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What's wrong in consuming reactive power?

EM News Bureau ,  Monday, June 23, 2014, 14:28 Hrs  [IST]

 
Raviteja
Raviteja Chivukula


If millions of users start consuming considerable amount of reactive power, the power utility company will collapse in losses.
We keep hearing regularly that the low power factor is bad, and that power factor should be kept greater than 0.95, low power factor is inefficient, and so on. Many of us usually accept it as a thumb rule. Rather than accepting it, let us go a step further in rigorously understanding why low power factor is bad? Whom does it affect the most? What happens if you don't worry about low power factor? How do you correct low power factor? etc. To answer these questions, let us understand some commonly used terms like instantaneous power, real power, reactive power, apparent power and power factor.

Simply put, instantaneous power is given by P(t) = V(t) x I(t) . Instantaneous power is a function of time. So if I measure the voltage level given to the load at this instant and measure the current drawn by the load at the same instant and multiply them, I get instantaneous power. Though it is easy to measure, instantaneous power may not be all that useful. In a typical AC power system, the instantaneous power may not always be positive. In a time of 20ms (corresponding to 50Hz), the instantaneous power may change multiple times between positive and negative as seen in Figure 1. How much time does it remain positive, how much time does it remain negative depends on the kind of load connected.

For purely resistive loads (like bulbs, electric iron etc), the voltage and current are perfectly in phase with each other. This means that the instantaneous power is always positive. The flow of electrical power is always from the grid to the load and not in the other direction.

 
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Figure 1: Voltage, Current and Instantaneous Power Waveforms
On the other extreme, if the load is purely reactive, the voltage and current are 90 degrees out of phase.

If V(t) x I(t) is computed over time, it can be seen that the instantaneous power changes direction every quarter cycle (5ms for 50 Hz AC). This means, the average power transferred over time, from the supply to the load is zero.

Typical loads however are not purely reactive. In fact, if a load is purely reactive, on an average, it cannot do any useful work for us. Typical loads are a combination of resistive and reactive loads. The resistive load component gives us useful work, while the reactive load component is just an overhead. The Pythagorean sum of real power and reactive power is termed as the apparent power and the ratio of real power to apparent power is termed as power factor.

Now, one may ask, if I am consuming power for 5ms and then pumping back power for 5ms, what is the problem? In order to answer this question, let us go back to the concept of reactive power. If a load is purely reactive, it consumes energy from grid for 5ms and then pumps energy back into the grid for 5ms. This means, I am not getting any useful work from this load. But still, current has to flow back and forth into the load from the grid. And this current travels all the way from the generating power station to my load. Due to this flow of current, all the way from the generating station to the load, heating losses occur in the transmission lines. So the bottom line is: if the utility company just charges for the 'real power' that you consumed, it will not be able to bear the losses occurred due to reactive power. And if millions of users start consuming considerable amount of reactive power, the utility company will collapse in losses. Hence the utility companies need to penalize low power factor.

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Figure 2: NI CompactRIO is a generic embedded platform that can programmed for various functionalities like Power Factor Correction, PMU, Power Quality Analyzer, Smart Recloser etc.

One thing to note here is that, power factor is entirely a consequence of the load connected. The generation side does not govern the power factor. So, the onus is upon the load to correct it to make sure that the reactive power is very small as compared to the apparent power. i.e. the power factor should be close to 1.

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 need it and vice versa. So the best way to correct the lagging current due to inductive load is to add capacitor banks to the circuit. So, instead of borrowing the reactive current all the way from the generation site through the transmission lines, we are borrowing it from the locally present capacitors. For non-linear loads 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 nonlinear 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 computation to accurately perform power quality analysis and provide control output signals to take corrective actions on a realtime basis for e.g. controlling a thyristor-switched capacitor banks on PF correction on fast changing dynamic load circuits. The rigidity and reliability of this hardware allows it to be used in hazardous condition continuously for year's together. National Instruments LabVIEW electrical power suite software provides ready to use functions which helps to compute various power quality measurements like frequency variation, flicker, voltage dips, harmonics etc. as per IEC standards.

The CompactRIO system is programmed using NI LabVIEW Graphical System Design platform, and it can be customized as well as upgraded in the field. With minimal/no addition to the hardware, it can be programmed to behave as a number of different "personalities", such as Power Factor Correction System, Phasor Measurement Unit, Power Quality Analyzer, Smart Switch, Recloser, etc. Due to the modular nature of the hardware & remotely upgradeable software, NI CompactRIO is an ideal approach for smart grid applications that demand evolving functionality and requirements.


( Raviteja Chivukula is Technical Marketing Engineer, National Instruments )
 
                 
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