Interconnected Distribution Network Research Articles

Flexibility-Constrained Energy Storage System Placement for Flexible Interconnected Distribution Networks

Configuring energy storage systems (ESSs) in distribution networks is an effective way to alleviate issues induced by intermittent distributed generation such as transformer overloading and line congestion. However, flexibility has not been fully taken into account when placing ESSs. This paper proposes a novel ESS placement method for flexible interconnected distribution networks considering flexibility constraints. An ESS siting and sizing model is formulated aiming to minimize the life-cycle cost of ESSs along with the annual network loss cost, electricity purchasing cost from the upper-level power grid, photovoltaic (PV) curtailment cost, and ESS scheduling cost while fulfilling various security constraints. Flexible ramp-up/-down constraints of the system are added to improve the ability to adapt to random changes in both power supply and demand sides, while a fluctuation rate of net load constraints is also added for each bus to reduce the net load fluctuation. The nonconvex model is then converted into a second-order cone programming formulation, which can be solved in an efficient manner. The proposed method is evaluated on a modified 33-bus flexible distribution network. The simulation results show that better flexibility can be achieved with slightly increased ESS investment costs. However, a large ESS capacity is needed to reduce the net load fluctuation to low levels, especially when the PV capacity is large.

Optimization scheduling strategy for flexible interconnected distribution network considering transformer offline

Based on the flexible interconnection technology with intelligent soft open point (SOP), new opportunities have arisen to address issues such as complex power flow and uneven load rates in existing distribution networks. However, most existing research focuses on load balancing strategies and N-1 fault tolerance, without fully considering the impact of flexible access and offline of distribution transformers on the losses in the distribution network. Therefore, this paper analyzes the impact of structural adjustments to the distribution transformer interconnection system on the comprehensive losses of the distribution network, and concludes that under low-load conditions, the offline of lightly loaded transformers optimizes the comprehensive losses of the distribution network. However, the offline of lightly loaded transformers leads to an increase in the remaining transformer load rates in the distribution station areas. To address this shortfall, this paper proposes an optimization scheduling strategy for flexible interconnected distribution networks considering transformer offline, by simultaneously utilizing shared energy storage on the DC side to balance transformer load rates and minimize losses. Finally, case studies based on an IEEE 18-node interconnected system are conducted to validate the proposed approach under two different scenarios. The results demonstrate that the proposed control strategy outperforms existing strategies by optimizing the comprehensive losses of the system, thereby enhancing its operational economy and reliability.

Low-voltage ride-through strategy for an integrated permanent magnet synchronous generator in a flexible interconnected distribution network

For the permanent magnet synchronous generator (PMSG) integrated into the flexible interconnected distribution network (FIDN), its low-voltage ride-through (LVRT) strategy needs to be designed to enhance the transient operation capability of the FIDN. The design of the LVRT strategy also needs to take the fault location of the FIDN into consideration. To deal with faults occurring on the integrated feeder of the PMSG, the PMSG needs to realize successful LVRT using the hardware protection equipment. To deal with faults occurring on the feeder adjacent to the integrated feeder of the PMSG, the PMSG needs to release its stored energy to temporarily increase its active power output, which is then supplied to the loads on the faulted feeder that are isolated from the fault through the soft open point (SOP). In this paper, a novel LVRT strategy designed for the PMSG integrated into the FIDN is proposed, which includes dual operation modes that are separately applied to different fault locations. If PMSG is on the faulted feeder, the DC-link voltage of the PMSG is maintained with the controllable resistive fault current limiter (CRFCL), while the maximized stored kinetic energy is reserved to enhance the generation efficiency during the LVRT. If PMSG is on the feeder adjacent to the faulted feeder, the control strategy of the converters of the PMSG is adjusted in response to the power regulation goal at the SOP. Meanwhile, the maximum releasable kinetic energy of the PMSG is considered when increasing its active power output. The feasibility and effectiveness of the LVRT strategy for the PMSG are verified based on the numerical analysis.

