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Voltage Stability Of Electric Power Systems

This paper proposes an adaptive droop control (ADC) strategy for the voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) system, which enables the VSC-station to provide frequency regulation for onshore ac grids. A V-I-f characteristic is derived to establish the relationship between the frequency deviation and dc voltage. A VSC-station working with the V-I-f characteristic is able to adjust its dc voltage reference autonomously according to the grid frequency deviation. Thus, the power flow of the VSC-MTDC system will be redistributed and more power is going to be injected into the adjacent grid of which the frequency deviation exceeds the threshold value. The ADC strategy inherits the advantage of the traditional droop control (TDC) that there is no need for communication system. An ac/dc hybrid system that includes VSC-MTDC system, onshore grids, and offshore wind farms is built in DIgSILENT/Powerfactory. Modal analysis and nonlinear simulations are performed to obtain the optimal control parameters and demonstrate the performance of the ADC strategy.

Adaptive Droop Control of VSC-MTDC System for Frequency Support and Power Sharing

The continuing increase of wind energy penetration into power systems, in combination with the retirement of conventional generation, raises new challenges for the maintenance of power system stability. This paper presents a comprehensive review of wind power plant capabilities to provide frequency support and the corresponding methods available in the published literature are thoroughly analysed and compared. The topic is covered from different perspectives giving a comprehensive overview on the work carried out in this field. In addition, the integration of energy storage technologies and dispatching of wind farms during frequency deviations are thoroughly discussed. Finally, technical challenges, future research lines and general recommendations are provided.

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A review on frequency support provision by wind power plants: Current and future challenges

With the rapid increase of renewable energy sources and power electronic converters in the power system, the inertia of the power system is declined and the stability problem is more serious than ever before. In this paper, a virtual synchronous generator (VSG) control strategy is proposed to enable the voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) system to damp the low-frequency oscillation modes. Both rotational inertia part and governor part are modeled in the VSG controller. The $V^2-P-\omega$ characteristic is developed based on the $V^2-P$ droop control to emulate the action of the speed governor when the frequency of ac grid deviates from the nominal value. The virtual inertia part is used to emulate the inertia response of the synchronous generator. The communication between VSC stations is unnecessary with the autonomous power allocation ability of the $V^2-P-\omega$ characteristic. Modal analysis is utilized to test the system stability and optimize the controller parameters. Two test systems which include offshore wind farms, VSC-MTDC systems, and onshore power grids are built in the DIgSIENT/PowerFactory. The simulation results verify the effectiveness of the proposed VSG strategy.

A Virtual Synchronous Generator Control Strategy for VSC-MTDC Systems

Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

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Coordinated Control Strategies for Offshore Wind Farm Integration via VSC-HVDC for System Frequency Support

Modern large wind farms are required to provide frequency regulation service like conventional synchronous generation units. The frequency support capability of modern wind farms has been widely investigated and implemented. Remotely located large offshore wind farms are probably connected to the onshore system grid through voltage-source converter-based–high voltage direct current (VSC-HVdc) transmission systems. Due to the decoupling of VSC-HVdc and signal transmission delay, offshore wind farms may not be able to respond to the onshore grid frequency excursion in time and, consequently, the stability and security of the power system will be put at risk, especially for those with high wind penetration. This paper proposes a coordinated control scheme to allow VSC-HVdc link to contribute to the system frequency regulation by adjusting its dc-link voltage. By means of this approach, the dc capacitors of VSC-HVdc are controlled to absorb or release energy so as to provide frequency support. To further enhance the system frequency response, the frequency support from VSC-HVdc is also finely coordinated with that from offshore wind farm according to the latency of offshore wind farm responding to onshore grid frequency excursion. The control scheme is evaluated for both underfrequency and overfrequency events, and results are presented to demonstrate its effectiveness.

Contribution of VSC-HVDC to Frequency Regulation of Power Systems With Offshore Wind Generation

Wind energy is consistently attracting great research effort and actively developed in many countries. As a result, the penetration level of wind power in the power grid is increasing as well as the size of wind farms. A large-scale wind farm may consist of hundreds of wind turbines and its total installed capacity could be at a level of 1000MW or even more. Consequently, the large-scale wind farm could seriously impact the operation and control of the grid. To represent a large-scale wind farm, aggregated modelling takes advantage of fast computation and simplified implementation compared to detailed modelling that models every wind turbines individually and the interconnections among them. In this paper, three aggregated modelling techniques, namely, multi-machine equivalent aggregation, full aggregation and semi-aggregation are presented for power system transient stability studies based on a wind farm with permanent magnet synchronous generator (PMSG) wind turbines. Simulation results of the aggregated models and the detailed model are compared and analyzed respectively to prove the effectiveness of the aggregating techniques.

Aggregated Modelling for Wind Farms for Power System Transient Stability Studies

The potential of offshore wind energy has been commonly recognized and explored globally. Many countries have implemented and planned offshore wind farms to meet their increasing electricity demands and public environmental appeals, especially in Europe. With relatively less space limitation, an offshore wind farm could have a capacity rating to hundreds of MWs or even GWs that is large enough to compete with conventional power plants. Thus the impacts of a large offshore wind farm on power system operation and security should be thoroughly studied and understood. This paper investigates the impacts of integrating a large-scale offshore wind farm into the transmission system of a power grid through VSC-HVDC connection. The concerns are focused on steady-state voltage stability, dynamic voltage stability and transient angle stability. Simulation results based on an exemplary power system are presented and analyzed.

Impacts of large-scale offshore wind farm integration on power systems through VSC-HVDC

This paper describes fault ride-through and grid support of offshore wind farms based on permanent magnet synchronous generator (PMSG) wind turbines connected to the onshore AC network through two alternative transmission systems: high voltage AC (HVAC) or high voltage DC (HVDC) based on voltage source converters (VSC). The proposed configurations of the PMSG-based offshore wind farm and VSC-based HVDC are given as well as their control strategies under both steady state and fault state. The PMSG-based offshore wind farm is integrated into a test power transmission system via either HVAC or VSC-HVDC for comparison. DIgSILENT simulations are presented to demonstrate the performance of the proposed systems during a severe onshore AC fault at the point of common coupling (PCC).

Fault ride-through and grid support of permanent magnet synchronous generator-based wind farms with HVAC and VSC-HVDC transmission systems

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Hongzhi Liu

Papers

Contribution of VSC-HVDC to Frequency Regulation of Power Systems With Offshore Wind Generation

Aggregated Modelling for Wind Farms for Power System Transient Stability Studies

Impacts of large-scale offshore wind farm integration on power systems through VSC-HVDC

Fault ride-through and grid support of permanent magnet synchronous generator-based wind farms with HVAC and VSC-HVDC transmission systems

Grid Integration of Offshore Wind Farms via VSC-HVDC - Dynamic Stability Study