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Correcting Power Factor: An important savings factor

Pradeep Nair ,  Saturday, June 15, 2013, 10:53 Hrs  [IST]

Pradeep NairAdvanced PFC systems are very important these days as most of the utilities around the world impose huge penalties for industrial consumers with very low power factor.

These days, harmonic capacitors or “power savers” are very popular devices sold on teleshopping channels. They are being projected as the Holy Grail discovery to trim down your home electric bills. Unfortunately, the promises they make of 60 per cent reduction in residential power consumption are not true. One way to ensure that the household users don’t fall prey to these is to understand a term called power factor correction and how it impacts their bill. Power factor correction systems bring huge value to industrial power users and this is mainly due to the difference in the way industrial users and residential users are billed. To understand this, it is important to figure out how AC power works.

Power is the rate of flow of energy through a given point in the circuit. In AC circuits, the load can be resistive (e.g. bulb, electric iron) or reactive (e.g. air-conditioner, motor pumps, welding machines). The voltage and current are perfectly synchronized through a purely resistive load ensuring a unidirectional and positive power (product of voltage and current). This is called as real power (W). In a purely reactive load the voltage and current are perfectly 90 degrees out of phase. Thus their product reverses in direction over half cycle and the net power transferred is zero over a complete power cycle. This is reactive power (VAR). It doesn’t have a magnitude and thus it is scaled only in the imaginary axis in the power triangle. The power which completes this Pythagoras theorem is the apparent power (VA) which is the product of root mean square value of voltage and current. Thus apparent power is a combination of the real power that gets transferred through the load and the reactive power which gets pushed back to the source from the capacitor and the inductor in the load. The power circuit in our house and industries are never purely resistive or reactive and thus apparent power provides the real picture of the power consumed by the load. An ideal power circuit will keep the apparent power as close as possible to the real power but this is never the case and thus the true power consumed is always more than the ideal value. It is this factor which provides a very good indication of this deviation and is called the power factor.

National InstrumentsPower Factor
Power factor is the ratio of real power to apparent power. It fluctuates between -1 and 1 with the ideal value as 1 (Real power = Apparent power).It is also equal to the cosine value of the phase difference between voltage and current. Thus you will also see it being referred to as Cos f in various electrical equipment. When the phase angle difference between voltage and current is 25° then 90 per cent power is transferred efficiently, with 60° the work done by power is only 50 per cent and as we saw earlier at 90° the work done by the power on the load is zero (imaginary). A good analogy to this is a man pulling a big log of wood using a rope. If he pulls the log with the rope parallel to the ground, maximum work is done in moving the log ahead. But as he starts to create an angle between the rope and the ground, the harder will it be to pull the log. When the rope is horizontal to the ground (90°) no matter how much energy he puts in he won’t be able to move the log ahead i.e. work = 0. Hence, the best way to use power in the most efficient way is to reduce reactive power in the circuit, thus bringing the apparent power to as close as the real power. This is done by power factor correction systems.

Power factor correction systems
The way to correct power factor is to reduce the reactive power in the circuit. Most of the industrial loads have extensive inductive load giving rise to a lagging current, reducing the power factor. Inductive current and capacitive current have an inverse relationship as capacitors discharge current when inductors conduct it. So the best way to correct the lagging current due to inductive load is to add capacitor banks to the circuit. But at times the reactive elements starts resonating with each other causing voltage fluctuations and hence leaving the system unstable. Thus it is very important to conduct a complete engineering survey by an expert before PFC systems are installed specially on non-linear loads. For non-linear loads a passive PFC systems alone doesn’t solve the problem. Active PFC systems which can re-shape the current waveform through a load provide better power factor correction capabilities for circuits with non-linear loads. They also work in multiple stages providing automatic power factor correction and continuously targeting an optimum PF value. They perform high end online data processing to provide real-time PF correction.

Advanced embedded systems like National Instruments CompactRIO platform are used to make Smart Active Power Factor Correction systems. The versatility, modularity and flexibility of NI CompactRIO allow it to directly acquire voltage and current signals from multiple power lines simultaneously. The availability of FPGA on these systems allow it to perform high-end number crunching to calculate the derived values, compute complex analysis and provide control output signals to take corrective actions on a real-time basis for e.g. controlling a thyristor-switched capacitor banks on PF correction on fast changing dynamic load circuits. The ruggedness and reliability of this hardware allows it to be used in hazardous condition continuously for years together. National Instruments LabVIEW electrical power suite software provides advanced analytics tools which help to program these PFC systems as per IEC standards.

National InstrumentsBuilt on such advanced technology there are various types of smart PFC systems supplied to global market which can perform automatic and static power factor correction. As these systems are modular and flexible, they can be added with additional I/O’s to perform other power quality measurements like frequency variations, flickering, voltage dips, swells, unbalances and harmonics. This helps to identify and correct the power circuits of various potential faults which can cause a reduction in the PF value. For example they are used in heavy harmonic infused power distribution systems to detect and prevent harmonic amplifications providing advanced and fast detuned power factor correction.

Such advanced PFC systems are very important these days as most of the utilities around the world impose huge penalties for industrial consumers with very low power factor. This is because the utilities have to provide the additional reactive power while they charge only for the real power. Also apart from the average energy consumption measured in kWh, industries are also charged for maximum demand consumption which is calculated in kVA (apparent power). Thus power factor correction is very effective and helpful in reducing the electric bill for industrial users compared to residential users where the savings are minimalistic to the cost of the equipment.

In addition to these benefits, PFC also helps to curb line losses, voltage fluctuations and meet contractual and regulatory agreements. So if you operate a factory and haven’t still implemented a power factor correction system, it will be worth spending some time evaluating your options because correcting power factor is a very important savings factor for your business.

(Pradeep Nair is Business Development Manager – Energy, National Instruments)
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