DC Distribution Network Research Articles

Study on magnetically blown arc characteristics and the resulting contact erosion mechanisms of high-power DC relays

High-power DC relays are widely used in new energy power systems and DC distribution networks, and commonly use transverse magnetic fields to accelerate arc extinguishing. In this paper, the magnetically blown arc characteristics and the resulting contact erosion mechanisms of high-power DC relays are studied by experiments and simulations. The experiments are carried out to analyze the arc motion, contact erosion mass, and contact morphologies, and to verify the simulation model. Considering the energy transfer source term, the establishment of a more perfect magnetohydrodynamic arc model is accomplished, and an erosion model is further established. The results indicate that the magnetically blown arcing process has three stages while the arc shapes and motion characteristics are different, and the heat flux acting on the contact can be accurately obtained by the arc model. The greater external transverse magnetic field is more conducive to extinguishing the arc and reducing the arc erosion time. In contrast, the Archimedes force plays a very small positive role and the self-induced magnetic field of the arc has a negative effect. For contact erosion, the temperature distribution of the molten pool appears trailing effect. Moreover, the advanced mathematical model proposed for erosion mass has good accuracy. The calculation results of erosion rates show that Cr reduces erosion while the current increases erosion nonlinearly. Besides, the flow and deformation characteristics of the molten pool will lead to convex strip-shaped erosion traces and arcing position transfer, and the moving erosion will cause the lateral accretion effect. Furthermore, the analysis shows that the reason for edge droplet sputtering is the pressure difference caused by Plateau–Rayleigh instability, and optimization suggestions for reducing contact erosion are given.

A Novel Protection Strategy for Single Pole-to-Ground Fault in Multi-Terminal DC Distribution Network

The single pole-to-ground (SPG) fault is one of critical failures which will have a serious impact on the stable operation of the multi-terminal DC distribution network based on the modular multilevel converter (MMC). It is very significant to analyze fault characteristics for detecting faults and protection design. This paper established the DC SPG fault model, which showed that in the presence of a reactor, the short-circuit current was reduced from 2.3 kA to 1 kA at 6 ms after the fault. Then, a novel SPG fault protection strategy was proposed, which detected the current derivative in connection transformer grounding branch. When the value increases past the threshold of current derivative, small resistance was switched on to increase fault current. Thus, the reliability of differential protection was enhanced. Compared with the traditional protection method, the proposed method does not need communication, and improved the speed of protection. Finally, the simulation model was established in PSCAD/EMTDC. The model included three converter stations: T1, T2 and T3. Among them, T1 outputs power, and T2 and T3 receive power. The results of RTDS showed that the DC circuit breaker operated within 3 ms, the three-port circuit breaker worked within 50 ms, which proves that the proposed strategy was effective. At this time, the system switched from the T1–T2–T3 three-terminal networking operation mode to the T1–T2 two-terminal hand-in-hand operation mode. Since the T3 terminal no longer received power, the transmission power of the T1 terminal decreased, and the received power of the T2 terminal remained unchanged.

Study of the impact of DC distribution network under large-scale electric-powered vehicle access

As a high-standard verdant method of transportation, electric-powered-vehicles have become the development hotspot of the automotive business in recent years. Large scale access to electric-powered vehicles can form a distributed energy storage system, which can improve the operational performance of power grid peak shaving and frequency modulation, and improve the comprehensive utilization efficiency of system energy. But at the same time, large-scale electric-powered vehicles connected to the DC distribution network will have a certain impact on its topology. Therefore, this paper studies the impact of the access location and number of electric-powered vehicles on the network topology and establishes a multi-dimensional evaluation system of the DC distribution network topology. The application of this result can play an evaluation and guidance role in the operation stability of the DC distribution network in the case of large-scale access to electric-powered vehicles.

Complete Large-Signal Stability Analysis of DC Distribution Network via Brayton-Moser’s Mixed Potential Theory

For a nonlinear RLC network, Brayton and Moser have proposed the so-called general mixed potential function (GMPF) whose time-derivative is negative semi-definite. Then, the equilibrium of the RLC network is stable if it is a local minimum of the GMPF. Therefore, Brayton-Moser’s mixed potential theory is a powerful methodology, which has been widely used in the stability analysis of DC microgrid. However, most of the results in existing references are flawed and incomplete. This paper carries out the complete stability analysis of the DC distribution network with constant power loads via Brayton-Moser’s mixed potential theory. Firstly, several critical points in this theory that are often mistaken are emphasized. Secondly, based on Brayton-Moser’s mixed potential theory, the condition that the equilibrium is a local minimum is proposed. All the low-voltage equilibria are proved to be unstable, and only the high-voltage equilibrium can be stabilizable and the complete stability conditions are provided. Thirdly, some unsolved problems about the stability issue of DC microgrid via Brayton-Moser’s mixed potential theory are presented. Finally, hardware-in-the-loop (HIL) experimental results verify the proposed stability conditions.

