Voltage Multiple-Zero-Crossings at Buses Feeding Large Triac-Controlled Loads

Triac-controlled loads operating in three-phase four-wire electric networks produce large zero-sequence current harmonics that cause large currents in the neutral conductor. Under certain conditions, the phase-to-neutral voltage (PTNV) at the bus feeding a large triac-controlled load is distorted to such a degree that multiple zero crossings (MZC) appear in the voltage waveform. This situation may cause malfunction of the triac control systems or any other control systems that are based on the detection of voltage zero crossings. The influence of various parameters of the triac-controlled load and of the cable feeding it, on the appearance of voltage MZC, is investigated. A simulation model is built with alternative transients program version of Electromagnetic Transients Program and is validated by comparison with measurements taken at a real theatrical lighting installation controlled by triacs. The occurrence of MZC in the voltage waveform is also explained by an analytical approximation. It is shown that certain cable configurations should be avoided when feeding large triac-controlled loads, or otherwise, MZC will appear in the PTNV.