspots in the transformer and gradually lead to degradation of the equipment and its insulation.

 It should be noted that Inter-turn faults are also known as incipient faults which means that such faults are not much dangerous in the beginning but gradually progress into extreme faults overtime. Therefore, a buchholz relay is employed to protect from these faults.

Working of a Buchholz Relay:

Incipient faults in a transformer result in excess heat generated, this heat causes the oil to greatly heat up. At such high temperatures, the oil starts to decompose and liberates gases at temperatures of up to 350^o C and oil pressure is built up.

This sudden buildup of oil pressure results in the oil to rush into the conservator. A vane or a lever of sort is placed between the oil tank and the conservator and it operates a set of trip contacts. The vane operates once the oil surge pushes it into closing the contacts and sends a trip signal to the circuit breaker.

A buchholz relay also contains a second set of contacts which are operated by a float that floats over the oil. In the event of oil leakage or decomposition of oil, the oil level decreases, and the float also falls closing the contacts. These contacts do not issue a trip signal, however, they activate an alarm or a warning signal as loss of oil does not require immediate tripping.

The nature of the fault and the degree of damage can be predicted by the analysis of trapped gases, formally known as Dissolved Gas Analysis (DGA). This is because the different gases released depend upon the insulation area that was heated.

Over-Fluxing Protection using a Volt per Hertz Relay:

The magnetic flux in a transformer is given by:

Where,

V = RMS Voltage

f = Frequency

N = Number of turns in the winding

So, we can see that an over voltage at a fixed frequency or an under frequency at a fixed voltage can result in the transformer to over flux. A power transformer is already operating at its knee point of saturation curve and any further increase in its magnetic flux causes it to reach further saturation.

This saturation leads to the transformer core drawing more magnetization current, this is known as over excitation. The more magnetization current the transformer draws, the more core losses occur which eventually leads to overheating of the transformer.

This can be further explained by the effect of oversaturation on the shape of the sine wave. Oversaturation gives rise to 3^rd and 5^th harmonics which distorts the sinusoidal current waveform as shown below:

This distortion ultimately draws more current resulting in increased power losses and overheating of the transformer.

Eddy current losses also increase because the additional flux due to saturation flows into other parts of the transformer and other conducting equipment nearby.

Therefore, to maintain the transformer flux within permissible operating limits the V/f ratio must not be more than the allowed value. For example, a transformer rated at 1.5 per unit voltage (150%) at a rated frequency will over flux once the Volts/Hertz ratio exceeds 1.5(150%). This will also be true if the frequency falls by almost 68% at rated voltage.

To detect these changes, a microprocessor-based relay known as the Volts/Hertz relay is used which measure the V/f ratio of the transformer and sends a warning signal if the transformer is over fluxing. This problem does not require immediate tripping as the voltage and frequency are then adjusted accordingly by shedding loads or correcting any problems in the supply.

Protection of Transformers using Time Current Curves (TCCs):

An electric power system has transformers along with their protective devices, 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.

Summary of Transformer Faults and their Protection:

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

Engineers at AllumiaX performs Power System Studies, to provide best engineering methods and solutions for transformers and other sensitive components present in power systems. Contact us for the best-in-the-market power system consulting and field services.

Do let us know of your feedback and feel free to ask any questions.

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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.