Continuously expanding deployments of distributed power-generation systems (DPGSs) are transforming the conventional centralized power grid into a mixed distributed electrical network. The modern power grid requires flexible energy utilization but presents challenges in the case of a high penetration degree of renewable energy, among which wind and solar photovoltaics are typical sources. The integration level of the DPGS into the grid plays a critical role in developing sustainable and resilient power systems, especially with highly intermittent renewable energy resources. To address the challenging issues and, more importantly, to leverage the energy generation, stringent demands from both utility operators and consumers have been imposed on the DPGS. Furthermore, as the core of energy conversion, numerous power electronic converters employing advanced control techniques have been developed for the DPGS to consolidate the integration. In light of the above, this paper reviews the power-conversion and control technologies used for DPGSs. The impacts of the DPGS on the distributed grid are also examined, and more importantly, strategies for enhancing the connection and protection of the DPGS are discussed.

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Distributed Power-Generation Systems and Protection

The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.

Autonomous Control of Interlinking Converters with Energy Storages in Hybrid AC-DC Microgrids

Model predictive control (MPC) has established itself as a promising control methodology in power electronics. This survey paper highlights the most relevant MPC techniques for power electronic systems. These can be classified into two major groups, namely, MPC without modulator, referred to as direct MPC, and MPC with a subsequent modulation stage, known as indirect MPC. Design choices, and parameters that affect the system performance, closed-loop stability, and controller robustness are discussed. Moreover, solvers, and control platforms that can be employed for the real-time implementation of MPC algorithms are presented. Finally, the MPC schemes in question are assessed, among others, in terms of design, and computational complexity, along with their performance, and applicability depending on the power electronic system at hand.

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Model Predictive Control of Power Electronic Systems: Methods, Results, and Challenges

Power quality and voltage control are among the most important aspects of the grid-connected power converter operation under faults. Nonsinusoidal current may be injected during unbalanced voltage sag, and active or/and reactive power may include double frequency content. This paper introduces a novel control strategy to mitigate the double grid frequency oscillations in the active power and dc-link voltage of the two-stage three-phase grid-connected photovoltaic (PV) inverters during unbalanced faults. With the proposed control method, PV inverter injects sinusoidal currents under unbalanced grid faults. In addition, an efficient and easy-to-implement current limitation method is introduced, which can effectively limit the injected currents to the rated value during faults. In this case, the fault-ride-through operation is ensured, and it will not trigger the overcurrent protection. A non-maximum power point tracking (non-MPPT) operation mode is proposed for the dc–dc converter. The mode is enabled under severe faults when the converter cannot handle the maximum PV power. Finally, experimental validation is provided by implementing a method in an experimental setup, including a 2 kW PV inverter.

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Control Strategy for Three-Phase Grid-Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Grid integrated renewable DG systems: A review of power quality challenges and state-of-the-art mitigation techniques

This paper proposes an adaptive dc-link voltage control method for the two-stage photovoltaic inverter during the low voltage ride-through (LVRT) operation period. The dc-link voltage will be controlled to follow the change of grid voltage during the LVRT operation to maintain the high modulation ratio so that the high frequency harmonics injected into the grid can be attenuated significantly. Besides, when suffering the asymmetrical grid faults, the proposed control method could to some extent attenuate the double-line-frequency dc-link voltage ripple to keep the dc-link voltage in the safe operational range by shifting the double-line-frequency power ripple to the front-end dc input source, which can be achieved by intentionally fluctuating the dc input power or employing a bidirectional dc–dc converter depending on the voltage drop ratio and the input power level. The theoretical findings were verified by MATLAB simulations and the constructed experimental prototype.

