Grid Connection Point Research Articles

Optimal Locating and Sizing of BESSs in Distribution Network Based on Multi-Objective Memetic Salp Swarm Algorithm

Battery energy storage systems (BESSs) are a key technology to accommodate the uncertainties of RESs and load demand. However, BESSs at an improper location and size may result in no-reasonable investment costs and even unsafe system operation. To realize the economic and reliable operation of BESSs in the distribution network (DN), this paper establishes a multi-objective optimization model for the optimal locating and sizing of BESSs, which aims at minimizing the total investment cost of BESSs, the power loss cost of DN and the power fluctuation of the grid connection point. Firstly, a multi-objective memetic salp swarm algorithm (MMSSA) was designed to derive a set of uniformly distributed non-dominated Pareto solutions of the BESSs allocation scheme, and accumulate them in a retention called a repository. Next, the best compromised Pareto solution was objectively selected from the repository via the ideal-point decision method (IPDM), where the best trade-off among different objectives was achieved. Finally, the effectiveness of the proposed algorithm was verified based on the extended IEEE 33-bus test system. Simulation results demonstrate that the proposed method not only effectively improves the economy of BESSs investment but also significantly reduces power loss and power fluctuation.

Analysis of Voltage and Frequency Influenced by Control Function Selection of Smart Inverter based on Voltage Class of Grid Connection Point

In recent, smart inverter is focused on as a solution for the problems on voltage and frequency stability caused by mass introduction of PV. Smart inverter has many functions to keep voltage and frequency within the proper range. Its effectiveness significantly depends on which function is activated but it has not been clarified sufficiently. This paper investigates influence on voltage and frequency of combined use of voltage regulation function and frequency regulation function of smart inverter. In the analysis, smart inverters connected to transmission network and ones connected to distribution network are modeled separately, and appropriate function activation in each network is investigated. Results of simulation study shows that precedence setting in the combined use of multiple function sometimes makes interference among them and deteriorates performance of smart inverter. The dependencies on the performance of introduction in transmission and distribution network by the analysis, and importance of selection of activating function considering introduction amount in transmission and distribution network is recognized.

Adaptive Online-Learning Volt-Var Control for Smart Inverters Using Deep Reinforcement Learning

The increasing penetration of the power grid with renewable distributed generation causes significant voltage fluctuations. Providing reactive power helps balancing the voltage in the grid. This paper proposes a novel adaptive volt-var control algorithm on the basis of deep reinforcement learning. The learning agent is an online-learning deep deterministic policy gradient that is applicable under real-time conditions in smart inverters for reactive power management. The algorithm only uses input data from the grid connection point of the inverter itself; thus, no additional communication devices are needed and it can be applied individually to any inverter in the grid. The proposed volt-var control is successfully simulated at various grid connection points in a 21-bus low-voltage distribution test feeder. The resulting voltage behavior is analyzed and a systematic voltage reduction is observed both in a static grid environment and a dynamic environment. The proposed algorithm enables flexible adaption to changing environments through continuous exploration during the learning process and, thus, contributes to a decentralized, automated voltage control in future power grids.

Reactive PowerControl of Grid-Connected Photovoltaic Power Generation

In order to solve the problem of grid-connected point voltage exceeding the limit caused by large-scale photovoltaic power stations connected to the grid, and to increase the penetration rate of photovoltaics in the grid, photovoltaic power stations should have more flexible reactive voltage regulation capabilities to provide reactive power support to the grid. To this end, this paper establishes an equivalent aggregation model for large-scale photovoltaic power plants. Under the premise that the photovoltaic power station is not equipped with reactive power compensation devices and the inverter reactive power output is zero, it is analyzed that due to the existence of line and transformer impedance, photovoltaic access reduces the grid the problem of voltage stability. Based on the above reasons, this paper proposes a three-layer reactive power control strategy for photovoltaic power plants from the perspective of the cooperation between the reactive power compensation device and the grid-connected inverter. This strategy coordinates the reactive power output between the reactive power compensation device and the photovoltaic power generation unit and between the inverters of a single photovoltaic power generation unit. Under this control strategy, the photovoltaic power plant can regulate the grid voltage more effectively, and the active and reactive power losses of the grid are minimized on the premise that the grid voltage is maintained within the required range. Combined with the photovoltaic array power reduction operation strategy, the stable operation of the power grid is ensured under the premise of a certain reactive power output capacity of the photovoltaic power station. Finally, an example is used to verify the correctness and effectiveness of the strategy.

