Traditionally, the emphasis in performance assessment of circuit-breakers is on their capability to interrupt fault currents. Although this is the prime task of a circuit-breaker, more routine based switching of lower current can present significant concerns in distribution and transmission networks.
One example of such a duty is inductive load switching, where very small to modest inductive current, usually from unloaded transformers, starting motors and shunt reactors, is interrupted (typically a few amps to several hundred amperes). The small magnitude of current can lead to interruption after a short arcing time. When the contact gap is not yet long enough to withstand the transient recover voltage (TRV), re-ignition will follow. TRV is of high-frequency (1 – 10 kHz, depending on rated voltage), since it is basically determined by the reactor’s inductance and its (small) stray capacitance only.
Re-ignitions, even multiple re-ignitions, are allowed by the standard provided they occur at the first current zero only. Prolonged re-ignitions at multiple current zeros are of major concern in shunt reactor switching, and testing is aimed to quantify the re-ignition behaviour. The parameters of the interruption process, more specifically the re-ignition pattern, can be used to predict overvoltage in practical situations.
Another feature of small current interruption is that current will be interrupted slightly before natural current zero. This is called "current chopping" and is due to the very strong SF6 gas blast needed for fault-current interruption. Since at the point of tripping the breaker is not aware of the magnitude of current to be interrupted, its action is always prepared for the maximum current. This causes (much) smaller current to be chopped at a level of a few amps (chopped current) before actually reaching power frequency zero.
In inductive load switching, the sometimes large magnetic energy stored in the inductive load, even at chopped current of few amperes, increases the magnitude of the subsequent TRV.
Concluding, the small interrupted current is rather a disadvantage than an advantage. In fault current interruption a much larger contact distance is needed for interruption, allowing the transient recovery voltage to always develop across a wider contact gap.
Recently, a high demand for shunt reactor switching tests has been observed in the market. In response to this trend, KEMA Laboratories have expanded their possibilities to perform shunt reactor switching tests to a rated voltage of 550 kV full-pole and 800 kV half pole. One challenge is the availability of sufficiently large test reactors, while still having low enough stray capacitance to match the standard’s requirements, and beyond. Although the lowest standardized high-voltage (HV) shunt reactor current is 100 A, in many cases tests are requested at lower current. This is because at a given rated voltage the load reactor is larger and traps more inductive energy in the switching process, causing higher switching transients. The higher end of the standardized HV shunt reactor current (315 A) is to demonstrate the switching behaviour with the highest frequency TRV. Also from a measurement point of view, EHV shunt reactor switching testing is a major challenge. The fastest transients must be measured, recorded and characterized, as these are necessary input to calculation models that estimate the overvoltages in service.
Shunt reactor current switching tests with HV and UHV breakers of 550 kV and 800 kV are performed now on a routine basis at KEMA Laboratories, thanks to effective investments in reactors and measurement systems.