DC Distribution System Research Articles

A Novel DC Transmission and Distribution System with 100% New Energy Consumption

With the continuous development of new energy technology and the continuous improvement of people’s green and low-carbon awareness, distributed photovoltaic, wind power, and other new energy are more and more widely used. On the other hand, the development of new industries, such as electric vehicle charging stations and data centers, has increased the DC load significantly, showing the characteristics of the coexistence of AC and DC user loads. Therefore, it is of great practical significance to study how to effectively utilize new energy and realize the direct power supply of AC and DC loads to achieve the dual carbon goal. Combined with the development trend of new energy and DC transmission, this paper proposes a ±10 kV DC transmission and distribution architecture with 100% new energy consumption, which can realize the complete local consumption of new energy and reliable power supply for diversified AC and DC loads, maximizing the reduction of power loss caused by AC-DC conversion. This paper discusses the key devices and key technologies in the above architecture and analyzes the advantages and disadvantages and application scope of various methods in the current research results.

Multi-level Topology and Dynamic Power Sharing of Household DC Distribution System

With the popularity of electronic information products and the gradual adoption of DC speed control in electric household appliances, more and more household appliances are equipped with rectifiers or inverters. Moreover, household rooftop PV, wind turbines and energy storage devices also output electric energy in the form of DC. This paper presents a topology of household DC distribution system, which not only improves the reliability of power supply from the power consumption mode, reduces the loss in the process of power form conversion, but also provides users with safer voltage level and more reasonable fault protection. On this basis, this paper proposes an instantaneous power sharing scheme based on droop control, which enables each power module of the system to achieve steady-state and dynamic power sharing.

Voltage-Balancing Dual-Active-Bridge (VB-DAB) Converter for Bipolar DC Distribution System

This article proposes a new voltage-balancing dual-active-bridge (VB-DAB) converter for the bipolar dc distribution system. The proposed converter comprises the same components as those of the conventional DAB converter, and it integrates functions of a VB without additional components. The proposed converter transmits power using the same principle and control as the conventional DAB converter. Furthermore, it regulates both output voltages equally and redistributes the unbalanced load current. Therefore, the proposed converter neither requires additional components nor results in losses by them. Consequently, it can achieve high efficiency and high power density. The effectiveness of the proposed converter is verified by 3-kW prototypes with a 380-V input and ±190-V outputs conditions.

전기저장장치의 배터리 기생커패시턴스 및 SOC에 따른 절연저항 변동 메커니즘 및 모델링에 관한 연구

Recently, due to the significant growth of RES(renewable energy source), the capacity of ESS (energy storage system) interconnected to RES is increasing rapidly. Especially, it is mandatory to install IMD (insulation monitoring device) in order to monitor insulation resistance of ESS and detect faults such as ground fault. However, there are no specific regulations on numerous IMD being operated in actual ESS, and particularly the issues that insulation resistance dramatically varies depending on the stray capacitance and SOC (state of charge) of batteries are being occurred in large scaled ESS. Therefore, in order to operate ESS in a stable manner, this paper performs surveys on actual measurement data of ESS insulation resistance, which varies according to the operation environment and conditions in distribution system. Based on the evaluation on influence of stray capacitance and SOC to insulation resistance from the survey results, this paper proposes a mechanism of variation in insulation resistance depending on the stray capacitance and SOC of battery and performs modeling of DC distribution system with ESS to evaluate the variation characteristics of insulation resistance according to each mechanism. From the simulation result based on the proposed mechanism and modeling, it is confirmed that the leakage current and CMV (common mode voltage) increases along with stray capacitance and SOC but insulation resistance decreases. Therefore, in order to operate large scaled ESS in a stable manner, it is confirmed that proper measures to enhance insulation level depending on the capacity and SOC of battery and mitigate leakage current and CMV.

Power flow analysis of DC distribution system in a ship with non-electric propulsion

<span lang="EN-US">Direct current (DC) shipboard power distribution system offers higher power efficiency and voltage stability compared to the alternating current (AC) systems due to lower impedance. The implementation of DC distribution system in all-electric ship seems to be worthy since the reduction of power loss and voltage drop could overcome the drawback of DC system. However, the effectiveness of DC distribution system in ship with non-electric propulsion has not been investigated yet. Unlike in an all-electric ship, electric power flow in the distribution system of a ship with mechanic propulsion is considerably lower. The study aims to provide numerical analysis of power loss and voltage drop reduction on DC distribution system that applied to a ship with mechanic propulsion. The power flow analysis is performed on a tanker ship. Contrary to the hypothesis, the results show that the DC power distribution increase the power losses about 15% compared to AC system due to the addition of rectifier and inverter. However, the voltage drops are decreased in DC distribution system. Further investigation in the other aspects should be performed before concludes whether DC distribution system is worthy to be used in the aforementioned ship.</span>

