Study On The Key Technologies For Harmonic Suppression And Reactive Power Compensation

The increase in coal mine enterprises’ output leads to the increase in the motor’s capacity. The improvement in the automation level of coal mining as well as the advancement in its production technology requires more variable-frequency drives in the coal production. However, the impact of reactive power caused by large-capacity motors and the current harmonics brought about by variable-frequency drives resulted in the mining area’s power quality declining. The research background is electric distribution system of mining area. The hybrid active power filter(HAPF) and cascaded H-bridges static synchronous compensator(STATCOM) were taken as objects of research.The parameters of passive part were designed for two typical HAPF topologies.The mathematic relation between the parameters of LC and the capacity of converter was deduced for the first topology which connects a converter to a LC filter in series.The parameters of LC were designed using the mathematic relation. The second HAPF topology injects current of a converter into a LC filter. The effect of LC parameters on the amplitude of branch currents was analyzed. With the objective which is to make the currents of the converter and the reactance L smaller than the compensation current, the principle for selecting the LC parameters was built. By comparing the simulation results, the first HAPF topology is conductive to make the breakdown voltage of device lower; the other HAPF topology is easy to reduce the converter’s current. The results of simulation and experiment verify the effectiveness of the designing parameters.Transfer function of the ip-iq harmonic detection method was built. Based on the analysis of frequency domain, the idea of composite harmonic detection was proposed.The specific harmonic detection method in the synchronous rotating reference frame is combined with the ip-iq harmonic detection method to generate the composite harmonic detection. Low-order harmonic currents are extracted by specific harmonics extraction, that reduces the AC components in ip and iq components under steady conditions and allows the selection of low-pass filter with high cut-off frequency for extracting fundamental current to make the dynamic response fast with certain precision. The precision of composite harmonic detection was analyzed by transfer function. The comparisons between the ip-iq method and the composite harmonic detection were conducted by simulation and experiment to verify the effectiveness.Combined with the model prediction control theory, the virtual flux-based model prediction pulse-width modulation(MPC-PWM) algorithm was presented. The cost function was constructed for tracking virtual flux. The cost function is used to obtain the optimal voltage vector. The converter with inductive loads was used to comparatively research the modulation effect of space vector PWM and MPC-PWM,the results of which show that load currents with the MPC-PWM algorithm displays lower harmonic distortion under conditions of a certain average switching frequency.The MPC-PWM with delay compensation is applied to the HAPF. The experimental study indicates that the MPC-PWM algorithm has low switching frequency under conditions of a certain distortion rate.When the MPC-PWM is applied to the cascaded H-bridge STATCOM, a simplified optimization method was presented. The ideal voltage vector is projected to the abc coordinate axis to obtain the initial conditions for optimizing. Optimizing set contains three voltage vectors obtained by the initial conditions for optimizing. The simplified method reduces the number of vectors in the optimizing set. Based on the linear programming theory, it was proved that the optimal vector can be accurately selected through the simplified method. Balancing the capacitor voltage of each module and restraining average switching frequency, a kind of voltage balancing strategy sorted as per the cost function was established. The effectiveness was verified by simulation and experiment.When the H-bridge cascaded STATCOM adopts the carrier phase disposition PWM algorithm, it is impossible for the pulse cycle rotation to realize the voltage balance of the modules. In accordance with the five-level H-bridge cascaded STATCOM, the charging and discharging characteristics of capacitors were analyzed to improve the pulse rotation strategy. The improved pulse rotation strategy, which realizes module’s voltage balance by adjusting a bit of pulse-rotation time, was analyzed and verified by simulations and experiments.Integrating the asymmetry of grid voltage and the negative sequence components of STATCOM command current, a general power model was built, so that the unbalanced power among phases due to the asymmetry of grid voltage and the output negative sequence components of STATCOM could be compensated directly through the calculations of power model, thus promoting rapidity of DC voltage balance among phases. Four possible methods of DC voltage balance among phases were derived. In order to avoid the calculations of phase locked and trigonometric function,simplify the calculation, a method for real-time calculation based on the theory of virtual instantaneous reactive power and virtual coordinate transformation was proposed. The method only needs to multiplication and addition of instantaneous value. The simulations and experimental results proved that power balance strategy and simplified algorithm are effective.