Direct Current Power Grid Research Articles

Research on CNN-LSTM DC power system fault diagnosis and differential protection strategy based on reinforcement learning

Introduction: With the development of artificial intelligence technology, more and more fields are applying deep learning and reinforcement learning techniques to solve practical problems. In the power system, both the direct current (DC) power system and the power grid substation are important components, and their reliability and stability are crucial for production efficiency and safety. The power grid substation is used to convert power from high-voltage transmission lines to low-voltage transmission lines, or from alternating current to direct current (or vice versa), in order to efficiently transmit and distribute power in the power system. However, diagnosing faults and designing cascaded protection strategies has always been a challenge due to the complexityand uncertainty of the DC power system.Methods: To improve the reliability and stability of the DC power system and power grid substation, this paper aims to develop an intelligent fault diagnosis system and cascaded protection strategy to reduce faults and downtime, lower maintenance costs, and increase production efficiency. We propose a method based on reinforcement learning and a convolutional neural network-long short-term memory (CNN-LSTM) model for fault diagnosis and cascaded protection strategy design in the DC power system. CNN is used to extract features from raw data, while LSTM models time-series data. In addition, we use reinforcement learning to design cascaded protection strategies to protect the power system from the impact of faults.Results: We tested our method using real 220V DC power system data in experiments. The results show that our method can effectively diagnose faults in the DC power system and formulate effective cascaded protection strategies.Discussion: Compared with traditional methods, this intelligent method can diagnose faults faster and more accurately, and formulate better cascaded protection strategies. This method helps reduce maintenance costs, increase production efficiency, and can be applied to other fields.

Modeling of Ship DC Power Grid and Research on Secondary Control Strategy

Compared to alternating current (AC) grids, direct current (DC) grids are becoming more and more popular. A power distribution approach is suggested in order to solve the issue of uneven power distribution of distributed generation (DG) in a ship DC microgrid. Power control is carried out using a tracking differentiator (TD), while the output power change rate is not greater than the maximum power ramp rate permitted by the battery, and state-of-charge balance is attained quickly. The proposed strategy also reduces the communication pressure on the power grid. A distributed hierarchical control model of a DC microgrid based on a consensus algorithm is created in order to validate the suggested methodology. The simulation results demonstrate that the established model is capable of simulating the DC microgrid accurately, that the states of charge values of the five batteries gradually converge under the adjustment of the secondary strategy, and that the suggested strategy is reasonable and efficient.

Special Issue: OPERATION AND CONTROL OF MODULAR DC TRANSFORMER APPLIED TO ENERGY INTERNET, 61-65.

Bidirectional direct current (DC)-DC converter has a broad appli-cation prospect in the interconnection of high-voltage and high-power DC power grid, which is helpful to promote the integrateddevelopment of DC power grid.

Research on Improved Fault Current Analysis Method for Flexible Direct Current Power Grid Considering Alternating Current Feed

Flexible direct current (DC) grid can realize large-scale renewable energy, wide-area coordinated complementation, and reliable power transmission. It is an important development that can be used to support high-voltage and large-capacity flexible DC transmission in the future. The short-circuit current of the DC line is one of the important bases for the selection of key main equipment parameters such as converter valves, DC circuit breakers, and reactors in the flexible DC grid. In this paper, a flexible DC grid equivalent circuit network model with alternating current (AC) feed-in is established. Aiming at the monopolar ground fault of the flexible DC grid grounded through the metal loop, an optimized traditional matrix calculation method is proposed to obtain the accurate line fault current value. On this basis, with an actual engineering background, the equivalent circuit model of the four-terminal bipolar flexible DC power grid is established, and the influence of grounding position, grounding parameters, and current-limiting reactor on the fault current of the DC line is analyzed. Finally, simulation using the PSCAD software verifies the effectiveness and accuracy of the proposed method. The method proposed in this paper can provide the necessary bases and references for the selection of flexible DC grid equipment.

