Stabilizing power systems

Before a power system's reliability can begin to be guaranteed, a detailed study must be made of its stability—that is, its ability to return to normal or near-normal operating conditions after disturbances. The disturbances may be classified as brief or sustained. Steady-state stability, as it is called, describes the system's swift recovery from the relatively small disturbances that occur everywhere in power systems owing mainly to the dynamic nature of power loads and generation. To maintain steady-state stability, constraints on operating parameters—basically voltage levels and power flows—are tightened.

The transient stability of a power system is the term for its ability to return to normal after a fairly large disturbance, such as the outage of a transmission line or a generating unit. If a power system seems too vulnerable to transients, additional means of regulation are considered: the activation of more generation units, for example, or added operational constraints. Evaluation of those constraints in real time can prevent a power system from entering an unstable state.

In the Soviet Union, the power systems had limited transmission capabilities [see "Why the Soviets centralized emergency control"]. What the power engineers developed there was, in its final form, an advanced version of a centralized control system for the automatic prevention of emergencies. Power systems under this kind of centralized control are capable of operating with power flows just shy of the limits set for stability, as emergencies and overloading of transmission lines promptly trigger compensatory control actions.

The numerous unsatisfactory features of the Soviet systems rendered them less than immune to instabilities. But the control techniques were themselves sound and ought to be even more effective in systems where real-time data and opportunities for automated intervention abound.

Note that this kind of emergency prevention protects the power system as a whole from cascading degradation, not just individual transmission lines from overloading. Together with the use of relays to protect transmission lines and generator excitation systems, automatic preventive control forms a crucial component of emergency management.

The actions used in centralized emergency prevention consist of the outage and fast unloading of generating units, and load shedding. For long high-voltage compensated transmission lines, the outage of reactors or tuning of line compensation can sometimes help when trouble looms. Regulation of dc-line power flow and use of thyristor-controlled regulators also can be useful. In fact, experience has shown that applying only one type of control action—only generator outages, or only load shedding—is in the majority of cases unacceptable for large power grids [Fig. 4]. A balanced mix of actions must be applied.

Since it is next to impossible to compare the performance of systems in the former Soviet Union having centralized emergency preventive automated control (Cepac) with similar systems without Cepac, the authors hesitate to make sweeping claims for the Soviet grid. But both dealt in person with a 750-kV line that crossed the Ukraine from Russia to Hungary and which routinely tripped more than a dozen times a year during the 1980s, without ever seriously interrupting service in the Ukrainian system. At least in this situation, Cepac appears to have performed well.

In the authors' view, the last 15 years of relatively reliable operation of the Russian power systems without large-scale blackouts attests to the efficacy of Cepac systems. The authors' own experience with those power systems is their source for the automatic control methods and approaches to emergency prevention described hereafter.

Existing control systems

Assorted types of preventive controls, mostly local, exist in North America, Japan, and Europe. By and large, they address the overloading of individual transmission lines or individual events in a power system. Canada's Ontario-Hydro and Hydro-Quebec, however, have developed preventive automatic systems. In New York State, Consolidated Edison's emergency control system includes elements of centralization and calls for automatic load shedding whenever lines bringing power from utilities elsewhere are overloaded. In the western states, automatic emergency control of a 400-kV dc line is employed to damp system oscillations.

But last year's large-scale blackouts in the western part of the United States have pointed up a need to consider the reliability of the power system as a whole, rather than just portions of it, and hence a need to centralize the strategies used to prevent emergencies. Suppose a centralized control system had existed during the emergency on 10 August 1996—it would have eliminated the dangerous overloading of the networks in Oregon and hence the cascading outages that led to the Pacific Intertie's disconnection.

In fact, even in the final stage of that emergency, a Cepac system could have initiated actions to unload power flowing through the Intertie, so as to provide a normal power flow stability reserve of around 8 percent. This move would probably have required shedding about 300­400 MW of load (0.3­0.4 percent of the total load of the western states) in the southern part of system, for however long a time was needed (probably about a half hour) to bring power generation reserves from the south on stream. In this way, 14 000 MW of load lost in the blackouts of 10 August could have been saved.

Relevant models

The typical Cepac system developed in the former USSR selects control actions on the basis of their integrated effect on power systems and involves two or more local preventive control systems. Coordinated systems of this type were implemented in the '70s in all the power pools of the then USSR as well as in the largest regional power systems. Their effectiveness in containing an outage before it could cascade was proved in practice. Further coordination of automatic versions of these systems culminated in the '80s in a Cepac system. Its purpose was to guarantee the stable operation of the entire power system by using all the means of emergency prevention control present in that system.

The most advanced Cepac system was set up in the Urals Power Pool, Russia. It went into action whenever the adaptive estimation of steady-state and transient stability indicated it should. The pool includes more than 70 power stations with an installed capacity of 40 500 MW, supplying the country's largest industrial region with a population of about 28 million. It is therefore comparable with large power pools in the United States.

In the Cepac systems, emergency prevention was based on the on-line evaluation of stability (steady-state and transient) for a set of contingencies, plus the calculation of the control actions needed for reliable operation. If line or generating unit outages led to the overloading of a transmission connection (with steady-state or transient stability or thermal limits being exceeded), the Cepac system would initiate actions designed to avert cascading. (Thermal limits are defined as enough current for normal operation of transmission lines but not enough to damage them.)

Hardware implementation of Cepac systems depends on the availability of computers, of communications and control channels, and the data acquisition system. Control actions are set in motion by special controllers and local automatic control systems [Fig. 5]. In the USSR, special controllers for load shedding were developed to distribute among individual loads however much was to be shed.

Data acquisition and state estimation systems were relatively primitive in the former Soviet states, though, hampering the setting up of centralized and coordinated emergency prevention systems in Russia. For this and other reasons, they were implemented separately from energy management systems and relied on a separate computer control center.

Particularly gratifying to the authors of this article was the adoption of the Cepac approach in 1995 by Chubu Electric Power Co., Japan. This Cepac uses on-line analysis of transient stability for a power system with 100 generating units.