An improved distributed secondary control for multi‐voltage‐level DC distribution networks with virtual impedance arrangement

AbstractThe distributed secondary control of the DC distribution network is becoming increasingly important in modern power systems to achieve accurate power sharing of each distributed unit. Here, an improved secondary control strategy for energy storage systems (ESSs) in the multi‐voltage‐level DC distribution network (MVL‐DCDN) is proposed. At the primary control level, the interlinked DC transformers (DCT) are controlled by difference‐of‐v2 to support bidirectional voltage making the MVL‐DCDN regarded as a single‐voltage‐level one using only the local information of DCT. At the secondary control level, it can be observed that the voltage correction term and virtual impedance correction term have a coupling relationship. In order to tackle this coupling effect, a distributed secondary control is imposed in an MVL‐DCDN based on improved droop control. Concretely, this proposed secondary control strategy contains a power sharing controller, a virtual impedance controller, an average voltage restoration controller, and a reference voltage consensus controller. With the proposed method, this coupling effect can be eliminated to ensure the steady operation of the virtual impedance. Furthermore, a steady‐state analysis is performed to discuss the unique solution of the DC distribution network and conduct hardware‐in‐loop experiments to demonstrate that this method achieves effective results.

Fault location discrimination and protection strategy for DC distribution network based on PET

After the bipolar short circuit fault occurs on the low-voltage DC side of the low-voltage DC distribution network with a power electronic transformer, the fault current amplitude is high and the rise speed is fast, which leads to the rapid locking of power electronic devices, and the loss of fault information of line protection. Based on the multi-branch DC distribution network with CHB-type PET, this paper proposes a fault current limiting strategy. PET starts current limiting measures after detecting the DC side fault, which can provide stable and continuous fault transient information for line protection while providing a small amount of short circuit current. On this basis, using the fault current at both ends of the line, longitudinal polarity protection without strict communication synchronization is proposed. The simulation analysis based on PSCAD/EMTDC software shows that the scheme can realize fast and reliable fault line selection and has strong resistance to transition resistance.

Power Disequilibrium Suppression in Bipolar DC Distribution Grids by Using a Series-Parallel Power Flow Controller

The unbalanced power between positive and negative poles in a bipolar DC distribution network (DC-DN) generates an unbalanced current at the neutral line, which enlarges the power losses of the system and the voltage deviation of DC loads. An unbalanced power suppression strategy based on a series-parallel power flow controller (SP-PFC) is proposed in this paper. The SP-PFC is adopted as the interconnection between two different DC-DNs. The topology and operating modes of SP-PFC are analyzed. Subsequently, SP-PFC output voltage and line current expressions under constant power control are derived. The nonlinear relationship between the output voltage and line current is linearized at the operating point. On this basis, the influences of unbalanced load and receiving-end voltage on the SP-PFC are investigated. A small-signal model of bipolar DC-DN containing an SP-PFC is established, and the system stability is analyzed. A simulation model of the bipolar DC distribution network containing an SP-PFC is built up in MATLAB/Simulink, and the effectiveness of the SP-PFC in the suppression of unbalanced power is verified.

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.

A Two-Step Stability Assessment Method for Interconnection of DC Distribution Networks

Interconnection of DC distribution networks is raising concerns over their stability. With the increasing number of power electronics converters, the conventional stability assessment methods require detailed internal information of the converters and passive network and/or large storage and computing power. In this paper, to fully protect the highest level of trade secret and user privacy and to avoid the curse of dimensionality, an improved pre-processing method is first developed to deal with the distribution networks prior to their interconnection. Then, a two-step stability assessment method is formulated to analyze the stability of the anticipated distribution networks subsequent to their interconnection. By comparing with the existing methods, the proposed one simultaneously achieves three objectives: i) it regards both the converters and passive network as black boxes and assesses the stability of the interconnected network solely based on the measured impedance/admittance; ii) it assesses system stability with a reduced-order system minor-loop gain and therefore, overcomes some of the difficulties of dimensionality curse; and iii) it prevents from the internal instability and guarantees the global stability of the interconnected distribution network. Finally, a modified IEEE 37-bus DC distribution network is implemented in the Matlab/Simulink environment to verify the validity of the proposed method.

An Efficient EMS for BESS in Monopolar DC Networks with High Penetration of Renewable Generation: A Convex Approximation

This research presents an efficient energy management system (EMS) for battery energy storage systems (BESS) connected to monopolar DC distribution networks which considers a high penetration of photovoltaic generation. The optimization model that expresses the EMS system with the BESS and renewable generation can be classified as a nonlinear programming (NLP) model. This study reformulates the NLP model as a recursive convex approximation (RCA) model. The proposed RCA model is developed by applying a linear approximation for the voltage magnitudes only at nodes that include constant power loads. The nodes with BESS and renewables are approximated through the relaxation of their voltage magnitude. Numerical results obtained in the monopolar version of a 33-bus system, which included three generators and three BESS, demonstrate the effectiveness of the RCA reformulation when compared to the solution of the exact NLP model via combinatorial optimization techniques. Additional simulations considering wind power and diesel generators allow one to verify the effectiveness of the proposed RCA in dealing with the efficient operation of distributed energy resources in monopolar DC networks via recursive convex programming.

