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Appendix A - Glossary
- AC
- Alternating current.
- Ancillary service
- A range of services acquired by Transpower to help ensure grid security:
- Instantaneous reserves (IR)
- Regulation (also known as frequency keeping)
- Voltage support
- Black start
- Over-frequency arming
- Asynchronous generator
- An AC generator in which the average rotational speed of the rotor in normal operation is not proportional to the system frequency. Note that the generator has stationary and rotating electrical windings, the former being part of the stator and the latter part of the rotor.
- Black start
- This is an ancillary service which enables the grid to recover from a total shutdown. Power station auxiliary equipment must be supplied with power before the main generators can be started, power stations that provide black start capability are fitted with stand-by generators which can self-start and so energise the grid.
- Block dispatch
- The EGRs contain rules that allow large hydro systems to offer by station but dispatch themselves as a single block, on the proviso that the total output of the system equals the total dispatch of all the stations.
- Carbon credits
- A number of these have been issued to emission-reducing projects in New Zealand. They are internationally tradeable and assist marginal projects become viable. Meridian Energy, for example, was awarded carbon credits for its Te Apiti wind farm and subsequently sold them to the Netherlands government for $10.50 per credit.
- Constrained on
- Dispatch is undertaken in dispatch time using forecast demand, but spot prices used for settlement, known as final spot prices, are calculated using SPD the first business day after real time using actual demand. Because of the differences between forecast and actual demand (and for other, more complex reasons) the final spot prices do not always match the spot prices that would be calculated at dispatch time using SPD. Hence generators may be dispatched at a price lower than their offer price, in which case they receive a constrained on payment to compensate for the difference between their offer and the spot price they received. Constrained on payments are aggregated up and paid by spot purchasers.
Generators may also be constrained off, in which case they did not generate as much as they would have expected given that their offer price was lower than the spot price they received. They are not currently compensated for being constrained off. - Contingency (or contingent event)
- A sudden and unexpected event which disrupts normal operation of the grid. Examples include the loss of a generator, the loss of one or two poles on the HVDC link, the loss of an AC line, the loss of a large amount of load, or the loss of other equipment on the grid.
- Converter
- A device which converts AC power to DC power or vice versa. In the context of WTGs, the converter actually does an AC-DC-AC conversion. A converter is typically constructed using power electronics and is also known as a variable speed drive or VSD.
- Contingent event
- In relation to IR, an event where resources are able to be economically provided without disconnecting demand, typically the loss of an AC transmission circuit or the loss of a single energy in-feed to an AC system resulting from a generator trip.
- Dispatch
- Generation is dispatched when the SO advises the generator to operate at a particular power output from a particular time to a particular time. For example, a generator might be dispatched at 100 MW from 5 pm to 6 pm on a particular day. Unde the rules of the electricity market, the generator is obliged to maintain 100 MW during that hour, or risk being fined for breaching the rules.
- Double fed induction generator (DFIG)
- In the context of WTGs, an asynchronous generator in which the stator windings are connected directly to the grid while the rotor windings are connected through an AC-DC-AC converter. Two thirds of the energy output of the WTG is typically transferred from the stator, one third form the rotor. The DFIG is also known as the wound rotor induction generator or WRIG.
- EC
- Electricity Commission
- EECA
- Energy Efficiency and Conservation Authority
- Electricity Governance Rules (EGRs)
- Rules which form part of the Electricity Governance Regulations and which govern many aspects of electricity supply including the operation of the wholesale electricity market.
- Embedded generator
- A generator that is connected to a local network rather than directly to the grid.
- Extended contingent event
- In relation to IR, a contingent event where supply is maintained only after disconnecting some demand, e.g. an event involving loss of both poles of the HVDC link.
- Fast instantaneous reserve (FIR)
- SR that increases output within the first few seconds of a contingent event involving a frequency drop.
