Abstract:With the load growth and the power grid expansion, the problem of short-circuit current (SCC) exceeding the secure limit in large-scale power grids has become more serious, which poses great challenge to the optimal secure operation. Aiming at the SCC limitations, we use multiple back-to-back voltage source converter based (B2B VSC) systems to separate a large-scale AC power grid into two asynchronous power grids. A multi-objective robust optimal secure operation model of large-scale power grid with multiple B2B VSC systems considering the SCC limitation is established based on the AC power flow equations. The decision variables include the on/off states of synchronous generators, power output， terminal voltage, transmission switching, bus sectionalization, and modulation ratios of B2B VSC systems. The influence of inner current sources of renewable energy generators on the system SCC is also considered. To improve the computational efficiency, a mixed-integer convex programming (MICP) framework based on convex relaxation methods including the inscribed N-sided approximation for the nonlinear SCC limitation constraints is proposed. Moreover, combined with the column-and-constraint generation (C&CG) algorithm, a method to directly solve the compromise optimal solution (COS) of the multi-objective robust optimal secure operation model is proposed. Finally, the effectiveness and computational efficiency of the proposed solution method is demonstrated by an actual 4407-bus provincial power grid and the modified IEEE 39-bus power grid, which can reduce the consumed CPU time of solving the COS by more than 90% and obtain a better COS.

Abstract:Fault current limiting is a critical technology to ensure the safe operation of modular multilevel converter based multi-terminal direct current (MMC-MTDC) grids. This paper proposes a fault severity classification based coordination control strategy of fault current limiter (FCL) and MMC for adaptive fault current limiting. The proposed strategy reduces the investment in FCL, and keeps the bus voltages of non-faulty lines at reasonable values. Firstly, a rapid fault circuit parameter estimation (FCPE) method using initial fault current information is proposed. With this method, the fault distance and fault transition resistance can be quickly estimated, which are used for a quantitative indication of the fault severity. Subsequently, the coordination control strategy of FCL and MMC is proposed, in which the FCL action is prioritized, while the control of MMC is complementary for current limiting. Based on the proposed strategy, fault severity phase planes (FSPPs) are constructed to assess fault severity and calculate the activation time of FCL and voltage regulation factor of MMC. Therefore, the FCL activation and MMC control are matched to the fault severity. The effectiveness and advantages of the proposed strategy are validated by the simulations in PSCAD/EMTDC.

Abstract:This paper develops a detailed equivalent model for modular multilevel converters with partially-integrated battery energy storage. The proposed model gains computational efficiency in two ways. Firstly, it markedly reduces the large number of nodes in the conventional switching model of the converter, thereby shrinking the size of its admittance matrix. Secondly, it avoids computationally expensive re-triangularization of the admittance matrix during the normal operation of the converter and restricts it only to the rare occasions of converter blocking. Mathematical derivation of the model is carried out using differential equations of the converter. The computational efficiency and accuracy of the proposed model are confirmed by comparison of the results from its implementation in the PSCAD/EMTDC simulator against conventional detailed switching models and measurements from a single-phase scaled-down laboratory setup. This paper also shows a case study wherein a converter with partially-integrated batteries is included in the CIGRE B4-5 benchmark system.

Abstract:This paper proposes an advanced voltage source converter (VSC)-driven model for soft open points (SOPs) and battery energy storage systems (BESSs) to actively balance three-phase distribution networks. The proposed model addresses the phase imbalance caused by the increasing integration of renewable energy and distributed generation. Unlike traditional models, which mainly focus on AC capacity constraints, the proposed model explores the complexities of the DC-link. This allows for a thorough examination of the interactions between active and reactive power, as well as the voltage levels on both the AC and DC sides of VSCs. The relationship between pulse width modulation (PWM) control configurations and VSC power outputs is discussed, enhancing control on both sides of the converters. This improvement also facilitates better cross-phase power transfer through SOPs and enhances the overall balance among the three phases. In addition, the proposed model incorporates the cooperative functionality of VSC-driven BESSs to sustain the phase balance. To further optimize the load distribution, phase-specific dispatching (PSD) is introduced, allowing for the flexible allocation of individual loads to distinct phases. Together, these coordinated technical solutions constitute a systematic optimization strategy. An algorithm is developed to harmonize the VSC-driven modeling for SOPs and BESSs with PSD, thereby improving the computational efficiency in managing power flow and phase balance. The results show that the proposed model significantly reduces losses and enhances the phase balance.

