DC Distribution System Research Articles

A reduced-order model for high-frequency oscillation mechanism analysis of droop control based flexible DC distribution system

High-frequency oscillation (HFO) is easy to occur in a flexible DC distribution system, due to the influence of the interaction of multiple time scale control links and physical circuits. This paper studies the mechanism of HFO and establishes a reduced-order mathematical model of a flexible DC distribution system based on droop control. Sensitivity analysis has been used to obtain a second-order system for mechanism analysis. The analytical equation of HFO is deduced in time domain considering the system parameters’ influence. This paper summarizes the influence law of the filter capacitor, equivalent inductance, and controller parameters on HFO. Finally, a simulation model of a low-voltage flexible DC distribution system is built based on Matlab/Simulink, and the simulation result verifies the validity of the theoretical analysis.

An Efficient and Reliable Solid-State Circuit Breaker Based on Mixture Device

In this letter, a novel solid-state circuit breaker (SSCB) based on mixture device (MD-SSCB) is proposed for dc distribution systems. The proposed MD-SSCB can actively interrupt faults without introducing additional thyristor, charging power supply, and charging strategies. Besides, the interruption capability of the MD-SSCB is not affected by the parameters of the outer circuit. As a result, the MD-SSCB has advantages of compact structure, economic design, high efficiency, and high reliability. Moreover, the MD-SSCB can turn off its thyristors without reverse voltage stress, which eliminates the severe reverse overvoltage problem in the fault-interrupting process, and hence further enhances the reliability of the breaker. Detailed operation principle and design guidelines of the MD-SSCB are discussed through mathematical modeling. The feasibility of the proposed breaker is validated by experimental results.

Calculation of Short-Circuit Current in DC Distribution System Based on MMC Linearization

The calculation of the short-circuit current is an important basis for fault detection and equipment selection in the DC distribution system. This paper proposes a linearized model for modular multilevel converter (MMC) considering different grounding methods and different failure scenarios. This model can be used in different fault conditions before MMC’s block. Under different fault forms, the model has different manifestations. This paper analyzes and models the DC distribution network with two types of faults: inter-pole short circuit and single-pole grounding short circuit. Among them, the modeling and analysis of single-pole grounding short-circuit uses the method of common- and differential-mode (CDM) transformation. To solve such a model, an analytical calculation method is proposed. As a mean of evaluating the effectiveness and accuracy of the proposed model, the analytical calculation solution is compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.

VMD-Enabled Current-Based Fast Fault Detection Scheme for DC Microgrid

The utilization of low-voltage direct current (LVdc) distribution system is gaining popularity in recent years due to its numerous advantages. However, the development of efficient fault detection scheme for an LVdc microgrid is a challenging problem. This article proposes a fast and effective variational mode decomposition (VMD)-based fault detection technique for the LVdc distribution system with penetration of renewable sources using the local end current measurements only. This scheme processes the rate of change of current signal of the distribution lines through the VMD algorithm that decomposes it to different modes. The cumulative energy of the most significant mode is being computed and used as a fault detection index. The proposed scheme is tested for pole to pole and pole to ground fault with broad variation in fault location, fault resistance, and distributed generations penetration in grid-connected and islanding mode of microgrid operation. The performance of the proposed technique is compared with the existing differential current and overcurrent relaying scheme as well as other existing schemes. The testing has been done in MATLAB/Simulink and Typhoon hardware-in-loop testbed. The proposed scheme detects all types of fault with wide variation in fault and operating conditions of the microgrid with faster response time. The test results indicate that the proposed protection scheme is a potential candidate for providing effective protection measure for LVdc microgrid.

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.

