Paralleling CAT Alternators with VR6 AVR - High Circulating Currents

[MJELECT]

Hi,
I have 3 CAT generators which I am trying to set-up. They are in a bit of a mess and while the KW are the same there seems to be high reactive currents in the system.
All the generators run OK when taking load on their own but when paralleled the currents are all over the place.
I've not had alot of experience setting up AVR's but so far I have the below:
One at a time....
1. Set DRP pot fully CCW.
2. Set the no-load voltage to nominal voltage.
3. Load up the gen-set to full load and adjust the DRP pot to nominal voltage plus 3%.

Can anyone confirm that this would be the right procedure?

I know that it has worked before so the installation should be OK.

Thanks

 

[catserveng]

Sounds like you may have the droop set incorrectly.

First, is this a droop parallel system, meaning you don't have any interconnecting wiring between the droop circuits on all the VAR's, correct?

Since you didn't provide a lot of detail, I have to ask, do all the units actually have a droop CT and is it properly sized?

When you initially setup the AVR, jumper out the remote voltage adjust, use the on board voltage trim pot to adjust nominal voltage, set stability pot to midrange, set droop pot to midrange. Remove the trim pot jumper and make sure the remote voltage adjust pot works as expected. Nominal voltage should be at midrange of trim pot.

Load unit with a load that has reactive content, means not a pure resistive load, be nice if you could get close to .9 or .85 lagging PF if possible.

Assuming all three units same sized, if so, use same load and adjust the voltage droop to see a 3% DROP in nominal voltage from no load to full load. If the units are different sized, use a proper load proportion to make these adjustments.

Parallel units and make MINOR adjustments to voltage trim pots, remember that VAR share will not be exact, especially if these are older units with the CAT bar type droop CT, but should be within 5% if everythung is working correctly.

About 90% of the time when I've had to set units up no reliable test load exists, setting droop pots to midrange usually is a good starting point but may result in more droop than desired, but never use less than 25% from full CCW of the droop pot for paralleling applications. Just try to start with the droop pots as close to the same as possible.

Here is a download from Basler that you may find helpful,

http://www.basler.com/downloads/VR_parallel.pdf

Here is a Basler manual for the Basler version of the VR6, the AVC63-12 (I think it's a better manual than the one CAT gives you with the AVR)

http://www.basler.com/downloads/9337200991E.pdf

You may have to register with Basler to get to the papers but it's free.

Hope that helps, Mike L.

 

[MJELECT]

Hi,
Thanks for the reply and sorry about the lack of detail.

I checked the system over today and have the below:

1. The units have a droop CT and are connected to terminals 5+6, so i'm assuming they are the 1A secondary type.
2. There is no remote voltage pot in the main switchboard.
3. Yes, there is no interconnection wiring between the gen-sets.

One thing that threw me right out was the one that I was having issues with had the CT connection on the AVR reversed. I guessed after they recently changed the bearings whoever re-connected it swapped those 2 wires so when I wound the DRP pot CW the voltage was increasing (that's why I wrote nominal plus 3% in my first post as this was the only one I was able to work on).

After discovering the wiring fault I managed to get the droops set so that the system was reasonably stable and evened out. Unfortunately, my load was not constant the whole time and what load they could give was a PF of about 0.4 (propulsion motors controlled by SCR variable speed drive). This didn't help with the set-up, especially for someone that doesn't really know what they are doing !!!!

My question now is, what would I recommend to them if they want a more accurate way of paralleling their gen-sets? Is cross-current a better way to go or is there any module that works with the AVR that can control the kVAR's a lot more accurately?

Thanks for your help,

Michael

 

[FreddyNurk]

Basler has some excellent documentation regarding cross current compensation for running generators in parallel, certainly in terms of advantages and disadvantages.

Its used on most of the remote stations I work on here, along with an allowance for bypassing the CTs of generators not operating.

Another option depending on what you're using to manage the load sharing is to use whatever generator controller (Woodward, Deif, Comap and others) you are using, if it has the option for VAr sharing and has provision for controlling the remote voltage of the AVR. Its quite likely to be the most expensive if you don't already have the equipment.

 

[catserveng]

Funny how just swapping two wires can cause so much grief, it's probably the number one issue I deal with in working on parallel application generators.

If your units are the same, then cross current may work better than droop, but a word of advice on that, change the droop CT's to 5 amp secondaries properly sized for your generator rating and wire the system per the Basler instructions.

If you want the best possible control for a diesel electric propulsion bus, my best luck has been with Woodward DSLC based systems, especially if you have different sized units, the controls can be tuned to provide very good proportional sharing of both real power and VAR's between the units.

I've was also involved on a retrofit project on a AGOR class research vessel where we upgraded all the AVR's to Basler DECS 200's, pretty expensive, but when we got them tuned the tracking of the AVR's was VERY good and we were down to about 1% droop, VAR share bas normally better than 2%.

There are a number of options, but since this system feeds a propulsion bus you'll likely need to assure best possible reliability. So your best bet would be to find a marine engineer with diesel electric experience and let him review your system and offer possible options.

Hope that helps,

Mike L.