Decentralized control for multi-terminal cascaded medium-voltage converters considering multiple crossovers

Decentralized control with multiple droop characteristics can significantly improve the accuracy of power flow in medium-voltage direct-current (MVdc) networks. However, multiple crossovers caused by different control characteristics can lead to the drifts of power and voltage and instability issues. When this type of control is implemented in the cascaded three-level neutral-point-clamped (C3L-NPC) converters, on one hand, the mechanism of such the power and voltage drifts was not investigated. On the other hand, power control accuracy, dc voltage balancing across submodules (SMs) and multiple crossovers should all be considered, which requires suitable control methods. To address the challenges, firstly, the mechanism behind the power and dc voltage drifts is analyzed. Secondly, a control scheme is presented to improve the power control accuracy and dc voltage balancing and concurrently, to avoid the multiple crossovers. This is achieved by suitable droop gain design and adding a secondary power compensator. The presented control scheme is verified in MATLAB/Simulink simulation and experimentally validated in a three-terminal MVdc testbed. Results show that the accuracy of steady-state power flow is improved by 15% due to the elimination of multiple crossovers, while the power accuracy at dynamics improved by 13% with the secondary power compensato

Analysis and mitigation of DC voltage imbalance for medium-voltage cascaded three-level neutral-point-clamped converters

The cascaded three-level neutral-point-clamped (3L-NPC) converter and the modular multi-level converter (MMC) are attractive solutions for medium-voltage direct-current (MVDC) applications. Due to their low cost compared to MMCs, cascaded 3L-NPC converters have been adopted in ANGLE-DCa 30 MVA MVDC link demonstration project in North Wales, UK. DC voltage imbalance across submodules (SMs) is a common challenge for both types of MVDC converters. Such imbalance is topology dependent and remains under-researched for cascaded 3L-NPC converters. In this paper, small-signal model-based analysis has been done to reveal that the dc voltage imbalance in cascaded 3L-NPC converters is caused by an unstable system pole. Two voltage balancing methods are presented. The first method is based on PI controllers to precisely regulate SMs voltages without influencing output power. However, it relies on communication between a central controller and local controllers within SMs. The second method uses inverse-droop based control to take over the dc voltage regulation upon loss of communication. Both balancing methods are experimentally validated using a 30 kVA testbed based on the ANGLE-DC project. It has been demonstrated that the dc voltages of SMs can be effectively balanced with both methods during changes of load conditions and dc bus voltages

DC/DC converter for offshore DC collection network

Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes.Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes

Active control of medium-voltage cascaded three-level neutral-point-clamped converters

Three-Level Neutral-Point-Clamped (3L-NPC) converters have been widely used in the high-power motor drives. In recent years, a novel cascaded 3L-NPC converter has been developed and adopted in the ANGLE-DC project − a 30 MVA MVDC link demonstration project in North Wales, UK. This cascaded configuration provides exceptional waveform quality, modular design and a cost-effective solution to MVDC applications. Although the control strategy for a single 3L-NPC converter has been well established, control of the cascaded 3L-NPC converter is still under-researched. The potential challenges to control strategy design arising from their cascaded connections need to be specifically explored. In particular, due to the series DC connection, the voltage imbalance across 3L-NPC submodules (SMs) may occur and influence the system stability. This issue may occur in converter stations where power is controlled in either point-to-point or multi-terminal systems. Beyond the electric characteristic, thermal characteristic is also vital to the performance of system. Thermal imbalance of 3L-NPC SMs may occur in a cascaded 3L-NPC converter even the voltage and power are equally shared, which poses great challenges to the system reliability. To address aforementioned challenges, this thesis developed suitable control schemes for the cascaded 3L-NPC converter system and demonstrated their operation using a 30 kVA MVDC testbed based on the real ANGLE-DC project. The DC voltage imbalance was analysed through a small-signal model-based approach. Two DC voltage balancing methods with and without communications were presented. The PI-based method can automatically switch to the droop-based method upon failures of communication. The DC voltage imbalance of the cascaded 3L-NPC converter is further investigated in a three-terminal MVDC network in consideration together with the interactions of control characteristics between different converter stations and the power control accuracy. Then suitable control scheme was proposed. Multiple crossovers due to the interactions are avoided while DC voltage balance and power control accuracy are achieved as well. To mitigate the thermal imbalance, a thermal sharing controller was superposed on the DC voltage balancing controller to regulate the active and reactive power of each SM according to their individual junction temperatures. The thermal stresses are hence equally shared in presence of mismatched component parameters and cooling system failures. The effectiveness of presented methods in the thesis has been verified in MATLAB/Simulink simulation and experimentally validated

Modular DC/DC converter topologies for off-shore DC collection point

PhD ThesisWith the development of the economy, the demand for energy has been substantially growing. Wind power, particularly from offshore wind farms, is one of the best solutions. In Europe, installed capacity had exceeded 8GW by 2105 and will probably reach 40GW in 2020. These ambitious plans require the establishment of large-scale wind farms with more efficient transmission systems. The high voltage direct current (HVDC) transmission system is an effective way to deliver large-scale energy over long distances with lower power losses. However, due to the growth of large-scale offshore wind energy system, the connection between the farms and HVDC transmission lines is more challenging. Medium-voltage DC (MVDC) collection networks are a promising technology for such integration, aiming to eliminate voltage difference. High-voltage high-power DC/DC converters are the key enabler for MVDC grids. But the present lack of suitable high-voltage high-power DC/DC converter topologies is preventing the development of DC networks. Several high-voltage high-power topologies have proposed in previous studies, but most such topologies involve design compromises in terms of switching losses, limited conversion ratios, and lack of modular design, lack of electrical isolation features. This thesis presents three novel modular DC/DC topologies, which are developed based on the conventional modular multilevel converter (MMC), to enable the integration of off-shore wind farms with high voltage direct current (HVDC) transmission systems. The first topology is a unidirectional single-phase modular DC/DC converter, and the second is a unidirectional threephase modular DC/DC converter consisting of a windfarm-side three-phase modular multilevel (MMC) inverter and a series-connected diode rectifier module linked by a special decoupled medium frequency transformer. The third topology is a bidirectional three-phase modular DC/DC converter where a three-phase MMC inverter produces a controllable AC voltage connected at the primary side of a three-phase decoupled medium frequency transformer. The secondary output voltages decoupled into three identical but 120-degree phase-shifted voltages. Simulations using MATLAB/Simulink are reported to demonstrate the effectiveness of the proposed converters. Moreover, a low voltage scaled-down prototype of the unidirectional II single-phase modular DC/DC converter is developed to validate its feasibility experimentally. Keywords: Modular multilevel converter, offshore wind farm, unidirectional/bidirectional modular DC/DC converters, HVDC system.Scottish Energ