Optimal Control Strategy for Distributed Energy Resources in a DC Microgrid for Energy Cost Reduction and Voltage Regulation

Distributed energy resources (DERs), including renewable energy resources (RESs) and electric vehicles (EVs), have a significant impact on distribution systems because they can cause bi-directional power flow in the distribution lines. Thus, the voltage regulation and thermal limits of the distribution system to mitigate from the excessive power generation or consumption should be considered. The focus of this study is on a control strategy for DERs in low-voltage DC microgrids to minimize the operating costs and maintain the distribution voltage within the normal range based on intelligent scheduling of the charging and discharging of EVs, and to take advantage of RESs such as photovoltaic (PV) plants. By considering the time-of-use electricity rates, we also propose a 24-h sliding window to mitigate uncertainties in loads and PV plants in which the output is time-varied and the EV arrival cannot be predicted. After obtaining a request from the EV owner, the proposed optimal DER control method satisfies the state-of-charge level for their next journey. We applied the voltage sensitivity factor obtained from a load-flow analysis to effectively maintain voltage profiles for the overall DC distribution system. The performance of the proposed optimal DER control method was evaluated with case studies and by comparison with conventional methods.

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.

Hierarchical virtual inertia control of DC distribution system for plug-and-play electric vehicle integration

DC distribution system will play a significant role in future urban distribution system, while the integration of electric vehicle has a tremendous influence on the stability of DC distribution system. This paper presents an assessment and mitigation strategy of such negative impact on DC distribution system. Mathematical modelling and analysis of the load/source admittances are provided to evaluate the overall system stability based on the Nyquist admittance ratio criterion. The theoretical analysis shows that the integration of electric vehicle introduces incremental negative admittances which significantly degrades the system stability. Therefore, a hierarchical virtual inertia control strategy, consisted of a local-level control and a system-level control, is proposed to improve the system damping and suppress the negative influence. The local-level virtual inertia control can suppress the instability caused by electric vehicle integration, while the disturbance caused by the fluctuations of other DC loads or sources can be suppressed by the system-level virtual inertia control. Numerical examples verify the effectiveness of the method, which can improve the stability of DC distribution system and satisfy the requirement of electric vehicle plug-and-play.

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.

저압직류 배전계통에서 PV가 전압 불평형에 미치는 영향에 관한 연구

Since the multiple PVs will be connected with bipolar Low-Voltage DC (LVDC) distribution system in the future, several problems caused by PV should be analyzed. In order to speed up to introduction of LVDC distribution system, the effect of PV on the voltage unbalance, which is one of the several issues, should be surely studied because it can lead to the power loss and debase the power quality of LVDC distribution system. This paper has analyzed the voltage unbalance by classifying upstream and downstream of PV. As a result, PV contributes the voltage unbalance to upstream area of Point of Common Coupling (PCC). For downstream area of PCC, however, the voltage unbalance caused by PV is absent theoretically and the voltage unbalance occurred by upstream is just sustained in downstream. Finally, this paper has modeled a test system using ElectroMagnetic Transients Program (EMTP) and the proposed results are verified by the simulation for several cases.

Modeling and Analysis of DC Microgrids as Stochastic Hybrid Systems

This article proposes a method of predicting the influence of random load behavior on the dynamics of dc microgrids and distribution systems. This is accomplished by combining stochastic load models and deterministic microgrid models. Together, these elements constitute a stochastic hybrid system. The resulting model enables straightforward calculation of dynamic state moments, which are used to assess the probability of desirable operating conditions. Specific consideration is given to systems based on the dual active bridge (DAB) topology. Bounds are derived for the probability of zero voltage switching (ZVS) in DAB converters. A simple example is presented to demonstrate how these bounds may be used to improve ZVS performance as an optimization problem. Predictions of state moment dynamics and ZVS probability assessments are verified through comparisons to Monte Carlo simulations.

