Extremely long—hundreds of kilometers—untransposed high and extra-high voltage transmission lines with a flat, horizontal conductor configuration significantly contribute to negative-sequence voltages and, to a lesser extent, zero-sequence voltages. The incidence of negative-sequence voltages rises with increasing electrical load. In the late 1970s and early 1980s, Eskom operated two 400kV transmission lines to the Western Cape: one transposed, the other untransposed. The concept of Negative Phase Sequencing was not well understood among engineers at that time, nor were its contributing factors.
Negative Phase Sequencing
Maintenance Challenges
Before the practice of live-line maintenance was established, it was necessary to periodically shut down each of the two transmission lines for maintenance. 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 “unexplained” shutdowns.
Investigating the Issue
This initiated an investigation, which included consulting a professor from the University of Cape Town (UCT), who provided valuable insights into Negative Phase Sequencing. Armed with this new understanding, I promptly instructed the maintenance team to halt their work on the transposed line and wait for further guidance. Once the line was restored to service, the frequent and previously “unexplained” shutdowns at the customer’s plant ceased entirely.
Coordinating with Customers
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.
Preventing Future Incidents
To prevent similar incidents, we mandated that the untransposed line should not be the sole power source for the Western Cape. This proactive measure ensured a more stable and reliable power supply for the region.
Automatic Change-Over
Reflecting on Past Events
Reflecting on past events, it became evident that Negative Phase Sequencing was responsible for the numerous unexplained electrical disturbances I had been tasked with resolving.
A Notable Incident
Years earlier, a colleague and I were summoned to investigate why the standby generator at the undersea cable terminal was erratically activating and deactivating. Observing the voltmeter, we noticed sudden spikes in one phase voltage followed by swift drops in another.
Initial Confusion
At the time, this phenomenon was baffling, and we failed to connect it to upstream occurrences. However, with the new understanding of Negative Phase Sequencing, it became clear that this was the underlying cause of the erratic behavior, shedding light on the previously unexplained incidents.
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In the video below, the three-phase waves are displayed with the phase-displacement 120-degrees, but Phase 2 (yellow) voltage is reduced by 6%, which may result in 3rd harmonics disturbance represented by the thinner solid line (5% 3rd harmonic) and dotted line representing a 2% 5th Hamonic.
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Navigating Power Imbalances