Frequency Relay

[rockman7892]

On our man 6500hp 4.16kV wound rotor motor (which I have posted about before) we have an 81 frequency relay. We have recently had what appears to be nusience trips on this relay. The relay I am referring to is a Multilin MIV relay that has this 81 frequency setting. The underfreucncy settings in the relay are set to 59.50Hz with no delay and the overfrequcncy setting is set to 62Hz.

This motor has run twice with no issues and after running the 5kV circuit breaker controlling the motor is racked out. The relay trips seem to be occuring when the motor is offline and sitting idle on the system.

Unfortunately when I connected to the relay to pull the event reports I accidently deleted the event reports ( I am now kicking myself) so I cannot see exactly what frequcncy the relay was at when it tripped.

The questions that I have regarding this relay are two fold:

1) What is the purpose of using this frequency relay on a wound rotor motor? Does anyone have any information as far as reasons why a frequency relay is used. The reasons I can think are that the motor will obviously slow down, and loose power.

2) Do these settings I mentioned above seem o.k.? They seem very tight for the underfrequcny but then again I have heead that you want to keep them as tight to 60hz as possible.

3) The other strange thing is that this motor is operating on a plant that is tied to the same utility point as another plant here On-Site. The two plants are tied together right now so are all operating off of the same utility transformer. On the other plant we have an idential motor with an underfrequency relay with the same setting of 59.50 hz. I would expect to see the same issue in both places however I only see it on the one motor which seems strange.

 

[cranky108]

It is somewhat uncommon, but not heard of for under frequency relays with little or no delay to misoperate during voltage diversions, like system faults. This primarly caused by the way the relays work and the shift in the voltage zero crossing.

In general a frequency relay should have a delay applied, or at least a guard contact or relay, such as a breaker contact.

For non-critical applications, using a frequency relay with no delay is acceptable. Just know that at some time it may misoperate.

However, it is not uncommon for consultants to cheep out on protective applications, because they don't have to live with the results, or the application and results aren't apperently clear (same with plant managers).

 

[itsmoked]

cranky108; I have to take umbrage at the swipe at consultants.

I can't see any reason or even imagine any consultant "cheeping" out nor even cheaping out on any solution.

Consultants usually tell it like it is. The customer is the cheap skate that responds with, "what is the very least we can spend to limp by".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Or...
The customer provides the consultant with a hard cost they must maintain, so the consultant does the best they can with that constraint, picking the places where "cheaping out" can be tolerated. However, the 81 protection function only comes with a Multilin 469 or 369, neither of which I would classify as "cheaping out" by any stretch.

Frequency protection for a motor however is something I would only use when there was a generator supply or the possibility of having 2 supplies that can be transferred from. Is your power coming from a reliable utility through a single substation? If you have a generator, does it have it's own frequency relay? If so, is the redundancy necessary?

In several applications I have done, the consultant specified a Multilin 469, which has the 81 function built-in, but did not specifically state it needed to be used. Then someone comes along later and decides to turn it on "just because", without considering the nuisance tripping consequences. I would start with trying to determine the rationalization of those settings.

By the way, the 81 function on a 369/469 will trip if you have control power to it AND the line voltage in Phase A drops below 30%. So I have seen it where the control power, which from just poor planning, is sourced from phases B and C. Then when the breaker is racked out, the A stab is slightly shorted so control power loss is slightly behind the power loss in phase A. With the 469 still powered for a fraction of a second when the breaker is racked out, all kinds of trip functions including an 81 with be triggered. Having a trip delay would solve that too.


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[cranky108]

OK the consultants I am refering to were in the low bid package. I've just had to fix to many problems, which makes me a little bitter.

Most 81 relays have built in time delays, and under voltage dropout. So I believe there is no reason these were not used.

However, I haven't worked with the Multilin 469 or 369 relays.

The 81 relay need not be applied to most loads, unless there is a vibration, or load problem that needs to be solved.

 

[pittengineer]

The power for this motor is indeed coming from a reliable utility source through a single substation. This motor will never be run off of a generator.

I have questioned the engineering company who specified this relay and their response was simply that it is specified on all of their motors. (A third pary engineer implemented the settings)

So with this source information I am questioning what the purpose of this relay typically is on a motor? What negative effects can happen if the frequency starts to drop? The ones I can think of are loss of HP to the load and possible overflux of the windings? With these tight settings on these relays, is there really possible negative effects with the frequency dropping .5HZ to 59.5?

 

[cranky108]

It's not that the settings may be wrong. It is a matter that the delay and under voltage dropout will make it more stable, and provide fewer misoperations.

 

[jraef]

You are essentially correct in your assessment of the reasoning behind applying an 81 to a motor from a single stable source. Just keep in mind that in many centrifugal pump applications, an increase in speed creates a cubed increase in loading. At the same time, an increase in frequency without an increase in voltage begins to diminish torque. So the combination adds up fairly quickly. So to validate what cranky108 said, the settings are not unreasonable for some applications, but you need to have a delay to avoid nuisance trips. A short duration frequency change is not going to hurt the motor.

So why one and not the other? Did you check to see if there is a delay programmed in the other motor? Does this motor always come on after the other motor is already running, or vice versa? Do you have an energy recovery system on these WR motors? Lots of possibilities there.


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[electricpete]

I am still struggling with the reason for the underfrequency function. 3 ideas from most likely to least likely.

1 - If it is a high-speed underfrequencfy, it might be used for reclosure protection. On loss of power you want to take the machine off-line because re-application of power could result in out of phase with residual voltage. If this is the function, a delay is not tolerable.

2 - Volts/hz protection. More direct is a volts/hz relay, but if no voltage input is available I guess you could settle for frequency. NEMA MG-1 allops 5% variation in freuqency at namteplate voltage so 0.5hz seems very tight. If this is the function, a delay is certainly tolerable as pointed out above should resolve the nuisance tripping.