The Control Strategy for the Adaptive Reactive Voltage of Low and Medium Voltage Distribution Network with High Photovoltaic Penetration

The high permeability of photovoltaic power generation systems in medium and low voltage distribution networks brings challenges such as reverse power flow and voltage rise. Voltage violation becomes an important factor of restricting the massive access of distributed photovoltaics. In this paper, the reactive voltage control problem caused by the high-permeability photovoltaic access to the distribution network is studied. Based on the inverter reactive voltage control strategy, a distributed photovoltaic cosφ (U, P) control model is proposed, and based on the grid-connected point voltage and the actual active output of photovoltaics, a distributed photovoltaic in-situ adaptive reactive voltage control strategy is proposed. After simulation verification, this control strategy can automatically adjust photovoltaic output make full use of the voltage support capability of distributed photovoltaics at various locations and periods, effectively make up for the shortcomings of traditional voltage regulation capabilities, and improve reactive voltage control capabilities.

Research on voltage control strategy optimization of wind farm cluster

With the continuous expansion of the grid-connected scale of wind farms, the problem of the stability of the grid-connected point voltage of wind farm cluster has become increasingly prominent. However, the current reactive voltage control strategy of wind farm cluster has less research on the optimization of the internal node voltage and grid loss of wind farms, and the problem of the reduction of wind turbine life caused by the full reactive power of doubly-fed wind turbines needs further research. In view of the above problems, this paper proposes a wind farm cluster reactive voltage hierarchical control strategy based on the reactive power margin of doubly-fed wind turbines. The strategy improves the traditional artificial fish algorithm and realizes the reactive voltage control of wind farm cluster. As a result, the node and network loss have reached a certain degree of optimization.

Maximization of Distribution Network Hosting Capacity through Optimal Grid Reconfiguration and Distributed Generation Capacity Allocation/Control

High penetration of small-scale distributed energy sources into the distribution network increase negative impacts related to power quality causing adverse conditions. This paper presents a mathematical model that maximizes distribution network hosting capacity through optimal distributed generation capacity allocation and control and grid reconfiguration. In addition to this, the model includes on-load tap changer control for stabilization of grid voltage conditions primarily in grid operating conditions related to voltage rise problems, which can limit grid hosting capacity. Moreover, the objective function allows the possibility of energy transfer between distribution and transmission grids. The proposed model considers alternative grid connection points for distributed generation and determines optimal connection points as well as install capacity while considering network operating limits. The model is cast as a multiperiod second-order cone linear program and involves aspects of active power management. The model is tested on a modified IEEE 33 bus test network.

Multi objective transmission network expansion planning considering wind power uncertainty and scenario reduction

Under the background of large-scale new energy grid connection, the selection of grid connection point affects the grid structure to be expanded. The intermittence of wind power and the correlation with load also affect the voltage and line safety of each node in the grid. In order to accurately describe the uncertainty factors such as wind power output, multi scenario stochastic simulation technology and reduction method are used to establish multi scenario to accurately describe multi-point wind power and load uncertainty; At the same time, a multi-objective transmission network expansion planning model considering the cost of investment, operation and reliability is established. The effectiveness of the model is verified by IEEE 30 bus simulation. It shows that the accuracy and rationality of transmission network expansion planning can be improved by scenario reduction.

