DC Distribution Network Research Articles

Reliability modeling of multi-terminal DC circuit breaker and its impact on multi-terminal DC distribution network

In the medium-voltage distribution network, the multi-terminal DC circuit breaker (MTCB) is one of the key pieces of equipment for the connection of multi-terminal converter stations and rapid fault isolation in the future. The reliability level of the multi-terminal DC distribution network is directly impacted by reliability of MTCB. In this paper, the main topological structure and basic working principle of MTCB are systematically analyzed. Then, taking the three-terminal hybrid DC circuit breaker which has been applied in the Zhuhai Tangjiawan “Internet plus” Demonstration Project of China Southern Power Grid (hereafter referred to as Zhuhai Demonstration Project) as an example, the reliability evaluation model of the MTCB is established based on the engineering reliability principle and considering the redundancy level of device. By calculating multi-dimensional reliability index, the reliability of MTCB under different design modes and redundancy levels is analyzed, and the influence of MTCB on the reliability of multi-terminal DC distribution network is further studied based on the minimum cut set method. The influence of the MTCB on the reliability of multi-terminal DC distribution network of Zhuhai Demonstration Project is then studied. Finally, an example of the three-terminal DC distribution network structure of Zhuhai Demonstration Project is analyzed to confirm the accuracy and effectiveness of the proposed model.

Research on source-load coordinated dispatching of flexible DC distribution network based on big data

In this new round of power network development, the concept of coordinated and optimized operation of power network interconnection and smart grid “source network load” has gradually attracted attention. Based on the analysis of the impact of the flexible DC distribution network on the complex energy system with multiple data sources and large data volume under the big data platform, the coordinated dispatching of the source and load of the big data flexible DC distribution network is studied. Therefore, a source-load matching index that can evaluate the impact of different types of loads on the stability of the flexible DC grid is constructed and incorporated into the load recovery optimization model. First, use the scene reduction method to process all generated scenes; next, take the reduction technology and the scene generation method into account of the uncertainty caused by the prediction error and incorporate them into the multi-objective function optimization model; finally, the membership function of fuzzy numbers is used to model uncertainty. So as to construct a load recovery model that can coordinate the load recovery amount, importance and system dynamic response. The actual meaning of the matching index is explained through model solving and actual case analysis.

Transient Fault Analysis Method for VSC-Based DC Distribution Networks With Multi-DGs

Flexible direct current (dc) distribution networks are the development trend for the future distribution system. However, the protection and control of these networks are vulnerable to complicated dc faults, accurate fault analysis is required. In this article, we propose a three-stage transient fault analysis method to improve the accuracy of fault characteristic calculation for the voltage source converter based dc distribution network with multi distributed generators, which can unify and simplify the calculation process of different transient fault stages. An impedance parameter decoupling approach is proposed based on the dc line parameters, which can avoid the influence of the coupling impedance on fault response. The decoupled equivalent fault circuits are established for accurate fault analysis with flexibility and independence. In addition, a three-stage fault analysis is proposed, which is able to unify the calculation process. Finally, numerical simulations based on PSCAD/EMTDC have been carried out, which well demonstrates the effectiveness of the proposed impedance decoupling approach and the accuracy of the proposed method for fault analysis. Compared with the traditional fault analysis without decoupling, the proposed method stands out the conciseness and correctness.

A Coordinated Restoration Method of Hybrid AC–DC Distribution Network With Electric Buses Considering Transportation System Influence

The postdisaster restoration capabilities for critical loads in distribution networks need to be enhanced. In recent years, electric buses (EBs) have been widely used with the carbon-neutral target, which can be dispatched and discharged for load restoration considering their mobility and the V2G technology. However, the restoration capabilities of EBs are limited by the transportation system while the ac distribution network can hardly achieve power transfer between lines due to the radial constraint. In this article, a coordinated restoration method of hybrid ac–dc distribution network with EBs is proposed, which has flexible power transfer capability. The control modes of voltage source converters (VSCs) during the postdisaster period as well as the EB restoration capability considering transportation system constraints are analyzed in detail. Then, a bilevel programming model is developed to optimize the network reconfiguration, VSC outputs, and the proposed EBs dispatching scheme simultaneously, in order to achieve a better power transfer capability for load restoration considering their coordination. Simulation studies are performed to verify the proposed method.

