Medium Voltage DC Distribution System Research Articles

High-speed directional pilot protection for MVDC distribution systems

Selective dc protection with high speed is the key technology for the dc distribution system. The single-ended protections applied in high voltage dc (HVDC) grid are not suitable for the dc distribution system, due to the absence of the boundary elements. Differently, the pilot protection shows a good application prospect in dc distribution systems, because it can protect a clear bounded zone. In this paper, the characteristic of backward traveling waves on the fault cable and healthy cable is analyzed in detail. Then a high-speed directional pilot protection is proposed for dc distribution systems, which identifies the fault direction mainly according to the high-frequency backward traveling wave. Compared with the traditional pilot protections, such as the current differential protection, directional protections based on current change value (Δi) or change rate (di/dt), the proposed protection is immune to the cable distributed capacitor current. In addition, the proposed method does not depend on the boundary of the line terminals, thus being more suitable for the medium voltage dc (MVDC) distribution system. Simulation cases based on the PSCAD/EMTDC are carried out to verify the feasibility and advantages of the proposed scheme.

Reduced-order modeling and dynamic stability mechanism analysis of MTDC system in DC voltage control timescale

An equivalent source-load MTDC system including DC voltage control units, power control units and interconnected DC lines is considered in this paper, which can be regarded as a generic structure of low-voltage DC microgrids, mediumvoltage DC distribution systems or HVDC transmission systems with a common DC bus. A reduced-order model is proposed with a circuit structure of a resistor, inductor and capacitor in parallel for dynamic stability analysis of the system in DC voltage control timescale. The relationship between control parameters and physical parameters of the equivalent circuit can be found, which provides an intuitive insight into the physical meaning of control parameters. Employing this model, a second-order characteristic equation is further derived to investigate system dynamic stability mechanisms in an analytical approach. As a result, the system oscillation frequency and damping are characterized in a straight forward manner, and the role of electrical and control parameters and different system-level control strategies in system dynamic stability in DC voltage control timescale is defined. The effectiveness of the proposed reduced-order model and the correctness of the theoretical analysis are verified by simulation based on PSCAD/EMTDC and an experiment based on a hardware low-voltage MTDC system platform.

A New Protection Scheme for MMC-Based MVdc Distribution Systems With Complete Converter Fault Current Handling Capability

DC fault handling is a critical aspect for designing and operating dc distribution systems. DC distribution protection must be rapid to avoid the damages caused by the fast growth of dc fault currents. For medium-voltage dc (MVdc) distribution systems, low-cost high-speed dc breakers are still challenging. A hybrid modular multilevel converter (MMC)-based dc fault protection and restoration method is proposed for a cost-effective dc fault protection solution for MMC-based MVdc systems. In the proposed protection solution, the converter topology and associated fault handling can bypass and interrupt the fault currents from both ac and dc sources, and thus allowing fast dc fault isolation by low-cost no-load dc disconnectors. Furthermore, the proposed dc protection solution facilitates converter fault ride-through and system restoration after fault removal. Simulation results of the pole-to-pole fault protection show the satisfactory performance of the proposed protection solution and scheme.

Fault-Tolerant Oriented Hierarchical Control and Configuration of Modular Multilevel Converter for Shipboard MVdc System

Medium-voltage dc (MVdc) distribution system is considered as the promising power architecture of future shipboard power system. Fault-tolerance ability is of great importance for power system on ships. In this paper, zonal dc–dc conversion system based on modular multilevel converter (MMC) and three-phase bridge rectifier are proposed with its hierarchical fault-tolerant scheme. The topology configurations of the front-end MMC with multiwinding medium frequency transformer under normal and postfault circumstances are presented. The reconfiguration scheme and fault-tolerant control strategy are also proposed to ride-through submodule (SM) level fault, phase level fault and bus level fault. Based on the hierarchical fault-tolerant scheme, redundant configuration of the MMC SM is further analyzed with the hierarchical reliability modeling. The effectiveness of the proposed control strategy is verified by the experimental results.

