Reactive Power Support Capability Research Articles

Collaborative scheduling method of active-reactive power for rural distribution systems with a high proportion of renewable energy

Large-scale distributed renewable energy connected to the rural distribution network has given birth to a new rural distribution system with a high proportion of new energy typical characteristics, and the optimal scheduling of the new rural distribution system has become an important issue to ensure the safe and stable operation of the power grid. This paper proposes a method of active-reactive power collaborative optimization scheduling for rural power distribution system with a high proportion of renewable energy. Firstly, the active support capability evaluation model is established, and the active power support capability and reactive power support capability of rural power distribution system are quantitatively evaluated, which provides data basis and boundary conditions for the scheduling part. Then, considering power-loss cost, distribution network operation cost, and penalty cost, a method of active-reactive power collaborative optimization scheduling for rural power distribution systems with a high proportion of renewable energy is proposed. Finally, the active support capability evaluation and regulation platform of the rural power distribution system is built to provide technical support services for the safe and stable operation of the rural power distribution system. Given the problems of overload and overvoltage faced by rural power distribution systems with a high proportion of renewable energy, this paper aims to solve the key technical challenges of optimization and regulation of new rural power distribution systems. The results show that the optimized control method proposed in this paper has better security and economy, and is conducive to promoting the construction and operation of the new rural power distribution system.

Adaptive control of DFIG-based wind turbines for fault ride-through and reactive power support based on stator current transient component

The fault ride-through performance is improved through the control of the transient components of the stator current. The reactive power provided by the DFIG within the margin of the converter can significantly enhance the grid-connected safety of the DFIG. Firstly, the reasons for the fault overcurrent of the DFIG’s rotor are analyzed. Secondly, according to the operating conditions of the wind turbine, the reactive power support limit of the DFIG-based wind turbines based on the stator current transient component control is analyzed, and an adaptive reactive power support control suitable for fault ride-through is proposed to maximize its support potential for grid-connected voltage. Finally, the DFIG can not only suppress the rotor overcurrent through the transient component of the stator current but also have a reactive power support capability that is superior to the grid-connected standard.

A fast linearized AC power flow-constrained robust unit commitment approach with customized redundant constraint identification method

The large-scale integration of renewable energy resources in the power system challenges its economic and secure operation. Particularly, the increasing penetration of renewable energy will result in insufficient system voltage regulation and reactive power support capabilities, and may cause high risks of nodal voltage and branch flow violations. Therefore, to hedge the operational risks under the worst realization of uncertainties of renewable energy sources, a two-stage robust unit commitment (UC) model is developed. Meanwhile, the convexified AC power flow model is incorporated in the robust UC model to more accurately characterize the real-time operating status of power systems. On this basis, an AC power flow-constrained robust unit commitment (ACRUC) model is formulated. A circular linearization method is then adopted to handle the quadratic constraints in the original AC power flow model, transforming them into tractable linear constraints. Furthermore, to reduce the computational complexity caused by the large-scale newly-added constraints after the linearization process, a customized redundant constraint identification (RCI) method is developed, in which two different modes (i.e., cold and warm start modes) are designed considering the difference in base case system operating condition for linearizing branch losses. Then, the redundant network security constraints could be identified by solving a series of relatively simple optimization subproblems. Numerical results on the modified NERL-118 test system indicate that the proposed model could accurately depict actual operation and scheduling conditions, and also verify that the proposed customized RCI method could effectively reduce the problem scale and improve the solution efficiency.

A General Self-Adaptive DC Current Control of LCC-HVDC for Eliminating Subsequent Commutation Failure

With more renewable resources are integrated into the receiving-end AC grid through the LCC-HVDC link, a single-infeed LCC-HVDC (SIDC) system is facing a critical subsequent commutation failure (SCF), which stems from the poor dynamic reactive power support capability of the existing LCC-HVDC link's control strategies. This letter proposes a general self-adaptive DC current control (GSDC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of LCC-HVDC to eliminate the SCF, based on the dynamic reactive power interaction mechanism. Then, multiple comparative studies with the existing control strategies are implemented to verify the validation of the proposed control strategy by PSCAD/EMTDC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> . A fast evaluation index, called general commutation failure immunity index (GCFII), is also defined to show the performance of the proposed control strategy in eliminating the SCF.

