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

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

A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

Hybrid modular multilevel converter (MMC) applications under over-modulation

The Modular Multilevel Converter (MMC) has become a prominent converter topology due to its several advantages: high modularity, high scalability, low harmonic distortion, and high efficiency. In particular, due to its modular structure, it is possible to increase its voltage rating by stacking extra cells. The MMC has been successfully applied to high voltage direct current (HVDC) transmission systems and drive applications. Most MMC-based projects use the half-bridge sub-module (HBSM) as a building block to reduce semiconductor losses. Despite the MMC advantages, there are still open challenges regarding its control and operation. For instance, in the HBSM-based MMC, the minimum dc-port voltage cannot be lower than two times the AC output voltage. The over-modulation operation of the MMC, i.e. operation with reduced dc-link voltage, has shown some benefits. For instance, the MMC can operate with a reduced dc-port voltage in HVDC applications to avoid flashovers under extreme atmospheric conditions. In addition, for back-to-back MMC-based drive applications, it is possible to reduce the energy arm oscillations by controlling the dc-port voltage. The operation with a reduced dc-port voltage can be accomplished by using full-bridge sub-modules (FBSM) instead of the HBSMs. However, this solution has higher semiconductor losses. A possible alternative is to use a hybrid MMC. In this case, each arm is composed of HBSMs and FBSMs. However, in the hybrid MMC, the capacitor voltages of the HBSMs and FBSMs may drift apart if the converter operates with reduced dc-port voltage because the arm current becomes unipolar, i.e. the arm currents do not have zero-crossing angles. This thesis presents two control strategies to ensure the cell capacitor voltage balance of the hybrid MMC operating in over-modulation. A decoupled control is developed and shown to regulate the inner and outer converter variables independently. An optimisation problem is proposed to ensure the local balance between HBSMs and FBSMs. In addition, a closed-loop controller is considered to correct any mismatch between the control and actual system parameters. The proposed controller is validated through simulation and experimental results. In particular, a 5 kW hybrid MMC of 18 cells has been built to validate the proposed strategies. Finally, this thesis presents the control systems and experimental evaluation of a hybrid back-to-back (BTB) modular multilevel converter (MMC) for drive applications. The grid-side converter is a hybrid MMC composed of half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs), while the drive-side converter is an HBSM-based MMC. The proposed topology can operate with a variable dc-port voltage. By controlling the dc-port voltage as a function of the machine operational point, it is possible to reduce the high sub-module capacitor voltage oscillations in the machine-side MMC during low machine speed. An experimental rig composed of 36 cells was built and tested to validate the proposed control

An energy absorbing method for hybrid mmcs to avoid full-bridge submodule overvoltage during DC fault blocking

The full-bridge submodules (FB-SMs) in hybrid modular multilevel converters need to absorb the enormous energy stored in dc side and arm inductors during dc fault blocking, which may lead to severe overvoltage. An energy absorbing branch (EAB) composed of metal oxide varistor (MOV) and thyristors is proposed in this letter. No extra power loss is produced by the EAB during normal operation. The EAB can absorb a part of the energy and reduce the energy absorbed by FB-SMs suffering from severe overvoltage by clamping the dc voltage. Thus, the maximum FB-SM overvoltage is reduced. Turning off the EAB several milliseconds after the blocking of the converter can accelerate the decaying of the dc fault in the transmission line and reduce the required energy volume of MOV with minimal effect on the maximum FB-SM overvoltage. The proposed EAB shows better technical and economic performance than existing methods. The proposed EAB is validated by simulations and experiments

An auxiliary circuit enhancing the DC fault clearing capability of hybrid MMCs with low proportion of FB-SMs

The hybrid modular multilevel converter (HMMC) composed of half-bridge (HB) and full-bridge (FB) submodules (SMs) is an alternative to the FB-MMC with lower loss and cost. However, the HMMCs maximum reverse-biased voltage (RBV) is lower than the FB-MMC, so does the dc fault clearing capability (DCFCC). Reduced RBV will prolong the fault clearing time, especially when the dc side inductance is large. In this letter, an auxiliary circuit is proposed for HMMCs, which can change the fault current paths and enable both HB- and FB-SMs to participate in the fault clearing. Thus, the maximum RBV of the HMMC is increased to be equal to an FB-MMC. Moreover, the proportion of FB-SMs can be reduced. With the auxiliary circuit, the DCFCC of the HMMC is enhanced with the reduction of the power losses and semiconductor costs. Simulations and scaled-down experiments validate the proposed method

Providing Virtual Inertia Through Power Electronics

VSC-HVDC (voltage source converter based HVDC) system with its inherent merits for renewable energy integration has captured increasing research attentions. However, compared with AC systems dominated by synchronous generators (SGs), VSC-HVDC systems with general vector control cannot provide inertia for the grid due to lack of kinetic energy. This tends to degrade the safety and stability of the grid with the increasing penetration of renewable energy sources. To cope with this issue, virtual synchronous generator (VSG) has been proposed. In this thesis, firstly, a comprehensive introduction of various typologies of VSG schemes is made to illustrate their deficiencies and merits. The simulation results established in Simulink/Plecs show that VSG can not only participate into the regulation of frequency and voltage in case of power disturbances but guarantee the inertia provision for the grid. Although the integration of VSG control enhances the inertia and damping response of inverts, researches show that plenty of issues relative with VSG should be ameliorated. The fluctuation performances of SGs are introduced into the output active power and current of inverters when incorporates VSG control. This threatens the stability and safety of VSG operation, for power electronic based inverters are more vulnerable during the oscillations of current and frequency. Hence, to solve these issues, various enhanced VSG strategies have been constructed to improve its robustness and output performance. In this thesis, the structures and properties of enhanced VSG schemes are fully discussed. The results show that the dynamic properties of VSG during transient periods are enhanced in comparison of that of normal VSG. Modular multilevel converters (MMC) and alternate arm converters (AAC), as the representatives for enhanced topologies of VSC-HVDC system, have more complicated inner structures in comparison with 2/3 level converters. In this thesis, VSG control is applied into MMC/AAC models to strengthen their power and frequency regulation ability. In addition, a four-terminal multi terminal direct current (MTDC) system is incorporated with VSG control to provide primary frequency and voltage response for the grid. The results show that the integration of VSG improves the stability operation and inertia response of MMC/AAC/MTDC systems

A hybrid modular multilevel converter with reduced full-bridge submodules

A hybrid modular multilevel converter (MMC) with reduced full-bridge (FB) submodules (SMs) is proposed, where a high voltage rating half-bridge (HB) based MMC is connected in series with a low voltage rating FB-MMC in parallel with a fault breaking circuit on its DC side. Unlike conventional hybrid MMCs with mixed HB and FB SMs, the proposed topology uses the DC capacitor in the fault breaking circuit to block DC faults, while the FB-MMC only commutates the fault current from the FB-MMC to the fault breaking circuit. Thus, the proposed converter only requires around 10%-20% FB SMs, leading to reduced capital cost and losses compared to typical hybrid MMC. The optimal ratio of the FB-MMC and HB-MMC is assessed and comparative studies show superiority of the proposed topology over other alternatives. A case study with 10% FB SMs demonstrates the validity of the proposed hybrid MMC for DC fault blocking and post-fault system restart