Generator Trips on High Neutral Current 1

[nickoliver]

A year an a half ago I installed a cogeneration plant at a hospital. The scope involved combining three separate services into one 4000A 277/480V service and all work associated with a 600KW 277/480V 4W generator that parallels with the local utility. There is a heat exchanger that heats a boiler to provide the steam for the central plant as well as 600KW of the campus power needs.
Everything went through commissioning and testing just fine. The unit ran with no problem for a few months. The utility required the hospital to change the power factor of the unit for some reason and after they did the generator constantly trips on high neutral current. The neutral current relay is set for 50A and the generator manufacturer will not budge on that value. After investigating the loads in the hospital it was found they have 277V heaters all over the place in the HVAC system. They turn on and off as needed so the load, and consequently the unbalanced neutral current changes all the time.
The company that has the maintenance contract to service the co gen suggested a neutral current resistor to fix the problem. I have been adamant that that is not only a code violation in this application, that is not what neutral grounding resistors are for. They are to limit current in a ground fault situation. It will not work. The generator manufacturer will not give any reason why there generator can only handle 50A of return current when the alternator itself is rated at 1 MW. (The unit is rated at 600KW because of the engine size) Since changing the load is not an option I am saying the only options are increasing the relay setting to something reasonable or installing a 480-277/480V isolation transformer between the co gen and the hospital service. That is a really expensive option that no one wants to pay for but I can't think of any other way to solve it. It may be cheaper to find the 277/480V panel with the worst current imbalance and install a smaller transformer there but then we have to go through OSHPOD approval and that can take up to a year sometimes.
Is there anything I am not thinking of? Am I unreasonable to call BS on the generator manufactures claim that the neutral current relay has to be set so low? This has been going on over a year and no one will make a decision on what to do. It is all about placing blame right now.

PS: After sketching it out I believe adding a transformer between the co gen and the service won't work either. The grounded neutral on the secondary would create a load side neutral to ground bond of the main service de sensitizing the ground fault system.

 

[edison123]

May be the generator OEM is trying to protect the machine against excessive negative sequence current (which will overheat the rotor)due to unbalance loads.

Can the user change the single phase heaters to 3 phase heaters ? This will eliminate the unbalanced loading.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[Alex68]

The key point was the change of the power factor as required by the utility.
I suppose that after and before that event the number and the use of heaters didn't change.
But the protection started tripping after that event.

The 3rd hamonic production of a synchro generator changes with the working point of the gen itself. And the 3rd harmonic is a zero sequence harmonic which flows through the neutral if it finds a path.
Probably your protection is not filtered against 3rd harmonic!!
So the use of 1ph heaters causes the flow of zero sequence current and the relay is excited but the setting is not exceeded. But if you add the 3rd harmonic, the setting is exceeded sometimes and the relay trips.

In my opinion you should verify if the relay is equipped with 3rd harmonic filters and the cheapest solution should be adding filters or changing relay.
Let us konw the results of your checks.

Regards

 

[subtech]

hello nickoliver

Enlighten me please. You stated that the poco required the owner to "change the power factor of the unit". Exactly what did the owner do to comply with this request/demand?
Also, can you tell us please the maker and type/model
of protective relay(s) for the new gen.

 

[nickoliver]

Thank you for the replies. So if it is third harmonic current putting the relay over the threshold, we should be able to measure the neutral current with a regular amp meter and a true RMS amp meter and verify this right? We are scheduling one of or service people to go up and take a bunch of different readings to I will have him look into this.
I was out of the loop but I believe they raised the power factor. If I understand this correctly this is done by reducing the DC excitation of the rotor which results in contributing more KW and less KVAR's to the load, correct??
I will make an inquiry tomorrow to the generator manufacturer about the relay type and weather or not there are third harmonic filters.

 

[Alex68]

Instead of an amp meter, it should be better using an oscilloscope or something like a spectrometer. In this way you can check also the distortion.
Please note that varying the excitation you modify the voltage and the reactive power but not the active power.

 

[jghrist]

What kind of connection is used on the utility transformers? If they are Grd Y - Grd Y, you may be feeding unbalanced current on the utility distribution system. Generators have relatively low zero-sequence impedance compared to utility sources which can lead to the generators contributing a disproportionate share of zero-sequence current. A change in power factor may have made the situation worse. Does the problem occur when serving loads not in parallel with the utility?

