Power Factor Correction for Drilling Rigs
SCR systems deliver a notoriously poor power factor. Compensation is built into the design of the engine and generator combination, so the question that is often asked is: Can I get more power to my Mud Pumps with Power Factor Correction?
Before getting started it's worth reviewing what we mean by Power Factor. Figure 1 shows a typical industrial installation where the generator KVA is matched to the engine KW. The blue line represents a load which contains an inductive component, typical of installations with a large number of induction motors. The power factor is expressed as the ratio of the real power (or current) to the total power (or current), and is the cosine of the angle Φ. In this case the angle is 30 degrees, a power factor of 0.866.
Please note that throughout this article we move freely between KVA and current. This is because the voltage is a constant when calculating real, reactive or total power and can be ignored for the most part when discussing the ratios between each.
Drilling Rig Power Limit
Most drilling rigs are designed with the poor power factor in mind, and for this reason the generator is oversized so that the engine KW is about 0.7 of the generator KVA. In other words, power factor compensation is in-built for a power factor equal to or better than 0.7. If the power factor is less than 0.7 the limiting component is the engine KW. Less than 0.7 then the limiting component is the generator current capability or KVA. It's worth mentioning at this point that the rig should have a Power Limit system fitted which limits the SCR output to keep the total rig load within these limits. It goes without saying that if the Power Limit is not correctly calibrated then you will not achieve full power output (or conversely you risk blacking out the rig).
Power Factor on a Drilling Rig
Figure 3 shows a typical drilling rig power limit envelope with the vectors of two different load power factors. The blue line shows a power factor of 0.866, and the length of the line represents the maximum available current (or total power) available at this installation with this power factor. The limiting component here is the engine size because the power factor is greater than our built-in compensation of 0.7.
The purple line shows a worse power factor of 0.5, and in this case the limiting component is the generator current capability or KVA. Now it looks like there is unused capacity in the engine, which is what prompts the question about getting more power.
Power Factor CorrectionIn Figure 4 we have added a 'base load' as an orange line consisting of a bank of capacitors. With no other loads connected the load will be purely capacitive, and the power factor will be very poor but in the opposite direction to the power factor caused by inductive loads and the SCR system.
When the SCR load is added in, it can be seen that the orange vector is much greater in length than the uncompensated purple line before it hits the conjunction of the engine KW and the generator KVA ratings. It is this increase in length that leads to more power and allows the Mud Pumps (for example) to be driven harder.
This final diagram illustrates some important points:
- Firstly, the amount of capacitance is optimised for one power factor only, in this case it's 0.5. At other power factors the angle of the orange vector will be different, and the engine or generator power limit comes in to play.
- If we have a fixed level of capacitive compensation and the power factor is better than the optimum level all that is happening is that I am adding KW to my engine load which means increased fuel costs. The same occurs if the load is not up against the limits: the extra capacitance adds fuel cost if it is not required or is over-compensating. This makes a case for dynamic compensation using contactors or a thyristor controlled regulator.
- On this diagram, which is a good proportional representation, it can be seen that the amount of capacitive compensation required to compensate fully for a power factor 0.5 is quite a significant proportion of the generator KVA rating, about 40% in this case. This translates to about 600KVAR of capacitance PER GENERATOR, and the load current at 600V will be around 600 amps. It may be that a design compromise has to be made between space available and level of compensation.