Loss of Field Excitation on Basler DECS 200

[hle42000]

I tried to synchronize the turbine generator with Utility. But, after 30 seconds of synchronizing with utility. I got an alarm and it opened the turbine generator's breaker. I went to check the DECS 200, it displayed a message Loss of Field Excitation.
It happened like this three times now. What can I do to troubleshoot this voltage regulator? I am think of KVar is too low comparing with utility of 1Mvar. Our turbines has 1268Kvar, and we set 450Kvar on the DECS 200. Would somebody please help me if this setting is good. The system has been working for over four years now.

 

[catserveng]

Is this a new system or a new problem on an existing system?

The DECS200 is a very good AVR in my experience, but it has a number of things that need to be addressed in setup and it has one very important problem, NEVER open the field output under load. If this was a retrofit, (or sometimes new installations by older designers)there may be a field breaker or contactor in the field output, IT IS NOT SUPPOSED TO BE THERE! It will either kill the field output or make it not work correctly. If you need to shutdown the AVR use a contact in the power to the power supply and use the excitation disable function. The manual is real clear on that. It is the number one thing I find when I get a report of a DECS200 not working.

Second possible problem, could be a problem with the power supply in the AVR or the source of power, such as a PM. Best way to troubleshoot is to use the metering function in the Bestcoms software and watch the inputs and outputs while the units runs and parallels. There are also some pretty good logging features in the DECS, set up those to help you see what is going on.

I had one with a similar problem about 2 months ago, turned out to be the voltage lower input was hanging, bad relay, driving the setpoint down after the breaker closed.

I find the software is the best troubleshooting tool, if you didn't get a copy it's a free download from Basler, but you will have to register.

Hope that helps, Mike L.

 

[hle42000]

I decided to increase the loss of field value from 450 to 750kvar on the DECS 200. The turbine generator was able to synchronize with the utility for now. However, I read the multimeter on the turbine generator side, it gave me a power factor of -0.971 with a Kvar of -0.350 Mvar. Would somebody explain to me why I got this reading? I am expecting a power factor of 0.8

 

[davidbeach]

Why would you be expecting a power factor of 0.8? Just because the generator might be capable of that value, doesn't mean that's where it will run when connected to the grid unless it is in a PF control mode and the set point is 0.8, or in a Watt/VAr control mode with Watt and VAr set points to get a 0.8 pf. Generally it will be in a droop mode or a Watt control mode and the power factor will be what ever works out to maintain output where it needs to be.

 

[catserveng]

Do you have it configured for Power Factor control and a setpoint of .8 lagging? Are the digital inputs correctly wired so that it goes into the desired mode after breaker close?

These regulators are usually pretty good at controlling where you want them to if properly installed and setup. Just changing a protective setting level may be masking another problem.

Does the meter you're reading match with what the DECS is reading?

 

[hle42000]

Yes, I had the pf set point of 0.8. Before the turbine generator is synchronzing with grid, the DECS 200 displayed -0.26. Right after generator is paralled with grid, the DECS 200 displayed 0.86. But then the pf kept decreasing to become a negative number of -0.98.
Can somebody explain to me what is the technical meaning of negative Kvar? negative power factor for this application?

 

[catserveng]

In both the manual and the software for the DECS200 it describes a power factor number with a minus sign as a leading power factor (generator absorbing VARs).

I strongly urge you to get the software, hookup to the AVR and go to the metering function. When at rated speed and voltage before trying to sync, note the field output and status indications. Are you using the DECS200 to perform the voltage matching for paralleling? If so you should be able to watch it change setpoint to match the bus. After breaker closes watch the field output, if your setpoint is properly set, and the DECS is correctly wired and the controlling inputs corrrect, the field output should increase slowly (based on the PF gain rate)until the DECS thinks it's at 0.80 lagging PF (programmed setpoint). You can also use the datalog function built into the software if things happen too quickly for you or you want to review with someone else.