A Series Arc Fault Diagnosis Method in DC Distribution Systems Based on Multiscale Features and Random Forests

Series arc faults (SAFs) are able to cause electrical fires, which threaten the security of dc distribution systems critically. The noise of SAFs can interfere with the adjacent lines, and the SAF detectors based on the information in a single line are prone to trip. Therefore, to correctly diagnose SAFs, it is of significant value to consider the centralized SAF diagnosis methods that analyze the information in the entire system. This article analyzes the mechanism of arc noise interfering with normal lines in detail and proposes an SAF diagnosis method that comprehensively considers the information in the entire system. In the proposed method, the time-domain features, frequency-domain features, and singular values obtained from different scales of current are utilized to construct high-dimensional feature vectors. Then, to reduce the redundant information in feature vectors, feature importance analysis and feature selection are realized using random forests (RFs). Finally, RFs are employed to fuse the information of feature vectors for SAF diagnosis. The offline experimental results indicate that the diagnostic accuracy of the proposed method can reach up to 98.93%, and the effectiveness and advancement of the proposed method are verified by comparing it with different state-of-the-art methods. Besides, the proposed method is implemented in the microcontroller for real-time operations. The results of the online experiment show that the proposed method is of outstanding reliability and robustness under transient conditions, and, at the same time, the diagnostic speed is less than 250 ms and can satisfy the requirements of the standard.

Active Damping Control and Parameter Design Method for ISOP-Based Load Converter in DC Distribution System

To meet the diversified requirements of active damping configuration in a dc distribution system, the input-series-output-parallel (ISOP) load converter based on phase-shifted full-bridge (PSFB) modules is investigated in this article. The reduced-order impedance model of the PSFB-based ISOP converter with common-duty-ratio control is established first, and then the active damping control strategies using the primary- and secondary-side voltages are proposed, respectively. By reformulating the stability criterion in the form of impedance-frequency curves and using the impedance sensitivity, the sensitivity of resonance frequency is derived to develop a method for designing the parameters of active compensators. Finally, the experimental results demonstrate the effectiveness of the proposed active damping control strategies and parameter design method.

Energy Loss Savings Using Direct Current Distribution in a Residential Building with Solar Photovoltaic and Battery Storage

This work presents a comparison of alternating current (AC) and direct current (DC) distribution systems for a residential building equipped with solar photovoltaic (PV) generation and battery storage. Using measured PV and load data from a residential building in Sweden, the study evaluated the annual losses, PV utilization, and energy savings of the two topologies. The analysis considered the load-dependent efficiency characteristics of power electronic converters (PECs) and battery storage to account for variations in operating conditions. The results show that DC distribution, coupled with PV generation and battery storage, offered significant loss savings due to lower conversion losses than the AC case. Assuming fixed efficiency for conversion gave a 34% yearly loss discrepancy compared with the case of implementing load-dependent losses. The results also highlight the effect on annual system losses of adding PV and battery storage of varying sizes. A yearly loss reduction of 15.8% was achieved with DC operation for the studied residential building when adding PV and battery storage. Additionally, the analysis of daily and seasonal variations in performance revealed under what circumstances DC could outperform AC and how the magnitude of the savings could vary with time.

Protection Scheme Based on Fault Area Estimation in Loop-Type Low Voltage DC Distribution System with Photovoltaic System using Derivative of Current and Power at Tie Switch

Protection Scheme Based on Fault Area Estimation in Loop-Type Low Voltage DC Distribution System with Photovoltaic System using Derivative of Current and Power at Tie Switch

A Low-Voltage AC, Low-Voltage DC, and High-Voltage DC Power Distribution System with Grid: Design and Analysis

Low-voltage (LV) and high-voltage (HV) DC distribution systems are being investigated as alternatives due to the growth of DC distribution energy resources (DER), DC loads such as solar and wind power systems, and energy storage sources (ESSs). Furthermore, an HV/LV DC distribution system offers various advantages, including lower conversion losses, an easier connecting strategy for DC DERs, and less complex power management techniques. As renewable energy sources are increasingly incorporated into the electrical grid, it is important to create novel, effective approaches for connecting such sources and loads. It would hence be effective to merge DC distribution with AC distribution to fulfill the energy demands of both DC and AC consumers. To this end, this study proposes a multizone design with four buses: low-voltage direct current (LVDC), high-voltage direct current (HVDC), low-voltage alternating current (LVAC), and an electrical grid. A model of this system that covers crucial elements, including power systems, DER systems, and power electronic devices, to serve as a foundation for the analysis and design of this architecture is proposed. MATLAB/Simulink is used to conduct a simulation study to verify the performance of the proposed design. In this study, a hybrid electrical grid with an LVDC, HVDC, and LVAC distribution network test is used and implemented. Additionally, a transient and steady-state characteristic analysis of the test system is performed.

