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Alternator tripped by reverse reactive power

Alternator tripped by reverse reactive power

Alternator tripped by reverse reactive power


An alternator 950 kW 400V synchronized with utility network has tripped by reverse reactive power. There was no disturbance on the network. Plz see attached in the doc list of protection in the alternator relay Micom and the sequence of event.
Just before the trip, the boiler was operating well. There was no drop in the turbine steam pressure.
There are reverse active power and reverse reactive power protection in the relay.
Can you tell us what may be the cause of reverse reactive power?

RE: Alternator tripped by reverse reactive power

Generator voltage too low at the time of synchronization is the classic cause.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Alternator tripped by reverse reactive power

The problem did not arise when we were synchronising the alternator.
The problem occurred when the plant was in normal operation. Everything was stable on the alternator, boiler and the network.

After the trip, we resynchronised the alternator and everything is working well up to now for 3 days.

RE: Alternator tripped by reverse reactive power

The one I'm familiar with is long, lightly loaded overhead feeders and a single set running (of multiple sets) in an islanded remote network.
Not much load on the far end of the feeder tends to end up with a rise in voltage which gets seen as an excess import of vars and off it goes.

Whether that applies to your situation is unclear from your description of your plant.

EDMS Australia

RE: Alternator tripped by reverse reactive power

Do you have a Sequence of Events Recorder [SER ] to offer further enlightenment? FreddyNurk's suggestion is a highly plausible possibility, but you'll need more evidence, viz., relay targets, SER traces and the like.


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Alternator tripped by reverse reactive power

An import of reactive power by an already synchronized generator can only
happen if you loose the FIELD (DC excitation) to the machine. When the DC field is lost,
the sync generator becomes an induction motor, which draws a large amount of reactive
power to create the flux. ANSI 40 (LOF) protection will look after this incident.
I am not sure whether the ansi function of your relay had been enabled.
Hope this helps.

RE: Alternator tripped by reverse reactive power

Or the field is too low because of a failed field controller.
Or loss of power to the field, such as a loss of station power, such as a loss of power to the field controller.

I have seen the AC power turned off to the field controller, because it was not labeled in the AC panel.

RE: Alternator tripped by reverse reactive power

The incident has relationship to the
* utility condition at the time of occurrence - was there overvoltage?
* Alternator AVR operation mode - was it in 'Manual' mode or any of the excitation limiter alarm existing already
* protection settings for Reverse reactive power protection - What was the basis for the reactive power pickup threshold setting & Was there time delay provided
You can take above points for further investigation.

RE: Alternator tripped by reverse reactive power

Reactive power flow is a result of a voltage mismatch between the generator commanded voltage and the grid voltage.
When the generator voltage setpoint equals the grid voltage, there is no reactive power flow.
The generator is not able to control the voltage of the grid.
If the generator voltage setpoint does not match the grid voltage, there will be a compensating flow of reactive power.
The root cause of a reactive power reverse flow trip is either:
1. The grid voltage has gone up.
2. The generator field excitation has gone down.
3. A relay is failing.
- Check the relay including the wiring and CTs and PTs.
- Investigate the possible cause of a voltage mismatch.

Ohm's law
Not just a good idea;
It's the LAW!

RE: Alternator tripped by reverse reactive power

Be aware that by grid voltage, the meaning is the high voltage terminals of the generator, and not the high side of the generator step up transformer.
The step up transformer will consume vars itself, and if that is not considered, your calculations can be wrong.
This is important, because many times the meters only show the high side of the step up transformer, and not what is happening between the generator and the step up.

RE: Alternator tripped by reverse reactive power

Good catch Cranky. One of those small things that may become a big thing.

Ohm's law
Not just a good idea;
It's the LAW!

RE: Alternator tripped by reverse reactive power

Faulty PT signals incorrectly indicating a higher-than-actual bus voltage, faulty AVR retarding the firing pulse too much or external system conditions. Narrow it down to inputs, outputs, or control/feedback issue. Our generators regularly take in VARs as part of system operating conditions. We are on the supply side of a HVDC link to the load center, so our "normal" operating conditions are not always normal.

Problem with metering inputs feeding into the AVR, problem with the AVR itself, or normal response to abnormal system conditions. Figure out which and troubleshoot from there.

Or possibly faulty protection as well. Protective relays should be tested.

RE: Alternator tripped by reverse reactive power

I will also add that I likely wouldn't recommend an attempted re-sync to the grid following a protection trip like this.

Excessive VARs out (over-excitation condition) had a basic worst case scenario of overheating and insulation breakdown over time, possibly volts/hertz damage.

Excessive VARs in (under-excitation condition) has a worst case scenario of loss-of-sync and pole slipping, the result of which can be catastrophic damage to the stator, and this can happen very, very quickly, almost instantaneously.

RE: Alternator tripped by reverse reactive power

After a check we found that there is a difference in the setting of reverse reactive power and voltage in the AVR and the alternator relay as follows:
Alternator relay voltage max. 432 V
AVR voltage max. 440 V
Alternator max. reverse reactive power: -34.2 kVAr
AVR max. reverse reactive power as attached table.

Do you think if we set the max AVR voltage and reverse reactive power slightly less than the alternator relay this may solve the problem? e.g. 430 V & -33 kVAr

RE: Alternator tripped by reverse reactive power

What is the model of MICOM relay installed?

RE: Alternator tripped by reverse reactive power

Micom P40 Agile

RE: Alternator tripped by reverse reactive power


RE: Alternator tripped by reverse reactive power

From your photo the relay is MICOM/ P343 Gen Prot relay.
As I mentioned before it is having ANSI#40 loss of field relay.
There is no other function in the relay for reverse reactive power.
Therefore, pl. confirm,
1) ANSI#40 function is programmed in P343
2) The relay tripped by that function.
While running, this function looks after the generator from absorbing reactive power.
Because when the excitation DC field is lost while running the sync gen
becomes an induction motor and absorbs reactive power for its excitation.

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