■ Key words: Non-ideal Grid Condition, Grid-tied Inverter, Power Quality, HIL Simulation, Real-Time Simulator, IEEE Transactions on Sustainable Energy

Introduction of users and research results: 

To address the non-ideal grid conditions, such as grid impedance uncertainty and grid voltage interference, etc., a research team led by Professor Li Guojie and Wang Keyou from Shanghai Jiaotong University has proposed the design of a robust current controller for three-phase grid-tied inverter based on filtered tracking error, This design focuses on the grid-tied inverter control and aims to make the grid-tied inverter under the non-ideal grid conditions more robust. The proposed controller performed well in the current tracking and is able to directly control the three-phase balanced current input to the grid. Given the elimination of all phase-locked loops and multi-loop controllers, the controller gains extra benefits in high dynamic and tracking performance. Moreover, The Lyapunov function is used to prove the stability of the system, which can improve the quality of the incoming current and realize the active improvement of the power quality of the isolated microgrid.

The research team verified the performance and effectiveness of the proposed control strategy by using offline simulation and hardware-in-the-loop experiments (based on the hardware-in-the-loop simulation of Shanghai ModelingTech’s StarSim series software/hardware), and the corresponding results were summarized and published in the SCI’s journal - IEEE Transactions on Sustainable Energy：

Xin Huang, Keyou Wang, Bo Fan, et al. Robust Current Control of Grid-tied Inverters for Renewable Energy Integration under Non-Ideal Grid Conditions[J]. IEEE Transactions on Sustainable Energy, 2020, 11(1): 477-488.  DOI: 10.1109/TSTE.2019.2895601.

Application Background

The current typical microgrid structure presents the following characteristics:  

Affected by this, under some non-ideal grid conditions, the microgrid will have power quality problems. Specifically, it includes two situations. The first is the unstable operation of the grid-tied inverter caused by the grid impedance. The grid impedance in the microgrid is affected by many factors and changes in a wide range, resulting in the original controller parameters of the grid-tied inverter no longer applicable, thereby reducing the control performance and weakening the original damping effect, resulting in the distortion of the grid-connected current and even the system destabilized. The second is the grid-connected current distortion caused by the background harmonics and unbalanced components of the grid voltage. Generally, harmonics and unbalanced currents generated by nonlinear and unbalanced loads flow through the grid impedance, causing bus voltage distortion. In addition, due to the small inertia of the system, high-power load switching will also cause system voltage fluctuations, resulting in grid-connected current distortion, affecting equipment operation and system stability.

To address this problem, a research team led by Professor Li Guojie and Wang Keyou from Shanghai Jiaotong University has built a model incorporating grid impedance uncertainty and voltage interference, and proposed a robust current control strategy based on filtered tracking error. Under such non-ideal power grid conditions, the real-time simulation, as an authoritative verification method widely recognized by academia, can simulate various fault conditions. The research team used offline simulation and HIL simulation to double-verify the control strategy to prove the effectiveness of its strategy.

Application Achievements

Simulation analysis and HIL verification

Testing the proposed robust current control algorithm was conducted under Matlab/Simulink offline conditions and NI-PXI-based HIL conditions, respectively. The test system for simulations and HIL testing is illustrated below, in which the PV cell panel and Boost circuit serve as the preceding grid-tied inverter. An energy storage device (such as a battery, etc.) is also connected to the DC bus of the inverter, which might be used as an ideal DC source under the appropriate conditions. Simulating the inverter operations at varied modes and conditions is possible by placing the switch (SW) at On/Off. For example, the inverter may operate at MPPT mode when SW1 is Off. The inverter may operate as instructed to run at the ideal DC source mode when SW1 is On. SW2 and SW3 opening or closing may achieve switching between the Line #1 and #2, presenting a simulated case corresponding to the grid impedance variation. Moreover, the local load can be transferred to the system via SW4 and SW5. Simulating the grid-side voltage interference (harmonic, 3-phase unbalance, etc.) can be made via the programmable voltage source.

Current tracking performance test. Considering different grid impedance conditions, the current reference waveform has a step, and the results are shown in the following figure by comparing the PI controllers with different parameters.

It can be seen from the figure that the current controller under the proposed control strategy can take into account both dynamic tracking performance and robustness under the change of grid impedance.

Harmonic suppression performance testing. The testing was conducted to analyze the total harmonic distortion (THD) of the proposed controller’s grid current under the grid voltage harmonic interference. The results after comparing PI and PR controller grid current waveform quality under the same condition are given below. 

It can be seen from the figure that the proposed robust current control strategy can adapt to a wide range of grid impedance changes, and the quality of the incoming current is the highest.

HIL simulation test verification. Build a hardware-in-the-loop test platform as shown in the figure below, use StarSim simulation software to build a system model, peripheral controller (control algorithm) + NI-PXI chassis FPGA (StarSim HIL circuit simulation), and form a hardware-in-the-loop closed loop system through peripheral IO. The host computer performs online program debugging and real-time observation of running results, and the oscilloscope records real-time waveforms.

The simulation results taking into account the grid impedance variation are illustrated below. To address the grid current distortion arising from the presence of tremendous harmonics in PCC voltage under PI/PR controllers, the proposed control strategy is able to maintain stable inverter operations under a wide range of grid impedance variations, and also assure grid current quality.

It can be further verified by simulation experiments that the proposed controller can ensure the three-phase balance and sine wave of the grid current under harmonic and three-phase unbalanced disturbances; considering the grid frequency and phase offset, the proposed controller can ensure that the frequency changes and Control system robustness under phase offset conditions.

Finally, considering unbalanced/non-linear loads, the simulation results are shown in the figure below. It can be seen from the results that in the case of load changes, the inverter can provide the corresponding reactive power, negative sequence and harmonic currents required by the local load for compensation, which verifies the robustness of the proposed control strategy and improves the power quality of microgrid.