Voltage frequency-based adaptive reclosing strategy for flexible DC power grids

Overhead lines (OHLs) in High Voltage Direct Current (HVDC) systems are vulnerable to temporary faults. Recovering power transmission by reclosing is of considerable significance to the grid reliability. However, the traditional auto-reclosing strategy will result in a secondary shock to the direct current (DC) system if the fault is permanent. Besides, a long delay is required to ensure that all types of temporary faults are cleared. To address these two challenges, this paper proposes an adaptive reclosing algorithm which can actively identify the fault property and fault disappearance time. Firstly, the discharging process of the fault point is analyzed, and the dominant frequency characteristics of the fault voltage under different fault properties are revealed. Then, the fault property and the fault disappearance time are identified based on the change time of the dominant frequency. Moreover, the influencing factors, including the fault resistance and the dead zone, are also analyzed. Finally, the algorithm is verified under different fault locations, fault resistances, arc lengths, noises, sampling frequencies and dead zone in PSCAD/EMTDC. The results show that the algorithm is effective, and its dead zone is less than 8% of the line.

A loop-analysis theory based power flow method and its linear formulation for low-voltage DC grid

As in alternative-current (AC) networks, the power flow (PF) task is the backbone of direct-current (DC) networks with the development of DC power system. In this paper, it proposes a loop-analysis theory based iterative power flow method for the low-voltage DC (LVDC) power grids. And further the linear power flow model via Taylor's series expansion has also been developed. Based on the node-branch incidence matrix, the relationship between the branch powers and node injection powers has been established. The formation of the proposed iterative method and its linear approximation are derived based on the branch powers rather than the injected currents. ZIP loads and V buses (multi-slack buses) have been taken into account. Simulation results verified the accuracy and speed of the proposed methods in comparison with the classical Newton-Raphson method and two other existing linear methods for both radial and meshed networks.

Multi-time Scale Optimal Power Flow Strategy for Medium-voltage DC Power Grid Considering Different Operation Modes

Direct current (DC) power grids based on flexible high-voltage DC technology have become a common solution of facilitating the large-scale integration of distributed energy resources (DERs) and the construction of advanced urban power grids. In this study, a typical topology analysis is performed for an advanced urban medium-voltage DC (MVDC) distribution network with DERs, including wind, photovoltaic, and electrical energy storage elements. Then, a multi-time scale optimal power flow (OPF) strategy is proposed for the MVDC network in different operation modes, including utility grid-connected and off-grid operation modes. In the utility grid-connected operation mode, the day-ahead optimization objective minimizes both the DER power curtailment and the network power loss. In addition, in the off-grid operation mode, the day-ahead optimization objective prioritizes the satisfaction of loads, and the DER power curtailment and the network power loss are minimized. A dynamic weighting method is employed to transform the multi-objective optimization problem into a quadratically constrained quadratic programming (QCQP) problem, which is solvable via standard methods. During intraday scheduling, the optimization objective gives priority to ensure minimum deviation between the actual and predicted values of the state of charge of the battery, and then seeks to minimize the DER power curtailment and the network power loss. Model predictive control (MPC) is used to correct deviations according to the results of ultra short-term load forecasting. Furthermore, an improved particle swarm optimization (PSO) algorithm is applied for global intraday optimization, which effectively increases the convergence rate to obtain solutions. MATLAB simulation results indicate that the proposed optimization strategy is effective and efficient.

Optimal Power Flow in Direct-Current Power Grids via Black Hole Optimization

This paper addresses the Optimal Power Flow (OPF) problem in DC power microgrids via a combinatorial optimization technique known as Black Hole Optimization (BHO). Such optimization method allows to solve OPF problems via algorithmic strategies trough a master-slave formulation. In the master stage, the total power generated by each Distributed Generator (DG) is determined by the BHO, while the slave strategy is entrusted with solving the resulting conventional power flow problem via a classical Gauss-Seidel (GS) numerical method. For comparison purposes, this work uses nonlinear optimization methods available in General Algebraic Modeling System (GAMS) as well as continuous metaheuristic optimization techniques. Two test feeders with 21 and 69 nodes were considered for validating the proposed hybrid BHO-GS optimization method, which enables to demonstrate its applicability, robustness and efficiency compared to conventional approaches. The results of all the simulations were obtained via MATLAB 2017a.

