| With the increasing of nonlinear loads in power system, a large amount ofharmonic currents were injected into power systems, so the system voltages weredistorted seriously. And large mounts of reactive power loads in power system wentagainst the stability of voltage and efficiency of power system. So research on harmonicand power was carried out widely. The main two aspects of article are presented asfollow:①To harmonic current suppression and reactive power compensation,the activepower filter (APF) is an important development trend. As far as principles of APF areconcerned, harmonic currents of compensated target are detected, then APF generatescompensating currents which are equal to harmonic currents in amplitude and conversein polarity to purify power currents. After compensation, the power currents onlycontain fundamental components. So real-time and exact harmonic current detection isthe key.Frequency domain analysis of harmonic current detection: for the problems that thenon-integer harmonic cannot be detected by FFT transformation and the application ofpower harmonic measurement based on windows and interpolated FFT is difficult. Thispaper introduces two methods for detecting harmonics in power network signals bymeans of wavelet transform (WT) and wavelet packet transform (WPT). Becausewavelet packet transform is based on binary discrete wavelet transform, it candecompose the spectrum of a signal into uniform subbands, then improve the accuracyof detecting harmonic time frequency. The simulating result shows that the estimationof the harmonics'frequency and amplitude based on WPT reached a more reasonableaccuracy than FFT and WT.Time domain analysis of harmonic current detection: The p ,q method and i_p ,i_q method based on instantaneous reactive power theory were used as harmonic currentdetecting method of APF. This paper analyzes the detection principles,characteristicsand appliance localization of the two methods. A conclusion was drawn by analysis andsimulation that: As a harmonic current method of APF, the i_p ,i_q method was moreaccurate than p ,q method.In addition, this paper proposes a new real-time method which is a modified onefor the three-phase four-wire harmonic current detection based on the instantaneousreactive power theory. Two modified points are presented in the paper. One is adding aPI regulator after LPF; the other is putting a fundamental positive sequence voltageselected unit instead of traditional PLL. By the simulation of MATLAB it is proved tobe able to improve the accuracy of harmonic current detection and meet the request ofdynamic compensation. ②In a long time, engineers and researchers work with the assumption that systemvoltage was sinusoidal. However, nowadays the voltage distortion was more and moreserious, so research on power components and power definition when voltage wasdistorted was very meaningful. Unfortunately, nothing but active power had beenaccepted widely. Research on power components definition was made in the paperagainst these problems. The paper introduced the power components of single-phase and three-phasethree-wire load circuit, then individually represented the physical meaning andcharacteristics about active power ,reactive power ,positive-sequence power ,negative-sequence power and zero-sequence power in these circuits. Finally, This paper extrapolated similarities and differences of conventionalinstantaneous power theory and improving power theory in three-phase four-wiresystem. According to the existence of e0 and i0 , the article individually represented thephysical meaning and characteristics about active power ,reactive power ,positive-sequence power,negative-sequence power and zero-sequence power . Then,the paper introduce a newest theory——the instantaneous power theory on the rotatingp ? q ? r reference frames. Through a table, the paper compares the three powertheories (conventional instantaneous power theory,improving power theory andinstantaneous power theory based on p ? q ? r reference frames).
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