Medium Voltage DC Distribution System Research Articles

An Improved AC-Link Voltage Matching Control for the Multiport Modular Multilevel DC Transformer in MVDC Applications

In this paper, an improved AC-link voltage matching control (IVM) strategy is proposed for the multiport modular multilevel DC transformer (M3DCT), which comprises a single-phase modular multilevel converter (MMC) and a series of cascaded H-bridge units. The objective of the proposed IVM strategy is to address the AC-link voltage mismatch phenomenon. Distinct from existing control methods, such as various phase-shifting control methods and the conventional AC-link voltage matching control strategy, the proposed IVM strategy orchestrates the operation of the M3DCT in an innovative fashion. It allows the sum of inserted submodule (SM) numbers in the upper and lower arms to be flexible, no longer confined to a specific SM number per arm. Consequently, the AC-link voltage of the M3DCT is maintained proximate to the matched operating condition, regardless of the degree of mismatch in the DC side voltage of the M3DCT. This enables the enhancement of the M3DCT’s overall operational performance, particularly under conditions of light loads within medium-voltage DC (MVDC) distribution systems. The correctness and effectiveness of the proposed control strategy and the corresponding analysis are substantiated through simulation results.

A Three-Port PSFB/DAB-MMC PET With Inertia Enhancement Under LVDC Disturbance

DC distribution system has been popular in research and industry applications in recent years. The topology of the three-port modular multilevel converter (MMC) was proposed to interconnect the medium voltage DC (MVDC) distribution system, the medium voltage AC (MVAC) distribution system and the low voltage DC (LVDC). Compared to classical two-port MMC, a three-port MMC substitutes the tranditional submodules with bidirectional isolated converters. There are two kinds of existing three-port MMC topologies. One topology uses dual active bridge (DAB) converters to transfer power from the medium voltage side to the LVDC system, which is called DAB-MMC. The other topology utilizes bidirectional phase shift full bridge (PSFB) converters as submodules, which is called isolated MMC (I-MMC). Compared to the DAB-MMC, the I-MMC reduces the cost of the submodule capacitors and the power switches. But the LVDC inertia of the I-MMC is low due to the common submodule capacitors, especially during disturbances. In this article, a hybrid three-port MMC topology with inertia enhancement is proposed. With the combination of the PSFB and DAB submodules, the proposed MMC increases the equivalent inertia of the LVDC system using a novel virtual control method. The simulation results in MATLAB/Simulink and real-time RT-LAB prove the feasibility and the accuracy of the proposed inertia enhancing method.

High speed protection of medium voltage DC distribution system using modified mathematical morphology

One of the challenging issues in the realization of medium-voltage DC (MVDC) distribution system is high-speed protection scheme design against quickly rising short circuit fault-DC current. This paper proposes a high-speed protection scheme based on the processing of energy signals through mathematical morphology (MM). The voltage and current signals measured at a distribution line section (DLS) are used to obtain the energy signal. The proposed scheme develops a modified MM-based filter to extract the transient information in the energy signal rapidly and distinctly in the form of positive/negative polarity. The polarity detection of any disturbance in the analysed energy signal indicates the occurrence of a sudden disturbance in the system. The sudden disturbance occurrence confirmation triggers the faulty DLS identification and classification algorithms of the proposed scheme. The faulty DLS identification and classification algorithms are based on the detected polarities of the energy signal. The protection scheme requires low bandwidth communication. The proposed protection scheme is evaluated by using a ±2.5 kV radial distribution network containing 4 feeders under different fault conditions in MATLAB/SIMULINK software. Simulation results verify that the proposed scheme can isolate the faulty portion within a few milliseconds after fault inception under a variety of fault cases.

An improved energy management for MVDC distribution system based on exponential droop control

The energy management optimization and DC voltage stability are the main challenges when loads surging and renewable energy sources (RESs) fluctuations occur in the medium-voltage DC distribution system (MVDC-DS). To solve these issues, this paper proposes a dual-time scale energy management method based on improved droop control for the MVDC-DS. Firstly, a ±10 kV three-terminal hybrid modular multilevel converter (MMC)-based ring MVDC-DS with DC fault ride-through capability is constructed, which including electric vehicle charging stations, energy storage systems, multiple loads and RESs. Secondly, the droop control adopted by the hybrid MMC is improved by exp-function to achieve higher power quality. On this basis, a dual-time scale energy management method is proposed to minimize the electricity purchasing cost. The reference power of each controllable unit is optimized in the long time scale, and the parameters of improved droop control are optimized in the short time scale. Finally, a case study is conducted on the ±10 kV three-terminal hybrid MMC-based ring MVDC-DS, and the results indicate that the proposed method can improve the economy and power quality of system, and help to realize the normal system operation under different conditions.