Ripple Analysis and Suppression Method Research of Direct Drive Permanent Magnet Synchronous Wind Turbine under Wind Shear

Wind shear is among the important sources of torque fluctuation on mechanical side and output power fluctuation on electromagnetic side and grid connection point, which frequency are triple turbine rotation frequency. With the increase in wind turbine single capacity and wind power integration scale, the power fluctuation caused by wind shear cannot be ignored. The paper takes a direct-drive permanent magnet synchronous wind turbine (PMSWT) as the research object. Firstly, the fluctuation transfer mechanism model is established; then, the current ripple is extracted by a notch filter to calculate the voltage control signal to suppress power fluctuation actively by the DC link capacitor which absorbs fluctuation energy. The simulation experiment shows that the suppression strategy is effective and reliable.

A Modified Transient Current Limiter to Mitigate Energization Inrush Current in Wind Turbine Transformers

Energization of wind turbine transformers (WTTs) in wind power plants (WPPs) leads to high transient inrush current, which imposes high mechanical and thermal stresses on transformer windings and voltage sag problems at grid connection point (GCP). Therefore, to solve these problems, the inrush current should be suppressed during the energization time. The current research presents a modified transient current limiter (MTCL) for suppressing WTTs’ inrush current. Two types of MTCL such as resistor type (R-type) and reactor type (L-type) are considered for this purpose. To investigate the efficiency of the MTCL, a WPP includes five wind turbines which are simulated by PSCAD/EMTDC software. The simulation results reveal that the MTCL is an effective means for suppressing the WTTs’ inrush current and mitigating GCP voltage sag during energization. Besides, to show the effectiveness of the proposed MTCL, the performance of the MTCL is compared with the conventional TCL through simulation.

Research on Distributed Photovoltaic Grid-connected Voltage Cooperative Control Strategy Considering Local Load

The remaining capacity of the photovoltaic inverter has achieved good results in solving the problem of the voltage limit of the grid-connected point of the distributed photovoltaic power generation system. But at present, in order to increase the reactive power capacity of the inverter, related research mainly focuses on limiting the power output of the inverter, without considering the impact on the penetration rate of the distributed photovoltaic power generation system. This paper clarifies the mechanism of the voltage limit of the grid-connected point of the distributed photovoltaic power generation system, and proposes a coordinated control strategy for the voltage of the distributed photovoltaic grid-connected point that takes into account the local load. Use back-to-back converters to control the local load reactive power of photovoltaic power generation, and change the output power of photovoltaic power generation by controlling the working status of the grid-connected inverter. Set up the brake control link. When the limit is exceeded, the local load reactive power control will be given priority to adjust the grid-connected point voltage, thereby avoiding the impact on the photovoltaic power generation capacity of the distribution network. Finally, experiments based on Matlab/Simulink simulation platform verify the feasibility and effectiveness of the proposed control strategy.

Economical layout optimization of wave energy parks clustered in electrical subsystems

To obtain a maximum power output and minimized capital and operational costs, the layout of wave energy parks needs to be optimally designed. An economical model for large-scale wave energy systems is built and merged into an evolutionary optimization routine for arrays of point-absorbing energy converters. The model includes all the parameters that affect the total system revenue such as electrical cable lengths, distance from grid connection point, number of substations and hydrodynamic interaction among the devices, with the goal to find the optimal layout which minimizes the levelized cost of electricity. Converters inside the park are grouped in clusters via a k-means clustering algorithm, which allows to minimize the intra-array cable length under the input of real wave climates. The results show that the hydrodynamical interaction has a large impact on the optimal design of wave energy parks, and that the length of the intra-array cable does not play a significant role in the economical layout optimization routine for the studied wave energy park system.

