AC-DC Distribution Network Research Articles

Dispatchable region for distributed renewable energy generation in reconfigurable AC–DC distribution networks with soft open points

The dispatchable region (DR) describes the ability of a power system to adapt to the variability of distributed renewable energy (DRE) generation. Switchable devices such as soft open points (SOPs) and tie switches enable the AC–DC distribution network to flexibly cope with DRE power fluctuations. Currently, research on DRs seldom considers the effects of SOPs and reconfiguration with tie switches in AC–DC distribution networks, which prevents accurate evaluation of DR for this type of network. To bridge this gap, this paper formulates DRs for DRE generation in an AC–DC distribution network considering SOPs and reconfiguration with tie switches. The original model for deriving DRs is typically nonconvex, and the DR boundaries are intractable because of nonlinear power flow equations and binary variables of tie switches. Accordingly, this paper employs the outer polyhedral approximation and convex hull relaxation to approximate the original DR. An efficient adaptive constraint generation algorithm is developed to search for boundaries of the approximated DR. The numerical results show that the proposed model can effectively adapt to the DR of reconfigurable AC–DC distribution networks with SOPs and verify the accuracy and computational efficiency of the proposed method.

A new iterative mixed integer linear programming based real time energy efficient management of AC-DC distribution networks

In the smart grid era, AC-DC distribution networks are evolving for integrating massive amounts of distributed energy resources, especially battery associated solar power generation units, into distribution networks in most energy efficient ways. Performing real time energy management for such networks is challenging due to presence of different power electronic converters, grid-edge devices and voltage controlling equipment. Existing studies either focused on developing offline day ahead methods, or shortsighted greedy strategies by neglecting the offline beneficial attributes in real time optimization frameworks. On the contrary, this article proposes a novel real time AC-DC distribution network energy management portfolio by merging load control and conservation voltage reduction techniques to achieve energy efficient operation of the network by regulating the operation of the controllable resources. Initially, the overall optimization problem is developed as a mixed integer non-convex programming by modelling the offline benefits as time coupled stochastic expressions in real time optimization platform. The non-convex expressions are later simplified by adopting different linearization techniques and the entire energy management framework is revised as a simple mixed integer linear programming (MILP). The original problem is decoupled for each time step by simplifying the time coupled expressions using virtual queues and Lyapunov functions. Additionally, a new iterative MILP algorithm is proposed to solve the revised energy management problem with less computation complexity and time. Unlike the previous solution methods, the proposed algorithm initializes the variables by their median values and handles the infeasibilities by adding tangential cutting plane constraints and penalty variable minimization objective. Demonstrating on 33-bus AC-DC distribution network, the superiority of the proposed real time energy management process and iterative MILP algorithm is established after comparing with standard energy management portfolios and existing solution methods.© 2017 Elsevier Inc. All rights reserved.

Power Quality Enhancement by DC Distribution

Most of the electric equipment used in residential buildings operate with electric energy in the form of direct current (dc) in their internal circuits. The advent of many modern equipment in residential buildings which operate on dc has necessitated an in-depth study of their impact on the distribution system. This paper presents an analysis of the performance of distribution system if these appliances are directly supplied by dc. As a part of this investigation a prototype conventional alternating current (ac) distribution system is analysed for different combinations of utility and load conditions by simulation. A hybrid ac-dc distribution network with minimum conversion stages is proposed.

A Collaborative Governance Strategy for Power Quality in AC/DC Distribution Networks Considering the Coupling Characteristics of Both Sides.

The interactions between power quality in the AC-DC distribution network segments contribute to the distributed propagation of power quality anomalies throughout the entire network. Focusing on the photovoltaic multifunctional grid-connected inverter (PVMFGCI), this study deeply explores a collaborative governance strategy for optimizing regional power quality. Initially, the analysis dissects the DC ripple generation mechanism corresponding to harmonics and asymmetry in AC subnetwork voltages. Subsequently, a strategy is proposed for partitioning comprehensive control regions for AC-side power quality, taking into account photovoltaic governance resources based on insights from photovoltaic control realms and power quality classifications. Further, a collaborative allocation model for governance resources incorporating active optimization potentials of grid-connected converters is established based on the governance capabilities and residual capacities of PVMFGCI. Finally, the effectiveness of the proposed approach is validated through a MATLAB-based example analysis.

Two-stage robust optimal operation of AC/DC distribution networks with power electronic transformers

Power electronic transformers (PET) are a new type of power electronic equipment with a multi-port flexible dispatch function, which can play the role of a power hub in a system composed of multiple AC-DC hybrid distribution grids for interactive sharing of power in multiple regions. In this study, a two-stage robust optimization operation model of a hybrid AC-DC distribution network with PET is proposed based on PET power transmission and transformation characteristics. The stochastic uncertainty of the distributed renewable energy output in the AC-DC grid is handled by a two-stage robust optimization method to determine the minimum total system operation cost under the worst case of distributed renewable energy output. Finally, a constrained column generation algorithm is used to solve the two-stage robust optimization model in the min-max-min form and verifies the validity of the model in this study.

