Periodic Reporting for period 3 - MVDC-ERS (Flexible medium voltage DC electric railway systems)
Berichtszeitraum: 2021-06-01 bis 2022-04-30
Railway electrification provides faster and reliable train journeys compared to those of diesel trains and a strong reduction of pollution in busy stations and the country-side. However, many national programmes for the electrification of new and existing railway lines have required a substantial investment for the railway infrastructure. This is because railway electrification uses AC single-phase power that requires connection to high-voltage transmission lines, which are not always available in the intended places where the railway feeder stations should be located and usually require complicated and expensive modifications of the existing layouts.
In addition, the typical power levels of heavy railways and even high-speed railways is compatible with the capabilities of medium-voltage distribution systems. However, the connection of railway feeder stations to the power distribution network would be possible only by adopting schemes that does not introduce any imbalance in the power distribution system. In contrast to the single-phase AC electrification system, DC systems satisfy this requirement, However, the level of the DC voltage is limited to around 3 kV for the limitation on the maximum short-circuit breaking current of circuit breakers, which in turns limit the maximum power of the railway. Additionally, a higher voltage of the power supply would pose problems for the traction system of the trains, which operates at voltage levels of few kV.
The aim of Medium Voltage DC Electric Railway Systems (MVDC-ERS) project is to propose a new type of MVDC traction power supply based on controlled bidirectional converters to improve the connectivity of the railway to the grid and to integrate renewable power sources to the railway electrification system. This would not only improve the efficiency of the railway supply, but it will give additional capacity to the power distribution grid, as railway electrification lines could be used to provide extra capacity between the nodes where the substations are connected. This would be especially important for future scenarios where a higher proportion of renewable energy sources will be introduced in the power system and the control of the power flows will be vital to maintain the correct functionality of the power system. With reference to the on-board traction system, the project investigates DC Power Electronic Traction Transformers (PETT) to adapt the catenary voltage for the traction system of the trains.
The key objectives of the project are to introduce new high-efficiency topologies of power conversion systems to convert medium-voltage AC power into medium-voltage DC power with the capability of limiting the short-circuit current, to introduce high-power density topologies of power converters for on-board DC transformers, to investigate the impact of the forthcoming wide band-gap semiconductor devices in terms of efficiency and voltage level for the converters of the feeder stations and in terms of weight and volume for the traction converters, to understand how the new railway electrification system should be controlled and protected when renewable power sources are integrated, to understand how on-board energy storage can be exploited and how it can optimise the operations of the network, and to work with industrial stakeholders to investigate the marketability of the new electrification system and trains.
In addition, the typical power levels of heavy railways and even high-speed railways is compatible with the capabilities of medium-voltage distribution systems. However, the connection of railway feeder stations to the power distribution network would be possible only by adopting schemes that does not introduce any imbalance in the power distribution system. In contrast to the single-phase AC electrification system, DC systems satisfy this requirement, However, the level of the DC voltage is limited to around 3 kV for the limitation on the maximum short-circuit breaking current of circuit breakers, which in turns limit the maximum power of the railway. Additionally, a higher voltage of the power supply would pose problems for the traction system of the trains, which operates at voltage levels of few kV.
The aim of Medium Voltage DC Electric Railway Systems (MVDC-ERS) project is to propose a new type of MVDC traction power supply based on controlled bidirectional converters to improve the connectivity of the railway to the grid and to integrate renewable power sources to the railway electrification system. This would not only improve the efficiency of the railway supply, but it will give additional capacity to the power distribution grid, as railway electrification lines could be used to provide extra capacity between the nodes where the substations are connected. This would be especially important for future scenarios where a higher proportion of renewable energy sources will be introduced in the power system and the control of the power flows will be vital to maintain the correct functionality of the power system. With reference to the on-board traction system, the project investigates DC Power Electronic Traction Transformers (PETT) to adapt the catenary voltage for the traction system of the trains.
The key objectives of the project are to introduce new high-efficiency topologies of power conversion systems to convert medium-voltage AC power into medium-voltage DC power with the capability of limiting the short-circuit current, to introduce high-power density topologies of power converters for on-board DC transformers, to investigate the impact of the forthcoming wide band-gap semiconductor devices in terms of efficiency and voltage level for the converters of the feeder stations and in terms of weight and volume for the traction converters, to understand how the new railway electrification system should be controlled and protected when renewable power sources are integrated, to understand how on-board energy storage can be exploited and how it can optimise the operations of the network, and to work with industrial stakeholders to investigate the marketability of the new electrification system and trains.
The specifications and requirements of the MVDC substations were defined, considering only the topologies with inherent capability of limiting DC fault current. Three topologies, i.e. 12-pulse thyristor rectifier with anti-parallel inverter, two-level voltage source converter and modular multilevel converter (MMC) with full-bridge sub-modules were considered suitable for use in MVDC substations.
The comparison of these topologies shown that the MMC with full-bridge submodules is preferable. The simulation model for MMC with full-bridge submodules included the DC fault current controller (Fig. 1 and Fig. 2). The analyses show that that the MVDC TPS meets the desired quality measures in both normal and abnormal conditions.
