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The technique for correction of a harmonically distorted input current waveform in passive power factor correction is the addition of passive components to create filters; these filters are tuned to sink or prevent the propagation of harmonic currents [48].

This section will outline the case of a bridge rectifier connected to the mains supply, and the various filter configurations that can be applied to correct harmonic currents. These filters will be discussed based on their strengths and weaknesses.

Figure 4-9 illustrates a rectified mains supply powering a resistive load:

Figure 4-9: Bridge Rectified AC Supply With Smoothing Capacitor

The presence of the smoothing capacitor C1 in the bridge rectifier will cause a phase difference between the voltage and current waveforms, but this behaviour will be ignored for the purpose of illustrating harmonic correction; when a harmonic correction technique also benefits the phase angle separation it will be noted. The harmonic filtering can be used in conjunction with reactance balancing of the load to achieve minimal current phase angle and maximum purity factor.

The capacitance of the smoothing capacitor C1 will have an effect on the amplitude of the harmonic current drawn when the bridge rectifier diodes forwards conduct; the larger the value of the capacitor the greater the amplitude of the harmonic current. Reduction in the size of the capacitance will reduce the peak amplitude of the harmonic current, but significantly increase voltage ripple which is undesirable.

53 | P a g e C h a p t e r 4 |Power Supply and Electronic Converter

The harmonic filter circuits which will be discussed are: x AC Side Series Inductance

x Series Band-pass filtering x Harmonic trap filtering x Capacitor-fed rectifier

4.4.2.1AC Side Series inductance

The AC side series inductance harmonic correction is shown in Figure 4-10:

Figure 4-10: AC Side Series Harmonic Distortion Filter

The presence of the inductor between the mains supply and the rectifier smooths the current waveform and reduces the current phase angle created by the smoothing capacitor C1. The design consideration for the inductor is such that the magnitude of the current flowing is zero when the voltage waveform crosses the zero axis. The main points of comparison for this harmonic correction technique can be summarised by:

x Low-cost and high reliability due to single component x Moderate correction to the current phase angle x Moderate correction to the harmonic currents

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4.4.2.2Series Band-pass filtering

The series band-stop filter is shown in Figure 4-11 and is present on the AC side of the rectifier:

Figure 4-11: AC Side Series Band-Pass Harmonic Distortion Filter

The frequency response of the filter is described by the following equation:

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The band-pass filter consists of an Inductor L and a capacitor C2, and is tuned to allow the line frequency to pass, and to attenuate frequencies either side of the line frequency; as line frequencies are low this tends to result in large values for the band-pass components. The band-pass harmonic filter effectively eliminates harmonic currents and provides good current phase angle correction; this phase angle change can be further corrected by reactance balancing of the load.

The main points of comparison for this technique can be summarised by: x Moderate cost due to two high value components

x Good correction to the current phase angle x Excellent correction of harmonic currents

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4.4.2.3Harmonic trap filtering

A modification to the series band-pass filter is the harmonic trap filter. This approach is shown in Figure 4-12 and is present on the AC side of the rectifier:

Figure 4-12: AC Side Harmonic Distortion Trap Filtering

The difference between this filter and the series band-pass filter is the series LC network is providing a very low impedance path around the bridge rectifier; this prevents the harmonic current from flowing through the load. The resistor is present in each trap filter to limit the peak value of the current flowing through the filter. The presence of this resistor reduces the efficiency of this approach.

The frequency response equation of each harmonic trap filter is same as the series band- pass. Each harmonic trap filter is tuned to a specific harmonic frequency, therefore this approach is expensive due to the high component count.

The harmonic trap filter exhibits excellent harmonic current correction, and excellent current phase angle balancing when tuned appropriately; the current phase angle can be further corrected by reactance balancing of the load.

The main points of comparison for this technique can be summarised by: x Highest cost due to individual trap filters for each harmonic frequency x Excellent correction of current phase angle

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4.4.2.4Capacitor Fed Rectifier

The capacitor-fed rectifier is shown in Figure 4-13, and is present on the AC side of the rectifier:

Figure 4-13: AC Side Capacitor-Fed Bridge Rectifier Harmonic Distortion Filter

The capacitor-fed rectifier is a very simple circuit which reduces harmonic current amplitudes. The harmonics are still present as steps in the sinusoidal current waveform when the rectifier switches.

The current phase angle correction of this technique is poor and will require reactance balancing of the load. There is also a significant decrease in available voltage across the load.

The main points of comparison for this technique can be summarised by: x Very low cost due to single component

x Poor correction of current phase angle x Poor correction of harmonic currents x Significant reduction in output voltage

57 | P a g e C h a p t e r 4 |Power Supply and Electronic Converter