Olimpo Anaya-Lara

Control of DFIG-based wind generation for power network support

This paper addresses the design and implementation of a novel control scheme for a doubly fed induction generator (DFIG), of the type employed with wind turbines, to provide support to power system operation. It is shown that this controller provides a DFIG-based wind farm with operational and control compatibility with conventional power stations, the ability to contribute to voltage support and recovery following network faults, the ability to provide a power system stabilizer capability that improves overall system damping, and the capability of contributing short-term frequency support following loss of network generation. A simple but realistic test network that combines synchronous and wind farm generation has been modeled and used to assess dynamic performance. Simulation results are presented and discussed that demonstrate the capabilities and contributions of the new DFIG controller to network support.

A power system stabilizer for DFIG-based wind generation

A power system stabilizer (PSS) for a wind turbine employing a doubly fed induction generator (DFIG) is presented. It is shown that this PSS can significantly influence the contribution that a DFIG-based wind farm can make to network damping. A simple, generic test network that combines synchronous and wind farm generation is used to demonstrate system performance contributions. The results of both eigenvalue analysis and time response simulation studies are presented to illustrate contributions to network dynamic and transient performance that the DFIG controller with its PSS can make. Performance capabilities superior to those provided by synchronous generation with automatic voltage regulator and PSS control are demonstrated.

Small-Signal Stability Analysis of Multi-Terminal VSC-Based DC Transmission Systems

A model suitable for small-signal stability analysis and control design of multi-terminal dc networks is presented. A generic test network that combines conventional synchronous and offshore wind generation connected to shore via a dc network is used to illustrate the design of enhanced voltage source converter (VSC) controllers. The impact of VSC control parameters on network stability is discussed and the overall network dynamic performance assessed in the event of small and large perturbations. Time-domain simulations conducted in Matlab/Simulink are used to validate the operational limits of the VSC controllers obtained from the small-signal stability analysis.

Fault Ride-Through Improvement of DFIG-WT by Integrating a Two-Degrees-of-Freedom Internal Model Control

A novel two-degree-of-freedom internal model control (IMC) controller that improves the fault ride-through (FRT) capabilities and crowbar dynamics of doubly fed induction generator (DFIG) wind turbines is presented. As opposed to other control strategies available in the open literature, the proposed IMC controller takes into account the power limit characteristic of the DFIG back-to-back converters and their dc-link voltage response in the event of a fault and consequent crowbar operation. Results from a digital model implemented in Matlab/Simulink and verified by a laboratory scale-down prototype demonstrate the improved DFIG FRT performance with the proposed controller.

Influence of Windfarms on Power System Dynamic and Transient Stability

The interaction between bulk windfarm generation and conventional generation and its influence on network dynamic characteristics are investigated. A simple three-generator system having characteristics that may be considered representative of the major areas of the UK network is used for the dynamic studies. Time response simulation and eigenvalue analysis are used to establish basic transient and dynamic stability characteristics. The wind generation is provided either by windfarms based on Fixed Speed Induction Generators (FSIGs) or Doubly Fed Induction Generators (DFIGs). In addition, in order to provide a base-line, against which windfarm influence on network dynamics can be judged, the case where the power is provided only by conventional synchronous generation is also considered.

Impacts of High Penetration of DFIG Wind Turbines on Rotor Angle Stability of Power Systems

With the integration of wind power into power systems continues to increase, the impact of high penetration of wind power on power system stability becomes a very important issue. This paper investigates the impact of doubly fed induction generator (DFIG) control and operation on rotor angle stability. Acontrol strategy for both the rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG is proposed to mitigate DFIGs impacts on the system stability. DFIG-GSC is utilized to be controlled as static synchronous compensator (STATCOM) to provide reactive power support during grid faults. In addition, a power system stabilizer (PSS) is implemented in the reactive power control loop of DFIG-RSC. The proposed approaches are validated on a realistic Western System Coordinating Council (WSCC) power system under both small and large disturbances. The simulation results show the effectiveness and robustness of both DFIG-GSC control strategy and PSS to enhance rotor angle stability of power system.

Influence of Tower Shadow and Wind Turbulence on the Performance of Power System Stabilizers for DFIG-Based Wind Farms

The aim of the paper is to demonstrate the way in which mechanical power variations, due to tower shadow and wind turbulence, influence control performance of power system stabilizer (PSS) loops for doubly-fed induction generators (DFIGs). The PSS auxiliary loops are applied on a specific DFIG control scheme, the flux magnitude and angle controller (FMAC). However, since the PSS signal is applied at the output of the basic controller, the PSS performance characteristics displayed are deemed typical for DFIG control schemes in general. The relative capabilities of PSS controllers based on stator power, rotor speed, and network frequency, when the DFIG turbine is subjected to aerodynamic torque variations, are investigated via simulation studies. A two-generator aggregate model of a wind farm is introduced, which enables the influence of tower shadow and wind turbulence on both an individual turbine and on the overall wind farm itself to be assessed.

Aggregated Wind Turbine Models for Power System Dynamic Studies

The interaction between a power system and a wind-farm can be studied using either a simplified ‘aggregated’ or a complex ‘detailed’ model. The detailed-model results compare well with the actual, therefore, provided that both models have similar dynamic characteristics, the aggregated model provides a rapid and cost-effective way of representing large wind-farms in power system dynamic studies. A brief overview of wind-farm aggregation methods is given and their application in power systems analysis is discussed. Aggregation techniques are then applied to a wind-farm with thirty wind turbines. It is shown that the aggregation of the swing equation and equivalent representation of network and generator impedances can be used for aggregated representation of fixed speed wind turbines. A similar approach applied to the d-and q-axis controller of a doubly-fed induction generator (DFIG) failed initially to provide satisfactory dynamic characteristics with the aggregated model. Consequently, a simple method was used to scale the rotor currents of the controller, so the dynamic characteristics with DFIGs became acceptable.

Control strategies of VSC-HVDC transmission system for wind power integration to meet GB grid code requirements

This paper presents control strategies for a DC transmission system connecting onshore/offshore wind farms to the power network. These control strategies aim to allow the VSC-HVDC transmission system to meet the Great Britain (GB) Grid Code regarding frequency support, voltage/or reactive power support and Fault Ride-Through capability. This paper utilizes the kinetic energy of the wind turbine generators and the energy stored in the DC link capacitors of wind farm side converter to provide short to medium term frequency support to the AC network. The grid side converter is used to provide reactive power/or voltage support to the AC network during AC faults which cause voltage dips as low as 0.15pu (retained voltage). A reduced network model of Scotland and England has been implemented in Matlab/Simulink to assess control performance. Key results are presented and discussed.

Adaptive Zone Identification for Voltage Level Control in Distribution Networks With DG

A decentralized voltage control is proposed for distributed generation (DG) units to provide short and long-term voltage support in distribution networks. Local controllable zones are used to determine the voltage control boundaries for each DG unit. The number of zones and their size depend on the number, location and size of the DG units, and can be reconfigured in real time in response to network topology changes. The performance and value of the proposed control approach are demonstrated under various operating scenarios. The study is based on the IEEE 33-bus radial distribution network implemented in DIgSILENT PowerFactory.