What’s causing creeping time error?


Transpower is New Zealand’s electricity system operator. One of our functions is to dispatch generation to meet load. Any imbalance between load and generation causes a variation in the system frequency away from its nominal 50 Hz. Frequency deviations from 50 Hz are integrated into a measure called ‘time error’. Time error is in effect the amount in seconds that an electric clock connected to the system would drift.

The system operator is mandated to manage frequency and time error within set limitations.

When Transpower commissioned Pole 3, the new inter-island HVDC link, in 2014 it upgraded the control systems and introduced a Frequency Keeping Control (FKC) mode of operation, which linked the two islands’ frequency. However, turning on this controller had some unexpected impact on the pattern of time error drift, presenting new challenges for the system co-ordinators to manage it.

With FKC in operation the time error has a seemingly randomly occurring positive bias. The time error will frequently ramp up at a significant rate, as illustrated below (NI = North Island, SI = South Island):


This rapid ramp in time error prompts a reset by the system co-ordinators to take corrective action with extra dispatches, which are both time consuming and costly. The time error ramp up happens seemingly randomly, exhibited some days more than others, and the times in which it occurs do not have an obvious association to a particular time of day, load pattern or generation mix. We seek a better understanding of the dynamics of what causes this and how it can be mitigated.

The FKC control system balances the frequency of the two islands, and does not have any control related to time error. The issue observed with time error may be an interaction between the FKC control system and other phenomena on the power system. It may be the interaction with certain loads, the pattern of load changes throughout the day, certain generation, something inherent in the FKC control system, a combination of all or something different entirely.

Transpower is submitting this challenge to the Mathematics in Industry New Zealand event to answer this question:
What are the causes of time error creeping positive while operating in Frequency Keeping Control mode and how might the time error creep be mitigated?

To assist in answering this question we will provide relevant data (lots of it!), control system schemas, and an expert in the subject.