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Power systems are undergoing fundamental changes with increasing penetration of renewable energies. A reactive power ancillary service market is of significance to incentivize private providers, such as thermal power units and renewable power plants (RPPs), to participate in maintaining voltage security. Because of the local nature of reactive power, one of the recognized issues is that monopolistic participants may have great market power and strategically manipulate their costs. To pursue economic efficiency, this paper proposes an incentive-compatible mechanism based on Vickery-Clarke-Groves (VCG) auctions for the day-ahead reactive power ancillary service market. The obligations of reactive power ancillary service are quantified in the VCG-based market mechanism. Depending on the expected performance of participants compared with the corresponding obligations, payments are obtained for thermal power units, RPPs, and loads. Hence, providers have the incentive to declare their true costs and undertake obligations. Furthermore, a scarcity price is employed to reward the provision of scarce reactive power resources and ensure reliable market clearing. The proposed mechanism is illustrated using the IEEE 24-bus and 118-bus systems. The results verify the effectiveness of the proposed mechanism. Moreover, high rewards or penalties are observed for voltage weak areas, thereby inducing appropriate investments in reactive power capacities. [ABSTRACT FROM AUTHOR]

Inverter-based Volt/Var control (VVC) methods can effectively address issues of voltage deviations and constraint violations, as well as reduce network power losses in active distribution networks. Conventional VVC mainly relies on rule-based, mathematical, or heuristic methods. However, these methods can be inefficient or even infeasible for large-scale systems, especially when operational uncertainties are considered at both spatial and temporal scales. To this end, firstly, this paper formulates a multi-region coordinated VVC optimization problem to simultaneously minimize bus voltage deviations and network power losses. Furthermore, with the problem being formulated as a partially observable Markov Game, a multi-agent deep deterministic policy gradient (MADDPG) algorithm is modified to solve this problem. Each region, which is a sub-network with a control center, is modeled as an agent and learns to optimize inverter reactive power output setpoints through exploration in virtual environment and improved neural network training procedure. Spatial and temporal uncertainties of photovoltaic power generation and loads are modeled by stochastic programming as scenarios in the MADDPG algorithm. This algorithm can address the uncertainties to guarantee solution robustness. Thus, this paper eventually proposes a robust regionally coordinated VVC method, and numerical simulations indicate high computing efficiency and robust optimum. [ABSTRACT FROM AUTHOR]

With the increasing development of AC/DC hybrid power systems, local synchronous generators in receiving-end grids are being replaced by high-voltage direct current (HVDC) power. The risk of short-term voltage instability exists because of the decline in dynamic reactive power resources from generators, the presence of high-proportioned induction motors and the demand for vast reactive power of HVDC under AC faults. In this situation, unit commitment (UC) should take short-term voltage stability into account. Hence, crucial generators can be retained to provide sufficient dynamic reactive power. This paper proposes a UC model incorporating short-term voltage stability constraints (SVSCUC). Furthermore, a decomposition-based solution approach is presented, which involves an alternate iterative solving of UC and differential-algebraic equations (DAEs). Time-domain simulation (TDS) is utilized to quantify the stability with an indicator. Linear stable cuts are formulated through trajectory sensitivity analysis and added into the UC. The UC is decomposed into a master problem based on mixed-integer linear programming (MILP) and subproblems. The SVSCUC is illustrated using benchmark systems and a real system in eastern China. The results show the effectiveness and efficiency of the proposed methods. Violations of short-term voltage stability for credible contingencies are eliminated under the commitment of SVSCUC. [ABSTRACT FROM AUTHOR]

