Research on an Asymmetric Fault Control Strategy for an AC/AC System Based on a Modular Multilevel Matrix Converter

This paper studies control strategies for an AC/AC system based on a modular multilevel matrix converter (M3C) when an asymmetric fault occurs in the secondary side ac system. Firstly, the operating principle of M3C is briefly introduced and verified by simulation. Then, based on its mathematical model by double αβ0 transformation, the decoupled control strategies for the primary side and secondary side systems are designed. In view of the asymmetric fault condition of the secondary side system, the positive sequence and negative sequence components of voltages and currents are separated and extracted, and then a proportional resonant controller (PR) is used to regulate the positive and negative sequence currents at the same time to realize decoupled current control in the αβ reference frames. The capacitor voltage balancing control, which consists of an inter-subconverter balancing control and an inner-subconverter balancing control, is realized by adjusting four circulating currents. Finally, the proposed control strategy is validated by simulation in the PSCAD/EMTDC software (Manitoba HVDC Research Center, Canada). The result shows that during the period of the BC-phase short-circuit fault occurring in the secondary side system, the whole system can still operate stably and transmit a certain amount of active power, according to their set values. Furthermore, the capacitor voltages are balanced, with a slight increase during the fault period. The simulation results verify the effectiveness of the proposed control strategy

Transient stability improvement of grid-forming voltage source converters considering current limitation

Transient instability issues of grid-forming voltage source converters (VSCs) considering current limitation strategies are addressed in this paper. Take the virtual synchronous generator (VSG) as the representative grid-forming control, qualitative analysis of transient instability mechanism of the grid-forming VSC is performed firstly. After that, a novel synchronization method named hybrid synchronization control combining the synchronous generator swing equation and the phase-locked loop (PLL) principle is designed, which not only makes the VSC retain its grid-forming characteristics, but also greatly improves its transient stability characteristics. Furthermore, an improved current limiting strategy (ICLS) that can reliably enable and disable current reference limiting loop is proposed, overcoming the drawback that the existing strategy fails to recover from current saturation mode effectively. Finally, simulation and experiments are built to verify the accuracy of the transient stability analysis and the effectiveness of the proposed transient stability improvement scheme.</p

A Transient Stability Numerical Integration Algorithm for Variable Step Sizes Based on Virtual Input

In order to reduce the online calculations for power system simulations of transient stability, and dramatically improve numerical integration efficiency, a transient stability numerical integration algorithm for variable step sizes based on virtual input is proposed. The method for fully constructing the nonhomogeneous virtual input for a certain integration scheme is given, and the calculation method for the local truncation error of the power angle for the corresponding integration scheme is derived. A step size control strategy based on the predictor corrector variable step size method is proposed, which performs an adaptive control of the step size in the numerical integration process. The proposed algorithm was applied to both the IEEE39 system and a regional power system (5075 nodes, 496 generators) in China, and demonstrated a high level of accuracy and efficiency in practical simulations compared to the conventional numerical integration algorithm

Research on the transformerless connection mode for DC power distribution system

The connection mode of the flexible DC distribution network and the AC network is not only the basis of the system design but also is one of the key technologies in the DC distribution. This paper demonstrates the feasibility of the transformerless system using modular multilevel converters (MMC) in the power distribution system. Here, in allusion to the DC power distribution network without transformer, the influence of AC system fault on DC system and the influence of DC system fault on AC system are analysed in detail separately. By establishing the simulation model with the PSCAD/EMTDC software, the transformerless system proposed is verified feasible at the appropriate voltage and the appropriate earthing way of AC system. The research proposed a solution of saving economic cost and space which is the critical issue in the conventional DC power distribution system with the transformer

Calculation method for equivalent model of AC filters in HVDC transmission system

