Medium Voltage DC Networks Research Articles

A Novel Hybrid DC Transformer Combining Modular Multilevel Converter Structure and Series-Connected Semiconductor Switches

A hybrid dc transformer (DCT) combining the features of modular multilevel converter (MMC) and series-connected semiconductor switches (SSs) is proposed in this article, which is suitable for interconnection between medium-voltage dc networks and low-voltage dc networks. Compared with the conventional MMC-based DCT, the number of submodules (SMs) of the proposed DCT can be reduced under the same device voltage stress. A quasi-square-wave modulation is employed, which can ensure the high-reliability drive of SSs with the realization of zero voltage switching in a wide power and MV voltage range. The voltage self-balancing characteristic of the SM capacitor can be achieved in the proposed DCT, with no requirement of voltage sensor or specific modulation pattern. All the SSs are turned <small>on&#x002F;off</small> at zero voltage, the dynamic voltage balancing of the SSs is ensured naturally. Based on the analysis of the operation principle, the control scheme and parameters design are elaborated. The theoretical analysis is verified by the simulation and a downscaled 4-kW prototype experiment.

Fault Location and Classification for MVDC Networks

This paper proposes a two step fault detection scheme for medium-voltage dc (MVDC) networks based on wavelet analysis. The proposed Grid Transients Classifier (GTC) is used to continuously monitor energy of wavelet packet decomposition levels (WPDL) for voltage and current signals to determine any transient disturbance like load change, pole to pole (PP) or pole to ground (PG) faults, pole to pole to ground (PPG), voltage sags, switching, etc. Subsequently, type of disturbance is identified through an artificial neural network (ANN) classifier. Then, proposed Active Grid Impedance Estimator (AGIE) injects a signal with two fundamental frequencies to evaluate grid impedance from the measurement point in new condition. Using the obtained data, fault type, location and its severity are specified. Hardware-in-the-Loop (HIL) results are provided for a shipboard grid case study to validate the real-time application of the proposed method and demonstrate its effectiveness.

Sizing Algorithm for a Photovoltaic System along an Urban Railway Network towards Net Zero Emission

A reliable transportation system is essential for the development of a community. Especially in urban transportation, rail transportation is a faster, more comfortable way to travel for the commuters. These benefits can be valued further when the rail transportation system is with zero emissions. Electric trains can be considered a zero-emission transportation method. However, a rail transportation system operates with net-zero emissions when electricity is generated from zero-emission-based sources. Photovoltaic systems have already been integrated into railway stations and spare land owned by railways to achieve net-zero emissions. Furthermore, medium-voltage DC network and microgrid concepts have been proposed to incorporate more renewable energy sources into railway electrification systems. However, the energy generated from those systems is not enough to realise net-zero emissions, as the power requirements of an urban railway electrification system are high. Accordingly, this article investigates the possibility of implementing a photovoltaic system along the railway tracks to meet the energy demands of an urban railway electrification system so that net-zero emissions can be achieved. Other significant advantages of the proposed photovoltaic system are lower feeder losses due to distributed photovoltaic systems integrated into the railway electrification system, lower conversion losses due to the direct integration of the photovoltaic system into the railway electrification system, and the nonrequirement of additional space to install the photovoltaic system. In this paper, a photovoltaic system capacity sizing algorithm is proposed and presented by considering a railway electrification system, the daily schedule of trains, and historical photovoltaic weather data. This proposed photovoltaic system capacity sizing algorithm was evaluated considering a section of the urban railway network of Sri Lanka and a three-year, 2017-2020, photovoltaic weather data. The results indicated that the potential for photovoltaic generation by installing photovoltaic systems along a railway track is much higher than the requirement, and it is possible to meet the required train scheduling options with proper sizing. Furthermore, in the three-year analysis, it is possible to achieve 90% of the energy required for the railway electrification system with effective train scheduling methods.

Innovative Energy Management System for MVDC Networks with Black-Start Capabilities

Medium voltage DC (MVDC) networks are attracting more attention amid increased renewables penetration. The reliability of these DC systems is critical, especially following grid contingencies to maintain critical loads supply and provide ancillary services, such as black-start. This paper proposes an innovative energy management system (EMS) to maintain reliable MVDC network operation under prolonged AC grid contingencies. Similar EMS designs in literature tend to focus on limited operating modes and fall short of covering comprehensive elongated blackout considerations. The proposed EMS in this paper aims to preserve the distribution network functionality of the impacted MVDC system through maintaining a constant DC bus voltage, maximizing critical load supply duration, and maintaining the MVDC system black-start readiness. These objectives are achieved through controlling generation units between Maximum Power Point Tracking (MPPT) and Voltage Regulation (VR) modes, and implementing a smart load shedding and restoration algorithm based on network parameters feedback, such as storage State of Change (SoC) and available resources. Practical design considerations for MVDC network participation in AC network black start, and the following grid synchronization steps are presented and tested as part of the EMS. The proposed system is validated through simulations and scaled lab setup experimental scenarios.

Resonant Modular Multilevel DC–DC Converters for Both High and Low Step-Ratio Connections in MVDC Distribution Systems

DC transformers based on power electronics are key items of equipment for medium voltage dc (MVDC) distribution systems. Both high and low step-ratio dc-dc conversions are required to interface dc links at different voltages to form an integrated dc distribution system. The transformer-coupled resonant modular multilevel dc-dc converter (RMMC) is well-suited to high step-ratio connection between an MVDC network and a low voltage dc network but it is not suitable in low step-ratio conversion for linking two MVDC networks with similar but not identical voltages. This article presents a circuit evolution of the high step-ratio transformer-coupled RMMC into its low step-ratio transformer-less RMMC counterpart. These two RMMCs share the same structure and the same resonant process within the resonant submodule stack giving rise to the same operational advantages. Also, the circuit design experience can be readily transferred from one to the other. From these two base RMMC circuits, a family of RMMCs with further configurations is elaborated that provide a wider variety of connection options for MVDC distribution systems. In this circuit family, each high step-ratio transformer-coupled RMMC has a low step-ratio transformer-less RMMC counterpart and one can be transformed into the other via the circuit evolution process presented. The theoretical analysis for both high and low step-ratio RMMCs has been verified through full-scale simulations of medium voltage examples and further verified through down-scaled experiments on laboratory prototypes.

Multiterminal Medium Voltage DC Distribution Network Hierarchical Control

In this research study, a multiterminal voltage source converter (VSC) medium voltage DC (MVDC) distribution network hierarchical control scheme is proposed for renewable energy (RE) integration in a co-simulation environment of MATLAB and PSCAD/EMTDC. A DC optimal power flow (DC OPF) secondary controller is created in MATLAB. In PSCAD/EMTDC, the main circuit containing the adaptive DC voltage droop with a dead band and virtual synchronous generator (VSG) based primary controller for the VSCs is implemented. The simulation of the MVDC network under the proposed hierarchical control scheme is investigated considering variations in wind and solar photovoltaic (PV) power. The network is also connected to the standard IEEE-39 bus system and the hierarchical scheme tested by assessing the effect of tripping as well as restoration of the REs. The results show that during random variations in active power such as increasing wind and PV power generation, a sudden reduction or tripping of wind and PV power, the primary controller ensures accurate active power sharing amongst the droop-based VSCs as well as regulates DC voltage deviations within the set range of 0.98–1.02 pu with an enhanced dynamic response. The DC OPF secondary control optimizes the system’s losses by 38% regularly giving optimal droop settings to the primary controllers to ensure proper active power balance and DC voltage stability. This study demonstrates that the hierarchical control strategy is effective for RE integration in the MVDC distribution network.