Study On Harmonic Currents Detecting Methods In Active Power Filters

Active power filter is an advanced power electronic device, which can be used for integrated compensating harmonic. Compared with some conventional measures, the active power filter has many advantages and favorable prospects. Because of the characteristics, real-time and accurate compensation, it is possible to take Ml advantage of digital signal processing and control technologies. If so, not only can the functions of active power filters be optimized, but also the performances of active power filters can be improved significantly. But the instantaneous reactive power theory need twice coordinates transformation; the calculation will take much time. So the control system can't be a really instantaneous system. This thesis studied mainly the following three parts:1. A kind of harmonic detecting methods based on resampling theory and the mean filtering. Firstly, this paper discusses the instantaneous reactive power theory in three-phase and single-phase system respectively. From this discuss, it has be broken that the instantaneous reactive power theory is only used in three-phase system for a long time. This paper proposed a method for digital harmonic detecting based on resampling theory and the mean filtering. The filter is a finite impulse response (FIR) digital filter. It makes that the control system can follow the power system after one three period in three-phase system, one period in single-phase system. In resampling theory, the signal contains the useful signal that can't be overlapped and useless signal that can be overlapped. Based these theory analysis, simulation has been done. The Simulation shown that this filter not only take advantages of digital filters in accuracy, but also overcomes the bad performance of digital filters in dynamics, and can fellow the measured signal in real time. This part contains three points below:(1) Based on the instantaneous reactive power theory, three-phase currents in power system can be transformed into the active current i_p and the reactive current i_q in α - βcoordinates. If the three-phase currents are symmetric, i_p(i_q) contains only 3n harmonics(n is integer). According to this principle, this paper designs a new low-pass digital filter. It is a finite impulse response (FIR) digital filter, and its order is N/3(N is the samplingnumber in a period ).The direct current in i_p ( i_q ) can pass through this filterdamplessly, while 3/j harmonics damp thoroughly. So the control system can follow the power system after one three period. In single-phase system, the active current ip and thereactive current iq contain all harmonics, and the order of FIR filter is N. So controlsystem can follow the power system after one period in single-phase system.(2) A method for harmonic detecting based on resampling theory is proposed. It can improve the dynamic characteristics of digital filters. If the highest frequency in signal spectrum iso, the first sampling frequency will be e>f >4a>. When the active power filter- APF is designed to compensate N' harmonic currents, there sampling frequency should not be less than (iV' + l + ?)H'o, where w0 is the fundamental frequency. If the sampled data is Nl in a period, there sampling frequency is w^ =Nl/4*w0. Furthermore, a digital harmonic detecting system based on resampling, inheriting the core idea of the instantaneous detecting method, is designed. That is, transform the fundamental currents into DC component. Simulation shows that this system not only has accuracy and stability, but also overcomes the long existent weakness of large amount of calculation for the design of digital filter and poor on-line response characteristics and bad dynamic following performance of digital filters.(3) A method for digital harmonic detecting based on resampling theory and the mean filtering is proposed, and a digital harmonic detecting system is designed. Because of containing only 3? harmonics (n is integer) in ip and iq, the filter can be designed as a mean filter to take the direct current Ip and / , and / and iq before the low-pass filter can be resampled based on the resampling theory. Through this digital system the harmonic currents in power system can be detected timely and accurately.2. Another kind of harmonic detecting methods based on the band-filters harmonic current detecting. This band filters are simple and practical FIR filters. The amplitude of this filter is 1 in first-harmonic frequency, the amplitude is 0 in other harmonic, and the phase of the filter is 0 in first-harmonic frequency. The filter can follow the fundamental wave in one period. Then the filtering algorithm can be simplified according to the character of the filter, so the number of addition and multiplication is less than one in the direct algorithm. This method is new harmonic current detecting. The control system is more reliable than other system. The theoretical analysis and simulation have shown hat this filter not only take advantages of digital filters in the accuracy, but also overcomes the bad dynamics performance. The digital harmonic detecting method based on this kind of filter can be used in any kind of power harmonic compensation system. This part contains three points below:(1)A simple and practical instantaneous harmonics current detecting method is introduced, which realizes a special filter based on DFT (Discrete Fourier Transform). The filter has no fundamental frequency decay and -InIN fundamental phase delay. When N is enough large, the phase delay can be omitted. And the harmonics above 2nd order are filtered. The control system can follow the power system after one period. The recursion of filtering algorithm is obtained according to filter's response character and the calculation is thus greatly simplified. One output only needs two multiplications and two additions.(2) A simple and practical instantaneous harmonics current detecting method is introduced, which realizes a special filter based on exponential finite impulse response. The filter has on fundamental frequency decay and phase delay, and all harmonics above 2nd order are filtered. The control system can follow the power system after one period. The recursion of filtering algorithm is obtained according to filter's response character and the calculation is thus greatly simplified. One output only needs two multiplications and two additions.(3) A simple and practical instantaneous harmonics current detecting method is introduced, which realizes a special filter based on cosine finite impulse response. The difference with above methods is the filter's coefficients are real numbers. The filter has on fundamental frequency decay and phase decay, and all harmonics above 2nd order are filtered. The control system can follow the power system after one period too. One output only needs two multiplications and two additions.As we known, the factors that affect power quality are not only harmonics. The factors have such as reactive, unbalanced three-phase, negative sequence etc. In harmonic detecting methods based on instantaneous reactive power theory, all or some factors that designer wants to be compensated can be done. But the methods in this part process the fundamental frequency and only compensate the harmonic currents.3. Experiments based on data acquisition card and virtual instrument. Finally, based PC—6330D data acquisition card and virtual instrument, some experiments of various control systems are developed in single-phase. The software can use Labview(Laboratory Virtual Instrument Engineering Workbench) developed by NI(National Instrument) company. The results of experiments show that every simulation is accurate. The dynamic performances between simulations and experiments are different. According to two kinds of harmonic detecting methods, there are the own advantages and disadvantages:The instantaneous reactive theory has developed for years, and it is almost perfect. All factors that affect power quality can be compensated if the principle figures are little changed. This paper develops the instantaneous reactive power theory so that the control system can follow the power system after one three period in three-phase system, one period in single-phase system. At same time it improves the calculation for the instantaneous reactive theory. But the fatal weakness of the instantaneous reactive power theory is the large amount of calculation. Even if the methods in first part are used, the calculation is an important factor affected power quality. And the detecting system based on instantaneous reactive power theory is designed for one kind of power systems.The harmonic current detecting methods based the band-filters compensate the harmonic currents only. The design is very simple and can be used in any kind of power harmonic compensation system. One output only needs two multiplications and two additions. It is almost instantaneous detecting. But the control system follows the power system after one period. The following performance is not as good as that based on instantaneous reactive power in three-phase.In practice, proper method should be chosen according practical need.