Effects of Reactive Power Flow Power Factor Correction

Figure A.21 represents a basic circuit of energy transportation from a generator to a consumer through a transmission line, which is simply represented by a series inductance and resistance. The consumer is represented by an inductive resistive impedance. The reason for these representations are explained in Chapter 5.

The consumer 'absorbs' both PL and QL. The generator or 'the utility' has to supply both the PL of the load and the transmission loss given by I2Rt, where Rt is the resistance of the transmission line. It also has to supply the QL of the load plus the I2Xt absorbed by the reactance of the transmission line. The transmission loss is therefore strongly influenced by the transfer of QL.

To appreciate this, assume that the load consists solely of an inductor, which absorbs Q but not P . In this case the energy meter at the consumer premises, which records kilowatt-hours (i.e. energy purchased), indicates zero but the finite current in the transmission system results in /2Rt. which has to be supplied by the utility. Clearly this is a most undesirable scenario which the utility endeavours to discourage, in the case of large consumers, through special tariffs that penalize the absorption of Q.

Generator

Consumer (load)

Figure A.21 Simple transmission system b a

Generator

Consumer (load)

Figure A.21 Simple transmission system b a

Additionally, the utility has the legal obligation to supply power to the consumer at a more or less fixed voltage. In Chapter 5 it is shown that for transmission lines, with Xt > Rt, the voltage VL at the consumer terminals is particularly sensitive to changes in Q rather than P. It should now be obvious that the utility encourages consumers to draw P at minimum Q, i.e. with cos $ as close to unity as possible.

If a capacitor C were to be connected across the load terminals and sized to generate QC so that QC = Ql, then the consumer will appear to the utility as having a unity power factor. The utility would be delighted with such an arrangement as this will minimize losses in the transmission line resistance Rt and will ensure minimal variation in consumer voltage. This procedure is known as power factor correction. Such capacitors are used extensively in power systems to generate or 'inject' reactive power at strategic points of the network.

This discussion also highlights why power system equipment is rated in terms of apparent power VA rather than P or Q. As an example take the transmission line in Figure A.21. This line is suspended from insulators that have been designed to withstand (including a safety factor) the nominal voltage of transmission VL. It is also made of a conductor that has a specific resistance per km and chosen so that when the rated current is carried the heat generated does not exceed a level at which the expansion of the line produces an illegal sag at the lowest point of the catenary. With a purely inductive consumer load absorbing the rated current at nominal voltage the transmission line will be fully loaded in spite of the fact that it transports no useful power. Similar considerations apply to other power system equipment.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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