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This paper investigates the Multilevel Current Source Converter (MCSC) STATCOM. It proposes a new method of modulation. The new modulation method produces less THD and di/dt at the terminals of the MCSC STATCOM when compared to the widely used Phase-Shifted Carrier SPWM (PSC-SPWM). To ensure the inductors' currents remain balanced, a Current Balancing Algorithm (CBA) is presented. The CBA does not require any controls. It relies on assigning optimum switching states to each CSC module in real-time. In addition, two control strategies are presented in this paper. The first one assumes the STATCOM to be a SISO. Therefore, it only requires one PI controller. However, the gains for the controller must be changed if the STATCOM was to transition from capacitive mode to inductive mode, or vice versa. The second control strategy overcomes this problem by implementing a TITO integral state feedback controller. The contributions of this paper are validated via simulation and experimental results. [ABSTRACT FROM AUTHOR]

With the development of the MMC-HVDC, the lightweight requirements of the converter valves have become increasingly urgent. The reduction of submodule (SM) capacitance is difficult because the ripple of capacitor voltage cannot be too large, which is one of the key factors that affect the weight and the volume of the MMC. On the basis of the fundamental frequency and double fundamental frequency components are the critical factors for voltage ripples, this paper proposes an arm inductor switching control (AISC) strategy to reduce the voltage ripples of SM capacitor by controlling the inductance value of the bridge arm. The proposed strategy and the existing circulating current suppression controller (CCSC) are researched in terms of mathematical principles. A two terminal DC transmission model is built in PSCAD/EMTDC simulation environment, then the capacitor voltage fluctuation curves of non-capacitance-reducing control, CCSC, and proposed strategy with the same parameters are simulated. Afterwards, an experiment prototype is built to verify the simulation results. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

A special stress for circuit breaker is the occurrence of evolving fault or parallel switching condition. These switching cases are not standardized by international standards, but may occur in service and may create problems for the breaker. In the past, it was a problem mainly with oil circuit breakers and some types of air-blast circuit breakers but lesser known with SF6 circuit breakers. In the meantime, with upcoming new switching technologies along with changed operating duties, some doubts exist that these switching cases may become an issue again. This paper explains in detail the background of these stresses and presents a method to evaluate the capability of SF6 circuit breakers for such situations. The theoretical evaluation involves network calculations, gas flow simulations, and dynamic dielectric analysis. It is important to note here that several tests have been done in the past for evolving and parallel switching faults for both puffer as well as self-blast circuit breakers with the knowledge available at that time. These test results are used to validate the outcome of the theoretical evaluation which will help in better prediction of the circuit breaker behavior before performing actual tests. It can be concluded that theoretical simulation may, in majority of the cases, serve as sufficient proof to show feasibility for such cases. [ABSTRACT FROM AUTHOR]

This paper proposes a transformerless dc–dc converter, which features a large voltage ratio, for medium-voltage dc (MVDC) applications. The proposed converter combines a boost converter with a series-parallel resonant converter which enables soft switching of the power switches and diodes. Further, the power switches and diodes of the proposed converter experience lower voltage stresses compared to the other topologies. The proposed converter is modular and, therefore, permits series and/or parallel connection of multiple units for the desired voltage and/or power ratings. This paper discusses the principles of operation, analysis, and design of the proposed converter. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed converter. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC or M2C) is an emerging attractive multilevel topology for medium-voltage high-power applications. The capacitor voltage balancing control and the improvement of output current rating are two challenging issues of MMC. This paper proposed a new parallel-connected diode-clamped M2C (DCM2C), in which each arm consists of two parallel-connected clusters. In this topology, the capacitor voltages can be balanced automatically without any balancing control algorithms. Theoretically, no voltage sensors are needed to measure the capacitor voltages theoretically. Thus, the control of the converter can be simplified greatly. With a simple current-sharing control, an equal current distribution can be achieved between the two clusters. Furthermore, an extended parallel-DCM2C is proposed for larger current applications, and the corresponding current-sharing control method is developed. Experimental results validated the voltage self-balancing capability of parallel-DCM2C and the effectiveness of the proposed current-sharing control. [ABSTRACT FROM PUBLISHER]

This paper shows two applications for using vacuum circuit breakers (VCBs) in flexible ac transmission systems in the high-voltage (HV) power grid. This is first a mechanically switched reactor, which is series connected to a thyristor-controlled series compensator in order to enhance the inductive working range. Nevertheless, the switching frequency of VCBs is limited by their mechanical properties. In order to achieve a higher switching frequency, more VCBs are switched in parallel to each other. Second, a device is investigated, which consists of mechanically switched capacitors and a mechanically switched reactor. It is connected to a node in the HV system in order to enhance the voltage quality and to avoid a voltage collapse. Since VCBs are mainly available in the medium-voltage range, a transformer is used to connect the mechanically switched device to the HV system. In this paper, it could be shown that the mechanically switched series reactor is able to enhance the damping progress of power oscillations and that the mechanically switched device using VCBs can stabilize the voltage of the node in case of disturbances, such as faults in the power system. Furthermore, the electrical stress on the VCBs has been assessed for different simulation cases with PSCAD. [ABSTRACT FROM PUBLISHER]

This paper proposes an LCL dc-dc converter concept which is capable of achieving very high stepping ratios with megawatt-level power transfers. The converter can find potential application in connecting high-power dc sources to high-voltage dc transmission including future dc transmission networks. This converter is based on two ac/dc insulated-gate bipolar transistor-based converters and an internal passive LCL circuit without internal ac transformers. The LCL circuit is designed to enable voltage stepping without any reactive power circulation and with potentially soft switching operation which minimizes the switching losses. The designed converter has the ability to achieve current regulation even under extreme external dc faults and, therefore, the converter can operate through dc faults. The switch utilization is better than similar topologies and losses are reasonably low. A dual-active-bridge transformer-based converter design is presented to compare with the proposed LCL converter. A detailed PSCAD model confirms conclusions using a 100-MW 20-kV/300 kV test system. An LCL 200-W 20-V/100-V dc/dc prototype converter is built to validate the proposed topology. [ABSTRACT FROM AUTHOR]