Why Snubber Circuits are Required for Transformer Protection?

Switching Transients or Transient Recovery Voltages may occur in certain situations and events occurring frequently in normal operations of a MV or HV industrial or commercial Power Systems. They may not be fatal in their first occurrence but prolonged exposure to transients will reduce lifespan of power system equipment. They may also disturb the stability of the power system putting the entire installation at a risk of shutdown or failure of certain equipment.

A snubber circuit is a proven way of reducing these transients and ensuring the protection of individual equipment and the wider system. Therefore, snubbers find their applications in many areas in a power system, in most cases becoming an indispensable component of it.

What are Snubber Circuits?

The size and ratings of the components depends upon the type of application and power requirement. Each individual component in a snubber is carefully selected and its rating is determined through a complex design process.

Snubbers are applicable to a wide range of power applications ranging from a few hundred watts in power electronic devices to several MVAs in large power transformers. Sizes of the components employed will also change accordingly. A small snubber capacitor may serve well in a power electronics application whereas a large capacitor may be needed for a power transformer.

Broadly speaking, when you connect a snubber to a device of choice, it can absorb voltage spikes and normalize any abnormality or disturbance in the voltage waveform. They are most effective in mitigating fast-rising transient voltages and in attenuating reflections and resonances before they have a chance to build up. They are also employed to reduce the frequency of an electrical disturbance.

Such a disturbance, abnormality or voltage spikes more specifically known as transient recovery voltages (TRV) and power frequency over-voltages (PFOV) may arise in a power system as a result of:

• A lightning strike.
• A circuit breaker interrupting a transmission line.
• Switching in or switching out of a lightly loaded transformers.
• Switching of a heavily inductively loaded transformer for example in accelerating or stalled motors, arc furnace transformers and reactors.
• Flashovers and restrikes after closing of circuit breakers.
• Faults in the system.
• Loss of generation and sudden changes in load.
• Ferro-resonances.
• Ferranti effect.

Working Principle of Snubber Circuits

Snubbers usually consist of a capacitor connected in series with a resistor and both are to be connected in parallel with the desired connection point.

Under normal operating conditions, impedance of the capacitor is very high, effectively "isolating the resistor R from the system at normal power frequencies, and minimizing heat dissipation during normal operation. Under high frequency transient conditions, the capacitor offers very low impedance, thus effectively "inserting" the resistor R in the power system as cable terminating resistor, thus minimizing reflection of the steep wave-fronts of the voltage transients and effectively dampening them.

Application of Snubber Circuits

One may segregate the applications of snubbers into two broad categories:

• Applications in Power electronics.
• Applications for the protection in power system.

Our discussion in this article will focus on the applications in power systems with particular focus on transformers and their protection.

Snubbers for the Protection of Power Transformers

Transformers being an integral part of the power system are susceptible to such transient voltages and disturbances. In a case where these transient recovery voltages and disturbances come close to the basic insulation level (BIL) of the transformer, if for a short time and within the permissible level, they will not be fatal at their first occurrence but repeated occurrences will damage the insulation and windings overtime and eventually cause transformer failure leading to damage and loss of costly equipment.

In some situations, voltage transients may be significantly higher than the BIL of the transformer where their occurrence will immediately disturb the stability of the system causing more circuit breakers to trip along the system and escalating the fault.

In addition to transient voltages, transformers also exhibit voltage amplification due to high-frequency voltage transients from switching, there is a possibility that these frequencies may excite a resonance in the transformer windings if the frequency of the transients and the natural frequency of the transformer match. This resonance may cause a higher turn–turn voltage to be developed internally in the transformer winding. This may be true even in the situation where the transient voltage itself is within the capabilities of the transformer insulation system to withstand. (1)

A transformer is most susceptible or most likely to face these disturbances when:

• It is lightly loaded.
• It is driving a highly inductive load such as arc furnaces, stalled or accelerating motors and motor starting reactors.
• It is driving shunt reactors.
• There is a short distance between transformer and circuit breaker (less than 200 ft.)
• The transformer is a dry type or cast coil transformer with a low BIL.
• Transformer is loaded with a rapidly switching load such as VFDs and data centers.
• There are vacuum and SF-6 circuit breakers in the system.

Adequate protection of valuable assets is highly advised in these scenarios, you may be able to achieve this with a high-rated circuit breaker or a high-BIL rated power transformers. However, Snubbers circuits are a proven, inexpensive, and effective way to provide this protection as well. When connected to the terminals of a transformer they will serve to:

• Absorb the TRVs and keep their magnitude within the permissible level.
• Keep the frequency of the disturbances below the first resonant frequency level of the transformer thereby preventing voltage amplification from damaging the transformer's internal winding.

Snubbers for Protection of other Power System Equipment

With Circuit breakers, transmission lines, generators, large motors and reactors, they are used in parallel with surge arrestors to keep voltage transients within a safe level.

Snubber and Transient Recovery Voltage Studies

Power systems are dynamic and every power system set-up varies from the other in the sense that loadings, equipment ratings, manufacturer type, configuration, insulation levels, voltage levels and interconnection of different equipment may be different from the other.

While benchmarks and thumb rules exist for snubber designs, complete analysis and transient recovery voltage studies are needed to correctly predict the transient voltage levels and its frequency that may be unique to the system under study due to the inherent fact that no two power systems are exactly same.

We can thus, employ the result of TRV studies to size and rate the snubber equipment in a most cost effective manner that enhances protection level at the same time.

In complex scenarios where multiple snubbers are present and non-linear loads exist, a careful study is required to ensure that the increase in capacitance does not change the impedance of the harmonic orders and not shift the resonant frequency to harmonic orders (2).

Thus, In conclusion a carefully designed snubber circuit preferably through a Transient Stability Study can offer a cost effective solution for transformer protection from switching transients.