Baoze Wei

A Circulating-Current Suppression Method for Parallel-Connected Voltage-Source Inverters With Common DC and AC Buses

This paper presents a theoretical study with experimental validation of a circulating-current suppression method for parallel operation of three-phase voltage-source inverters (VSI), which may be suitable for modular parallel uninterruptible power supply systems or hybrid ac/dc microgrid applications. The basic concept of the proposed circulating-current suppression method is to modify the original current references by using the current difference among the parallel inverters. In the proposed approach, both cross circulating current and zero-sequence circulating current are considered, and are added into the conventional droop plus virtual impedance control. In the control architecture, the reference voltages of the inverters are generated by the primary control loop, which consists of a droop control and a virtual impedance. The secondary control is used to compensate the voltage drop on the virtual impedance. Furthermore, a circulating-current control loop is added to improve the average current-sharing performance among parallel VSIs. Experimental results are presented to show the effectiveness of the proposed control method to suppress both cross and zero-sequence circulating currents.

Analysis and Experimental Verification of New Power Flow Control for Grid-Connected Inverter with LCL Filter in Microgrid

Microgrid is an effective way to integrate the distributed energy resources into the utility networks. One of the most important issues is the power flow control of grid-connected voltage-source inverter in microgrid. In this paper, the small-signal model of the power flow control for the grid-connected inverter is established, from which it can be observed that the conventional power flow control may suffer from the poor damping and slow transient response. While the new power flow control can mitigate these problems without affecting the steady-state power flow regulation. Results of continuous-domain simulations in MATLAB and digital control experiments based on a 32-bit fixed-point TMS320F2812 DSP are in good agreement, which verify the small signal model analysis and effectiveness of the proposed method.

A circulating current suppression method for parallel connected voltage-source-inverters (VSI) with common DC and AC buses

This paper describes a theoretical with experiment study on a control strategy for the parallel operation of three-phase voltage source inverters (VSI), to be applied to uninterruptible power systems (UPS). A circulating current suppression strategy for parallel VSIs is proposed in this paper based on circulating current control loops used to modify the reference currents by compensating the error currents among parallel inverters. Both of the cross and zero-sequence circulating currents are considered. The proposed method is coordinated together with droop and virtual impedance control. In this paper, droop control is used to generate the reference voltage of each inverter, and the virtual impedance is used to fix the output impedance of the inverters. In addition, a secondary control is used in order to recover the voltage deviation caused by the virtual impedance. And the auxiliary current control loop is added to acquire a better average current sharing performance among parallel VSIs, which can effectively suppress both of the cross and zero-sequence circulating currents. Experimental results are presented in order to verify the effectiveness of the proposed control strategy.

Control architecture for paralleled current-source-inverter (CSI) based uninterruptible power systems (UPS)

This paper describes a theoretical and simulation study on a control strategy for the parallel operation of three-phase current source inverters (CSI), to be applied to uninterruptible power systems (UPS). A circulating current suppression strategy for parallel CSIs is proposed in this paper based on an auxiliary current control loop used to modify the reference currents by compensating the error currents between parallel inverters. The proposed method is coordinated together with droop and virtual impedance control. In this paper, droop control is used to generate the reference voltage of each inverter, and the virtual impedance is used to fix the output impedance of the inverters. In addition, a secondary control is used in order to recover the voltage deviation caused by the virtual impedance. and the auxiliary current control loop is added to acquire a better average current sharing performance among parallel CSIs, which can effectively suppress the circulating current. Simulation results are presented in order to verify the effectiveness of the proposed control methodology.

Enhanced power quality and minimized peak current control in an inverter based microgrid under unbalanced grid faults

The microgrid inverter experiences the power oscillations and current harmonics in case of the unbalanced grid voltage faults. However, there is a trade-off between power oscillations and current harmonics should be considered in three phase three wire inverter systems during the conventional fault ride through control. In order to solve this problem, a novel control strategy is proposed to enhance the output current quality while mitigating the active and reactive output power oscillations. Moreover, a simple current-limited control strategy can be achieved without the necessity of the voltage/current positive/negative sequence extraction. Finally, the simulation tests of the conventional and proposed control solutions are carried out. The results verify the effectiveness of the proposed strategy.

A power sharing method based on modified droop control for modular UPS

An average power sharing control strategy for parallel operation of voltage source inverter (VSI) based modular Uninterruptible Power Systems (UPSs) is proposed in this paper. The presented method is based on a modified droop control. The proposed method conquers the drawback of conventional droop plus virtual impedance control, and at the same time some challenges in the real applications have been considered. Such as the mismatch of output impedance of the parallel modules, the different calibration accuracy of the sample circuit, and the offset of the voltage reference from the control unit. Simulation has been presented in order to verify the effectiveness of the proposed control methodology under these three different conditions. The simulation results demonstrate that a better power sharing performance is obtained and successfully prevent of negative power which can protect the DC bus.

Control method of single-phase inverter based grounding system in distribution networks

The asymmetry of the inherent distributed capacitances causes the rise of neutral-to-ground voltage in ungrounded system or high resistance grounded system. Overvoltage may occur in resonant grounded system if Petersen coil is resonant with the distributed capacitances. Thus, the restraint of neutral-to-ground voltage is critical for the safety of distribution networks. An active grounding system based on single-phase inverter is proposed to achieve this objective. Relationship between output current of the system and neutral-to-ground voltage is derived to explain the principle of neutral-to-ground voltage compensation. Then, a current control method consisting of proportional resonant (PR) and proportional integral (PI) with capacitive current feedback is then proposed to guarantee sufficient output current accuracy and stability margin subjecting to large range of load change. The performance of the control method is presented in detail. Experimental results prove the effectiveness and novelty of the proposed grounding system and control method.

A modified droop control method for parallel-connected current source inverters

In this paper, a novel control method was proposed for current source inverters under the grid-connected working mode. The control scheme is based on a modified droop control method, with an additional current reference signal that will be generated instead of the voltage reference. Hence, there is only a current control loop with droop control in the whole control scheme without voltage control loop. So it is very suitable for grid-connected current source inverter which will simplify the design of the control scheme and combine the advantage of droop control. The parallel configuration is widely used to acquire high power demand, but the circulating current problem is a key issue that should be considered. In this paper, a simulation based on parallel current source inverters using the proposed control scheme is provided. Simulation results showed that a good circulating current suppression capability of the proposed control scheme is obtained with a proper controller design.

Leakage current suppression with a novel six-switch photovoltaic grid-connected inverter

In order to solve the problem of the leakage current in non-isolated photovoltaic (PV) systems, a novel six-switch topology and control strategy are proposed in this paper. The inductor-bypass strategy solves the common-mode voltage limitation of the conventional six-switch topology in case of unmatched inductances. And the stray capacitor voltage of the non-isolated photovoltaic system is free of high frequency ripples. Theoretical analysis and simulation are carried out to verify the proposed topology and its control strategy. Results indicate that the leakage current suppression can be achieved with the proposed solution.