Reactive droop compensation issues with three 250 KW Cats
Reactive droop compensation issues with three 250 KW Cats
(OP)
thread238-172850: power factor not stable: power factor not stable
Hello. I am in need of some advice on an issue we are having with two of three Caterpillar 250Kw units that are paralleled to supply a stand alone floating maintenance barge. The condition we are experiencing is that units #one and #two together run well and share the load equally and power factors adjust well. When you parallel either #one or #two with #three individually the power factors do not react well and the loads are not shared equally. We have surmised that it is cross current based on the bus amperage and individual unit amperage gages comparisons. And the fact that the power factor meters lag and lead respectively.
As I read through some of the threads, which were very helpful, I have some questions.
The gensets are SR4 CATS with upfitted VR6 regulators. We have had the Cat techs in to work on the system. They tuned in the 2301a controls and got the units to share pretty well. But, told us based on the age of the equipment that the power factors are as good as it gets.
Finally,
Question one: Can anyone tell me if the burdeon resistor is internal to the VR6?
Question two: How can you tell if the polarity of the CTs are correct?
I suspect that the polarity of the CT on unit #three is not correct but am leary of reversing it without some more information.
Sorry this got so long. Wanted everyone to have as much info as possible
Hello. I am in need of some advice on an issue we are having with two of three Caterpillar 250Kw units that are paralleled to supply a stand alone floating maintenance barge. The condition we are experiencing is that units #one and #two together run well and share the load equally and power factors adjust well. When you parallel either #one or #two with #three individually the power factors do not react well and the loads are not shared equally. We have surmised that it is cross current based on the bus amperage and individual unit amperage gages comparisons. And the fact that the power factor meters lag and lead respectively.
As I read through some of the threads, which were very helpful, I have some questions.
The gensets are SR4 CATS with upfitted VR6 regulators. We have had the Cat techs in to work on the system. They tuned in the 2301a controls and got the units to share pretty well. But, told us based on the age of the equipment that the power factors are as good as it gets.
Finally,
Question one: Can anyone tell me if the burdeon resistor is internal to the VR6?
Question two: How can you tell if the polarity of the CTs are correct?
I suspect that the polarity of the CT on unit #three is not correct but am leary of reversing it without some more information.
Sorry this got so long. Wanted everyone to have as much info as possible





RE: Reactive droop compensation issues with three 250 KW Cats
2. Easiest way is to set the droop pot to about mid position and add load to a stand alone unit that has some reactive power content and watch the voltage droop as the load is added, if the CT is reversed the voltage will increase instead of drop. The CT being reversed is not likely based on your description since usually if it is when paralleled one unit will hog all the reactive load and the other will go into loss of excitation, especially on these size units.
Go to Basler's website, get the manual for an AVC63-12 (the Basler version of the VR6), also in the downloads area next to the manual you'll see a link for an application note for VAR/PF in Parallel, get that as well, an excellent reference on how to setup a cross current circuit.
When the units had the VR6's installed, did they replace the droop CT's or leave the old ones? The original CAT bar typr CT installed in most older SR4's could be a little inaccurate, and not provide a good enough signal to make VAR sharing work correctly. When I upgrade I also install new droop CT's with 5 amp secondaries and sized for full load current.
Do the units have some voltage droop? If I don't have a resistvie/reactive load bank to set a system up I start with the droop pots set all the same, usually start at mid scale.
A properly installed cross current or droop VAR share system on three like sized units with the same AVR's and droop circuits should be able to proportionally VAR share within 2%. Differing AVR's and droop circuits will make that worse but have been able to get them within 10% in most cases.
Hope that helps, Mike L.
RE: Reactive droop compensation issues with three 250 KW Cats
When the VR6's were installed we did not replace the CT's. We used what was on the machine. I recently had the CAT parts guy research based on arrangement number what size the CT's were and he informs me that the CT's are the one amp variety. At one point the CAT Techs replaced one of the CT's (#2unit). I assumed that he replaced like for like.
The units are set for some droop. Not sure, but I want to say around 3 or 4%. I will look when I return to work this evening.
The upgrade of CT's seems like an extremly good idea. I will make that happen.
We do have access to resistive load banks. And I think we will address this as soon as we are able to shut down for maintenance and I have the replacement CT's in hand. Any other input/suggestions would be greatly appreicated.
RE: Reactive droop compensation issues with three 250 KW Cats
If you don't have a load bank, use your own load, just come up with some larger loads that equal at least 50% rated power. Apply it to each unit and assure the voltage droops are as close as possible, about 3% voltage droop will work fine for most cross current systems.
Also make sure the Volts/Hz (Basler calls it UF Knee)setting is the same on all units, I usually set it for 59.5 Hz on a 60 Hz system, otherwise if you hit it with a big load they won't respond the same and share like they should.
Link to AVC63-12 manual, http://www.basler.com/downloads/9337200991E.pdf
Link to Parallel Paper
http://www.basler.com/downloads/VR_parallel.pdf
Good Luck! Mike L.
RE: Reactive droop compensation issues with three 250 KW Cats
Thanks for all of the great advice. I will let you know how it goes and if any of this effects a positive change. I am not satisfied with the approach that this is as good as it gets in terms of power factor adjustment. This system worked pretty well until we had to change the VR3s to the VR6s.
RE: Reactive droop compensation issues with three 250 KW Cats
RE: Reactive droop compensation issues with three 250 KW Cats
RE: Reactive droop compensation issues with three 250 KW Cats
I do go behind a lot of people working on systems that don't work right for a number of reasons, mainly poor installation and wiring practises, too little droop, and improper or incomplete intial setup.
e5fornow makes a good point about the isolation transformers, some systems got them installed, some didn't. Burden resistor sizing has a lot to do with CT match and where your loads are most of the time, and adjustable resistor is usually the best, and get one In most retrofits I clean up I find they were done half assed by people who really didn't know how to do it in the first place, Frankly, I find if they don't know about and use the Basler article, they shouldn't be doing it, at least in my opinion. Your comment about not being sure if everything is hooked up right is pretty typical. There needs to be a good overall drawings, the wiring needs to be correctly marked, the droop's need to be correctly set, manual voltage adjust pots should be locked with stem locks, and to do it right you really either need a proper test load and develop a good test plan using your loads you can do repeatedly.
A lot of systems can be marginally done and work ok. A lot of systems are really operating in droop because they are poorly done and the operators really don't know any differently.
I've also done a lot of systems in droop, an they also work just fine as long as the droops are set the same and no one messes with the voltage adjustments. The actual system droop will be slightly less than the single unit droop. Not sure why, maybe one of the other folks here can explain that, but in most cases that is what I've seen.
I rarely do droop or cross current control systems on new installations any more, active VAR/PF control using a Woodward, Basler or PLC based type control is really where most systems I deal with are going, especially mixed systems.
Hope that helps, good luck! Mike L.
RE: Reactive droop compensation issues with three 250 KW Cats
RE: Reactive droop compensation issues with three 250 KW Cats
RE: Reactive droop compensation issues with three 250 KW Cats
Other possiblity, I've only seen two in 30 years, is a mismarked CT. You are talking about the droop adjustment pot on the VR6, right? The old VR3 droop pots are gone, correct?
Do you have an as-built drawing of your system?
If you'd like to get a hold of me direct go to www.pactectric.com, maybe I can walk you tru some thigns more directly.
Mike L.