Paralleling Isoch Mode

[car81]

When paralleling a diesel generator which is in Isoch Mode to a bus fed by a station service transformer, why does the source with a higher frequency take on all the load? That is, for example, the diesel will either overload or it will drop all the load, depending if the system frequency is lower or higher than its own setting. Thanks

 

[coulter]

When paralleling a diesel generator which is in Isoch Mode to a bus fed by a station service transformer, why does the source with a higher frequency take on all the load? That is, for example, the diesel will either overload or it will drop all the load, depending if the system frequency is lower or higher than its own setting. Thanks

I'm going to assume the station service bus is pretty stiff - either a utility, or a large generation capacity.

Let's look at the generator controls:
There are only two, the throttle and the DC rotating field drive. They react differently depending on if the gen is islanded or parallel.

Island operation:
The throttle controls the frequency. If the load increases, the gen slows down and the governor opens the throttle to increase speed back up to setpoint.

The DC field drive controls the voltage. If the voltage sags, the voltage regulator increases the field drive to raise the voltage back to setpoint.

Parallel operation.: When the gen is paralleled with a (fairly stiff) bus, consider the frequency is stuck and the voltage is stuck. The generator doesn't have enough power to pull the bus (utility) around. The two controls react differently than when islanded.

The throttle now controls load share. If the governor opens the throttle, the gen assumes more load.

If the field drive increases, the generator puts out more vars. I'll leave this out for the rest of the discussion. It doesn' t have much to do with your question.

Now let's look at governor control modes:
Isoc is generally defined as the generator control algolrithm does all it can to hold frequency at setpoint.

Droop uses a linear freqency/kw equation. If the frequency drops the throttle opens up, but not enough to pull the frequency back to setpoint. Depending on the droop slope and the frequency setpoint the gen should settle out some where at middle of the governor control range. This mode can be used when paralleling.

The only other common control mode is constant gen kw. The throttle is essentially fixed. This is the most common to use when paralleling

Isoc is never used when paralleling. As you noted, the gen is always fighting the utility and generally loses.

hopefully this make sense

carl

 

[nawao]

Paralleling generators

Paralleling generators

You can use governor Isoch mode when paralleling generators in an isolated bus (island mode). Paralleling to a Grid can only be done in governor droop mode. In both cases the alternator AVR will be in droop mode.