Increasing distributed energy resources (DERs) may result in reactive power imbalance in a transmission power system (TPS). An active distribution power system (DPS) having DERs reportedly can work as a reactive power prosumer to help balance the reactive power in the TPS. The reactive power potential (RPP) of a DPS, which is the range between the maximal inductive and capacitive reactive power that the DPS can reliably provide, should be accurately estimated. However, an accurate estimation is difficult because of the network constraints, mixed discrete and continuous variables, and the considerable uncertainty in the DPS. To solve this problem, this paper proposes a robust RPP estimation method based on two-stage robust optimization, where the uncertainty in DERs and the boundary-bus voltage is considered. In this two-stage robust model, the RPP is pre-estimated in the first stage and its robust feasibility for any possible instance of the uncertainty is checked via a tractable problem in the second stage. The column-and-constraint generation algorithm is adopted, which solves this model in finite iterations. Case studies show that this robust method yields a completely reliable RPP, and also that a DPS, even under the uncertainty, is still an effective reactive power prosumer for the TPS. [ABSTRACT FROM AUTHOR]

In this paper, we review the emerging challenges and research opportunities for voltage control in smart grids. For transmission grids, the voltage control for accommodating wind and solar power, fault-induced delayed voltage recovery, and measurement-based Thévenin equivalent for voltage stability analysis are reviewed. For distribution grids, the impact of high penetration of distributed energy resources is analyzed, typical control strategies are reviewed, and the challenges for local inverter Volt–Var control is discussed. In addition, the motivation, state-of-the-art, and future directions of the coordination of transmission system operators (TSO) and distribution system operators (DSO) are also thoroughly discussed. [ABSTRACT FROM AUTHOR]

Line-commutated-converter-based high-voltage direct current (LCC-HVDC) is an important and attractive method of transmitting large-scale integrated wind power. However, the contingencies induced by LCC-HVDCs and nearby areas usually lead to an overvoltage problem in hybrid AC/DC sending-side systems, which may result in severe cascading trip problems. Therefore, a robust voltage control strategy for hybrid AC/DC sending-side systems is proposed in this paper to prevent cascading trip failures and accommodate more wind power. The proposed strategies address the voltage control complexity of LCC-HVDCs and converter stations, appropriately formulate the different locally fast-response switching strategies of filter banks and different control modes of the LCC-HVDCs in the automatic voltage control system. To compute the formulated problem efficiently and quickly for online application, a method of transforming the robust optimization problem and the Benders decomposition method are introduced. Based on case studies, the computational efficiency and accuracy of the proposed method are determined using operational data for the TianShan District in the Northwest China Power Grid. [ABSTRACT FROM AUTHOR]

It is desirable to develop robust controllers for fast frequency/voltage regulation and active/reactive power sharing in an islanded microgrid with high penetration level of renewable energy sources. This paper proposes a fully distributed discrete two-level control strategy for islanded microgrids. The upper-level control determines the active/reactive power generation references to minimize the overall voltage deviations while achieving accurate active/reactive power sharing. The lower-level control enables the inverter to track the power references by adjusting inverter output voltage magnitude and angle. A flexible finite-step convergence tracking controller, which considers the uncertainties and disturbances, is introduced to improve the robustness and control accuracy. The convergence and optimality of the proposed discrete control algorithm are validated by rigorous analysis. The effectiveness of the proposed distributed control approach is verified by simulation results. [ABSTRACT FROM AUTHOR]

The recent rapid development of distributed generation (DG) necessitates the consideration of transmission–distribution interactions in voltage stability assessment (VSA), and how to perform a transmission–distribution-coupled static VSA (TDVSA) has become a pressing problem. To resolve this urgent issue, this paper first theoretically analyzes the impacts of the voltage-maintaining capacity and low-voltage tripping of DG units on the static voltage stability of an integrated transmission–distribution system and then proposes a distributed TDVSA method that fully considers these impacts when assessing the voltage stability of the integrated system. As the core of the method, a distributed continuation power flow algorithm is designed that includes a distribution-equivalencing-based predictor, a distributed transmission–distribution corrector, and a step-length regulator and requires only a limited amount of data to be exchanged between the transmission system operator (TSO) and the distribution system operator (DSO). Numerical simulations show that even in the case of low DG penetration, a cascading low-voltage tripping event can significantly degrade an integrated system's voltage stability, and only the proposed distributed TDVSA method can produce an accurate assessment. In addition, the number of iterations between the TSO and DSO is shown to be limited, indicating the potential of applying the proposed method to future grids. [ABSTRACT FROM PUBLISHER]