Stable Operation Domain Analysis of Back-to-Back Converter Based Flexible Multi-State Switches Considering PLL Dynamics

Flexible multi-state switches (FMSSs), also called soft open point (SOP), are regarded as the core devices in flexible interconnected distribution network. However, due to their more complex control structure and nonlinear characteristics, there is a tight and complex coupling relationship between the transmission power limit and stability of two (or more) converter sides, which brings difficulties to the planning and power flow dispatching control of flexible interconnected distribution networks. In this paper, taking the back-to-back converter based FMSS (BTBC-FMSS) as the research object, the dynamic equivalent circuit connected to the distribution network is constructed, and the transmission active and reactive power expression of the converter on both sides considering PLL dynamics is deduced. The variation rules of transmission power limits on both sides of the device with various structures and control parameters are summarized. Furthermore, the stable operation domain of the BTBC-FMSS is constructed, and the effective measures to improve the transmission power stability of the device are given according to the analysis of the stable operation domain. Finally, a simulation model is built in Matlab/Simulink to verify the effectiveness of the analysis results.

Future‐proofing city power grids: FID‐based efficient interconnection strategies for major load‐centred environments

AbstractThe flexible interconnection device (FID) offers significant advantages for interconnecting different distribution networks flexibly. This paper focuses on the significant advantages offered by the FID for interconnecting different distribution networks flexibly. It specifically delves into FID‐based multi‐voltage and multi‐substation distribution networks, proposing a preferable scheme applicable to major load‐centred cities. Beginning with an analysis of constructed FID‐based flexible interconnected distribution network projects, key configurations and features are summarized. Subsequently, typical configurations, electrical parameters, facilities, relevant power functionalities, and application scenarios of multi‐voltage multi‐substation distribution networks are outlined. Building upon this foundation, a suitable interconnection scheme tailored for current urban use is explored to meet the specific needs of load‐centred cities, while incorporating recent advancements in high‐power‐density IGCT technology. An EMT model of a 10 kV/10 MW IGCT‐based four‐substation distribution network is developed in PSCAD/EMTDC. Through thorough analysis under different conditions, the operational performance and benefits are evaluated, providing insights into the efficiency and resilience of the proposed FID‐based interconnection. Lastly, challenges and prospects are discussed from various perspectives to advance the development of FID‐based flexible interconnection solutions. This study aims to contribute to the advancement and implementation of robust interconnection solutions to meet the evolving needs of major load‐centred cities.

Optimal allocation strategy of SOP in flexible interconnected distribution network oriented high proportion renewable energy distribution generation

Focusing on the issues of high distributed generation in the distribution network, this paper presents an SOP capacity allocation method for flexible interconnected distribution networks based on sensitivity analysis. Firstly, the improved sensitivity analysis is used to determine the best SOP installation position; Then, the number of SOPs is set in advance, and the location and capacity of SOPs are optimized using the second-order cone programming algorithm, with the minimum annual cost of distribution network as the optimization goal. Finally, this optimization method is verified on the modified IEEE 33-node distribution system model. The results show that the application of the optimization strategy proposed in this paper can significantly reduce the annual operating cost of distribution network and improve voltage stability, and the proposed model can effectively adjust the location and capacity of SOP access, thus improving the economic benefits and reliability of active distribution network.

Review on development prospect of operation scheduling strategies for flexible interconnected distribution network in renewable energy-penetrated power system

As the renewable energy integrates in the power system, the interactions between source, load, and storage equipment in the distribution network become frequent. The original distribution network could not meet the growing demand of the power grid. The use of controlled power electronic devices for flexible interconnection transformation of the distribution network could help improve the controllability, reliability, and safety of the power system and promote the consumption of distributed power sources. It is an important means of evolving towards future intelligent distribution networks. Typical devices for flexible interconnection are discussed of the operating characteristics. In addition, the core technologies of flexible interconnected distribution network system are introduced from the view of point of dispatching. Finally, to achieve a wider application of flexible interconnected distribution networks, the development trend is prospected.

Robust fault recovery strategy for multi-source flexibly interconnected distribution networks in extreme disaster scenarios

To enhance the resilience of power distribution networks against extreme natural disasters, this article introduces a robust fault recovery strategy for multi-source, flexible interconnected power distribution networks, particularly under scenarios of extreme disasters. Initially, the comprehensive risk of system failure due to ice load on distribution lines and poles is fully considered, and a model for the overall failure rate of lines is constructed. This model addresses the diverse failure scenarios triggered by various meteorological conditions. Through the use of information entropy, typical extreme disaster failure scenarios are identified, and lines with high failure rates under these scenarios are determined. Subsequently, a box-type interval model is developed to represent the uncertainty in the output of distributed generation (DG), and on this basis, a robust fault recovery model for multi-source power distribution networks interconnected through soft open points (SOPs) is established, and use the Column and Constraint Generation (C&CG) algorithm to solve the problem. Finally, the fault recovery model and strategy proposed are validated through an illustrative example based on a modified IEEE 33-node interconnected system.

