Medium-voltage DC Distribution Network Research Articles

Solid-State Circuit Breaker Based on Cascaded Normally-On SiC JFETs for Medium-Voltage DC Distribution Networks

Solid-State Circuit Breaker Based on Cascaded Normally-On SiC JFETs for Medium-Voltage DC Distribution Networks

Enhanced Fault Detection and Localization Strategy for High-Speed Protection in Medium-Voltage DC Distribution Networks Using Extended Kalman Filtering Algorithm

Enhanced Fault Detection and Localization Strategy for High-Speed Protection in Medium-Voltage DC Distribution Networks Using Extended Kalman Filtering Algorithm

An adaptive reclosing scheme for MVDC distribution networks with mechanical DCCBs

The DC distribution network is an effective solution to distributed renewable energy applications. After a fault occurs in the network, it is necessary to quickly identify and isolate it to protect system components. Subsequent to fault isolation by DC circuit breakers (DCCBs), power transmission should be swiftly restored by reclosing to improve system reliability. However, conventional reclosing schemes risk a second strike on the grids and endanger the safety of the networks. Given those drawbacks, an adaptive reclosing scheme for medium-voltage DC (MVDC) networks is proposed in this paper. An additional signal injection branch with a simple control strategy is appended to the existing commutation branch of a mechanical DCCB for reclosing. The fault nature can be identified by analyzing the magnitudes of one know frequency of the voltage signals injected into the faulty lines while fault locations under permanent faults can also be calculated before reclosing by applying this scheme. Compared with currently widely used methods, this scheme is characterized by its simplicity in structure and affordability in costs. Simulation results by PSCAD/EMTDC verify the efficacy and robustness of the proposed scheme.

Dual-transformer-based hybrid resonant three-level ZCS converter

A hybrid resonant three-level converter comprised of dual transformers is proposed in this paper, which is suitable for the application of distributed photovoltaic power accessing the medium voltage dc distribution network. The proposed converter can be obtained by adding a control circuit into the traditional neutral point clamped (NPC) three-level (TL) circuit, achieving the basic TL circuit operating with a fixed duty cycle. Pulse width modulation (PWM) is adopted for the control circuit to realize zero current switchings for the basic TL circuit, which delivers most of the power, under full load range. As a result, the switching loss of the converter can be significantly reduced. The influences of the turn ratio of the second transformer and resonant capacitance on the switch current, the peak value of resonant voltage, and the value of the resonant inductance value are discussed in detail, and the parameters design principles are put forward. Finally, a prototype is built to verify the performance of the proposed converter.

Research on Voltage Control of Multi Terminal Flexible Medium Voltage DC Distribution Network

In order to solve the problem that the original control strategy cannot meet the voltage quality requirements of the system under various operation modes due to the expansion of DC distribution network scale, this paper studies the voltage control strategy of a ring type medium voltage DC distribution network. Firstly, the traditional control method is improved, the droop control is added to the margin control as the slave station control method. At the same time, the DC transformer connected with the energy storage is used as the backup voltage regulating station in case of power shortage to meet the voltage quality requirements of the system in normal operation and fault. Then, a DC voltage control strategy based on depth first search is proposed to solve the problem of voltage deviation caused by system fault, so that the distribution network can automatically adjust the control according to the real-time structure in case of fault and improve the voltage quality. Finally, the proposed control strategy is simulated in PSCAD / EMTDC to verify the effectiveness of the control strategy.

Hybrid resonant three‐level ZCS converter suitable for photovoltaic power MVDC distribution network

This study presents a hybrid resonant three-level (TL) zero-current-switching (ZCS) converter for distributed photovoltaic power applications accessing medium-voltage DC (MVDC) distribution network. The main constituents of the converter is a neutral point clamped TL circuit, an auxiliary circuit along with two transformers and six switches. The ZCS condition of the TL circuit is reached by operating the auxiliary circuit utilising pulse width modulation technique. It delivers the majority of the power and the switching loss is remarkably reduced. A section on the influence of the turn's ratio of the auxiliary transformer on the current is also presented. The piecewise linear electrical circuit simulation simulation results validate the designed parameter principles. For the sake of comparison, an experimental prototype is developed of 300–1500 V/1500 W to validate the performance of the proposed converter.