3 - at our plant the large synchronous turbo-generator is protected by an underfrequency trip. The purpose in that case is to avoid prolonged operating at resonance. Yes, the machine passes through that region during startup and shutdown, but only briefly. But I have a hard time imagining that resonance would be as much a concern for your machine. And the relationship between frequency and mechanical speed is of course a little more complicated for wound rotor asych motor than for sync generator. And the fact that it is used on every large motor doens't seem consisten with protecting from a resonance.

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[electricpete]

Should have added to the 3rd item - if it is for resonance protection, a delay would probably be acceptable.

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[cranky108]

Typical delays from what I have seen is 6 to 15 cycles. But these are utility applications for load shedding.
Shorter delays can and will misoperate.

At 6 cycles delay the relay should have 12 samples, where at 15 cycles the relay will have 30 samples.
Below 3 cycle delay the relay can be fooled by voltage shifts.

Even induction motors will support the voltage for a number of cycles, and if capacitors are applied they can self excite, but because of the load this will be a decreasing frequency.

If this is to protect for reclosing, a delay of 8 cycles won't hurt, as reclosing should not be taking place for at least 20 to 30 cycles to allow for the arc plasma to disapate from the fault.

 

[electricpete]

Thanks. There are good points. It certainly takes some finite period of time to examine the waveform and conclude with high certainly that frequency is low or power is lost. I definitely shouldn't have said a delay "is not tolerable".... what I was trying to express was in the first case you need to be careful that introducing a delay without understanding the function because it could defeat the purpose of the protection. I know if the interruption/reclose is very very brief than the machine doesn't have time to drift out of sync and there is no problem to reclose. As the interval gets longer the machine has chance to drift out and you can get trouble. Exactly how long an interruption would be tolerable before we get in the danger zone.... I've read some discussion on that would have to go back and refresh my memory.

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[electricpete]

hurt, as reclosing should not be taking place for at least 20 to 30 cycles to allow for the arc plasma to disapate from the fault.

As I recall some power stations have a fast bus transfer scheme where a momentary power interruption can occur as a result of an in-house power transfer among alternate sources (fast bus transfer). We don't have it at our station, but I've read about it at other stations. I think in that case the duration of the interruption can be much shorter. Maybe someone else is familiar with that and can comment. Also are there other examples where we cannot assume duration of interruption will be at least 20 cycles?

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[oldfieldguy]

I don't know about "wound-rotor" motors, but on large synchronous motors underfrequency and reverse power trips are not uncommon. They're in place to actively open the feeder in case of loss of power supply due to a fault where the spinning mass of the excited synchronous motor would generate several cycles of power back into the system. Naturally this would be reverse power and rapidly decaying frequency.

old field guy

 

[RRaghunath]

rockman7892,
I agree with oldfieldguy. Are you sure the motor is not synchronous type (Synchronous motor could also be called wound rotor motor I guess)!!
Under frequency / over frequency protections for an induction motor (squirrel cage or wound rotor type) do not seem much relevant.

 

[waross]

Electric Pete;
In regards to your question concerning the reson for under-frequency trip:
I worked at a very large mine mill on an large island. The utility had a lot of generation in a number of locations on the island. There were also undersea links supplying energy from the mainland. If for any reson the undersea links were lst, te island would then become "islanded". Our plant was one of the largest consumers on the island. The utility installed an under-frequency relay on our plant (And probably on other large industrial plants, but I can't say for sure about the others) to trip us off line if the frequency dropped below about 57 Hz. We had our own under-frequency relays that would trip us off at about 58.5 Hz. If we tripped ourselves we could put ourselves back online at reduced capacity (with probably a call to the utility load dispatch center.) If the utility relay tripped at57 Hz., getting back online was a lot more onerous and time consuming.
This may not apply to Rockman.
As has been suggested, they may be there because they are a feature of the multilin and no-one has ever set them up properly.
Rockman, you mentioned that the issues appear to be happening when the unit is offline and the breaker is racked out. I like jraef's suggestion of uneven breaking of the control contacts.
If the motor is off-line, can you try racking out and in and see if this is initiating the trip?



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill. That is another possible reason for underfrequency protection along with the ones I have listed: load shedding to protect the system. In that case it would be a utility relay and would seem to me very unusual to install it for an individual load.



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[waross]

Pete;
The idea was to shed your load ahead of the utility trip to try to avoid a utility trip. Both the plant and the utility were a lot happier if enough load could be shed by the customer to avoid the utility relay tripping.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

To answer some of the questions above:

Yes this motor is indeed a wound rotor Induction motor.

I checked to see if there was a delay on the other motors frequency relay and found that it is also set with no delay. This relay however is an electromechanical type relay. The relay I am using on the new motor is an MIFII Multilin relay.

We have racked the breaker in and out and were not able to see the frequency relay pick up when doing so.

The breaker stayed racked in over the weekend and in the open position, and I came in this morning to find this MIVII relay tripped on a frequency pickup. I looked at the event file for this occurance and found that the frequency reported when the relay tripped was 58.922Hz. All voltages at the time of trip looked o.k. The event report doesnt give you much information nor does it give any waveforms. The time that this relay tripped was after the motor had been running and then was stopped.

Usually after this motor is stopped the other similar motor is started. I'm not sure if there is any coorolation there but will monitor it. I dont know that this has happened everytime so will have to go back and look at this if it happens again.

I took the advice of others above and set a time delay in the relay. I went with about roughly 15 cycles as a delay so I set the time delay to 250ms or .25sec. Hopefully this delay will help overide these nusciance trips. I will continue to monitor this relay and report any other findings.