A high voltage circuit breaker plays a crucial role in power systems by protecting electrical equipment from damage due to electrical faults in the system. One of the key challenges in circuit breaker design is dealing with the transient recovery voltage (TRV), a phenomenon that occurs immediately after current interruption. When a fault current gets interrupted in between the opening contacts of a circuit breaker, a high-frequency transient voltage appears across the contacts. For a certain fault current, the frequency, amplitude and shape of the waveform of this transient voltage mainly depends on inductance and capacitance of the system. The system inductance resists current changes and capacitance resists voltage changes. Because these properties of inductance and capacitance the high peak, high frequency transient voltage appears across the opening contacts of a circuit breaker. After current interruption, the inductance drives a voltage surge, while the capacitance causes oscillations. The result is a rapidly rising voltage that challenges the dielectric strength of the CB.
What is Transient Recovery Voltage (TRV)?
As already told, a Transient Recovery Voltage (TRV) is the voltage that appears across a circuit breaker immediately after it interrupts a fault current. This voltage is transient in nature, meaning it is rapidly damped out and gets stabilized at the system’s steady-state voltage. TRV is influenced by several factors, including:
- System voltage and configuration – If the system voltage is higher, naturally transient recovery voltage will also be higher.
- Fault current magnitude – If the fault current is higher, the rate of current drops to zero during interruption is higher hence TRV is higher.
- Circuit inductance and capacitance – As we have already told, the shape and frequency of TRV mainly depends on system inductance and capacitance.
- Load characteristics Load characteristics – If the connected load to the system is inductive in nature, it will further increase and influence the transient recovery voltage.
When a circuit breaker interrupts a fault current, the system can often be approximated as a parallel L – C network. Because of the inherent inductances (L) of the power network, the current cannot vanish instantly. Instead, it is diverted through the parallel capacitive path (C) associated with the circuit breaker. However, there is no physical capacitor connected across the circuit breaker; rather, this is due to busbars, cables, and stray capacitances inherent in the system.
The TRV appearing across the breaker contacts has two parameters, one is voltage and other is frequency.
The voltage can be derive as
$$ v(t) = L\frac{di(t)}{dt} $$
The frequency of transient oscillation can be represented as
$$ f_o = \frac{1}{\sqrt{LC}} $$
Rated Transient Recovery Voltage
The Rated Transient Recovery Voltage is the highest TRV that a circuit breaker is designed to withstand under specified test conditions. It is a standard parameter defined by IEC 62271-100 and IEEE C37.04 standards to ensure uniformity in circuit breaker performance.
- Peak Value: The maximum amplitude of the TRV.
- Rate of Rise of Recovery Voltage (RRRV): The slope of the voltage increase immediately after current interruption.
- Time Delay: The time taken for the voltage to reach its peak.
- Oscillatory Nature: The shape of the TRV waveform, which can be characterized as a single-frequency oscillation or a multi-frequency waveform.