Soft Open Point Research Articles

Distribution System Reconfiguration with Soft Open Point for Power Loss Reduction in Distribution Systems Based on Hybrid Water Cycle Algorithm

In this paper, the role of soft open point (SOP) is investigated with and without system re-configuration (SR) in reducing overall system power losses and improving voltage profile, as well as the effect of increasing the number of SOPs connected to distribution systems under different scenarios using a proposed hybrid water cycle algorithm (HWCA). The HWCA is formulated to enhance the water cycle algorithm (WCA) search performance based on the genetic algorithm (GA) for a complex nonlinear problem with discrete and continuous variables represented in this paper by SOP installation and SR. The WCA is one of the most effective optimization algorithms, however, it may have difficulty striking a balance between exploration and exploitation due to the nature of the proposed nonlinear optimization problem, which mostly causes slow convergence and poor robustness. Consequently, the HWCA proposed in this paper is an efficient solution to improve the balance between exploration and exploitation, which in turn leads to improving the WCA’s overall performance without the possibility of getting trapped in local minima. Several cases are studied and conducted on an IEEE 33-node and the IEEE 69-node to investigate the real benefit gained from using SOPs alone or simultaneously with the SR. Based on the obtained results, the proposed HWCA succeeds in enhancing the performance of the proposed test systems considerably in terms of loss reduction (e.g., 31.1–63.3% for IEEE 33-node and 55.7–82.1% for IEEE 69-node compared to the base case) and voltage profile when compared to the base case while maintaining acceptable voltage magnitudes in most cases. Furthermore, the superiority of the proposed method based on the HWCA is validated when compared with the GA and WCA separately for both test systems. The obtained results show the outperformance of the proposed HWCA in attaining the best optimal solution with the least number of iterations.

A Chance-constrained Optimization Combined with Soft Open Points and Volt/Var Control under Uncertainties in Distribution Networks

With more and more photovoltaics (PVs) integrated into distribution networks, their drastically fluctuating nature may lead to voltage limits violation. However, the existing studies only focus on capacitor bank dispatching, which cannot address the drastic fluctuation of PV outputs. To adapt to the fast and continuous variation of PVs outputs in the distribution networks, this paper proposes a joint optimization of capacitor banks (CBs) and soft open points (SOPs). A chance-constrained model is established including the nonlinear equations of SOPs. Then, the second-order conic is applied to SOPs equations for efficient and accurate solving of the model. Finally, a cluster satisfying the given confidence level is formed by a proposed heuristic method to improve calculation efficiency. The numerical simulations on distribution networks show the efficiency and accuracy of the proposed algorithm over the existing methods.

Simultaneous siting and sizing of Soft Open Points and the allocation of tie switches in active distribution network considering network reconfiguration

Soft Open Points (SOPs) are power electronic devices and emerging flexible interconnections. The active and reactive power flow flexible controllability of SOP facilitates to improve the performance characteristics of the networks. To evaluate the contribution of SOPs to operational characteristics improvement, instead of a fixed number and an optimized configuration of SOPs is necessary. In addition to optimal siting and sizing of SOPs in distribution networks, considering the smart switching capability of SOPs compared to conventional switching through tie switches is important. This issue affects the decision of simultaneous investment for these types of equipment. Existing studies have not considered the coordinated presence of SOPs and candidate tie switches in a planning scheme for allocating them simultaneously. Here, a stochastic scenario-based planning model is proposed to determine the optimal sites and sizes of SOPs and the allocation of tie switches simultaneously while obtaining the optimal operation strategy of these devices during the network reconfiguration process. The model is formulated using the second-order cone programming approach. The optimization problem is solved considering renewable power production scenarios and different loading conditions. Finally, the proposed model is conducted on the IEEE 33-bus and 69-bus systems, and the results are presented.

Two-layer operation optimization of concentrated solar power with thermal energy storage system and soft open point

In order to improve the dispatching and grid-connected capacity of new energy, enhance the comprehensive economic benefits, and reduce the voltage offset and fluctuation of the distribution network, this paper proposes a two-layer operational optimization model of concentrated solar power (CSP) with thermal energy storage system (TESS) and soft open point (SOP). Firstly, according to the demand of load, the upper-layer distribution network formulates the optimal scheduling strategy of photovoltaic (PV), wind power (WP), CSP, energy storage system (ESS) and power purchase through the schedulability of CSP with TESS. Secondly, the upper-layer scheduling results are connected to the lower-layer distribution network, and coordinated control is carried out through SOP, on-load tap changer (OLTC) and capacitor bank (CB). Then, the lower model is converted into a linear model, a quadratic cone model and a rotated quadratic cone model, followed by absolute value removal, so as to rapidly and accurately produce the solution of the model. Finally, the model is analyzed on the improved IEEE33 node system. The results show that the proposed model in this paper not only ensures the economy of the power grid but also improves the stability of the voltage.

