Inertia emulation in inverter control strategies benefits in improving dynamic frequency performance in microgrid. Therefore, some inverter control strategies introduce virtual inertia, and the corresponding performance is investigated. These strategies mainly focus on improving frequency dynamics from the perspective of inverter, while their dynamic influence to ac bus and the interaction among different inverters are still unclear. This paper aims to investigate the dynamic frequency performance of multiple generation units in both grid-connected and islanded mode considering inertia interaction. A transfer function model of paralleled inverters with emulated inertia is set up for theoretical analysis. With this model, frequency and phase response of ac bus is studied under several cases, which reveals the interactive frequency performance of voltage-controlled inverters. Furthermore, inertia interaction among different inverters is analyzed in detail, which indicates the reaction from grid to inverter. Consequently, the effectiveness and ineffectiveness of virtual inertia in improving frequency dynamics are demonstrated, which is practical and helpful for virtual inertia imitation in microgrid. Simulation and experimental results are provided to validate its effectiveness.

The active power control of increasing renewable energy resources is a growing concern. For example, solar energy exploitation is highly dependent on the central controller and other resources. Previous research has introduced some solutions for this problem, but the performance is usually unsatisfactory in power allocation, communication dependence, mode switch, and algorithm complexity. This article proposes a straightforward but effective strategy for the two-stage photovoltaic (PV) inverter, which uses the voltage-control method to adjust the PV inverter’s output power and match the load demand. Moreover, reference power tracking is used instead of traditional maximum power point tracking. The mode detection and switch strategies are proposed to solve the power shortage problem, making the PV inverter maintain the voltage-control method even in the power shortage state. Even without adequate support from other facilities, the proposed method can control the PV inverter to organize an islanded microgrid if PV arrays’ maximum power is larger than the load demands. Therefore, solar energy can further participate in grid support and ensure that the microgrid is more stable, robust, and economical. Finally, the proposed methods are verified in simulation and experiment.

With the employment of power electronic converters, renewable energy resources introduce noticeable diversity into different generation units and dramatically change the characteristics of the supply side. Cooperation of different kinds of generation units in steady and dynamic states hence becomes important issues for grid integration. This paper studies transient active power circulation caused by parameter difference among different kinds of generation units, like synchronous generator and voltage-controlled power inverters. Parameter differences brought by power electronic inverters are analyzed, including capacity, inertia and grid-side inductance. Moreover, a simplified transfer function model is utilized for the detailed analysis, which reveals how the transient active power circulation is influenced by the parameter difference. The theoretical analysis is verified with both simulation and experiment. Consequently, some important conjoint influences of inertia, impedance and capacity difference are clarified, where the effectiveness and ineffectiveness of inertia and impedance imitation are concluded in transient active power allocation. Inertia and Impedance design that attenuates transient active power circulation is proposed in several certain situations for inverter cooperated with other generation units.

The hybrid soft open point (HSOP), formed by the parallel combination of the normally open point (NOP) and the soft open point (SOP), combines the advantages of flexibility and high cost-efficiency. At the power outage moment, the seamless switching process of HSOP contains a co-operation state with NOP and SOP parallel working together. This state involves current transitions between NOP and SOP, which may cause overcurrent and trigger the protection shutdown. To address this issue, a wide-band mathematical model is established to analyze and evaluate the transiting current, including high-frequency transiting current caused by the phase difference of pulsewidth modulation, and low-frequency overcurrent caused by a grid voltage step change. Based on the analysis, a high-frequency overcurrent limitation strategy is introduced, relying on the synchronization modulation strategy, and a low-frequency overcurrent limitation strategy is proposed, based on virtual impedance. The effectiveness of the proposed strategy is validated by operating the whole switching process with the simulation and a downscaled experimental platform. According to the experimental results, the SOP can support the load voltage within 20 ms after the power outage. Moreover, the high-frequency currents can be suppressed to around 10% compared to extreme cases, and the low-frequency overcurrent can be completely inhibited.

The dc-link voltage synchronization (DCVS) can be applied in the grid-forming static synchronous compensator (STATCOM) to realize dc-link voltage regulation and synchronization with the grid simultaneously. However, DCVS may lead to low frequency oscillation (LFO) of grid-forming STATCOM. To analyze and resolve the LFO issues of grid-forming STATCOM, this article built the small signal model of grid-forming STATCOM with consideration of DCVS, ac voltage and current dual close loop control (VCDC) and reactive power control (RPC). It is revealed that the LFO is mainly caused by the −180° phase characteristic of DCVS and phase lag introduced by VCDC. Besides, RPC may bring right half plan poles to the close loop and lead to subsynchronous oscillation. Based on the analysis, a lead-lag compensator is utilized and designed in this article to compensate phase lag in the close loop, which is simple but can effectively damp the LFO of grid-forming STATCOM. Besides, the proposed strategy is compared with the existing methods and shows the advantage of inertia provision to decrease the rate of change of frequency in the synchronization. Simulation and Experimental results validate the stability analysis and effectiveness of the proposed method.

The growing installation capacity of distributed photovoltaic inverter needs change of the operation method in the modern power grid. Recently, to better integrate into the grid, the active power control is investigated in photovoltaic inverter. Meanwhile, without energy storage, the photovoltaic inverter can use voltage control method to further support the grid voltage and frequency. Due to the stochastic photovoltaic power, the mode switch is needed when the maximum power still cannot meet the grid demands. The operation of PV inverter in this power shortage mode has not been sufficiently studied. Therefore, this paper investigates the mode switch method for voltage-controlled two-stage photovoltaic inverter. The power conversion principle of synchronous generator is realized in the inverter and the dc/dc converter maintains the dc-side voltage. The mode switch method between normal operation and power-shortage state is proposed. With this method, the PV inverter can real-time adjust the output power of photovoltaic battery and match the load demand without support from other energy source even under PV output power's intense change.