A Backward-Forward Sweep Based Power Flow Algorithm for Generalized AC-DC Distribution Network with Distributed Generations

Developing a power flow (PF) methodology for the AC-DC distribution network is complex but important, especially due to the involvement of power converters. In this work, a Backward-Forward Sweep (BFS) approach developed based on graph theory and matrix algebra is proposed for solving the PF for both the radial and mesh configured AC-DC distribution networks with distributed energy resources (DERs). For this purpose, first, the per-unit current, voltage, and power model of the 3- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> pulse width modulated (PWM) rectifier, 3- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> PWM inverter, and DC/DC power converter have been developed. Then, a generalized sensitivity matrix has been developed for computing the loop breakpoint, PV breakpoint, and V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{dc}$</tex-math></inline-formula> breakpoint injections simultaneously allowing the approach to work for both mesh and radial networks with DERs modeled as PQ, P, PV, and V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{dc}$</tex-math></inline-formula> buses. The major benefits of the proposed algorithm are that it can incorporate various functional/control modes of the aforementioned converters. The feasibility of the proposed power flow algorithm has been explored on AC-DC distribution test networks. The test results show that the approach is feasible and precise.

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.

Risk Assessment of AC/DC Hybrid Distribution Network considering New Energy and Electric Vehicle Access

In view of the operational risk issues such as safety and economy caused by the connection of new energy and multiple types of electric vehicles (EVs) to the AC–DC distribution network, an AC–DC distribution network operational risk assessment method that takes into account multiple risk factors is proposed. First, a probability distribution model of scenery output and EV timing is constructed, and the starting charge state of multiple types of EVs is replaced by the number of daily driving miles subjectively set; then, based on the complex network theory, timing safety indicators, such as voltage overrun risk and branch power overload operation risk, are proposed, and the economic risk is established according to the economic operation of the distribution grid; Furthermore, a risk assessment matrix for grid-connected EVs with different capacities is constructed, and the principal component analysis (PCA) method is used to reduce the dimension of the risk assessment matrix and calculate the objective weight coefficient; finally, taking the improved IEEE 33 node AC/DC power distribution system as an example, the comprehensive risk evaluation based on PCA is compared with the traditional one, and the results show that when the safety and economic risk factors are considered at the same time, the operation risk in a certain range has a downward trend when the proposed method is adopted, which has a positive guiding significance for the planning of EV capacity in a certain area.

A novel unbalanced power flow analysis in active AC-DC distribution networks considering PWM convertors and distributed generations

Recent major achievements in power electronic technologies have expedited the development and proliferation of small-scale distributed generations (DGs). AC-DC distribution network creates a novel paradigm to attain the utmost benefits through smoothing the operation of different types of DGs. However, due to the asymmetrical electrifying of single-phase loads with different consumptions, variations in output power of the small-scale DGs connected to each phase, and disparate line parameters of the network, operation under unbalanced condition is inevitable. Therefore, performing AC-DC load flow analysis considering an unbalanced condition is indispensable to estimate the system states. In this paper, a novel direct approach based on backward-forward sweep (BFS) algorithm is proposed for power flow analysis in AC-DC distribution networks that applies Clarke’s transformation to model the operation of pulse width modulation (PWM) converters at unbalanced operation condition. Different control strategies including input power control (IPC), input output power control (IOPC), and output power control (OPC) are considered in the proposed formulation. Furthermore, to investigate the impacts of DGs on AC-DC distribution networks, an effective approach based on sensitivity matrix calculation is proposed to approximately update the outputs of DGs in each iteration. To evaluate the performance of the approach and examine its efficiency, the proposed formulations are applied to a hypothetical 123-bus AC-DC test system, containing different type of DGs and power converters, and compared with a heuristic method. The simulation results verify the effectiveness and applicability of the proposed method in load flow analysis of AC-DC distribution networks in unbalanced condition.

A fuzzy coordinated energy management strategy for energy storage units in DC multiport energy router

As the proportion of new energy in power systems continues to increase, the widespread use of solar and wind-based distributed generation (DG) power in DC distribution networks will face problems such as new energy consumption, grid stability, and economical benefits in the future. At present, energy management strategies for photovoltaic (PV) grid-connected power generation systems are mainly researched in the field of AC–DC distribution network. This paper provides an advanced energy management strategy of the DC multiport energy router. Considering the battery state of charge (SOC), real-time electricity price (EP), and user-side electricity charge (EC), an energy management scheme based on the fuzzy coordinated control strategy is designed to improve the utilization of energy storage batteries and the economic benefits. A 20 kVA multi-port DC energy router simulation platform is established and verified in MATLAB/Simulink. The results show that the energy management strategies based on fuzzy coordinated controller can solve the multi-objective problem. Compared with traditional control strategies, the fuzzy collaborative energy management strategy has a more comprehensive optimization effect.

