AC-DC Hybrid Distribution Network Research Articles

Distributed optimal Volt/Var control in power electronics dominated AC/DC hybrid distribution network

AbstractThe integration of large‐scale distributed power sources increases the voltage fluctuation in AC/DC hybrid distribution network (AD‐HDN). Power electronics devices such as photovoltaic (PV) inverters, soft open point (SOP), and voltage source converters (VSCs) can be utilized for voltage/var control (VVC) to alleviate the risk of voltage fluctuation and violation. This paper proposes a distributed optimal VVC method in power electronics dominated AD‐HDN. Firstly, the reactive power and voltage characteristics of PV inverters, SOP, and VSCs are analysed, and an optimal VVC optimization model for AD‐HDN to minimize node voltage deviation, PV curtailment, and network loss is proposed. Then, the second‐order cone (SOC) relaxation technique is used to re‐formulate the model into a convex optimization model. A distributed optimal VVC framework based on the alternating direction method of multipliers (ADMM) is constructed. Based on the residual balance principle and relaxation technique, an accelerated ADMM method is further proposed to solve the proposed model. Finally, case studies are conducted on the IEEE 33‐node and 85‐node systems to verify the superiority and effectiveness of the proposed method.

A Non-Iterative Coordinated Scheduling Method for a AC-DC Hybrid Distribution Network Based on a Projection of the Feasible Region of Tie Line Transmission Power

AC-DC hybrid distribution grids realize power transmission through tie lines. Accurately characterizing the power exchange capacity between regional grids while ensuring safe grid operation is the basis for the coordinated scheduling of resources in interconnected distribution grids. However, most of the current AC/DC hybrid models are linear, and it is challenging to ensure the accuracy criteria of the obtained feasible regions. In this paper, a two-stage multi-segment boundary approximation method is proposed to characterize the feasible region of hybrid distribution grid tie line operation. Information such as security operation constraints are mapped to the feasible region of the boundary tie line to accurately characterize the transmission exchange capacity of the tie line. To avoid the limitations of linear models, the method uses a nonlinear model to iteratively search for boundary points of the feasible region. This ensures high accuracy in approximating the real feasible region shape and capacity limitations. A convolutional neural network (CNN) is then utilized to map the given boundary and cost information to obtain an estimated equivalent operating cost function for the contact line, overcoming the inability of previous methods to capture nonlinear cost relationships. This provides the necessary cost information in a data-driven manner for the economic dispatch of hybrid AC-DC distribution networks. Numerical tests demonstrate the effectiveness of the method in improving coordination accuracy while preserving regional grid privacy. The key innovations are nonlinear modeling of the feasible domain of the contact line and nonlinear cost fitting for high-accuracy dispatch.

A Fully Decentralized Optimal Dispatch Scheme for an AC–DC Hybrid Distribution Network Formed by Flexible Interconnected Distribution Station Areas

A Fully Decentralized Optimal Dispatch Scheme for an AC–DC Hybrid Distribution Network Formed by Flexible Interconnected Distribution Station Areas

Optimal Scheduling of AC–DC Hybrid Distribution Network Considering the Control Mode of a Converter Station

Due to the difference in types of loads between regions and the increasing integration of random elements such as electric vehicles (EVs) and distributed generations (DGs), distribution station areas (DSAs) are facing challenges such as unbalanced load rates and voltage violations. An AC–DC hybrid distribution network formed by interconnecting AC-DSAs using flexible DC technology can not only address these issues, but also offer more efficient interfaces for EV charging piles and DC devices on the DC side. To fully leverage the advantages of the technology and coordinate dispatchable elements within each DSA, this paper proposes an optimal scheduling model, which balances the load rate between DSAs, improves voltage profiles, and considers the control mode of the converter station as a dispatchable element, taking into account its impact on the voltage deviation on the DC side. Simulation results demonstrate the effectiveness of the proposed model in balancing load rate and improving voltage profiles. Moreover, rational decision-making regarding the selection of the control mode for converter stations can effectively mitigate the voltage deviation on the DC side without deteriorating the voltage deviation on the AC side.

A novel PET model based volt/var control of AC–DC hybrid distribution network

With the explosive growth of distributed renewable energy connected to the power system, the application of AC–DC hybrid distribution network with power electronic transformer (PET) becomes more and more extensive. An equivalent loss model is formulated from the perspective of the internal topology of PET. Then, a novel PET power flow model is established to obtain the relationship between the three ports. On this basis, reactive power of PV nodes connecting PET and distributed renewable generation is modified by establishing an equivalent impedance matrix, thus the forward–backward sweep load flow calculation method is improved for power flow calculation and volt/var control of distribution network. Different from traditional reactive optimization methods, a multi-objective reactive power optimization model considering the PET and two types of photovoltaic converters is established in this paper, with the targets of minimizing voltage deviation and power loss simultaneously. Finally, the numerical simulation shows the accuracy, rationality, and feasibility of the proposed method.

