Using Ehv/hv Load Tap Changers To Correct Regional Voltage Unbalance

Transformer banks that intertie EHV systems and HV systems are often comprised of three or more single-phase units which may consist of autotransformers or three winding transformers connected in wye-wye-delta. Further, these units are sometimes equipped with on-load load-tap-changers (LTCs) on the EHV winding which can be used to regulate the voltage of the HV bus. With single phase transformers, it is possible to intentionally operate the LTCs on different taps to lower voltage unbalance levels on the HV system where most customers are typically interconnected. This methodology can mitigate for unbalance due to phase magnitude differences but does not mitigate for unbalance due to phase angle displacements. Nevertheless, trial testing at multiple EHV / HV intertie banks has shown that this strategy can appreciably lower the negative sequence to positive sequence voltage unbalance ratio on the HV system. The effectiveness in unbalance reduction is greatest at the HV bus directly tied to the intertie bank for which the tap adjustment is made and becomes less effective moving further away from this bus. Trial testing has shown that this strategy is more effective when simultaneously employing it across multiple intertie banks within a region. Other considerations that should be made include increased circulating current in the intertie bank delta tertiary winding and increased current on the primary and secondary winding neutral connections; however, trial testing has shown that such increased currents are manageable. Increase of the voltage unbalance on the EHV bus should also be considered, but trial testing has shown that it is minimal compared to the voltage unbalance reduction on the HV bus. Another consideration is implementation of LTC controls by individual phase as traditionally LTCs have been controlled together instead of by individual phases.