Synchronizing 2 generator problem, pls help 1

[michelin3x]

Hi
I am new to this field, so my explanation may not sound right, so here it goes.
Once I put one generator online (Gen 1) and try to sync the other one (Gen 2) with the help of a synchroscope, it stays on for 8 secs and then trips. No other fault indications. There is a synchronizing relay if that might be a cause.

Things to note:
Before CTs directions change
1. If I sync Gen 1 to Gen 3 or vice versa, Gen 3 KVAR and Ampere meters go really high.
After CTs directions reversal
2. Reversed all 3 CTs on both Gen, now the breaker trips as mentioned earlier. The KVAR meter don't show any reading now and the ampere meter shows around 150 amps on no load.

Also, How do I identify if the CTs are in the right direction with the arrow mark on it?

 

[dpc]

Check the settings on your voltage regulators and CT compensation circuit wiring. It sounds like a problem with the voltage regulation of one or both of the generators. If you synch and stay on-line for 8 seconds, I'd guess the synchronizing circuit and generator phasing is correct. If the phasing was wrong, it would be much more exciting at the instant the breaker closed.

There has to be some method provided to allow the generators to operate in parallel and share the reactive load. Otherwise the regulators will fight each other, especially if there is a wiring error in the cross-compensation CTs.

 

[michelin3x]

Thank you!!!
And what about the CT directions...???!!!

 

[waross]

Are you tripping on reverse power? The synchro-scope should be turning slowly clockwise when you close in. Your voltages should be matched before closing in.
Put everything back the way it was before you started guessing aimlessly.
Match your voltages.
Adjust your voltages so they match.
Adjust the governor on the incoming generator so that the 'scope is turning slowly clockwise.
Close in at 12:00 o-clock.

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

 

[dpc]

You need to look at the CT polarities indicated by the voltage regulator installation manual. Monitor the voltage and current of both generators during the time period it remains synchronized.

As Bill said, if you just start blindly changing CT polarities, etc, you'll make more work for yourself and possibly damage something.

Maybe this will help as background info:

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

Good luck.

 

[michelin3x]

Alright I did what exactly Bill has told without missing out a thing also. Alright these a re a few values I have noted down on the meters showing on the Gen panels
Before Sync:
Gen 1
Power: A few KW's
KVAR : 0
Amperes: 0

Gen 2:
Power: Zero
KVAR: 0
Amperes: 0

After sync(no load),
Gen 1
Power: 140 KW
KVAR: 0
Amperes: 80 A

Gen 2
Power: Going negative
KVAR: 0
Amperes: 80 A

Checked for reverse power on the test points, not showing even though the meter is showing the other way.

PS: There are no loads put on.

 

[xnuke]

You need load.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[michelin3x]

So u mean sync'ing w/o a load will give me a negative kw on one??!!!! Do u think my gen tripping cud be related to the master slave circuitry??

 

[davidbeach]

Far more difficult to keep unloaded generators running in parallel and stable than it is to have loaded generators paralleled.

 

[michelin3x]

Alright thanks guys for all the help, will do a little more of study, just cant figure out why my breaker is tripping off?!!

 

[dpc]

Well something is tripping it. Generally undervoltage/overvoltage, over/under frequency protection, and reverse/low forward power trips the generator breaker directly rather than via a lockout relay. You should review the breaker control schematics to determine all of the contacts on the trip bus and try to narrow it down.

If you have no load, it could be reverse power or low forward power protection tripping. Sometimes the governor has protection trips to the generator breaker also.

 

[waross]

When syncing a generator, the incoming set should be running a little faster to pick up some load and prevent motoring the incoming set.
If I understand your description, the incoming set is running a little slow and is being motored slightly. It should trip off in that case.
Heed the other posters; Put some load on the set before trying to sync a second generator.
A voltage difference between the sets will upset the VAR sharing. Too much voltage difference will drive circulating reactive currents between the machines. The characteristics and settings of the governors will determine the kW load sharing between the machines.
Any voltage difference will upset the VAR sharing of the machines. The cross current schemes monitor the current in C phase and any reactive current will be at 90 degrees. This will put it in phase with the voltage across A and B phases. The current is dropped across a resistor and the resulting voltage is used to bias the sense voltage in the AVR so that the VARs are shared.