Fault Location Based on Comprehensive Grey Correlation Degree Analysis for Flexible DC Distribution Network

Flexible DC distribution networks have a strong capacity for new energy consumption and have received extensive attention from scholars in recent years, but fault location for DC distribution lines is a difficult task. To solve this problem, we propose using a comprehensive grey correlation degree analysis to analyze the similarity of aerial mode traveling differential current waveforms at the first and last ends of a DC line, thus achieving fault location by obtaining the optimal time shift. Additionally, we built a six-terminal flexible DC distribution network model in Matlab/Simulink for simulation and validation, showing that the method can complete the rapid and accurate location of all types of faults on a DC line, and that it possesses an anti-transition resistance capability, making it suitable and highly reliable for cases of low sampling frequency.

Improved Laplacian Matrix based power flow solver for DC distribution networks

Distribution networks feature distinct topologies than transmission networks, such as radial or weakly meshed structures with tens of thousands of nodes. They have more points of power injection owing to the integration of distributed generators and high R/X ratios. Furthermore, there has recently been a surge of interest in DC distribution networks. In the planning and operation of modern distribution systems, load flow needs to be executed in series considering short intervals of time in the order of minutes or even less. Hence, these networks require a load flow solver that can converge fast with low computational burden. In this paper, we propose a unique iterative power flow solver based on graph theory for DC distribution networks. The proposed formulation is flexible and can handle both radial and mesh configurations with just one connectivity matrix. To validate the proposed method, we used the IEEE 33 bus test feeder and compared the results with an existing methodology. Results suggest that the proposed method is robust and possesses fast convergence.

Monopolar Grounding Fault Location Method of DC Distribution Network Based on Improved ReliefF and Weighted Random Forest

Compared with the pole-to-pole short circuit, the fault characteristics are not obvious when a monopolar grounding fault occurs in a DC distribution network, and it is difficult to locate the fault accurately. To solve this problem, this paper proposes a fault location method based on improved ReliefF and Weighted random forest (WRF). The 24 time and frequency-domain fault features of the postfault aerial mode current are calculated, and the most useful features are selected to form the optimal feature subset for input to the fault location estimator. In this paper, the ReliefF algorithm is utilized for automatic feature selection and obtaining the weights of features. In addition, the WRF algorithm is used to build the fault location estimator. Considering the fault location, fault resistance, noise and time window length, the Matlab/Simulink simulation platform is used to simulate the fault situation and compare it with other algorithms. The simulation results show that the average positioning error of the fault location method is less than 0.1%, which is not affected by the fault resistance and has strong robustness.

Voltage Regulation Strategy for DC Distribution Networks Based on Coordination of Centralized Control and Adaptive Droop Control

In DC distribution networks, the uncertainties of renewable energies and loads cause frequent voltage fluctuations. In this paper, a voltage regulation strategy based on the combined coordination of centralized optimal control (COC) and adaptive droop control (ADC) is proposed for droop-controlled DC distribution networks. In the COC strategy, the operation reference power of voltage source converters (VSCs) is optimized using the bus injection power measurements to improve the voltage profile and balance the power loading rates of different VSCs. The VSC capacity margin constraint is established to ensure that the capacity margin of each VSC is sufficient to cope with the unexpected fluctuation of loads and renewable energies. Then, an ADC strategy is adopted to handle voltage fluctuation between adjacent two COCs. In the ADC strategy, the operation reference power of the VSCs is further adjusted to recover the voltages. The voltage regulation is achieved based on the relationship among the VSC voltages and the minimum/maximum voltages of the whole network, which does not depend on the information of each nodal voltage and thus real-time network communication is avoided. The simulation results show that the proposed voltage regulation strategies can effectively improve the voltage profile under various uncertain environments.

