Introduction to Power System Analysis:
In Power System Analysis we have been primarily concerned with the economical operation of a power system. An equally important factor in the operation of a power system is the desire to maintain system security. System security involves practices suitably designed to keep the system operating when components fail. Besides economizing on fuel cost and minimizing emission of gases (CO, CO2, NOx, SO2), the power system should be operationally “secure”. An operationally “secure” power system is one with low probability of system black out (collapse) or equipment damage. If the process of cascading failures continues, the system as a whole or its major parts may completely collapse. This is normally referred to as system blackout. All these aspects require security constrained power system optimization (SCO).
Since security and economy normally have conflicting requirements, it is inappropriate to treat them separately. The final aim of economy is the security function of the utility company. The energy management system (EMS) is to operate the system at minimum cost, with the guaranteed alleviation of emergency conditions. The emergency condition will depend on the severity of violations of operating limits (branch flows and bus voltage limits). The most severe violations result from contingencies. An important part of security study, therefore, moves around the power system’s ability to withstand the effects of contingencies. A particular system state is said to be secure only with reference to one or more specific contingency cases, and a given set of quantities monitored for violation. Most power systems are operated in such a way that any single contingency will not leave other components heavily overloaded, so that cascading failures are avoided.
Most of the security related functions deal with static “snapshots” of the power system. They have to be executed at intervals compatible with the rate of change of system state. This quasi-static approach is, to a large extent, the only practical approach at present, since dynamic analysis and optimization are considerably more difficult and computationally more time consuming.
Power System Analysis can be said to comprise of three major functions that are carried out in an energy control centre:
(i) system monitoring,
(ii) contingency analysis, and
(iii) corrective action analysis.
System monitoring supplies the power system operators or dispatchers with pertinent up-to-date information on the conditions of the power system on real time basis as load and generation change. Telemetry systems measure, monitor and transmit the data, voltages, currents, current flows and the status of circuit breakers and switches in every substation in a transmission network. Further, other critical and important information such as frequency, generator outputs and transformer tap positions can also be telemetered. Digital computers in a control centre then process the telemetered data and place them in a data base form and inform the operators in case of an overload or out of limit voltage. Important data are also displayed on large size monitors. Alarms or warnings may be given if required.
State estimation is normally used in such systems to combine telemetered data to give the best estimate (in statistical sense) of the current system condition or “state”. Such systems often work with supervisory control systems to help operators control circuit breakers and operate switches and taps remotely. These systems together are called SCADA (supervisory control and data acquisition) systems.
The second major security function is contingency analysis. Modern operation computers have contingency analysis programs stored in them. These foresee possible system troubles (outages) before they occur. They study outage events and alert the operators to any potential overloads or serious voltage violations. For example, the simplest form of contingency analysis can be put together with a standard LF program, along with procedures to set up the load flow data for each outage to be studied by the LF program. This allows the system operators to locate defensive operating states where no single contingency event will generate overloads and/or voltage violations. This analysis thus evolves operating constraints which may be employed in the ED (economic dispatch) and UC (unit commitment) program. Thus contingency analysis carries out emergency identification and “what if’ simulations.
The third major Power System Analysis function, corrective action analysis, permits the operator to change the operation of the power system if a contingency analysis program predicts a serious problem in the event of the occurrence of a certain outage. Thus this provides preventive and post-contingency control. A simple example of corrective action is the shifting of generation from one station to another. This may result in change in power flows and causing a change in loading on overloaded lines.
These three functions together consist of a very complex set of tools that help in the secure operation of a power system.