Zero-sequence current measurement uncertainty in three-phase power systems during normal operation

Zero-sequence currents in high-voltage power systems during normal operation can have a significant influence on nearby infrastructure. It is therefore necessary to gain insight into typical levels of zero-sequence currents in a variety of operational situations. Measurements can provide this insight. When performing measurements, one needs to know the uncertainty associated with the measurement to draw accurate conclusions. The measurement uncertainty associated with three different practical strategies to measure zero-sequence currents during normal operation are studied: a direct and indirect measurement strategy using current clamps and oscilloscopes and a measurement strategy using phasor measurement units (PMU). Measurement uncertainty is studied using two methods: by analytical expressions and by Monte Carlo simulations. Both methods give consistent outcomes for the measurement uncertainty and show the same tendencies of the zero-sequence current measurement uncertainty as a function of the positive-sequence current. This paper studies measurement strategies with error sources based on realistic measurement devices that could be used in practice. The following outcomes were found: in the direct strategy, the primary current transformers are the largest uncertainty sources; in the indirect strategy, the current clamps are the largest uncertainty sources; in the PMU strategy, the PMU total vector error is the largest uncertainty source