Abstract:The transformer-less configuration is regarded as a preferred solution for soft-open-point (SOP)-based flexible interconnected distribution networks (FIDNs). This study proposes a reliable and cost-effective method for handling single-line-to-ground (SLG) faults in transformer-less SOP-based FIDNs. Firstly, the zero-sequence fault equivalent circuit of FIDNs is established, and the fault propagation characteristics between the grids interconnected by the SOP are analyzed. Secondly, a zero-sequence current isolation strategy based on proportional-resonant control is proposed to prevent fault propagation toward the healthy-side grid. An active injection-based grounding parameter identification (GPI) approach is then proposed, enabling accurate calculation of the compensation current for arc suppression. Finally, the selection of injected signal parameters is discussed, and the complete timing flow of the SLG fault handling method is presented. A 10 kV SOP-based FIDN model configured with a transformer-less topology is developed in PSCAD/EMTDC. The performance of the SLG fault handling method is verified under transition resistances ranging from 10 Ω to 10 kΩ. The results reveal that the average relative error in GPI is less than 4%. Furthermore, the compensated currents of faulty branch remain below 10 A across various fault conditions.

Abstract:High proportion of renewable energies and the installation of power electronic devices (PEDs) pose tough challenges to the operation of power systems. In this paper, the remote coordination adjustment (RCA) of PEDs in stochastic scenarios is studied. The steady-state model for the AC/DC system with PEDs is first established, and the alternate iteration method based on linearization (AIML) is adopted, especially for efficient deterministic power flow calculation. Then, the RCA is proposed using a modular local sensitivity method combined with AIML, which can adjust the electrical variables by diverse PEDs with high efficiency. Additionally, the probabilistic power flow calculation using the quasi-Monte Carlo method with the adaptive sampling number (ASN-QMC) is introduced to keep the balance between the computational efficiency and accuracy, as well as demonstrating the positive impact of RCA by the PEDs in stochastic scenarios. The effectiveness of the proposed RCA is validated by a series of modified IEEE test systems.

Abstract:Two-level totem-pole power factor correction (PFC) converters in critical conduction mode (CRM) suffer from the wide regulation range of switching frequency. Besides, in high-frequency applications, the number of switching times increases, resulting in significant switching losses. To solve these issues, this paper proposes an improved modulation strategy for the single-phase three-level neutral-point-clamped (NPC) converter in CRM with PFC. By optimizing the discharging strategy and switching state sequence, the switching frequency and its variation range have been efficiently reduced. The detailed performance analysis is also presented regarding the switching frequency, the average switching times, and the effect of voltage gain. A 2 kW prototype is built to verify the effectiveness of the proposed modulation strategy and analysis results. Compared with the totem-pole PFC converter, the switching frequency regulation range of the three-level PFC converter is reduced by 36%, and the average switching times is reduced by 45%. The experimental result also shows a 1.2% higher efficiency for the three-level PFC converter in the full load range.

Abstract:To tackle emerging power system small-signal stability problems such as wideband oscillations induced by the large-scale integration of renewable energy and power electronics, it is crucial to review and compare existing small-signal stability analysis methods. On this basis, guidance can be provided on determining suitable analysis methods to solve relevant small-signal stability problems in power electronics-dominated power systems (PEDPSs). Various mature methods have been developed to analyze the small-signal stability of PEDPSs, including eigenvalue-based methods, Routh stability criterion, Nyquist/Bode plot based methods, passivity-based methods, positive-net-damping method, lumped impedance-based methods, bifurcation-based methods, etc. In this paper, the application conditions, advantages, and limitations of these criteria in identifying oscillation frequencies and stability margins are reviewed and compared to reveal and explain connections and discrepancies among them. Especially, efforts are devoted to mathematically proving the equivalence between these small-signal stability criteria. Finally, the performance of these criteria is demonstrated and compared in a 4-machine 2-area power system with a wind farm and an IEEE 39-bus power system with 3 wind farms.

Abstract:The capacitor is one of the most important components in a modular multilevel converter (MMC). Due to the chemical process and the aging effect, the capacitor is subject to deterioration over time which is usually manifested by a drop in capacitance. To identify the abnormal capacitors and enhance the reliability of MMCs, an improved submodule (SM) capacitor condition monitoring method is proposed in this paper. The proposed method estimates the capacitance during each control cycle based on the switching states of SMs, offering advantages such as high accuracy and no adverse influence on the operation of MMCs. Firstly, the aging differences of capacitors in different SMs per arm of MMC are analyzed. Then, the capacitances of SMs that switch on the state are calculated based on the relationship between the capacitor voltage and current during each control cycle. A data processing algorithm is proposed to improve the accuracy of capacitance estimation. Finally, the simulation and the real-time control hardware-in-the loop test results based on real-time digital simulator (RTDS) show the effectiveness of the proposed method.