Hierarchical automatic voltage control for integration of large-scale wind power: Design and implementation

Abstract Emerging sub-synchronous interactions (SSI) in wind-integrated power systems have added intense attention after numerous incidents in the US and China due to the involvement of series compensated transmission lines and power electronics devices. SSI phenomenon occurs when two power system elements exchange energy below the synchronous frequency. SSI phenomenon related to wind power plants is one of the most significant challenges to maintaining stability, while SSI phenomenon in practical wind farms, which has been observed recently, has not yet been described on the source of conventional SSI literature. This paper first explains the traditional development of SSI and its classification as given by the IEEE, and then it proposes a classification of SSI according to the current research status, reviews several mitigation techniques and challenges, and discusses analysis techniques for SSI. The paper also describes the effect of the active damping controllers, control scheme parameters, degree of series compensation, and various techniques used in wind power plants (WPPs). In particular, a supplementary damping controller with converter controllers in Doubly Fed Induction Generator based WPPs is briefly pronounced. This paper provides a realistic viewpoint and a potential outlook for the readers to properly deal with SSI and its mitigation techniques, which can help power engineers for the planning, economical operation, and future expansion of sustainable development. 

/pdf/review-of-sub-synchronous-interaction-in-wind-integrated-1wgq4n5p.pdf

Review of sub-synchronous interaction in wind integrated power systems: classification, challenges, and mitigation techniques

Due to growing environmental concerns, wind power has taken a larger share of the generation market. While the increase in wind generation provides diversification of the supply, it also undermines system reliability by introducing uncertainty in power supply and decreasing the maneuverability of the grid in terms of VAR absorbance and provision. To restore the reliability of high wind penetrated systems, this paper proposes hierarchical voltage control (HVC). The proposed pattern generalization methodology is based on principle component analysis and a three-layer feedback architecture utilizing online state estimation. The proposed control includes both steady-state and dynamic constraints. Governed by an optimization trained with cases representing operating snapshots and solved via an active-set algorithm adopting stepwise linearization, the proposed HVC provides efficiency and adaptability. As a result, the proposed control provides adequate system support under a wide range of contingencies and operating conditions to enhance grid reliability. Its capability to regulate yearly round operations in real time is validated using a reduced Western Electricity Coordinating Council power system model.

Wide Area Hierarchical Voltage Control to Improve Security Margin for Systems With High Wind Penetration

In Part I of this work, a static voltage security region was introduced to guarantee the safety of wind farm reactive power outputs under both base conditions and N-1 contingency. In this paper, a mathematical representation of the approximate N-1 security region has further studied to provide better coordination among wind farms and help prevent cascading tripping following a single wind farm trip. Besides, the influence of active power on the security region is studied. The proposed methods are demonstrated for N-1 contingency cases in a nine-bus system. The simulations verify that the N-1 security region is a small subset of the security region under base conditions. They also illustrate the fact that if the system is simply operated below the reactive power limits, without coordination among the wind farms, the static voltage is likely to exceed its limit. A two-step optimal adjustment strategy is introduced to shift insecure operating points into the security region under N-1 contingency. Through extensive numerical studies, the effectiveness of the proposed technique is confirmed.

https://www.mdpi.com/1996-1073/7/1/444/pdf

A Static Voltage Security Region for Centralized Wind Power Integration-Part II: Applications

Connecting high penetration of wind power into power grid usually makes voltage fluctuate, due to the volatile nature of wind power injection. This paper therefore proposes a quadratic robust optimization model to guarantee the voltage of each wind unit within the security region, no matter how the wind power varies. Based on the wind power prediction, the prediction error is regarded as uncertainties, and the robust solution can be found by regulating the reactive power equipment and each wind unit using the duality filter method. In the proposed model, linearized derivation instead of original non-linear expressions in the objective function and constraints has been utilized. Furthermore, inner loop iterative method is introduced to reduce the linearized error, which divides the optimal voltage control into multiple steps with piecewise values and updates the sensitivity at each step, according to different wind farm size and condition. A test system with 36 wind units has been simulated, and the result using Monte Carlo simulation. Comparison with traditional method shows the effectiveness of proposed method.