To prevent the wind units from cascading trip faults, the concept of an autonomous voltage security region (AVSR) has been discussed in previous studies. However, the AVSR is sometimes not sufficiently large due to the insufficient dynamic reactive power reserve (DRPR) under normal conditions, resulting in secure operation problems and a large amount of wind curtailment. Therefore, reservation of an appropriate capacity of dynamic reactive power is a crucial issue in wind farms, so a DRPR optimisation model to enlarge the AVSR is proposed in this work. The computation of the optimal DRPR includes six steps and involves the exchange of information between wind farms and the control centre. The computation is rapid and can be applied to real wind systems online. The proposed method demonstrated good performance in a simple test case and a real system in Northern China. [ABSTRACT FROM AUTHOR]

Autonomous voltage security regions (AVSRs) have been proposed to prevent cascading trip faults in wind farms. However, due to insufficient dynamic reactive power (DRP) support and unreasonable DRP allocation in large‐scale, centralised wind power integration areas, especially for high wind power penetrations, AVSRs may be too small to guarantee secure wind farm operation, resulting in large amounts of wind power curtailment. DRP allocation is crucial for wind farms to accommodate more wind power and decrease curtailment. Therefore, a DRP allocation scheme based on the AVSR to accommodate more wind power is proposed in this work. An improved AVSR considering wind power curtailment is first derived; the function between the AVSR, DRP reserves and curtailment is then obtained, and an optimal DRP allocation scheme using Benders decomposition is finally proposed. Good performance is observed when tested on a real large‐scale wind power base in Northern China. [ABSTRACT FROM AUTHOR]

This paper presents an autonomous wind farm voltage controller based on model predictive control. The reactive power compensation and voltage regulation devices of the wind farm include static Var compensators, static Var generators, wind turbine generators and on-load tap changing transformer, and they are coordinated to keep the voltages of all the buses within the feasible range. Moreover, the reactive power distribution is optimized throughout the wind farm in order to maximize the dynamic reactive power reserve. The sensitivity coefficients are calculated based on an analytical method to improve the computation efficiency and overcome the convergence problem. Two control modes are designed for both voltage violated and normal operation conditions. A wind farm with 20 wind turbines was used to conduct case studies to verify the proposed coordinated voltage control scheme under both normal and disturbance conditions. [ABSTRACT FROM PUBLISHER]

Cascading trip faults in large-scale wind power centralized integration areas bring new challenges to the secure operation of power systems. In order to deal with the complexity of voltage security regions and the computation difficulty, this paper proposes an autonomous voltage security region (AVSR) for each wind farm and the point of common coupling (PCC) substation, whose voltage can be controlled in a decoupled way. The computation of the AVSR can be completed using a stepwise search method exchanging voltage and power information between the control center and the wind farms. At each wind farm, an AVSR is determined to guarantee the normal operation of each wind turbine generator (WTG), while in the control center, each region is designed in order to guarantee secure operation both under normal conditions and after an N-1 contingency. A real system in Northern China was used to carry out case studies to verify the effectiveness of the AVSRs proposed, and good performance was demonstrated using the Monte Carlo method. [ABSTRACT FROM PUBLISHER]

