Excitation of synchronous generator and Generator’s Breaker 4

[xalouris23]

Hello all,

I am new to group. I am experiencing a design problem where in case of failure of protection relay of a High voltage circuit breaker of synchronous generator, the breaker remains closed (since protection relay is in failure, of course no protection is active and breaker cannot be opened except locally) and due to “poor” design I would say, AVR is also receiving signal from protection relay to de excite the generator.

6 Generators are in parallel to the busbar. That means in short that in case of protection relay failure of one HV generator, generator breaker will remain closed, prime mover diesel engine will be running and excitation of generator will be off!

Any thoughts on the effect the generator will have without excitation while still connected to busbar and prime mover still running?

My understanding is that in this case, generator stator, as is still connected to the grid, will become field excitation and rotor windings will become armature. Prime mover RPM will not be exactly at synchronous speed with busbar and some slip is to be expected (droop mode) so I guess induced AC voltage will be generated in rotor’s poles and potentially big current throughout the rotor windings, rectifier and excitation’s armature windings. Basically creating an induction generator. Any input on this? Do you agree?
Is this a potential situation that rotor can be damaged due to excessive heat after some time?

DG is 20MVA @11kV
Any input is appreciated.

 

[waross]

Amortisseur Windings

Amortisseur windings are bars which are found in the rotor of synchronous motors. These bars are short circuited similar to the rotor windings in a squirrel cage motor. The function of these windings is to dampen the torsional oscillations in the rotor that may occur as a result of load fluctuations. They are also known as damper windings.

The Amortisseur windings can also be used to start the synchronous motors. The Synchronous motor is not self-starting. Hence, motor starts initially as an induction motor through the action of the amortisseur windings. When the sufficient speed has been attained, the excitation to the rotor of the synchronous machine is switched on. The motor then runs at the synchronous speed as a synchronous machine.

The generator will be running on the damper winding as an induction motor as you anticipated.
I seem to remember a warning that synchronous motors may overheat if left running too long as an induction motor.
However, the energy required to spin the diesel is so little compared to the rating of the machine that it probably will not overhat.
Of more concern is the prime mover.
Diesel engines depend on the pressure of combustion to force the rings into good contact with the cylinder walls.
In some designs there are small ports to direct the combustion pressure to the back of the rings.
With light load or even worse, no load, the rings may not seal well and the engine may start to pump oil out of the exhaust.
This is called "wet stacking".
There are two dangers;
1. Fire from the oil pumped out of the exhaust stack.
2. The engine running out of oil. I have seen a small diesel lose enough oil to trip off on low oil pressure in an hour or so. That was just a light load, motoring is worse.
Not all engines do this.
I have cured wet stacking in new engines by load banking for several days.
There may not be a cure for an older engine that is wet stacking.
If motoring continues, wet stacking may run to destruction of the diesel engine.
Have you considered a stand alone "Reverse Power Relay" as backup protection?


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

 

[RRaghunath]

Agree if the excitation is OFF but the prime mover is available, the Synchronous generator will continue to work providing watts to the system and drawing VARs from the system (for providing excitation).
Under such circumstances, there is no impact on the prime mover and the generator is also capable of working as induction generator for a few minutes without suffering any damage. It is the power system which is supplying VARs to the generator which is likely to suffer with severe voltage dip. Of course, it depends on how strong the system is.
It is expected that the Loss of excitation protection will operate and isolate the machine in a matter of a few seconds.
If the protection relay is faulty and there is no Main-2 protection, it becomes the operator's responsibility to notice the abnormality and isolate the machine.
There also seems to be an error in the Generator control scheme. In all the machines I have seen the excitation breaker opening scheme is enabled only after confirming the main circuit breaker is open. This is so even though the commands from protection relay are issued simultaneously to both main and excitation breakers. You may want to check.

R Raghunath

 

[xalouris23]

Agree that reactive power will be drawn from network from other connected genset of course. Voltage fluctuation are expected but I don’t know if the other power plant gen sets will trip considering 5 more genset running in parallel of same rating. I suspect not…
Excitation breaker is closed but AVR is receiving faulty signal for de exciting similar to a normal d’excitation in case of differential protection. So it will de excite normally the machine.

Reverse reactive backup relay is not an option.
There as way to fix it by adding output contact from protection relay to tripping coil etc but my point is not to solve the problem but the consequences to the generator if this happens.

But your input is good and I will check if there is any function in AVR that blocks de excitation if generator breaker is still closed and for the moment I don’t see anything like this in the drawings.

