parallel generators donot divide reactive KVAR load equally problem

[osamaelkaey]

we have three generators cat 3412 ,800 kw at our power station, they loaded in synchronization, we have aproblem in the sharing of the reactive power KVAR.the voltage regulator used in the system model VR6, and the sharing used cross current compensation. the problem happened when disconnect any of the three generators for maintenance and loaded again, as we found the pf rapidally change between them.we had performed the following:
1- checked all the connection again for any wiring loose but fount it is ok
2- 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
thanks

 

[catserveng]

Has this been a long term problem or just show up recently? Any work been done on any of the units that may have contributed to the problem. Since 3412's have been out of production for a while I'm assuming this is a system that has been in service for some time.

Attached is a Basler publication that discusses cross current compensation, on page 17 second paragraph it discusses the importance of the unit parallel contact in the circuit. This is usually the most likely cause of the problem you describe. Also for the system to work correctly some droop must be adjusted into each regulator, and the same percentage for all units, if the droop pots are fully counterclockwise (typical in most system for a zero droop), you can also have system instability that may change depending on how many units are online. On page 16 is a typical cross current circuit for three units.

Hope that helps, Mike L.

 

[waross]

Check the polarity of the quadrature CT. It may have been reversed when the unit was reconnected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[osamaelkaey]

this problem showed up recently, no work done for the generator, we made more check ,kindly find below check sequences, at the end we isolated the cross current connections, and tested the system when the VRS connected on droop mode , and the system is stable. But still did not know the root cause of the instability of the PF when the VRS connected on cross current compensation connection , may the harmonics cause this problem? , as the power station supply pumps worked by VSD (variable speed drive), find below the check sequence:
1. Check voltage adjusting resistor and make sure that they operated well.
2. Checked AVR (VR6) with the same result so we replaced AVR of the three gensets generator by another with the same result.
3. Checked CCCT by:
• Checked cables between CCCT OK.
• Checked CT polarity and connection which connected in CCCT OK.
• Checked relays used in the connections OK.

4. Remove CCCT wiring connections between generators with the same result .
5. So we operated in droop mode of AVR with this readings:

Unloaded G1
G2: 405V, 495A , 0.9 PF.
G3: 405V, 505A , 0.8 PF.
Reloaded G1
G1: 402v , 0.8PF , 222 KW , 352A.
G2: 404v , 0.9PF , 178 KW , 290A.
G3: 404v , 0.9PF , 210 KW , 348A.

Unloaded G2
G1: 404V, 522A , 0.8 PF, 303KW.
G3: 406V, 477A , 0.85 PF, 309KW.
Reloaded G2
G1: 403v , 0.8PF , 193 KW , 322A.
G2: 404v , 0.8PF , 202 KW , 325A.
G3: 404v , 0.8PF , 206 KW , 347A.

Unloaded G3
G1: 404V, 531A , 0.8 PF, 308KW.
G3: 406V, 453A , 0.9 PF, 293KW.
Reloaded G3
G1: 402v , 0.8PF , 211 KW , 329A.
G2: 404v , 0.8PF , 202 KW , 321A.
G3: 404v , 0.8PF , 194 KW , 341A.