On the 13th December 2011, Huntly power station lost its connection to the national grid. Huntly is a major contributer to New Zealand’s electricity supply, so naturally this was a significant event. At the time of writing, the connection still hasn’t been restored.
In an electricity grid, generation must match demand almost exactly at all times, otherwise the system will become unstable. Since Huntly is a major New Zealand generator, the result was widespread power outages ranging from the upper North Island, right down to Wellington. Not all power in these areas was lost, however. This load shedding, as it’s called, is absolutely necessary to avoid even bigger problems; if everyone stayed connected, a total blackout would be the result.
In New Zealand, the grid frequency is normally 50 Hz. It is quite normal for the frequency to oscillate around this value, as generators are constantly balancing their output to match demand. The frequency must be within 0.75 Hz of 50 Hz, but is normally a lot closer – within 0.2 Hz or so. But today at 12:33:38, the loss of Huntly’s input means that there is a serious shortage of energy going into the grid, and consumers start sucking the kinetic energy from the remaining generators in the country instead. This causes the grid frequency to slow down rapidly.
Electricity distributors (the “lines company”) are responsible for disconnecting consumers to avoid the frequency going too low. The lowest frequency observed here was 47.691 Hz, at about 5 seconds after the failure. The frequency was back within the normal range about 7 seconds later. This is remarkable considering the sheer size and complexity of the system we’re dealing with here. What does this mean for consumers? Not a lot really. Devices with motors may have slowed down slightly during this period, but it would have mostly been unnoticed.
New Zealand’s normal voltage is 230 V, plus or minus 6%. The loss of Huntly caused a drop in voltage (as observed from the University of Waikato) to around 216 – right on the limit of tolerable range. The voltage recovered from the worst of it within 30 seconds, and was back to normal in slightly less than 2.5 minutes. What does this mean for consumers? Probably nothing much either, but this variation in voltage may have upset sensitive equipment.
Grid frequency is the same throughout the system, which in this case means the North Island, whereas voltage is a more local phenomena, hence why voltage and frequency excursions recovered at different rates. “Recover” doesn’t mean that the problem has been fixed; it simply means that supply and demand are back in balance. Many consumers were still without power. For a more detailed discussion about frequency and voltage, as well as real-time measurements, see the NZ Grid Monitor page.