nickoliver
Electrical
- Aug 20, 2004
- 27
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.
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.