Modelling Major Hydro Systems

As at April 2007

Disclaimer

Reasonable care has been taken to ensure that the information in this paper is up to date at the time of issue. Potential users of EMarket should, however, ensure that they evaluate EMarket and this paper through an appropriate evaluation process in consultation with Energy Link. The authors are also reliant on certain information external to EMarket and Energy Link, for which no liability or responsibility can be accepted.

Introduction

This technical bulletin is intended to provide users and interested parties with a detailed explanation of how EMarket’s four major hydro systems are modelled.

This paper includes a brief overview of other features in EMarket and a note on enhancements planned in the short to medium term.

Main Hydro Localisation

Each major hydro system may be represented by one more resources, and the appropriate resource used in each particular simulation run. In this way, each grid may have its own version of each of the main hydros. Each of these may be created as the thermal stations are but each of the main hydros has a separate category on the main screen. For example, you might want to run a simulation with a station added to a major hydro – in this case you would create a new resource of the hydro concerned, add the station, attach the resource to the grid you are using and do the run. This gives you a high degree of flexibility in how you model hydro systems.

The diagrams of the four major hydro systems in this technical bulletin show the default configurations but the user is able to change these, add or delete stations, in order to investigate upgrade or alternative scenarios, e.g. the addition of the now abandoned Project Aqua to the Waitaki hydro system.

In each of the separate hydro entities must run and spill prices and minimum flow are accessible. This facility allows the running of multiple configurations of the main hydros (see New Zealand Major Hydro Systems) without having to change the model's global variables.

Summary

IMPORTANT: The material in this section on offer formulation for the major hydro systems provides background information and relates primarily to the "old code" from v1.3.6.

The following is an overview of the operation and modelling of the main hydro systems. The level of detail has been deliberately chosen to give the user an appreciation of the offering and acceptance dispatch algorithms. It is not intended to detail in extenso the precise workings of each major hydro system's algorithm, nor is it intended to be a full account of the operation of the hydro systems in practice. Rather, it is intended to give the user an insight into the behaviour of the major hydro systems, both as modelled and in practice. Note that if a major hydro system uses Short term River Chain Optimisation then its smaller storages are modelled and its offers are calculated by the optimiser rather than as described here.

In each of the following sections an overview of the system is given, with a diagram showing the principle parts. The key for these is shown on the Waitaki diagram. This is followed by an explanation of the algorithms used to model the offers and post dispatch evaluation. Unless otherwise stated, the information used to put together the major hydro models is taken from resource consent applications, brochures and other sources of information in the public domain.

Note that, in all cases, flow delays are ignored, thus avoiding the need to include inter-period hydro modelling in EMarket.

Manapouri

The Manapouri hydro-electric system consists of two storage lakes in series, Te Anau and Manapouri, a hydro-electric station at West Arm of Lake Manapouri and two parallel tail race tunnels to Deep Cove on the West Coast. The two lakes are linked by the Upper Waiau River with discharges from the system occurring either through the tail race at Deep Cove or through the Waiau-Mararoa control structure into the Lower Waiau River.

Minimum flows in the Lower Waiau River are required and include normal minimum flows, recreational, flushing and river mouth opening flows. Only the minimum flows are modelled, which captures 89% of the average annual loss in GWh.

The operation of the lakes themselves are constrained by the Guidelines agreed between the Guardians of Lakes Manapouri and Te Anau and ECNZ. These Guidelines are restrictive and specify inter alia how long the lakes may remain at any given level and the interval before returning to that level. The intention of these Guidelines is to mimic the natural rise and fall of the lakes in their uncontrolled state and thereby to preserve the shoreline ecology.

Since the ratio of storage to inflows is low and inflows are great, the station is virtually run of river and in practice is operated to use the inflows as they arrive. This produces a storage profile in EMarket that is close to natural without enforcing the Guidelines explicitly.

Aside from the above complications, the Manapouri system is quite simple. It is modelled as one storage lake, Manapouri in the inflows file, and one generator. Spill and must run flows both pass down the Waiau River.