Integrated reactive power optimization for distribution systems considering electric vehicle dis-/charging support

Electric vehicle (EV) charging, and discharging have an important reactive power support capability for the distribution network, for which a reactive power optimization method considering EV dis-/charging is proposed. Firstly, the principle of charger’s reactive power regulation is analysed,​ and a charger state constraint model is established to satisfy the constraints on the power factor angle by modifying the charger state for the battery capacity and the target power of the users. Secondly, a quantification method for user reactive power optimization subsidies is proposed to establish network-side and user-side demand indicators respectively, and to integrate reactive power optimization target function for the distribution system. Then, bi-directional charger’s PF is taken as the optimization variable, and the charger power factor angle optimization method is proposed based on the Particle Swarm Optimization (PSO) algorithm, so as to establish a reactive power optimization model considered the orderly dis-/charging, in order to realize the active/reactive power regulation between the distribution network and Owners of the EV. Finally, simulations are carried out using the IEEE-33 model as an example. Results proved that presented orderly dis-/charging method can increase voltage profile and decrease loss rate of the network to 40%, but also significantly decrease customer’s expenditure to 13%, while satisfying the temporary travel demand of customers.

Analysis of the Route Selection of HVDC in the Yangtze River Delta Load Center under the Background of the New Power System

As the load center in East China, the receiving-end power grid in the Yangtze River Delta is densely distributed with conventional DC and faces great security and stability risks in the context of the development of dual-carbon and new power systems. Based on the existing problems in the Yangtze River Delta power grid, this paper draws up different DC technology routes and compares and selects them based on the safety and stability support capabilities and reactive power support capabilities of the receiving-end power grid, the impact on the short-circuit current of the system, technology maturity, land occupation, and economy, and finally put forward the recommended technical route for a foreign power to enter Shanghai.

Research on the Fault Diagnosis Method of a Synchronous Condenser Based on the Multi-Scale Zooming Learning Framework

Under the background of the “strong direct current and weak alternating current” large power grid, the synchronous condenser with dynamic reactive power support capability becomes more important. Due to factors such as manufacturing, installation, and changes in operating conditions, there are many faults associated with the synchronous condenser. This paper studies a fault diagnosis method based on multi-scale zooming learning framework. First, through the energy fully connected (energy FC) layer, the synchronous condenser feature components of the fault signal of the camera are learned, and the transient features of the signal are enhanced. At the same time, the data is adaptively compressed and the effective features are mapped in a distributed manner. The faults are effectively diagnosed and isolated in advance. Secondly, a multi-scale learning framework is constructed to learn the multi-frequency features in the vibration signal. Finally, experiments show that the proposed method has certain advantages over the existing excellent models. The accuracy rate of diagnosis is higher than 99%.

A Novel Coordinated Control System to Reactive Power Compensation of Photovoltaic Inverter Clusters

With the development of new energy, a cost-effective reactive power compensation scheme is essential to the voltage stability of the power system for small-capacity distributed generation. This paper proposes a coordinated control scheme of inverter cluster which is based on the reactive power support capability of the photovoltaic inverter. Moreover, by using power angle vectors, a reactive power distribution algorithm is proposed to solve the poor power quality of the point of common coupling connecting source and load in the distributed generation station. Simulations verify the performance of the algorithm is better than the conventional static capacity distribute algorithm and dynamic residual margin distribute algorithm. Finally, the effectiveness of the reactive power compensation scheme and distribution strategy for improving power quality and regulation ability proposed in this paper is verified by operation experiments in the actual power station.

