Series-connected Submodules Research Articles

Circulating current suppression method for MMC-HVDC systems with asymmetric arm impedance

The modular multilevel converter (MMC) has been a highly promising topology in the high-voltage direct-current (HVDC) transmission area, where each arm of the MMC may consist of hundreds of series-connected submodules and an inductor. Due to its parameter inaccuracy, component aging, and so on, the component parameter in different arms of the MMC may be different, which may cause circulating current in the MMC-HVDC transmission system, and result in current deterioration, power losses, and electromagnetic interference, etc. In this paper, the circulating current suppressing (CCS) in the MMC-HVDC system, due to asymmetric arm impedance, is analyzed. Based on the mathematical analysis, a method of using an auxiliary circuit is proposed for the MMC to realize the CCS and improve the performance of the MMC-HVDC system. Simulation studies are conducted with PSCAD/EMTDC in the HVDC system, which confirms the feasibility of the proposed method.

Low-Capacitance Modular Multilevel Converter Operating With High Capacitor Voltage Ripples

To provide a reliable approach that can significantly decrease energy storage requirements, this paper proposes a low-capacitance modular multilevel converter (MMC) operating with high capacitor voltage ripples. In the proposed MMC with high ripples (HR-MMC), the dc value of the capacitor voltage is slightly decreased by slightly increasing the number of series-connected submodules in each arm. Therefore, the MMC can operate under a higher capacitor voltage ripple without changing the peak capacitor voltage. Evaluation shows that the cost and volume of the HR-MMC can be reduced due to the reduction of energy storage requirements. The volume reduction effect is particularly obvious. The impacts of the capacitor voltage ripple on the linear modulation region are also analyzed. The obtained active and reactive power diagram shows that the active power and capacitive reactive power ratings are not affected by the high capacitor voltage ripple. The high capacitor voltage ripple only results in a reduction of inductive reactive power rating. The correctness of the theoretical analysis and the feasibility of the HR-MMC were verified through simulations and experiments.

Input-Independent and Output-Series Connected Modular DC–DC Converter With Intermodule Power Balancing Units for MVdc Integration of Distributed PV

In this paper, a novel topology of modular dc–dc converter has been proposed for medium voltage dc (MVdc) integration of high-penetration distributed PV. The proposed topology is featured with input-independent and output-series connected configuration and intermodule power balancing units (PBUs). A high voltage gain could be achieved with series-connected submodules (SMs), while each SM's input port is connected with PV array to realize the independent maximum power point tracking (MPPT) control function. PBUs with bidirectional power conversion ability are installed between adjacent SMs for the elimination of intermodule power mismatch. The operation principles of the proposed converter have been explained in detail along with the corresponding control strategies. The design principles of major passive components have been investigated on the basis of boundary operation conditions, while a quantitative analysis of steady-state power losses has been performed. Moreover, a comparative study is conducted from the aspects of operation range, topology complexity, component stress, and power efficiency. The theoretical results have been validated by both simulations and down-scaled experiments.

Discrete-Time Sliding-Mode Observer for Capacitor Voltage Control in Modular Multilevel Converters

This paper proposes a discrete-time sliding-mode observer (SMO) for equivalent capacitor voltages control in modular multilevel converters, aiming to reduce the communication complexity in applications with several series-connected submodules. The observed voltages of the equivalent capacitors are used in the voltage control loops, resulting in a simpler implementation, which demands low communication effort, since the individual capacitor voltages are not used in the central processor. The proposed SMO was obtained from discrete-time analysis with a stability improvement, differently from discretized observers previously proposed in the literature. A discrete-time methodology is presented to design the observer and ensure the proper operation of the control system. The proposed SMO design methodology in the discrete-time domain enables a simple design of the observer, ensuring transient and steady-state observer performances. Simulation results are presented to demonstrate an example of the proposed methodology, where the observer operates with the control system under distinct conditions. Additionally, experimental results are included to demonstrate the effectiveness of the proposed control system with discrete-time SMO for the equivalent capacitor voltages.

A Transformerless Bipolar Modular Multilevel DC–DC Converter With Wide Voltage Ratios

This paper presents a transformerless bipolar dc–dc converter based on series-connected submodules. It is intended for transforming dc voltage and managing power flow for medium/high-voltage dc grids. The dc–dc converter is composed of positive-pole and negative-pole subsystems, each of which consists of six arms/branches. Every two arms and one branch are constructed into a T-type circuit with the outer terminals interface to primary- and secondary-side dc buses. The two T-type circuits in one subsystem are connected in parallel to transmit power between primary and secondary sides. The transformerless structure (none bulky coupled inductors as well) solves the challengeable insulation and cooling issues of the extremely high-power magnetics in prior arts, while not sacrificing the power efficiency and system reliability. A $ \pm $ 10-kV 2-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the bipolar dc–dc converter. Experimental results obtained from a laboratory setup also confirm the validation of the proposal.

A Flying-Capacitor Modular Multilevel Converter for Medium-Voltage Motor Drive

This paper presents a flying-capacitor modular multilevel converter (FC-MMC) based on series-connected submodules. It is intended for completely improving the performance of a medium-voltage motor drive system in the entire speed range especially at zero/low speed under rated torque condition. The proposed FC-MMC circuit is characterized by the cross connection of upper and lower arm middle taps through a flying capacitor in per phase leg. By properly controlling the ac current flowing through the flying capacitor, the power balance between upper and lower arms is achieved, leading to very small voltage ripples on submodule dc capacitors in the entire speed range from standstill to rated speed even under the rated torque condition. Meanwhile, no common-mode voltage is injected. Simulation results obtained from a 4160-V 1-MW model show that the proposed FC-MMC along with the proposed control method performances satisfactorily in dynamic and static state even when operated at zero/low speed. Experiments on a downscaled prototype also prove the effectiveness of the proposal.

A Transformerless Bipolar Multistring DC–DC Converter Based on Series-Connected Modules

This paper proposes a transformerless bipolar multistring dc–dc converter based on series-connected submodules. It is intended for converting dc voltage and managing power flow for medium-voltage high-power dc networks. This new converter has a flexible structure of multistrings and each string consists of four valves with their middle points crossly connected through two cost-effect dc capacitors to form power balancing channels between valves. Its transformerless solution and single-stage configuration greatly reduce the converter volume, weight, cost, and power loss. In this paper, voltage ratio range and its influence on submodule design are analyzed. Moreover, a control strategy with focus on current sharing control and submodule capacitor voltage control is proposed. A simulation model rated at ±10 kV 2 MW verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1.5-kW laboratory setup also confirm the validation of the proposal.

High current, thin silicon-on-ceramic solar cell

Thin-film polycrystalline silicon solar cells offer the potential to achieve 19% efficient photovoltaic power conversion. Well-designed, 20-100 micron thick, thin-film silicon solar cells can achieve high efficiency by employing light trapping and back surface passivation. Low cost is achieved by minimizing the amount of feedstock silicon required per watt of power output. Electrically insulating supporting substrates enable monolithic, series-connected submodules. A solar cell device comprised of a 20 micron thick layer of silicon grown on an insulating ceramic substrate, designed to effect light-trapping and back surface passivation, has resulted in an independently verified short circuit current of 25.8 mA/cm/sup 2/. Analysis of the spectral response of the solar cell indicates the presence of both light-trapping and back surface passivation with an effective diffusion length in excess of twice the device thickness.