Adaptive DC-Link Voltage Control of Two-Stage Photovoltaic Inverter During Low Voltage Ride-Through Operation

Today, conventional power systems are evolving to smart grids, which encompass clusters of AC/DC microgrids, interfaced through power electronics converters. In such systems, increasing penetration of the power electronics-based distributed generations, energy storages, and modern loads provide a great opportunity for power quality control. In this paper, an overview of the power quality control of smart hybrid AC/DC microgrids is presented. Different types of power quality issues are studied first, with consideration of real-world hybrid microgrid examples, including data centers, electric railway systems, and electric vehicles charging stations. It shows that compared to traditional centralized power quality compensations, smart interfacing power converters from distributed generations, energy storages, and loads, and the AC and DC subgrids interfacing converters are promising candidates for power quality control. To realize the smart interfacing converters' power quality control, both primary converters control and secondary system coordination are required. In this paper, a thorough review of the primary control of interfacing converters to integrate the power quality compensation are presented, with a focus on the hybrid AC/DC microgrid harmonics compensation and unbalance compensation. For multiple interfacing converters, the secondary control with system-level coordination and optimization for harmonics and unbalance compensation (considering both unbalance and harmonics in single-phase and three-phase systems) are also presented. Challenges like low switching frequency of interfacing converters, parallel interfacing converters operation, and interfacing converters communications are discussed, and typical solutions for primary and secondary controls to deal with them are presented. The paper also includes rich case study results.

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Power Quality Control of Smart Hybrid AC/DC Microgrids: An Overview

Supercapacitors (SCs) are increasingly used in energy storage system to tackle the fast power transients. They can be used individually or together with other energy storage units like batteries. In a microgrid, multiple SCs could be installed at different locations without communications among them. This paper focuses on the energy management strategy (EMS) of multiple SCs in a dc microgrid, where the distributed generation units and energy storage units are controlled with the plug-and-play feature. A novel EMS is proposed, which uses the state-of-charge-based adaptive virtual impedance to facilitate the transient power sharing among the parallel SCs without physical communications. The detailed analysis and design procedure are explained based on an example microgrid system with two SCs and one battery energy storage connected in parallel through the dc/dc converters. The effectiveness of the proposed method is verified by both simulation and experimental results.

Energy Management Strategy of Multiple Supercapacitors in a DC Microgrid Using Adaptive Virtual Impedance

This letter proposes a novel seven-level (7L) hybrid-clamped converter, which has competitive performance and lower device count compared to existing topologies. It can easily balance the floating capacitor voltage at the switching frequency, and therefore ensure low capacitor voltage ripples even under very low fundamental frequencies. Both multipulse diode front-end and active front-end structures can be used with the proposed 7L converter, where the 7L active front end has the ability to balance the dc-link capacitors. The proposed 7L converter is very suitable for medium voltage drives (4.16 to 6.6 kV) that feature low costs, high-performance, and low-speed operation capability. Simulation and experiment results are obtained to verify the feasibility of the proposed topology.

A Novel Seven-Level Hybrid-Clamped (HC) Topology for Medium-Voltage Motor Drives

The nonlinear loads in distribution grid may generate harmful low-order harmonics, polluting the grid and deteriorating the voltage quality particularly when the grid is weak. The grid interfacing converters in the distribution system, such as the distributed generation (DG) interfacing converters or hybrid ac/dc grid interlinking converters, can participate in distribution grid harmonic control. In this paper, two virtual-impedance-based harmonics control methods are developed for grid interfacing voltage-source inverters (VSIs) to improve the power quality of the distribution grid. As the control parameters are designed based on the virtual impedance theory, clear physical meanings are given to explain their impacts on the system. Also, the proposed methods do not rely on the closed loop feedback control and, therefore, are very suitable for VSIs with low switching frequency (such as those for high-power DGs or interlinking converters for hybrid ac/dc systems), whose closed-loop control feedback bandwidth may be limited for harmonic regulation. The influence of system delay and feedback control loop is considered and modeled in the design procedure; thus more accurate virtual impedance control is achieved for low-switching-frequency VSIs. Moreover, a comprehensive comparison of the two methods, including stability and harmonic compensation performance, are given. Their effectiveness is verified by experiment results.

Hao Tian

Papers

Adaptive DC-Link Voltage Control of Two-Stage Photovoltaic Inverter During Low Voltage Ride-Through Operation

Power Quality Control of Smart Hybrid AC/DC Microgrids: An Overview

Energy Management Strategy of Multiple Supercapacitors in a DC Microgrid Using Adaptive Virtual Impedance

A Novel Seven-Level Hybrid-Clamped (HC) Topology for Medium-Voltage Motor Drives

Hybrid AC/DC System Harmonics Control Through Grid Interfacing Converters With Low Switching Frequency