A voltage coordination control strategy based on the reactive power-active network loss partitioned aggregation domain

As the scale of the power system continues to expand, it becomes more difficult to regulate the voltage of large grids in a unified manner by traditional control methods. In this paper, based on the theory of modularity function model in complex networks, a voltage coordination control method of partitioning the aggregated domain of reactive-voltage sensitivity weights and active network loss-voltage sensitivity weights of power systems is proposed. Firstly, the system is partitioned according to the modularity function model and the concept of aggregation domain is proposed. Secondly, the reasonableness of the aggregation domain is judged by using the proposed reactive power support capability index, and the partitioned voltage control strategy is proposed to realize the optimal and reasonable control of the voltage crossing point. Finally, the simulation experiment compares the voltage regulation results of the centralized control method and the zonal control method. The zoned control method provides a relative voltage improvement of 3.27% over the centralized control method, the active network loss of the zoned control method is 3.30% less than that of the centralized control method, and a relative reduction in time of use of 90.70%. The simulation results show that the proposed method can effectively reduce the search range of reactive power compensation devices, reduce the number of nodes for coordinated control, reduce the active network loss of the system due to voltage regulation, and ensure the control capability of reactive power compensation devices within the partition.

Optimal Power Coordinated Control Strategy for DFIG-Based Wind Farm to Increase Transmission Capacity of the LCC-HVDC System Considering Commutation Failure

The line commutated converter (LCC)-based high voltage direct current (HVDC) transmission system is an effective scheme for long-distance transmission of wind power. Commutation failure is a common failure of LCC-HVDC transmission system. During the commutation failure, the overvoltage of the sending alternating current (ac) grid will occur to disconnect the wind farm from the utility grid and limit the transmission capacity of the system. It is necessary to suppress the overvoltage of the sending ac grid under commutation failure to increase the transmission capacity of the LCC-HVDC system. In this article, a mathematical model of the transient overvoltage peak value of the sending ac grid under commutation failure is established. The mathematical model can quantitatively analyze the influence of the reactive power of the wind farm and the transmission capacity of the LCC-HVDC system on the overvoltage peak value of the sending ac grid. Based on the mathematical model, a power coordinated control strategy for wind farms is proposed to improve the reactive power support capability under grid overvoltage. The proposed control strategy can effectively suppress the overvoltage of the sending ac grid and increase the transmission capacity of the LCC-HVDC system. The accuracy of the mathematical model is verified by simulation, and the superiority of the power coordinated control strategy is verified by the control hardware-in-the-loop (CHIL) experiment.

Recent Advances in Energy Storage Systems for Renewable Source Grid Integration: A Comprehensive Review

The reduction of greenhouse gas emissions and strengthening the security of electric energy have gained enormous momentum recently. Integrating intermittent renewable energy sources (RESs) such as PV and wind into the existing grid has increased significantly in the last decade. However, this integration hampers the reliable and stable operation of the grid by posing many operational and control challenges. Generation uncertainty, voltage and angular stability, power quality issues, reactive power support and fault ride-through capability are some of the various challenges. The power generated from RESs fluctuates due to unpredictable weather conditions such as wind speed and sunshine. Energy storage systems (ESSs) play a vital role in mitigating the fluctuation by storing the excess generated power and then making it accessible on demand. This paper presents a review of energy storage systems covering several aspects including their main applications for grid integration, the type of storage technology and the power converters used to operate some of the energy storage technologies. This comprehensive review of energy storage systems will guide power utilities; the researchers select the best and the most recent energy storage device based on their effectiveness and economic feasibility.

