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.

Other Documents

This bulletin is one of a series of technical bulletins relating to Energy Link’s EMarket model. Taken together, the bulletins replace the old EMarket User Guide. The full series of bulletins covers an overview of the EMarket model, the details of the four major New Zealand hydro systems modelled in EMarket, water values and hydro offers, power flows, dispatch and nodal pricing, short term river chain optimisation and company optimisation.

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.