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.

A current-limiting and energy-transferring DC circuit breaker topology for DC distribution network

With the development of power electronic technologies and distributed power generation, DC distribution networks attract increasing attention due to their various advantages compared with traditional AC distribution networks. However, DC fault protection is one of the major issues in DC distribution networks. To improve their reliability and protect the semiconductor devices under DC faults, a current-limiting and energy-transferring DC circuit breaker topology is proposed in this paper. By applying passive components and thyristors, the proposed topology is capable of quickly limiting the fault current and transferring the faulty energy. The working principle, mathematical model and parameter designing method of the proposed topology are presented in this paper. The simulation results verify that the proposed DC circuit breaker could effectively limit the fault current and quickly interrupt the fault current. Cost and conduction power loss evaluation proves the practicality of the proposed topology in medium-voltage DC distribution networks.

Stability Analysis of Multi-Port MVDC Distribution Networks for All-Electric Ships

Advances in the power electronics technologies, over the years, have opened up the possibilities to consider medium-voltage dc (MVDC) distribution networks as possible evolutions of the existing medium-voltage ac (MVAC) distribution networks on large ships. The MVDC distribution networks provide the possibilities to increase fuel efficiency and remove bulky transformers. However, to implement industrial-scale MVDC distribution networks, some critical challenges exist, such as lack of standardized equipment and system-level stability. This paper studies the feasibility and stability of MVDC distribution networks when a distributed layout is considered. Due to the nature of the network, a multiple-input-multiple-output (MIMO) impedance stability approach is employed for modeling and assessment. The different components are modeled according to the existing industrial medium-voltage technologies. The theoretical analysis is verified by full-model time-domain simulations. As a summary of the contribution, the main features of the proposed study are: 1) the realistic identification of feasibility limits; 2) the definition of design rules for capacitance sizing and best placement; and 3) the evaluation of distances for dc distribution cables and their corresponding inductances.

New modelling of photovoltaic power integration system and its control strategy for medium‐voltage DC distribution network

Here, a novel photovoltaic power integration system based on LLC resonant converters is proposed for medium-voltage DC distribution network, which exhibits a feasible and efficient solution for large-scale solar energy integrating into DC grids. A control system with the voltage signal generation part and the frequency control part is designed to realise the current and voltage sharing of the sub-module, while achieving the maximum power point tracking (MPPT) of the PV arrays. To verify the practicability of the system with many switching devices, this paper proposes an optimisation model to accelerate the simulation speed of this system without losing accuracy. Finally, the feasibility of the proposed system is validated by the simulation based on PSCAD/EMTDC.

Small‐signal stability analysis for the multi‐terminal VSC MVDC distribution network; a review

The stable operation and control of the proposed terrestrial multi-terminal Voltage Source Converter (VSC) medium voltage DC (MVDC) distribution network is a remarkable motivation towards developing the future universal DC grid interconnecting the low voltage DC systems on one side and the high voltage DC systems on the other. The small-signal stability analysis are important in designing these power electronic based systems (PEBS). In this study, small-signal stability approaches for PEBS are reviewed with the aim of identifying suitable methods for the proposed MVDC distribution network. It is established that the impedance-based methods are suitable for local stability analysis complementing very well with the state-space modelling-eigenvalue analysis method best for global stability analysis. The Lyapunov linearisation method offers simple, adaptable, and accurate small-signal and large-signal stability study. On the other hand, the bifurcation analysis is an emerging non-linear approach with promising efficiency and precision. In conclusion, there is no single small-signal stability criterion that encompass an acceptable level of accuracy, adaptability, and efficiency. Thus, the impedance-based methods combined with state-space modelling-eigenvalue analysis, Lyapunov linearisation, and bifurcation theory can be suitable small-signal stability approaches for the multi-terminal MVDC distribution network.

