A quick introduction might be a good place to start.
First, this is a probably a good time to point out that everything I write in this blog is my own opinion and does not represent The Lines Company's official position in any way. Talk to The Lines Company (TLC) to get their version or opinion. It may be quite different from mine in many cases. I am an independent software developer/consultant who has been doing some data processing for them (since December 2009) but am not an employee. I'm also a TLC customer.
Of course I will also try to get my facts straight, but no doubt I will sometimes be wrong or make a mistake and in other cases what I write may be somewhat simplified if only to avoid writing a small book. If I find an error or someone points out a mistake then I will try to correct it and make a note to show that has happened when it's something significant.
So, here's some background info in case you aren't familiar with the situation.
"The Lines Company" (henceforth TLC) is the electricity distribution company in the area where I live (in the central North Island of NZ). They are currently the only lines company in New Zealand that bill their customers directly and also have a unique (and rather controversial method) of determining some of their charges.
Changes are likely in the future but for now the official methodology is still based on the idea that a significant portion of each customer's monthly bill will based on their maximum single average three hour load (or demand) in kilowatts (kW) during a period when TLC's network was being "load controlled" . These charges (proportional to that single peak value) are fixed for a year and come from consumption data of one kind or another collected over the previous winter period.
Currently only a small percentage of TLC's customers have a meter than can actually directly measure the load (or "demand") they impose on the system at any particular time. I am one of them. The vast majority still have old style meters that only track total usage, so for all of these some kind of estimate has to used.
Load control is also known as "ripple control". TLC have equipment that can send out signals across relevant parts of their network and if your home has a ripple control receiver (sometimes also referred to as a "relay") that is on the appropriate channel then this would typically turn off the power to your hot water cylinder (and perhaps other things also such as underfloor heating). Then at some later stage TLC send out another signal to turn the power to those appliances back on. Basically it's a remote control on/off switch to some appliances or parts of the electrical system in your home or business. Not all homes or business have a ripple control receiver installed. If not, they are "uncontrolled". TLC are trying to encourage their customers to put as much of their load as is practical onto a a controlled circuit as this means they can more effectively reduce load on their network at peak times. This is also called "load shedding". It can reduce the charges levied on them by Transpower (who run the national grid) and it can also help them defer upgrades.
There are typically a number of different channels used in each area for load controlling. They can be turned on and off independently so there's no guarantee that any two homes or businesses will always be controlled at the same time. However, as far as I know, when the network load is particularly heavy then it is quite likely that all or most channels will end up being controlled together.
Now, if you are a TLC customer (apart from a few larger customers that have negotiated contracts directly with TLC) then you either have a time of use (TOU) meter (sometimes also known as a "demand meter") or else you or have one or more older types of meter that only record total power used ("units" or kWh). There are also different models of TOU meter - not everybody has the same thing.
A TOU meter records how much power is used over each clock half hour (according to their own internal clock), say from 6pm to 6:30pm, and then from 6:30pm to 7:00pm, etc. The model I have at my house (EM1000) has enough memory to store a little over nine months of half hourly consumption data. These meters typically have a LCD display that shows some information but not (yet) anything like your highest three hour demand. To get that kind information, first the half hour data has to be downloaded from the meter (to a laptop or special hand-held reader device) and then processed later.
The older, non TOU meters are like a simple car odometer. They just track your total usage. They can be used to determine an average kW load or demand over some particular period but to do that you'd typically have to take a reading at the start of the period and then again three hours later, before finally subtracting the first value from the second, and dividing by three (or however many hours elapsed between your readings). That kind of approach is possible if you are keen enough to do it, but it isn't a practical method for TLC when they have (currently) something like 20,000 plus customers still using that kind of meter - plus of course their meter readers probably won't want to be lurking around outside your house for three hours on some cold winter's night just to get such a reading!
So, how does TLC work out a load or demand value for each customer?
If you have a TOU meter (hopefully you know this but it's also possible that you don't!) then in the months following each winter TLC sends out meter readers to download all the half hour data. Eventually that data finds its way to me (for now at least) as I process that data for TLC as an independent contractor. TLC also provide me with additional information required to do that, including the date and time their records show each ripple control channel was being "controlled" over the relevant period (the last winter) plus the load control channel relevant to each installation. Basically my job is to use the periods of load controlling for the channel corresponding to the relay in your house (but only if they are long enough, meaning they will cover at least six consecutive full half hours of data from your meter) and calculate the average load over the three hour period covered by those half hours. This is done for all the qualifying periods of load controlling falling in the 1 June to 30 September winter period. Finally the highest single three hour load is found from that list and that is the "load" (in kW) you should see on your invoice from TLC for the following year (starting in April in theory). One last thing - "uncontrolled installations" (without any ripple control receiver) are treated as if they are on channel 1 when it comes to determining their peak three hour load. So the processing I do looks at the half hour data from those installations at the times that "channel 1" is being controlled i the relevant area. There are some other complications but that's the basic story for TOU meters.
If you don't have a TOU meter then TLC are forced to estimate (and mostly it's a pretty "rough estimate") your maximum three hour load during a controlled period. In other words, it's meant to be their best shot at estimating the peak average three hour load your home or business would produce if it actually had a real TOU meter installed. There are many many potential problems with getting such an estimate because there is very little hard information to work from and many variables. But basically, TLC (or their statistician) determine a "formula" which attempts to produce such an estimate. Once determined, the input to the formula for each home or business is essentially their average daily power consumption over some (hopefully typical) months in or around the peak winter period. That input comes from the normal readings taken recorded by your electricity retailer (King Country Energy and others).
Currently "the formula" is usually updated (perhaps "recreated" is a better description) each year so there isn't really just one. TLC have a sample of homes (only 150 of them which is a pretty small number in my opinion) and currently all from the Waitomo/Otorohanga area that provides the data to do this. Measurements are made in these homes of their total uncontrolled power consumption over some months of the winter plus their corresponding half hour data during load controlling, and then a curve is fitted to that data. Basically the formula for that curve is "the formula" that is then applied to the other 20,000 odd customers who don't have TOU meters installed (yet). For each of these, the formula takes their average daily "uncontrolled" consumption (over some months) as input and outputs an estimated peak three hour load. Here's the graph from TLC's 2010 methodology paper that shows the data from this sample and the curve that has been fitted to it:
So, cutting to the chase, currently something like 95% of TLC's customers have their demand or load values estimated via one of the several variations of the formula. Oops. I forgot to mention there is also a different version for dairy farm milking sheds (and that they are "measured" over the September through December period instead of the standard June through September winter period). TLC does this because dairy farms apparently tend to generate their maximum load over this later period. Of course, this made me wonder about sheep farms (shearing?) and any number of other kinds of businesses with different peak periods but I can only presume TLC doesn't single these other groups out because they don't currently have the same kind of identifiable impact on TLC's network as a whole.
Finally I should also point out that TLC's whole network is actually generally viewed as five or six separate regions when it comes to the details of billing and load controlling and possibly other technical stuff that I know very little or nothing about. These mostly correspond to certain "Grid Exit Points" (GXPs) and are: Waitomo/Otorohanga (Hangatiki GXP), around Taumarunui (Ongarue GXP), National Park and Ohakune (each with their own GXP's), Turangi (Tokaanu GXP) and Whakamaru/Arohena (which has some more complicated set-up that I think we can safely ignore for now). A Grid Exit Point is a substation where Transpower provides a connection from the main grid to one or more lines companies in that area so they can then distribute the electricity across their own smaller networks to people like you and me.
Phew. That's a bit longer than I thought it would be when I started and definitely enough for now. I hope at least some of it makes sense!