A Surgeless Diode-Clamped Multilevel Solid-State Circuit Breaker for Medium-Voltage DC Distribution Systems

In conventional medium-voltage solid-state circuit breakers (SSCBs), series-connected semiconductor switches are used with metal oxide varistors (MOVs) in parallel with each of them to extend the voltage ratings of the breakers without incurring the dynamic voltage unbalance. Besides, for isolating the faults at faster speeds, the clamped surge voltages of the MOVs are always much higher than the system voltages. For those reasons, the voltage ratings of all the switches in the SSCBs have to be overdesigned according to the surge voltage values, which causes high material costs along with nonideal conduction losses in the SSCBs employed for medium-voltage dc distribution systems. To solve this dilemma, a surgeless diode-clamped multilevel SSCB is proposed in this article. It offers an ultrafast isolation speed with no significant surge voltages or dynamic unbalanced voltage across the semiconductor switches. With these attributes, the proposed SSCB can potentially use the semiconductor switches rated at lower voltages compared to conventional SSCBs and have higher efficiency along with faster isolation speed. The simulation and experimental results are presented to validate the technical feasibility and practical values of the proposed SSCB.

Input-Series-Output-Parallel DC Transformer Impedance Modeling and Phase Reshaping for Rapid Stabilization of MVDC Distribution Systems

This paper focuses on the instability problem of the medium-voltage DC (MVDC) distribution system and proposes an impedance phase reshaping (IPR) method. To obtain the load impedance model of the MVDC distribution system, the input impedance of the input-series-output-parallel (ISOP) DC transformer (DCT) is derived by the generalized average modeling (GAM). Based on the obtained model, the traditional ISOP DCT controller optimization (IDCO) approach is discussed and the IPR method is developed. An impedance phase controller is introduced based on the original control method. According to the optimized impedance stability criterion, the parameters of the impedance phase controller are determined. Compared with the IDCO approach, the proposed method weakens the negative resistance characteristic of the load impedance at the resonant frequency. Therefore, the MV bus voltage oscillation is rapidly mitigated. Besides, the dynamic performance of the system using the IPR method can be classified as good. The simulation results show that the mathematical model is correct, and the proposed method is effective for the rapid stabilization of MVDC distribution systems.

Adaptive Master-Slave Control Strategy for Medium Voltage DC Distribution Systems Based on a Novel Nonlinear Droop Controller

To ensure accurate active power-sharing and an improved dynamic response to DC voltages in a medium voltage DC distribution system (MVDCDS), this paper presents an adaptive master-slave (AMS) control strategy based on a novel nonlinear droop (ND) controller. This ND controller, which is based on an improved sigmoid nonlinear function, is applied to the slave converter stations. An improved sigmoid function is designed to achieve a continuous and nonlinear droop curve. Through use of the proposed ND controller, the proposed AMS strategy can achieve seamless switching between the master-slave control and droop control modes automatically. Consequently, the AMS control strategy not only guarantees precise control of the active power in the steady state but also allows a superior dynamic performance to be achieved in the transient state, during which the system suffers from both large-scale disturbances and communication failures. The effectiveness and superiority of the proposed AMS control strategy are illustrated by theoretical analyses and validated by performing hardware-in-the-loop (HIL) tests in a ±10 kV MVDCDS.

Medium voltage DC distribution system simulation based on average-value model

Transient simulations with of DC distribution system based on switch model (SM) are computationally expensive because of a large number of switching events in power electronics converters. As an alternative, average-value model (AVM) replaces the switch with controlled source, which can effectively improve the simulation efficiency. This paper takes a typical double-ended DC distribution system as an example and introduces the key equipment selected in the system. Then AVMs of corresponding equipment are given. The DC distribution system based on AVM and SM is built respectively in the PLECS software. The simulation result shows that simulation based on AVM can effectively reflect the steady-state and dynamic characteristics of the system and greatly improve the simulation speed, which can provide an effective and fast reference for the stable operation of DC distribution system in different working modes.

Fault identification of medium and low voltage DC distribution system based on high frequency fault component

The protection technology of medium and low voltage DC distribution system is still in its infancy, and its accuracy is not high in the process of practical application. Therefore, a fault identification method of medium and low voltage DC distribution system based on high-frequency fault component is proposed, and the topology, grounding mode, protection equipment Fault type and fault transient characteristics are closely related to the design of protection scheme. Based on the high-frequency fault component, the fault identification algorithm of medium and low-voltage DC distribution system is optimized, and different fault categories of AC and medium and low-voltage DC distribution system are divided, so as to achieve the research goal of accurate positioning of different fault types. Finally, it is confirmed by experiments. The fault identification method of medium and low voltage DC distribution system based on high frequency fault component has high accuracy and fully meets the research requirements.

