Voltage-source converters (VSCs) are essential for renewable energy. However, the oscillatory behavior generated by dynamic interaction between VSCs and the grid is a challenging issue due to the interaction's intricacy and unknown system details. After introducing VSCs into the power system, stability assessment is often wanted. "Perturbation-based impedance measuring + Middle-Brook criterion" is a well-known approach. However, this approach is rarely adopted due to the need for perturbation devices (PDs). So this article proposes a stability assessment method to obtain the system eigenvalue map. This method does not need PDs, so is in-field adaptive. The eigenvalue map is evaluated from closed-loop frequency responses (FRs) by vector fitting (VF), and the FRs are measured by the VSC itself. To support the method theoretically, a modularized model of the VSC-grid system is derived. The model indicates how each closed-loop FR of the VSC(s) reflects all the eigenvalues of the VSC-grid system. Because the transfer zeros of some FRs reduce the observability of some eigenvalues, weighted combination of VF results from multiple FRs is adopted in the proposed method to guarantee the precision. The accuracy of the proposed assessment method and investigation results is verified by both simulations and experiments.

With the increasing application of renewable energy sources (RES), the randomness and volatility of RES power leads to severe power balancing issues, which may cause power quality issues like voltage deviation. Currently, the photovoltaic inverter may directly disconnect from the grid to solve the problem, which results in serious waste of power. Especially, in distributed networks, this may result in conflicts among different users. To address these issues, this paper proposes a control method and switching strategy for photovoltaic (PV) inverters under different operating conditions. The control method achieves the current-controlled operation when PV inverters connect to the grid, ensuring stable power output. When power quality issues arise on the grid side, the PV inverters operate in voltage-controlled mode, running in island mode to power the loads even without energy storage. Besides, a switching control strategy has been formulated to reduce the power impact and oscillations caused by the mode transition. Experimental verification has shown that the photovoltaic inverter can operate stably under both operating modes, with minimal power fluctuations during the switching process, proving the effectiveness of the proposed method.

In the normal process of operating, SAPF (shunt active power filter) injects the compensation current into power grid to maintain the power quality, which may induce the harmonic instability in the whole system. It has been revealed that the instability is induced by the resonance characteristics of grid impedance and loads, which is dominated by the phase characteristics. However, traditional active damper stabilizes the system by increasing the real component of the load’s admittance, which is inefficient when the resonance is severe. Therefore, this paper proposes a novel APF-type active damper to stabilize the system by phase reshaping. The proposed active damper is designed to operate as an APF, which detects the harmonic current as the current reference. And the compensation ratio determines the effect of phase reshaping. Additionally, a PI controller is incorporated to dynamically modify the compensation ratio, which manages to effectively stabilize the system. Compared with traditional active dampers, the APF-type active damper presents an alternative approach of system stabilization with less power loss. Simulations are conducted to verify the theoretical analysis and proposed novel APF-type active damper.

This paper builds a unified model from the perspective of active power and power angle to compare the stability of virtual synchronous generator (VSG) and current-controlled inverter (CCI) when connected to weak grid. Under the unified model, the effect of grid strength on the stability and dynamic response of CCI and VSG is analyzed. It is found that VSG better damps active power perturbation while CCI will have right half plane poles (RHP) under weak grid, which result in stronger stability of VSG than CCI under weak grid. Then an active power feedforward control is proposed to improve the stability of CCI under weak grid. Through feedforward the input of active power controller to the amplitude command of AC voltage control, the low frequency oscillation of CCI under weak grid is well suppressed. Simulation validates the theoretical analysis and effectiveness of the proposed control.

The wide frequency range of harmonic components introduced by the extensive access of power electronic equipment to the power grid will induce the phenomenon of under-damped high-frequency oscillation and harmonic amplification in the increasingly power electronic power grid system, which threatens the safety and reliability operation of the system. With the continuous improvement of the response frequency band of the power electronic grid system, it is necessary to conduct a more comprehensive research and description of the broadband port characteristics of the thyristor rectifier, to provide technical support for the broadband interactive response analysis of power electronic devices. So this paper analyzes the relationship between the port voltage and current of the thyristor rectifier, and establishes a small-signal model based on the harmonic transfer function matrix (HTF). The model can accurately describe the frequency coupling characteristics in a wider frequency range. On this basis, an on-demand truncated HTF description method suitable for usage scenarios is proposed. At last, the accuracy of the proposed HTF modeling results is verified by simulation and experiments.

With the rapid development of social economy and industry, electricity requirement of customers has greatly increased. Thus, huge power demands and peak-valley characteristic of load make much trouble to distribution network. To expand the capacity of distribution network, many existed methods have been proposed such as rebuilding transmission lines, expanding transformer capacity, using energy storage device. Battery storage has the advantages of low cost, easy to control, small size and high utilization rate. Besides, unbalance loads and loads with reactive power have a negative impact on the power quality of distribution network. Therefore, research on capacity expansion and power quality governance is urgently needed. In this paper, multi-function control strategy for battery storage is proposed, which expand the capacity of distribution network and ensure the power quality. According to peak-valley characteristics of loads, battery storage can expand the capacity. By compensation for negative sequence current and reactive power, power quality is ensured. Meanwhile, in some extreme cases, the battery storage can be used as an emergency power supply, providing power for important loads and sufficient response time for other power failure protection measures of the system. The principle and topology of the control strategy are verified in simulation. The simulation results show that multi-function control strategy we propose can expand capacity of distribution network, ensure power quality and serve as power supply in emergency. Experiment is in progress.

A design for secondary system of grid-connected large-scale PV power plant according to its operating characteristics is proposed to improve efficiency and reduce operating costs.In addition,new methods for efficient operation are actively explored on the basis of safety and stability of power plant.The design and related methods will be used as reference in future engineering design and technical upgrade.