Transient Recovery Voltage (TRV) is perhaps an inevitable and un-welcomed problem of the Power Gird. In medium and high voltage networks, the circuit breaker interrupts the current flow to the system during either normal switching operation or fault interruption. This leads to a rapid increase in the voltage across the breaker poles, as it become almost twice as high as operating.
Despite having an interrupting chamber with sufficient dielectric strength inside the breaker, the problem needs a timely solution since the recovery voltage rise results are overwhelming of the insulating medium.
Transient Recovery Voltage can lead to breaker failure; therefore, understanding the concept and its effect is important. We will be discussing the following.
2: Transient Recovery Voltage on Systems
3: Factors affecting TRV
4: What are the effects of TRV on a Circuit Breaker?
Transient voltage occurs whenever there is a sudden change in the state of the current. It is safe to say that the transient voltage across the circuit breaker poles is Transient Recovery voltage. Since TRV has can damage the circuit breaker to a great extent, therefore, it is important to learn about its effects.
Transient Recovery Voltage on Systems
TRV is the transient voltage that appears across a circuit breaker poles after a switching action.
The figure below shows how a sudden change of current can lead to a transient restriking voltage that oscillates to values higher than the nominal operating voltage of the system until it is finally damped (recovered) to the operating voltage.
Factors Affecting Transient Recovery Voltage
There are several factors of the system that have a direct or indirect impact of the TRV, however, below we have listed the most prominent ones:
- System Grounding
- Internal factors like the first pole to clear the fault.
- The number of transmission lines that tend to be terminated at a bus, along with their ability to resist the altering current.
- Bushing capacitance of voltage transformers, circuit breaks, etc.
- The level of the fault current at the time of the TRV Study.
- Capacitance and the Inductance in the system
What are the effects of TRV on a Circuit Breaker?
Despite the fact that Transient Recovery voltage occurs in a system for a very short time, it does increase rapidly. The fast increase leads to a number of adverse effects on system components. Below are some of these effects.
The transient recovery voltage comes into play due to the system’s inability to provide a constant flow of current. Things become more complicated due to the interruption of short circuits. Therefore, some precautionary measures should be in place to avoid a bigger problem.
Unchecked TRV will lead to insulation breakdown because it will stress out the circuit breaker insulation. Common examples of circuit breaker’ insulation breakdown are flashover between one phase and another or external flashover between the phase and the ground.
The arching in the interrupting medium occurs during circuit breaker switching. At this time, the focus of the arching medium is to regain the insulation property. When the interrupting medium is able to withstand the rapidly increasing voltage, it means the interruption was successful.
To ensure the success of the interruption, the speed of the medium to go from conducting state to the insulating state should be higher than the rate of rising in the Transient Recovery Voltage. However, in case the rate of rise in TRV is more than the speed at which the medium returns to the insulating state, the current will keep on flowing into the circuit.
The establishment of the current is referred to as the re-ignition. The main reason for re-ignition is the arc plasma that has the conducting ions that reestablish the current. Re-ignition always occurs right after the current zero. A high TRV will force the medium to return to the insulating state, thus making it more stressed.
When there is a dielectric breakdown that occurs because of the high field created across contacts, it allows the medium to not only become conductive but also to carry the current. This results in the reestablishment of the current.
Restrike is the process of establishing the current. It mostly occurs almost about half a cycle after there has been a current interruption.
A TRV leads to both re-ignition as well as restrike. When TRV challenges the longitudinal voltage between the poles of the breakers and it reaches the maximum voltage that the gaps can withstand, there will be a longitudinal breakdown or re-ignition. Restrike on the other hand is when a break down takes place about half a quarter cycle after the current interruption.
Since both concepts can lead to failure, therefore it is imperative to avoid them by design. Ensure that you get a thorough TRV analysis done with functionalities and built-in standards to get a seamless comparison of a system’s prospect of having a TRV along with breaker innate TRV envelope and obtained simulation.
The team at Mechatronica Engineering Research & Development Ltd. Has vast experience in performing TRV studies for power systems during normal switching and when the system is subject to fault in accordance with IEEE C37.011. Our studies are not only conducted using well known transient analysis software packages such as EMTP and ATP, but we also have our own tools that were developed in-house by our experienced engineers to perform these studies for cases not covered by these softwares.