Computer pulls hot water plug

Graham Petrie, Vector network operations manager. Picture / Richard Robinson

At 6pm yesterday, three men in the operations rooms at Vector knew Aucklanders were arriving home because their computer screens told them heaters and bath taps were turning on.

At the same time, Vector's computer was turning the hot water heaters off.

The computer screens kept an eye on how much power was being used. As demand rocketed, the computer turned off hot water heaters in different areas around town to save power.

Graham Petrie, Vector's network operations manager, has stayed late to explain all the blinking lights and how they control the hot water cylinders.

Mr Petrie has worked at Vector for the past 30 years, and says using the hot water cylinder system is an age-old remedy used in many countries to control the load on the power system.

"We have been doing this for at least 60 years. It's just the technology is a bit different."

A computerised system keeps a watch on how much power each area is using and cuts off hot water heaters intermittently to keep the demand under the maximum limit.

In most areas, the signal is sent from the GXP (grid exit points), the point at which power is sent out to zone substations before travelling to streetside transformers and then to individual houses.

In Auckland there are six GXPs spread from Wiri in the south to Pakuranga in the east, Takanini in the west. From the North Shore to Rodney there are a further five. In areas of Auckland south of the Harbour Bridge, once demand reaches a certain point the system sends out pulses of a different frequency over the top of the usual frequency. Known as "ripple control", the frequency with a code attached travels through the wires until it hits a household meter box.

At this point a relay for the hot water cylinder recognises the signal and switches off the cylinder.

The meters in different areas controlled by each GXP recognise different codes so, if the demand is not too high, only the hot water heaters in small areas need to be cut off instead of entire suburbs.

When power demand eases , another signal is sent out to turn the cylinders back on.

Mr Petrie said they were turned on in stages because, for every kilowatt of power saved by switching off , it takes between three and five kilowatts to catch up.

Just to confuse matters, a different system is used in North Shore and Waitakere. Known as the pilot wire system, this involves a computerised "switch" at the zone substations.

This sends the word out via a separate wire from the mains supply, which runs down to a relay wire in every meter.