Optimization modelling of HCMC-STATCOMs for offshore wind power based on PSCAD

Under normal circumstances, the voltage fluctuation of the wind farm’s grid connection point is mainly caused by the volatility of wind power output. In recent years, with the clustering development of offshore wind power, the voltage fluctuation brought by the output fluctuation of wind power is becoming more and more notable. The main method of improving the reactive voltage problem of wind farms is to add more static and dynamic reactive power compensation devices. Hybrid cascaded multilevel converter STATCOMs (HCMC-STATCOMs) don’t need transformers to increase the device capacity and decrease the harmonics on the output voltage, which is effective in reducing the device cost and volume. This paper firstly introduces the topology and operation principles of an HCMC-STATCOM and then carries out a comparative study on HCMC-STATCOMs and traditional cascaded STATCOMs. Finally PSCAD/EMTDC is applied to model and simulate HCMC-STATCOMs. The simulation result shows that the reactive power of HCMC-STATCOMs can accurately vary according to the system’s demand on reactive power. This means that HCMC-STATCOMs have good dynamic performance and the harmonic contents in the output phase voltage and AC current are quite small. As a dynamic reactive power compensation device, HCMC-STATCOM can be adjusted in real time according to the system’s reactive power requirements, and has the advantages of small volume and low cost. The simulation results show that the properties of HCMC-STATCOMs are advantageous for the operation and control of the system and applicable for the dynamic reactive power configuration of offshore wind power.

Research on Reactive Power and Voltage Control for Wind Farm Based on coordinate control of DFIGs and SVG

Abstract Wind farm generally uses long-distance transmission mode to transmit electricity, because the grid can not fully support the grid voltage of wind farm. When the wind speed changes, there is a problem of voltage instability at the grid-connection point. However, the reactive power compensation device of the power grid can not meet the requirements of fast reactive power response and compensation. Although doubly-fed induction generators (DFIGs) have fast power decoupling control, its reactive power is limited by active power output. In order to meet the reactive power demand of the power grid, the instability of the voltage at the grid-connection point is solved by coordinating the reactive power output capacity of DFIGs and the Static Var Generator (SVG). This paper proposes a coordinated control method of reactive voltage in wind power plant based on Starsim, which uses the upper computer to send voltage control instructions to control the reactive output of the simulation model in Starsim. The simulation model is built in MATLAB, and the simulation model is put into Starsim. Through the simulation, it is concluded that the control method could stabilize the grid voltage of wind farm.

Application and machine learning methods for dynamic load point controls of electric vehicles (xEVs)

From the customer's perspective, the appeal of electric vehicles depends on the simplicity and ease of their use, such as flexible access to electric power from the grid to recharge the batteries of their vehicles. Therefore, the expansion of charging infrastructure will be an important part of electric mobility. The related charging infrastructure is a big challenge for the load capacity of the grid connection without additional intelligent charge management: if the control of the charging process is not implemented, it is necessary to ensure the total of the maximum output of all xEVs at the grid connection point, which requires huge costs. This paper proposes to build a prediction module for forecasting dynamic charging load using machine learning (ML) techniques. The module will be integrated into a real charge management concept with optimization procedures for controlling the dynamic load point. The value of load forecasting through practical load data of a car park were taken to illustrate the proposed methods. The prediction performance of different ML methods under the same data condition (e.g., holiday data) are compared and evaluated.

Monitoring concept suitable for utilising flexibilities in the low-voltage distribution grid: Learning from implementation in Greencity Zurich

The electricity utility of Zurich (ewz) and Venios GmbH set up real-time monitoring in the Greencity test area to gain more insights into the low-voltage grid and experience with the utilisation of flexibilities such as charging stations for electrical vehicles. Based on a digital model of the physical power grid, measurements in the field, load models and specific grid limits, the grid state is determined every 5 min by load flow calculations. Measurement data from the field is streamed to a Microsoft Azure cloud environment. The underlying monitoring concept developed by ewz for the low-voltage grid addresses use cases to all selected measurement points and parameters. Experience gained in information technology (IT) architecture has proven a scalable cloud environment that fulfils IT security. By aggregating personal load profiles on the grid connection point, data protection and thus the regulatory framework can be met.