Normally I would say that AVR should be programmed in a way not to de excite if genset breaker is still closed.

 

[edison123]

IEEE 492

Complete loss of excitation on an operating generator results in dangerous overheating of its rotor within a few seconds unless the machine is disconnected from the system. The degree to which this heating will occur depends on the initial load on the generator and the manner in which the generator is connected to the system. When excitation is lost, the generator tends to overspeed and operates as an induction generator.

This overspeed normally results in a reduction in load due to the characteristics of the turbine and governor, an increase in stator current associated with low voltage at the generator terminals, and high rotor currents. These rotor currents will flow through the field winding (provided the field circuit is not open), and also through the amortisseur windings and rotor pole faces. The amortisseur winding and rotor pole face currents will cause high and possibly dangerous temperatures in a few seconds.

If the loss-of-field condition has persisted for some considerable or unknown length of time, the rotor should be inspected before operating again.


We did a rewind of a 15 MW two pole rotor whose winding and pole faces got cooked when it ran without excitation for a few minutes. The damper winding and a few wedges got melted and molten metal was sprayed around the stator end winding.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Thanks Muthu Sir, for sharing your valuable experience.
True, it depends on what was the load prior to the event and how long the event was allowed to last before the machine is isolated.

R Raghunath

 

[edison123]

You are welcome, Mr Raghunath. The worst case I have seen is a 50 MW hydro generator completely destroyed (stator & rotor windings, stator core, DC exciter, bearings, turbine everything damaged beyond repair) due to asynchronous operation with field maintained for hours due to simultaneous governor & breakers malfunction and the operation & maintenance team just stood by and watched the destruction of machine from afar. Text book case of what IEEE defines as the most dangerous operating condition.

Muthu

http://www.edison.co.in

 

[xalouris23]

Very nice input and experience Mr Muthu! Basically as I thought and expected actually confirmed in real incident!
Appreciated!

 

[xalouris23]

@Mr Muthu:
But this is for hydro generator and I don’t know how hydro is working as prime mover… For a gen set of 6 DGs, the prime mover, in any case even without excitation, will rotate close to synchronous speed. Is it the same for a hydro? Is it in hydro that you have severe difference of RPM if excitation is lost?

We will for sure not test but maybe our case with diesel generator is a bit softer since prime mover will oscillate I suspect around synchronous speed.
Maybe coupling might break…
@Mr Muthu: How is it possible that nobody manually operated the breaker? Was is “welded” or had any mechanical problem on the tripping mechanism?

 

[RRaghunath]

Thanks Muthu Sir!
The rotor in a Hydro machine is supposed to have higher thermal inertia. It takes a lot to destroy that rotor.
I can understand, such things do happen in the industry.

R Raghunath

 

[xalouris23]

Also if I understand well your incident is a bit different.

In IEEE 492 it is considering asynchronous machine and not regular synchronous self excited generators with cylindrical poles (depending on engine usually 10 or 12)and diesel engine as prime mover:

Reference:
General recommendations for the operation, loading, and maintenance of synchronous hydro-generators and generator/motors are covered. This guide does not apply to synchronous machines having cylindrical rotors. In this guide, the term hydro-generator is used to describe asynchronous machine coupled to a hydraulic turbine or pump-turbine.

I have no idea how hydro gen set is operating….Are you having synchronous generator?

 

[Gr8blu]

There are four basic rotor winding configurations for an AC generator application, independent of prime mover type. These are: 1) synchronous salient pole, 2) squirrel cage induction, 3) wound rotor induction, and 4) permanent magnet rotor. In the synchronous rotor, excitation comes from a separate source (which may be a dedicated external source feeding through a sliding mechanical contact OR an on-shaft rotating rectifier). For squirrel cage, the excitation comes from the current induced by the magnetic field originating in the stator winding. For wound rotor-induction, it could be either (depending on whether the rotor is operating short-circuit or not). And for the PM design, the excitation is always "on" as it is a function of the magnetic field of the PM material.

If the synchronous rotor loses excitation, it operates just like the squirrel cage induction machine (i.e., on the "cage" winding) for a short period of time. When it does so, it is trying to create the full field effect from a winding that was never meant to do so. As it tries, current flows in the cage winding, resulting in heating of both the bars and shorting rings. More importantly, it also heats up the area of the braze where the bar and ring are joined. This brazed joint has a much lower thermal capability than either the bar or ring - which means it is the first point of failure. The generator will try to "push" power to the connected bus as long as the prime mover can physically turn the rotor at sufficient speed. Only after the speed drops to something below synchronism will the machine change over to motoring and begin drawing power from the bus. For rough orders of magnitude: the bars may see temperatures in the 300 C range while rings see 50 C - which translates into a joint temperature somewhere between 650 and 825 C (which is hot enough to melt the actual braze filler material).

