Unstable Voltage

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

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.

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.

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.

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.

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

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.

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.

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.

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.

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.