Wavelet-Based Time-Domain Solutions for Common-Mode Currents on Open-Form Conductors Modeled With Band-Limited Scattering Parameters

One of the technical challenges of future naval surface combatants in utilizing medium-voltage dc distribution systems is common-mode coupling and galvanic isolation for managing faults and personal safety. The behavioral modeling reported in this paper is computationally efficient to be used in modeling distributed common-mode currents and potentials throughout the open-form conductor, which in this case is the ship structure. Physics-based modeling is used to form a multiport S-parameter behavioral model of the ship structure that complements the S-parameter models of parasitic paths to grounded equipment enclosures and cables sheaths. A time-domain common-mode conduction example is presented to illustrate the workflow that converts physics-based models of elementary hull elements into a complete S-parameter behavioral model of an arbitrary hull structure with power cables running along it. Because of the measurement and the simulation bandwidth limitation, the frequency domain model based on S-Parameters is band-limited. A novel approach for reconstructing the system impulse response in the time domain from its band-limited S-parameter model is introduced using continuous wavelet transform utilizing fast Fourier transform.

Integrated Optimization of Network Topology and DG Outputs for MVDC Distribution Systems

Medium-voltage DC (MVDC) distribution systems are receiving more and more attractions. For a MVDC distribution system, less network power losses can be obtained if the network topology and the outputs of distributed generations are optimized together. However, this is a tough optimization problem due to the mixed integer non-convex programming property. To efficiently address this problem, a convex mixed-integer quadratic programming (MIQP) based integrated optimization approach is proposed in this letter, which is validated via three MVDC systems.

Full-Process Operation, Control, and Experiments of Modular High-Frequency-Link DC Transformer Based on Dual Active Bridge for Flexible MVDC Distribution: A Practical Tutorial

DC transformer (DCT) will be the key device for medium-voltage dc (MVDC) power distribution system. This paper gives a practical tutorial on full-process operation, control, and experiments for application of DCT based on dual active bridge in flexible MVDC distribution system. The operation of DCT for MVDC distribution is designed to three modes: MVDC, low-voltage dc (LVDC), and power control modes. Three optimal modulation methods during startup, steady, and transient processes are proposed which can reduce current impact in practice. A full-process control strategy during operation state is proposed to achieve flexible control and fast management of voltage and power in MVDC distribution system, especially an optimal balance control during block process is proposed to reduce power losses. On this basis, a fault-handling solution is proposed to improve reliability of DCT in practice, and a hardware design method is proposed to enhance flexibility and modularity. Finally, an industrial prototype is built, and comprehensive experiments verify the validity and effectiveness of the proposed solution. The practical application of DCT in MVDC distribution are expected, and the exploration of this study could provide valuable references for the practical DCT design.

Multi‐terminal medium voltage DC grids fault location and isolation

Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi-terminal VSC-based DC networks. This study proposes a protection strategy for medium voltage DC distribution systems. The strategy consists of a communication-assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers. This study also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analysed through real-time simulation studies by use of the hardware-in-the-loop approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi-terminal DC distribution networks and VSC stations against different types of faults.

Power Flow and Generator-Converter Schemes Studies in Ship MVDC Distribution Systems

The medium-voltage dc (MVDC) distribution system is a new shipboard power distribution technology that has been extensively discussed and studied by many shipbuilding corporations and ship-owners in recent years. As different types of power converters and ac and dc power distributions simultaneously involved in the system, the system performance for different connections of generators to MVDC converters under varying steady-state operating conditions is essential and important in the design stage of the electrical power distribution system. This type of problem can be analyzed by performing power flow studies. This paper aims presenting a methodology for power flow studies in ship MVDC distribution systems for system planning purpose. A power flow model of the power converters in steady-state is derived to consider medium and low voltage characteristics of electric power system on this type of ship. An ac/dc power flow solution approach then is developed to incorporate ac-system and dc-system models and the ac/dc interface buses in the analysis for determining an appropriate connection of generators and MVDC converters and power converter setting values. Test results of two alternatives for the direct connection of generators to MVDC converters under varying steady-state operating conditions are presented and compared to determine the proper generator-power converter scheme.

Design of Smart MVDC Power Grid Protection

Improved reliability and safety of the medium-voltage dc power distribution systems on board of all electric ships are the objectives of this paper. The authors propose the integration of the self-healing capability against faults of the measurement system in power system fault detection and protection systems. While most of previous work in the literature focuses on either one aspect independently, here, the two are integrated. On one hand, our approach addresses also the case of concurrent power system fault and measurement system fault. On the other hand, the proposed approach must be capable of distinguishing between the two types of failure. The proposed architecture is based on exchange of information between energy conversion and measurement devices. This makes the impact of communication delays critical, so its analysis is provided for the proposed case study. The impact on the performance of the measurement validation and protection systems is derived and can provide hints on the design. The protection method used as case study consists in controlling power converters to ride through the power system fault while maintaining power supply to the vital loads. To overcome failures of the measurement system, invalid data were detected and reconstructed through their expected value.