With the advancement of the construction of photovoltaic, energy storage, DC and flexibility, the problem of harmonics in a low-voltage DC (LVDC) power supply system has received widespread attention. Compared with those of an AC system, the characteristics of a DC system are that the power supply and load are more decentralized, the proportion of power electronic equipment is higher, and the AC-DC coupling effect caused by interface converters is strong. Therefore, the problem of harmonics in the DC system indicates that both the low- and high-frequency harmonic peak-to-peak values may be high, and thus the classic harmonic analysis and control methods for the AC system are not completely applicable. To this end, the main harmonic sources for the LVDC power supply system are analyzed, and the mechanism and characteristics of the DC bus voltage harmonic problem caused by DC sources and load are revealed. Accordingly, a hybrid active power filtering harmonic voltage control technology is proposed for the harmonic sources of a broadband and decentralized DC system, which achieves the control of harmonics in the case of uncertain disturbance location and high- and low-frequency mixing, thus improving the power quality level of the LVDC power supply system. Finally, simulation results prove the effectiveness of the proposed scheme.

In the context of a high-proportion of new energy integration into the distribution network, the interaction between a grid-connected converter and background disturbances increases the risk of harmonic resonance due to the emission of primary and secondary harmonics over a wide frequency band. Therefore, an indiscriminate risk-domain assessment method considering the severity of secondary harmonics and a temporal harmonic risk assessment method are proposed. First, based on the harmonic amplification factor, the severity indexes of harmonic amplification and secondary harmonics are defined, and the risk of harmonic resonance in all the nodes and branches of the distribution network across the entire frequency band is evaluated through indiscriminate sweep injection. Second, considering the temporal characteristics of actual harmonic sources and taking into account the superposition of primary and secondary harmonics, a risk level matrix containing three dimensions of over-limit probability, over-limit severity and continuous over-limit probability is constructed to quantify the temporal over-voltage and over-current risks of the distribution network. Finally, the feasibility of the proposed method was verified through a test case and an actual engineering case.

To solve the resonance problem in grid-connected offshore wind power delivered by long-distance AC submarine cables, the typical equipment models are established at first, and the resonant mechanism is analyzed by taking into account the capacitance-reactance characteristics of submarine cables. On this basis, a resonance suppression technology for offshore wind power based on high-voltage active resonance suppressor (HARS) is proposed. A two-stage transformation structure consisting of a Buck transformer and a H-bridge cascade valve bank is adopted to realize reliable grid-connection at 220 kV high-voltage level. According to the impedance reshaping theory, a resonant damping control strategy based on the feedback of resonant component of grid-side current is designed, and the feasible region analysis method for the damping coefficient and band-pass center frequency is also given. Finally, the feasibility and effectiveness of the proposed technology are verified by simulations and engineering practice.

There are extensive ranges and long electrical distances in the sending-end system of the large-scale photovoltaic (PV) base transmission by VSC-HVDC. A voltage dip will occur on the DC side of PV inverters after the short-circuit fault recovery at the sending-end, which is a new power quality problem the new power system faces. Aimed at this issue, the impact of the balance between output powers from PV arrays and inverters on the DC-side voltage is analyzed at first, and the voltage-power characteristics of PV arrays and the output power characteristics of inverters under different control strategies are mainly taken into consideration. Second, four kinds of operating conditions for inverters are formulated according to the operating point of PV arrays and the control strategies for PV inverters, and the evolution characteristics of DC-side voltage of inverters under each operating condition are also analyzed. It is revealed the mechanism is combined effects of movements of the operation point of PV arrays and the changes of control strategies of the inverter because of the power imbalance during the fault recovery. Finally, simulations are performed, and results verify the correctness of the mechanism analysis.

The neutral-point clamped (NPC) three-level converter is extensively used in low-voltage distribution networks with distributed energy resources. In a three-phase-four-wire system, the DC bus neutral-point (NP) of a three-phase converter is commonly connected to the N-wire, creating a zero-sequence current path in the system. This configuration leads to output voltage distortions caused by DC NP voltage fluctuations and switching dead-time, and further causes harmonic emission from the converter itself. First, the coupling relationship between NP voltage fluctuations and output current is analyzed, laying a foundation for establishing a quantitative model of NP voltage fluctuations. Based on this model, an active filtering circuit control system for DC bus is designed to suppress both the NP voltage fluctuations and the induced harmonics. Then, the influence of switching dead-time under various operating conditions of the three-phase converter is investigated. A quantitative relationship between the switching dead-time and output voltage distortions is derived, guiding the design of a modulation wave compensation strategy for mitigating the harmonic emission caused by switching dead-time. Finally, simulation models and an experimental platform were developed to reproduce the harmonic emission issues of the three-phase converter, and results validated the effectiveness of the proposed scheme.