- Fault ride-through
- The ability of a generator connected to the grid to withstand a voltage drop during a short circuit of a transmission line, e.g. caused by a tree falling across a line. Also known as "low voltage ride-through". Contrast this with local networks where the generators need to trip off during a fault as the chance of being "islanded" is high since local networks are usually poorly interconnected. An island in a local network occurs when some or all of the network disconnects from the rest of the network and from the grid.
- Frequency keeping
- Refer to Regulation
- Governor (or speed governor)
- A control device which controls the power output and the speed of rotation of a synchronous generator, the latter in response to changes in system frequency.
- Grid
- The network of high voltage transmission lines operated by Transpower in New Zealand, or the equivalent in other countries or states. In New Zealand, the grid is primarily made up of AC lines at voltages between 66 kV and 220 kV but also includes the HVDC link which runs from the Benmore dam in the South Island to Haywards, just north of Wellington in the North Island. The HVDC link runs for most of its length on towers above ground but also includes the Cook Strait cables.
- Hedging contract
- In the electricity industry, a hedging contract is typically a financial contract whose payments either completely or partially offset the risks of purchasing from or selling into the electricity spot market.
- HVDC link
- HVDC stands for high voltage direct current. The HVDC link carries up to 1,040 MW from the South Island to the North Island or up to 626 MW in the other direction. The link is configured in two "poles" or in other words in two halves. Pole 1 operates at 270 kV and uses old mercury arc valve technology for inversion and conversion. Pole 2 operates at 350 kV and uses modern thyristor power electronics.
- Intermittent generation (IG)
- A term currently used in the context of wind generation.
- Instantaneous reserves (IR)
- An ancillary service provided either by spare synchronised generation known as spinning reserve (SR) or by interruptible load reserve (ILR) that is available to the grid in case of an under-frequency event which typically occurs when a generator is suddenly and unexpectedly lost. The first manifestation of the loss of generation is a drop in the frequency of the grid below 50 Hz. Spinning reserves increase their output when the frequency drops. Interruptible load is disconnected from the grid when the frequency drops below a preset limit which is currently 49.25 Hz.
- Inverter
- A device which converts DC power to AC power. Typically it is constructed of power electronics.
- Load factor
- The average energy output in MW of a generator over the course of a year divided by its total rated output in MW.
- Local network
- A distribution network, owned by a lines company, and connected to the grid, which serves a local area down to the residential level. Local networks typically operate at between 33 kV and 400 kV, the latter being the supply voltage for a typical residential street.
- Marginal generator
- A marginal generator is one who's output is dispatched up or down by the SO in response to changes in load while the output of other generators is held constant. This is different to a generator performing regulation where output moves in response to frequency variations.
- MED
- Ministry of Economic Development.
- Must-run dispatch auction (MRDA)
- An auction run every day in the wholesale electricity market in which base-load generators can bid for the right to offer their plant at zero, giving them the highest degree of certainty possible of being dispatched. Once the right is won they must pay the bid price, i.e. they are prepared to pay to generate.
- N-1
- An operating principle that states that the grid should be configured and generators dispatched to ensure that the system can survive a contingency where one unit or one line is lost.
- Node
- A point on the grid where demand is taken from the grid, generation is injected on to the grid, or two or more lines connect to each other.
- NZEM
- The New Zealand Electricity Market was the voluntary spot market which preceded the EC which took over from the NZEM on 1 March 2004.
- Over-frequency arming
- An ancillary service provided by generators that have specially fitted relays which activate and disconnect the generator to prevent frequency rising to excessive levels in the South Island. The relays would typically activate if the HVDC link disconnected whilst sending significant energy northward. South Island generators would trip to slow rising frequency levels in the South Island.
- Pre-dispatch schedule
- A schedule of forecast spot prices and dispatch quantities which are published every two hours by the SO starting at 1 pm each day to cover the remainder of the current day plus all of the following day.
- Risk adjustment factor (RAF)
- A factor that scales the basic N-1 risk up or down to correct for the differences between the reserve risk calculated by SPD and the risk calculated by RMT. Since the end of 2003 the RAFs have all been one and the adjustments are now made using risk offset values.