Optimal Selection and Integration of Batteries and Renewable Generators in DC Distribution Systems through a Mixed-Integer Convex Formulation

The problem concerning the optimal placement and sizing of renewable energy resources and battery energy storage systems in electrical DC distribution networks is addressed in this research by proposing a new mathematical formulation. The exact mixed-integer nonlinear programming (MINLP) model is transformed into a mixed-integer convex model using McCormick envelopes regarding the product between two positive variables. Convex theory allows ensuring that the global optimum is found due to the linear equivalent structure of the solution space and the quadratic structure of the objective function when all the binary variables are defined. Numerical results in the 21-bus system demonstrate the effectiveness and robustness of the proposed solution methodology when compared to the solution reached by solving the exact MINLP model. Numerical results showed that the simultaneous allocation of batteries and renewable energy resources allows for the best improvements in the daily operating costs, i.e., about 53.29% with respect to the benchmark case of the 21-bus grid, followed by the scenario where the renewable energy resources are reallocated while considering a fixed location for the batteries, with an improvement of 43.33%. In addition, the main result is that the difference between the exact modeling and the proposed formulation regarding the final objective function was less than 3.90% for all the simulation cases, which demonstrated the effectiveness of the proposed approach for operating distributed energy resources in monopolar DC networks.

Fault detection method of flexible DC distribution network based on color relation analysis classifier

Fault detection method of flexible DC distribution network based on color relation analysis classifier

Power Flow in Bipolar DC Distribution Networks Considering Current Limits

Power electronics converters are equipped with current controls that protect the converter from over-currents. This protection introduces non-differentiable equations into the power flow problem. The conventional Newton's method is not suitable in that conditions. This letter proposes a fixed-point iteration to overcome this difficulty. The technique is derivative-free, and hence, it can naturally include the saturation given by the converters’ current protection. Exact conditions for convergence and uniqueness of the solution are demonstrated using Banach's fixed point theorem. Numerical experiments in Matlab complement the analysis.

Research of metering scheme key techniques for medium and low voltage DC distribution network

DC power grid has received increasing attentions recently, and with it comes the demand for highly reliable and stable metering solutions. At present, in the DC distribution projects that have been implemented, some DC metering devices have been put into use. However, from the national and industrial level, there is no clear standard for the design of the metering scheme for the medium and low-voltage DC distribution network, and there is a situation of imperfect metering device configuration. Therefore, based on the analysis of DC metering devices, algorithms and location selection, this paper takes the Hangzhou Jiangdong flexible DC distribution network as an example to analyze the error distribution caused by metering algorithm selection and device configuration. In addition, the influence of harmonics on DC measurement is analyzed to provide the suggestions for DC measurement solutions. The analysis has been verified by MATLAB/Simulink, which provides references for DC metering standards.

DC Soft Open Points for Resilient and Reconfigurable DC Distribution Networks

This paper introduces the concept, theory of operation and applications of soft open points for direct current networks (DCSOPs). The DCSOP is based on a bidirectional DC–DC converter actively controlled to behave like a normal conductor. Unlike the normal conductor, the DCSOP can transfer the electric power between nodes at different voltage levels. With this advantage, the DCSOP can effectively control the power flow direction. Thus, DCSOPs can play a vital role in the reconfiguration of DC distribution networks. The operation and control of the DCSOP device was investigated, both in transient and steady-state conditions. Then, a DCSOP was integrated into a DC microgrid model to validate its ability to change the power flow through the modelled feeders. In addition, a set of reliability indicators was calculated for the DC microgrid under different reconfiguration scenarios. It was shown that reliability is improved when the DCSOP device implements network reconfiguration. Finally, an agent-based framework for controlling the DCSOP in a DC microgrid is presented. A fundamental implementation was created for reconfiguring a DC microgrid with a DCSOP controlled by an agent, proving that the agent-based system can effectively control the DCSOP device for reconfiguration and voltage regulation.

