In this paper, the small-signal stability of a DC current flow controller (CFC) integrated meshed multi-terminal HVDC (MTDC) system is investigated with unique contributions: (1) The dynamics of a multi-line CFC, interconnected voltage- source converters (VSCs) and the DC transmission lines are considered in the small-signal modelling. (2) The switching effect of the capacitive components in the π-type DC transmission lines is identified. The comparative studies of modelling the DC transmission lines using π-type/T-type equivalent circuit are conducted. (3) The impact of the CFC on the AC side of the MTDC system is taken into account with analysis of the critical short-circuit ratio (CSCR) through root-locus analysis and participation factors. The relationship of the SCR with VSC outer-loop control and power transfer is analyzed. (4) The influence of the CFC control system/structural parameters on the small-signal stability of the entire system is investigated. The relationship of the CFC integration with the VSC outer-loop control is analyzed. The adjustment of single/multiple parameters for enhancing the system stability and a sequential parameter tuning approach for multiple parameters are explained. The effectiveness of the proposed method applied in different MTDC systems with CFC is verified. It would be useful for the configuration and parametric design of the CFC integrated meshed MTDC systems.

There are various types of distributed generators (DGs) with different grid integration strategies. The transient characteristics of the fault currents provided by the DGs are different to those of conventional synchronous generators. In this paper, a distribution network with multi-type DGs is investigated, including consideration of DG low-voltage ride through (LVRT). The fault current characteristics of two typical DGs, i.e. an inverter-interfaced distributed generator (IIDG) and a doubly-fed induction generator (DFIG), are analyzed, considering the specific operation modes. Based on analysis of the fault characteristics, an equivalent model of the multi-type DGs under symmetrical/asymmetrical fault conditions is established. A fast-iterative fault calculation method for enhancing the calculation efficiency while avoiding local convergence is then proposed using an improved particle swarm optimization (PSO) algorithm. A simulation system of the distribution network with multi-type DGs is established in PSCAD/EMTDC. The simulation results validate the high accuracy and calculation efficiency of the proposed calculation method of the fault components. This can assist in the settings of the protection threshold.

In order to improve the small-signal stability of AC/DC hybrid systems containing voltage-source converter based high-voltage direct current (VSC-HVDC), this paper extends damping torque analysis (DTA) theory to VSC-HVDC supplementary damping controller to analyze its dynamic impacts and inhibit oscillations. Firstly, this paper presents the mechanism and procedure of VSC supplementary damping controller and establishes the linearized model of AC/DC system with damping controllers. On this basis, the DTA theory of VSC supplementary damping controller and the equation of damping torque index (DTI) are derived. The theory can be applied to the design of the damping controller including the selection of installation location and channel as well as the parameter tuning. Finally, case studies are carried out in two AC/DC hybrid systems containing VSC-HVDC. The simulation results verify the correctness and feasibility of DTI analysis. VSC damping controllers designed by the proposed method can effectively suppress oscillations and improve the dynamic stability of the AC/DC system.

When a synchronous condenser connected power network encounters a non-full phase fault, the amplitude of the stator current is severely suppressed by the magnetic field generated inside the synchronous condenser, in which the sensitivity of failure protection of circuit breakers are affected resulting in rejection of failure protection. In severe cases, it may endanger the secure and stable operation of the synchronous condenser and the related equipment. In this paper, dynamic behaviors of a synchronous condenser without/with de-excitation action under non-full phase faults are investigated with the following four contributions: 1) A mathematical model under non-full phase fault conditions is established. The theoretical value of fault currents is calculated when the synchronous condenser is without/with de-excitation action. 2) The inferiority/superiority without/with de-excitation action is analyzed. The tendency of dynamic behaviors of variables, including the phase current, negative-sequence, and zero-sequence currents, after de-excitation action, is obtained. A selection criterion of failure protection is proposed according to the dynamic behaviors of negative- and zero-sequence currents. 3) Based on the consistent characteristics of the theoretical calculation, simulations in PSCAD/EMTDC and recorded on-site data from WaveEx, the fact that the simulation model established in PSCAD/EMTDC can provide an effective verification platform for practical applications and data analysis of a synchronous condenser integrated power grid under non-full phase faults is justified. 4) An improved operational threshold of failure protection of circuit breakers is proposed. It can reduce the rejection operation probability of failure protection, which improves the protection sensitivity and enhances the security of the system operation. The simulation results justify the necessity and reliability of the criterion setting proposed for failure protection.

In the traditional non-invasive load monitoring (NILM) algorithms, the identification accuracy is enhanced with the increased network scale while sacrificing the calculation speed, which restricts the efficiency of the load identification. In this study, a multi-feature (active/reactive power and current peak-to-peak value) fusion algorithm is proposed, which can achieve enhanced identification accuracy with a smaller network scale while maintaining the calculation speed. The features of the power and current amplitudes of the loads are transformed into the values of red-green-blue (RGB) color channels by color coding and then fused into the V-I trajectory features. After that, the true-color feature image with higher discrimination is generated and input into the convolutional neural network (CNN). The testing results on the PLAID data set indicate that in comparison with the traditional load identification algorithm, the algorithm proposed in this study performs higher identification accuracy with a smaller neural network parameter scale, which significantly improves the identification efficiency.