Topology Simplification and Parameter Design of Z/T/Γ-Source Circuit Breakers

Z/T/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma $ </tex-math></inline-formula> -source circuit breakers used in flexible dc distribution systems have been rapidly developed due to their advantages of high speed and low cost. However, the precise model of the circuit breaker is generally a high-order circuit, which is difficult to solve. The lack of simplified topology and parameter design considering system parameters (line inductance, line resistance, and load capacitance) restricts their applications in practical engineering. Therefore, based on the equivalent mathematical model, a simplified topology that can represent Z/T/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma $ </tex-math></inline-formula> -source circuit breakers is proposed and their working principles are revealed in this article. Considering system parameters, the parameters of Z/T/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma $ </tex-math></inline-formula> -source circuit breakers are designed to take care of short circuit fault and step changes in load. Finally, simulation and experiment show that parameters designed by the proposed method can ensure the reliable operation for Z/T/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma $ </tex-math></inline-formula> -source circuit breakers.

Bidirectional Power Sharing for DC Microgrid Enabled by Dual Active Bridge DC-DC Converter

Currently, high-performance power conversion requirements are of increasing interest in microgrid applications. In fact, isolated bidirectional dc-dc converters are widely used in modern dc distribution systems. The dual active bridge (DAB) dc-dc converter is identified as one of the most promising converter topology for the mentioned applications, due to its benefits of high power density, electrical isolation, bidirectional power flow, zero-voltage switching, and symmetrical structure. This study presents a power management control scheme in order to ensure the power balance of a dc microgrid in stand-alone operation, where the renewable energy source (RES) and the battery energy storage (BES) unit are interfaced by DAB converters. The power management algorithm, as introduced in this work, selects the proper operation of the RES system and BES system, based on load/generation power and state-of-charge of the battery conditions. Moreover, a nonlinear robust control strategy is proposed when the DAB converters are in voltage-mode-control in order to enhance the dynamic performance and robustness of the common dc-bus voltage, in addition to overcoming the instability problems that are caused by constant power loads and the dynamic interactions of power electronic converters. The simulation platform is developed in MATLAB/Simulink, where a photovoltaic system and battery system are selected as the typical RES and BES, respectively. Assessments on the performance of the proposed control scheme are conducted. Comparisons with the other control method are also provided.

A DCM Single-Controlled Three-Phase SEPIC-Type Rectifier

A discontinuous conduction mode (DCM) three-phase single-ended primary-inductor converter (SEPIC) is presented in this article. The analyzed converter operates as a high-power factor stage in AC–DC conversion systems. As its main features, it presents three controlled switches and a single control signal with simple implementation and low-current harmonic distortion. The converter topology, its design equations, and its operation modes are presented as well as a simulation analysis considering a 3 kW–220 V three-phase input to 400 V DC output converter. The experimental results are included, considering as an application the rectifier stage in low-power wind energy conversion systems (WECS) based on a 1 kW permanent magnet synchronous generator (PMSG) with variable voltage frequencies. From the analysis performed in the paper and the simulation and experimental results revealed, it is concluded that the converter is indicated to be employed in any AC–DC low-power conversion system, such as DC distribution systems, and distributed generation or hybrid systems containing variable-frequency generation.

Influence Analysis of Current Limiting Reactor on Pole-to-Pole Short Circuit Fault in DC Distribution System

With the development of power electronic technology, flexible DC distribution system has gradually become a research hotspot. Pole-to-pole short circuit fault is the most serious fault type in DC distribution system. When pole-to-pole short circuit fault occurs in DC side, it will produce a large fault current, which will cause serious harm to DC distribution system. Therefore, it is urgent to install current limiting reactor to limit fault over-current. Aiming at the DC distribution system based on half bridge MMC, this paper analyses the fault mechanism and fault characteristics of pole-to-pole short circuit fault, studies the current limiting effect of the installation position and capacity of current limiting reactor on fault current, and carries out simulation verification in PSCAD / EMTDC platform. The analysis and simulation results show that installing a current limiting reactor at the outlet of the DC side can reduce the peak value of capacitor discharge current, and installing a current limiting reactor at the outlet of the AC side can reduce the overall amplitude of fault current.

