AVR Basler DECS200(redundancy), VARs moving

[ViCH]

I think this question is special for Mr.Catserveng, as he is a super specialist for this AVR model, as I notice.
We had situation long time ago, with two same size genset running, equipped DECS200. After few days of stable running, VARs from one generator moved to another, and we got 0.9MW/0.6MVAR and 0.9MW/0.01MVAR. It was continuosly running without trip, until we had to stop them for service. Temperatures of windings were OK, AVR also did not show any alarm.
Our electrical engineer said that it is not problem since KW is sharing, so we forgot about it. But after that several times we noticed same scenario.
Is it really OK?
We used DEIF parallel controller for sync. Decs in AVR mode (AUTO). Voltage matching OFF.
Later I found suggested setting list from comissioning engineer, where AVR is STOP, voltage matching is ON. I read manuals, but did not get point for VAR walking. Could that two settings make sence for that?

Viktor
Electrical Technician II

 

[waross]

Voltage matching may be another name for parallel.
There is a quadrature circuit that is used for VAR sharing.
It is often turned off when a set is islanded.
If the voltages do not match exactly, the higher voltage set will produce more VARs.
A CT on C phase is used to produce a current that is 90 degrees out of phase with A-B phase.
This current develops a bias voltage across a resistor in series with the A-B voltage sense terminals of the AVR on the opposite machine.
The action is to slightly reduce the voltage on one set and slightly raise the voltage on the other set so that the VARs balance.
If the switch is off, that won't happen.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ViCH]

So its normal that it suddenly happen? Because after start up it was shared VARs correctly several days.

Viktor
Electrical Technician II

 

[catserveng]

Usually the DECS200's are pretty stable, but a few things,

First, Voltage Match is a function of matching generator the bus voltage, when DECS is in AVR Mode (52L/M and 52J/K contacts both closed), if enabled DECS will try to match gen voltage to sensed bus voltage.

What exactly is your operation condition? Two units online supplying an island, in parallel to grid or what?

If you're using a DEIF controller, are you using it only for sync? Or does it do real and reactive power sharing as well? (Deif makes a lot of different controllers, would help to know which one you have and what you are actually trying to control with it, and how it is interfaced with the DECS, AUX input or raise/lower contacts).

What mode are you trying to operate the DECS in? AVR mode, Droop mode, VAR control mode or Power Factor control mode?

Have you tried to run a datalog? The datalogging feature in the DECS is actually pretty good. I would look at setting up a data log with gen voltage, field volts and amps, 52J/K and L/M positions, and real and reactive power levels. That way you can get a trend and see what is driving what.

If this was happening early in the systems life it is usually a setup/programming problem, a lot of things to go thru and do right. If this has been operating fine for some time and is an intermittent type problem I usually start with taking a hard look a the input contact wiring. Had a similar issue with a DECS150 recently where the VAR's would drop and then creep back up, eventually found that the PLC was glitching the Pre-Position input and setting the regulator back to it nominal voltage setting.

The two contact inputs, 52J/K and 52L/M determine the mode of operation, if one of those starts going intermittent then you may see a problem similar to yours based on what settings you have.

Hope that helps, MikeL

 

[ViCH]

DEIF Multi Line 2. It was using for constant control of parallel work two gensets, without grid.
DECS was AVR mode with 3% droop.
Now I dont have access to them (however I still keeps docs and reports of it), but I just wondering to know was it problem, or no. It was "my small frog, which I had not ate". And if it is problem, then could be VOLTAGE MATCHING OFF configuration be guilty or not.

Viktor
Electrical Technician II

 

[catserveng]

The voltage matching OFF should not be a problem.

The DECS in droop mode usually tracks really well, so if you're seeing the VAR share not working as expected I'd make sure the droop CT circuits are ok, the droop CT's are in good shape (especially if there are mounted in directly in the gen enclosure) and that their wiring is ok.

If the Deif controller is doing the voltage matching, what can happen sometimes, especially if you are using the raise/lower inputs instead of the aux input, is that the voltage setpoint doesn't return to nominal after matching voltage, so the voltage setpoint may be slightly lower or higher than the other unit, causing the VAR's to not share equally. That's where using the PrePosition input can help by returning the setpoint to nominal when the breaker closes. Not sure how you have it installed, but if it continues to be an issue I'd do the datalogging as a first step.

MikeL.

 

[ViCH]

Thanks Mikel, for sharing Your experience. I think we stop here, since I heard the possibilities, but cannt check it on practice. At least next time I will know what to do.

Viktor
Electrical Technician II

 

[ViCH]

Hi again.. This DEIF GPC2 controller... I have rechecked drawings and found interesting thing. It has modes for governor (fixed F, base load, droop, load sharing), and for AVR (fixed V, fixed VAR, fixed PF, VAR sharing).. By drawings, its always fixed voltage modes for both engine. For governors - its Load sharing mode while running in parallel, and Base load if only one running. Its island.
Questions -
1) can be VAR problems be due to fixed voltage mode? May VAR sharing mode fix issue?
2) For stand alone generator - would be fixed Frequency/fixed Voltage be the best option or not?

Viktor
Electrical Technician II

 

[catserveng]

The DEIF GPC2 is a very capable controller, right up there with the Woodward and ComAp load sharing controls. But the most common problems with them are the setup and tuning. Along with the DEIF controller you also have an AVR with broad functionality as well. So getting to the root cause of your problem requires you to fully understand what the initial design intent was and then decide how you really want it to work.

So for your first question, it all depends on how your system was designed, it isn't uncommon to have systems with these type controls with the VAR share still working in droop, or maybe cross current compensation. If that was the intent, then the DEIF controller should just stay in the fixed voltage mode so it doesn't bias the AVR and let the AVR control the way it wants to. Depending on the version of the GPC2 you have there is also a setting for "Voltage Trim", if enabled even in the fixed voltage mode it will try and maintain the setpoint voltage. If the AVR is trying to work in droop sometimes they will fight and cause voltage or VAR fluctuations depending on what is really going on. Leads me back to using the datalog feature on the AVR, try to see if the VAR fluctuation you're seeing is due to an AVR driven change, or if the biasing from the DEIF is causing it. You also need to make sure you understand how the DEIF in interacting with the DECS AVR, it can either be with raise/lower contact inputs, or by using an analog voltage bias, usually +/- 10vdc, that way you know what to look for when troubleshooting.

You can use the VAR sharing mode, even with the DECS in droop, although I usually set the DECS to 2% and let the biasing have a little more authority. You need to make sure that you are NOT using cross current compensation and then enable VAR sharing from the DEIF, things could get erratic and voltage instability or a trip.

For your second question, in a single unit in standalone ideally fixed frequency and voltage from the DIEF would be ideal in my opinion, and let the engine governor and the AVR manage speed and voltage control to maintain the desired nominal settings.

Your comment about currently being in baseload mode while in a single unit is running is a bit confusing, usually baseload is used while in parallel with a utility or a paralleled network to maintain a constant kW load setpoint. Although the DEIF controller should know if a unit can really go into baseload based on breaker inputs and communications with the DEIF controller on the other units.

Hope that helps, MikeL

 

[ViCH]

Thanks for response, Mikel.
I think, they just design it only for running in parallel.. So base load was using as a temporary mode, while genset was syncing to gas turbine generator (MAIN SUPPLY). Then, once breaker for GTG open, second diesel sync with first, and DEIF switchs to Load sharing mode. Looks like they never think about stand alone operation.
There are no cross current compensation, as per the drawings.
Control for Governor and AVR - digital output relays.

Viktor
Electrical Technician II