Research On Control Technology Of Shunt Active Power Filter

With the increasing prevalence of power electronics device applications, the power quality of the grid has been severely polluted. Comparing with traditional passive compensators, shunt active power filter (APF) is a novel power quality compensation device with fast dynamic response, high precision of steady state compensation, flexible compensation manners, and less probability having resonance with the grid impedance. APF is capable of comprehensively tackling power quality problems associated with harmonics, reactive power and unbalanced grid, which is crucial for improving power quality and maintaining the stability of power system. There are still some challenges for the digital implementation and industrial application of APF. For example, the analysis precision of commonly used Fourier mathematical tools is sensitive to grid frequency; if control strategy ensures good control performance, it is difficult to give consideration to rapidity and robust stability simultaneously; design of switching noise filter lacks specific theory; system is prone to oscillate in the situation of capacitive load, and so on.In order to resolve the above problems, this paper thoroughly researches on the key technologies of synchronous phase-locked loop (PLL), harmonic detection, control strategies, switching noise filter design and damping control to ensure an excellent compensation performance and a robust stability of APF system. Specific research issues run as follows:Obtaining accurate synchronized phasor of the grid is essential for APF to achieve grid-side unit power factor control. The hardware PLL technology is susceptible to high frequency noise, resulting in a false zero-crossing signal. This paper proposes a synchronized phasor measurement method based on recursive discrete Fourier (RDFT) algorithm, possessing higher precision while taking up fewer resources. However, large errors might be expected for the amplitude and phase detection of RDFT algorithm when the grid frequency shifts, so a direct correction algorithm of RDFT is also proposed. This algorithm obtains the real grid frequency through piecewise calculation of the phase angle difference for two consecutive periods, and then employs the current phase shift and recursive pointer value to correct the detection results gathered by the traditional RDFT algorithm, which will yield the accurate amplitude and phase information of the grid. The total amount of calculation for this algorithm is rather small, while it is still able to maintain a high precision with dynamic fluctuations of grid frequency. This research also established a single-phase harmonic detection method for correcting RDFT algorithm, which could significantly reduce the harmonic measurement errors due to shifts of frequency.The traditional application of every-order compensation based on instantaneous reactive power theory is not capable of accurately limiting the amplitude or restricting compensation bandwidth, this paper introduced a selective harmonic compensation strategy to improve the flexibility of APF compensation and further facilitate its industrial application. Moreover, a three-phase selective harmonic detection method based on synchronous rotating coordinate transformation is also put forward, and a sliding window averaging filter with data window length of1/6frequency cycle enables APF to complete the dynamic tracking of load harmonics in only3.3ms for the typical rectifier bridge. Instantaneous current value comparison is able to maintain self-stability of the current loop, and the impacts on the control performance of APF as positioning of feedback current and system delay are also analyzed. In addition, a multi-sampling rate instantaneous comparison technique is adopted to effectively enhance the switching frequency and control performance of the APF system. The instantaneous current comparison technique may experience obstacles as “climbing ripple effect” and large switching ripples, so a multi-sampling rate instantaneous space vector comparison algorithm is also proposed to realize three-phase decoupling and improve output waveforms.Inserting reactance loads exerts a large influence on the performance of APF, this paper presents a systematic and simple design connection reactance loads. According to the compensation capacity and load prediction of APF, the system is designed to meet the traceability with optimal reactance value. In order to reduce the high-frequency pollution to the grid, APF need to install an additional switching noise filter. This paper analyzes the characteristics of LCL filter system with different types of load, and proposes a set of scientific LCL filter design methods based on impedance analysis to grant the system with both excellent switching noise filtering and high damping ratio. However, LCL filter may also amplify output current and give rise to extensive phase lag problems; therefore, a corresponding feed-forward compensation algorithm is presented to reduce the above negative effects of LCL filter.Overheating is a major problem for passive damping resistors of LCL filter; this research focuses on a LCL parallel virtual damping strategy in accordance with the characteristics of instantaneous current value comparison, which does not require complex differential operations and is basically simple to implement. Sampling frequency and stable domain of virtual damping ratio is also presented by analyzing the system stability in discrete domain. As the APF system would become unstable with the incorporation of switching noise filter, the implementation of parallel virtual damping control could suppress system oscillations with uniform damping effect. Generally speaking, there are always concerns for stability issues of feed-forward control strategies when the load for APF contains high frequency paths. In this paper, the mechanism of self-excited oscillation and suppression method of APF system is proposed, and meanwhile the hybrid control strategy of simultaneously detecting the load current and grid current is also put forward to inhibit system oscillation and improving compensation accuracy. The APF and TSC hybrid compensation system is in favor of combining the advantages of both, and the stability of a hybrid compensation system which TSC performs as the load of APF is also analyzed. The experimental results have demonstrated that the system could effectively eliminate the TSC grouping differential, and the system possesses fast dynamic response and high precisions in steady-state performance.