ПАРАМЕТРИЧЕСКОЕ СОГЛАСОВАНИЕ УСТРОЙСТВ КОНТРОЛЯ ИЗОЛЯЦИИ И СОПТ

The aim of the study is to develop a method for parametric matching of insulation monitoringdevices, unearthed DC distribution systems and discrete inputs of relay protection.The use of unearthed DC distribution systems for power supply to relay protection devicesand other responsible consumers makes it possible to ensure high reliability and safety during their operation. It is necessary to ensure uninterrupted power supply to these consumerseven in the event of a ground fault in one of the DC distribution network poles. Insulationmonitoring of the network poles and the prompt execution of repair work when the insulationresistance falls below the critical level set by the response value, are a guarantee of highreliability of power supply. The article discusses the reasons leading to dangerous levels ofpole voltage unbalance and false triggering of relay protection discrete inputs during groundfaults of the signal lines connected to them. A method based on multivariate DC analysis isproposed to determine the matched insulation monitoring response value with the DC networkparameters and discrete inputs parameters, when the conditions for the occurrence ofsuch accidents are excluded. The influence of insulation monitoring devices in a DC systemon pole voltages unbalance and relay protection operation is also considered. A modificationof the insulation monitoring active method is proposed, which does not create additional polevoltage unbalance and risks of relay protection misoperation. The operating range of thestatic characteristics of the pole voltages and leakage current measuring transducers is narrowedin the grid unbalanced modes by using a modified active insulation monitoring met hod.Estimates of the increase in the insulation resistance measurement error arising in thesemodes are obtained.

Energy Efficiency Evaluation and Revenue Distribution of DC Power Distribution Systems in Nearly Zero Energy Buildings

With the advantage of integrating distributed energy, storage and DC load with high efficiency, the DC distribution network recently attracted wide attention in the field of nearly zero energy buildings. Considering the large number of buildings and the enormous energy-saving potential, the distribution form and the revenue distribution are key factors affecting energy efficiency and operational economy. Aiming at the characteristics of AC and DC distribution topologies in nearly zero energy buildings, an energy efficiency evaluation method is proposed based on time-sequential power flow simulation, where the piecewise linear efficiency function of converters and dynamic line power loss model are established. Based on a DC distribution demonstration project, an AC power distribution system with the same boundary conditions is designed. The results show that the efficiency of DC distribution topology is about 6% higher than the AC system, while the efficiency advantage in office, hotel, commercial, educational and residential buildings in Beijing, Shenzhen and Shanghai varies from 0.6–4.96%, which shows a high correlation with the proportion of a high-power load, with Pearson correlation coefficient of 0.794. With the increase of DC load ratio and distributed power supply access capacity, the efficiency of the system will be improved accordingly. On the basis of this, a method to equalize DC distribution income in the nearly zero energy buildings power market is proposed. In addition, 15 typical scenarios of different cities were selected to evaluate efficiency and influencing factors. Finally, assessing the multi-stakeholder impact of DC retrofitting to balance the revenue generated by the DC distribution model suggests that transport costs should be raised by 8.2–13.1% to balance revenue with the equipment prices decreased by about 0.1 yuan/W.

Solar Photovoltaic Integration in Monopolar DC Networks via the GNDO Algorithm

This paper focuses on minimizing the annual operative costs in monopolar DC distribution networks with the inclusion of solar photovoltaic (PV) generators while considering a planning period of 20 years. This problem is formulated through a mixed-integer nonlinear programming (MINLP) model, in which binary variables define the nodes where the PV generators must be located, and continuous variables are related to the power flow solution and the optimal sizes of the PV sources. The implementation of a master–slave optimization approach is proposed in order to address the complexity of the MINLP formulation. In the master stage, the discrete-continuous generalized normal distribution optimizer (DCGNDO) is implemented to define the nodes for the PV sources along with their sizes. The slave stage corresponds to a specialized power flow approach for monopolar DC networks known as the successive approximation power flow method, which helps determine the total energy generation at the substation terminals and its expected operative costs in the planning period. Numerical results in the 33- and 69-bus grids demonstrate the effectiveness of the DCGNDO optimizer compared to the discrete-continuous versions of the Chu and Beasley genetic algorithm and the vortex search algorithm.

