Paralleled Generator Protection

[Josh111]

What is the difference between a reverse power relay and a reverse KVAR relay?

 

[subtech]

The reverse power relay(32R) s a standard protection feature for most all synchronous generators. It measures and monitors direction of power flow at the generator terminals.
Power (watts) from the gen to the system is normal. Power flow from the system into the gen is abnormal and very much undesirable. The reverse power relay (if set correctly) will sense the flow of real power into the gen and trip a breaker to disconnect the gen from the system. Power can flow back into the gen if drive from the prime mover is lost (no steam, diesel engine dies, etc.)

The reverse VAr relay protects the gen from a loss of excitation. When the excitation system is lost, VAr will flow from the system back into the gen. When this happens, induced currents at twice the system frequency will flow in the rotor windings and rotor structure. This is very bad because the impedance of the rotor structure is generally high. When AC currents flow there, much heating results.
The heat can and will damage the rotor windings, and possibly even the rotor forging.
So, VAr out of the gen is good/normal
VAr into the gen is BAD/abnormal. The loss of excitation relay should disconnect the gen from the system when VAr flows into the gen.

Hope this helps

 

[Josh111]

Many people with previous generator experience tell me reverse KVAR relay is not necessary.Could this be because possibly the reverse KVAR relay is only necessary for brushless synchronous generators which are relatively new?

 

[m.s.j]

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 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.


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[Josh111]

is reverse KVAR relay needed when a generator is run alone (island mode)?

 

[m.s.j]

About LOF relay, YES, Because of electrical system inertia.

 

[Josh111]

please explain. It seems that a under/overvoltage relay would be adequate when generators are ran alone.

 

[m.s.j]

Protection against loss of field is provided using an impedance relay with an offset circular characteristic on the R-X diagram.
It is possible to provide the same function using a reverse VARs power relay, which should be time delayed sufficiently to obtain stability under conditions of reactive power oscillation.
When a direct measurement of field current is possible using a shunt, a current relay may be used, giving over and under current protection.
In the islanding condition, the problem of reverse VAR is cable capacitance where supplies the load VAR demand allowing the generator to continue at unity power factor. This scenario is feasible where feeder have long cable runs and low load density and could also arise overrated or non-switched customer power factor corrector capacitors are installed.
Application of overhead cables is being considered in rural networks. This will combine the issues of low load density and circuit capacitance.
A further and more limiting problem with reverse VAR protection is its application where the island zone consists more than one generation site. The reason for this problem is that each generator in the island zone will have independent and differing AVR response characteristics. The response of each generator depends on its location in relation to the load and the other generators and the gradient of its voltage droop charecterisic. Under certain conditions it is clearly possible that one or more generators in the island do not experience a voltage depression and hence do not export VAR into the island.
In light of the problem of parallel generation and possibility of supporting VAR demand from other sources , it is considered that reverse VAR relays cannot be widely used for loss of mains protection.

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[Josh111]

If no capacitor banks are present and the power factor is less than 1 but is .8, is reverse KVAR relay still necessary if generator is islanded and not even ran in parallel with other generators?

 

[m.s.j]

Excuse me, I cannot recommend what you want; I just have some general information. For technical recommendation you must refer to generator manufacturer.
According to some normal practice, the illustrated ANSI code function in attached figure are used for low and medium power generators.
Field loss is detected either by a delayed reactive overpower protection unit (ANSI 32Q) for high power rating systems or by an under impedance protection unit (ANSI 40) for “islanded” power systems with generators, or by direct monitoring of the excitation circuit if it is accessible (ANSI 40DC).

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