Research On DC Bus Voltage Control Method Considering Line Impedance

When the distributed power generation device provides electric energy to the DC microgrid load,the steady-state performance involved is mainly: the balanced distribution error of the output power(output current)of multiple parallel converters;the steady-state error of the bus voltage.The most commonly used control strategy in the DC microgrid system is the droop control method,but with its development,the limitations of this method have become increasingly prominent:(1)The fixed droop coefficient cannot realize the precise distribution of the output current of multiple parallel converters;(2)The drop of bus voltage is not only affected by the droop coefficient,but also related to other factors.After analysis,it can be seen that the line impedance between each converter and the DC bus is inconsistent,resulting in uneven distribution of output current;and the line impedance itself is also an important cause of the secondary drop of the bus voltage.In order to solve the influence of line impedance on the steady-state performance of the system,this paper proposes a new type of droop control method that actively measures line impedance.The measured line impedance value is introduced into the calculation of the droop coefficient in the form of negative feedback,so that a new droop coefficient is obtained to eliminate the influence of the line impedance on the output current distribution,and the bus voltage can also be picked up to a certain extent.Two methods can be used to measure line impedance:(1)Series disturbance voltage source method.This method is suitable for DC microgrid systems with long distances and large line impedance.(2)Bus voltage measurement method.This method is suitable for DC microgrid systems with short distances and low line impedance.The adjustment of the droop coefficient cannot completely solve the problem of bus voltage drop.On the basis of the new droop control,the line impedance voltage drop is calculated according to the measured line impedance value,the convergence factor is introduced,and the voltage compensation amount is calculated by integration.Similar to translation compensation,the compensation amount of the bus voltage is adjusted while the current rate of change remains unchanged,so as to ensure that the bus voltage is increased without affecting the current distribution accuracy.By building a simulation model composed of two parallel converters of the same capacity,the traditional droop control and the new droop control are compared.The simulation result shows: the new droop control can realize the precise distribution of output current,the distribution error is less than 5.7%;the bus voltage deviation rate drops from 8.2% to 1.2%.After adding the voltage compensation link,under the premise of maintaining the output current distribution accuracy,the bus voltage deviation rate is almost reduced to zero.In order to verify the adaptability of the new droop control,simulations were carried out for different load types and different working conditions.The results show that the steady-state average value of different types of loads has no effect,only the ripple of the current and voltage;the load power is gradually increased twice with a sudden change of 25%,the overshoot of the output current is less than 6.5%,and the transition process time Less than 0.6ms,the dynamic speed drop of the bus voltage is less than 1.3%,and the recovery time is less than 0.06 s.