27-03-2012, 12:29 PM
reconciling pool and contract dispatch in open access transmission operation
00707096.pdf (Size: 594.25 KB / Downloads: 193)
Introduction
Operating the transmission system in a fair and open
manner is a challenging issue in encouraging competition
in electricity supply. The transmission network has
more pronounced natural monopoly features, both
technical and economic, than any other portion of the
electricity supply industry and therefore its operation
and the pricing of transmission services pose many as
yet unresolved issues in the new competitive environment.
For this reason, in an unbundled system, the grid
is often vested in an entity that is answerable to a public
regulator (the NGC, for example) [l], or is a public
corporation, as in the Nordel countries [2], while in the
USA [3] the precise structure of ‘open access’ is now
being actively debated and developed.
Mathematical models
2. I The pool model
The formulation given here assumes bid-price order
dispatch, includes provision for system operating constraints
and allows demand-price elasticity to be modelled.
Demand at any time, however, is a function not
only of contemporaneous price but also of prices at
other times. Indeed, the intertemporal price [4] dependence
of demand lies at the heart of all dynamic tariff
theory and is central to pool dispatch.
2.2 Bilateral dispatch
The conceptual model of bilateral dispatch is that sellers
and buyers enter into transactions where the quantities
traded and the trade prices are at the discretion of
these parties and not a matter for the ISO. These transactions
are then brought to the IS0 with a request that
transmission facilities for the relevant amount of power
be provided. If there is no congestion on the system the
IS0 simply dispatches all requested transactions and
charges for the service.
3 Prioritisation and co-ordination
In the open access systems of the future, provision will
have to be made for the simultaneous existence of both
pool and bilateral (and for that matter brokered multilateral,
though this is not considered here) activities.
There will be requests for point-to-point MW transport,
utilities connected to the grid may have groups of
generating plant from which they intend to serve their
own group of customers, and finally there will be
uncommitted customers who wish to purchase power at
the prevailing pool price.
4 Example of co-ordinated operation
A simple example is now provided to illustrate the
mechanisms of prioritisation and co-ordination
described in Section 3. The 5-bus system of Fig. 1 consists
of two economic generating stations G1 and G2 in
the north, and loads L1 and L2 in the south. A third,
more expensive, generating station G3 is located in the
south. The three generators bid into the pool and the
loads take their power from the pool at pool prices.
5 Conclusions
The paper has developed a theoretical basis for co-ordinating
pool and contracted bilateral dispatches in an
open access transmission environment. The specific
problem addressed in depth is operation strategies for
congested transmission conditions. Conceptually reasonable
and computationally feasible approaches for
the solution of this problem have been developed and
are illustrated by a simple example.