Multi-stage expansion planning of energy storage integrated soft open points considering tie-line reconstruction

With the rapid development of flexible interconnection technology in active distribution networks (ADNs), many power electronic devices have been employed to improve system operational performance. As a novel fully-controlled power electronic device, energy storage integrated soft open point (ESOP) is gradually replacing traditional switches. This can significantly enhance the controllability of ADNs. To facilitate the utilization of ESOP, device locations and capacities should be configured optimally. Thus, this paper proposes a multi-stage expansion planning method of ESOP with the consideration of tie-line reconstruction. First, based on multi-terminal modular design characteristics, the ESOP planning model is established. A multi-stage planning framework of ESOP is then presented, in which the evolutionary relationship among different planning schemes is analyzed. Based on this framework, a multi-stage planning method of ESOP with consideration of tie-line reconstruction is subsequently proposed. Finally, case studies are conducted on a modified practical distribution network, and the cost–benefit analysis of device and multiple impact factors are given to prove the effectiveness of the proposed method.

Bi-stage operation optimization of active distribution networks with soft open point considering violation risk

With a high proportion of distributed renewable energy source (DRES) integrated into the electrical distribution network, the problems of voltage violations and branch overload, stemming from the randomness and volatility of DRES, have gradually become substantial. To solve such issues, a bi-stage optimization model is proposed for the active distribution networks with soft open point (SOP), which considers the violation risk, thus enhancing the economics and security of distribution network operation. Stage 1 corresponds to a reconfiguration model focusing on the economics and safety of operation, while stage 2 refers to a stochastic optimization model that manages the coordination and optimization of multiple adjustment methods. Meanwhile, to simulate the output uncertainty of distributed power supply, optimal scenarios are constructed based on Wasserstein distance. To limit the computational complexity, a hybrid algorithm Lévy flight and chaos disturbed beetle antennae search based on grey target decision technology and second-order cone optimization was used to solve the proposed model. Finally, the correctness and superiority of the proposed bi-stage optimization model are verified on the modified IEEE 33 and 123-bus distribution systems. The simulation results show that compared with the original system, the economic risk is reduced by 7.51% and 96.92%, and the security risk is reduced by 69.64% and 76.56%, respectively.

Maximum Hosting Capacity of Photovoltaic Generation in SOP-Based Power Distribution Network Integrated With Electric Vehicles

Photovoltaic generation is playing a significant role in realizing the target of carbon neutralization. This article proposes a two-stage robust optimization model to assess the maximum hosting capacity of photovoltaic generation in power distribution network (PDN) considering the integration of soft open point (SOP) and electric vehicles (EVs). Convex relaxation method is applied to convert the two-stage robust optimization model to the form of mixed-integer second-order cone programming. A solving strategy based on the column-and-constraint generation algorithm is developed to solve the model, in which linearized difference-of-convex strategy is proposed to deal with the relaxation gap. Case studies are carried out on the modified IEEE 33-bus PDN integrated with SOP and EVs, and the results show the advantages of the proposed model and solving strategy.

Coordinated Control Strategy and Physical Integration of a Multistation Integrated System

The integration of distributed photovoltaic and new energy vehicles could further aggravate the over-voltage, over-load, and three-phase voltage unbalance in distribution networks. Multistation integrated system (MSIS) can resolve the aforementioned power quality issues. In this article, a new topology for a MSIS is proposed, which uses soft open point as the key equipment of the system and adopts the hierarchical power supply mode for a data center, electric vehicle charging pile, and other function stations. Based on the topology of the MSIS and smooth switching of the converter control mode, a coordinated control strategy is proposed to control the working mode of each function station and further optimize the system operation conditions. The small signal model of the MSIS is derived, which can provide theoretical support for the stable operation of the system. The simulation results of two different application scenarios verify that the proposed topology of the MSIS and coordinated control strategy effectively improve the voltage level of the distribution network, enhance the reliability of sensitive load power supply, and ensure safe and reliable operation of the MSIS. Finally, the fusion and integration functions of the MSIS are tested and verified. The on-site demonstration indicated a mutual win–win situation for the power supply company, energy supplier, equipment provider and users.

