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Generator Reverse Power Trip on synchronizing 9

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edison123

edison123

Electrical
Oct 23, 2002
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Trying to commission a 11 KV, 2.5 MW steam turbine driven generator. The generator recently rewound and tested OK. Phase sequence also checked ok.

On synchronizing, the generator trips on reverse power every time. The turbine engineer says the turbine, the governor and the steam valve are ok.

I noted that on closing the breaker, the power factor goes haywire and the Woodwards Multifunction Relay reads about 100 amps in all 3 phases but the power is reading (-) 358 KW and the excitation is reduced to zero. (I will post two photos of the control board meters - before and after synch)in the following posts.

The AVR is Basler. I think the AVR is malfunctioning. Any other possible reasons for the RPR to act every time ?

Muthu
 
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Just back from the site. Apparently, they blew two of the rotating diodes yesterday trying to synchronize. As before the reactive power and pf went haywire. The turbine engineer claims the active power went from 75 KW to 1000 KW in a short time. The machine tripped on overload since the reverse power setting was increased from 3 seconds to 9 seconds (10 seconds is the max setting).

Today, the Basler AVR engineer found out that the PF control of AVR was not activated. New diodes are expected in 2 - 3 days.

I also checked the steam control valve manually from the 0 to 100% opening. The valve stem play was 11/4 inch for the entire range. I was assured the actual valve opening was checked and found ok.

I advised the client to disable the AVR auto mode and synch the machine manually and control the var load manually. Asked the turbine engineer to set the load at about 200 KW and see what happens to the stability of the machine. If it is still hunting, then it must be the governor issue. There is no setting for the governor to lock the load at a particular KW since there is no pressure or flow rate feedback loop.

Will post the AVR photos in the next posts.

Muthu
 
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Just what I said about the rotating diodes. If they keep up going on as they have been on that commissioning, they will need still more diodes and likely some more expensive items as well.

A couple more bad syncs or losses of excitation, etc. may result in some heavy damage both electrical ond/or mechanical. Is this a geared turbine? Possibly a new gearbox will be needed also, that is not a joke. A certain GT gearbox disintegrated, exploded that is, after one bad sync too many ...

The reverse power relay is not supposed to trip on overload, that implies positive power output to the busbar.

The PF control on the AVR is not relevant, or should not be. Normally that is a slow-acting correction on the voltage setpoint. Disable that for now.

Yes, see if you can run it manual excitation. But make sure the overvoltage protection is working first as well as overcurrent protection. Set that down to a conservative level in case it all goes bonkers again. Likely...

regards, rasevskii
 
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rasevskii - Thanks. I conveyed the same thing about the damages to the generator if they keep playing with it. We want to protect the stator rewind done by us. Apparently, this is a new turbine-GB from a not well known OEM and they blew the stator last time within 3 minutes of "commissioning". I told them our warranty will be void if they keep doing what they are doing now.

Muthu
 
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"There is no setting for the governor to lock the load at a particular KW since there is no pressure or flow rate feedback loop."
Put the governor in droop mode. The setpoint will then control the load level.
Try really hard to get the generator voltage up to the grid voltage before the next attempt. Be aware that the calibration of at least one of your voltmeters may be off.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Bill - The generator is synched automatically using a synch check relay. The photo I posted was when the generator voltage was still being built up manually before switch over to auto mode. The client has employed another contractor to check and calibrate all relays, instrument transformers and the instruments recently and he says everything is fine.

Muthu
 
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Forget about the Power Factor, for now. The PF meter of that type will only indicate correctly when the stator current is above about 10%. Below that the pointer is swinging all over the scale, meaning more or less nothing.

It looks like the whole scheme has to be recommissioned from the ground up.

Obviously each "Expert" will claim his part is perfect.

Is there really someone there from Basler?

regards, rasevskii

 
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Basler authorized service rep. Yound kid. Pity I can't be there for another 'commissioning' since I am off to another country in two days. Anyhoo, I tested all the generator windings (stator, rotor, exciter field and armature) today and they are still good. Keeping the fingers crossed.

Muthu
 
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This appears to be a Basler DECS 125-15 AVR which is possibly obsolete having been replaced by the DECS-100. The DECS 100 has software configuration under Windows but the 125 has configuration under some other software.

This is only a guess on my part having looked at the Basler website.

Can anyone clarify this? Possibly that is why there are problems with the AVR configuration.

Yes in fact there is cross-current compensation via a CT in the center phase, as well as PF, VAR, and manual control.

The manuals can be downloaded from the Basler website in pdf. Well worth a look.

regards, rasevskii
 
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Yes, it's a DECS15-32, it is an obsolete part, Basler recommended replacment is the DECS200. Uses a serial cable and Bestcoms software (free download from Basler).

The field output on these has relatively large steps, I used to get a lot of trouble calls because at low loads the power factor would appear to have large swings. Things would usually be fine after load gets up abit.

Have you tried using the AVR in Manual Mode? This is a great troubleshooting tool. From Bestcoms, on the configuration page, select Manual On, then use the settings page to raise and lower field output.

Who setup the initial PID settings? I have never had much luck with the calculator, and my experience with these is on larger CAT units and a couple of small PERP hydro units.

Before trying to parallel, use the Step Resp tab to evaluate the units response, a bad set of PID settings can be a contributor to your problems.

