# How are power flows between multiple power systems (tie-line flows) determined?

In Europe, for example, there are multiple transmission system operators (TSOs), which are responsible for their own region of the grid. These TSOs must exchange power to ensure reliable operation of the whole system.

Does anyone know how the amount of power exchanged is determined? Is there some forecasting of demand in each region that gives each TSO and ballpark of how much power they need/how much excess power (generation capacity) they have?

If anyone has any reports that describe this process it would be greatly appreciated!

Edit: A reason that I'm wondering is because it seems like in some regions tie line flows (between areas) are scheduled as a set point to be maintained. Any idea how these scheduled flows are determined? Specifically the note "maintenance of the scheduled interchanges" in this link.

In Australia we have the National Electricity Market, which is run by the Australian Energy Market Operator. Wikipedia describes the operation of the NEM as follows:

Exchange between electricity producers and electricity consumers is facilitated through a spot market where the output from all generators is aggregated and instantaneously scheduled to meet demand through a centrally-coordinated dispatch process. This process is operated by the Australian Energy Market Operator (AEMO) in accordance with the provisions of Australian National Electricity Law and Australian National Electricity Rules.

Scheduled generators submit offers every five minutes of every day. From all offers submitted, AEMO’s systems determine the generators required to produce electricity based on the principle of meeting prevailing demand in the most cost-efficient way (see also Economic dispatch). AEMO then dispatches these generators into production by sending automatic generation control (AGC) target signals to each generating unit.

The interchange of energy between regions is based on 'economic dispatch'. Every five minutes, each generator submits a bid to the market, i.e. "I will generate 10 MW for $10" or "I will generate 20 MW for$30". The market operator then calculates which combination of generators to run so that the total cost of meeting all customer's loads is minimised. This includes minimisation of transmission line losses and accounts for the maximum capacities of the interconnecting lines.

To answer your original question, the 'amount of power exchanged between areas' is not a controlled quantity. The electricity market is based on meeting the consumer's loads using the cheapest generators, and the power exchange between areas is a consequence of that.

You may wish to read further on 'economic dispatch' as it is a fascinating window into the generation and transmission industries.

• Thanks for the informative answer! Maybe it's different in Australia, but it seems in some regions, tie line flows (between areas) are scheduled as a set point to be maintained. Any idea how these scheduled flows are determined? Specifically the note "maintenance of the scheduled interchanges" in this link: books.google.com/… – Erik M Dec 3 '14 at 17:16
• @ErikM I believe the schedule you are talking about come from the security-constrained economic dispatch problem. – xuva Mar 19 at 21:44