Don't fade to black

Optimizing protective relaying prevents your electrical system from vanishing into darkness

By By John P. Nelson, P.E.

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The recent Northeast Blackout should be a wake-up call for the electric utility industry. Fifty million people were without power for as long as thirty hours. Early reports indicated that at least three people died as a direct result of the blackout. Economic losses from the event have been estimated to be well into the billions of dollars. If you have any doubt about the blackout's economic impact, just look at what happened to your gasoline prices.

I hope the blackout didn't affect your plant. But, even if you didn't experience an outage, you may have seen delays in your plant's supply chain. There are lessons to be learned, including how to better prepare a plant for a repeat incident. The blackout was caused by a series of events tied closely to the protective relays on a number of utility systems. Knowledge of protective relaying will help you to understand your plant's protective relaying, and its role in the utility system supplying power to your plant.

Power system stability and protection are complex. However, there's little doubt that a series of events led to the blackout. Lacking proper system protection schemes allowed some disturbance to cause electric generators to "swing" or oscillate, causing large amounts of power to slosh back and forth from one part of the grid to the other. The oscillation was so great that generators started tripping off line. And once a coal-fired or nuclear plant trips off-line, it can take hours, even days, to get it back on-line.

If a plant's electrical system is operating at its limit, it can fall apart, just like the Northeast utility grid. Preventing some small electrical system component from darkening the entire plant requires an understanding of protective relays.

 

Table 1. Example of under-frequency load shedding.

Note: These setpoints are for illustrative purposes only and may not reflect a particular utility's setpoints.

The devices that could have prevented the blackout were either not installed or, if they were, didn't operate properly. You're likely to hear a detailed explanation of what went wrong, and the U. S. government plans to pass utility reliability legislation.

What you probably won't hear about is the hardware that operated properly and contained the outage. The cascade of events would have been worse if devices called protective relays didn't isolate parts of the utility grid so that Chicago and Washington D.C. could keep their lights burning and motors turning. Nevertheless, these lit cities and other parts of the grid experienced power swings. Every utility interconnected to the northeast felt the system swing. But the utilities adjacent to the outages isolated themselves from the disaster.

 

Table 2. Protective relaying functions.

Meet the protective relay

According to IEEE 100, a protective relay is a device that detects defective lines, apparatus and other abnormal or dangerous power system conditions and initiates appropriate control circuit action when it does. The most common protective relay is the overcurrent relay that trips a circuit breaker when it senses overloads and shorts. Other relays operate on voltage, frequency, impedance, pressure and temperature, to name a few variables.

Many relays operate with currents and voltages that are proportional to those on the power system. Current transformers (CTs) and voltage transformers (VTs) convert grid currents and voltages into manageable secondary quantities in the range of five amperes and 120 volts. On older relays, these fed electro-mechanical coils, armatures and induction disks to close a contact that tripped the circuit breaker. Modern microprocessor relays manipulate the electrical signals from the current and voltage transformers mathematically and cause a contact to close and trip the circuit breaker. The microprocessor-based relay is a computer and, therefore, can perform other control and protection functions, such as metering, sequencing events and locating faults.

The Northeast power grid had been stressed to its maximum limits and some event started the system "swinging." The protective relays that electric utilities use for isolating faulty parts of the system should have minimized system instability and prevented the cascading blackout. But, for whatever reason, the whole electric grid started swinging.

The subsystems couldn't be separated from each other. The generators that had excess capacity should have been able to recover from the over-speed/over-frequency condition. Other overloaded system segments were unable to shed loads quickly.

Load shedding

A series of under-frequency load-shedding relays set at various frequencies and time delays can disconnect excess load. For example, under frequency set points may be similar to those in Table 1.

Shedding loads keeps the generators on-line by matching capacity to load. When a utility is overloaded or has insufficient generating capacity, the frequency decays. Quickly shedding small amounts of load at 59 Hz to 59.5 Hz may restore the match. However, if the frequency decays below 59 Hz, even more load must be removed to restore balance quickly and prevent the generators from tripping off-line.

A load-shedding program must be pre-planned and initiated automatically. The timing shown in Table 1 is in the range of 0.1 second to 0.2 second before corrective action takes place, much faster than any human can react.

Functionality

Protective relay functions are identified by a set of standard ANSI/IEEE device numbers. For example, the frequency relay for load shedding is identified by the number "81," which signifies frequency, and "U," which signifies under. Other common numbers include:

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