| Power Factor Correction |
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EXPLANATION When an electric load has a p.f. lower than unity, the Apparent Power delivered to the load is greater than the Real Power that the load consumes. Only the real power is capable of doing work, but the apparent power determines the amount of Current that flows in to the load, for a given load voltage. Energy losses in transmission lines increase with increasing current. Power companies therefore require that customers, especially those with large loads, maintain the power factors of their respective loads within specified limits or be subject to additional charges. Engineers are often interested in the power factor of a load as one of the factors that affect the efficiency of power transmission. For a Linear Circuit operating from a sinusoidal voltage, the current must be a sinusoid at the same frequency. When the current is exactly in Phase with the voltage, the power factor is 1. This corresponds to a purely Resistive load. A toaster is an example of an approximately linear device, an electric motor is not. For a non-linear circuit, such as a switchmode power supply, the current is not necessarily sinusoidal. In that case there is current at Harmonic s of the voltage frequency. PFC returns the power factor of an electric AC power transmission system to very near unity by switching in or out banks of capacitors or inductors which act to cancel the inductive or capacitive effects of the load. For example, the inductive effect of motor loads may be offset by locally connected capacitors. It is not possible to cancel out harmonic current using this technique, so different techniques must be used to power factor correct nonlinear loads. ELECTRICITY INDUSTRY ASPECTS PFC is desirable because the source of electrical energy must be capable of supplying real power as well as any Reactive power demanded by the load. This can require larger, more expensive Power Plant equipment, Transmission Line s, Transformer s, Switch es, etc. than would be necessary for only real power delivered. Also, resistive losses in the transmission lines mean that some of the generated power is wasted because the extra current needed to supply reactive power only serves to heat up the power lines. The electric Utilities therefore put a limit on the power factor of the loads that they will supply. The ideal figure for load power factor is Unity (that is, a purely resistive load). Real loads deviate from this ideal. Electric Motor loads are Phase Lag ging ( Inductive ), therefore requiring Capacitor banks to counter this inductance. Sometimes, when the power factor is Leading due to Capacitive loading, inductors (also known as ''reactors'' in this context) are used to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth between each AC cycle. Unity Power Factor gives rise to the greatest utilization of the plant. Electricity utilities measure reactive power used by high demand customers and charge higher rates accordingly. Some Consumer s install power factor correction schemes at their factories to cut down on these higher costs. The units of reactive power is ''volt-amperes reactive'' ( VAr ). APPLICATION Electricity industry: power factor correction of linear loads Power factor correction is achieved by complementing an Inductive or a capacitive circuit with a (locally connected) reactance of opposite phase. For a typical Phase Lag ging p.f. load, such as a large induction motor, this would consist of a Capacitor 'bank' in the form of several parallel capacitors at the power input to the device. Instead of using a Capacitor , it is possible to use an unloaded Synchronous Motor . This is referred to as a Synchronous Condenser . It is started and connected to the Electrical Network . It operates at full leading Power Factor and puts VAR s onto the network as required to support a system’s Voltage or to maintain the system power factor at a specified level. The condenser’s installation and operation are identical to large Electric Motor s. The Reactive Power drawn by the synchronous motor is a function of its Field Excitation . Its principal advantage is the ease with which the amount of correction can be adjusted; it behaves like an electrically variable capacitor. Switchmode power supplies: power factor correction of non-linear loads A typical Switchmode Power Supply first makes a DC bus, using a Bridge Rectifier or similar circuit. The output voltage is then derived from this DC bus. The problem with this is that the Rectifier is a non-linear device, so the input current is highly non-linear. That means that the input current has energy at Harmonic s of the frequency of the voltage. This presents a particular problem for the power companies, because they cannot compensate for the harmonic current by adding capacitors or inductors, as they could for the reactive power drawn by a linear load. Many jurisdictions are beginning to legally require PFC for all power supplies above a certain power level. The simplest way to control the load. At this point the power factor can be brought to near unity, using capacitors or inductors as required. This filter requires large-value high-current inductors, however, which are bulky and expensive. It is also possible to perform Active PFC . In this case, the bridge rectifier is replaced with a switchmode preregulator. The preregulator attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switchmode converter inside the power supply produces the desired output voltage from the DC bus. This approach requires additional semiconductor switches and control electronics, but it permits cheaper and smaller passive components. It is frequently used in practice. REFERENCES
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