 

[rbulsara]

Are you actually talking neutral current or ground current? There is no such thing as tripping on neutral current, there is no overcurrent protecton in neutral current.

If this is a solidly grounded system, even a 4000A of neutral current should not trip anything. Manufacuter's suggestion of neutral resistor is full of BS and suggests that they have no clue what they are talking about.

Please look at the one line and acual installation and tell us where the current sensor for this "neutral" current is located. Involve someone more knowledgeable. I suspect this is a grroun fault relay, needs some review and also there may be neutral to ground connections downstream in the field which needs to be found and corrected.

 

[davidbeach]

Alex68, your statement "varying the excitation you modify the voltage and the reactive power but not the active power" is possibly misleading. Agreed that the excitation system has no significant impact on active power. The excitation system can vary either the voltage or the reactive power, but can seldom affect both at the same time.

When running islanded, the system will provide (or absorb) however much reactive power is necessary to achieve the desired voltage with the connected load. Causing the generator to produce more reactive power (without changing the load) will cause the voltage to increase. Likewise a reduction in reactive power will lower the voltage.

When running in parallel with the grid, the generator is unable to control the voltage, so the terminal voltage matches the local grid voltage and the excitation system control the amount of reactive power. Generators parallel like this are generally run in VAr control or Power Factor control. The difference between the two relates to how the VAr output varies when the Watt output is varied.

I've seen the statement about the excitation not impacting the active power output many times, and it is a very good first approximation, but it is not absolutely true. When the excitation system draws its power from elsewhere and not from the prime mover then the statement is true. But when the excitation system draws its power from the prime mover, more power into the excitation system (more field current, more reactive power out) that power comes from somewhere. If the prime mover power output is not increased for a given increase in excitation the electrical power out of the generator will decrease. If the electrical power output is held constant while the excitation is increased the power output of the prime mover must increase.

 

[waross]

A generator manufacturer may want a ground fault current current trip set at 50 amps to limit possible core damage in the event of an arcing failure of the generator windings.
This is not equivalent to ground fault current.
It sounds as if there may be some confusion as to neutral current and internal ground fault current.
I understand the problem.
With a set in parallel with the utility, the neutral will be grounded at a location remote from the generator.
With an islanded generator you can ground the neutral and then monitor the ground conductor for internal ground fault current.
With the generator in parallel with the utility, your neutral connection to ground will most likely be at the main service panel.
I would suggest first try to break the "blaming" atmosphere. We have a problem, let's solve the problem and move on.
I appreciate the generator manufacturers desire to trip on any internal current to ground in excess of 50 amps so as to limit possible arc damage to the laminations.
Simply measuring the current at the neutral terminal is a good way to provide this protection but not the only way.
Also, using the neutral current to infer internal faults will not work when the load consists of line to neutral loads.
Consider installing differential protection on the windings. You may be able to set differential protection at much less than 50 amps, and I would argue this point to the manufacturer when asking for approval of a protection scheme that doesn't monitor neutral current.
To monitor ground fault current you must monitor the conductor from the system neutral to the ground grid. By code there may only be one connection. This conductor will carry any ground fault currents and you will not be able to differentiate system faults from generator faults.
Monitoring neutral current to provide a generator with protection and limit arcing damage works well for three phase loads, and line to line loads.
It will not work with a system with line to neutral loads.
For line to neutral loads you must use a differential scheme.
I have seen 600KW and 1200KW generators running without protection from internal ground faults. I have seen several burnouts on 600KW generators that were repairable. 600KW may be on the small end of the scale of machines that require such protection.
By the way, if the generator was ordered as a 4 wire machine, the 50 amp limit on neutral current seems to be unrealistic.
I believe the change in power factor was coincidental to the tripping problem.
However, if the generator was running with high reactive current and little active current, the generators share of the active load may have been small enough that the unbalanced current was below 50 amps. This is quite possible if the person setting up the generator is not familliar with generators.
If the governor setting was slow, an inexperienced operator may have attempted to increase the load by increasing excitation instead of with the governor setting. This could give a high generator current, but not enough real power to exceed 50 amps unbalance on the neutral.
The same thing may happen if an operator tries to adjust the voltage on a paralleled set.
respectfully

 

[Alex68]

Davidbeach
I'm sorry if I "disgusted" you with my statement. It was just a simplification of the phenomeno for non expert people. The power consumed by the excitation system is negligible compared to the whole produced power only in first approximation.
Your explanation is very good and I agree completely.