Backup protection with accelerating strategy based on fault control of modular multilevel converters in medium‐voltage direct current distribution network

AbstractThe medium‐voltage DC distribution network with multiple DC feeders is more cost‐effective when equipped with switches than DC circuit breakers. Under such conditions, the converters with fault‐handling capacity are blocked to clear the fault, which will cause the rapid disappearance of the fault current. Therefore, the primary protection has to pursue high speed at the expense of reliability, which threatens the network's safety. To construct a reliable protection system, backup protection with an accelerating strategy based on the fault control of converters is proposed. The current is controlled first by the fault control of converters to ensure the fault characteristics during backup protection. Then the DC switches on the healthy line are blocked by the polarity of the current integral, and the distribution network turns into the close‐loop mode to provide a fault current path. When the DC switches on the faulty line disconnect according to the predefined time delay, the current change time is recorded and thus the switches in the network are accelerated without communication. The proposed scheme only depends on the measured local current and the backup function still exists after acceleration. The effectiveness of the proposed scheme is verified in power systems computer aided design/electromagnetic transient including DC (PSCAD/EMTDC).

Efficient Day-Ahead Dispatch of Photovoltaic Sources in Monopolar DC Networks via an Iterative Convex Approximation

The objective of this research is to propose an efficient energy management system for photovoltaic (PV) generation units connected to monopolar DC distribution networks via convex optimization while considering a day-ahead dispatch operation scenario. A convex approximation is used which is based on linearization via Taylor’s series expansion to the hyperbolic relations between voltages and powers in the demand nodes. A recursive solution methodology is introduced via sequential convex programming to minimize the errors introduced by the linear approximation in the power balance constraints. Numerical results in the DC version of the IEEE 33-bus grid demonstrate the effectiveness of the proposed convex model when compared to different combinatorial optimization methods, with the main advantage that the optimal global solution is found thanks to the convexity of the solution space and the reduction of the error via an iterative solution approach. Different objective functions are analyzed to validate the effectiveness of the proposed iterative convex methodology (ICM), which corresponds to technical (energy losses reduction), economic (energy purchasing and maintenance costs), and environmental (equivalent emissions of CO2 to the atmosphere in conventional sources) factors. The proposed ICM finds reductions of about 43.9754% in daily energy losses, 26.9957% in energy purchasing and operating costs, and 27.3771% in CO2 emissions when compared to the benchmark case in the DC version of the IEEE 33-bus grid. All numerical validations were carried out in the MATLAB programming environment using the SEDUMI and SDPT3 tools for convex programming and our own scripts for metaheuristic methods.

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.

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.

Multi-objective Day-ahead Polarity Switching Optimization for Voltage Unbalance in Bipolar DC Distribution Networks

Multi-objective Day-ahead Polarity Switching Optimization for Voltage Unbalance in Bipolar DC Distribution Networks

Line Fault Detection of DC Distribution Networks Using the Artificial Neural Network

Line Fault Detection of DC Distribution Networks Using the Artificial Neural Network

Research on Fault Location in DC Distribution Network Based on Adaptive Artificial Bee Colony Slime Mould Algorithm

Research on Fault Location in DC Distribution Network Based on Adaptive Artificial Bee Colony Slime Mould Algorithm

Fault Characteristic Analysis and a Novel Pilot Protection Scheme for Pole-to-Ground Fault in Flexible DC Distribution Network

Fault Characteristic Analysis and a Novel Pilot Protection Scheme for Pole-to-Ground Fault in Flexible DC Distribution Network

Design Analysis of SiC-MOSFET Based Bidirectional SSPC for Aircraft High Voltage DC Distribution Network

Design Analysis of SiC-MOSFET Based Bidirectional SSPC for Aircraft High Voltage DC Distribution Network

Editorial Special Section on “Power Quality in the Energy Transition: Selected Papers From ICHQP 2022”

Electrical power quality is a vital aspect when designing or assessing the operation of all modern power systems and forms an important part of the ongoing energy transition to more efficient and multi-vector systems. However, the ongoing proliferation and changing functionality of power electronic devices, coupled with new grid operating paradigms, such as renewable energy sources integration, microgrids, low-voltage dc distribution networks, and the large-scale integration of electric vehicles, present unique opportunities and challenges for grid operators the world over and require new assessment methods and fresh perspectives on the role of power quality.