Since the over-modulation can lead to the unstable operation of converters, the over-modulation risk evaluation is important in the circuit parameter selection, operating region design, and so forth. However, the wildly used evaluation method of modular multilevel converters (MMCs) comes from two-level converters; and the article found that its obtained result is not always accurate for MMCs because four assumptions will be introduced when the conventional modulation ratio (CMR) is used as the indicator. To solve the problem, a new concept, namely dynamic modulation ratio (DMR), is proposed. Comparing with the CMR, the DMR can dynamically change according to the variation of operating conditions and can reflect the modulation characteristics more accurately. Quantitative comparisons are carried out between the CMR-based and the DMR-based evaluation methods in various operating conditions. The comparisons show that the CMR-based method is only accurate under a few situations for MMCs, and the obtained operating region of MMCs is not optimal when the CMR is used as the indicator in the parameter design of MMCs. In contrast, the DMR-based evaluation method is high-accurate under all situations. Finally, experiments are carried out in a down-scaled MMC prototype for further validating the method.

Grounding fault analysis is of vital importance for low voltage direct current (LVDC) supply and utilization systems. However, the existing DC grounding fault model is inappropriate for LVDC supply and utilization system. In order to provide an appropriate assessment model for the DC grounding fault impact on the LVDC supply and utilization system, an LVDC supply and utilization system grounding fault model is proposed in this paper. Firstly, the model is derived by utilizing capacitor current and voltage as the system state variable, which considers the impact of the converter switch state on the topology of the fault circuit. The variation of system state parameters under various fault conditions can be easily obtained by inputting system state data in normal conditions as the initial value. Then, a model solution algorithm for the proposed model is utilized to calculated the maximum fault current, the system maximum fault current with different grounding resistance is simple to acquired based on the solution algorithm. The calculation results demonstrate that grounding resistance and structure of LVDC supply and utilization system have remarkable impacts on the transient current. The effectiveness of the proposed model is verified in PSCAD/EMTDC. The simulation results indicate that the proposed method is appropriate for the system fault analysis under various fault conditions with different grounding resistance and the proposed model can offer theoretical guidance for system fault protection.

To solve the wrong response of the fast frequency response(FFR) control system under the cyber attack so that enhancing the system security, in this paper, a FFR cyber attack defense control strategy considering with the network-based false data injection(NFDI) attack was proposed. This strategy first performs time-frequency analysis on NFDI attack using continuous wavelet transform, then the attack-based shuffle convolutional neural networks(ASCNN) was proposed to detect the NFDI attack. Based on the detected results and the end time of the cyber attack, a novel FFR cyber attack defense control strategy was developed to reduce the influence caused by FFR wrong response while recovering FFR wrong response. The experiment based on the actual frequency data and the PSCAD simulation indicates that the proposed strategies could detect the cyber attack in a short time and realize effective FFR output regulation so that enhancing the system operating. © 2021 Chin. Soc. for Elec. Eng.

针对风力发电接入微电网后产生的频率波动问题,提出了一种基于模型预测控制（model predictive control,MPC）的微电网频率协调控制策略,将风机和插电式混合动力汽车（plug-in hybrid electric vehicle,PHEV）纳入频率控制体系,通过对风机的浆距角与PHEV的充放电进行控制,来调节电网频率,对微电网的调频资源进行补充。对WTG桨距角控制系统与PHEV功率控制系统进行建模,阐述其控制原理。为了防止在调频过程中过多地使用PHEV,引入模型预测控制（model predictive control,MPC）理论,设计2个独立的MPC控制器来分别控制桨距角和PHEV功率输出。在MATLAB/simulink平台上验证了所提策略的有效性。结果表明:所提策略可以有效减小微电网的频率偏差和大型微电网中PHEV的用量,相比于传统PID控制具有更快的响应特性。

In this paper, a model is proposed for the optimal operation of multi-energy microgrids (MEMGs) in the presence of solar photovoltaics (PV), heterogeneous energy storage (HES) and integrated demand response (IDR), considering technical and economic ties among the resources. Uncertainty of solar power as well as the flexibility of electrical, cooling and heat load demand are taken into account. A p-efficient point method is applied to compute PV power at different confidence levels based on historical data. This method converts the uncertain PV energy from stochastic to deterministic to be included in the optimization model. The concept of demand response is extended and mathematically modeled using a linear function based on the quantized flexibility interval of multi-energy load demand. As a result, the overall model is formulated as a mixed-integer linear program, which can be effectively solved by the commercial solvers. The proposed model is implemented on two typical summer and winter days for various cases. Results of case studies show the important benefits for maximum PV utilization, energy efficiency and economic system operation. Moreover, the influence of the different confidence levels of PV power and effectiveness of IDR in the stochastic circumstances are addressed in the optimization-based operation.

The coastal urban multi-terminal DC (CU-MTDC) is a prospective solution for enhancing the power supply security of the coastal urban power (CUP) grid and integrating the large-scale offshore wind farm. However, the large DC disturbances may significantly impact the CUP power supply security. The existing DC unbalanced power distribution methods are difficult to be applied in the CU-MTDC because of the complicated optimization process and the expansion of the influence range of a DC fault. To solve the above problems, this paper proposes a coordinated control strategy of CU-MTDC under abnormal conditions. First, calculation principles for the active power reference of center coastal urban power (CCUP) grid converters are proposed. Second, a DC unbalanced power coordinated distribution strategy under abnormal conditions is used based on the dynamic priority control to ensure power supply security of the critical AC lines. Third, the controller parameters of CCUP grid converters are calculated. Through a simple control process, the number of the regulation converters is dynamically scheduled according to the DC unbalanced power. The DC fault influence range on the urban power grid can be limited in a sufficiently small area. Simulation verified the effectiveness of the proposed control strategy.

The steady-state performance of modular multilevel converter (MMC) can be optimized by the following methods: circulating current injection, third-harmonic signal injection, and average capacitor voltage control. However, most of the papers, due to the deficiencies of their MMC models, are limited to the study of adopting just one method and only optimizing a single-aspect of the performance. Thus, this article proposes a general MMC model for the comprehensive study of the steady-state performance optimization. The general model is built based on an equilibrium equation between the internal quantities and the external quantities of the MMC. This general model is capable of analyzing the optimization where the optimizing methods are combined and used to improve the multiaspect performance of the MMC. In addition, more influence factors are taken into account; hence, the calculation error can be smaller than 1% and even the minor effect of each optimizing method can be reflected. The proposed model is compared with traditional models; and it is found that both dc and fundamental components in modulation signals will be affected by the injected circulating current, rather than the constant values in the traditional models. Finally, the proposed MMC model is verified by both simulation and experiment.

针对广域测量系统中的测量数据受到攻击时,快速频率响应(fast frequency response,FFR)控制系统被欺骗而生成错误控制命令进而危害电网安全的问题,该文提出一种面向虚假数据注入攻击的新型FFR网络安全防御控制策略。该策略首先利用连续