The power fluctuations of grid-connected photovoltaic (PV) systems have negative impacts on the power quality and stability of the utility grid. In this study, the combinations of a battery/supercapacitor hybrid energy storage system (HESS) and the PV power curtailment are used to smooth PV power fluctuations. A PV power curtailment algorithm is developed to limit PV power when power fluctuation exceeds the power capacity of the HESS. A multi-objective optimization model is established to dispatch the HESS power, considering energy losses and the state of charge (SOC) of the supercapacitor. To prevent the SOCs of the HESS from approaching their lower limits, a SOC correction strategy is proposed to correct the SOCs of the HESS. Moreover, this paper also investigates the performances (such as the smoothing effects, losses and lifetime of energy storage, and system net profits) of two different smoothing strategies, including the method of using the HESS and the proposed strategy. Finally, numerous simulations are carried out based on data obtained from a 750 kWp PV plant. Simulation results indicate that the proposed method is more economical and can effectively smooth power fluctuations compared with the method of using the HESS.

Single-phase differential boost inverter, built from two identical DC-DC boost converters, has been suggested as an alternative to the usual connection of a DC-DC boost converter to a standard full-bridge inverter. Some control schemes for controlling it have also been proposed, but mostly for voltage regulation, supplying a standalone load, rather than grid-current control. Moreover, it has never been identified that two LCL filters are hidden in the differential boost topology. Problems related to their associated resonances have hence not been discussed, especially their variations with duty ratios. It is thus necessary here to unmask the hidden LCL filters, which upon clarified, permits vast knowledge from grid-current control of a standard LCL-filtered inverter to be extended for differential inverter control. However, the extension involves many challenges introduced by the varying resonance frequencies, nonlinearity and many control options permitted by the differential boost inverter. These challenges are addressed now with simulation and experimental results provided for verifying direct grid-current regulation of a differential boost inverter.

The loads of dc–dc converter (DDC) are mostly constant power loads (CPLs) in dc microgrid. Due to the negative impedance characteristic, CPLs may lead to the nonpassive factor in DDC. Thus, it may result in DDC, not operating reliably at the desired operating point. In this article, a nonlinear controller based on passivity-based stability theory is proposed for the DDC with CPL, which will improve the stability of the converter. Through damping and interconnection injecting, the alternating component passivity-based controller (PBC) is designed to realize the unified control of output voltage and inductor current. It could realize the passivation of alternating component. Furthermore, this controller can solve the problem of traditional PBCs to control inductor current or output voltage only. Moreover, a power observer is adopted, which could fast estimate the variation of the CPL and improve the adaptability of DDC when CPL changes. The proposed controller not only ensures the stability of DDC under large variation of CPL, but also features fast dynamic response with accurate tracking over wide operating range. Finally, the feasibility and effectiveness of the proposed controller are verified by simulation and experimental results.

The three level converters have an inherent problem of the neutral point (NP) voltage unbalance due to the split dc-link capacitors. This NP voltage unbalance degrades the output voltage/current waveform quality. A carrier based discontinuous pulse width modulation (CB-DPWM) with NP voltage balancing strategy- that involves the injection of calculated zero-sequence voltages to the original modulation signal - is implemented for the three level T-type converter in this paper. The zero-sequence voltages are determined from the space vector synthesis of the converter output voltages. This method allows the determination of the zero-sequence voltages with few judgment conditions. Unlike the space vector based discontinuous pulse width modulation (SVB-DPWM), the CB-DPWM is easier to realize in practical applications. Moreover, the proposed CB-DPWM can also be directly used in parallel systems, which can suppress NP voltage ripples and circulating currents simultaneously by a coordinated strategy. The efficacy of the proposed method in balancing the neutral point voltage in single and parallel systems is demonstrated by simulations and experiments.

DC-DC converters with constant power loads are mostly used in DC microgrids. Negative impedance and large disturbances of constant power loads may lead to the instability of DC-DC converters. To address this issue, a nonlinear control strategy consisting of an improved passivity-based controller and nonlinear power observer is proposed in this paper. First, an improved passivity-based controller is designed based on the port-controlled Hamiltonian with dissipation model. By proper damping and interconnection injecting, the fast dynamic response of output voltage and stability of the DC-DC converter is achieved. Second, the constant power load is observed by a nonlinear power observer, which is adopted to estimate the power variation of the constant power load within a small settling time and improve the adaptability of the DC-DC converter under power disturbance. Finally, the simulation and experimental results are presented, which illustrate the proposed control strategy not only ensures the stability of the DC-DC converter under large disturbances, but also can track the desired operating point with low voltage overshoot in no more than 10 milliseconds.

Second-order oscillating power is always generated by a single-phase converter. To decouple this oscillating power from its dc-link then requires a power decoupling circuit to operate continuously. In contrast, ac grid voltages supplying a three-phase converter are usually balanced. There will hence be no oscillating power, if grid currents are balanced too. However, grid voltages may become unbalanced during fault or other disturbances, which when multiplied with balanced currents, will result in oscillating power. Power decoupling for a three-phase converter is thus only necessary during fault, rather than continuously. Its target should also be to improve power quality even during fault, rather than to prolong lifetimes of capacitors claimed with single-phase decoupling. Those differences cause criteria applicable to selecting a power decoupling circuit for single- and three-phase converters to be different, especially when there are seven different types of faulted voltage conditions to consider. These considerations are now studied, before presenting experimental results for verifying effectiveness of all studied three-phase power-decoupled converters.