AC filter is an important part of HVDC system, which can restrain the AC-side harmonics effectively and compensate for the reactive power consumed by converter station. Due to the harmonic characters of converter, double-tuned filter and triple-tuned filter are widely used in HVDC system. There is no model of double-tuned filter or triple-tuned filter in the common simulation software such as ETAP and DIgSILENT, which brought some difficulties to the simulation of HVDC especially harmonic analysis. To solve this problem, the accurate and simplified calculation method for equivalent models which instead double-tuned filter and triple-tuned filter by two parallel single-tuned filters or three parallel single-tuned filters are proposed. Furthermore, the accurate formulas and simplified formulas are given which can convert the parameters of double-tuned filter or triple-tuned filter to the parameters of parallel single-tuned filters. Taking into account of the complexity of the accurate formulas, the resonant frequencies of double-tuned filter and triple-tuned filter are considered in the simplified formulas, which simplified the computational complexity. Finally, the feasibility of the equating method and the validity of parameters calculation method are verified

A double-break current injection vacuum DCCB: Principle and analysis

To improve the breaking performances of current injection vacuum DC circuit breaker (DCCB), a double-break current injection DCCB proposed. The opening moments of the two breaks are not simultaneously, one of the breaks is arc breaking, which is used for cut off the fault current, and the other is non-arc breaking, which is used to enhance the transient recovery voltage (TRV) withstand capability. During the interruption process, let the arc-breaking contact first open, the contact starts to separate and produce arcs, then the pre-charged capacitor injects a reverse current to extinguish the arc, meanwhile, the current commutation of non-arc-breaking contact is realized by a coupling inductor. In addition, freewheeling diodes connected in parallel at the contacts to delay the occurrence of transient recovery voltage, thereby further increasing the voltage withstand capability of the CB. The principle and analysis of proposed DCCB scheme are discussed. Finally, the effectiveness of the proposed DCCB scheme is verified by simulatio

Modified Modeling and System Stabilization of Shunt Active Power Filter Compensating Loads with μF Capacitance

The interactions between shunt active power filter (APF) and capacitance load tend to result in stability problems and resonance. The conventional model of a shunt APF is not precise enough to reflect this phenomenon. To address it, this paper proposes a modified shunt APF system model to accurately reflect various stability problems. This paper also studies the mechanism of positive feedback resonance brought by capacitance load and proposes a modified hybrid controller to improve the stable margin of the system, making the shunt APF work stably under different working conditions where there are &mu;F capacitors on the demand side. The correctness and validity of the proposed strategy are verified by simulation analysis and prototype experiments

Self-energy device for HVDC breakers and its control strategy

In order to reduce the cost of energy devices for IGBT drive circuits of the hybrid HVDC breaker, a self-energy device for the HVDC breaker is proposed here, which connects to DC lines and takes power from DC lines directly. The device consists of three operating modes: the start-up charging mode, the steady-state charging mode, and the maintenance restart charging mode. In view of these modes, corresponding control strategies are proposed to ensure the stability of power supply to drive circuits, and the simulation model is built and studied. For multi-terminal HVDC systems or HVDC grids, several self-energy devices in different lines can be connected through a certain combination, which can get the DC current flow regulation function

Single-phase grouding fault analysis and control scheme of transformerless flexible multistate switch

The compact design is suitable for the urban areas with intensive loads. The flexible multistate switch (FMS) can effectively reduce the footprint by removing transformers, which is the development trend of the flexible AC distribution equipment. In PSCAD/EMTDC, a simulation model containing two supply areas embedded in a transformerless FMS is built. Through the single-phase grounding fault simulation under the arc suppression coil grounding, the characteristics of zero-sequence component of FMS are analysed. By means of circuit analysis and sequence component decomposition, the intrinsic mechanism of AC/DC voltage fluctuation in the non-fault supply area caused by the zero-sequence component at the fault area is analysed, and the simulation result is explained. Based on the feeder transformation, a new protection and control scheme is proposed to overcome the problem of the zero-sequence ride through among different supply areas