We have three generators cat 3412 800kW at our power station. They load in synchronization. But we have a problem in the sharing of the reactive power KVAR.
The voltage regulator used in the system is a model VR6, and the sharing used cross current compensation. The problem happened when we disconnect any of the three generators for maintenance and loaded again, as we found that the pf rapidally changes between them.
We had performed the following:
- Checked all the connection again for any wiring loose but fount it is ok
- Replaced the voltage regulator for all the generator step by step and nothing happened.
Can any on help me to find the root cause for this problem?
Best Answer
Quick Summary: Check your reactive droop control setpoints on all three machines and make sure they match.
Forgive me; I don't know how much you've worked with this, so I'm going to write this for the uninitiated.
When machines are in parallel, they have to share load. If their controllers are set to maintain one, specific voltage, they don't share very well, shown in the next figure.
Here, the reactive load sharing pretty much depends on tiny little differences between the machines. In general, my experience has been that the machine that starts taking on the load will try to take all of it, and there isn't a great way to tell which one will do it.
The VR6 voltage regulator you mentioned has something called reactive droop control. The figure below shows how that cleans things up a bit.
The red line represents the total amount of reactive load in the system between the machines.
If both machines are set with the same droop percentage, then they will share the reactive loading in proportion to their KVAR ratings. The no-load set point can be adjusted up or down to increase or decrease the proportion of the reactive load taken - just imagine the red load lines sliding left or right when the black droop lines move up or down.
Looking at the VR6 manual that I googled, It doesn't seem to actually make you set the droop rheostat, so much as tell you that you can if you want to. Your three parallel machines all probably have slightly different droop characteristics. In this case, the one with the shortest droop compensation will eat up the most load, demonstrated below.
You can probably get pretty close to matched droop by just making sure all three machines droop rheostats have been turned by the same amount.