Pre-charging and DC fault ride-through of hybrid MMC based HVDC systems

Compared to half-bridge based MMCs, full-bridge based systems have the advantage of blocking dc fault, but at the expense of increased power semiconductors and power losses. In view of the relationships among ac/dc voltages and currents in full-bridge based MMC with the negative voltage state, this paper provides a detailed analysis on the link between capacitor voltage variation and the maximum modulation index. A hybrid MMC, consisting of mixed half-bridge and full-bridge circuits to combine their respective advantages is investigated in terms of its pre-charging process and transient dc fault ride-through capability. Simulation and experiment results demonstrate the feasibility and validity of the proposed strategy for a full-bridge based MMC and the hybrid MMC

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Enhanced independent pole control of hybrid MMC-HVDC system

This paper presents an enhanced independent pole control scheme for hybrid modular multilevel converter (MMC) based on full bridge sub-module (FBSM) and half bridge sub-module (HBSM). A detailed analysis of power distribution between upper and lower arms under asymmetrical DC pole voltages is presented. It is found that the fundamental AC currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical DC pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two DC control loops, two AC control loops and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the DC voltage operating region is significantly extended. To ride through pole to ground DC fault without bringing DC bias at the neutral point of interface transformer, a pole to ground DC fault ride through strategy is proposed. Feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC

DC/DC converters based on hybrid MMC for HVDC grid interconnection

This paper presents a multi-terminal high-power DC/DC converter configuration based on hybrid MMC topology with fault blocking capability for interconnecting HVDC systems. Its main functions include bidirectional power flow, step-up and step-down operation and fault isolation equivalent to a DC circuit breaker. By contrast to the conventional MMC based DC/DC converter, the proposed DC/DC converter with hybrid MMC configuration has the advantage of being able to block the DC/DC converter terminal connecting to faulty DC grid section, while continue operating the other terminals connected to healthy DC grid sections. The proposed DC/DC converter operation is analyzed and its control is described. Simulation results using Matlab/Simulink are presented to demonstrate the robust performance during dc fault conditions

Dual harmonic injection for reducing the sub-module capacitor voltage ripples of hybrid MMC

Reducing the capacitor voltage ripples of the half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) in a hybrid modular multilevel converter (MMC) is expected to reduce the capacitance, volume and costs. To address this issue, this paper proposes a dual harmonic injection method which injects the second harmonic circulating current and third order harmonic voltage into the conventional MMC control. Firstly, the mathematical model of the proposed control is established and analyzed. Then, the general strategy of determining the amplitude and phase angle of each injection component is proposed to suppress the fluctuations of the fundamental and double frequency instantaneous power. The proposed strategy can achieve the optimal power fluctuation suppression under various operating conditions, which also has the advantage of reducing the voltage fluctuation difference between HB and FB SMs. The correctness and effectiveness of the proposed strategy are verified in simulations in PSCAD/EMTDC

Hybrid MMC based multi-terminal DC/DC converter with minimized FBSMs ratio considering DC fault isolation

An isolated high-power multi-terminal DC/DC converter is studied in this paper, based on hybrid MMC configuration consisting of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs). To decrease the investment and power losses, a reduced arm FBSMs ratio (less than 0.5) scheme is adopted. A detailed analysis on the relationship of the DC/DC converter inner AC voltage and the arm FBSMs ratio under reduced DC voltage is presented. Based on this, a control strategy during DC fault is proposed which continues operating the converter connected to the faulty DC side with reactive current absorption. Under the same arm FBSMs ratio, compared to the conventional strategy of blocking the faulty side converter during a DC fault, the proposed unblocking method with reactive current injection can not only achieve greater DC fault current declining rate, but also ensure maximum power transfer between the interconnected healthy DC grids by maintaining a higher inner AC voltage in the DC/DC converter. The two strategies are compared and validated by simulations using PSCAD/EMTDC under different arm FBSMs ratio

Fast DC Fault Current Suppression and Fault Ride Through in Full-Bridge MMCs via Regulation of Submodule Capacitor Discharge

