voltage variation with load variation on generators


Thread Starter

henry lob

i am working on an land oil rig having 5 cat generators controlled inside a vfd house by ross hill ac module. these generators voltage is 600 Vac.

the generators when in sync are sharing equally the load and the voltage is steady on 600 volt. once the load is increased by the use of a tool affecting the total load like a drawworks hoisting a heavy load, the load is increased on all generators the same. but the issue is I am having also a increase in voltage from 600 to a range of 20 to 40 volts which is making other tools trip. the voltage should drop on a load increase; thus the ac module stabilized it on 600 volt automatically. does any know what is the problem here? my frequency is remaining same 60 hz. i checked everything concerning the speed of the engines and their throttle signal they are ok.

please if anyone can help

thank you
henry lob,

My first question would be: What has changed?

Presumably, things were working fine, and then something changed.?.?.?

Generators have exciters to control voltage. When generators are operated in parallel as you are describing the exciters have to be tuned so that they will all work together. Diesel governors are no different; I would presume that there is some kind of overall control system that's coordinating the diesels (which control the frequency in response to load changes) and the exciters (controlling the voltage in response to load changes).

My guess is that something has been misadjusted or needs to be re-tuned to restore the voltage control to normal. And, I'm betting it's something to do with the "load-sharing" system, sometimes called a "PMS" (Power Management System) which is coordinating the diesels and the generator exciters.

Another common cause of this kind of problem is that individual controls (like the individual exciter regulators) get misadjusted or "bumped" and need re-adjusting.

Finally, sometimes when things like this start occurring it's discovered that operation has recently changed--a new tool with a different load (real and reactive), or a different operator who operates the equipment differently than other operators (some are harder on equipment than others; some are more gentle; most are just rough--"It's the technician's job to keep it running, not mine!"; sound familiar?)

Please write back to let us know what you find!