Transmission-distribution long-term volt-var planning considering reactive power support capability of distributed PV

High penetration of grid-edge, inverter-based photovoltaic (PV) can cause significant voltage fluctuations not only at the distribution but also at the sub-transmission levels due to PV output intermittency. This paper proposes a reactive power planning tool for sub-transmission systems to mitigate voltage violations and fluctuations caused by high PV penetration and intermittency with a minimum investment cost. The planning tool considers all existing var assets in both sub-transmission and distribution systems to reduce the need of new equipment. It also closely coordinates with an optimization-based volt-var operational tool for selecting a set of scenarios with voltage violations due to PV intermittency and testing the final investment decision. The planning tool obtains an investment need for each scenario based on a proposed optimal power-flow framework. Because of a high number of discrete variables in this optimization problem, an efficient technique to recover the feasibility for the solution of the relaxed optimization problem is included. After obtaining separated solutions for all scenarios, two options are provided for making final planning decision: i) a conservative direct combination of investment need solutions and ii) a machine learning-based selection of representative investment needs at most time steps. The final investment decision options are verified using a realistic large-scale sub-transmission system and 5-min PV and load data. The results show a significant voltage performance improvement with a lower investment cost for additional var equipment compared to conventional approaches.

Comprehensive Evaluation of the Transient Reactive Power Support Capability of New Energy Stations Based on TOPSIS

In order to effectively evaluate the reactive power support capability of new energy stations, an evaluation system is established with the high voltage ride through(HVRT) capability, low voltage ride through (LVRT) capability and reactive power support capability selected as the evaluation index. And then, a comprehensive evaluation method to effectively evaluate the transient reactive support capability of new energy stations is proposed in this paper. The weight of a single index was determined by the coefficient of variation method and then the comprehensive evaluation index was determined based on TOPSIS(Technique for Order Preference by Similarity to an Ideal Solution). Finally, the rationality and effectiveness of the proposed evaluation method are verified by the measured data of a wind farm.

An Active Voltage Coordinate Control Strategy of DFIG-Based Wind Farm with Hybrid Energy Storage System

In a power system with wind farms, the point of common coupling (PCC) usually suffers from voltage instability under large wind speed variations and the load impact. Using the internal converter of a doubly fed induction generator (DFIG)-based wind turbine to provide voltage support auxiliary service is an effective scheme to suppress the voltage fluctuation at PCC. To satisfy the reactive power demand of the connected grid, an active voltage coordinate control strategy with the hybrid energy storage system of the wind farm is proposed. The dynamic reactive power balance model is established to show the interaction between the reactive power limitation of the wind farm and the reactive power compensation demand of the grid. This indicates the initial conditions of the active voltage coordinate control strategy. According to the critical operating point and the operation state of the DFIG, the active and reactive power coordinate control strategy composed of active ω-β coordinate control and active β control is proposed to enhance the reactive power support capability and stabilize the grid voltage. To compensate the active power shortage, an auxiliary control strategy based on the hybrid energy storage system is introduced. The simulation results show that the proposed strategy can suppress the voltage fluctuation effectively and make full use of primary energy.

An MVDC-Based Meshed Hybrid Microgrid Enabled Using Smart Transformers

Decentralized integration of distributed generation units and loads is increasing in the modern distribution grid. Maintaining the power flow and power quality within their accepted limits is a challenging task. The formation of meshed hybrid microgrids is an effective method to improve the power system. The smart transformer (ST) is a promising solution to establish meshed hybrid microgrids in the distribution system. This article analyzes the performance of an ST-based meshed hybrid microgrid interconnected to the main grid feeder through medium voltage (MV) dc-link of a second ST. The coordinated operation of interconnected ST system is proposed to explore the features of the configuration. During normal operation, the MVdc bus voltage is controlled by one ST, and this reduces the complexity of the overall control. The main grid and microgrid MVac source failure and converter fault conditions are explored to analyze the reliability of the proposed microgrid structure. Moreover, the reactive power support capability and active power losses for the proposed system are compared with the existing solutions. Simulation and experimental results are presented to show the operation of the proposed system.

