Reactive power compensation of DG sets

Whenever an industry is drawing power from utility, there are no major complications in managing the reactive power and close to unity power factor can be maintained. But when diesel generators are operating, some precautions need to be taken care for managing the reactive power. This article briefs the performance of the DG sets at various power factors and thereby showing the optimal manner of power factor compensation of generators with some examples.

Normally DG sets are rated in apparent power (kVA) along with power factor and typical rated power factor is 0.8 lag (considering the power factor of motors, without any capacitors) irrespective of alternator's apparent power. The diesel engine's mechanical output power (bhp / kW) is designed to match the electrical real power (kW = kVA x PF) of the alternator.

For example, consider a generator rated for 1000 kVA and lagging power factor of 0.8. The maximum possible real power (kW) the generator can supply is 800 kW (rated current = 1739 A). Thus, the diesel engine will also be rated to deliver 800 kW equivalent mechanical power to the alternator. The following cases show the operation of the alternator at different power factor.

Apparent power = 1000 kVA; connected load = 800 kW; power factor = 0.8

Current drawn, I₁ = 1739 A 

• Here, the generator is operated at the rated name plate values. The load draws maximum rated current from the generator and the generator draws the maximum permissible mechanical power (800 kW) from the diesel engine.

Apparent power = 1000 kVA; connected load = 800 kW; power factor = 0.6

Current drawn, I₂ = 1000*1000/ (1.732*415*0.6) = 2318.9 A

• In the case 2, the connected load is 800 kW (operated at 0.6 PF) which is equal to the maximum mechanical power that the engine can deliver. But the actual current drawn by the load is greater than the rated alternator current. This results in generator overloading. In order to avoid generator overloading, the maximum load (at the same PF) that can be connected to the generator is 600 kW at 0.6 PF.
• Conversely, if the connected load is 600 kW and if the power factor is at 0.6, the entire generator capacity (1000 kVA) is blocked for this partial load. If the power factor was to be improved to 0.8, then as in case 1 additional 200 kW load could have been connected to the same generator, thereby increasing the capacity and productivity of the industry.

Moreover, if 600 kW load is operated at unity power factor, the load current (1391 A) will be reduced by 40%, thereby 64% reduction in the copper loss / cable loss. This will result in fuel savings of the engine. This is what is described in case 3 below. 

Apparent power = 1000 kVA; connected load = 800 kW; power factor = 1.0 Current drawn, I3 = 1391 A

• The genset is connected to its full capacity of 800 kW and operating at unity power factor. Unlike case 1, the load draws 20% less current at UPF (1391 A). This in turn results in significant reduction in copper loss and thereby saving some fuel.
• Here, at UPF, even though the generator can support a load of 1000 kW, the diesel engine is rated to deliver a maximum mechanical power of 800 kW.
• Even though the current drawn is less than the rated current (1739 A), the genset is delivering its maximum real power (800 kW).
• Hence at UPF, utmost care should be taken such that the total connected electrical load must not exceed 800 kW. Otherwise the diesel engine will be overloaded.

• Close to unity power factor shall be maintained for the loads connected to the DG sets by using capacitors. This will result in the reduction of copper loss and hence savings in fuel.
• The load (active power) connected to the generator must not exceed the engine's equivalent kW rating.