Distributed Offers from Hydro Systems in EMarket

Aim

Previously EMarket has injected all generation from any given hydro system into one node on the transmission grid. In order to improve constraint and locational pricing modeling EMarket has now been upgraded to model the distributed injection of this generation at each station in the system.

Overview

There is a fundamental difficulty with modeling hydro systems operating in the NZEM. This difficulty arises from the fact that the hydrological constraints under which stations in a system operate are not modeled in SPD. In practice this problem is dealt with by ‘block dispatch’ which allows the system operator to redistribute dispatched quantity within the system. This arrangement is relies on a certain amount of human judgment, and so to model it requires some assumptions about the aims and methods of the system and grid operators.

These assumptions are made:

1. Offers for each station will be made so that the redistribution of generation will be kept to a minimum.
2. Offers, nodal prices and dispatch reflect the effect of transmission constraints as closely as possible.
3. Offers reflect the optimal operation of the hydro system.

Given these assumptions, the following requirements for EMarket should be sufficient to ensure good modeling of the hydro system within the market:

1. Generation within a hydro system is subject to all relevant hydrological constraints.
2. Generation reflects optimal operation given the nodal prices produced.
3. Generation is subject to all transmission constraints.

The Solution

Within the new version of EMarket each station within a Hydro Systems may have an injection node defined. EMarket will then generate offers as it always has - first ascertaining optimal operation and marginal ‘cost’ of water usage given different levels of total generation, then dividing this into bands of significantly different marginal cost and creating an offer for each of these. When these offers are presented to the dispatch engine they are not associated with a single node, but instead with a distribution across the station nodes in the system. The dispatch engine then dispatches these distributed offers with regard to all transmission constraints, and calculates line flows consistent with the distributed generation.

Example:

The following table represents optimal output of the stations in the Waitaki chain at the upper end of four price steps.

Price $/MWh	TKA	TKB	OHA	OHB	OHC	BEN	AVI	WTK
0	0.00	0.00	44.84	38.67	37.59	34.18	13.92	24.45
7	23.94	142.42	44.84	38.67	37.59	34.18	13.92	24.45
80	25.00	160.00	245.82	212.00	206.10	530.11	215.97	105.00
260	25.00	160.00	248.00	212.00	212.00	540.00	220.00	105.00