Automatic Voltage Control of Large-scale Synchronous Condensers in UHV-DC Converter Station

Large-scale synchronous condenser can provide a powerful dynamic reactive power for the Ultra high-voltage (UHV) Direct Current (DC) transmission system. The dynamic characteristics of synchronous condenser ensure the reactive power support capability in the case of grid fault. In this paper, the control strategy and deployment of the synchronous condensers in East China Power Grid are presented. Taking converter station as an example, this paper introduces the main architecture of automatic voltage control (AVC) system, and the relationship between host computers and slave computers. In the substation, it describes the mechanism of several control modes, and designs the on-site test scheme of the control modes. After on-site testing, the correct operation of the large-scale synchronous condensers can be guaranteed.

Multi‐objective control scheme for operation of parallel inverter‐based microgrids during asymmetrical grid faults

The growing interest in connecting more distributed generation (DG) units to the utility grid, microgrids deal with the various challenges to satisfy a sufficient level of ancillary services such as active power oscillations (APOs), reactive power oscillations (RPOs), fault ride-through (FRT) capability, and overcurrent problem. Hence, for parallel operated grid-connected inverters (GCIs) based MG, this study presents a multi-objective control scheme that simultaneously ensures elimination of the collective APOs/RPOs at point of common coupling (PCC), overcurrent protection and reactive power injection. One of the significant parts of this study compared with similar existing studies is that provides reactive power support capability to fulfil the FRT requirements of the grid-connected multi-DG units and to remain grid-connected during asymmetrical grid faults. A current restraining control is also presented to ensure the safe operation of the MG system and to avoid overcurrent. The cancellation of the collective APOs and RPOs at the PCC for parallel operation of the GCIs has been achieved by using adjustable control coefficients and demonstrated with theoretical analyses in detail. Extensive case studies are presented and discussed to demonstrate the performance of the proposed ideas and to meet the shortcomings of the previous studies.

Voltage Control and Power Loss Reduction in an Active Distribution Network using Solid State Transformers

Power losses in a distribution network accounts for a major part of energy loss in the overall system. Reduction in power losses can make the power system more energy efficient. This paper analyzes the reactive power support capabilities of Solid-State Transformer (SST) by adopting controls on both sides of SST. Moreover, a Moth swarm algorithm is used for selecting the optimal values of phase angles and modulation coefficients of the Voltage Source Converters (VSCs) on both sides of SST. Results show superior performance of the presented reactive power management scheme in terms of voltage violation and power loss of the system. Moreover, bidirectional power flow enables flexible operation of renewable energy sources (RES). Finally, the performance of the control scheme and optimization algorithm is tested on a modified IEEE 33-bus system using MATLAB.

Resilient decentralised control of inverter‐interfaced distributed energy sources in low‐voltage distribution grids

This paper shows that a relation can be found between the voltage at the terminals of an inverter-interfaced Renewable Energy Source RES and its optimal reactive power support. This relationship, known as Volt-Var Curve VVC, enables the decentral operation of RES for Active Voltage Management (AVM). In this paper, the decentralized AVM technique is modified to consider the effects of the realistic operational constraints of RES. The AVM technique capitalizes on the reactive power support capabilities of inverters to achieve the desired objective in unbalanced active Low-Voltage Distribution Systems LVDSs. However, as the results show, this AVM technique fails to satisfy the operator objective when the network structure dynamically changes. By updating the VVCs according to the system configuration and components availability, the objective functions will be significantly improved, and the AVM method remains resilient against the network changes. To keep the decentralized structure, the impedance identification capability of inverters is used to find the system configuration locally. Adaptive VVCs enable the decentralized control of inverters in an online setting. A real-life suburban residential LV-DS in Dublin, Ireland is used to showcasing the proposed method, and the effectiveness of proposed resilient active voltage management technique is demonstrated.

Evaluation of Reactive Power Support Capability of Wind Turbines

Evaluation of Reactive Power Support Capability of Wind Turbines