A simplified control scheme for electric vehicle-power grid circuit with DC distribution and battery storage systems

The direct current (DC) system is becoming the major trend for future internal power grid of electric vehicles (EVs). Since DC power grid system has a different nature to conventional alternating current (AC) grid system, appropriate design of the controller for EV-grid circuit is mandatory. In this paper, an EV employing a pure DC grid circuit with battery storage system (BSS) is considered as a study case. To enable a more efficient use of BSS, a flyback DC-DC converter for batteries charger/or discharger strategy is selected, which satisfies the power flows requirements. The dynamic and control performances of the combined system, i.e. ‘BSS-flyback DC-DC converter connected to a DC motor’, is investigated in terms of voltage/current signal fluctuations. The small-signal based control method is used, which limits the small-signal variations to about zero. To verify the effectiveness of the control strategy, several simulations are done using Matlab. The simulation results illustrate the performances obtained.

Electric Field Characteristics of an Arc-Induction-Type DC Circuit Breaker Combined with a Magnet for Stability in the DC Power Grid

For stability in the direct current (DC) power grid, it is very important to limit the current of the fault section and to protect the system in the fault condition. That is the core technology of the DC power grid. This paper suggests characteristics and a limited range of the fault current for the new model as the significant protecting technology in the DC power grid. Such new model is the arc-induction-type DC circuit breaker (CB) combined with a magnet. The magnetic arc-induction-type direct-current circuit breaker (DCCB) consists of a neodymium magnet, an induction needle, and electrical contacts. During fault operation, the arc occurring between the contacts is induced to the induction needle. To increase the induction effect, Lorentz force is applied to blow the arc. A magnet was applied to reduce the initial fault current and to induce magnetic flux. The simulation data were analyzed using the Maxwell 3D software program. The electric field included the electric force and the electric charge. Therefore, it could analyze the arc as the electric field. The simulation was performed based on the preceding study on induction needle. As a result of the simulation, it was confirmed that the electric field value of the induction needle increased when the DCCB was combined with a magnet.

Pole-to-Ground Fault Analysis and Fast Protection Scheme for HVDC Based on Overhead Transmission Lines

Flexible direct current (DC) transmission network technology is an effective method for large capacity clean energy access to power grids, but the DC short-circuit fault detection for it is a difficult problem. In this paper, the pole-to-ground fault transient characteristics in a multi-terminal DC power grid, based on overhead transmission lines and DC circuit breakers, are analyzed firstly. Then, a fast protection scheme is proposed according to the fault transient characteristics. Only local information is utilized for fault detection and location in the proposed scheme. Moreover, the scheme is verified to have the advantages of fast action speed, high reliability and the ability to resist the transition resistance. A four terminal DC power grid model based on actual engineering parameters is established in PSCAD/EMTDC, and the validity of the protection scheme under different fault conditions is verified by simulation results.

Research of UHV DC Transmission Based on the Current Control

Compared with AC (alternating current) transmission, DC(direct current) transmission has obvious superiority at high power, long distance, submarine cable transmission and asynchronous interconnected between exchange systems etc.. This paper constructed a kind of 12 pulse bridge UHV DC transmission system that includes control protection function through MATLAB/Simulink tool. By setting the breaker parameters, this paper studied the 12 pulse bridge UHV DC transmission system AC/DC side in case of setting down time, type and cleared. The simulation results demonstrate that this system has a good effect in harmonic control and it has no power compensations in realizing HV DC process, and it can completely work as an important reference in constructing HV DC power grid.