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Difference between Excitation Limiters and Protection 1

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oresakri

oresakri

Electrical
Joined
Dec 19, 2017
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22
Location
GR
Hello Guys,
Limiters do not replace the protection system but only prevent the protection system from tripping unsecessarily under extreme transient conditions. But how the limiters and protection system cooperate? The limiters operates faster than protection for the same fault condition?
 
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Faster, or at a lower setpoint, or both.
 
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So, why we use both limiters and protection system? Is it possible to use only limiters or protection system by adjusting them ( time and setpoint ) so they operate for transient and steady state?
 
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Well, if you omit the limiters then you will instead rack up trip events if you hit the capability limit of the rotor, which isn't generally a good thing operationally.

If you omit the protection then what happens when the limiter fails to operate correctly and your machine gets in trouble? Could turn very expensive very easily...
 
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Scotty, thanks a lot. So if I'm not mistaken, it's more like first and second zone of protection.
 
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Zone may not be the best word. More like levels of protection or backup protection.
Some would say that a protection scheme for the exciter and another protection scheme for the rotor would be described as zones.
Apart from nitpicking about words, I believe that you undestand the concept.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Yeah, you have the right idea. I agree with Bill that layers is a better choice of words: if the limiters work correctly then the trips should never be called upon.
 
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Let us take in order:

The excitation controller (aka AVR) controls the generator (or motor) excitation (ultimately the rotor field current) to control the generator terminal voyage to a defined level (when generator is single running)

The excitation controller may be fitted with excitation limiters which can over ride the basic settings in order to return the generators operating position to a safe level. Scotty uk posted an example of an operating diagram a few years ago)

The above are control devices.

An excitation monitor can watch the operation of the generator and if the generator does not return to a safe position, it can close a contact to send an alarm or trip signal to the monitoring / protection system. The monitor curve will be set outside the limiter settings.
The monitor operation may be time delayed to prevent spurious operation (false trips)

The above is part of the protection system.

This all has to be co-ordinated by a design engineer.

I hope this helps
 
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Hoxton, you said that " limiters which can over ride the basic settings ". So, limiters don't allow protection to operate(e.g. send a signal to the protection system and for a specific time it is forbidden to trip)? This is little different with Scotty and waross view.
 
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No, I think Hoxton's comments are correct.

If the AVR's primary purpose is to maintain a constant voltage at the machine terminals then the AVR will, at some set of load conditions, reach the maximum current limit of the rotor. At this point the excitation limiter will intervene and override the voltage regulator loop. The terminal voltage will begin to deviate from setpoint if the load increases further as the limiter prevents the rotor current increasing to maintain the terminal voltage at setpoint.

The supervisory trip function is often part of the AVR but is usually constructed to be relatively independent of the AVR control loops, so if the control loop develops a problem then the protection is not compromised. An excitation trip is tied into the generator main breaker trip circuit to prevent the machine operating as an induction generator. It isn't normally tied into the prime mover trip scheme, so that if the AVR can be restored to operation the generator can be put into service, or else it can be brought to a controlled shutdown. In my experience AVR's don't repair themselves, but better a controlled shutdown than racking up a baseload trip and eating up hot parts life.
 
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My utility has a Protection and Control group, and has for years, and in alignment with that thinking I tend to classify schemes as for either protection or control. True enough, there are sometimes interfaces and overlaps; nevertheless...

In line with what Hoxton wrote, I would call a limiter one possible part of a control or operating scheme, as opposed to a relaying or protection scheme. As such, I would not designate these discrete components as comparative layers of protection.

Certainly, as noted by Scotty, control and protection schemes are typically co-ordinated in such a manner that actual equipment trips from protection schemes are avoided such that, to borrow some of the OP's words, the control scheme prevents the equipment from tripping except under extreme conditions.

As to the OP's question "So, limiters don't allow protection to operate(e.g. send a signal to the protection system and for a specific time it is forbidden to trip)?" I would describe that as a very dangerous choice of words! I would much rather say that limiters impose control actions in such a manner as to anticipate and preclude the development of such undesirable conditions as would otherwise cause the equipment to trip completely out of service.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
 
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The generator protection system knows nothing of the excitation system and sits there patiently waiting for conditions to be such that the protection system is called upon to operate. The excitation system does its own thing and when system conditions are such that the base algorithm would have too much or too little current into the field the limiter does its job. If the limiter does its job there's nothing for the protection system to do. If the excitation system fails, the protection system is there to provide a backstop keep things from getting too bad. Bad is bad enough, don't want things to get to horrible.

Trips from the excitation system become very problematic if NERC PRC-005 applies. A Protection System (as defined) monitors electrical quantities (currents and/or voltages) and makes trip decisions. Where Protection Systems exist associated with Elements subject to PRC-005, those Protection Systems have to be periodically maintained. Protection Systems embedded in excitation system may be very difficult to properly test. Best to keep protection out of the control systems.
 
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So the key words i think are, "override the voltage regulator loop". That means that its (limeters) role isn't to protect the generator from overcurrent or overheating, but to prevent an unpleasant situation.
 
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The area of the capability curve where the overall machine capability is limited by the rotor thermal limit is far out to the lagging side of the curve at a fairly low power factor, normally less than 0.6pf. It's not a normal operating point for a baseload generator, but if the machine is being used as a synchronous condenser or to provide reactive support at a weak system node then it is a credible operating condition.

The limiter absolutely is there to prevent the rotor overheating. A burnt out field winding would certainly qualify as an 'unpleasant situation'. It does that by overriding the voltage regulator loop and preventing the field current exceeding the thermal limit of the rotor if the voltage regulator tries to boost it into an unsustainable area of operation.

The rotor is a big heavy lump of copper and iron and has a long thermal time-constant, so a short duration over-current won't have a detrimental effect, and the limiter has a characteristic which allows a short excursion above the maximum continuous rating (MCR) while preventing long-term operation above the MCR. Most AVR's can deliver considerably more current than the rotor's MCR and this additional current is available to provide improved response to transient conditions such as faults on the system, switching events, and the like.

If the field regulator and limiters suffer an internal fault which results in excessive field current (or dangerously low field current, we haven't really mentioned that limiter yet) then the independent supervisory trip will operate to protect the machine when the regulator and limiter is in the faulted state.

 
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Here are some very good papers on the subject I have used in the past for these kinds of discussions,





Here is a paper that directly discusses UEL's and thier system impact,

Kestrel also has some other papers on related topics,
At a Basler conference many years ago a presentation was made discussing this issue as it affected distributed generation, I have looked for that paper but can't find it, it did break down the discussion that the SEL paper discusses above, how the AVR limiters interact with protection settings and impact on the system, as applied to generators in distributed generation applications. Have to thank David Beach (and others at Basler at that time) for that one, helped me a lot in dealing with many issues as we applied DG systems. Digital excitation controls were coming into the size range units we were dealing with at the time and going from a simple OEL in an analog AVR to multiple OEL types and UEL's and the impact of their settings in some of the systems we did presented some interesting problems.

From a general standpoint I'd think the discussion would apply across all sizes of generators, but in practise I have found that some info i get derived from large utility power sources doesn't apply to smaller salient pole machines. What size units and in what application are you dealing with?

MikeL.
 
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Mikel, thank you for your assistance. I found these papers very helpful. Also i would like to add one more. As for the size of my generator, it is a 7MVA, salient pole with 4 poles.
 
 http://files.engineering.com/getfile.aspx?folder=5ec08ce6-1114-4c09-a5e6-f4d4a8bab7f1&file=Coordination_of_Overexcitation_Limiter,_Field_Overcurrent_Protection_and_Generator_Control.pdf
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