Understanding Power System Models and Predicting the Next Blackout

By Tony Sleva, P.E., Sleva Associates

How can we ensure that the United States doesn’t suffer another major blackout in the coming years? The answer to this question is multi-faceted and not easy to arrive at. We can predict load growth based on historical averages. We can inspect and maintain components so the likelihood of preventable failures is minimized. We can develop power system models that uncover weaknesses and vulnerabilities in the foreseeable future. And, we can use the study results to provide a basis for system reinforcements.

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Are Power System Models and Motor Models Adequate?

Power system models include detailed representations of generators, transformers, buses, transmission lines, distribution lines, and other components that are designed and/or specified by electric companies. Resistive loads, shunt and series capacitors, and shunt and series reactors are modeled as constant impedance loads. Motors and power supplies are modeled as constant power loads.

For general purpose motors, models utilized in power system models are probably good enough, as the required motor torque decreases when motor speed decreases. For air conditioners and similar loads, power system models need to be revised to reflect the fact that after a compressor is operating, compressor torque does not decrease as motor speed decreases, and the margin between motor torque and load torque may not be sufficient to assure that motors do not stall before faults are cleared.

Distribution system voltage recovery can be the constraint that limits transmission system operation. The concern is that load flow models are developed assuming constant impedance and/or constant power loads, whereas voltage recovery analysis requires the use of models that include motor re-acceleration.

Air Conditioner Specifics

Air conditioners, like many electrical loads, cycle on and off by individual controls in the load units. During voltage recovery transients, air conditioners are especially troublesome as recovery torque requirements are significantly greater than cold start torque requirements. (Refer to Figure 1, which illustrates motor and compressor speed torque curves.) The compressor cold start curve is the characteristic that is expected after an air conditioner has been off for several minutes. The compressor voltage recovery curve is the characteristic that is expected when an air conditioner motor stalls and restarts before the refrigerant returns to the cold start condition. The compressor voltage recovery characteristic is transient since it only persists for a few minutes until refrigerant returns to the cold start condition. Unfortunately, many air conditioners may be simultaneously subjected to a recovery voltage transient as motors attempt to reaccelerate. The impact of this transient characteristic is that voltage dips can be a greater concern than complete loss of voltage-especially if other air conditioners that were in the standby mode start during the transient.

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Significance of Voltage Sags

When voltage sags, VARs provided by shunt capacitors decrease at the same time that VAR load increases. If a three-phase fault persists for more than a few cycles, the speed of many air conditioner motors will drop to less than 80 percent of nominal-which means many motors will stall and draw locked rotor current when voltage recovers-and the margin between motor torque and compressor demand may decrease to the point that motor recovery start time will increase tenfold or more. In the worst case, air conditioner motors will not restart. The impact on power system operation is illustrated in Figure 2 and Table 1 (Aggregate Load during Normal Operation) and Figure 3 and Table 2 (Aggregate Load during Voltage Recovery).

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For the conditions illustrated in these figures and tables, MW load remains constant while MVAR load increases substantially during the period when air conditioner motors are attempting to reaccelerate. This explains the events that are being observed on distribution lines: Namely, air conditioner motors stall when relatively short voltage transients occur. Voltage does not recover until motors trip via overload protection or compressor torque requirements decrease as refrigerant returns to the cold start condition.

In the worst case, a three-phase fault can occur on a transmission line, degrade voltage in a wide area, cause many air conditioner motors to stall, and lead to system collapse. (We have tested window unit air conditioners and observed that when voltage is reduced to 50 percent, a running air conditioner stalls, and the air conditioner will not restart when voltage recovers to 80 percent of rated even though during cold start conditions the same air conditioner will start with 70 percent voltage.)

Spinning Reserve Requirements

Traditionally, spinning reserves have been thought of as reserve “watt” requirements. Spinning reserve requirements have traditionally been equal to the largest operating generator. During summer load periods, both spinning watt reserves and spinning VAR reserves are required.

