Harmonic Reductin,Low pass (L-C) filter circuit on ac side & Active Shaping of Input (line) Current
The important point to be noted is that, recently due to increasing use of power electronic units, utility or electricity supply agencies (boards), have restricted that the power is drawn by the consumers, so as to decrease the harmonic content in the input current, or make it sinusoidal, and at the same time, improved load power factor is achieved. Two schemes –
(a) Passive (filter) circuits and
(b)Active shaping of input line current
Low pass (LC) filter circuit:
Fig: Output & Input current waveforms and Low pass filter on source AC side
- Assuming that output (load) current is constant (dc) without any ripple, the ac input (source) current is square wave in, as this current changes sign, when the input voltage changes sign.
- If a Fourier analysis of the above current is done, there are harmonic components present in it. Just as filters have been used on the output (dc) side, a low pass (L-C) filter is used on the input (source) side to reduce the harmonic components in the input current.
- The inductors used tend both to improve the power factor and also reduce harmonics as given earlier. The overall energy efficiency remains the same, though additional losses occur in the inductors, but conduction losses in the diodes are reduced.
Active Shaping of Input (line) Current:
- By using a power electronic converter for current shaping, it is possible to shape the input current drawn by the single phase bridge converter (rectifier) to be sinusoidal and also in phase with the input voltage.
The choice of the power electronic converter is based on the following considerations:
- No need for electrical isolation between the input (dc) and output (dc) sides
- the power flow is always unidirectional from the utility side to the equipment
- the cost, power losses and size of the circuit used should be small.
- The basic principle of operation is as follows. At the input side, the current, is, is desired to be sinusoidal, and also in phase with the voltage, vs.
- At the full wave bridge converter output, iL and sv have the same waveform The control used is constant tolerance-band one. Here, the current. iL, is controlled, such that peak-to-peak ripple Irip in iL remains constant.
- The reference input, i*L , is made sinusoidal having same (line) frequency. With a pre-selected value of Irip, iL is forced to be in tolerance band (iL Irip/2) and (iL – Irip/2) by controlling the status of the switch, S.
- So, the input current, iL, follows the reference input, , which is sinusoidal.
Fig: Active harmonic filtering: (a) Step-up converter for current shaping (b) line waveforms (c) |Vs| and iL