Control scheme for multi‐terminal VSC‐based medium‐voltage DC distribution networks

With the development and application of voltage source converter (VSC) high voltage direct current (HVDC) technology, DC distribution is gradually attracting the attention of researchers in recent years. However, many studies are still in the theoretical and exploratory stages. Compared with DC transmission networks, medium-voltage DC (MVDC) distribution networks are more sophisticated with respect to the complex operation modes, high penetration of renewable energy resource, and use of diverse power electronic devices. Therefore, master–slave control strategy which has been widely used in HVDC transmission projects may not be the first choice for DC distribution networks. In this study, a three-terminal DC distribution dynamic simulation platform is initially established and droop control strategy is adopted due to its flexible scalability. Secondly, plug and play solutions for key equipment such as VSCs and DC solid-state transformers are proposed. Thirdly, the response characteristics of a droop control system under high renewable energy resource penetration and frequent transition of operation mode are studied. Fourthly, the system fault isolation and recovery strategy under DC pole-to-pole fault is proposed. Finally, the whole control scheme is validated through experiments using the dynamic simulation platform. In the future, this proposed scheme can be used in the control of VSC-based MVDC distribution networks.

Planning and analysis of the demonstration project of the MVDC distribution network in Zhuhai

The DC distribution system is an important development direction of the distribution system, which can improve the reliability and the quality of the power supply, and support the new energy, the energy storage, the electric vehicles, and the flexible access of AC and DC loads to grid. To realize the demonstration application of the DC distribution technology, China’s first demonstration project of the medium voltage DC distribution network will be built in Zhuhai, Guangdong Province to support the construction of the city energy internet. First, this paper analyzes the demand of the DC distribution network project, and puts forward the construction content and construction target. Then, it designs and analyzes the electrical connection mode, system operation mode, and startup and shutdown mode of the DC distribution network, and proposes the overall project construction plan. Finally, it conducts the specific project design and analysis, which mainly include the selection of equipment such as inverters, DC transformers and DC circuit breakers, the design and analysis of the DC control and protection system, the design and analysis of the over-voltage protection and the configuration scheme of the lightning arrester, and analysis of the system transient characteristics. The design and analysis of the engineering program is a combination of China’s distribution network engineering practice and technical characteristics, which lays a solid foundation for the advancement of the DC power distribution technology in China, and has reference value and demonstration effect for the design and construction of other projects.

Hierarchical control strategy for MVDC distribution network under large disturbance

The DC distribution technology brings challenges and opportunities to the penetration of large-scale distributed renewable energy sources. In a medium-voltage DC (MVDC) distribution network, operation mode switch occurs owing to some large disturbances. To manage these disturbances and achieve seamless switch of the operation mode, a hierarchical control strategy with two layers, namely local and global layers, is proposed in this work. The novel local layer controller called P–U–I controller is designed to enhance the voltage stability, improve system controllability and suppress overcurrent under large disturbances. This controller only needs local information and does not rely on fast communication. Furthermore, as the scale of the DC distribution network expands continuously, the number of system operation mode increases in geometric progression. Therefore, a global layer controller based on breadth-first search algorithm is proposed. This controller can automatically identify the system topology and adjust the control mode of converters to optimise the system operating characteristics. Thus, this hierarchical control strategy can accurately control the system power in steady state, suppress overcurrent under large disturbances and suit large-scale DC distribution networks. Finally, a three-terminal MVDC distribution network simulation model established on power systems computer-aided design and RT-LAB validates the proposed strategy.

Analysis and optimisation of modulation strategy based on dual‐phase‐shift for modular multilevel high‐frequency‐link DC transformer in medium‐voltage DC distribution network

DC/DC converters are crucial in DC distribution networks for converting voltage and interconnecting different voltage links. The modular multilevel high-frequency-link DC transformer (MDCT), which employs multiplex conversion principle and provides many advantages, realises voltage conversion, power transfer and electrical isolation between links in medium-voltage DC distribution networks. To improve the performance of MDCT, this study investigates the modulation with the phase-shift strategy and its influence on current stress, power characteristic and efficiency characteristic of MDCT. Besides, to simultaneously achieve an optimal solution for both current stress and efficiency characteristics of MDCT, an optimal modulation strategy based on dual-phase-shift is proposed. Finally, an MDCT prototype is constructed and experimental results verify the correctness and effectiveness of the analysis and proposed scheme.