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

 

[catserveng]

A lot of good direction here, but most assumes this system may be real power sharing in droop and VAR sharing in cross current. Maybe the OP could actually describe what he is using for the governors, how real power sharing is being done (droop, isochronus using a real power controller, or a combination type system)? How is the system doing VAR sharing? Is it in voltage droop, cross current, or using some form of reactive power controller?

Since these seem to be smaller size units, he may have genset controls like a Woodward EasyGen or ComAP. I just had a similar situation with units using Woodward Load Share Modules over the engines electronic governors, the load share lines were reversed at on section.

With the current offerings of really nice controllers at much lower prices these days, most units I see are using active real and reactive load sharing. I haven't done a new installation using cross current for VAR share for a couple of years, same goes for using droop for real power sharing. We are also seeing a large number of mostly standby systems that require very good sharing, voltage and frequency control with multiple units on the bus and no bus load. Not easy but can be achieved with newer controls and well done setup and testing. The bulk of my experience is with Woodward based controls, like the DSLC and MSLC, but they are expensive and mostly used for larger units, now with the EasyGen and others like Basler, Dief, ComAp and DeepSea, I see lots of systems using these types of controls on smaller units.

Could you provide some details about your installed system? Is this a new problem on a new system, or a new problem on a system that has been in service for awhile? Any work done on the engines, generators or controls if this is an existing system before this problem occured?

Mike L.

 

[michelin3x]

Thank you guys for the valuable information!!!

@Mike L
This is a real old system, no modern electronics like the easy gens. Got the AC modules, AVR, Master slave circuitry etc

This SCR house was dated at least 30 years back. I am working on the refurbishment.

Initially when I tried to sync, the amperes and the kvars meter used to shoot high the moment I brought the second generator online. This problem stays with the generator however I try to sync it, whether it is 1 to 2 or 2 to 1.

So as suggested, I reversed the CTs and then the kvars stayed at 0 at no load and the amperes would go upto 80amps on both generators but stay for around 8 sec and trip the Gen 2 off always.

 

[catserveng]

OK, an older system, still could use some more detail,

What kind of governors on the prime movers?
How are you doing load sharing?
What kind of AVR's
How are you doing VAR sharing?

Have you tried individually loading each unit one at a time with a known load (load bank with reactive load would be preferred)? Do the units respond the same way? Ideally they would have the same voltage droop slope and the governors would respond close to the same.

Did the system ever work right? If so when did it start not working? If it's 30 years old likely the generators have been on and off a couple of times at least, droop CT's don't get backwards unless they we either not installed right the first time or put back wrong later in life.

If it has cross current compensation there are multiple places in the circuit the problem could be, and swapping a DCT around at one point may or may not fix the overall issue. How did you "reverse the CT's"? Did you swap the secondaries or flip them on the phase leads?

In taking another look at you info provided, as soon as you close the incoming unit breaker Gen 1 picks up load, even though no kW on the bus, and Gen 2 has no load and appears to be going in reverse. What are the governors doing? Based on the info Gen 1 governor should be adding fuel and Gen 2 governor will likely be at minimum fuel. I think knowing what kind of governing you have and how real power sharing is done is needed to help with the problem described based on your info so far.

Hope that helps, Mike L

 

[michelin3x]

Thanks Mike!!!!
Alright today, I had to replace an AC module as there was a problem with one of them. And now its a new result.

Now all I see the KVARs going negative once i sync on Gen 2 only and generator trips off within 8sec... Help me... This trouble is killing me...

 

[ScottyUK]

All] the kVars go negative - as in both sets together, or the kVars on set 2 go negative? If you are not attached to a larger system then the other machine must see an equal increase in kVars. The likely cause is a voltage mis-match, with the machine displaying negative kVars being the lower of the two. How are your AVRs configured - for islanded operation or to operate in parallel with another set or a larger system?