Soft normally open point (SNOP) is an emerging solution for distributed networks (DNs) to address voltage violation and feeder congestion caused by the increasing integration of distributed energy ...resources and new types of loads. Based on power electronics, SNOPs can substitute traditional tie switches for power flow regulation, voltage adjustment, and power quality improvement, enhancing the DNs' controllability and flexibility. Existing SNOPs found in demonstration projects and the literature are generally based on the back-to-back voltage-source converter (VSC). Because it requires more full-power-rating VSCs as the number of connected feeders increases, this solution is uneconomical in multifeeder flexible interconnection scenarios due to the high device cost and volume. Alternatively, this article proposes a series-shunt multiport SNOP (S 2 -MSNOP) based on a cascaded H-bridge structure and series-shunt arrangement. The proposed topology can easily extend its ports by increasing the number of submodules with small power ratings, reducing the device cost and volume. The operation principles and control strategies of the S 2 -MSNOP are elaborated. Verifications on both a 1-MVA simulation model and a 3.3-kVA scaled-down experimental platform prove the feasibility and effectiveness of the S 2 -MSNOP.
As a new type of power electronic device, Soft Normally Open Point (SNOP) can replace the traditional tie switch in distribution network to realise uninterrupted flexible regulation of power flow and ...optimise voltage profile. Several possible joining topologies and working modes of the three-port SNOP in the distribution network is discussed here. For the back-to-back three-port SNOP topology, a three-port SNOP coordination control strategy based on Finite-Control-Set Model-Predictive Control (FCS-MPC) is proposed, which has many advantages of clear principle, simple implementation, and fast dynamic response. The dynamic mathematical model of three-port SNOP is established, and the working mode set is proposed. Based on FCS-MPC, the control algorithms of UdcQ mode, PQ mode, and CVCf mode are implemented. Accordingly, coordination control strategy between the modes of three ports is presented. In order to verify the feasibility of the proposed scheme, a three-port back-to-back SNOP simulation model was built using Matlab/Simulink. The simulation results show that the proposed coordination control strategy can effectively achieve multi-port coordinated control which can provide a flexible solution for applications of multi-port SNOP.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This paper considers the effects of various voltage control solutions on facilitating an increase in allowable levels of distributed generation installation before voltage violations occur. In ...particular, the voltage control solution that is focused on is the implementation of `soft' normally-open points (SNOPs), a term which refers to power electronic devices installed in place of a normally-open point in a medium-voltage distribution network which allows for control of real and reactive power flows between each end point of its installation sites. While other benefits of SNOP installation are discussed, the intent of this paper is to determine whether SNOPs are a viable alternative to other voltage control strategies for this particular application. As such, the SNOPs ability to affect the voltage profile along feeders within a distribution system is focused on with other voltage control options used for comparative purposes. Results from studies on multiple network models with varying topologies are presented and a case study which considers economic benefits of increasing feasible DG penetration is also given.
With the rising penetration levels of random resources such as renewable energy (RE) and electric vehicles (EVs), the uncertainty of active distribution network (ADN) is inherently increasing. To ...deal with disturbances and enhance stable operation, many power electronic devices such as Soft Normally Open Point (SNOP) have been integrated to ADN. This paper proposes a novel node flexibility evaluation method of ADN for SNOP integration. Firstly, operation scenarios of ADN are generated based on the k-medoids and k-means clustering method. A hybrid algorithm based on particle swarm optimization (PSO) and simulated annealing (SA) is used in the scenarios. The above is the demand assessment for the flexibility evaluation. Then, RE consumption, load satisfaction, voltage deviation and netload fluctuation of each node are obtained from scenarios and weighted by the information entropy weight method (EWM) to form the comprehensive flexibility index. Nodes are classified according to the evaluation results, which provide the integrated location of SNOP. Finally, a case study showed the differences between the proposed method and others.
This paper focuses on the soft normally open point (SNOP) which has no transformers and connects to the 10 kV distribution network through the arc suppression coil grounding mode. A mathematical ...model containing positive-, negative-, and zero-sequence components under AC fault is established by the circuit analysis and the sequence component decomposition. Then the voltage fluctuation mechanism at the DC side and the non-fault AC side caused by the zero-sequence voltage on the fault side is analyzed under the case without transformers. Taking the single-phase grounding fault as an example, the fluctuation formula of the DC voltage and AC voltage is deduced in detail. Based on the theoretical analysis, the zero-sequence control strategy and the sub-module improvement strategy of the modular multilevel converter (MMC) are proposed. A back-to-back SNOP simulation model is established in PSCAD/EMTDC. The simulation results verify the accuracy of the theoretical analysis, as well as the validity of zero-sequence control and MMC topology improvement strategies.
Soft normally open point (SNOP) is a power electronic device installed at normally open point (NOP) in a distribution network which can greatly promote the flexibility and controllability of ...distribution network. This paper introduces the typical installation of SNOP, and take the three-terminal SNOP converter station demonstration project in Hangzhou as case study. This paper is divided into two parts. Firstly, a new strategy which uses power of feeders' load to balance power flow among feeders is proposed. Then under fault circumstances, voltage support strategy of injecting positive active and reactive current based on line resistance-reactance ratio (R/X) is proposed. Simulation results verify the accuracy and effectiveness of the strategies adopted.
To meet the requirements of maximum consumption of new energy and non-cut load during the islanding, a control scheme of smooth transition through soft normally open point (SNOP) for the active ...distribution networks is proposed, which includes the coordinated control strategy, the stability range analysis within the control delay, and the grid-connected/islanded seamless transition. The control strategies of the SNOP, distributed generation (DG) and energy storage system (ESS) are coordinated to suppresses power fluctuations. The threshold of the post-fault power loss is derived to avoid the action of the anti-islanding device. The phase angle considering the time delay is compensated, the reference value of the control inner loop is solved with the steady-state inverse model, and the pre-synchronous grid-connection is carried out, so as to realize the seamless transition. Simulation based on PSCAD/EMTDC verifies the effectiveness of the proposed strategy.
The penetration of renewable energy (RE) has been increasing in recent years, which leads to the increment of the uncertainty of the distribution networks, as well as the power quality problems. Many ...power electronic devices, such as soft normally open points (SNOPs), have been applied in the distribution networks to deal with the disturbances causing by RE and thus enhance the operation stabilities consequently. This paper presents a multiple time-scale flexibilities evaluation method for accessing the SNOPs into the distribution network. There are four sub-indicators are employed in the evaluation method, which are net load fluctuations, voltage deviations, distortion rate of harmonic voltage, and RE ramp rates. The subindicators are weighted by the information entropy weight method (EWM) under each typical scenario. Distribution network nodes are partitioned according to the evaluation results, which provide the access basis of SNOP. A case study is performed to show the nodes flexibility improvement of a distribution network with SNOP integrate.
Soft normally open point (SNOP) is a novel power electronic device installed in place of normally-open point. The application of SNOP will greatly promote the flexibility and controllability of ...distribution network. Considering the high investment of SNOP, both tie switch and SNOP should be taken into account in the coordinated operation problem of distribution network. Firstly, the optimization model for distribution network coordinated operation with SNOP and tie switch is proposed. Then, combining the simulated annealing method with conic programming, this paper proposes a hybrid optimization algorithm, involving simulated annealing method to obtain the switch states and conic programming to optimize the transmitted power of SNOP. This hybrid algorithm can solve the above large-scale mixed-integer nonlinear problem accurately and rapidly, while satisfying the demand of distribution network coordinated operation. Finally, the IEEE 69-node system is used to demonstrate the effectiveness of the proposed hybrid algorithm.
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