A Benefit Evaluation Model for Peak-Clipping Benefits of Distributed Photovoltaic Grids Taking into Account Uncertain Low-Voltage Distribution

Due to the high energy consumption in the traditional AC-DC hybrid microgrid, the benefit evaluation may be difficult. In this paper, an AC-DC low-voltage hybrid distribution network suitable for distributed photovoltaic power generation access is proposed. DC power directly supplies DC household appliances, and AC power grid directly supplies AC appliances. In this way, the loss of electric energy in the process of energy conversion can be reduced, and good economy can be achieved. The 24-pulse thyristor bidirectional converter is used in the connection between AC and DC power grids, which can reduce the loss and prevent the failure of benefit evaluation. The photovoltaic low-voltage distribution tracking adopts conductance increment method, and the converter control adopts constant voltage control to maintain the bus voltage stability. The simulation results prove the feasibility of the scheme.

Research on Typical Connection and Operation Mode of AC-DC Hybrid Distribution Network

Research on Typical Connection and Operation Mode of AC-DC Hybrid Distribution Network

Energy Efficiency Evaluation of Distribution Network Considering Power Quality Conditions

There are three-phase imbalance, voltage deviation, frequency fluctuation and other power quality problems in the traditional AC distribution network, which are difficult to meet the demand of power users for high power quality. The AC-DC hybrid distribution network is superior to the AC distribution system in terms of reducing power loss and improving power supply quality. Based on the research status of energy-saving indexes of distribution network, considering the characteristics of AC-DC hybrid distribution network, this paper analyzes the factors that affect the energy efficiency level of AC-DC hybrid distribution system, constructs the evaluation index system, determines the classification evaluation standards of different indexes according to the relevant standards, and puts forward the quantitative evaluation method of the energy efficiency level of AC-DC hybrid distribution network.

Study on Power Supply Structure of AC / DC Distribution Network Considering Different Proportion of New Energy Access

Considering the diversification of energy and load, according to the different situation of new energy access to distribution network, three typical AC-DC hybrid distribution network power supply modes are summarized. On the basis of these three power supply modes, the indexes of AC-DC hybrid distribution network are extracted, and the comprehensive evaluation index system is established. A comprehensive evaluation model of AC / DC hybrid distribution network suitable for new energy access is proposed, which aims to maximize economic benefits. The example shows that the optimal power supply mode can be selected through the evaluation model when the proportion of new energy is different, which can lay a foundation for the future distribution network to better serve users on the premise of distribution network profitability.

Coordinated Control Strategy Based on MMC and DFIG

The AC-DC hybrid distribution network based on modular multilevel converters (MMC) is beneficial for receiving distributed generation (DG), energy storage devices and DC loads. Therefore, the coordinated control of MMC and DG is very important for its safe operation. In this paper, doubly-fed induction generator (DFIG) are used as DG, and the control strategies of DFIG and MMC systems are analyzed. The system modeling research is carried out on a real time digital simulator (RTDS) platform. Then based on MMC control, an improved adaptive Crowbar circuit is proposed to improve inverter AC system low-voltage ride-through (LVRT) capability. The results show that the coordinated control strategy can better compensate the reactive power of the system to achieve LVRT, and verify the effectiveness of the control strategy adopted by the converter station.

A Robust Optimal Coordinated Droop Control Method for Multiple VSCs in AC–DC Distribution Network

AC–DC hybrid distribution network is viewed as a featured system to efficiently connect to all kinds of power sources and loads. In order to improve the system robustness, adapting to random fluctuations of renewable energy sources, this paper proposed a robust optimal coordinated control method for multiple voltage source converters (VSCs) in AC–DC distribution network. A robust optimization model was established to co-optimize the slopes of active and reactive power droop control of VSCs with the aim to minimize the total network loss, while ensuring the system security. VSC capacity coupling and power flow constraints were both considered. Then, to enhance the computational tractability of the problem, based on the column-and-constraint generation algorithm, ${V^2} - P$ and ${V^2} - Q$ were implemented in the droop control schemes holding the linear feedback characteristic so that the non-convex max–min sub-problem can be transformed into a mixed integer second-order cone (SOC) programming problem. Furthermore, to increase the accuracy of SOC relaxation in the sub-problem, a current limit strategy was designed where the maximum possible network loss was pre-identified to restrain the feasible range of branch current. Numerical experiments suggest that the proposed coordinated control method can improve the system security with little extra economic cost, and the reliability of the results is enhanced by the proposed current limit strategy.

