Tao Xu

Conventional soft open points (SOPs) suffer from limited transfer capacity during distribution network faults. To address this issue, this paper proposes an integrated energy storage system and soft open point (ES-SOP) along with a power path dynamic reconfiguration method. The device consists of an M × N AC switch matrix, N AC/DC converters, and a common DC bus with energy storage. This structure provides three distinct power paths: a mechanical direct path, a third-party grid path, and an energy storage path. A seamless reconfiguration technology is developed to eliminate inrush currents during mechanical switching. It combines multi-unit virtual synchronous generator (VSG) pre-synchronization with a DC bus voltage droop coordination mechanism. The overall control follows a two-time-scale strategy. On a long time scale, a heuristic rule selects the most suitable healthy grid as the mechanical source. On a short time scale, the droop parameters of the converters are optimized to autonomously share the remaining power between the third-party grid path and the energy storage path. This allocation minimizes losses and requires no fast communication. Hardware-in-the-loop experiments verify the performance: the proposed method completely suppresses inrush current, keeps DC bus voltage fluctuation below 20 V during mode transitions, and achieves a transfer efficiency of approximately 98.5%.

Existing decentralized secondary frequency control methods can be mainly classified into two categories from the perspective of the actual regulation effect of the droop curve: intercept adjustment methods and slope adjustment methods. As the name implies, the intercept adjustment method restores system frequency by adjusting the intercept of the droop curve, which reduces the regulation margin of the distributed generators' primary control in responding to subsequent load disturbances. On the other hand, the slope adjustment method restores system frequency by adjusting the slope of the droop curve. When the load power increases, the regulation margin of the primary control can be maintained unchanged by adopting this method. However, when the load power decreases, using this method will still reduce the regulation margin of the primary control. The decrease in regulation margin can adversely affect system stability and reliability. To address this issue, this paper proposes a novel decentralized secondary frequency control method. In the proposed approach, only the slope is adjusted when the load demand increases, while both the intercept and slope are adjusted when the load demand decreases. As a result, the regulation margin of primary control can be preserved under any load variation conditions. In addition, this paper establishes a synchronized adjustment mechanism and further investigates the case of consecutive application of the proposed method. The stability of the proposed method is analyzed, and its feasibility is verified through simulations and hardware experiments.

Coordinating PWM among multiple inverters can dramatically improve the performance of active thermal control (ATC). But as the number of inverters increases, their coordination with fixed grouping cannot further maximize the ATC performance due to the inconsistent power variation among inverters. This article therefore firstly proposes the dynamic coordination method for flexibly grouping inverters during their operation. Application wise, the inverters with similar loss trajectories will be grouped together. Then, the skillfully designed parameter calculation and jumping technique will keep the current harmonic qualified, which, in consequence, improves both regulation range and regulation correlation of ATC. A prototype with six inverters verified the proposed method. The results indicate that the proposed method can dramatically improve inverter reliability while keeping the output current qualified.

As a novel high-voltage and high-power multilevel topology with two sets of AC terminals, nine-arm modular multilevel converter (9A-MMC) can effectively reduce system volume, weight, and cost by multiplexing the middle arm. Steady-state analysis for 9A-MMC is crucial to its circuit parameters design, semiconductor devices selection, and operation performance evaluation, however, the existing steady-state analysis model of 9A-MMC is oversimplified and can only realize qualitative analysis. This paper proposes an improved steady-state analysis model by adopting more accurate modulation references and considering circulating current components in arm currents. Firstly, the fundamental circuit equations are established according to Kirchhoff’s laws of voltage and current. Then, the mathematical relationships among internal electrical parameters are derived from the internal circular interaction path. Next, according to the dynamic power equilibrium relationships, a set of equilibrium equations is constituted to solve the unknown variables in modulation references and arm currents, followed by all electrical parameters being obtained accurately. Finally, the feasibility and accuracy of the proposed analysis model are verified by both simulation and experimental results.

Recently, modular multilevel series/parallel converters (MMSPCs) have witnessed an upward trend, where the submodules (SMs) are allowed to be connected not only in series but also in parallel. The parallel operation of SMs offers significant advantages, including sensorless capacitor voltage balancing, arm current sharing, SM capacitance reduction, and system efficiency improvement. However, in the conventional MMSPC, parallel connection is typically limited to adjacent SMs. To achieve higher voltage and more output levels of MMSPC with a limited number of SMs, the SM capacitor voltages need to be configured differently, similar to those in an asymmetric cascaded H-bridge converter. This configuration means that parallel connection is not allowed between adjacent SMs, but across one or more SMs. Therefore, this paper proposes a novel MMSPC with the ability of parallelization across SMs, allowing each SM to be designedly connected in parallel with any other SM. Besides, the tailored modulation strategy is developed to arrange the connection sequence and state among SMs, which is crucial to the normal operation of the proposed MMSPC. Finally, the simulation and experimental results demonstrate the feasibility and effectiveness of the proposed MMSPC with the ability of parallelization across SMs.

