Bright Idea

Bright Idea

Using SPC could help prevent the next blackout

In 50 Words Or Less

• On Aug. 14, 2003, a power outage enveloped portions of the eastern United States and Canada, including New York and Toronto.

• Blackout costs in the United States that month were between $4 billion and $10 billion.

• Statistical process control could have helped avoid that blackout and others like it.

Statistical process control (SPC) has proven its value to industries for decades. Since it was pioneered by Walter Shewhart in the early 1920s, it has been used by W. Edwards Deming to improve the quality of U.S. munitions during World War II and to help Japanese industry recover after the war ended. Its application has a place in the manufacturing and service realms, among others.

But there is an opportunity to apply it where it can have an impact on our daily lives by providing warning signs of pending outages, signal waste and process problems in the world’s largest machine: the North American power system.

The impact of major disturbances to that system is significant. Blackout costs in the United States alone in August 2003 were between $4 billion and $10 billion, most of which was incurred during the Aug. 14 power outage that struck dozes of cities in the eastern United States and Canada.1 But the event that left cities such as New York, Cleveland, Detroit and Toronto in the dark could have been avoided if SPC had been used to provide an early warning sign.

Power points

The primary agency responsible for reliable performance of the U.S. electric power grid is the North American Electric Reliability Corporation (NERC), which sets performance requirements for planning and operation of the power system.2 The power system of North America is comprised of three major interconnections that can be thought of as independent islands (see Figure 1):

• Western—generally everything west of the Rockies.

• Electric Reliability Council of Texas (ERCOT).

• Eastern—generally everything east of the Rockies except Texas.

Figure 1

Each interconnection is actually a large machine, as all generators pull in tandem with the others to supply electricity to customers. The speed of the interconnection is frequency, which is measured in cycles per second or hertz (Hz). If the total interconnection generation exceeds customer demand, frequency increases beyond the target 60 Hz until a balance is achieved. Conversely, if there is a temporary generation deficiency, frequency declines until balance is again restored at a point below 60 Hz. Balance is required because electric motors use more energy if driven at a higher frequency.

The minute-to-minute operation of the interconnections is done by entities called balancing authorities, which dispatch generators and purchase and sell energy to meet their individual needs, reduce expenses or create revenue.

There are more than 140 balancing authorities in North America (see Figure 2). Each balancing authority in an interconnection is linked via high-voltage transmission lines (called tie-lines) to neighboring balancing authorities. Overseeing the balancing authorities are wide-area operators called reliability coordinators, who are to balancing authorities what air traffic controllers are to pilots.

Figure 2

To understand the workings of the grid, it helps to visualize a traditional water utility (see Figure 3). For a municipality to operate its own system effectively, it needs sufficient pumps (generation) to maintain the level (frequency) of a storage tank to serve its customers. If demand exceeds supply, the level drops. Level is the primary control parameter in an independent system.

Figure 3

Utilities quickly learned the benefits of connecting to neighboring systems. In our water example, an independent utility must have pumps equivalent to its largest online pump standing by if it wants its storage tank to remain level in the event of a pump failure. If utilities are connected together via pipelines (tie-lines), however, reliability and financial impact improve.

Once the systems are interconnected, the level is the same throughout. If one utility loses a pump, there is a drop in level, although it is much less than in an independent system. The balancing authority that needs water (energy) can purchase output from others.

Thus, there are two inputs to the interconnected balancing authorities’ control process: interchange error, which is the net outflow or inflow compared to what it is buying or selling, and frequency bias, which is the balancing authority’s obligation to stabilize frequency. In other words, if frequency gets low, each balancing authority is asked to contribute a small amount of extra generation in proportion to its size.

Each balancing authority shares a common meter with its neighbor on every tie-line. This is true for real-time control and accounting. If the balancing authority is not buying or selling energy, and if it is meeting its customers’ demand precisely, the net of the meters on its boundary is zero.

If, for example, the balancing authority chooses to buy 100 megawatts (MW) of energy, it tells its control system to allow 100 MW to flow in. Conversely, the seller will simultaneously tell its control system to allow 100 MW to flow out. The interconnection remains in balance, and frequency is stable. If an error in control occurs, it will show up as a change in frequency.

Customer demand and generation output are constantly changing for all balancing authorities. That means balancing authorities will have some unintentional outflow or inflow at any given moment. This mismatch in meeting a balancing authority’s internal obligations is measured via an instantaneous value called area control error (ACE), which is measured in MW.

Dispatchers at each balancing authority fulfill their NERC obligations by monitoring ACE and keeping the value within limits proportional to balancing authority size. This balancing is accomplished through a combination of computer-controlled adjustment of generators, telephone calls to power plants, and purchases and sales of electricity with other balancing authorities.

Conceptually, ACE is to a balancing authority what frequency is to the interconnection. Overgeneration results in a positive ACE and an increase in frequency. A large negative ACE due to undergeneration causes frequency to drop. A volatile ACE results in a volatile frequency.

Process failures

As noted earlier, frequency variation from the target is the result of operators not maintaining balance as load changes occur during the day. The target is normally 60 Hz but is occasionally adjusted in an interconnection

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