Research On Key Technologies Of Injection Type Hybrid Active Power Filter And Its Engineering Implementation

At present, nonlinear and time-varying loads are increasing in power grid. Harmonic pollution is getting worse and reactive power is insufficient in power grid. However, the requirement to electric power quality of customers is more and more strict. The matter of voltage quality and harmonic is getting more and more serious. For improving voltage quality and decreasing harmonic contents, a equipment with big capacity which can supressing harmonic and compensating reactive power is necessary in power grid. Passive filter (PF) is atraditional equipment for reactive power compensation and harmonic suppressing. However, PF can only suppress designated-order harmonic currents and is likely to parallel resonate with power grid impedance. It would be replaced by new equipment gradually. The typical device is hybride active ower filter (HAPF). It hold the strongpoints of APF and PF concurrently and already become the hotspot of researching of supressing harmonic and compensating reactive power.Firstly, this thesis proposed a new detection method for reactive power, negative sequence and harmonic current in unbalanced power grid. Then focusing on the research of harmonic suppression and certain-capacity reactive power compensation in one substation, an injection type HAPF (ITHAPF) was put forward. Then some issues of ITHAPF were discussed, such as topology structure, working principle, current controlling, DC voltage controlling, key techniques in engineering application and integrated design. Based on theoretical research, the actualized scheme of integrated system for harmonic suppressing and reactive power compensating was presented, and the engineering application of ITHAPF was implemented.Detecting reactive power, negative sequence current and harmonic components of power system current accurately and rapidly is an important factor for good operation performance of APF. However, the reactive power, negative sequence current and harmonic components detected by traditional ip-iq arithmetic will have amplitude and phase errors when the voltage of power networks is unbalanced or distorted. This has negative influence on the compensation performance of APF. Therefore this, this paper presents a new detection method which based on improved ip-iq arithmetic. This method is suitable for the unbalanced AC system and don't need PLL circuit to obtain synchronous rotate angle. Even though the voltage of power networks is unbalanced and distorted, it can works without phase locking, filtering and symmetrical decomposition to the voltage beforehand. The phase of the benchmark vector of the arithmetic is given by the digital detection system itself precisely. Therefore, the time delay caused by PLL and the filter is avoided and the error is reduced. Negative sequence current, reactive power and harmonic components can be detected precisely.Aiming at the requirement of suppressing harmonic current and compensating certain-capacity reactive power in one substation simultaneously, based on the analysis of the advantages and disadvantages of sigle injection active power filter and passive filter, this thesis proposed a resonace injection type hybrid active power filter which incudes passive filters, active power filter, injection circuit and so on. The active part connects with power grid parallel through the injection circuit. Because of the existace of fundamental series resonance ciruit (FSRC) in injection circuit, the fundamental voltage which the active part undertakes is small. This decreased the required rate of active part effectively. Therefore, the topology structure of ITH APF is very suitable for application in high voltage condition. The passive part of ITHAPF was used to suppress some specific-order harmonic currents and realize "rough filter"; the active part of of ITHAPF was used to suppress remanent harmonic currents dynamically and realize "precise filter". At the same time, passive part can compensate certain-capacity reactive power.Comparing with conventional APF, ITHAPF have more complicated structure model. Its system order gets higher beacause of increase of the number of inductance and capacitance. So the control system is more complex. If reference signal has error caused by inaccurate detection and this make output instructions contain a small fundamental component, the inverter will generate big fundamental current. This will influence the security and stability of ITHAPF. This paper established the control model of ITHAPF firstly. Then the mechanism and imperilment of fundamental current circumfluence were analysed. For eliminating fundamental component of the inverter output current, this paper proposed a double closed-loops control strategy which composed of outer control loop of injection circuit current and inner control loop for output current of inverter. The outer control loop can make the injected current track the load harmonic current precisely and the improve the performance of APF. The inner control loop can limit the output current of inverter, increase system damping and restrain system resonance. This will impove the security and reliability of the inverter. Generalized integral arithmetic used in outer loop controller will make the gain of the system becomce very big at the frequency of harmonic current. But the gains at other frequency only have small change. This will impove the performance of ITHAPF in tracking the harmonic currents wanted be filtered.Ther injection circuit of ITHAPF contains FSRC. Theoretically, the fundamental voltage undertaked by the active part is nought. So the DC-voltage of ITHAPF can hardly be maintained through controlling the converter's active current. The usual method is adopting uncontrollable rectifier. However, if the PCC voltage is not ideal, DC-voltage might be raised up by the energy from AC side and the performance and security of ITHAPF would be threated. Based on the analysis of the theory of the DC voltage rising, this thesis proposed four methods for stabilizing the DC voltage including adopting multiple converters, optimizing the injection circuit, adopting energy release circuit and selecting controllable PWM rectifier at DC side.In allusion to the problems in practical engineering such as electromagnetic interference (EMI), complex process and large computing of PWM modulation method, dead-time effect which influences working performance of device, this paper researched some key technologies of ITHAPF in engineering application. Firstly, the reason of EMI generated by PWM pulse, the harm of EMI and the way of EMI transmission were analysed. Then EMI suppression and electromagnetic compatibility (EMC) design were researched. it is pointed out that EMI suppression measures including decreasing intensity of interference source, cutting off EMI transmission route and improving the anti-interference ability of equipments. Subsequently, a mended regulation PWM modulation method was proposed. Comparing with regulation PWM modulation, this method is more precise than regulation PWM modulation and its calculation amount is smaller. Then dead-time effect on output precision of inverter was analysed. A compensation method of dead-time effect, namely, current feedback compensation method was proposed. This method was confirmed by experiment.The theoretical and simulation analysis in this paper shows that performance of ITHAPF correlate with the main circuit parameters. considering application of engineering, this paper proposed a set of method of parameter design of ITHAPF:For passive filter design, capacity, cost and quality factor shoud be considered mainly after tuned frequency was selected based on actual conditions; for injection ciruit design, in addition to the factors considered in passive filter design, harmonic current injection capability and the voltage which FSRC undertakes shoud be considered chiefly. For the output filter design, switching harmonic suppression, effective harmonic output capacity and dynamic tracking ability should be considered first.For coupling transformer design, after determining the capacity, the matching of current and voltage of active part shoud be optimized so that the capacity of the power switching devices can be utilized fully. When designing the DC capacitor of Inverter, its voltage should have a moderate value considering current tracking ability of voltage inverter and the installation costs. The values of its capacitance mainly decided by the inverter DC-bus voltage fluctuations allowed. Based on the previous principle, this paper designed a harmonic analysis and suppressing Integration system based on ITHAPF. This system has the functions of harmonic detection, analysis, suppressing and publication of harmonic data. It can suppress harmonic current dynamically and compensate large-capacity reactive power simultaneously. Besides, the system had excellent harmonic analysis ability, friendly human-machine interfaces and reliable harmonic-data share ability. This integrated system designed to put into operation in the field have good performance for harmonic suppressing and reactive power compensation.In short, this paper realized a integrated systems based on hierarchical distributed structure which has the functions of harmonic monitoring, analysis and suppressing. This will provide some useful practical references for promoting the high power hybrid active filter in China.