The simulation model was extended to double-end and mesh feeding schemes and the MVDC railway network was tested with photovoltaic panels (Fig. 3). Furthermore, a simulation case of a meshed MVDC network was created, in which the MVDC TPSs are connected to different distribution networks (Fig. 4). The work towards developing the small-scale prototype has been started with developing power circuit of an MMC with 12 full-bridge submodules (Figs. 5, 6 and 7).
On the basis of a comparative research between all the topologies of PETT presented in the literature, it was found that the modular input series - output parallel (ISOP) medium frequency transformer based topology is the most suitable for MVDC application, especially when silicon carbide (SiC) semiconductor devices are used as power switches. A mathematical model of two such topologies has been developed in MathCad and simulated using PSIM and Simulink. The first modelled topology is a Dual Active Bridge converter (Fig. 8) and the second a bidirectional phase-shift full-bridge (BPSFB) converter (Fig. 9). Furthermore, an inverter and a motor model were attached to the PETT in Simulink (Fig. 10). The system was tested with different load steps. Fig. 11 and 12 illustrate five load steps using the DAB converter as modules. The second simulated topology is the BPSFB converter. In the case of both converters, after the transfer functions were obtained, the controllers were designed based on the desired phase margin.
The BPSFB converter modules were simulated as well, applying five load steps (Fig. 13).
The comparison of these topologies shown that the MMC with full-bridge submodules is preferable. The simulation model for MMC with full-bridge submodules included the DC fault current controller (Fig. 1 and Fig. 2). The analyses show that that the MVDC TPS meets the desired quality measures in both normal and abnormal conditions.
The simulation model was extended to double-end and mesh feeding schemes and the MVDC railway network was tested with photovoltaic panels (Fig. 3). Furthermore, a simulation case of a meshed MVDC network was created, in which the MVDC TPSs are connected to different distribution networks (Fig. 4). The work towards developing the small-scale prototype has been started with developing power circuit of an MMC with 12 full-bridge submodules (Figs. 5, 6 and 7).
On the basis of a comparative research between all the topologies of PETT presented in the literature, it was found that the modular input series - output parallel (ISOP) medium frequency transformer based topology is the most suitable for MVDC application, especially when silicon carbide (SiC) semiconductor devices are used as power switches. A mathematical model of two such topologies has been developed in MathCad and simulated using PSIM and Simulink. The first modelled topology is a Dual Active Bridge converter (Fig. 8) and the second a bidirectional phase-shift full-bridge (BPSFB) converter (Fig. 9). Furthermore, an inverter and a motor model were attached to the PETT in Simulink (Fig. 10). The system was tested with different load steps. Fig. 11 and 12 illustrate five load steps using the DAB converter as modules. The second simulated topology is the BPSFB converter. In the case of both converters, after the transfer functions were obtained, the controllers were designed based on the desired phase margin.
The BPSFB converter modules were simulated as well, applying five load steps (Fig. 13).
The research has generated a new method for controlling and modulating MMCs with full-bridge submodules. The developed method enables the converter to operate with low DC side ripples (in rectifier mode of operation), unity power factor and acceptable voltage ripple across the submodule capacitors. The method is especially beneficial for MVDC MMCs which have low number of submodules.
In addition, a MVDC railway simulator has been developed in Matlab/Simulink environment. The simulator consists of MVDC traction substations (TPSs) and MVDC overhead lines. Each TPS model consists of a MMC and AC distribution network model. The railway simulator can also simulate double-end fed and meshed MVDC network.
Moreover, the simulation model of the novel DC-PETT has been developed in Matlab/Simulink and PSIM, presenting how a voltage control loop can be sufficient for a multi-modular ISOP connected traction transformer system.
In the next steps, the main characteristics of the MVDC railway electrification will be shown by small-scale lab demonstrators, indicating that technology has reached a sufficient level of maturity to feed electric railways with DC power and overcome the traditional disadvantages of single-phase AC supply used for many railways around the world. The potential impact is a mitigation of the problems related to the connection of electric railways to the public grid and corresponding connection costs. This will enable a wider use of electric trains and pave the way to cheaper electrification systems and, hence, reduced CO2 emission from the rail transport.
In addition, a MVDC railway simulator has been developed in Matlab/Simulink environment. The simulator consists of MVDC traction substations (TPSs) and MVDC overhead lines. Each TPS model consists of a MMC and AC distribution network model. The railway simulator can also simulate double-end fed and meshed MVDC network.
Moreover, the simulation model of the novel DC-PETT has been developed in Matlab/Simulink and PSIM, presenting how a voltage control loop can be sufficient for a multi-modular ISOP connected traction transformer system.
In the next steps, the main characteristics of the MVDC railway electrification will be shown by small-scale lab demonstrators, indicating that technology has reached a sufficient level of maturity to feed electric railways with DC power and overcome the traditional disadvantages of single-phase AC supply used for many railways around the world. The potential impact is a mitigation of the problems related to the connection of electric railways to the public grid and corresponding connection costs. This will enable a wider use of electric trains and pave the way to cheaper electrification systems and, hence, reduced CO2 emission from the rail transport.