■ Keywords: Microgrid, Power System Stability, HIL Simulation, Real-Time Simulator, Automation of Electric Power Systems

Introduction of users and research results:

The research team of the Energy Research Institute of the Hefei University of Technology has proposed a coordinated control algorithm of voltage unbalance compensation in islanded microgrid. This method is based on the hierarchical control idea, which has taken into account the loop current problem possibly present in the compensation process by focusing on the coordinated control of voltage unbalances in the islanded microgrid, and thereby a multi-objective optimization algorithm for unbalanced voltage compensation is proposed based on particle swarm algorithm. This new algorithm can handle PCC voltage unbalance compensation while putting the port voltage unbalances of the microgrid inverter into an acceptable range. The research team conducted testing with use of Shanghai ModelingTech’s StarSim power electronic small-step real-time simulator. The test results demonstrated that the proposed compensation approach is workable, and they were published in the Power System Automation：

LAI Jidong, XIE Tianyue, SU Jianhui, et al. Coordinated Control of Voltage Unbalance Compensation in Islanded Microgrid Based on Particle Swarm Optimization Algorithm[J]. Automation of Electric Power Systems, 2020, 44(16):121-129. DOI:10.7500/AEPS20200108004.

Application Background

Under an islanded microgrid operation mode, due to the lack of support from the large grid, the voltage of the bus and each node in the grid is completely supported by the coordinated work of many microgrid inverters in the grid. It means that the power voltage quality fully depends on the control performance of the microgrid inverter. In the low-voltage microgrid system, due to the existence of a large number of single-phase loads, the load in the grid is actually characterized as unbalanced. If these loads are not properly controlled, it is easy to appear voltage unbalance, which affects the quality of power supply, and even causes system instability in severe cases. The compensation of the voltage imbalance of the microgrid in island operation mode is a key issue related to the quality of the power supply voltage in the network and the stability of the system.

For the microgrid voltage imbalance compensation control, it needs to detect the instantaneous circuit response, rather than the average signal obtained from the large time-step. Therefore, the average large step method based on the PWM duty cycle is not suitable for this research. At the same time, the real-time simulator must be able to simulate multiple inverters, require certain performance, and be able to simulate various fault conditions.

The StarSim real-time simulator provided by ModelingTech (www.modeling-tech.com) is based on the detailed model of power electronic devices and utilizes the latest FPGA technology to realize power electronics with 1 microsecond step length, arbitrary topology, and arbitrary operating conditions for Real-time simulation of power electronic system, which is widely used in real-time simulation of traction power supply system fault diagnosis, control strategy verification, renewable energy grid connection, motor drive, etc. The research team of the Hefei University of Technology used the StarSim real-time simulator for power electronics for testing the coordinated control algorithm of voltage unbalance compensation in islanded microgrid.

Application Achievements 

Fig.1 shows the HIL+DSP test platform built by the Hefei University of Technology, consisting of the real-time simulator HIL and DSP28335 control board. The power electronic system uses the StarSim FPGA Solver to simulate in real time at a step of 1 us. The control algorithm model operates on the DSP28335 control board, and the real-time simulator and DSP28335 control board are interconnected via actual physical IO. Fig.2 shows the control block diagram of coordinated control of voltage unbalance compensation based on particle swarm algorithm. Fig.3 is the result of the algorithm waveform verification.

The following figure shows the experimental waveforms of the port voltage and PCC voltage waveforms before and after compensation when the capacity of the two inverters is 1:2.

Fig. (a) and (b) show possibility in making the voltage unbalance compensation at the inverter port and PCC with use of the particle swarm algorithm, in which the VUF (voltage unbalance factor)at the port and PCC can be corrected to within 2%, thus making the intention occur.