Energy Expo: How Does Load and Power Generation Balance in the Electric System?

The purpose of these blog posts is to explain engineering power system concepts to everyday day Albertans. If you have any questions that you would like answered in future blog posts, send them to info@skhoffman.com.

The Question

What happens when a power line goes down, a neighbourhood is cut off, and suddenly there is less load in the system?

Where does that extra power go?

What if the wind stops and there is less power going into the system?

Where does the extra power come from?

Economic Dispatch

For the most part, what power plants are on is determined ahead of time–based on their cost.

Part of the Alberta Electric System Operator’s (AESO’s) role is to do this planning. Seven days ahead of time, generators bid into the system and are ranked into a merit order which lists generator and how much power they can produce from the lowest to the highest price .

Then, based on system load, the AESO dispatches generation. The system operators go through the list until all of the load is served. The bid of the last dispatched generator is the power price.

Economic Dispatch Example

In this example, in order to generate enough power to meet the system load, all of the generation up to Gas Plant 1 must be online. Since Gas Plant 1 was the last generator to be dispatched.

Its bid of $25.00 sets the power price.

This is done on a minute by minute basis and the pool price is the average cost over one hour.

More Information

More information on how this is operated can be found on the AESO’s website under Market Basics.

You can see the real time power price as well as the forecast here.

And the real time supply and demand (which generators are online and what the current load is) here.

Real Time

But what happens in real time?

Operators can’t possibly know the second that you plug in your vacuum and need more power.

When load and generation are balanced, generators are designed to produce 60 Hz (the North American standard frequency). When there is an imbalance, the generators will automatically adjust their output to regain balance.

This action takes place without any action from an operator.

Changes in the System State

When frequency goes down, the generator will automatically produce more power and when frequency goes up, the generator will automatically produce less power.

So, if a generator goes offline, the system inertia causes the frequency to go down which pulls more power from the generators that are still online.

Similarly, if a line trips cutting off load, the system inertia causes the system frequency to increase which lowers the power output of the generators that are online.

Operating Reserve

In order to make sure that enough power is always available, the AESO ensures that there is an operating reserve–extra generating capacity should a generator go offline or load increase beyond the anticipated forecast.

Operating reserves need be able to come online quickly, for example, gas power plants.

Future Questions

Do you have any other burning questions about electricity or power generation for future Energy Expositions?

Send them to info@skhoffman.com

Further Reading

For more detailed information on this topic, check out these resources:

Practical Power System Operation Textbook by Ebrahim Vaahedi

Principles of Power System Operation by Western Electricity Coordinating Council (WECC)

Balancing and Frequency Control by North American Electricity Reliability Corporation (NERC)