A quadratic robust optimization model for automatic voltage control on wind farm side

The commutation failure (CF) mitigation effectiveness is normally restricted by the delay of extinction angle (EA) measurement or the errors of existing prediction methods for EA or firing angle (FA). For this purpose, this paper proposes a CF mitigation method based on the imaginary commutation process. For each sample point, an imaginary commutation process is constructed to simulate the actual commutation process. Then, the imaginary EA is calculated by comparing the imaginary supply voltage-time area and the imaginary demand voltage-time area, which can update the imaginary EA earlier than the measured EA. In addition, the proposed method considers the impacts of commutation voltage variation, DC current vari-ation, and phase angle shift of commutation voltage on the commutation process, which can ensure a more accurate EA calculation. Moreover, the DC current prediction is proposed to improve the CF mitigation performance under the single-phase AC faults. Finally, the simulation results based on CIGRE model prove that the proposed method has a good performance in CF mitigation. 

/pdf/commutation-failure-mitigation-method-based-on-imaginary-d2ubauf2.pdf

Commutation Failure Mitigation Method Based on Imaginary Commutation Process

Three critical cascading faults with nearly 2000 WTGs (Wind Turbine Generator) tripped took place in China on early 2011, from which the reactive power and voltage control issues for large wind farms are spotlighted. Some key factors inducing the cascading trip are surveyed by this paper firstly, including WTGs voltage distribution, dynamic reactive power reserves and coordination of tight-coupled wind farms. Rather than only focusing on the PCC (Point of Common Coupling) voltage, AVC (Automatic Voltage Control) in a wind farm is actually a network-based control considering voltage distribution and drop on the radial feeders. A hierarchical control AVC (Automatic Voltage Control) framework considering wind farms integration is presented and the detailed functions are introduced, involving network model, state estimation, 3-D visualization based monitoring, power flow & sensitivity calculation, security assessment & early warning and control strategy decision. Such an AVC system has been applied into several wind farms, one of which is selected as an example to introduce the configuration and deployment details. The distributed AVC system is also a mini-EMS (Energy Management System) prototype for large wind farms.

Distributed Automatic Voltage Control framework for large-scale wind integration in China

In China, wind power industry has undergone great development in the last few years, while several cascading faults happened in 2011, reveal that terminal voltage of wind turbine generator (WTG) should be monitored and controlled. Furthermore WTGs and static var compensators/generators (SVC/SVGs) need to be coordinated to provide voltage service for the whole network. In this paper, a network model based coordinated automatic voltage control (AVC) strategy for wind farm (WF) is proposed: three control modes out of eight control zones are defined and implemented to accomplish three control objectives. A test AVC system is built to prove the proposed characteristics, and on-site applications in WFs are ongoing now.

Network model based coordinated automatic voltage control strategy for wind farm

The invention relates to a method for calculating sensitivity of wind power plant reactive power on voltage based on a perturbation method and belongs to the technical field of automatic voltage control of a power system. The method comprises the following steps of: identifying the equivalent impedance of an external power grid accessing to the wind power plant according to the sensitivity of reactive power at a wind power plant grid-connected point on the voltage of the wind power plant through the perturbation method, and forming a complete power network together with a power network in the wind power plant; combining real-time measurement in the wind power plant to form a jacobian matrix, and calculating to obtain the sensitivity matrix of the reactive powers among the nodes in the wind power plant on the voltage; and finally, checking the calculation result by using the obtained sensitivity of the reactive power on the voltage through the perturbation method, if not, identifying the equivalent impedance of the external power grid again, and calculating the sensitivity matrix of the reactive power on the voltage until the sensitivity matrix passes the check. According to the method, the high-accuracy sensitivity matrix of the reactive power on the voltage is obtained finally and is used for voltage control in the wind power plant.

Method for calculating sensitivity of wind power plant reactive power on voltage based on perturbation method

Method for enhancing voltage safety margin of wind farm

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