Research On Power Balance Control And Short-circuit Fault Protection Strategy Of DC Multi-microgrids

The DC microgrid is an important component of the future power grid,and it corresponds to the development trend of new power systems that use renewable energy as the principal source.Bus voltage fluctuations are generally caused by source-load prower imbalances and line short circuit faults,and they are a crucial issue in the reliable functioning of DC microgrids and multi-microgrids.These faults can reduce the quality of load power supplies,resulting in protection misoperation and affecting the stability of DC microgrids and multi-microgrids,as well as posing a threat to the superior grid.As a result,the key to reducing bus voltage fluctuations and improving the stability of DC microgrids and multi-microgrids is the consideration of rapid power balancing between them,as well as the rapid identification and removal of short circuit faults.A control strategy based on node differential current is proposed to resolve the bus voltage fluctuation caused by the imbalance of the DC microgrid source and load power.By calculating the power of the DC microgrid equivalent micro source and load,solving the instantaneous expression of the power differential between the DC microgrid source and load,the internal relationship between bus voltage fluctuation and power regulation is obtained.According to the DC microgrid current micro increment equation,the energy storage power regulation is changed into the control of the energy storage converter’s conduction duty cycle to regulate the energy storage interaction power,and ensure the power balance of the DC microgrid.Finally,an experimental platform is built to verify the effectiveness of the proposed strategy.The experimental results show that the proposed strategy improves the response speed of energy storage and reduces the fluctuation of bus voltage.The power imbalance between the microgrid source and the load will cause the bus voltage to fluctuate in DC multi-microgrids when the power difference exceeds the regulatory range of the subnet,and the mutual support of microgrid power will increase network transmission loss.To solve the problem of bus voltage fluctuation and power loss caused by the DC multi-microgrids’ power imbalance,an optimization control strategy considering power loss based on model predictive control is presented.By analyzing the working mode of the bus converter,a model of microgrid interaction power is established to predict microgrid interaction power.In addition,the power loss model of the DC multi-microgrids with the goal of minimum loss is established to optimize the power distribution of the multi-microgrids and keep the voltage stability of DC multi-microgrids.Simulation and experimental results show that the proposed strategy reduces voltage fluctuation and power loss of the DC multi-microgrids.Another issue that causes bus voltage fluctuation in DC multi-microgrids is the short circuit faults.For the bus voltage fluctuation caused by short circuit faults in the DC multi-microgrids,a composite protection strategy based on power gradient and considering communication faults is proposed.By analyzing the characteristics of voltage and current during DC microgrid faults,the model of fluctuating voltage and the removal time of short-circuit faults is established to obtain the variation rule of the branch power gradient.The branched power gradient is solved and used as fault judgment parameters to adjust the unit protection and non-unit protection thresholds.When the communication is normal,the unit protection strategy consisting of the power gradient difference component is activated.In case of communication failure,the unit protection is blocked and the non-unit protection strategy is activated because the communication-based unit protection strategy fails.The proposed strategy combines both protection advantages without adding measurement devices and without changing the original topology.The experimental results show that the proposed strategy has better rapidity and sensitivity than the over-current protection strategy and current differential protection strategy,and is not affected by communication faults,thus reducing the bus voltage fluctuation.The power gradient differential protection strategy may fail due to the weakening of fault characteristics of voltage and current when the DC multi-microgrid is in a high resistance short circuit fault,making it difficult to detect the short circuit fault.A protection strategy based on the differential derivative of branch admittance is proposed as a solution to the issue of voltage fluctuation brought on by high resistance short-circuited.By analyzing the reverse variation characteristics of transient voltage and current,the equations of short circuit impedance and branch admittance are established,the variation law of line admittance derivative with short circuit impedance is obtained,and the threshold value of branch admittance derivative is established appropriately.The characteristics of DC multi-microgrids with different voltage levels are standardized,the branch transient admittance and derivative per unit value are calculated,and high resistance short-circuit fault protection requirements for DC multi-microgrids are provided.Compared with the over-current protection strategy,current differential protection strategy,and power gradient protection strategy,simulation and experimental results show that the proposed protection strategy increases the identification range of short-circuit impedance,speeds up the removal of short-circuit fault,improves the sensitivity and speed of protection,and reduces the voltage fluctuation of the DC multi-microgrids.