A Novel Start-up Strategy for MMC-based Medium-voltage DC Distribution System with Low Inrush Current

To realize medium-voltage dc distribution system, the modular multilevel converter (MMC) is commonly adopted due to its high-level scalability and reliability for medium-voltage high-power energy conversion. Since the capacitors’ voltages of the MMC are insufficient when the uncontrolled pre-charging stage is finished, large inrush current exists when the MMC is unlocked for active charging, which can cause malfunction of protection and threat the successful start-up of the whole system. This paper presents a detailed analysis on the causes of inrush current during the MMC’s active charging process. Based on the analysis, a novel start-up strategy of the MMC is proposed to achieve improved controllability of the MMC’s ac-side currents for inrush current suppression. The simulation results from a full-scale simulation model demonstrate that the inrush current can be effectively suppressed during the MMC’s active charging process.

Continuous operation of LVDC source/load under DC faults in MMC-DC distribution systems

With penetration and usage of renewable sources and loads based on power electronics increasing, the modular multi-level converter based direct current (MMC-DC) distribution system is getting broad attention. In MMC-DC system, DC/DC converters are employed to link the low voltage DC (LVDC) source/load and the medium-voltage DC (MVDC) distribution system. DC fault ride through capability at MVDC side is one of the technical difficulties, and has been the focus of previous studies. However, whether LVDC source/load could operate continuously under MVDC faults is still an open question. In this paper, key facilities, protection procedures and operating characteristics of LVDC source/load in two typical MMC-DC frameworks based on half bridge submodules (HB-MMC-DC) and full bridge submodules (FB-MMC-DC) are studied respectively. Analyses show LVDC source/load could operate continuously in HB-MMC-DC system with the help of DCCBs, but with voltage variation exceeding acceptable range, they fail to operate continuously in FB-MMC-DC systems. Therefore, a controllable storage unit is deployed at the LVDC side of the DC/DC converters to suppress voltage variation. The working principle and parameter design of the proposed solution are presented. Case studies are carried out in PSCAD/EMTDC for verification. Finally, comprehensive comparisons between the two MMC-DC systems are conducted.

Analysis of the Energy Dissipation Characteristic in an MMC-Based MVDC System With the Liquid Metal Current Limiter

Utilizing DCCBs to realize selective protection is a promising solution for dc fault isolation. While the large fault current and high fault energy make the dc fault interruption a crucial problem in the medium-voltage dc (MVDC) distribution system. A novel fault isolation strategy based on the liquid metal current limiter (LMCL) is proposed in this article to reduce the breaking current and dissipated energy of the dc circuit breakers (DCCBs). Based on a three-terminal MVDC distribution system, the fault current and energy dissipation characteristics for the reactor fault current limiter (L-FCL) and the LMCL are analyzed theoretically. The simulation results show that under the same breaking current, the dissipation energy and the dissipation time in the LMCL case are reduced by 48% and 41%, respectively. Besides, the influence of system parameters on breaking current and energy dissipation characteristics under the LMCL case is further analyzed. Finally, an experiment circuit is designed to verify the proposed scheme.

Key technologies for medium and low voltage DC distribution system

Development of the medium and low voltage DC distribution system is of great significance to a regional transmission of electric energy, increasing a penetration rate of new energy, and enhancing a safety of the operation of the AC/DC interconnected grid. This paper first summarizes the medium and low voltage DC distribution system schemes and plans put forward by many countries, and then elaborate status of under-construction medium and low voltage DC distribution system project cases in China. Based on these project cases, this paper analyzes key issues involved in the medium and low voltage DC distribution system topologies, equipment, operation control technologies and DC fault protections, in order to provide theoretical and technical reference for future medium and low voltage DC distribution system-related projects. Finally, this paper combines a current China research status to summarize and give a prediction about the future research direction of medium and low voltage DC distribution system, which can provide reference for the research of medium and low voltage DC distribution system.

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.

High-Capacity MVDC Interruption Based on IGCT Current Sharing Integrated With Freewheeling

Medium-voltage dc (MVdc) distribution systems have the advantages of high power density, low line loss and fast system response. However, the high rise rate and large peak value of short circuit current make the fault current interruption a critical problem in MVdc systems. To solve this problem, a novel circuit breaker scheme based on integrated gate-commutated thyristor (IGCTs) current sharing integrated with freewheeling is proposed in this article, which not only can ensure a high breaking capacity and short interruption time in the full current range, but also have superior vacuum interrupter recovery performance and high breaking reliability. The analysis of key factors, including stray parameters, inductance-capacitance (LC) parameters and arcing time, on the current commutation characteristics is presented, and the optimal design principle of the circuit breaker parameters is obtained. Finally, the proposed MVdc circuit breaker is verified with a 64.2 kA/10 kV fault current interruption experiment.

