In power system, operation in the leading mode (drawing Vars into the generator from the power system) can be required during periods of light load. High-voltage transmission lines are highly capacitive when lightly loaded. The Vars produced by these lines can raise system voltage to unacceptable levels. Under such conditions, generators are operated in the leading mode to absorb excess reactive power, thus controlling system voltage.
When a relatively large generator is connected to a weak system, the unit may be required to operate in the leading mode to prevent overvoltage caused by the transmission of real power across the system.
The most obvious limit to leading Var operation is the bottom portion of the generator’s capability curve, but several other limits exist. Var flow is dependent on the difference between the system and generator voltage and the impedance between the two. Leading Var operation requires generator voltage to be less than the system voltage. The minimum allowable continuous generator terminal voltage, which is 95% rated, can therefore limit the leading Vars a generator can absorb during normal operation.
The lower portion of the generator capability curve defines the leading Var limit of a generator.
Leading Var capabilities for round rotor and salient pole machines differ, because they are set by different physical limitations within the machine. In the salient pole machine, the ampere rating of the stator winding limits leading Vars, while heating in the ends of the stator core material limits the round rotor machine. Figure below is a generalized capability curve showing the relative limitation for a hydro generator, which is a salient pole machine, a steam-turbine generator, and a gas-turbine generator. The latter two machines, which have significantly less leading Var capability, are round rotor machines.
The minimum excitation limiter (MEL) is unique in that it is not a protective relay, but rather a control function integral to the automatic voltage regulator (AVR) circuitry. It acts to limit Var flow into the generator. During normal operation, the AVR maintains generator voltage at a preset value.
When system conditions attempt to produce Var flow into the generator in excess of the MEL setpoint, the MEL activates, overriding the AVR setpoint to increase terminal voltage which reduces the Var in-flow. The voltage increase continues until Var flow is reduced below the MEL setting.
Of course there is a impedance relay type for generator loss of field protection (LOF) .Impedance-type LOF relays are applied at the generator terminals to detect a complete failure of the generator excitation in the form of a loss of DC voltage or short circuit. Such a failure will collapse the internal generator voltage and result in Var flow into the generator far in excess of the generator rating. The LOF relay is designed to recognize this condition and trip the generator, usually within one second of the failure.
This type of LOF protection can miss operate when system disturbances caused large transient Var flow into the generator. Coordination of the MEL and LOF characteristic assures that any disturbance that would cause a large influx of Vars into the generator, thus emulating a loss of field, would first actuate the MEL.
The MEL shall be set to prevent operation beyond the leading Var capability of the generator. The MEL and LOF relay characteristics should not overlap. This will ensure that power system swings that produce leading Var transients actuate the MEL before the LOF relay. This does not ensure that the MEL will respond with sufficient speed to prevent a LOF trip for such as swing.