Application of Superconducting Fault Current Limiter as a Virtual Inertia for DC Distribution Systems

DC distribution systems can provide effective solutions for integrating renewable energy sources in future power systems. The low inertia of the DC distribution system causes several problems regarding stability and fault response. For stability, this low inertia represents a major cause for instability and voltage oscillations, especially with constant power loads. For fault response, the low inertia results in very high fault currents with a significant rate of rising and limited damping. This study aims to use superconducting fault current limiter (SFCL) as a virtual inertia for DC distribution systems under various disturbances and fault conditions. The system description and modeling are first presented, including the detailed dynamics of SFCL. The stability analysis for the DC system is carried out using the Hurwitz criterion, from which the suitable range of SFCL resistance is identified. Considering this range and fault current limitation, an SFCL resistance of $1.5~\Omega $ is adopted. The whole system is implemented using PSCAD/EMTDC software. A series of case studies are investigated to validate the effectiveness of SFCL in strengthening the inertia of the DC system. These case studies include sudden variations in supply voltage, sudden load changes, and faults. SFCL could successfully suppress voltage oscillations, keep voltage stability, and limit fault currents.

Comparison of Grounding Modes of MMC-Based Flexible DC Distribution System

DC distribution system has a high power quality and power reliability. It is conducive to the access of renewable energy and battery-based storage system. There are many fault types in DC distribution system. The fault characteristics are complex and closely related to the grounding mode. The investigation on the grounding mode is of great significance. Firstly, the basic structure, the operating principle and the control strategy of Modular Multilevel Converter (MMC) are briefly introduced. Secondly, the topology of DC distribution network is presented. The fault characteristics of single-phase-to-ground fault both in AC side and DC side are analyzed. The advantages and disadvantages of these grounding modes are discussed. The results provide references for flexible DC distribution system (FDDS) design.

Direct current distribution system

Direct current distribution system

Adaptive Droop Coefficient of DC Distribution System Based on Voltage Oscillation Characteristic

DC distribution system has become an important direction in improving the stability and reliability of the power system. Based on the research of the oscillation mechanism and frequency characteristics of DC voltage, a control method with variable droop coefficient is proposed to solve the voltage oscillation problem under droop control. The result of a differential operation on the voltage is used by the comparator and counter to compare with the threshold to judge whether the bus voltage oscillates or not. The oscillation frequency is calculated to change the droop coefficient adaptively based on the obtained analytical expression between the oscillation frequency and droop coefficient. The simulation model of the DC distribution system is built in Matlab/Simulink. The result verifies that the proposed method provides a new way in changing the droop coefficient adaptively to improve the voltage oscillation and enhance the system stability effectively.

Mitigation of Complex Non-Linear Dynamic Effects in Multiple Output Cascaded DC-DC Converters

In the modern world of technology, the cascaded DC-DC converters with multiple output configurations are contributing a dominant part in the DC distribution systems and DC micro-grids. An individual DC-DC converter of any configuration exhibits complex non-linear dynamic behavior resulting in instability. This paper presents a cascaded system with one source boost converter and three load converters including buck, Cuk, and Single-Ended Primary Inductance Converter (SEPIC) that are analyzed for the complex non-linear bifurcation phenomena. An outer voltage feedback loop along with an inner current feedback loop control strategy is used for all the sub-converters in the cascaded system. To explain the complex non-linear dynamic behavior, a discrete mapping model is developed for the proposed cascaded system and the Jacobian matrix’s eigenvalues are evaluated. For the simplification of the analysis, every load converter is regarded as a fixed power load (FPL) under reasonable assumptions such as fixed frequency and input voltage. The eigenvalues of period-1 and period-2 reveal that the source boost converter undergoes period-2 orbit and chaos whereas all the load converters operate in a stable period-1 orbit. The proposed configuration eliminates the period-2 and chaotic behavior from all the load converters and is also validated using simulation in MATLAB/Simulink and experimental results.