Grid-forming converters have been identified as an enabling technology in more-electronic power systems. Nevertheless, the reactive power control of grid-forming converters has often been ignored during transients. This article innovatively points out the transient angle and/or voltage instability of grid-forming converters with typical reactive power control schemes, comprising voltage, reactive power droop, and reactive power PI control schemes. We disclose that the grid-forming converter with the voltage, reactive power droop, or other zero-order reactive control schemes faces only the transient angle stability problem yet with a limited capacity for reactive power/power factor regulation. In contrast, the reactive power PI control scheme, as a first-order reactive control scheme, allows for constant reactive power/power factor operation. However, this control scheme or other first-order reactive control schemes may destabilize grid-forming converters through three transient stability problems—transient angle, voltage, and mixed stability problems. Moreover, we disclose the mechanisms behind these transient stability problems. Furthermore, we present a thorough comparison of typical reactive power control schemes. Finally, experimental results verify theoretical analyses.

This article proposes a new active thermal control method for power semiconductor devices in parallel grid-tied inverters using coordinated pulsewidth modulation (PWM), which breaks the limitation that the PWM-based active thermal control cannot reduce the switching frequency below rated value, and therefore provides more regulation freedom to improve the reliability of power semiconductor devices. In specific, both frequency and phase shift values of PWM sequences will be adjusted coordinately to relieve the junction temperature of parallel inverters while keeping the total high-frequency harmonics at point of common coupling (PCC) to meet the quality requirement. The presented method is applicable to parallel inverters with identical or different input power. Finally, experimental results verified the performance of the proposed method.

Multiple inverters connected at the same point of common coupling will generate the differential mode circulating switching harmonics (DM-CSH), which can worsen the output current quality and threaten the inverter safety. The behavior of DM-CSH is dominated by PWM phase shift values, quantity of inverters and grid impedance, where the time-varying output current harmonics and its boundary are hard to be estimated. To fill this gap, this article firstly builds the time-varying model that can precisely describe the RMS trajectory of output current harmonics. Then, the time-varying model is deeply analyzed by employing the particle swarm optimization to obtain the boundary model for different quantity and grid impedance. After that, the boundary model is further analyzed by the limit theory to obtain the extreme boundary for any quantity and grid impedance. Finally, the experimental results and measured field results verified the theoretical analysis. This article could help to estimate the amplification of current harmonics before installing the multiple inverters, where the possible overcurrent operation can be avoided.

The crystal oscillator in the microprocessor is susceptible to errors due to factors such as temperature, shocks, and electrostatic influences. That will lead to errors in the frequency of the generated PWM waves, then the deviations in the switching frequencies can cause sampling aliasing in the inverter, resulting in beat frequency oscillations. In the master-slave control microgrid, the main power source is controlled using droop control, while other distributed energy sources adopt PQ control, beat oscillations will flow into various parts of the droop control system, thus affecting system stability. This results in significant fluctuations or oscillations in the voltage and frequency support provided by the main source, subsequently leading to instability and even uncontrollability of other distributed power sources. In this paper, an investigation and analysis are carried out to examine the effects of varying degrees of inverter switching frequency errors on the stability of the system.

Transformer-less connection schemes can provide a feasible solution for lowering the economic cost, occupied space, and device weight of modular multilevel converter (MMC) systems. However, due to the reduction in the converter transformer, the current flow loop is changed; as a result, the existing MMC model is not suitable. In this paper, the ac- and dc-side equivalent circuit models of the MMC system using a transformer-less connection scheme are established in both a–b–c stationary and d–q rotating coordinate systems. Then, a highly accurate mathematical analysis model is proposed, in which the interactions among the electrical quantities can be fully seen. The mathematical model is established in the time domain, and hence the amplitude and phase angle of every harmonic component in each quantity can be directly obtained. The proposed model is verified under various typical situations by comparing the calculated values with the actual waveforms. The comparison results prove that the calculation error is small enough to be negligible. The mathematical model in this paper can provide a powerful tool in terms of the performance analysis and the main circuit parameter design for MMCs in transformer-less applications.