I usually set them up no load, test the step response at 5% up and down, then try it. If I'm having problems like you seem to be I'll parallel it in manual, and use the metering page to look at the parameters, this way you can tell if the readings are correct.

The Basler folks here in USA are usually pretty helpful, not sure about support where you're at.

Download and use the software, it's way better than setting and and reading from front, and you can save setting files and see the metering.

Hope that helps.
 
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I have a lot of customers still using the DECS 15, it was an early digital regulator but in this size range worked well. The best change came when Basler got rid of the DCIM module and made it so you could use a serial cable to use Bestcoms with.

I do a fair number of conversions from the older DECS to DECS200's every year, main reason is that end user wants better control or more features. The DECS 200 tracks very well and I have some island mode plants where the VAR share with minimum droop settings are excellent. I also did a diesel electric drive research vessel about 2 years ago, the older AVR's had a hard time with voltage stability in rough seas. The DECS200's did very well after carefull setup and adjustments.

One thing that will kill a DECS 15 almost everytime is an open in the field circuit. They will either fail completely or act goofy. I had sites where the protective relays operated a field breaker from the previous AVR install, first time the protection operated, the AVR was done. Make sure the field output goes direct to the field and there are no opens or bad connections. If you need to kill excitation, open the power leads to the power supply, not to the DECS from the power supply or in the field circuit.

A note of caution, not sure the technical savvy of your Basler rep, the DECS 200 is a VERY nice regulator, but has more menus and things to go thru, if your guy is having trouble with the DECS 15 the DECS 200 will likely be over his head. A DECS 100 will likely not work, they are a maximum 65 VDC regulator, suited for smaller gensets. The Marathon DVR is a branded Basler DECS100. Nice unit but likely not usable for yours unless you have a lower power excitaion system.

Hope that helps, good luck.

Mike L.
 
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An excellent post, catserveng.

Now we have an hands-on expert on Basler AVRs on the forum. You have summed it up nicely. Does anyone on the jobsite know all this?

Now I have looked up on the Woodward site the info as a pdf download on the MFR2 device on the panel that someone has a finger on there,in the photo.

It is a regulator, PF controller, synchronizer, certain relay protections, displays, and more...all depending on how it is programmed. It may be working against the Basler or with it...

Have a look at the Woodward download site. One has to register first. Well worth the time.

With all this fancy kit just how did they manage to do a bad sync maybe more than once...perhaps wrong connections, programming, or someone pushed a relay in by hand...not unknown in this business.

Just too many specialists on the site, and no-one overseeing the total job, possibly.

regards, rasevskii

 
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hi, on my opinion this could be only two things:
- governor problem, never could be avr problem, cause the reverse power is active reverse power, and it´s not dependent on AVR.
- error on current transformers connection to the reverse power reley, i have see this problem a lot ot times...

 
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and about decs200, i have mounted a lot of them and are really superior to decs15, and the control on PF is better on low charges... and about diodes, you could be sure you have a bad synchronizing, maybe the phases for syncronism on voltage are reversed and this explain why you broke diodes if you really syncronize out of phase.
for me, before synchro is good to open the star point on generator, and switch the circuit breaker (after megger) and cheek sequence of phases on all the voltage transformers, then you open CB and start unit and cheek the sequence again and must be the same, and so, then you take out the relay for closing CB and connect a analogic multimeter on volts and cheek the moment you give the commant for closing, in this moment, all the thres multimeters must be on 0, this means you could syncronize.
After syncronize, if your AVR is not on power factor control, in the beginning you are not gonna trip but then reactive will swing and trips.... but never, a reverse power trip.
i hope i could help and sorry for my english.
 
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Final feedback. After changing the diodes, we did the OCC today. The generator vibrations increased drastically to 7.8 mm/sec peak even as the voltage was built to 4 KV (we had less than 1 mm/sec at 11 KV in the earlier OCC) and the exciter current also increased to 0.8 amps as compared to the earlier current of 0.45 Amps at 4 KV. So, they fried the generator rotor during the last synchronizing blowing the diodes in the process.

I suspect the generator lost excitation and was running as induction generator. Does this Basler AVR DECS 15 have provision for sensing loss of excitation ? If yes, how does it sense it ?

Muthu
 
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Looks like a complete retesting of everything from the ground up on that site is needed. Not just some reprogamming of one thing or another.

What kind of plant is it? Are there other generators in service there also? We could help out if some dwgs and other factual information were made available, at least to see if the concept, choice of equipment, and connections appear to be serviceable.

There are a lot of experts available here on this forum. There do not seem to be any on that site.

How do they manage a bad sync with the Woodward MFR2 which is also a synchronizer? Programming wrong...? Were the syn PT conns properly checked to see that the synchroscope reads 12 oclock with the GCB closed with the same voltage on each side? That is the line side open elsewhere and using only the generator voltage? You said phase rotation was correct, that had to be or else it could never have been paralleled at all...

regards, rasevskii

 
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That's what I have advised the client - check everything ground up, upgrade the AVR and do a total system study with a professional engineer. The original drawings and settings are missing and turnover of operating staff is high leading to poor record keeping.

Now the client faces the question whether to dump the generator or rewind the rotor.

Muthu
 
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