Research On Unbalanced Load Control Strategy For Parallel Inverters In Islanded Micro-grid

In the island microgrid,the new energy generation units connected in parallel are affected by complex conditions,often unable to perform energy output in accordance with the ideal operating conditions,and in severe cases will cause system failures.The voltage imbalance and power sharing problem when the parallel inverter with unbalanced load in the island microgrid is the key to the safe and stable operation of the system.In this paper,the negative sequence voltage and power cycle suppression of parallel inverters with unbalanced loads in parallel island inverters are studied.The mechanism of the power cycle and negative sequence voltage of the parallel inverters in the island microgrid is analyzed systematically.Put forward some very practical control methods.The main research content is as follows.Firstly,the mathematical model of a two-phase rotating coordinate system of a single off-grid operation inverter is established,and a specific method of voltage and current control loop parameter design is obtained according to the pole configuration method.Then the dynamic vector method is used to establish the mathematical model of the droop control loop of the inverter.The stability and transient response characteristics of the inverter are analyzed by using the root trajectory method.Finally,the specific method for parameter setting of the droop control loop is given.Then discuss the power sharing control of parallel inverters.Firstly,the generation mechanism of the power flow in the parallel inverter with balanced load is analyzed.Then,a hybrid control strategy combining improved droop control and adaptive virtual impedance is proposed to achieve power sharing.Specifically,the common bus voltage feedback is added in the droop control reactive power loop to solve the parallel inverter voltage difference caused by the difference in the line impedance,so as to achieve power flow suppression between the inverters.Further,through the adaptive virtual inductance method,the decoupling of the output active power and reactive power is realized,and the output voltage amplitude difference of the parallel inverter is further reduced,thereby increasing the degree of equalization of the reactive power.After that,the parallel inverter with unbalanced load control strategy was studied.Firstly,the voltage and power output characteristics of the parallel inverter with unbalanced load are analyzed,and the causes of unbalanced voltage and power circulation are obtained.Then two improved control strategies are proposed to realize the suppression of negative sequence voltage and negative sequence power circulation.The positive-negative-sequence voltage sequence control strategy in the two-phase rotating coordinate system firstly separates the positive and negative output voltages by a second order generalized integrator(SOGI),and then realizes a positive sequence voltage control loop The voltage tracking,the positive sequence voltage command value is obtained by the compound control strategy when the parallel inverter is provided with the balanced load;the negative sequence voltage instruction is equal to 0,the negative sequence voltage is suppressed through the negative sequence voltage control loop,and finally the output voltage is realized.Three-phase balance and negative sequence power are equally divided.The proportional-integral resonance control strategy introduces a resonant controller on the traditional inverter control loop.The voltage command value is obtained from the compound control strategy when the parallel inverter is equipped with balanced load.It is not necessary to separate the positive and negative sequences to realize the negative-sequence voltage.And negative sequence circulation inhibition.Two control strategies were verified on the simulation and experimental platform respectively.