High Voltage Direct Current (HVDC) is more cost-effective than High Voltage Alternating Current (HVAC) for transmitting power over long distances, and therefore is ideal for bulk power transfer from wind, solar, hydroelectric, and tidal power plants located in offshore or remote locations to load centers. The use of Voltage-Sourced Converters (VSCs) in HVDC transmission systems offers greater flexibility when compared to their counterpart, Line Commutated Converters (LCCs), due to their smaller footprint, improved power quality, as well as decoupled active and reactive power control, voltage support, and black start capabilities. The most recent advancements in VSC technology have led to the emergence of a new converter topology known as the Modular Multilevel Converter (MMC). The simplest and most economical MMC cell structure is the Half-Bridge Submodule (HBSM), which is unable to prevent AC side contribution to DC side faults in HVDC systems. Therefore, DC fault protection in the HB-MMC requires either installation of expensive DC Circuit Breakers (DCCBs) or the opening of AC side breakers that are not adequately fast. Adding two extra switches to the HBSM results in the Full-Bridge Submodule (FBSM) configuration which ensures that, in the event of a DC side fault, there is a reverse voltage in the path of AC side current feeding the DC side fault through the antiparallel diodes in the SM switches. In addition, such fault blocking SMs capable of bipolar voltage generation equip the MMCs with Fault Ride Through (FRT) ability, thus allowing them to remain connected to both AC and DC networks during DC faults while operating as Static Compensators (STATCOMs) and exchanging reactive power with the AC network. A comprehensive review of notable fault blocking SM configurations and fault ride through techniques is presented in this thesis. In the event of a DC side fault, the fault current contributions are initially made by SM capacitor discharge, which occurs before the fault is detected, followed by the AC side contribution to the DC side fault. While the AC side currents can be regulated using fault blocking SMs with bipolar voltage generation capability, the initial discharge of the SM capacitors results in high DC fault currents, which can take several milliseconds to be brought under control. A method to actively influence the rate of rise of the DC fault current by regulating the discharge of SM capacitors in an HB-MMC system has been presented in the literature. In this thesis, the approach has been modified and adapted to a FB-MMC system. The discharge direction of the FBSM capacitors is inverted following the detection of a DC side fault which leads to a reversal in the fault current direction and a fast drop-off towards zero. The conventional FRT procedure where the DC fault is cleared by making adjustments to the MMC arm reference voltages followed by STATCOM operation of the MMC is initiated after the detection of zero-crossing of the DC fault current. The proposed control scheme provides significantly faster DC fault current suppression compared to the case where the conventional FRT procedure is initiated immediately upon DC fault detection. Simulations performed on a point-to-point FB-MMC test system are used to verify the theoretical analysis and to evaluate the DC-FRT performance of the proposed scheme

Prospective submodule topologies for MMC-BESS and its control analysis with HBSM

Battery energy storage systems and multilevel converters are the most essential constituents of modern medium voltage networks. In this regard, the modular multilevel converter offers numerous advantages over other multilevel converters. The key feature of modular multilevel converter is its capability to integrate small battery packs in a split manner, given the opportunity to submodules to operate at considerably low voltages. In this paper, we focus on study of potential SMs for modular multilevel converter based battery energy storage system while, keeping in view the inconsistency of secondary batteries. Although, selecting a submodule for modular multilevel converter based battery energy storage system, the state of charge control complexity is a key concern, which increases as the voltage levels increase. This study suggests that the half-bridge, clamped single, and full-bridge submodules are the most suitable submodules for modular multilevel converter based battery energy storage system since, they provide simplest state of charge control due to integration of one battery pack along with other advantages among all 24 submodule topologies. Depending on submodules analysis, the modular multilevel converter based battery energy storage system based on half-bridge submodules is investigated by splitting it into AC and DC equivalent circuits to acquire the AC and DC side power controls along with an state of charge control. Subsequently, to validate different control modes, a downscaled laboratory prototype has been developed