Based on what info provide so far I think you may have a wiring or sensing problem, my experience with DECS AVR's when properly installed and setup is that the control and metering functions work very well, so if you're seeing a large leading power factor before the breaker closes, the power factor indication is incorrect unless yo have some loads before the breaker (unlikely).

How about back to an earlier question, is this a new unit/site or an existing site with a new problem?

Mike L.

 

[catserveng]

Here is a link to the manual, your questions lead me to believe that maybe you haven't read it.

http://www.basler.com/downloads/9360100990J.pdf

I don't want to sound out of line here, but are you comfortable enough with this device to be troubleshooting and making adjustments to it? I would say this is a critical component in your turbine generator control system, and depending on your electrical system, you doing something icorrectly could lead to a pretty big problem.

 

[hle42000]

Mike, yes we are using Bestcoms software to do the set points. This is a new technical issue with an existing site. We didn't have this problem before.

Basically, the customer had this problem synchronized with grid after the grid came back from a failure. This is only once in awhile issue due to utility fluctuation. Sometimes the utility voltage is a little bit high like 14.3KV or low 13.3KV for this application of 13.8KV.
So, we tried to test this synchronizing issue. We started the turbine generator and it tried to sychronize with grid no problem at all. Then, we failed the grid and tried to bring it back again. They are synchronized in 3 seconds, and then we got an alarm DECS shutdown and the generator breaker got tripped. I went to check on the DECS, and it displayed loss of field.

We ran the test again. While the turbine generator trying to synchronize with the grid for about 3 seconds we got an alarm gain with DECS shutdown. I went to check to DECS display the same message. So, I decided to change the setting of LOSS OF FIELD LEVEL from 450Kvar to 750Kvar. We ran the test again. The generator was able to close with grid with no problem. But, I checked both the generator power meter and DECS display with this weird negative power factor of -.98

 

[hle42000]

catserveng (Electrical),

Yes, I read this DECS manual. But, it didn't help me to resolve the issue I had. Do you understand the LOSS OF FIELD LEVEL set point on this DECS 200? What does it do?

 

[catserveng]

Look at page 3-8 in the manual, you defined threshold for kVAR's being absorbed by the generator for the time delay you also select. So you had a setting of 450kVAR (leading) for some time delay, the generator exceeded that value for the time delay period, you had loss of field enabled, it shut down. So depending on the size of your machine (details you failed to provide) your .98 leading power factor may exceed that value. Instead of fixing the problem, you just increased the threshold level.

The DECS should go into the PF control mode when the breaker is closed if that is how you have it programmed. It apparently is not since instead of going to a 0.80 lagging power factor, it goes leading.

It sounds like the DECS is staying in AVR mode after the breaker closes, you may have the preposition wired to return it to nominal after the breaker closes. So depending on where the generator voltage level is compared to the utility may drive if you stay on line or not.

What controls voltage match (if anything)? What is the voltage match window? Are the control inputs to the DECS doing what they are supposed to? Look at the metering in Bestcoms, does it go to PF control mode when the breaker closes?

Get to know the datalog function and sequence of events log, can be your best friend in troubleshooting this AVR.

 

[rbulsara]

hle:

Firstly, with catserveng you are in best of the hands for opinions on gen controls, so if I were you I would heed his advice, especially the cautionary ones. I would even say, get someone who do generator paralleling service for living, on site.

Having said that here are my two cents:

1. Detecting reverse var (leading or negative power factor for a generator) is poor man's Loss of Field relay, meaning it is an indirect way of detecting loss of field. Reverse var on the other hand does not necessarily mean that the field is lost. It could be also activated if the generator actually sees capacitive loads.

2. By increasing the set point of kvar, you have just temporarily masked the symptom, not resolved the issue. Every generator has some (limited) ability to stay stable in leading power factor mode. You really do not want to exceed that or you might cause more serious issues. Check with the mfr.