 

[stevenal]

Davidbeach,

"But when the excitation system draws its power from the prime mover, more power into the excitation system (more field current, more reactive power out) that power comes from somewhere."

But this reactive power is not real, so no additional mechanical power from the prime mover is required except to overcome the additional losses from the additional current (both field and armature). Do you agree?

 

[davidbeach]

Alex68, I certainly would not say that I was "disgusted", don't know where that would have come from.

stevenal, I was referring to the power consumed in the field; but you are right that if there is more current out at the terminals there will be more real power losses in the generator.

Yes, as a first approximation, change in excitation produces no change in real power. But when you start looking at what's going on in the last percent, you have to consider the power (all real) consumed by the field.

 

[stevenal]

Davidbeach,

I find the the word "consumed" troubling. Power is not consumed, only converted. Any power going into the field must either be converted to heat as losses, or pass through the output terminals. Remember: Conservation of energy is not just a good idea, it's the law.

 

[davidbeach]

OK, point taken. Increased excitation = more field current = more I²T losses in the field = more electrical power/energy converted to heat. If prime mover shaft power does not correspondingly increase the electrical power out the terminals will decrease; if the power out the terminals does not decrease the power in from the prime mover goes up.

 

[GTstartup]

I think we are splitting hairs. In general it should be stated (as Alex68 did) that increasing or decreasing excitation has no (or neglible effect) on power output assuming no change in prime mover input (you cant get something for nothing).

For example, typical quantities of excitation current and voltage for a 100MVA generator with brushless exciter are 15A at 45V DC. If anything, you could get more power delta by the change in generator efficiency. If you saw the # 10 AWG wires running from the excitation system to the brushless exciter you would quickly realize that excitation power requirement should not enter into tis discussion except a purely academic level.

 

[davidbeach]

Your 15A at 45VDC is probably what is going into the rotary exciter, not what is flowing from the rotary exciter into the main field (the current that is actually providing the excitation for the generator). On your 100MVA unit, the main field will be converting about 100kW into heat. That would be something on the order of 200A at 500VDC.

 

[dpc]

Getting back to OP, if generator manufacturer insists on a 50A setting on the neutral current, they have provided you with a 3-wire system, not a 4-wire system. If you have any 277 V loads, the 50A neutral current setting is not going to work.

Why this suddenly became a problem after operating for 18 months is not clear. Is it possible that other changes were made about the same time as the power factor setting change you mentioned? I suppose overexcitation and increased third-harmonic current is a possibility, but most new digital relays will filter that out. But 50A of fundamental neutral current should not be a problem for a true 4-wire system.

If you specified the generator as a 4-wire system, you should asked the generator manuf how a 4-wire system is going to function with this 50A setting.

If you actually have got a 3-wire generator configuration instead of a 4-wire configuration, then that's another matter. Sometimes the generator neutral point is connected directly to the generator frame internally, which makes a 4-wire system a lot trickier to implement.

 

[waross]

Given that the unbalance of the heaters will cause a neutral current.
Part of that current will be supplied by the utility and part by the generator in appoximately the ratio of real power supplied by the generator and real power supplied by the utility.
If the utility requested a change in power factor, you must have been running either badly under-excited or badly over-excited.
Both conditions will create high generator reactive currents with little real power.
When the power factor was corrected, the same level of current would produce much more real power and the generators share of the unbalanced current could increase past the 50 amp set point.
Contact the generator manufacturer and discuss a change from neutral current sensing to differential protection to detect internal faults.
In regards to the change in power with a change in excitation; We don't know if the set was originally over excited or under-excited.
respectfully

 

[nickoliver]

Update:
I received an email today that the problem is resolved. Apparently there was a technician from the generator manufacturer out last week. Why it took the generator supplier 18 months to make this happen is unknown to me. In his report he states he found the neutral current trip "set extremely low at 50A." (Exactly what I have been saying all along) He recommended setting the trip for the maximum output of the generator (approx. 428A) but the supplier requested a more Conservative 120A. That is where it is set today and as far as I know all is well. All this is from the same people that said the manufacturer says it has to be set a 50A and thats it! Time will tell if this is the answer but I believe it is. Thanks for all the input.