Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Varjani, Ali Yazdian, Harmonic control techniques for inverters and adaptive active power filters, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 1998. http://ro.uow.edu.au/theses/1949
This thesis is concerned with the general issue of power quality. The specific areas of interest include harmonic distortion and its minimisation. In particular the thesis considers a PWM switching strategy which yields near optimal performance in terms of harmonic distortion as well as on-line harmonic detection mechanisms and adaptive active power filtering solutions.
For the purpose of load side harmonic reduction, a novel equal area based PWM (EAPWM) switching strategy is developed which is suitable for voltage source full bridge inverter applications. The objective of this strategy is to minimise both the harmonic distortion and the switching losses in the inverter. Switching losses in the inverter are minimised by developing a hybrid switching sequence. The harmonic distortion is minimised by adopting a technique which ensures that the PWM pulses are placed at appropriate positions of choice based on an equal area criterion so that their areas are better matched with the areas under the reference waveform.
The EAPWM technique is evaluated and its performance is compared with existing PWM techniques including natural and regular P W M switching strategies. The performance evaluation and comparison is based on the total harmonic distortion and maximum inverter fundamental output voltage. For a case where the ideal output waveform is sinusoidal it is shown through simulation that the proposed technique provides a PWM output with minimum harmonic distortion and maximum fundamental voltage.
The second issue addressed by the thesis is adaptive active power filtering. The objective is to develop an economical solution where a partial and flexible harmonic reduction technique is provided such that the established harmonic standards are satisfied. Partial and selective compensation of those individual harmonics which exceed the recommended levels as set by regulatory bodies reduces the rating of active power filters thus leading to cost savings. This approach contrasts with existing techniques where the objective is to reduce all possible harmonic components to zero.
A new control strategy for active power filters that combines adaptive online harmonic estimation with partial and selective harmonic compensation schemes has been implemented within an integrated controller. T o have an accurate online estimation of harmonic components, a new adaptive structure based on a combination of resonator filter bank and frequency demodulation frequency tracking is proposed.
Performance evaluation of the proposed technique for harmonic estimation for time-varying nonlinear load is carried out where the simulation results show that the proposed filter bank structure provides better performance when compared to widely used conventional technique such as short term Fourier transform. The proposed control strategy has been implemented using a digital signal processor. Experimental results from a laboratory prototype are presented showing steady state and transient performance. It is shown that the proposed harmonic estimation together with the flexible harmonic compensation scheme provides an efficient solution in reducing the power rating of the active power filter while limiting specific harmonics to desired levels of compensation.