3. You may in fact have weak field, AVR, mixed up wiring or some combination of them.
4. Typically AVR should have a selection of var mode or power factor mode of operation while paralleled with utility. While either should work in ideal conditions, if the load is relatively steady (not varying much), var mode is preferable. It sets a fixed var value to export. This ensures a lagging pf for the gen. Review these settings.

5. Also there would be a contact closure signal telling the AVR when the inter-tie breaker is closed. You may want to check that it is working properly.

6. The above is not an exhaustive list. It is possible that something else is also going on. You really need a good service engineer on site.

Rafiq Bulsara

http://www.srengineersct.com

 

[hle42000]

The former engineer who programed this AVR had chosen the following for mode of operation on DECS 200. It is working fine before until two days ago.

For Control Mode, he selected AVR. For Operating Status, he chose Off which means the DECS is not working in PF or VAR mode.

I asked this issue with Basler technical specialist. Here is his response.

Right now, is the DECS-200 is operating in AVR mode in droop if I go by the sequence of events you sent to me yesterday. Droop does not maintain a certain PF or Var level, this is the function of the PF/Var controller. Droop will lower the excitation level as the grid voltage goes up, causing Vars to be imported and the DECS-200 will register a negative PF. Likewise, the DECS-200 will raise excitation when utility voltage goes down and begin to export Vars, causing a positive PF. The extent at which it imports or exports Vars is dependent on the droop percentage that is programmed into the DECS-200. In droop, the import or export of Vars is not directly controlled by the DECS-200, it is dependent on the state of the utility voltage.

Referencing the screen shot when first synchronized, your system (utility) voltage is ~13.75kV. This doesn’t deviate much from the rated voltage of your generator 13.8kV, so the PF looks normal.

If I look at the screen shot 3 minutes after synchronization, I see that your system voltage has increased to ~13.9kV. Taking into account the fact that in droop the generator will import Vars when system voltage increases, it makes sense that your PF goes slightly leading at this voltage. However, the kVars you are importing in that screenshot are nowhere near your LOF threshold. If your system voltage reaches levels around 14.2kV, I could see much more Vars coming into the generator depending on your droop percentage and the LOF alarm kicking in.

It seems that the utility voltage may have fluctuated somewhat higher than in the past, causing your previous LOF threshold to alarm, or, you previously operated in PF or Var control mode when paralleled.

I would suggest trying to operate the unit in PF mode.

 

[rbulsara]

For Operating Status, he chose Off which means the DECS is not working in PF or VAR mode.

There lies the problem. You were lucky to survive for this long, now it shows. Many (erroneously) believe that since AVR mode is 'auto' it should work fine in parallel with utility, it is not the case.

Follow Basler specialist's advice. As I said either var or PF mode should work.

Rafiq Bulsara

http://www.srengineersct.com

 

[catserveng]

One of the reasons I like the DECS200 is how much info it gives you. The first time the OP hooked up to it he should have been able to figure out he was not in VAR or PF control.

Funny how a system can work for a time without really being right, and when the problems do occur, the answers are usually rith there.

Better lucky than good, hope OP fixes it right. Last month I had a call to retrofit a DECS because it was a "piece of crap". After 45 minutes going back thru the install and setup found it wasn't installed to print or programmed correctly, yet was kinda working for over two years. Fixed the wiring, fixed the programming, only charged about $1000 when I should of gone ahead and installed an analog AVR and PF controller like the customer asked for. Guess I suck at business.

To the OP, if it wasn't setup correctly to do power factor or VAR control, then I'll bet the PID settings aren't correct either. It may be in your best interest to get someone familiar with the DECS and your tail end and do it right, including setting up the protections so they work properly in concert with your other protective devices/relays. Don't know how big your unit is, what kind of systems it's in or how important it is, but you got lucky, found a problem without hurting something, next time you might not be.

My two cents worth, Mike L.