Two-Layer Cooperative Optimization of Flexible Interconnected Distribution Networks Considering Electric Vehicle User Satisfaction Degree

The scaled access of electric vehicles (EVs) exacerbates load fluctuations in distribution networks, which is not conducive to the stable and economic operation of the distribution networks. At present, user satisfaction degree is generally low. To avoid this problem, this paper proposed a two-layer cooperative optimization of flexible interconnected distribution networks considering EV user satisfaction degree. First, the EV user satisfaction degree model is established by considering EV users’ charging waiting time, charging power, and other indicators. At the same time, an EV charging mode switching model is constructed based on the number of EVs entering the network and their battery charge state. On this basis, the Monte Carlo algorithm is used to generate the results of the daily distribution of EV loads taking into account the user satisfaction degree, so as to improve the load ratio of the transformer in the distribution network. Further, a two-layer cooperative optimization of flexible interconnected distribution networks considering electric vehicle user satisfaction degree is developed with the daily operating cost of each network as the optimization objective. Finally, a flexible interconnected power distribution network consisting of three power distribution networks is used as an example for validation. The results show that this method is effective in improving EV user satisfaction degree and reducing the peak-to-valley ratio of the system load while taking into account the safe and economic operation of the distribution network, which greatly improves the reliability and economy of the operation of the flexible interconnected power distribution network.

An optimal scheduling of renewable energy in flexible interconnected distribution networks considering extreme scenarios

AbstractAs renewable power penetration gradually increases in hierarchical distribution networks, certain regions have started to lack the ability to consume. How to improve the consumption capacity of a hierarchical distribution network through optimal dispatching has become a hot topic in the current research on distribution system operation. Firstly, the article makes an accurate and rapid assessment of the consumption capacity of the station area through the limit scenario analysis method; secondly, based on the assessment results, the main network renewable power sources are prioritized for consumption, while the main network and the station area; thirdly, the main network renewable power sources are prioritized for consumption, considering the assessment results while the main network and the station area reach the optimal power flow simultaneously through a multi‐layer dispatching model. Then, through the flexible interconnection system, load balancing and energy optimization are performed for unconsumed distributed generation (DG) and incomplete energy storage system (ESS) in the station area to further improve the efficiency of renewable energy. Finally, the effectiveness of the proposed model is verified by simulation tests.

City of lights, city of pylons: Infrastructures of illumination in colonial Hanoi, 1880s–1920s

AbstractThis article traces the early stages of urban electrification in the French protectorate of Tonkin (in Vietnam’s north) from the late 1880s to the late 1920s. It focuses on Hanoi, where in 1895 the French entrepreneurs Hermenier and Planté secured a concession for lighting the streets of the soon-to-be capital of French Indochina. Before long, the city’s fast-paced development and the concomitant rise in demand for both public and private lighting necessitated contractual amendments and further capital investment in the upgrading of the power station and grid extensions. In 1902, Hermenier converted the business into a joint stock company named the Société Indochinoise d’Électricité with the aim of enabling further growth and geographical expansion. Contractual arrangements were frequently renegotiated and adjusted to new circumstances. However, electricity supplies kept lagging behind the fast pace of demand growth. During the post-First World War years of colonial economic expansion, power failures and blackouts became a routine occurrence and were a frequent target of press coverage. It was only in the late 1920s that electricity supplies improved and turned Hanoi into a city of lights. Although the majority of Vietnamese residents remained excluded from private electricity access throughout the colonial period, electric power quickly became a fact of everyday life for an emerging Vietnamese urban bourgeoisie and served as a marker of modern sophistication. Plans for an interconnected distribution network in the Tonkin delta subsequently also triggered hopes for an electrified future for the countryside.