Sequential Power Flow Calculation of AC/DC Hybrid System Considering Converter Station Loss and Its Capacity Constraints

In the power flow calculation of AC/DC hybrid distribution network, the loss and capacity of the converter station have a great influence on the power flow calculation, and the AC/DC alternately solves the power flow, which has the disadvantage of poor convergence performance. In this paper, the power flow model of AC/DC hybrid distribution network with VSC is established. The loss mechanism of the converter station is analyzed and the capacity limit of the converter station is fully considered. The sequential power flow algorithm with VSC AC-DC hybrid distribution network is proposed. Its convergence performance is improved. In the improved 33-node AC/DC hybrid power distribution network with multiple types of DG, the correctness, effectiveness and algorithm fastness of the algorithm are verified.

Fault Analysis of VSC Single-pole Ground Fault in Flexible AC/DC Hybrid Distribution Network

The stable operation of the flexible AC-DC hybrid distribution network is inseparable from the configuration of the relay protection device, while the fault analysis and simulation research is the basis for the protection scheme selection. This paper introduces the current situation of fault analysis and protection research of flexible AC-DC Hybrid distribution network, and set up a model of radiated flexible AC-DC hybrid distribution network on PSCAD/EMTDC platform, then deeply analyzed the grounding fault of VSC single-pole short circuit of DC side. According to the simulation results, the trend and characteristics of the transient state after the fault are studied, which lays a foundation for the research of the subsequent relay protection method and the configuration of the protection device.

A Fault Location Method for DC Lines Connected With DAB Terminal in Power Electronic Transformer

Power electronic transformer (PET) composed of modular dual active bridge (DAB) blocks has a promising future in ac-dc hybrid distribution network because of its energy routing ability. The key element that restrains the development of PET is the vulnerability of power electronic devices during faults, especially dc faults. DC faults (mostly line faults) have very short transient process and no obvious fundamental frequency, which means fault location is difficult to be realized for dc lines in distribution network. Considering that fault location plays an important role in realizing fast fault clearance and recovery, this paper analyzed fault characteristics of dc line faults with respect to the basic structure of modular DAB block and proposed a two-terminal fault location method based on transient voltages measured in dc line boundaries. To evaluate the validity of proposed method, numerical simulations with different fault conditions were implemented in PSCAD/EMTDC. The simulation results indicate that the proposed fault location method located the faulted point quickly and correctly.

Modelling and simulation of AC–DC hybrid distribution network based on flexible DC interconnection

AC and DC hybrid distribution network detailed modelling and multi-operation conditions of the simulation analysis will effectively support the distribution network project reliable and stable operation. However, at present, large-scale system-level simulation of AC–DC hybrid distribution network based on flexible DC is the lack of timeliness and practicability. Based on the real-time digital simulation platform RT-LAB, ACDC distribution network model is established, which includes ±20 kV flexible DC interconnection system, distributed photovoltaic, wind power, energy storage system, electric vehicle and the corresponding converter control detailed modelling. In view of the limitation of the balance of energy storage system, the flexible DC interconnection is applied to active distribution network, which can provide power supply when the power gap occurs. The conditions of consumptive mode by the energy storage system, power supply through flexible DC interconnection from external power grid were simulated and analysed. The model verifies the validity of the system application in distribution network. This study provides preliminary design reference for active distribution network construction.

Coordinating voltage regulation for an AC–DC hybrid distribution network with multiple SSTs

Coordinating voltage regulation for an AC–DC hybrid distribution network with multiple SSTs

Three-Stage Distributed State Estimation for AC-DC Hybrid Distribution Network Under Mixed Measurement Environment

The ac–dc hybrid distribution network is a credible path for the future evolution of distribution network. State estimation is a paramount foundation for the safe and stable operation of the complex distribution network. The application of the centralized state estimation method in an ac–dc distribution network has some obstacles, such as low computational efficiency, large communication capacity, and privacy protection problem. Based on the three-stage state estimation theory, this paper established a three-stage state estimation model for the ac–dc hybrid distribution network integrating supervisory control and data acquisition system and phasor measurement unit. The proposed three-stage estimation model achieves linearization of the nonlinear state estimation for the ac–dc hybrid distribution network. That is, the first and third stages simply solve the linear state estimation problem, and the second stage is a one-step nonlinear transform. Moreover, the alternating direction method of multipliers (ADMM) is applied to solving the distributed problem. Thus, an accurate and efficient three-stage distributed state estimation method for the ac–dc hybrid distribution network is proposed. In this method, ac subsystems and dc subsystems execute state estimation tasks based on local information and eventually achieve the overall consistency of the system state estimation through the transfer iteration of the boundary information, respectively. The combination of bilinear theory and ADMM ensures the convergence of distributed state estimation while improving computational efficiency. The simulation results verified the advantage of the proposal over the existing methods in many aspects.