- Risk offset
- A parameter that may reduce or increase the basic N-1 risk calculated in SPD to allow for the contribution of "free" reserve (IR that is not dispatched but which is nevertheless available during an under-frequency event) and other factors that reduce or increase the requirement for IR.
- Risk premium
- The amount of money that a risk-averse person would pay to avoid taking a risk. The size of the risk premium depends on the magnitude and probability of the alternatives that the person faces. A person who is risk averse prefers a certain outcome to an uncertain outcome with the same expected value.
- Regulation
- Also known as frequency keeping, this is an ancillary service provided by partly loaded plant with special governors that react faster to frequency changes than most other plant. This spare, synchronised capacity is used to control frequency of the grid, nominally 50 Hz, within normal limits of 49.8 to 50.2 Hz.
- Reserve management tool (RMT)
- A dynamic, non-linear software model of how the system frequency responds to a contingent event or extended contingent event.
- SPD (Scheduling, pricing and dispatch)
- A large scale linear programming model, in various versions for slightly different purposes, which is used by the SO and the electricity market's Pricing Manager, to dispatch generation and calculate spot prices.
- Spinning reserve (SR)
- Spare synchronised generation reserve that is available to the grid in case a generator is suddenly and unexpectedly lost. SR may be either partly loaded spinning reserve (PLSR) which is simply a generator that is running and generating, but at a lower output that allows it to increase output quickly in response to low frequency. Some hydro generators can also provide tail-water depressed (TWD) spinning reserve. In this case a hydro generator and turbine are run as a motor from the grid, which means the generator is spinning and synchronised. It can quickly be turned back into a generator in response to low frequency.
- Sustained instantaneous reserve (SIR)
- SR that increases output within roughly 20 - 60 seconds of a contingent event involving a frequency drop.
- Synchronise
- A generator is synchronised with the grid when its voltage, frequency and phase match that of the grid.
- Synchronous generator
- An AC generator in which the average rotational speed of the rotor in normal operation is exactly proportional to the system frequency. Note that the generator has stationary and rotating electrical windings, the former being part of the stator and the latter part of the rotor.
- System Operator (SO)
- The SO is responsible for ensuring the short term security of supply on the grid including dispatching offered generation to match demand and minimise the total cost of generation. Currently this is Transpower.
- Trading Period
- In the New Zealand electricity market each day is separated into 48 trading periods of one half hour in length. There are two exceptions which occur at either end of the daylight saving season, one having 46 trading periods and the other 50 trading periods.
- Under-frequency event
- A contingency in which the system frequency falls below its normal operating low range of 49.8 Hz.
- Unit commitment
- The process of preparing a unit to generate at some point in the future, typically within pre-dispatch time. Large thermal units such as those at Huntly power station can take several hours to start up from cold.
- Variable speed drive (VSD)
- See converter.
- Voltage support
- An ancillary service which is required in order to maintain the voltage on the grid within acceptable limits. The service is typically deemed to be provided free by generators by virtue of their being synchronous machines, though this assumption is breaking down in the case of WTGs which are more commonly variations of asynchronous machines.
- Wholesale electricity market
- A term used to refer principally to the spot market through which all electricity flowing on the grid is traded. Spot market participants offer to generate and bid to take demand on a half hourly basis for every half hour. The market rules specify how generation is to be dispatched and how spot prices are to be calculated.
- Wind energy market share
- The total annual wind generation in GWh divided by total national generation in GWh, expressed as a percentage.
- Wind energy integration
- The technical and economic ability of wind farms to connect to and operate within the New Zealand electricity supply network in a manner which is compatible with the operation of the electricity supply system as a whole.
- Wind energy penetration
- The ratio of installed wind capacity in MW to national peak generation in MW, expressed as a percentage value.
- Wind farm
- A collection of two or more WTGs located within a small geographical area and connected to the local or national supply grid at one point of connection.
- Wind turbine generator (WTG)
- An electricity generator powered by a wind turbine.
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