The equal area criterion is a simple graphical method for concluding the transient stability of two-machine systems or a single machine against an infinite bus. The Equal area criterion is a “graphical technique used to examine the transient stability of the machine systems (one or more than one) with an infinite bus”. PDF | The classic equal-area criterion (EAC) is of key importance in power system analysis, and provides a powerful, pictorial and quantitative.


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Since Pm remains constant, the accelerating power Pa becomes equal to Pm.

Equal Area Criterion

The difference in the power gives rise to the rate of change of stored kinetic energy in the rotor masses.

Thus the rotor will accelerate under equal area criterion constant influence of non-zero accelerating power and hence the load angle will increase.

The power will then revert back to the normal operating curve. At that point, the electrical power will be more than the mechanical equal area criterion and the accelerating power will be negative. The objective of this study is to find out whether the load angle comes back to steady value subsequently clearing of the disturbance.

  • Equal Area Criterion for Stability

Here, non-linear equations are solved to determine the stability. The Equal Area Criterion is concerned with transient stability. It equal area criterion in fact a very easy graphical method used. It is for deciding the transient stability of single machine or else two-machine system against infinite bus.


Equal Area Criterion for Stability Over a lossless line, the real power transmitted will be Consider a fault occurs in a synchronous machine which was operating in steady state.

Here, the power delivered is given by For clearing a fault, the circuit breaker in the faulted equal area criterion should have to be opened up.

Equal Area Criterion for Transient Stability - Circuit Globe

The equal area criterion mover which is giving the input power is driven with the steam turbine. For turbine mass system, the time constant is in the order of few seconds and for the electrical system, it is in milliseconds.

Thus, while the electric transients take place, the mechanical power remains stable. The power angle curve is considered which is shown in fig.

Power System Analysis

When a fault occurs; the circuit breakers opened and the real power is decreased to zero. In this case, the area A2 is less than the area A1; the system will become unstable.


The problems associated with the transient stability of the system is because of the following reasons. These are as follows Sudden change in load Switching off one of the lines which cause a change in the reactance of the system and hence equal area criterion change in load conditions.