Volts per Hertz protection needs generally arise from different situations than those for which turbine underfrequency protection is provided. Turbine-generator shutdown with the automatic voltage regulator left out of service, sudden load rejection with the automatic voltage regulator out of service, and manual excitation adjustment during startup with faulty metering are events that support the need for volts per Hertz protection.
The protection methods discussed here illustrate volts per Hertz protection schemes that provide protection for both the generator and transformer on unit connected generators. Modern transformers withstand relatively low overexcitation since the flux density is already high at rated values. The most commonly employed methods are discussed.
For hydrogenerators, generator high overvoltages may occur without exceeding the overexcitation capability of the generator. During load rejection the overspeed of hydrogenerator could exceed 150% of rated speed.
If an overvoltage condition is the result of a proportional increase in frequency, the V/Hz relaying will miss the event because the ratio of V/Hz may not have changed. It is general practice to provide overvoltage relaying to shutdown the generators to protect for these high-voltage levels if governor and excitation control fail to operate properly.
Volts per Hertz limiters in generator excitation system controls
The V/Hz limiter is a control feature within the automatic voltage regulator that limits generator field current to hold the generator output voltage to a safe V/Hz value. The limiter will limit the output of the machine to a set maximum V/Hz no matter what the speed of the unit.
This limiter functions only in the automatic control mode. To provide protection when the unit is under manual control, the limiter may also have a relay signal output which will activate any additional protective circuits to trip the generator.
In addition to a V/Hz limiter in the excitation control, it is recommended practice to provide separate V/Hz relaying to protect the unit transformers and the generator. This protection should be as independent as possible from the V/Hz control in the excitation system. If possible, it should be supplied from different VTs. Independent V/Hz relaying provides protection for voltage regulator malfunction or if the regulator is out of service.
EFFECTS OF CURRENT TRANSFORMER SATURATION ON OVERCURRENT PROTECTION COORDINATION
The function of a CT is to produce a
secondary current that is proportional in magnitude and in phase with
the primary current. This secondary current is applied to protective
relays of compatible range and load (or burden) characteristics.
When CTs are operated at or near the
knee of their excitation curve, small increases in current magnitude
can cause the flux density to increase substantially and cause
saturation. When saturation occurs, the secondary current wave shape
becomes distorted, and the signal to the protective relays is no
longer proportional to the current input.
In some cases of severe saturation, the
output current of the secondary could be near zero on one or more
phases. Depending on the level of distortion of the secondary
waveform and the design of the relay, the operation can be affected.
For electromechanical induction disk
relays, the effect of CT saturation is to slow the rotational speed
of the disk. When the CT becomes saturated, the actual secondary
relay current is less than it should be, its wave shape is distorted,
and the relay operates more slowly. This condition leads to longer
trip times and possible miscoordination.
Saturation can occur in CTs used to
measure low-voltage ground-fault current, especially in underdesigned
core-balance CTs in backup ground-fault relay applications.
Saturation has also occurred in the solid-state low-voltage trip
devices that use current sensors (which should not be confused with
CTs, except for the fact that they reduce phase currents to a value
compatible with their devices electronic circuitry).
These current sensors form a residual
circuit for the measurement of ground-fault current. Normal
equipment-starting current or downstream phase faults may produce an
unbalanced current that can cause a false groundfault current trip.
In most industrial systems, CT
saturation is significant only in circuits with relatively low ratio
CTs and high magnitude fault currents. In most cases, these circuits
feed utilization equipment; therefore, relays with instantaneous
settings below the CT saturation point can be applied.
As one progresses back toward the
source, the CT ratios get larger at the same voltage level. Also, the
CTs have more turns; develop higher voltages; and, therefore, are
less likely to saturate when standard burdens are applied. Saturation
of CTs due to the dc component of an asymmetrical fault current can
cause a delay in the operation of some instantaneous relays. It can
also cause false tripping of residually connected instantaneous
ground-fault relays.
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