3. Defining and Measuring Integration
The definition of wind energy integration we have used for this study is: "the ability of wind farms to connect to, and operate within, the New Zealand electricity supply network in a manner which is compatible with the day-to-day operation and short term security of the electricity supply system as a whole".
This definition is focussed on connection to the grid either directly, or via local networks, and operation and security of the grid and the wholesale electricity market. In this context, the terms "operation" and "security" are focussed on the short term, typically from real time until the end of the next day. The definition does not include, for example, limits to the total potential for wind energy which arise due to its "economics" in the wider sense, in particular its cost relative to other forms of generation. Nor does it include the impact of wind generation on dry year security of supply.
However, to focus purely on the ability to connect wind generation to the grid without any consideration of the economics of wind generation or the impact on the electricity market could produce overly optimistic assessments of the limits to wind energy integration. Therefore, the brief for this study allowed for the exclusion of technically and operationally feasible scenarios where there was reasonable doubt that wind energy would be viable, or even acceptable, based on either its cost relative to other forms of generation or on its impact on the electricity market. Even so, this study is more about what could happen rather than what might happen in the context of a healthy electricity market.
There does not appear to be a commonly accepted measure for wind energy integration even though the concept seems straightforward - the amount of wind generation compared to the total system generation. GE Power Consulting14 offers two alternatives: installed wind capacity divided by total installed capacity, or installed wind capacity divided by total system peak load. The latter is preferred by GE as capacity margins (i.e. the difference between installed capacity and peak load) vary between countries depending on a range of factors having nothing to do with wind energy.
For this study we also define wind energy penetration as the ratio of installed wind capacity in MW to peak generation in MW, expressed as a percentage value. Penetration is expressed relative to the peak demand of the country as a whole.
By this definition, the existing wind farms shown in Table 1 represent penetration of 2.5% relative to peak generation expected in 2005.
Penetration, however, cannot give the total picture of how wind energy could contribute to electricity supply because wind farms run at significantly less than their total rated output. In energy terms we define the load factor of a generator as its average energy output in MW over the course of a year divided by its total rated output in MW. Since the wind does not blow constantly at full strength, load factors for wind farms around the world typically fall into a range from 15% to 50%, New Zealand being at the higher end of the range.
We therefore also define wind energy market share as the total annual wind generation in GWh divided by total annual generation in GWh, expressed as a percentage. Market share is expressed relative to the total annual generation of the country as a whole.
3.1 Normal Operations - Time Frames
Connection to the grid, either direct or via a local network, takes place before a wind farm can operate, and the wind farm remains connected throughout its lifetime. This study, however, considered both connection and operational issues. When we considered operational limits on wind energy integration it was useful to define three operational time zones, described below.
3.1.1 Real Time
This is the time zone from now, let's call it time T, out to T + 5 minutes. Within the period from T to T + 5 minutes we can be sure that the SO will rerun SPD15 to calculate a new dispatch schedule.
Real time is the domain of frequency regulation and, to a lesser extent, the marginal generator. In real time it is the frequency keeping station whose output varies in order to maintain frequency within the normal operating band of 49.8 Hz - 50.2 Hz. There is some contribution from other stations due to their governor response, but the regulating station's governor responds faster than most and its output can swing up and down within a band of about 100 MW in total.
Because of the presence of the HVDC link, there must be one regulating station in each island.
3.1.2 Dispatch Time
This is the time zone from T + 5 minutes to T + 2 hours. T + 5 minutes is the latest that another dispatch calculation will be performed. Generators must submit firm offers at time T for generation at time T + 2 hours and beyond.
Dispatch time is the domain of the marginal generator or generators, also known overseas as the load following generator(s). If there are no lines constrained, there is only one marginal generator in the country, but each new line constraint brings at least one more generator onto the margin.16 In addition to the marginal generator for energy there will also be marginal generators for Fast Instantaneous Reserve (FIR) and Sustained Instantaneous Reserve (SIR) in each island.
The marginal generators are dispatched up and down at least every 5 minutes in response to changes in demand and the requirement for reserves.
Because the HVDC link is currently configured to have limited capacity to share Instantaneous Reserves (IR), IR is dispatched separately and independently in the two islands.
3.1.3 Pre-Dispatch Time
This is the time zone from T + 2 hours through to the end of tomorrow.
Pre-dispatch time is the domain of the pre-dispatch schedule (PDS) which is published every two hours starting at 1 pm today through until the end of tomorrow. The PDS provides generators with forecast prices and dispatch quantities and is their first indication of how much they may be required to generate.
Large thermal units such as those at Huntly power station may take many hours to start up from cold. Although hydro electric generators typically have a much higher degree of flexibility than large thermal generators, nevertheless in the complex river chains which make up our large hydro electric systems it can take a long time for water leaving storage lakes to pass through all stations downstream. In some cases these delays are up to 24 hours. These generators need adequate warning to allow them to perform and optimise river flow management.
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