Analysis and design of energy storage capacity of AC-DC hybrid power distribution unit with energy storage to improve the reliability of power grid

Based on the development of AC-DC distribution network, a new AC-DC distribution device with energy storage structure is designed in this paper. This paper first analyzes the existing AC-DC power distribution equipment and network reliability assessment methods. On this basis, the design is put forward, the energy storage link is placed at the DC high voltage side. The constraints of energy storage as an critical load backup power supply are defined and the mathematical model is established. In this paper the timing Monte Carlo method is used to calculate the reliability of the device and the power grid. The model in this paper can be applied to a variety of distribution network structure and energy storage scenarios, with high accuracy. It is concluded that the AC-DC distribution unit has remarkable effect in improving reliability.

Optimal operation of AC–DC distribution network with multi park integrated energy subnetworks considering flexibility

In order to improve the utilisation rate of renewable energy and flexibility of the system, the AC–DC distribution network with multiple park integrated energy subnetwork (PIESN) based on multi-port power electronic transformer (MPPET) is proposed. First, a power-sharing model of interconnected PIESNs based on MPPET is built, in which the port power of MPPET can be controlled. Then the flexibility demand and supply of electricity, heat and cooling are analysed and a flexibility-sharing model among multi-PIESN is established. Multi-energy flexibility conversion mechanism is proposed and the flexibility index is established. Moreover, the optimal operation problem is formulated and solved. The objective function is the comprehensive operation cost, which includes generation cost, energy cost of PIESN, network loss cost, MPPET loss cost, carbon emission cost and electricity storage system operation cost. Finally, simulation on the modified IEEE33 system shows that the proposed model can make the system be in an adequate flexibility, improve the utilisation rate of renewable energy, reduce load shedding and realise the power sharing among multi-PIESNs.

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.

A Network-Topology-Based Approach for the Load-Flow Solution of AC–DC Distribution System With Distributed Generations

The ac–dc distribution systems have recently gained huge popularity due to advancements in power converters, high penetration of renewable energy resources, and wide usages of dc loads. However, load flow in such systems is a challenging task due to nonlinear characteristics of power converters. This paper presents a novel load-flow algorithm for ac–dc distribution systems, utilizing the concept of graph theory and matrix algebra. Four developed matrices, loads beyond branch matrix, the path impedance matrix, the path drop matrix, and the slack bus to other buses drop matrix, and simple matrix operations are utilized to obtain load-flow solutions. These matrices reveal the network topology and relevant information about the behavior of ac–dc distribution network during load-flow studies. In contrast with traditional load-flow methods for HVDC systems, the proposed technique does not require any lower–upper decomposition, matrix inversion, and forward–backward substitution of the Jacobian matrix. Because of the aforementioned reasons, the developed technique is computationally efficient. The proposed method has been tested using several case studies of ac–dc distribution network, which includes different operating modes of various power converters. Results show the feasibility and authenticity of the proposed method.

Utilising a Lagrangian approach to compute maximum fault current in hybrid AC–DC distribution grids with MMC interface

Hybrid AC–DC networks are transforming high-voltage transmission and medium-voltage distribution grids by embracing the advantages of both AC and DC systems, which facilitates the inclusion of renewable energy sources and distributed generation. As modular multilevel converters (MMCs) are vastly employed in such hybrid networks, determining their maximal fault current in worst-case scenario is a critical design factor for planning and implementation of a reliable protection scheme. This study develops a novel mathematical framework that applies a Lagrangian energy method to calculate the maximal fault magnitude. This method allows to account for converter's internal energy and compute its impact on the amplitude of the fault current. It is shown when the converter is interfacing weak AC sources with high internal impedance such as wind farms or solar farms, dumping the internal energy of the converter into the fault is the salient contributing factor of the fault magnitude. Furthermore, to distinguish and classify the output overcurrent as either ignorable transients or destructive faults, a perceptron with sigmoid threshold is employed. The model is verified using a simulated medium-voltage hybrid AC–DC distribution network.

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.

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.

A Novel Method of Core Saturation Suppression for the AC+DC Distribution Network Transformer

With the rapid development of smart grids, the interconnection between the grid and distributed renewable energy is the inevitable trend of future study. Because of the existence of the DC source in the combined AC-DC distribution network, the transformer iron core is easily saturated generating lots of harmonics and increasing the loss of the transformer. This paper presents a novel method based on inductive filtering technology of core saturation suppression of the transformer in the combined AC-DC distribution network, this novel method can suppress the harmonics caused by the flux saturation and forbid the harmonics intruding into the ac grid. In the end we build the simulation model to prove the correctness and practicability of this novel method.