What is transient recovery voltage and how does it affect circuit breakers

Introduction to transients

It’s important to understand that transient is associated with every change of state of a circuit. Transient recovery voltage thus is transient across a circuit breaker. Transient recovery voltages are contingent upon the circuit conditions and parameters. They are essential to study for the application of circuit breakers.

Generally, circuit breakers in a system are applied based on available short circuit capability at that point in the circuit. But, when the circuit is interrupted, it results in a TRV, which has harmful effects on the circuit breaker.

TRV manifests in different ways depending on circuit configuration. Hence, this report aims to study the various parameters causing and affecting the TRV.

Transient Recovery Voltages on Systems

What is Transient Recovery Voltage?

It is ascertained that TRV is a resultant of the change in the state of the circuit. Thus TRV can be defined as voltage appearing across a circuit breaker after a switching action. Typical of every transient TRV also has high amplitude and high frequency. The figure below shows an example of TRV, figure is taken from the TRV explanation given by Dufornet et. al.

Figure 1 – Transient Recovery Voltage

TRV is a point by point difference of voltage at the incoming side and at the outgoing side of a circuit breaker. When a circuit breaker interrupts, the incoming side or the side to bus or supply is connected tries to return to power frequency voltage level and the outgoing side depending on what is connected also oscillates. The difference between these voltages is recovery voltage.

TRV is associated with every interruption, but the ones resulting because of interruption of fault current are the most ominous TRV. Thus the choice of circuit breakers or introduction of means and methods for safeguarding the circuit and circuit breaker has to be considered.

2.2. Factors affecting TRV

Transient recovery voltage is affected by various parameters of the system. Prominent among them are listed below:

1. Inductance and capacitance in the system
2. Fault current level of the system at the point of the study of TRV.
3. Bushing capacitance of circuit breakers, voltage transformers etc
4. A number of transmission lines terminating at a bus and their characteristics impedance.
5. Internal factors of the circuit breaker like the first pole to clear a fault etc.
6. System grounding.

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2.3. Types of TRV wave shapes

Transient recovery voltages manifesting across any circuit breaker in a system are seen to have some typical shapes. Waveshapes are classified into types:

1. 1-Cosine: Observed in case of transformer fed or reactor fed faults.
2. Exponential cosine: Observed when transformer fed faults on breaker terminals with transmission
   lines connected on the incoming side of the circuit breaker.
3. Triangular or Saw-tooth waveshape: Observed on line side when short transmission lines are connected.
4. Initial TRV: Observed when buswork of the substation is involved.

Figure 2 – 1-Cosine TRV, Exponential TRV, and Triangular TRV

Calculation of different types of TRV wave shapes

System Reduction

TRV can be analyzed by drawing a detailed system diagram and then reducing it to obtain an equivalent circuit across the breaker terminals. It is generally observed that the system can be reduced to a series or parallel R-L-C circuits. These equivalent RLC circuits can then be solved for TRV.

Let’s take the example of circuit reduction. The system is first drawn out as a single line diagram and afterwards, a single-phase RLC parallel equivalent is obtained at the breaker terminals. To obtain the resultant TRV it is assumed to inject short circuit current in the breaker terminals and a solution for voltage across the elements is obtained which is TRV.