Since the Manapouri system has no minimum generation requirement, (i.e. the minimum flow is not derived from a minimum generation as with most hydro systems) the must run price input is generally ignored in determining the offer.

One offer band is usually made which is at the current water value. If storage is too low then no offer is made but if storage is such that spilling is likely then two offer bands are used. The first offer band is the portion of storage which may be lost and is offered at the flood price, whilst the second, if any output remains, is offered as usual at the current water value.

Two further things may affect the offer: if the second band has a lower price than the first then the offers are combined at the lower price; if any offer price falls below a certain threshold then it is reset to the "avoid zero offer price." At present the "avoid zero offer price" is set to $1/MWh.

Waitaki

The Waitaki system is the most complex of the four main hydro electric systems in New Zealand. As can be seen from the following diagram, there is potential to use all of the inflows to the system in a relatively controlled manner compared with the Clutha system, for example. The Ohau River is not normally used in the course of generation since a release for generation flows down the adjacent canal, but flows are specified for summer and winter. The Tekapo and Pukaki rivers are normally dry.

The diagram shows storage lakes behind most of the stations in the system. Apart from Tekapo and Pukaki the storage is minimal. Ohau has some storage, about a dozen GWh at most, as does Benmore. However, Benmore's storage of 156 GWh is misleading in practice due to the requirements for a reasonably high head to retain efficiency in generation at Benmore and the requirement for a sufficiently high lake level so as not to inconvenience recreational users of the lake. The lakes at Aviemore and Waitaki are more in the nature of head ponds. All lakes except Tekapo and Pukaki are modelled with zero storage.

The system can be considered to consist of three parts in two ways: by the storage lakes of Tekapo, Pukaki and Ohau, or by the generation groups of Tekapo, Ohau and Lower Waitaki.

Lake Pukaki can hold the greatest amount of potential energy of any storage lake in New Zealand. It is important to note that flows from Tekapo which pass through Tekapo B station flow into Pukaki, thereby re-using Lake Tekapo water and increasing the inflows to Pukaki.

Lake Ohau has much less storage than Tekapo or Pukaki but due to its position in the hydro chain its water is worth the same energy per unit as water in Pukaki. Dividing the system by generation groups divides the total possible contribution to energy production approximately equally.

The offering algorithm works in the following way. Lake Tekapo's level is assessed and may fall into one of three categories: near to spilling, near to empty or at neither extreme. In the first case the release is set to the maximum canal flow, in the second case the release is set to zero. When the lake is at neither extreme a flag is set to indicate discretionary releases are possible and if the relative level of Tekapo is greater than Pukaki, Tekapo release is calculated to provide a minimum energy output from the system and to contribute to the minimum flow requirement down stream from the Waitaki station.

Lake Pukaki is then similarly assessed. If it is near to spilling then the release is set to equal the amount expected to be spilt, subject to meeting Waitaki minimum flows and allowing for the release from Tekapo and the uncontrolled flows in the Ahuriri River. In all cases, the maximum flows in the Pukaki canal exert an overall limit on the flow from Pukaki.

If Pukaki can meet the maximum canal flows for the length of the tick then the discretionary release flag is set to true and the initial release is set to whatever is required to meet the Waitaki minimum flow. If the lake is not capable of meeting the maximum canal flows then the release is set to zero, discretionary releases are not permitted and any further releases to meet the minimum flow are made from Tekapo.

The releases from this first stage are totalled to give the first, or must run, offer band. This offer is reduced to the flood price if either lake is, or is near to, spilling. If discretionary releases are permitted then either or both Tekapo and Pukaki's possible releases form the second offer band at the current water value. If the second offer is at a price less than the first offer then a combined offer is made at the lower price subject to not being lower than the "avoid zero offer price" of $1/MWh.