If the excitation (on a squirrel cage induction) design is turned "off", then all power to the stator winding is removed. There will still be residual magnetism in both the rotor and stator iron, which when coupled with the prime mover turning the rotor, will cause current to flow in the stator winding. If it is enough, power will flow from the generating unit to the bus; if not, the bus will supply power and the machine will attempt to motor against the prime mover.

If the machine was a permanent magnet design, the field strength (aka "excitation") cannot be simply "turned off". This is why extra precautions are required from a protection standpoint with PM designs in all rotating machine applications.

Regardless of the type of machine, the ability of the unit to produce at the correct line frequency (and voltage) is going to depend on the interaction between the automatic voltage regulator(AVR) and the prime mover speed controller. Usually, the prime mover will try to maintain speed (in order to maintain line frequency) up to the point it can no longer do so (i.e., mechanical overloading).

Last - to answer the OPs question on hydro units: until the upstream water inlet valve is closed and the incoming piping between the valve and impeller is drained of liquid, the impeller will continue to drive the generator rotor at some speed (which might be near synchronism and might be considerably above that), thereby "feeding" energy into a fault on the stator side, even when the stator winding is disconnected from the associated distribution bus.

Converting energy to motion for more than half a century

 

[edison123]

OP
There are two types of synch generators. Cylindrical rotor (mostly 2 pole and a few 4 pole of mega machines) and salient pole rotor (4 poles and above). Diesel generators are salient poles and same as hydro generators. Loss of excitation affects both type of rotors as shown above. Asynchronous operation with field maintained is very different from loss of excitation operation and the former is the most dangerous operation. Any prime mover can lose speed control due to governor malfunction. In the case I cited, there were a series of cascading mishaps including lack of human intervention leading to a disaster.

Raghunath
Rotor winding is subject to the same thermal limits as the stator. See below the rotor failure which occurred in multiple poles. I personally saw the shocking aftermath and submitted a failure analysis report to CEA. On my recommendation, they scrapped the entire unit and went for a new one. It was long time ago but I still remember the disaster.

Muthu

http://www.edison.co.in

 

[wcaseyharman]

Lots of great answers here: in summary, yes, it will eventually damage the generator rotor due to asynchronous operation.
You also might thermally damage the stator as well due to excess armature current and possible end iron heating if left long enough.
It may also cause major issues with your electrical system as the generator will absorb tons of VARs and pull the system voltage down.
Sounds like a redundant relay installation is in order, or at least a breaker trip based on relay failure.

Casey

 

[Hoxton123]

Surely what is being discussed is similar, if not identical to 'breaker fail philosophy'?

Breakers can fail to operate for a host of logic failures.

Protection relay systems can fail to operate for similar reasons.

We had a mines gas powered small power station in UK midlands, it comprised 3 x 4.125 MVA 1000 rpm 11kV generators, driven by low compression spark ignition gas engines. Connected at 11kV to the utilities 11/33 kV transformer.

Operation was base load 24/7, assuming that there was adequate gas pressure.

The station was operated on automatic, although there was an operator/ maintenance engineer on weekday day shift.
Early one evening there was a reduction in fuel gas pressure and the main control panel command one set to stop.
The sequence went correctly, load ramped down, breaker open command, fuel off, Auxiliaries run on to cool set down.

Only problem was that the breaker control handle was in ‘manual’ not ‘auto’…

So the salient pole generator ran on acting as a motor, probably a combination of remanent magnetism and the damper winding.

Being a low compression engine, the ‘motor’ powered the engine quite adequately…. All night.

Next morning the operator arrived and discovered the problem.

Generator engineer was summoned, and the rotor was removed for factory inspection, which it passed.

The engine had to be stripped at the top end as the lack of combustion pressure has resulted in lube oil blow by, and the manifolds were full of oil.

Quite a few lessons were learn from this, including the fact that a breaker not changing status when commanded was not issued as a fault to the remote monitoring station. It was a local alarm, but there was no one there to take action. So we added a breaker mal-operation protection to the logic so that if a generator breaker failed to operate after a definitive time (10 seconds I think) then the plant was shut down and the main outgoing breaker was opened.

No doubt this is one of those lessons that has to be relearnt every thirty years after the old guys retire!