Control.com - Nerds in Control

Scenario:

Two generators (equal, identical sets) are feeding power to a switchboard through ACBs, 415V, 50Hz, 150kVA each set. The system is a 3 phase-3 wire, insulated neutral system. The governor control system is rudimentary with a Bosch type fuel pump - rack. There is a motor hooked to the rack that is supposed to increase or decrease the rack setting based on frequency sensing through the switchboard controls.

Independently both generators appear to run fine taking about 50% of rated load without issues. Unfortunately we have no means of laoding beyond that.

When the generators are run in parallel however, it is always no. 1 generator that trips on reverse power. If we are fast enough a minor adjustment of fine volt control knob brings the balance of the KVAr to normal & power factor also to normal. This however lasts only for that particular load. The moment the load is changed by even the slightest bit (2kW) the meters fly off in opposite directions again with similar results.

I am suspecting cross-current issues.

If you are referring to a reverse power (real power, not reactive power), then your problem is most likely *NOT* the voltage regulator setting. Real power is directly proportional to the amount of fuel (or steam or water or wind, or whatever the prime mover converts to torque). So, if the unit is tripping on reverse (real) power, the problem is with the adjustment of the fuel pump racks.

Are these units supplying an electrical load independent of other generators (sometimes referred to as "island mode")? Are they connected to an electrical grid with other generators? What causes the load to change?

How do you start the units? Do you bring one unit up to rated speed, then close it's breaker? How much load does it accept at that time? How do you parallel the second unit? What happens to the load on the first unit when you parallel the second unit to it?

Responding to V. Chavan's 22-Dec (02:50) query... your description of the problem lacks sufficient information to make a meaningful analysis:

Q1) Is this a plant being commisioned for the first time? Or an existing plant that has exhibited the problem for the first time?

Q2) If a new plant, then have cross-connected CT's been checked regarding ratio and polarity? In fact have safety jumpers be removed?

Q3) If an existing plant, then has there been recent work done to the wiring?

Q4) Had wiring been checked? If so, how? With primary-current injection? Or with secondary-current injection?

Q5) Can you reveal some of the system parameters?

Regards,
Phil Corso (cepsicon@aol.com)

Thank you Phil. Incidentally the generators are feeding to a common bus bar. The power system is a 3 phase-3 wire, insulated neutral system. The two generators are connected through ACB's & a power management system.
The fuel racks are fine. Each generator will supply load independently to full capacity when running by itself & no other generator paralleled to it.

Thanks, Vaishakh

2X-May-08 (xx:xx) by Phil Corso, PE
Responding to Vaishakh’s 25-May-08 (18:40) problem... the observation is typical of a cross-current compensation circuit problem.

The two compensation circuit CTs and their associated voltage regulator wiring are especially sensitive to current-direction and polarity. This type of problem is usually related to a CT installed bakward or one that is reverse-wired!

A good start is to use two clamp-on ammeters to simultaneously compare the secondary currents of each. Then short-circuit both of them. If the problem disappears it will confirm that the circuit is at fault!

Regards,
Phil Corso (cepsicon@aol.com)

Assuming that you have checked and set the unit properly, the only reason I see is as follows;
Generator governors can be set for Isochronous operation or Droop operation.

Independently operating generators are usually set in the isochronous operation.

When you have two or more generators running in parallel, they need to equally share the load available as a percentage of their capacity and both operate like 'buddies' supporting each other. Hence on such applications the governors must be selected to 'Droop' mode and the percentage of Droop is to be set. This will enable each generator to share the load in spite of any load variations.

For more details of droop I will refer to my library and come back. It would help if you specify the make/model of generator/governor etc.

Best of luck
Regards.