Design and Research on DC Electric Leakage Protection Circuit Breaker

In recent years, DC power supply and distribution technology plays an increasingly significant role in power supply and distribution systems with a low loss and large power supply radius. However, the corresponding DC distribution network protection electric devices cannot satisfy the needs of the existing DC power supply and distribution network. Based on this, in this article, an intelligent/energy-efficient DC leakage protection circuit breaker based on two-phase magnetic nanomaterials and detection and control technology is proposed. Firstly, the core of an intelligent/energy-efficient DCCB is composed of two-phase magnetic nanomaterials, and the mathematical modeling process, preparation and processing method of two-phase magnetic nanomaterials are given. Secondly, the DC leakage detection technology is studied by fluxgate technology, and the design process of magnetic rings, winding and detection circuit is given. Thirdly, the simulation and experiment results of the detection system and actuator are given by the simulation and experiment of an intelligent/energy-efficient DCCB to verify the correctness of the proposed scheme.

Mechanism and Suppression of Frequency-Decrease Effect Related to the Low Modulation Frequency Ratio in a High-Ratio MMC-Based DC Transformer

The high-ratio MMC-based DC transformer is the crucial device for the interconnection between different DC distribution networks. Compared to the typical MMC, the transformer has a similar structure and low modulation frequency ratios (FR). Thus, it also has the severe frequency-decrease effect and similar operating performance deterioration. However, because of the extremely lower FRs that are close to 1, the feature of the frequency-decrease effect in the transformer has different manifestations. And because of the two-stage structure which combines the MMC and rectifiers, more factors emerged in the transformer to determine the frequency-decrease effect. Therefore, this study further develops the theory and elaborates the frequency effect's mechanism that is originally from the MMC to adapt to the more complex structure of the transformer. A quantitative analysis of the frequency-decrease effect is then provided, demonstrating that because of the transformer's two-stage structures, two types of FRs and concomitant factors exist in the transformer, thus having a combined effect on the harmonic distributions of capacitor voltages and causing more proportional voltages' harmonics distorting into the slope harmonics. Based on the analysis, the study entirely redesigns the FRs and regulates the modulation strategy of the transformer to fundamentally eliminate the frequency-decrease effect. The study also confirms that the FRs in the two sides cannot be set to the same, and their decimal part cannot cause extra low-frequency harmonics. The frequency-decrease effect, the deteriorations, and the efficiency and comparisons of the proposed methods are finally demonstrated via simulations and experiments.

A Fast Pilot Protection for DC Distribution Networks Considering the Whole Fault Process

To improve the consumption of renewable energy and to meet complex load demands. A model of a multi-terminal flexible DC distribution network with multiple overhead branch lines is constructed. Aiming at frequent non-permanent faults of overhead lines, a fast pilot protection scheme is proposed to cover the whole fault process. Using the hybrid DC circuit breaker (HDCCB) action as the boundary mark, the fast pilot protection can be divided into two phases: post-fault and fault restart. The first phase highlights rapidity, sending a trip command to HDCCB within 1-2 ms after the fault. The second stage emphasizes reliability, and points out that the data of the whole fault process should be fully utilized, and to fully excavate the characteristics of fault current. A multi-time scale and multi-criteria fusion protection scheme is proposed, which can effectively improve the reliability of fault area identification. Finally, a corresponding model was built based on PSCAD, a large number of simulation tests and robustness analysis proved that the proposed fast pilot protection scheme is feasible.

Flexible DC distribution line protection principle based on distance metric of clustering centres

Aiming at the issue of unsatisfactory selectivity of single-terminal protection, and poor speed and high requirements for data synchronization of double-terminal protection in flexible DC distribution networks, an artificial intelligence (AI)-based method is proposed to obtain the best clustering centres through unsupervised clustering analysis of historical data to realize the non-deterministic flexible DC distribution line protection principle. The method forms historical data samples by simulating different types of short-circuit faults at different locations of the line in advance, combining them with the actual faults, and collecting and processing the post-fault currents. The K-means clustering algorithm is then used to find the best clustering centres corresponding to different fault types, and fault identification and pole selection are realized by comparing the distance between real-time data and each clustering centre. The process relies merely on single-terminal current as the characteristic quantity, and it does not need complicated feature extraction and calculation. Thus, the cumbersome threshold setting in conventional current protections can be avoided. Finally, the case studies are carried out in PSCAD/EMTDC, and the results show that the proposed protection has good selectivity and rapidity, and the tolerance to fault resistance is improved compared with the conventional local-current-based protections.