Coordinated control and application of multi-terminal DC distribution system

Because of the rapid development of power electronics, DC loads, and distributed energy resources, the advantages of DC distribution systems are increasingly prominent. This paper provides a brief description of the concepts and functional advantages of DC distribution systems, introducing the main equipment of multi-terminal DC (MTDC) distribution system, the topologies, and the typical application scenarios of each topology. The coordinated control technology of MTDC distribution system is analysed and summarized in terms of control architecture, coordinated control, damping control, and operation mode. The existing MTDC distribution demonstration projects are summarized, and recent typical projects are introduced. Finally, the issues that can be studied regarding to the innovation and application of MTDC distribution system are pointed out.

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.

Bidirectional Synchronous H6 Inverter for Hybrid AC/DC Distribution System With Improved Light Load Efficiency

The overall efficiency of DC distribution systems may be impacted by the operation of the bidirectional inverter connected to the grid; one of its most critical components. When compared to high-demand industries, the typical load profile of such system in residential buildings is highly variable, including several periods of light-load. Therefore, this paper presents a method to overcome the reduced efficiency at light-load of bidirectional inverters integrated into a DC distribution system powered by renewable energy. Existing topologies are evaluated for obtaining optimal efficiency as well as an investigation of loss breakdown at light-load using analytic models and PSIM simulation. A novel synchronous H6 transformer-less topology is proposed. Simulation results show that the proposed topology can achieve an efficiency of 96.5% under full load and 98.3% under light-load. Efficiency improvements over a standard H6 topology are 4.5% under light-load and 1.5 % at full load. This is mostly due to reduced conduction and switching ON loss of MOSFETs used to replace diodes. When using the rectifier mode, similar levels of improvement are observed, with a 4% increase under light-load and a 1.2% improvement under full load conditions, respectively. Experimental results confirm the effectiveness of the proposed topology, showing nearly the same trend in improvement vs. the standard H6.

Real-Time Coordinated Control of a Grid-VSC and ESSs in a DC Distribution System for Total Power Loss Reduction Considering Variable Droop Using Voltage Sensitivities

Recent advances in direct current (DC) distribution systems that involve renewable energy sources (RESs) and energy storage systems (ESSs) have shown that DC voltage control improves operational stability. This paper proposes an efficient real-time DC voltage control strategy that reduces total power losses while maintaining the DC voltages within acceptable ranges via coordination of the grid-connected voltage source converter (GVSC) and the ESSs. A new strategy determines whether to invoke cooperative GVSC/ESS control or ESS voltage control alone. The GVSC and ESSs exhibit proportional-integral (PI) and variable droop controllers, respectively. In particular, the variable droop controllers are developed with consideration of the voltage sensitivities. The ESS outputs are then compensated by reference to the droop coefficients and the available ESS output powers. Case studies confirm that our voltage control strategy permits higher minimum DC voltages (and thus smaller total power losses) when consecutive events occur, compared with conventional strategies using PSCAD/EMTDC.

Stability Analysis and Control of DC Distribution System with Electric Vehicles

The DC distribution network system equipped with a large number of power electronic equipment exhibits weak damping characteristics and is prone to low-frequency and high-frequency unstable oscillations. The current interpretation of the oscillation mechanism has not been unified. Firstly, this paper established the complete state-space model of the distribution system consisting of a large number of electric vehicles, characteristic equation of the distribution network system is derived by establishing a state-space model, and simplified reduced-order equations describing the low-frequency oscillation and the high-frequency oscillation are obtained. Secondly, based on eigenvalue analysis, the oscillation modes and the influence of the key system parameters on the oscillation mode are studied. Besides, impacts of key factors, such as distribution network connection topology and number of dynamic loads, have been discussed to suppress oscillatory instability caused by inappropriate design or dynamic interactions. Finally, using the DC distribution example system, through model calculation and time-domain simulation analysis, the correctness of the aforementioned analysis is verified.