Coordinated allocation of soft open point and converter‐based energy storage systems in PV penetrated active distribution networks

Coordinated allocation of soft open point and converter‐based energy storage systems in PV penetrated active distribution networks

A Two-Layer Control Strategy for Soft Open Points Considering the Economical Operation Area of Transformers in Active Distribution Networks

A soft open point (SOP) can improve the flexibility of power control and thus further enhance the reliability and economy of distribution networks. However, existing SOP regulation strategies have mainly focused on feeder imbalance regulation and line loss optimization in medium-voltage distribution networks, and the loss of SOPs and distribution transformers are often ignored, which has poor applicability in low-voltage distribution network (LDN). Therefore, a novel two-layer control strategy is proposed for SOPs working in LDN, which can improve economy and reliability of the system simultaneously. In terms of the system layer, the proposed load regulation strategy considering the economical operation area of the transformers can improve the system economy while realizing load imbalance regulation. In terms of the equipment layer, the proposed peer-to-peer control of each port in the SOP can effectively suppress the voltage fluctuation on the DC side. Thus, the system layer commands can be quickly tracked and well conducted. Finally, the feasibility and effectiveness of the proposed two-layer control strategy are verified by simulation and experimental results.

Peer-to-Peer electricity trading of interconnected flexible distribution networks based on Non-Cooperative games

With the integration of power electronic devices represented by soft open points (SOPs), distribution networks have gradually evolved into interconnected flexible distribution networks (FDNs). Considering the deregulation of electricity market and user privacy, multiple stakeholders have participated in the operation of FDNs. Peer-to-peer (P2P) electricity trading is promising to alleviate operational problems of interconnected FDNs. As multiple regions pursue the maximum profits individually, non-cooperative game methods can be utilized to realize fair profit allocation in P2P trading. In this paper, a non-cooperative game-based P2P trading method is proposed to meet the electricity trading needs of multi-region interconnected FDNs. First, based on non-cooperative games, a two-layer P2P electricity trading framework is established to realize cost reduction and voltage profile improvement of multi-region interconnected FDNs. Then, a P2P trading adjustment mechanism is designed to improve the operational profits of SOP, in which spatial active power trading adjustment, temporal dispatching of energy storage (ES) link and reactive power support are incorporated. Finally, the effectiveness of the proposed method is verified based on a practical distribution network with four-terminal SOP in Tianjin. The results show that the proposed P2P electricity trading method can promote the economic operation performance of interconnected FDNs and improve the operational profit of SOP.

Maximization Approach of Hosting Capacity Based on Uncertain Renewable Energy Resources Using Network Reconfiguration and Soft Open Points

Nowadays, the continuous increase of renewable energy integration in power systems has become a global demand, especially after the urgent need to reduce the greenhouse gases resulting from fossil fuel fuels. Therefore, various methods have been presented to increase the hosting capacity (HC) that increases the system's ability to integrate more renewable energy resources (RERs) without exceeding the system's operational limits. In this paper, an optimization-based approach is proposed for assessing and maximizing the HC via network reconfiguration (NR) and/or soft open points (SOPs) techniques. The uncertain nature related to photovoltaic (PV) and wind turbine (WT) is considered and simulated based on hourly historical data in addition to taking into account the load variability. Various case studies are considered to evaluate the impact of the NR and SOPs combination on maximizing the system HC. The proposed approach has been tested with the standard IEEE 33-bus and IEEE 69-bus distribution systems. The obtained results confirmed the effectiveness of the proposed approach in maximizing the hosting capacity.

A Robust Mixed-Integer Convex Model for Optimal Scheduling of Integrated Energy Storage—Soft Open Point Devices

Soft open points (SOPs) are power electronic devices which can replace conventional normally open points in distribution networks. SOPs enable full control of active power flow between the interconnected feeders and can inject reactive power at each node to which they are connected. SOPs integrated with energy storage (ES) have been recently proposed to realize both spatial and temporal flexibility in active distribution networks. The flexibility provided by integrated ES-SOP devices will allow network operators to run their networks closer to their limits, but only if there is appropriate management of the uncertainty arising from demand and renewable generation. The only existing model of an ES-SOP uses nonconvex nonlinear equations, neglects uncertainty, and represents converter losses in an oversimplistic manner. This paper presents a robust mixed-integer convex model for the optimal scheduling of integrated ES-SOPs to ensure a zero probability of constraint violation. Losses of the subsystems comprising the ES-SOP are modeled using a proposed binary-polynomial model, enabling efficient scheduling of the energization state of subsystems to reduce no-load losses. The ES-SOP is considered in this paper to be owned by the network operator to: 1) manage power flow constraints, 2) minimize cost of losses, and 3) maximize arbitrage profit.

