Power systems are used to convert natural energy to convert natural energy to electric power .They transport electricity to factories and houses to satisfy all kinds of power needs .To optimise the performance of electrical equipment, it is impartment to ensure the quality of electrical power. It is well known that three phase alternating current is used to transport electricity .During the transportation both active power balance and reactive power balance are to be maintained between generating unit and the load. Those balances correspond to two equilibrium points: frequency and voltage. When either of the balances is broken ant reset at a new level the equilibrium points will float.

A good quality of electric power system requires both the voltage and frequency and voltage to remain at standard values during operation. For North America the standard values -

Power systems are used to convert natural energy to convert natural energy to electric power .They transport electricity to factories and houses to satisfy all kinds of power needs .To optimise the performance of electrical equipment, it is impartment to ensure the quality of electrical power.　It is well known that three phase alternating current is used to transport electricity .During the transportation both active power balance and reactive power balance are to be maintained between generating unit and the load. Those balances correspond to two equilibrium points: frequency and voltage. When either of the balances is broken ant reset at a new level the equilibrium points will float.

　A good quality of electric power system requires both the voltage and frequency and voltage to remain at standard values during operation. For North America the standard values