Stability analysis of multi-paralleled grid-connected inverters including the distribution parameter characteristics of transmission lines

The sparse distribution characteristics of renewable energy resources can lead to there being tens of kilometers of transmission lines between a grid-connected inverter and the actual grid. Accurate analysis of the stability of such grid-connected inverter systems currently involves using a complex hyperbolic function to shaped model of the transmission line circuit. This has proved to be problematic, so, drawing upon the distribution parameter characteristics of transmission lines, this paper looks at how to use impedance-based stability criteria to assess the stability of multi-paralleled grid-connected inverters. First, the topology of multi-paralleled inverters connected to the grid via transmission lines is established, using each transmission line terminal as a grid connection point. Each grid-connected system is taken to be equivalent to a small-signal circuit model of the source-grid. Euler's formula and the Nyquist stability criterion are combined to assess the stability of the associated grid-connected current transfer functions and evaluate the stability of the grid-connected current. Finally, a simulation analysis circuit is constructed to verify whether power line intervention will cause stability problems in the grid-connected system. Overall, it is found that long-distance transmission lines are more likely to cause unstable output of the grid-connected current. It is also found that the number of grid-connected inverters, the short-circuit ratio (SCR), the distorted grid and the inverter parameters can all have a significant impact on the stability of the grid-connected current.

Operational optimisation framework improving DSO/TSO coordination demonstrated in real network operation

The study at hand describes the state of the art regulatory framework for managing the distribution system operator (DSO)/transmission system operator (TSO) interface including information about the available flexibility data and access rights of the different grid operators. It describes a possible future procedure to manage this interface using software tools to support this coordination performing grid analysis and providing active as well as reactive power flexibility information at several TSO–DSO interconnection points. The proposed study gives a short overview of two optimisation approaches/software tools with different application functionalities, developed and adapted to meet the demands coming up with such tasks, performed within the European project ‘EU-SysFlex’. Both tools contribute to the objective of optimising available flexibility resources connected to meshed distribution grids with multiple grid connection points to the transmission grid. The investigations/calculations are performed using a real existing high-voltage grid of MITNETZ STROM. Using the outputs, provided by both tools, the demonstrator aims at providing beneficiary results for the discussion of how to evolve the regulatory framework. The functionality of the developed algorithms of both optimisation tools is evaluated and investigated by means of a live field test.

Study on Three-Phase Reclosing Strategy Applicable to Tie Line of Photovoltaic Power Station

When a fault occurring in the tie line of photovoltaic power station is cleared by the protection three-phase trip, photovoltaic power source (PVS) will form an unplanned island with the local load. However, since the voltage regulation and frequency control capability of PVS is generally poor, it is very difficult to satisfy the synchronous reclosing conditions, eventually resulting in an automatic reclosing failure. According to the islanding control strategy of PVS and the principle of phase-locked loop (PLL), a vector relationship model of PVS port actual voltage, port calculation voltage, grid-connected point voltage and so on before and after reclosing is established. Based on this, the influences of reclosing phase difference, amplitude difference and frequency difference on reclosing impulse voltage and current are analyzed, and a synchronous check reclosing strategy applicable to the tie line of PVS is proposed, which can effectively improve the success rate of the three-phase reclosing in tie line. Digital simulation results verify the correctness of the proposed reclosing strategy.

Steady-state Operation Area of VSC-HVDC Converter Station Connecting Renewable Energy Cluster by Isolated Network

When the large-scale renewable power island is connected to VSC-HVDC transmission system, we should figure out the steady-state operation area of VSC-HVDC converter station. Based on the equivalent model of renewable energy island, the constraints of VSC-HVDC converter station are analyzed, and a fast method for calculating steady-state operation area of converter station is presented in this paper. The influence of equivalent line impedance of renewable energy island, transformer ratio of converter station and voltage setting value of grid-connected point on steady-state operation area of converter station is analyzed. And the three-dimensional view of the steady-state operation area of the converter station under different grid-connected voltage is depicted.