Active power control strategy of DFIGs in flexible interconnected distribution network

In the development of new-type power system, the flexible interconnected distribution network becomes a better inclination. With the continuous increase of wind power generation in the flexible interconnected distribution network, the frequency stability issue caused by the high randomness of wind power generation has become increasingly prominent. This paper suggests an advanced active power strategy of double-fed induction generators (DFIGs) based on modifying power tracking curves in the interconnected distribution network for smoothing out the fluctuations of the system frequency. To this end, the various power tracking operation is used to participate in power smoothing instead of maximum power tracking operation so as to fully utilize the large rotor inertia of the double-fed induction generators for smoothing out the fluctuations of the power output of double-fed induction generators and system frequency. The control gain for the various power tracking operation is determined by the instantaneous system frequency deviation and rotor speed of the DFIG. Based on the modified IEEE 14 power system, simulation results successfully proved that the suggested advanced active power strategy is benefit for reducing the fluctuations of the output power of the DFIG and suppressing the maximum system frequency deviations with the scenario of continuously varying wind speed conditions.

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.

Feasibility analysis of PV and energy storage system integration for flexible distribution networks: A moment-based distributionally robust approach

With the requirements of carbon neutrality and scientific and technological progress, massive distributed renewable energy resources have been integrated in distribution networks. The flexible interconnected distribution networks (FDNs) provide an effective way to avoid the negative effects of power quality, network loss and relay protection. This paper proposes a feasibility analysis model of distributed photovoltaic power generations (PVs) integration for FDNs based on distributionally robust. It uses moment-based ambiguity sets to model the uncertainty of PV outputs. Based on the circular constraint linearization method and duality theory, the solution method of the above model is designed. In this paper, two typical FDN structures are considered, namely, the closed-loop flexible interconnected structure between feeders and the flexible interconnected structure between feeder loops. The hosting capacity of the PV and energy storage system of the two structures is analyzed in case study, and the effectiveness of the proposed model and method is verified.

Sensitivity Model-Based Optimal Decentralized Dispatching Strategy of Multi-Terminal DC Links for the Integration of Distributed Generations in Distribution Networks

This study proposes a novel decentralized operation strategy for a multi-terminal direct current (MTDC) link system to control the power flow between interconnected distribution networks. The proposed strategy is divided into two stages. First, by applying a curve-/surface-fitting technique to the network topology data, the formulas for the voltage and total line losses in the network are derived. Subsequently, the active/reactive power set-points of the MTDC link are optimized to minimize the network losses and balance the injected power from the upstream grid. The optimization model with a quadratically constrained quadratic problem is relaxed as a second-order constrained programming model using the proposed substitution process. The proposed strategy is implemented solely based on the information at the point of common coupling of the MTDC link and does not require the acquisition of any real-time data related to the distributed generator output and load demand. The effectiveness of the proposed method is verified by comparing it with a conventional centralized operation method using modified IEEE-33 test networks.

An auxiliary-power-compensation droop control strategy for multi-terminal flexible interconnected distribution network

The emergence of multi-terminal flexible interconnected distribution network can effectively improve the consumption level of new energy and enhance the power supply quality of distribution network. At present, the droop control strategy is widely used in the modular multilevel converter (MMC) in multi-terminal flexible interconnected distribution network, which can adjust and distribute DC voltage and power autonomously according to the droop characteristic curve. However, this type of strategy is challenging to cope with the sudden working conditions. These conditions include the power reversal, the outage of station and large fluctuation of new energy sources, which can cause abnormal fluctuation of DC voltage. Therefore, this paper proposes a control strategy based on the auxiliary power compensation in the flexible interconnected distribution network to solve the problem. This compensation strategy introduces an auxiliary power reference and simplifies the control loop, effectively suppressing the power fluctuation and optimizing the dynamic performance of the system. Compared with the traditional improved droop control strategy, this strategy does not need to change the parameters of the control system, has a stronger ability to suppress the voltage fluctuation, and the response speed of the control system is faster. The reliability and effectiveness of the proposed control strategy are verified by simulation.