Peer-to-Peer Electricity Trading of Interconnected Flexible Distribution Networks Based on Distributed Ledger

The condition of power imbalance in flexible distribution networks (FDNs) is deteriorated due to the high penetration of distributed generators. Through peer-to-peer (P2P) electricity trading, multiple regions interconnected by soft open points (SOPs) can flexibly exchange power to alleviate power imbalance. As the physical foundation of P2P transactions, SOP regulation guarantees accurate instruction execution. Besides, smart contracts based on distributed ledger technology (DLT) facilitate highly credible P2P transactions. Oriented for the economic operation of interconnected FDNs, this article proposes a DLT-based P2P electricity trading method based on intelligent SOP regulation. First, a smart contract is designed for P2P transactions of FDNs, in which trading solutions are automatically settled with a modified highest combined offer principle. Then, a trading platform is built for the P2P electricity trading of interconnected FDNs. Finally, the effectiveness of the proposed method is verified on a practical FDN with four-terminal SOPs in Tianjin.

Optimal operation method for load restoration of distribution networks under disturbance scenario

Extreme events can cause damages to transmission system equipment, resulting in a major power outage. In disturbance scenario, local distributed energy resources (DERs) and switches can be fully utilized to restore power loads, which is critical for enhancing the resiliency. A service restoration model for distribution network with soft open point (SOP) is proposed, in which the ZIP loads are considered. Traditional tie switch (TS) operations and the power output capabilities of local DERs are coordinated to restore loads in outage area. On this basis, SOP flexibility is explored to further improve the restoration performance. The restoration model includes nonlinear constraints and integer variables, which is usually a mixed-integer nonlinear programming (MINLP) problem, and is difficult to obtain a feasible solution within an acceptable time. In particular, the ZIP load model is not directly amenable with conic solvers. To improve solution efficiency, convex relaxation, linear approximation, and big-M methods are introduced to convert MINLP problem into a mixed-integer second-order cone programming (MISOCP) problem. Simulation tests are studied to verify the effectiveness of the proposed model.

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.

Design and operation of Hybrid Multi-Terminal Soft Open Points using Feeder Selector Switches for flexible distribution system interconnection

Distribution systems will require new cost-effective solutions to provide network capacity and increased flexibility to accommodate Low Carbon Technologies. To address this need, we propose the Hybrid Multi-Terminal Soft Open Point (Hybrid MT-SOP) to efficiently provide distribution system interconnection capacity. Each leg of the Hybrid MT-SOP has an AC/DC converter connected in series with a bank of AC switches (Feeder Selector Switches) to allow the converter to connect to any of the feeders at a node. Asymmetric converter sizing is shown to increase feasible power transfers by up to 50% in the three-terminal case, whilst a conic mixed-integer program is formulated to optimally select the device configuration and power transfers. A case study shows the Hybrid MT-SOP increasing utilization of the converters by more than one third, with a 13% increase in system loss reduction as compared to an equally-sized MT-SOP.

Full-Order Terminal Sliding-Mode Control for Soft Open Point

In this paper, a full-order terminal sliding-mode control method is proposed for the rectifier side and the inverter side of the soft open point (SOP). The rectifier-side DC voltage control system consists of the voltage- and current-loops with controllers which are designed using full-order sliding-mode (FOSM) to enhance the dynamic performances and anti-disturbance. The integral type virtual control signal without chattering is designed to compensate for the unmatched uncertainties including external disturbances and some parameter perturbations. The full-order terminal sliding-mode (FOTSM) controller for the current-loop can force the current response to track its reference in finite time. The inverter side power control system is designed to regulate the power. The FOTSM controller for the power-loop ensures the power-tracking accuracy under a disturbed condition. Finally, the simulations demonstrate the effectiveness of the proposed controllers for the rectifier and inverter sides in the soft open point (SOP).

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.

Probabilistic power management of a grid-connected microgrid considering electric vehicles, demand response, smart transformers, and soft open points

The uncertainty of renewable energy generation and additional EV charging load requirements pose challenges to power system operation. The present work proposes a novel probabilistic energy management strategy comprising a coordinated Volt/Var and active power management strategies to minimize the cost of operation, improve the voltage profile, and enhance the voltage stability of the microgrid. The proposed active power management strategy comprises an implementation of an optimal incentive-based demand response strategy, economic scheduling of dispatchable generation sources, and optimal dispatch of battery energy storage system. The optimal Volt/Var strategy comprises a coordinated operation of soft open points, network reconfiguration, and the use of smart transformers. Stochastic modeling of renewable generation uncertainties, load demand, plug-in hybrid electric vehicle charging load demand, and grid energy price are carried out using the “Hong’s 2m point estimate method”. The proposed approach is validated by simulation studies on a thirty-node grid-connected microgrid system. The proposed coordinated energy management strategy can reduce the daily expected cost by ∼19.36% and the daily expected network loss by ∼60.29%. The voltage profile and the voltage stability are also significantly improved.