After dispatch, the algorithm first determines the generation from the uncontrolled inflows from Ahuriri and Ohau and the must run releases from Tekapo and Pukaki. If this quantity exceeds the dispatched amount then ipso facto some water must have been unused which is calculated in terms of equivalent spill from Pukaki.

Spill may also have been required to meet the minimum flow if the dispatched generation used less water than the minimum flow. If the dispatched generation is greater than the generation from the must run releases then, depending on the relative storage of Tekapo and Pukaki, releases are deemed to have been made from either or both of these lakes, as required to meet the amount of generation dispatched. These calculations are subject to the usual constraints of canal capacity.

Results of the tick are contained in the hydrological output file. Both spill and release are in cumecs equivalent to releases from Pukaki.

Clutha

The Clutha system comprises the following:

the controlled storage of lake Hawea;
two uncontrolled lakes, Wanaka and Wakatipu;
two uncontrolled tributaries, Shotover and Manuherikia; and
two hydro electric stations, Clyde and Roxburgh.

As uncontrolled lakes, the outflows from both Lakes Wanaka and Wakatipu are determined by their inflows and current storage. That is, the outflow over a period of time is a function of the starting storage and the inflows over that period. Lake Hawea, while controlled, has a minimum release of five cumecs as well as a maximum release of 200 cumecs, the latter being an informal arrangement to limit bank erosion. In practice, high flows in the Shotover can cause flows up the Kawarau toward Lake Wakatipu but this is not modelled due to its minor overall effects.

The two dams which form Lakes Dunstan and Roxburgh contain the Clyde and Roxburgh generation stations, respectively. Neither lake has a large amount of storage and consequently the storage is assumed to be nil in the model to reflect their status as head ponds.

Minimum flows apply downstream of each dam:

At Clyde the requirement in 150 cumecs.
At Roxburgh the minimum flow is 150 cumecs, equivalent to 60 MW generation, between September and April and the minimum flow is 100 cumecs which is equivalent to 40 MW generation, for the rest of the year.

These flows may be met by generating to release the water or by spilling at the dam.

Inflows to the system are assumed to be known with a high accuracy. This is due to the forecasting mechanisms in place in practice and the assumption that it is in the interest of the system operator to predict hydrological conditions well to maximise revenue and minimise spill. From these predicted inflows the expected outflow from the uncontrolled lakes is calculated.

Since only water in Hawea can be stored under direct control, any other flows in the system must be offered as must run generation. If Hawea is close to full and may spill then the portion which might be spilt is added to the must run offer. If storage is low only the minimum release is permitted, if possible. When Hawea is at neither extreme the minimum release is added to the must run offer and further releases are made available for the discretionary offer.

The quantity in the first offer band is calculated by adding the release from all sources upstream of Clyde, multiplying it by the MW per cumec for Clyde, then adding the flow from the Manuherikia River and multiplying it by the MW per cumec for Roxburgh. This is offered at the must run price except when Hawea is close to spilling in which case it is offered at the flood price.

The second offer is only calculated if discretionary releases from Hawea are permitted. If this is so an offer is determined subject to the maximum release and maximum usable flows at both dams. If the current water value is less than the must run price then the two offers are totalled and are offered in at the lower price, subject to not being lower than the "avoid zero offer price" of $1/MWh.

After dispatch a check is made to see if the dispatched generation can be met by the uncontrolled flows. If further releases are required these are taken from Hawea. When generation is more than covered by the sum of uncontrolled flows and the minimum release from Hawea, then spill occurs. The release and spill amounts in the hydrological output file are in equivalent cumecs at any of the sources except the Manuherikia, since all sources except Manuherikia have the same MW per cumec for the system.

Note that in times of high inflows and low demand it is not uncommon to see high spill figures for the Clutha system combined with releases from Hawea. This is usually due to either the minimum release at Hawea being enforced or a requirement to release more water than usual because of high lake levels.

Waikato

The Waikato hydro electric system can be thought of as a single chain of stations stretching from Lake Taupo to Karapiro with a handful of tributaries along its length.

Modelling Major Hydro Systems

Contents