Neutral Current Trip

[Nick]

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.

 

[dbuckley]

A delty-wye tranny would lose the neutral current, but thats a high price to pay.

Pretend for a minute you only used one phase on the genset; it does seem absurd that a 1MVA genset can only deliver about 14KVA to a single phase load. You cant in reality expect to get 33% of the rated output on just a single phase load, but 1.4% is taking the mick.

A neutral resistor is a terrible idea; if you lose the supply and the cogen line breaker stays in you've got a wild neutral situation, and one you cant blame the poco for.

Interesting that it used to be OK till the PFC bank got changed....?

 

[robbietan]

could be some of the loads were affected by the power factor change and that the current they are drawing have also changed?

 

[kalanjeya]

If you feel it is happened only by heating load, you can add some capacitor bank to improve the power factor.

 

[kingpb]

Additional info required: How did the pf of the generator change? Did it go up or down? I am not aware of how you would change the rated pf of the machine, although you could certainly modify the controls to change the operating pf.

If your pushing more vars out, to support a poor plant pf, then you have to overexcite the unit, which will increase the terminal voltage. I doubt you are underexciting the unit to take in vars, this would very odd indeed for a genset.

As far as the manufacturer changing from the 50A, forget it, unless you want them to void the warranty. You have to look for the cause of the problem, not simply turn up the protective device and overlook the real issue.

Neutral ground resistors (low resistance) are used all the time for generators of all sizes (so are reactors, but investigate pros and cons). Not sure why you think this is not a good idea. High resistance can also be used, but it is not practical for this install since it requires an extensive system of fault indicators and alarms, with trained personnel available to quickly remove the fault.

From your description, it sounds like you are normally operating in parallel with the utility as a co-gen unit, while operating as CHP. The gen set is more then a peaker unit, or standby. Therefore, since it is running in parallel, it is very likely that you are picking up harmonics, circulating currents, or possibly unbalanced load that are causing the problems.

Work with the genset manufacturer, they encounter these issues all the time, and their engineers should be able to help you resolve the issue. The down side is you will probably have to add some additional equipment.

Think about putting in a grounding switch that can be be opened when in parallel, but will automatically close if the utility power is lost.

 

[don_resqcapt19]

king,

As far as the manufacturer changing from the 50A, forget it, unless you want them to void the warranty.

Why would anyone even make a 600kw four wire generator than cannot support 4 wire loads?

Neutral ground resistors (low resistance) are used all the time for generators of all sizes (so are reactors, but investigate pros and cons).

You can't use grounding resistance or impedance on a grounded conductor of a four wire system where the grounded conductor is used as a circuit conductor. The code requires this to be solidly grounded.
Don

 

[winnie]

1) What is the neutral to ground bonding arrangement that someone would suggest a neutral resistance? Surely they were not suggesting a resistor in _series_ with the neutral to reduce normal neutral current, so I presume that they are talking about a resistor between neutral and ground. Aside from the code issues of resistance grounding, for this to help at all suggests that they believe that the neutral is bonded both at the generator and at the service...and that latter would be a problem.

2) I've not worked with such systems, and don't know the engineering details, but wouldn't a 'zig-zag' autotransformer be another choice for deriving a neutral? A transformer sized to provide 100A at 277/480Vof neutral current would be about the size of a 50 KVA isolation transformer, if I understand how these things would be sized.

-Jon

 

[bob]

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.

I would check an see if the capacitors are causing the problem. Are the caps
at one location or distributed? Connected delta or wye? Have the caps ever tripped the OC device? Get a neutral reading and let us know.

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.

You are absolutely correct.

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). It may be cheaper to find the 277/480V panel with the worst current imbalance..

I would think you would have already done this. However, with the heaters
turning on and off these reading will change continuously

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.

I would arrange a meeting with the hospital management and generator
management and insist that this problem be solved. This is not something
where the generator people can just sit back and do nothing. They need to step up and assist in finding a solution to the problem.

 

[brian john]

This gets a bit out of my realm of expierence, but I had a similar problem before with a cogen facility. Their generators had a different winding pitch (our gen/paralleling tech figured this out and tried to explain it to me) from the utility's and as certain load were placed on line, the neutral current would increase. (the neutral current was high in harmonics). The repair was argured over (in court) and we were not involved after testing, but I was told much later the the gen manufacture unit did not meet required job spec.s and was going to replace the generator portion of the unit.

 

[templdl]

The first thing that I would do is to assure that the neutral was grounded properly. You have the normal source and the emergancy source which both feed into a common neutral "IF" the transfer switch is 3p with no switching neutral.
If the neutral is grounded at the generator as well as the sevice entrance then neutral current will flow though the grounding conductor to some degree unbalances the neutral which I found to be an application issued on occassion. What I'm trying to say is that the neutral may be grounded properly but, then again, please don't assume that it is. Some have found that the neutral has been bonded to the frame of the generator.

 

[Nick]

Just to clarify some of the questions below, the power factor of the generator is changed through the controller by varying the amount of dc excitation in the rotor. By adjusting this they can control how much of the total generator KVA is used to produce KW and how much is used to produce Kvars. The utility requires the generator to contribute a certain amount of Vars to the grid when operating in parallel. Unfortunately I don't know right now if they increased power factor (reduced Vars) or lowered it. The loads are what they are and have not changed. The only N-G bond at the facility is at the main service. When we re fed all the separate services we removed all the old services (now sub feeds) N-G main bonding jumpers. We removed the bond that came installed on the generator as well.

?Surely they were not suggesting a resistor in _series_ with the neutral to reduce normal neutral current,"

Unfortunately I believe that is exactly what is being suggested. This is the main reason I still have any involvement at all. I am long gone on another job and my office keeps getting pressed for a solution. They don't really see it as our problem but I don't want to see this owner or GC waste allot of money on bad advice. We may be drug into a law suite over this as well and I have a hard time just ignoring the issue. When I was out there I developed a good relationship with the owner and I want to see it work right.

 

[ghostbuster]

There is a device called a neutral current eliminator.It hooks in parallel to the load and primarily drains off zero sequence components(3,9,15 harmonics).I have included the link,Good Luck

http://www.mirusinternational.com/pages/eliminator.htm