How can I model a vacuum circuit breaker?
In EMT simulations, it is necessary to analyse the performance of a vacuum circuit breaker (VCB) with respect to various transients that will arise in any power system. For example, two key transients of interest are transient recovery voltage (TRV) analysis and ferroresonance analysis. The attached PowerFactory model (available in version 2016 SP2) presents a detailed circuit breaker model for this purpose.
DIgSILENT developed an EMT model of a VCB based on [1] (hereafter referred to as Greenwood’s model). Greenwood’s model simulates the voltage recovering characteristic of a VCB after contact separation and is considered adequate for the investigation of arc reignitions and voltage escalation problems during current interruption of switching transients.
The behaviour of the VCB is characterized by the rate of rise of its dielectric strength (k-factor) and the arcing time, which is the time between the contact separation and the effective current interruption (i.e. current zero crossing). The user can view this characteristic in the PowerFactory model by viewing the signals called s:Vbpos and s:Vbneg in the Mayr Arc Models (DSL).
If the transient recovery voltage after arc suppression exceeds the dielectric strength of the gap between the separating contacts (i.e. the amplitude is too high or the TRV rises too quickly), the arc will reignite (i.e. switch closes for that particular pole). Depending on the characteristics of the circuit (natural resonance frequencies), high frequency currents may occur, which superimposed to the power frequency current, may create virtual current zero-crossings, either on the same pole or in the other poles. The arc will be suppressed again at these virtual current zero-crossings. The process can repeat resulting in multiple arc re-ignitions leading to a voltage escalation.
For circuit breakers, the k-factor and TRVp are typically given by the manufacturer in datasheets. For contactors, this information is typically not available in manufacturer datasheets but the manufacturer instead guarantees a maximum value of the chopping current during inductive switching operation (e.g. motor switching). The arcing time was deliberately assumed to be very short, which implies an underestimation of the voltage withstand capability immediately after current interruption. This is a conservative assumption as it represents a higher risk of abnormal current interruption (i.e. arc re-ignition) and voltage escalation.
[1] Greenwood and M. Glinkowski, “Voltage Escalation in Vacuum Switching Operations,” IEEE Transactions on Power Delivery, vol. 3, no. October, pp. 1698-1706, 1988.