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This week’s question and answer:

Q: When energizing a distribution transformer 2.6MVA 33KV/690V in a windfarm at no load with 2KM of cabling from main sub is it normal to experience overvoltage transient spikes that could exceed 2.5KV (Surge arrestor rating) for a short period?

A: The question does not contain enough details to address the specifics of the overvoltage or circuit configuration. In general, there are two scenarios of energizing a distribution transformer: 1. from the 33 kV side, i.e. from the collector grid side and 2. from the 690V side, i.e. from the turbine side. Referring to scenario no 1. the transformer could be energized either by the 33 kV switch/circuit breaker adjacent to it (switchgear inside the tower) or remotely, from the main collector substation 2 km away. In either case closing of a circuit breaker or a switch with an inductance of the no-load transformer and capacitance of the 2 km MV cable might possibly create an overvoltage condition. This overvoltage condition can originate from a few distinct effects: a. resonant effect of large inductance and capacitance at a frequency very close to the power frequency and b. due to transients resulting from the traveling waves when a high frequency surge is generated at the switch end. The former is a likely condition. For example, a distribution transformer with a magnetizing inductance of a few Henry and 2 km of MV cable with capacitance of ~0.3-0. 4 mFarads per km can easily resonate at approx. 50 - 60 Hz frequency. In such a case a long lasting and dangerous overvoltage could arise. The solution to the problem could be, for example, detuning the resonating circuit (not always easily done), or eliminating switching no-load transformers (i.e. change the operational procedures).

Dr. Mietek Glinkowski, P.E.
Global Head of Technology, Data Centers
Director of Technology, Power Products

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