Under normal operation of a synchronous generator, the field created by the rotor windings locks in with the revolving mmf of the stator windings and the rotor moves at synchronous speed, at least in the steady state. When excitation is lost, the rotor field suddenly loses its mmf and the rotor begins to move away from synchronism, having lost its strong magnetic coupling with the stator mmf. During this time, the governor is still set to deliver a given amount of power to the generator, so the generator will accelerate, inducing large slip frequency currents in the rotor in order to maintain the power output as an induction generator.

Actually, the power requirement will be reduced as the slip increases due to the governor characteristic, the increase in stator current, and possibly the lowering of terminal voltage, but the total power is still quite large. Also, since the excitation has collapsed, the generator begins to absorb reactive power from the system in very large amounts, which depresses the voltage. This could lead to voltage collapse if the system is weak. The large increase in reactive power, at the leading power factor, creates large stator currents that may reach two to four times the rated current and the rotor begins to overheat.
The degree of rotor heating depends on several factors including the initial generator loading, the conditions causing the loss in excitation, and the way the generator is connected to the system. In a cylindrical rotor generator, the rotor currents will flow through the rotor body and through the field winding, if that winding has been shorted or is connected through a field discharge resistor, and will also flow through the rotor coil wedges.

These currents oscillate at slip frequencies and with magnitudes that are proportional to the generated power. These large rotor currents will cause very high and possibly dangerous temperatures in the rotor in a very short time. In most cases, the time required for these currents to cause serious damage is only a few seconds if the generator has suffered a complete loss of excitation

For hydro machines, there are almost always amortisseurs windings that are designed to carry slip currents, so these generators can continue to operate in the induction generator mode without damage. It is estimated that small solid rotor machines, of 50 MVA or so, can withstand induction generator operation for 3 to 5 minutes without damage but large machines of 500 MVA or more must be tripped in 20 seconds or less. This means that there is no time for an operator to evaluate the problem and determine the proper corrective action. The protective system must be fast in order to prevent severe damage.

There is also a potential for considerable stress to the power system during the transition. The generator that suffered the loss of excitation will draw its excitation from the power system as reactive power. This will depress the voltage at the generator terminals and possibly in the surrounding area if the system is relatively weak. This could cause problems in customer loads and could lead to voltage collapse.

Clearly, it is essential that large steam turbine generators that suffer a loss of excitation be tripped automatically and soon after the loss of excitation.

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