Transmission lines with high and extra-high voltage that are untransposed, meaning the conductors are not positioned at the corners of an equilateral triangle and lack an overhead grounding wire (OGW), significantly contribute to the generation of negative-sequence voltages and, to a lesser extent, zero-sequence voltages. As the electrical load increases, the incidence of negative-sequence voltages also rises.
During the late 1970s and early 1980s, Eskom operated only two extensively long 400kV transmission lines that delivered electricity to the Western Cape. One of these lines was transposed, while the other remained untransposed.
At that time, the concept of Negative Phase Sequencing was not widely understood among engineers, nor were the factors that caused it.
Negative Phase Sequencing
Before the practice of live-line maintenance was established, it was necessary to periodically shut down each of the two transmission lines for upkeep.
Confusion ensued at Eskom Western Cape when the transposed line was deactivated for maintenance. Despite an uninterrupted power supply, a major customer’s equipment, which was equipped with highly sensitive control systems, experienced frequent shutdowns.
This prompted an investigation, leading to a consultation with a professor from UCT who provided insights into Negative Phase Sequencing. With this new understanding, I immediately directed the maintenance team to cease work on the transposed line and await further instructions.
We then coordinated with the customer to align our maintenance schedule with their plant’s downtime.
The subsequent introduction of a third Transmission Line to the Western Cape significantly reduced the recurrence of such issues.
To prevent similar incidents, we mandated that the untransposed line should not be the sole power source for the Western Cape.
Automatic Change-Over
Reflecting on past events, it later became clear that Negative Phase Sequencing was behind the numerous unexplained electrical disturbances I had been tasked with resolving.
A notable incident occurred years earlier when a colleague and I were summoned to determine why the standby generator at the undersea cable terminal was erratically activating and deactivating. Observing the voltmeter, we noticed a sudden spike in one phase voltage followed by a swift drop in another. At the time, the phenomenon was baffling, and we failed to link it to upstream occurrences.
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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