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Power Factor Correction/Harmonic Reduction
In Modern telecommunication power supplies are among the devices that have a characteristic of distorted input current. Quite different from resistive heaters, toasters and tungsten light bulbs, typical switched-mode power supplies such as those in switching system draw input current in short pulses rather than in smooth sine waves. In order to deliver the same amount of power in short pulses, the current peaks are much higher. This puts more stress on the wiring in the circuit breakers, and even the generation and distribution equipment provided by the electric utilities. Figure 1 shows a typical distorted input current of an electronic load (a switched-mode power supply in this example).
To minimize these stresses and maximize the power handling capabilities of a switched-mode power supply, circuitry can be added to improve the shape of the input current. Ideally, the input current should have a sine waveshape, and be in phase with the input voltage. In this case, the maximum amount of power can be drawn from the ac line within the limits of power available from the source.
By definition, the input power factor is the real power divided by the apparent power (volts rms x amperes rms). The power factor will be unity (1.00) when the input current waveshape matches the input voltage waveshape. It also follows that the input current harmonics will be zero, since all of the power is concentrated in the fundamental frequency component. Without power factor correction (PFC) circuitry, typical switched-mode power supplies have power factors of approximately 0.6, and have considerable odd-order harmonic distortion (third harmonic often as large as the fundamental, with higherorder harmonics decreasing as their frequency increases). With power factor correction, today’s switched- mode power supplies can approach the ideal case, with power factors of 0.99 and harmonics well below 5%.
Power Factor Correction Control
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Present cosp
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Present L-L voltage + maximum
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Present current + maximum
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D kvar & S kvar
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Weekly average P.F. and present P.F.
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Capacitor overload % + maximum
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Present panel temperature + maximum
Windows Base Monitoring
3 Phase Capacitor
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