Closed-Loop Precharge Control of Modular Multilevel Converters During Start-Up Processes

Liu

IEEE Trans. Power Electron.

2019

A two-terminal active capacitor concept is proposed recently based on an active power electronic circuit with a voltage control method and self-power scheme. It retains the convenience of use as a passive capacitor with two power terminals only without any additional required connections, and has the potential to either increased power density or reduced design cost depending on the applications. Based on the previously proof-of-concept study, this paper addresses the design constraints, impedance modeling, and start-up solutions of two-terminal active capacitors. A design method for functionality, efficiency, lifetime and cost constraints application is applied to size the active components and passive elements. A voltage feed-forward control scheme is implemented to improve its dynamic response. Two start-up solutions are proposed to overcome the issues brought by the self-power scheme. A case study of an active capacitor for the DC link of a singlephase full-bridge rectifier is presented to demonstrate the theoretical analyses.

Section: Start-up Solutions For the Active Capacitormentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the Practical Design of a Two-Terminal Active Capacitor

Liu

IEEE Trans. Power Electron.

2019

“…In the control of the MMC capacitor voltage, in addition to the voltage equalization control of the sub-module during the steady state, it is necessary to perform a reasonable pre-charge control at the start of the MMC so that the capacitor voltage value reaches the rated value U dc /N that meets the normal working requirements to ensure that the system enters the normal operation phase safely and smoothly. Reference [27] gives the principle of pre-charging and its specific process. The experimental platform first disconnects the switches S 1 , S 2 , S 3 and S 4 .…”

Section: Experimental Verification and Analysismentioning

confidence: 99%

“…ensure that the system enters the normal operation phase safely and smoothly. Reference [27] gives the principle of pre-charging and its specific process. The experimental platform first disconnects the switches S1, S2, S3 and S4.…”

Section: Experimental Verification and Analysismentioning

confidence: 99%

Research on an Auto-Optimized Capacitor Voltage Balancing Control Strategy of MMC SM for Renewable Energy HVDC Transmission System

et al. 2019

The Modular Multilevel Converter (MMC) is one of the most attractive converter topologies in the High Voltage Direct Current (HVDC) transmission field. The latest widely used sorting method has a low algorithm complexity. It can effectively balance the sub-module (SM) capacitor voltages, but it would cause relatively high switching frequency and power loss. Aiming at the problem that the sub-module (SM) capacitor voltage sorting algorithm has a large switching loss due to the high switching frequency of the device, this paper proposes an auto-optimized capacitor voltage balancing control strategy. Firstly, the topology and operation principle of MMC are analyzed. Secondly, a SM capacitor voltage control method based on the dynamic deviation threshold is proposed. Considering the switch switching state of the SM and the difference between the voltages of each SM, the algorithm can obtain the dynamic deviation valve using the closed-loop control. The method can avoid the unnecessary repeated switching of the Insulated Gate Bipolar Transistor (IGBT) under the premise of ensuring that the capacitance voltages of the SMs are basically the same, which effectively result in reducing the switching frequency of the MMC SM and reducing the switching loss, thereby improving the operating efficiency of the system. Finally aiming at the proposed control strategy, the simulation and experimental verification are carried out which shows that the proposed algorithm can better control the system voltage deviation, reduce the switching loss of the system and improve the stability of the system.

“…In this condition, after the fault is cleared, the SM capacitors must be charged back to around the rated value as suggested in [16,17], and the MMC can then be restarted.…”

Section: Standby Operation Of the Dc/dc Converter During A Local Faultmentioning

confidence: 99%

AC Voltage Control of DC/DC Converters Based on Modular Multilevel Converters in Multi-Terminal High-Voltage Direct Current Transmission Systems

Rui

Fletcher

2016

Energies

Abstract:The AC voltage control of a DC/DC converter based on the modular multilevel converter (MMC) is considered under normal operation and during a local DC fault. By actively setting the AC voltage according to the two DC voltages of the DC/DC converter, the modulation index can be near unity, and the DC voltage is effectively utilized to output higher AC voltage. This significantly decreases submodule (SM) capacitance and conduction losses of the DC/DC converter, yielding reduced capital cost, volume, and higher efficiency. Additionally, the AC voltage is limited in the controllable range of both the MMCs in the DC/DC converter; thus, over-modulation and uncontrolled currents are actively avoided. The AC voltage control of the DC/DC converter during local DC faults, i.e., standby operation, is also proposed, where only the MMC connected on the faulty cable is blocked, while the other MMC remains operational with zero AC voltage output. Thus, the capacitor voltages can be regulated at the rated value and the decrease of the SM capacitor voltages after the blocking of the DC/DC converter is avoided. Moreover, the fault can still be isolated as quickly as the conventional approach, where both MMCs are blocked and the DC/DC converter is not exposed to the risk of overcurrent. The proposed AC voltage control strategy is assessed in a three-terminal high-voltage direct current (HVDC) system incorporating a DC/DC converter, and the simulation results confirm its feasibility.