High- and extra-high voltage untransposed Transmission Lines, where the conductors are not arranged in the vertices of equilateral triangle and without overhead grounding wire (OGW), is a major contributor to negative sequence voltage, and in lesser degree, zero sequence voltages. The occurrence of negative sequence voltages increase as the load increases.
In the late seventies and early eighties, Eskom had only two very long 400kV Transmission Lines supplying power to the Western Cape. The one line was transposed while the other one was not. Those days, very few engineers were aware of Negative Phase Sequencing let alone what causes that.
Negative Phase Sequencing
In the days before live-line maintenance, these two lines had to be switched off in turn for maintenance. It was when the transposed transmission line was switched off that utter confusion kicked at Eskom Western Cape. A major consumer’s machinery, with very sensitive control systems, kept tripping, even though the supply remained on. I started making enquiries and eventually consulted with a professor at UCT who briefed me about Negative Phase Sequencing and then it all started making sense. I went back to my office and instructed the maintenance crews to stop working on that Transmission Line immediately and wait for further instructions. We then liaised with the customer to synchronize our maintenance on that Transmission Line with their plant shut-down periods. When the third Transmission Line finally reach the Western Cape, we hardly ever had the same situation again. As a precaution a clear instruction was issued that the untransposed Transmission Line should never be the only one to feed the Western Cape.
Automatic Change-Over
As I said above, it all started making sense when I realized that the cause of many of the unexplained disturbances I was asked to investigate before then was because of Negative Phase Sequencing. Such an unexplained disturbance happened a few years before that when a friend and I was called to investigate why the standby generator at the terminal building of the undersea cable kept coming on and going off seconds later. While we were watching the voltmeter, we saw an instantaneous increase in one of the phase voltages and an instantaneous decrease in another several seconds later. At that time, we could not explain it and neither did we make any connection to what was happening upstream.
Consequences
Click here to read more about a recent incident and an explanation of the consequences of these type of network faults.
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