Flexible Interconnected Distribution Network with Embedded DC System and Its Dynamic Reconfiguration

Flexible interconnection transformation of a distribution network using all-control power electronics technology will help to overcome the technical bottleneck of distributed generation (DG) penetration and high-quality power supply in the traditional distribution network. However, the flexible interconnection form that uses back-to-back power electronic converters to replace tie switches has disadvantages such as high cost and large footprint, so its mass promotion and application are bound to be limited. Therefore, a new topology of flexible interconnected distribution network with embedded DC system (FDN+EDC) with a better economy is proposed. Firstly, the topology of FDN+EDC is introduced to improve the controllability level of the distribution network under high percentage DG penetration, and is compared with the conventional flexible interconnection method. Secondly, in order to realize the coordinated optimal economic operation of source, network and load in the FDN+EDC, a dynamic reconfiguration model of switches status and continuous adjustment of flexible interconnection device (FID) are proposed. Then, to achieve a fast and globally optimal solution of the model, a hierarchical coordinated optimization strategy is designed based on an improved particle swarm algorithm for the optimization of discrete and continuous variables. Finally, the validity of the algorithm is verified, and an 88-node FDN+EDC is constructed to verify the advantages of the proposed embedded DC flexible interconnection topology and realize the joint optimal dispatch of network-source-load.

The Impact of Distributed Energy Storage on Distribution and Transmission Networks’ Power Quality

This study investigates the effect of distributed Energy Storage Systems (ESSs) on the power quality of distribution and transmission networks. More specifically, this project aims to assess the impact of distributed ESS integration on power quality improvement in certain network topologies compared to typical centralized ESS architecture. Furthermore, an assessment is made to see if the network topology in which an ESS position supports its ability to restore node voltage magnitude within acceptable ranges. The power quality of a benchmark interconnected distribution and transmission network was determined using NEPLAN software. Following that, twelve variants of the benchmark were modeled, each with a different ESS integration architecture and (or) topology. Their power quality performance was compared to that of a benchmark network in addition to several cross analyses to determine the relative impact on power quality within the context of their respective ESS integration methodologies. The findings of this study buttress the understanding that the distributed ESS integration architecture within the distribution network topology, where the majority of consumer loads are connected, provides the strongest case for voltage magnitude power quality compensation, as required by the UK Electrical System Grid Code’s 5% rated node voltage compliance processes regulation.

A fault recovery strategy of flexible interconnected distribution network with SOP flexible closed-loop operation

With the continuous exploration of the application value of flexible interconnected distribution network, Soft Open Point (SOP) as its core device, how to fully explore the self-healing control support capability of SOP and improve its power supply reliability becomes particularly important. Aiming at the problem of fault recovery of flexible interconnected distribution network with multiple AC power supply areas, firstly, the topology of flexible interconnection of distribution network and the fault recovery model of SOP are described. Secondly, the stochastic nature of wind power, photovoltaic output and load is considered, and then the chance-constrained programming is introduced to deal with. Then, aiming at the coordinated operation of flexible closed-loop network in multiple AC power supply areas, and introducing the constraint on the acting number of switches, a stochastic optimization model of flexible interconnected distribution network fault recovery is established, and then the model is transformed into a deterministic mixed integer second-order cone programming model, which can be solved by CPLEX solver. Finally, the proposed model has been tested on a modified hand-in-hand IEEE33 system with dual-terminal SOP, verifying that the strategy proposed in this paper can effectively solve the fault recovery problem of flexible interconnected distribution network with multiple AC power supply areas considering uncertainty, and showing that this strategy can maximize the self-healing capability of distribution system.

Risk assessment of cascading failure of distribution network with flexible multi-state switch

With the implementation of carbon emission reduction policies in various countries, the penetration rate of distributed generation (DG) has gradually increased. Flexible multi-state switch (FMS), has attracted more attention as a new power device. This paper analyzes the cascading failure risk of flexible interconnected distribution system caused by the hidden failure of FMS protection. Firstly, based on the physical structure of FMS, the transient characteristics of AC feeder failure and the control and protection strategy of FMS are analyzed. Then, the uncertain factors causing protection misoperation are put forward and modelled. On this basis, the mechanism of cascading failures formation is analyzed. The risk index of cascading failure in flexible interconnected distribution network and the optimal load shedding based on failure consequences are established. Finally, the risk of system cascading failure is evaluated by the improved IEEE 33-bus system. Also, the influence of fault location and DG penetration on the risk is analyzed.