Why One Generator is Carrying the Entire Load?

There are various types of control and load sharing schemes that can be used when generators are operating in parallel which are droop mode and isochronous mode. Please see some of the various types below.

Droop Mode

I am making an assumption here that you need a little guidance for generator load sharing philosophy, so please disregard this section if you know about it already. I am explaining droop mode as it will help you understand isochronous mode.

Droop mode can be used on a generator when one or more generators are operating islanded or in parallel with the utility. A simple definition for this is that the engine speed/alternator terminal voltage decreases when the generator kW/kVAr load increases. A typical droop setting is that between 0%-100% of the generator kW/kVAr load, the engine speed/alternator terminal voltage will decrease (or droop) by 4% of the no load engine speed/alternator terminal voltage. This percentage can be call the "droop window %", but I am sure that there are other names for it. Droop can also be defined as "Varying load = varying engine speed/alternator terminal voltage" for short.

This control scheme can be used to load share with similar or different sized generators in an islanded configuration or in parallel with the utility supply.

Before operating multiple generators in an islanded scenario, to ensure that the generator equally share the kW/kVAr load the "droop window %" must be the same for the engine governor/voltage regulator. Also, the no load engine speed/alternator terminal voltage must also be identical. That way, whether the generators are of equal size or different size, they will share the load to the same percentage or their individual rated load.

Example

A one 4 pole generator is started and connected to a dead islanded network. The droop setting is 4% and the no load engine speed is 1560RPM and the no load alternator terminal voltage is 431.6V AC. The load is such it will be stepped up so that kW/kVAr output from the generator is equal to 100% of the it's capacity. So, at 100% of the generators capacity, the engine speed/alternator terminal voltage will now decrease to 1500RPM/415V AC, exactly 4% droop.

Now another identical generator with identical settings is synchronised and connected to the same network the fist generator is connected to, then the load on the first generator is decreased by half as it will be take up by the second generator. Now both generators will be supplying 50% of the load. In this case, the engine speed would now be 1530RPM and the alternator terminal voltage would be 423.3V AC. This equates to 2% droop as the generator load is at 50%.

When connected to the utility supply, the utility will dictate the frequency (and in turn the engine speed) and the alternator terminal voltage. So in this case, when the utility frequency/voltage increases, the generator frequency/voltage will stay the same as the utility supply and the kW/kVAr will decrease based on the droop settings The exact opposite will happen when the utility frequency/voltage decreases.

Isochronous

Isochronous mode can be used when one or more generator is operating islanded and has load sharing line connected (I will explain this detail later). You can think of isochronous mode as droop mode with a "droop window %" of 0%. So the engine speed/alternator terminal voltage will remain the same no matter what load is on the generator up to it capacity (0% - 100%). Isochronous can also be defined as "Varying load = Constant engine speed/alternator terminal voltage" for short.

Now when another generator using an isochronous load sharing scheme with the same engine speed (or frequency) and voltage set points are connected to each other without load sharing lines, a first impression would be that the load will be shared equally between the generators and the voltage and frequency would not change. However in the real world, this is not the case.

In reality, each controller would have some very slight errors in the frequency and voltage readings, and as such would control the output based on each individual reading.

Lets look at the above scenario without load sharing lines and looking at frequency and kW in the example only (the same observations can be made for the relationship between voltage and kVAr).

- Generator 1 would read 50Hz, but is actually operating at 50.001Hz.

- Generator 2 reads 50Hz, but is actually operating at 49.999Hz.

When they are synchronised and connected together, what will happen is the following:

- The actual frequency for generator 1 would decrease to 50Hz, but the controller would read approximately 49.999Hz. To compensate, the controller would increase the fuel demand to the governor to increase the frequency it is reading to 50Hz.

- The actual frequency for generator 2 would increase to 50Hz, but the controller would read approximately 50.001Hz. To compensate, the controller would decrease the fuel demand to the governor to decrease the frequency it is reading to 50Hz.

While this is happening, the load on generator 1 would increase until it reaches it's current limit. The protection relay would then open the circuit breaker and shutdown the generator if it is set correctly. In the mean time, the load on generator 2 would decrease until it starts motoring the engine. The protection relay would then open the circuit breaker and shutdown the generator if it is set correctly.

As mentioned at the start, the above also applies to the relationship between voltage and kVArs.

To prevent this error between controllers, load sharing lines are installed between each generator controller to make small adjustments so that both the frequency/alternator terminal voltage remain the same and the kW/kVArs are shared correctly (as per the settings).

Due to the nature of isochronous load sharing, this cannot be used when in parallel with the mains supply. You can either use the droop mode described above to do this, or you can look at using base load and power factor control load sharing modes.

I will explain in another post if you are interested.