The high-quality control of a single-phase grounding fault is important for supporting the safe operation of novel distribution network. However, the existing active-type arc suppression devices (AASDs) have some common problems, such as single function and high cost. To address these issues, a topological structure of AASD based on heterogeneous unit (HU-AASD) and its control strategy are proposed. First, the fourth arm is used to provide voltage support for the virtual neutral point of the HU-AASD, and the withstand voltage requirement of three-phase arms are reduced to the phase voltage, thus reducing the cost and volume of AASDs. Second, the capacitive and inductive reactive power compensation capabilities can be provided by the proposed scheme, and the HU-AASD can continuously provide reactive power compensation for the power grid after the single-phase grounding fault occurs, thus providing uninterrupted voltage support for the distribution network. Finally, simulation and experimental results demonstrate the validity and feasibility of the proposed topology and its control strategy.

The accurate assessment of system frequency modulation requirements and the alleviation of contradiction between steady-state output and dynamic regulation of PV-storage grid-connected units are crucial for realizing the rapid frequency support in grid-forming virtual synchronization PV-storage grid-connected systems. First, a model of a grid-forming virtual synchronization PV-storage grid-connected system is built, and the coupling characteristics among multiple control objectives are analyzed. Second, the traditional grid-forming control is improved based on the damping compensation principle to boost the steady-state control accuracy of PV-storage units. On this basis, frequency security early warning and the controller’s dynamic performance constraints are utilized to quantitatively evaluate the grid-forming control parameters, and a frequency support strategy for the grid-forming virtual synchronization PV-storage grid-connected system is proposed. Finally, a simulation system is constructed, and results verify that the PV-storage grid-connected units can swiftly support the system frequency and remarkably enhance the system frequency security under the proposed control strategy.

The widespread adoption of electric vehicles (EVs) has significantly accelerated the electrical transition of transportation, which brings challenges to distribution network such as "peak-on-peak" demand and deterioration of power quality. Moreover, the power and traffic systems belong to two different operators, so it is difficult to perform centralized optimization. To address these issues, a novel non-iterative coordinated optimization method for power-traffic based on the equivalent mapping theory is proposed. First, a decoupled framework for power-traffic incorporating the power at charging stations is established, and the optimal operation model of distribution network is transformed into a piecewise cost function with boundary charging station power and the corresponding equivalent constraint sets, thereby achieving the decoupled and coordinated optimization of power-traffic systems. Second, by exploiting the intrinsic relationship between boundary cost functions and nodal electricity prices, an optimized charging strategy for EVs which incorporates price signals is put forward, so as to alleviate the overload issues in distribution network. Finally, the effectiveness of the proposed method is validated through a case study, and results also demonstrate that the orderly charging strategy can effectively guide the EV charging behavior and improve the voltage quality.

Hybrid flexible interconnection devices (FIDs) combine the advantages of power electronics technology and electromagnetic interconnection devices in terms of regulation accuracy, overload capacity and economy, which have been favored in engineering applications. Aimed at the problems of power flow control and background harmonics in a hybrid FID under the condition of steady-state interconnection of power grid, as well as the problem of load voltage deviation and harmonic voltage under the condition of power-loss switching, the line power flow and harmonic distribution characteristics under these two operating conditions are analyzed, and the corresponding power flow control and multi-mode power quality management strategies are put forward. In addition, the feasibility of the proposed strategies is verified by simulation results.

To realize the effective control of power quality (PQ) in microgrid cluster (MGC) by means of PQ assessment of MGC and the cooperation with multiple converters, a novel strategy for MGC based on ultra-short-term PQ assessment is proposed. First, an ultra-short-term PQ assessment method is employed to evaluate and locate the PQ problems in MGC in real time. Then, according to the positioning results, a distributed consensus algorithm is utilized to coordinate the control of multiple converters, thus achieving the PQ task allocation and governance. After the completion of governance, the assessment and verification are performed again to ensure the governance effect and further form a closed-loop governance system at the level of MGC. Finally, the results of an example show that compared with the traditional method, the proposed strategy can realize the closed-loop control of PQ problems from the perspectives of identification, assessment and governance at the level of MGC while effectively reducing the harmonic distortion rate, thereby realizing the effective assessment and governance of PQ in MGC.