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Unstable Voltage

05/31/2011 5:53 PM

How do i get to control voltage on a cat 3516B generator,when running on island mode?When unit is in island mode the voltage shoots high or down and cant be controled unit trips on high/lowvoltage.Unit works normal when running in parallel with other unit.DVR has been replaced,and all exitation field checked ,pot is ok.

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#1

Re: Unstable Voltage

05/31/2011 9:39 PM

What is the load when this happens? Have you checked the settings on your AVR? Has all wiring from the PTs/CTs been checked?

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#4
In reply to #1

Re: Unstable Voltage

06/01/2011 9:22 AM

This happens at full load when the other units are taken off line and is left on its own to back the whole system..its has a load of 800kw.it begins to lose its voltage or gain V. all pt have been checked.droop ct has been checked,DVR settings have been checked.

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#7
In reply to #4

Re: Unstable Voltage

06/01/2011 1:12 PM

Much more data is required to diagnose this problem, though I suspect that there is a large mismatch between the voltage/load when connected to the grid and immediately thereafter.

What is the rating of the generator?

What is the percentages of load sharing just prior to disconnection? What procedure is used to ensure that the generator picks up the load prior to disconnecting from the grid?

What is the PF at the grid connection? At the generator terminals? Direction is important.

What type of loads are there? Motor, resistive, capacitive? Percentages?

I'm sure that there are members that will argue with me but simply opening the grid connection and hoping that the governor and AVR will respond fast enough does not guarantee a "bump-free" transfer, especially if the plant load is almost the same as the generator capacity.

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#2

Re: Unstable Voltage

06/01/2011 7:35 AM

Dear,

Most of the time this happens when Capacitors are in excess, taking alternator currents in leading mode w..r.t. Alternator volts. You mentioned this on no load ! Just check, the PF, at this condition.

You probably may not be having automatic PF control system, then switch off capacitors manually on no load.

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#5
In reply to #2

Re: Unstable Voltage

06/01/2011 9:28 AM

This occurs at full load just when other units are taken off line.The load is 800kw.The DVR is set at pf control of a .9 setting.

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#10
In reply to #5

Re: Unstable Voltage

06/02/2011 3:02 AM

when you are in island mode, DVR should be set to voltage mode not PF

you are requesting the alternator to provide a constant PF 0.9, when in parallel with another grid this is not a problem, any variation of the load PF will be compensated from other units, when n island mode, you cannot fix your PF, the load determine the power factor,your DVR will try always to reach 0.9 value of the PF by changing the voltage.

In Island mode you should preset the DVR in voltage mode, so that the DVR will regulate the voltae and not the PF

I think on DVR 2000 there is an external digital input two switch between two modes, you can use it to flip when other units are offgrid

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#3

Re: Unstable Voltage

06/01/2011 7:45 AM

What has the generator manufacturer got to say on the subject?

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#6

Re: Unstable Voltage

06/01/2011 1:07 PM

Power generation is not my field but this question just seems to me to be too imprecise for any analysis of your problem. How much of a voltage swing is too much, 1%, 0.1%, 50%? How much of a change in loading causes this voltage change? Are you turning OFF a few 40 watt fluorescent lamps lamps or the equivalent of Cleavland Ohio? How much reluctance is exhibited by the load? Does the Automatic Voltage Regulator (AVR) or the Dynamic (?) Voltage Regulator (DVR) start to respond to the excessive voltage prior to the safety trip? Is there any Power Factor Capacitors on the line?

Now once you know these things you might be able to troubleshoot the problem yourself. From my limited knowledge of power generation, I don't even know where to start with this imprecise of a problem.

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#8

Re: Unstable Voltage

06/01/2011 10:45 PM

I've had this problem but not on a Cat generator. In the end it was the pot. The generator would run fine for a few weeks then away it would go. The pot tested all right too at the time but after a while away it would go. Running a bit of electrical contact cleaner through it might tell a story and it won't do any harm.

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#9

Re: Unstable Voltage

06/01/2011 10:57 PM

I had the same problem with a CAT Gen set 50 KVA, the problem we found was the the AVR and Gen Control module were in a sealed box above the dynamo.

The AVR and Gen Control module were also locally built.

After many visits by the CAT Experts, we solved the problem by removing the Electronics and controls to a fan ventilated box mounted on the wall. Now no problem, temperature in the original enclosure was over 85 c when the system was running, our ambient is 35.

We have solved a lot of problems by adding Forced Air Flow.

Any way that solved the problem and allows for easier access to the controls.

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#11

Re: Unstable Voltage

06/02/2011 6:19 AM

A lot of good advice there - but one important question unanswered. Is this a new installation - has this condition always existed - or is it a recent departure from normal? If the latter, what have you changed recently in equipment or procedure? If not, then there's component failure, incrrect installation, or (perish the thought!) a design flaw! Good Luck BRW.

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#12

Re: Unstable Voltage

06/02/2011 10:08 AM

I see 2 likely problems.

First, as Le_Noble said, the regulator must be set to voltage mode when operating in island mode. There should be an input to the DVR to automatically switch modes when the unit is in island. If the input is not functioning or was not installed, the result described by Le_Noble will occur.

Second, the gain on the DVR may be set too high. If your 3516B was originally set up as a standby unit, the commissioning tests would have included a step load change of 2MW, which requires an extremely fast response with minimal damping. Your island load is less than 50% of rated capacity. When a relatively small load change occurs, the DVR control loop response is so fast it overshoots the setpoint, then drives back down and overshoots in the opposite direction. The cycling can continue and increase until the voltage reaches a shutdown point. Re-tuning the DVR with a lower gain and higher damp function should solve the problem.

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#13
In reply to #12

Re: Unstable Voltage

06/03/2011 2:21 AM

The purpose of AVR is to control the terminal voltage of the generator. The problem is occurring only in island mode of operation and indicates problems in AVR functioning.

In parallel mode no problem is faced, as the voltage may be controlled by other soruces and regulators in the grid.

Only when the operating conditions exceed the AVR capability limits, this type of problem may occur. However, it appears, that this may not be the case.

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