Spinning watt reserves can be located at remote points. Spinning VAR reserves, however, need to be geographically distributed so that sufficient VAR reserves are available in every given service area. A good rule of thumb is that spinning VAR reserves should be equal to 20 percent of the shunt capacitors that are in service and 200 percent of air conditioner motor VAR demand (running motor VARs, not starting motor VARs). Spinning VAR reserves need to be automatically dispatchable within a very short time, perhaps 50 milliseconds. With restructuring and divestiture taking place in the power industry, many utility companies do not have control of generation as they did in previous years. This could be a contributing factor that causes a relatively small event to become a critical event.

The spinning VAR reserve requirement is derived from considerations related to motor and load inertia, motor and load torque, and voltage recovery after delayed clearing of three-phase faults during peak load periods. That is, spinning VAR reserve requirements are driven by customer load, not by generator size or generator location.

In addition to spinning VAR reserves, fast-acting voltage comparison schemes need to be developed to shed load when voltage recovery is problematic. This is in addition to underfrequency load shedding and undervoltage load shedding schemes.

How Possible Is This Scenario?

Faults can occur at any time. Fortunately, most faults on transmission systems are not three-phase faults. Nevertheless, a quick review of power system incidents seems to indicate that the preceding discussion is a practical consideration. Consider the following events:

Event 1

A major city in California was blacked out after a bus sectionalizing circuit breaker was closed before safety grounds were removed (Figure 4). The impact of this event was limited by the uniqueness of location where the incident occurred. At a different location, the outcome would have been much worse.

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Event 2

Transmission system voltage sagged throughout the Northeast (Table 3). The event was mitigated when several 230-34-kV transformers failed due to a fault on the tertiary bushings of one transformer.

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Event 3

This event was little noticed, and not reported to the general public, as the system continued operating after three 500-kV lines tripped when a single phase-to-ground fault occurred on a 230-kV bus (refer to Figure 5, which shows a small portion of the power system).

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Event 4

Finally, consider events that have been reported in several states during summer peak load period. These events appear to be the result of low motor inertia in combination with mismatches between the torque capability of electric motors and the torque requirements of air conditioner compressors. The faults that initiate the events occur on adjacent transmission or distribution lines (Figure 6, at right).

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Predicting the Next Major Blackout

How can the events described in this article be used to predict the next major blackout? Events 1 and 2 demonstrate that delayed fault clearing-which is one of the precursors of a widespread blackout during summer peak load conditions-has been observed. Event 3 illustrates overtrips need to be a part of the analytical process. Event 4 provides a direct link between load, recovery voltage and spinning VAR requirements.

Where Do We Go From Here

In the immediate time frame, for summer 2007, we need to review spinning VAR requirements to assure that during July and August adequate VAR support is available to facilitate area-wide air conditioner restart subsequent to three-phase faults on 230-kV, 345-kV, 500-kV and 765-kV systems.

Beyond this summer, it is recommended that a forum be established that would facilitate an open dialogue of power system events and enhance cross-discipline communication among protective relay engineers, power system operators, equipment designers and futurists. The goal of this forum should be identification and elimination of recurring events that can lead to system collapse.

Membership in such a forum should be open to electric utilities, universities, transmission system operators, architect engineering firms, equipment designers, manufacturers, governmental agencies, and others with an interest in avoiding major power blackouts. One approach would be to categorize power system events on a scale of 0 to 4 with an urgency rating of low, medium or high. A category 1 means that if one more failure had occurred, widespread blackouts would have resulted. High urgency means that transmission system operators would be required to address the concern within 30 days.

The emphasis for the future should be tracing failures to the very structure of the power grid.

Tony Sleva is president of Sleva Associates Inc., an electrical consulting and training company located in Allentown, Pa. Tony is an ad doc instructor at University of Wisconsin – Milwaukee and author of a textbook entitled “Protective Relay Principles,” published by CRC Press. He can be reached at tony@sleva-associates.com

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