CAT SR4 Voltage Regulation Problems 1

[tivester]

I work at a data that is backed up by 3 CAT 3516 diesel engines with SR4 generators and VR3 voltage regulators. Two years ago in our 2nd generator we experienced a voltage regulation issue that was resolved by replacing the voltage regulator. We are now again having voltage regulation problems. After the problem started again a CAT service technician determined that one set of diodes was bad (a known issues for that model) and the problem seemed to have been resolved. A little over a month later the problem has returned. Here's the scenario:
While load bank testing, the generators are given a start signal to come online and the first one to 90% rated frequency and voltage closes to the bus and the other 2 sync through a Woodard synchonizer. During this time generators 1 and 3 come online and properly sync. Generator 2 however ramps up to 600V then drops to well below 480V. The breaker for the generator closes at approximately the same time as generator 1 and 2, staying closed with the engine continuing to run. The generator cubicle panel shows an over crank alarm and through our building automation system we recieve a day tank high fuel (although we have no day tanks), low oil pressure, and high water temp alarms.
Has anyone had problems similar to this?

 

[catserveng]

When they did the AVR replacement did they use another VR3 or install a VR6?

A large over and undershoot like you describe on that machine is likely due to the following reasons,

Stability setting way off, usually I start at mid span on the adjustment.

Loose or intermittant connection in the sensing, check the leads, fuse holders and connections.

Does this unit have the donut style PT's installed for sensing? I have seen these cause similar problems, frankly they weren't very good and gave lots of issues.

Intermittant or loose connections in the droop circuits, check the terminal blocks and the droop pot, if it's the style that plugs into the terminal block, you may want to consider replacing it, heat and vibration caused them to be intermittant. A bad droop CT can also cause these types of problems.

When they changed the diodes, did they use the latest part number, the one where the whole diode and heat sink assembly gets replaced? You're right, there was a lot of diode and surge suppresor issues a couple of years ago, and a flaky surge supressor could also cause the problem you report.

When the unit does get on line, does it VAR share properly with the other units?

On your alarms, is this unit equipped with an EMCP II control panel? Are you using a relay driver module to annunciate to your Building system, if so, it is likely you're losing communications to the RDM, a loss of comm signal with power still on the unit causes the output driver to flash at about a 1 second rate.

These are all issues an experienced CAT EPG Technician should be well aware of.

Hope that helps.

 

[tivester]

Thanks for all the information. We are having our Russelectric and CAT technicians out this week to collaborate on the issue. I will definitely look into all the points you brought up. Thanks again!

 

[38ChevyGuy]

This sounds similar to something we experienced. We had a VR3 voltage regulator go bad and it was replaced with the current model, the VR6. (VR3 is obsolete) Worked for a while, then during a utility outage, bad things happened. Our Catepillar dealer feels the VR6 does not "play well" with VR3's on the other engines. We replaced the other VR3's, so all are now VR6's. Seems to be OK so far. You have to be aware that the VR6 uses 240 volt control voltage, while some generators are 120 volts. So you have to insert a control transformer to make it work.

 

[tivester]

We haven't upgraded to a VR6 yet. That may be the next step if another VR3 fails. Good to know that we will want to upgrade them all at the same time. Thanks.