A Bipolar Hybrid Circuit Breaker for Low-Voltage DC Circuits

Presently available dc breakers fall into three categories, mechanical, static, and hybrid dc breakers. With the advent of dc generation, loads, and circuits dc breakers are required for system protection. This paper proposes an alternate method to break dc currents for low-voltage applications. Higher voltages have not been tested due to limited resources and safety concerns; however, the proposed mechanism can potentially be used at higher voltages. The proposed mechanism utilizes two switches along with two diodes, to reverse the direction of the current flow and clear the fault. This is different than the conventional single-switch commute-and-absorb method used in mechanical or static breakers in the sense that it reverses the current direction and delivers part of the fault energy back to the source. It is shown that the proposed method can break the fault current effectively with minimal arc and significantly reduces the voltage stress across the mechanical switches. The initial results of this work have been presented in the ECCE2021. Extended simulation and experimental results are provided to show the efficiency of the proposed dc breaker. In addition, the application of the proposed dc breaker is illustrated for dc distribution system via simulations and the design procedure is explained.

Power and Voltage Control Based on DC Offset Injection for Bipolar Low-voltage DC Distribution System

Power and Voltage Control Based on DC Offset Injection for Bipolar Low-voltage DC Distribution System

Coupled Inductor Based Voltage Balancing in Dual-Output CLL Resonant Converter for Bipolar DC Distribution System

Bipolar dc distribution systems face imbalances in voltages when unbalanced loads are connected at the outputs. To address imbalances in bipolar voltage levels, dedicated voltage balancers are required. However, they add extra active and/or passive components; including, magnetics, capacitors, etc. This increases the cost, reduces the conversion efficiency, and power density of dual-output converters. Therefore, to tackle the imbalances in output voltages without adding any extra active and/or passive components, this article presents coupled inductor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> ) based voltage balancing in dual-output <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> resonant converter. The main features of the proposed method are as follows: since <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> is the result of already present resonant inductors of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> tank circuit, therefore, it does not require extra magnetic components to balance the output voltages; with no additional components, it does not increase the magnetic volume, component count, cost, and complexity of the converter; due to the direct coupling, a half of circuit inductances are integrated to achieve the respective resonant inductance of the tank circuit; and it can also avoid complex control schemes applied to voltage balancing. Besides, with the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> , the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> converter maintains all of its good features including; zero-voltage and zero-current-switching (ZVZCS). The detailed analysis of the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> with different load conditions is discussed. Moreover, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> or Type-12 resonant tank circuit is presented to validate ZVZCS operation in the reverse power flow condition. To verify the effectiveness of the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> based voltage balancing, a 3.6-kW hardware prototype has been built and tested.

A Novel Peer-to-Peer Control Strategy for Multiterminal DC Distribution Systems

This paper proposes a novel peer-to-peer control strategy (PPCS) and then develops a generalized hierarchical control scheme (GHCS) based on the PPCS for DC voltage control and power sharing in multi-terminal DC distribution systems (MTDCDSs). At the primary level of the GHCS, the combination of the designed integration and deviation adjustment equation (IDAE) controllers, the ReLU-based standby controllers, and the peer-to-peer structure is developed to implement the PPCS. With the PPCS, not only is the mode switching of the voltage source converter (VSC) eliminated when the system’s operating conditions are changed, but also three objectives, including minimizing DC bus voltage deviation, achieving precise power sharing, and ensuring a fast dynamic response, are achieved at the same time. At the secondary control level, a simple optimization algorithm for voltage distribution and line losses is developed. The constrained optimization of the proposed GHCS does not require the consideration of the local controller’s structure because all voltage source converters with the PPCS perform constant power tracking control. As a result, the GHCS is simple and easy to implement. The optimal operation and satisfactory performance of the GHCS-controlled MTDCDS are demonstrated theoretically and validated through hardware-in-the-loop (HIL) tests in a ±10 kV MTDCDS.

Simulation Study of the Control Strategy of a DC Inverter Heat Pump Using a DC Distribution Network

Photovoltaics, energy storage, direct current and flexibility (PEDF) are important pillars of achievement on the path to manufacturing nearly zero energy buildings (NZEBs). HVAC systems, which are an important part of public buildings, play a key role in adapting to PDEF systems. This research studied the basic principles and operational control strategies of a DC inverter heat pump using a DC distribution network with the aim of contributing to the development and application of small DC distribution systems. Along with the characteristics of a DC distribution network and different operating conditions, a DC inverter heat pump has the ability to adapt to changes in the DC bus voltage and adds flexibility to the system. Theoretical models of the DC inverter heat pump integrated with an ice storage unit were developed. The control strategies of the DC inverter heat pump system considered the influence of both room temperature and varied bus voltage. A simulation study was conducted using MATLAB & Simulink software with simulation results validated by experimental data. The results showed that: (1) The bus fluctuation under the rated working voltage had little effect on the operation of the unit; (2) When the bus voltage was fluctuating from 80%–90% or 105%–107%, the heat pump could still operate normally by reducing the frequency; (3) When the bus voltage was less than 80% or more than 107%, the unit needed to be shut down for the sake of equipment safety, so that the energy storage device could adjust to the sharp decrease or rise of voltage.