Study on lightning protection scheme of multi‐terminal MMC‐MVDC distribution system

Based on a ±10 kV three-terminal MMC-MVDC (medium voltage DC distribution system based on modular multilevel converter) system, in this study, the authors mainly study on the influencing factors and protection schemes of the lightning intruding overvoltage (including direct lightning and back-flashover lightning). Firstly, a lightning intruding overvoltage simulation model of the MMC-MVDC distribution system using overhead lines is established based on the PSCAD/EMTDC program. The influence of steady-state operating voltage and the number of converter stations on the simulation results are simulated and analysed. Meanwhile, the proper simulation modelling method of the lightning intruding overvoltage for multi-terminal MMC-MVDC system is proposed. Secondly, the lightning intruding overvoltage is simulated when the line is struck by the lightning stroke current that occurs once in 20 years and the different lightning protection schemes are proposed. Finally, based on the lightning impulse withstand voltage, lightning trip-out rate and the cost of the protection schemes, the lightning protection schemes in different scenarios are proposed. The simulation results show that if the cost of the system is prioritised, the protection scheme of increasing the number of parallel stacks of the DL arresters can be selected, and if the power supply reliability of the system is prioritised, the protection scheme of installing the overhead ground wire as well as reducing grounding resistance can be selected.

Modeling, Control and Power Management of Six-Phase PMSM Based Shipboard MVDC Distribution System

In the modern shipboard power system, there is a growing concern about reduced fuel economy, efficiency improvement, and minimized emission in recent years. Besides, considering the islanded nature of the shipboard power system, ensuring the system reliability at both generation and load side is crucial. In this context, a hybrid medium voltage DC (MVDC) distribution system concept with diesel engine, PV system, and battery energy storage system (BESS) as the generators for the shipboard power system is proposed. Variable speed operation of a diesel engine is considered for the benefit of minimal specific fuel oil consumption (SFOC). Regarding the fault-tolerant characteristics of six-phase permanent magnet synchronous machine (PMSM), it is used for both generating and motoring application, thus maintaining system reliability at any time. A hierarchical control system is designed and implemented to ensure proper load power-sharing among the generators and to regulate the dc-link voltage of the proposed MVDC distribution system. To bolster the intent of minimal SFOC zone operation of diesel engines and coordination between the generators and load, a deterministic rule-based power management system is proposed and implemented. The simulation is carried out for potential operational modes of a cruise ship in the MATLAB/Simulink environment. Simulation results show that the proposed topological and control structure has satisfactory performance in terms of power delivery, stable dc-link dynamics, and overall system stability.

DC Solid State Transformer Based on Three-Level Power Module for Interconnecting MV and LV DC Distribution Systems

This article presents a dc solid state transformer based on three-level power module (PM) (TL-DCSST) for interconnection of medium-voltage dc (MVdc) and low-voltage dc distribution system. The proposed TL-DCSST adopts the low-voltage insulated-gate bipolar transistor (IGBTs) with excellent switching characteristics to build medium voltage PM and reduces the number of medium frequency transformer (MFT). This topology features inherent antibias mechanism for MFTs on the primary side, which saves the resonant capacitor at primary side. Meanwhile, the active front end is integrated to realize voltage and power regulation, low voltage ride-through and DC fault handling for MVdc side. In this article, the topology, operation principle, interleaving modulation, circuit parameter design, and control schemes are presented and analyzed comprehensively. At last, a prototype rated at 7 kW has been built and tested, and the experimental results verify effectiveness of the proposed TL-DCSST.

A Practical DC Fault Ride-Through Method for MMC Based MVDC Distribution Systems

Modular multilevel converter (MMC) based medium voltage DC (MVDC) distribution system has become a promising option in the future smart grid architecture due to many distinguishing features. However, MMC-MVDC distribution system is prone to face the short-term outage problem during bipolar DC faults, so how to implement DC fault ride-through (FRT) capability for MMC-MVDC is still a challenge. This paper intends to change the redundant half-bridge submodules (HBSMs) of the HB-MMC into the redundant full-bridge submodules (FBSMs), thus, a practical DC FRT method is proposed by using the combination of the hybrid MMC and the hybrid DC circuit breakers (CBs) for the MMC-MVDC system. Then, the DC fault current characteristics of the MMC with the proposed FRT method are discussed. Lastly, data analysis and simulations of a three-terminal MMC-MVDC system have been performed in PSCAD/EMTDC to validate the effectiveness of the proposed DC FRT method. The proposed DC FRT method is not only easy to implement, but also reduces the requirements and the investment of the DC CB.

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.