A multivariate statistical analysis (MSA)‐based power‐grid‐partitioning method is proposed. Considering the effectiveness of automatic voltage regulation of generators, network modelling is based on the quasi‐steady‐state sensitivity of voltage with respect to reactive power. The MSA‐based power‐grid‐partitioning method contains two stages: a quantitative method for determining the appropriate number of zones and allocation of nodes into a number of zones. The proposed method is applied to an automatic voltage control system and an N − 1 robust pilot node selection method is presented. Simulation studies on IEEE 14‐bus, 39‐bus systems and a real provincial power system in China are described to illustrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Integration of centralized wind farms may induce cascading tripping incidents, which brings about critical challenges to secure operation of power grid. One of the major risks is the lack of coordinated voltage/reactive power control for multiple wind farms. Therefore, a robust master-slave two-level coordinated static voltage security region (VSR) is proposed in this paper, along with detailed modeling of the network topology and wind farms to achieve coordination among wind farms. The slave VSR on the wind-farm-side is designed to guarantee the operation of each wind unit within a secure range in order to protect wind units from serious over-voltage and low-voltage problems, whereas the master system-wide VSR aims to coordinate centralized wind farms to ensure security both under normal operating conditions and \boldsymbol N - 1 contingency conditions. Furthermore, a hierarchically iterative coordination method is introduced to compute the voltage security range of each wind farm through the iteration process. A modified six-bus system with two wind farms, a real system from Northern China, and a large system with 972 wind units are studied, and results verify the effectiveness of the proposed two-level VSR and the hierarchically iterative coordination method. [ABSTRACT FROM PUBLISHER]

With increasingly large scale of intermittent and non-dispatchable resources being integrated into power systems, the power flow problem presents greater uncertainty. In order to obtain the upper and lower bounds of power flow solutions including voltage magnitudes, voltage angles and line flows, Cartesian coordinates-based power flow is utilized in this paper. A quadratically constrained quadratic programming (QCQP) model is then established to formulate the interval power flow problem. This non-convex QCQP model is relaxed to linear programming problem by introducing convex and concave enclosures of the original feasible region. To improve the solutions bounds while still encompassing the true interval solution, optimality-based bounds tightening (OBBT) method is employed to find a better outer hull of the feasible region. Numerical results on IEEE 9-bus, 30-bus, 57-bus, and 118-bus test systems validate the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

The power industry in China has grown significantly over the past decade, spurring the adoption of system-wide automatic voltage control (AVC) technology to meet stricter requirements for security and economical power system operation. To cope with the rapidly developing and frequently changing Chinese power grid, an AVC scheme based on adaptive zone division is introduced in this paper. Logically, this type of system has a three-level hierarchical structure, but, here, both secondary voltage control (SVC) and tertiary voltage control (TVC) are implemented via software at the same control center. The control zones are no longer fixed but are reconfigured online and updated in accordance with variations in the grid structure. The technical details of these procedures are presented here. The implementation of TVC (which is based on an online reactive optimal power flow) and SVC are also discussed, together with some technical details on improvements to their reliability and robustness. Some results obtained from field-site applications based on similar-days testing rather than simulations are employed to evaluate the performance and improvements of the AVC system. This system has been installed at over 20 control centers in China. [ABSTRACT FROM PUBLISHER]

With the integration of intermittent resources, more reserves are required for uncertainties. Traditionally, reactive power optimization in transmission network focuses on loss minimization problems, regarding loads as voltage-independent injections. In fact, the bus voltage magnitude affects the load active/reactive injection, providing a possibility for system operators to regulate the power of loads through voltage regulation. Inspired by this inherent feature of loads, this paper considers the regulation of voltage-dependent loads (VDLs) for fast reserves through reactive power optimization and voltage control under the coordination of transmission and distribution networks. A two-stage multi-objective optimal power flow model is developed to incorporate reserves from VDLs. The first stage is to optimize the minimization of losses and the maximization of the reserves that the VDLs can provide, which is modeled as an AC optimal power flow problem. In the second stage, it is verified whether the reserves from the first stage can be delivered by voltage regulation to settle down the possible imbalance power in the power system. Case studies show that the proposed method can estimate the amount of reserves from VDLs and optimize the bus voltages accordingly. [ABSTRACT FROM AUTHOR]