Development Of 380V/150kVA Broadband Impedance Measurement Equipment

With the massive consumption of traditional fossil energy sources,energy shortages,environmental pollution and other problems have become increasingly serious.New energy sources such as wind energy and light energy have been rapidly developed and utilized at home and abroad due to their clean and renewable characteristics.New energy power generation usually needs to be converted into stable power-frequency AC through grid-connected inverters and off-grid inverters,and then connected to a weak grid or provide power to local users.As the number of inverters connected to the weak power grid and supplying power to local users continues to increase,wide-band oscillations frequently occur,seriously affecting the stability of the power supply system.At present,the impedance analysis method has become an important method for studying the stability of interactive systems because of its simplicity of analysis.The impedance analysis method only needs to know the equivalent input impedance model of the inverter and the equivalent output impedance model of the weak power grid and user load to analyze and study broadband oscillation mechanism of new energy power generation system.Therefore,this paper developed a 380V/150 k VA,measurement frequency range of 10 Hz to 1000 Hz broadband impedance measurement equipment.The main innovations are as follows:1.The sequence impedance modeling of grid-connected and off-grid inverters in the new energy power generation system under equilibrium condition is carried out,and the influences of phase-locked loop,filter and dc side voltage on the characteristics of the sequence impedance of the two inverters are analyzed intuitively by Bode diagram.Aiming at the problem of unbalanced output voltage of off-grid inverters under three-phase unbalanced load conditions,a sequential impedance modeling method for off-grid inverters with positive and negative sequence dual branch control is proposed.The inverter sequence impedance characteristics and output voltage and current in traditional V/F control mode and positive and negative sequence dual branch control mode are compared and analyzed.In the traditional V/F control mode,the amplitude of the positive sequence impedance of the inverter at the fundamental frequency is small,but the amplitude of the negative sequence impedance is large,Therefore,the negative sequence current caused by the unbalanced load has a large voltage drop on the inverter impedance;The positive and negative sequence impedance amplitudes of the inverter in the mode of positive and negative sequence double-branch control are very small,so the small voltage drop generated by the negative sequence current on the inverter impedance has little effect on the output voltage balance of the inverter.2.An overall topology of low-voltage wide-band impedance measurement is proposed,in which the voltage disturbance series injection device adopts the structure of a transformer,PWM rectifier,PWM inverter and LC filter,which can emit a small disturbance voltage sine wave with a wide frequency band and low distortion rate.PWM rectifier adopts double closed-loop control to obtain stable dc side voltage,in which the outer voltage loop adopts the deadbeat control and the inner current loop adopts PI control.The PWM inverter adopts double closed-loop control to obtain the output voltage with good waveform,in which the outer voltage loop adopts the improved repetitive control and the inner current loop adopts the P control.Finally,the broadband impedance measurement of passive load,grid-connected inverter and off-grid inverter was carried out through Matlab simulation software,and the correctness of the impedance measurement equipment topology and measurement method proposed in this paper were verified.3.A 380V/150 k VA broadband online impedance measurement experimental equipment was developed with a measurement frequency range of 10 Hz to 1000 Hz.The software and hardware design of the impedance measurement equipment was introduced in detail.Based on Lab VIEW,a sequence impedance measurement and calculation procedures designed to facilitate human-computer interaction is designed on the industrial computer,and the sequence impedance characteristic measurement experiment was carried out on the passive load.Experiments have proved the rationality of the wide-band impedance measurement method and equipment design proposed in this paper.