Electrical Power Supply and Use for Dummies

The article excerpt below uses a simple analogy to explain how power is supplied, used, and paid for on the power grid. It may help to correct some common misconceptions which many people hold about the sourcing of electrical power on the grid.

It is excerpted from a guest article on Yes, Vermont Yankee, written by nuclear engineer Howard Shaffer. The article is also featured on the 121st edition of the Carnival of Nuclear Energy Bloggers.

To get a picture of what is going on physically, think of the electric power circuit connecting all the supplying plants and all the loads using power--think of them as all connected together by a giant bicycle chain.  All the suppliers and all the loads are connected to the chain by sprockets.  There are spare sprockets for suppliers and users who are not on the grid at the moment.

Each supplier and each user has a meter that shows how much they are supplying or using.  In this system, the chain needs to run at an almost constant speed.  In the U.S., it runs at 60 hertz (cycles per second for us old-timers). On this big chain, each supplier and user knows what they are doing, and they know the chain's speed.

No user can say: “My power is coming from that supplier over there, or those suppliers.”  All any supplier can say is: “I’m supplying my power to the chain (grid)."  All any user can know is: "I am getting my power from the chain (grid)."

Supplying Power, Using Power, Paying for Power

This can be maddening to explain.

Let's start with supplying and using power. The language has gotten scrambled by the way the chain system is used and paid for.

Some users are taking power all the time, some only part time.  Some suppliers provide power all the time, some only part time.  The chain must run at a nearly constant speed, and any supplier might break down and quit at any time.

To keep the chain going, the other suppliers must have some extra capacity to make up for the loss, at least for a short time.  In addition, the manager of the chain needs to be able to tell other suppliers to start when needed.

If the chain manager can’t get enough suppliers started and supplying, the chain will slow down (brownouts and low frequency).  In severe shortages some users may have to be disconnected from the chain (dropped load, rolling blackouts).  In the worst case, the chain stops (blackout).

Paying for Power

All the suppliers need to be paid.  All the users need to pay for what they get.  The chain owner and manager needs to be paid too.

Different kinds of supply and use cost different amounts.  If you are a supplier who is always ready to start supplying if another supplier breaks down, it costs money to stay ready.  The same is true if you back up a supplier that is only available for a few hours a day.  If you are a supplier who promises to stay ready to do backup, you usually get paid more.

If you are a user and know how much you want to use over time, it is cheaper for you to have a contract with certain suppliers.  Then these suppliers are  “on the chain” for you.  If you don’t have such a contract, you take the “chain’s price”-- a complicated average price from all the suppliers.

Careless Descriptions

People in the utility industry say: “X utility has a contract with Y plant for so much power.”  In reality the contract is for Y plant to supply power to the chain while X utility is using power from the chain.  Since everyone in the utility industry knows that power is put on the grid (on the chain), not sent specifically to a user, the shorter expression ("has a contract for so much power") came to be used.

The shorter expression was OK until the public wanted to know more.  Then the short expression caused confusion about how the physical system actually works.

Big Ben at Midnight

The Midnight Accountant

If you are a user, and your contract with a supplier ends, the chain will keep you going.  The chain doesn't know about your contract.  The chain just keeps moving, and you keep taking power off the chain the same way you did before.

However, when your supplier contract ends, you must pay the chain’s price. This price includes the higher cost "stay-ready" suppliers.

On March 21, at midnight Vermont Yankee kept right on powering the chain (grid).

No physical switch shifted.  The accountants shifted -- the billing.  At midnight, Vermont utilities no longer paid specifically for Vermont Yankee.  Vermont Yankee put power on the grid, and they took power off the grid, same as before.  However, at midnight the utilities began to pay specifically for other suppliers: Seabrook for example.  They pay the chain (grid) price when they don't have enough contracts with suppliers. Some of Vermont Yankee’s power is mixed in that price.

The accountants arrange the pricing, but the power to move the bicycle chain (grid) in Vermont was the same after midnight as before midnight.  The work of the accountants changed at midnight, but the work of the power plant continued.
_Howard Schaffer in Yes, Vermont Yankee

It might be helpful to read this article by Meredith Angwin which looks specifically at the supply of power in Vermont, following a recent change in contract by Vermont Utilities.

This type of confusion is particularly rampant among fans of "green power" who believe that they can pick and choose the sources of electrical power for their business or residence.

The dumbing down of cultures and educational systems in the western world has many sad side effects. One of them is the creation of a more sheep-like and compliant citizenry, generally ignorant of the underlying mechanisms of technological society.

But remember: It is never too late to have a dangerous childhood.