Research On Integrated Control Device Of Active Power In Distribution Network

With the rapid development of power electronics technology,the application of high-power semiconductor switching devices are widely used,and the large increase of nonlinear load leads to short-term voltage depression,three-phase imbalance,harmonic pollution and other power quality problems in the low voltage distribution network.As an active power comprehensive management device with a variety of compensation functions,the Unified Power Quality Regulator(UPQC)can effectively suppress harmonics and reactive,compensate for short-term voltage depressions,and solve various power quality problems.Due to the problems of high cost,low capacity and poor stability,the basic structure and working principle of UPQC are optimized,and the left-series three-phase four-bridge arm is optimized.The advantage of this structure is that it can directly control the midline current,so that the harmonic compensation effect is better,the DC side voltage is low,the requirements for switching devices and capacitor specifications are low,which greatly reduces the production cost of the device,and is conducive to the stability of the converter.In the detection and control strategy of compensation quantity,this paper deduces a transient Fourier harmonic detection method based on the principle of phase detection,which is simple and easy to program,and can accurately calculate and separate the amount of compensation.The algorithm is improved for the problem that detection delay can easily cause errors in the case of large load changes.The prediction method based on Newton’s difference algorithm is used to predict the compensation quantity of the next cycle of the linear change of the load,and the positive sequence component tracking method of the base wave is used to directly control the compensation of the nonlinear change of the load.According to the different characteristics of voltage current control,the tracking control of the compensation amount in this paper uses the triangular carrier pulse wide modulation method and the hysteresis comparison method respectively.Current tracking using the improved time-lag ring comparison method can solve the problem of switching frequency too large or too small due to fixed ring width.In order to maintain the stability of DC side voltage,this paper establishes a closed-loop control system with the active current of the grid as the input and the DC side voltage value as the output,and adds the PI link to adjust the DC side voltage.Through Simulink simulation,it is proved that the above method can effectively realize the detection and control of compensation amount.Combined with the above topology and detection control methods,this paper completes UPQC’s software design and hardware design,designs the control system with DSP and CPLD as the core,and finally builds the experimental platform and completes the experiment.According to the voltage current waveform before and after compensation,the performance of UPQC is analyzed,and the detection and control method proposed in this paper can realize compensation accurately and in real time,and has good dynamic and static characteristics.