Jiebei Zhu

A review of control methods for providing frequency response in VSC-HVDC transmission systems

With the ultimate aim to remove/reduce constraints on the amount of non-synchronous generators that may be connected to power systems, this paper identifies potential problems associated with system dynamics under high penetrations of converter-interfaced sources, especially from the perspective of inertia and responses to disturbances. The configuration of VSC-HVDC transmission systems and their controllers is introduced and analysed. Methods that have been proposed to enable VSC-HVDC systems to provide frequency response are reviewed, and a typical VSC-HVDC system with an associated control system is built and validated in Matlab Simulink. These models will form a key part of a modelling toolkit that is being developed to investigate optimal methods for reducing (or removing) future Non-Synchronous Generation (NSG) penetration constraints in the power system in the UK, which represents further work that will be conducted in this project.

Development of models to study VSC response to AC system faults and the potential impact on network protection

As the utilization of renewable energy sources (RES) and HVDC links is growing rapidly, many characteristics of the resulting power system can be seriously changed. HVDC links, as well as RES using converters as an interface with the main grid, can be all treated as non-synchronous sources. These sources are different from the traditional synchronous generators in many ways and bring significant challenges to the existing protection systems. Therefore, the aim of this paper is to explore power system protection performance issues under the context of the main characteristics of future power networks. In this paper the operating principles of converters is investigated. Initial equivalent models of Voltage Source Converters (VSC) in response to both symmetrical and unsymmetrical faults in the AC power systems are introduced and developed. Such models explain the characteristics of the future power networks with focuses on protection system performance. The VSC models will investigate system performance under fault conditions taking into account of European HVDC Grid Code requirements proposed by the European Network of Transmission System Operators for Electricity (ENTSO-E)[1].

Synthetic Inertia Control Strategy for Doubly Fed Induction Generator Wind Turbine Generators Using Lithium-Ion Supercapacitors

To address the system operability challenges due to the continuous reduction of system inertia by increasing penetration of renewable power generation, this paper proposes a new synthetic inertia control (SIC) strategy for doubly fed induction generator (DFIG)-based wind turbine generators. The proposed SIC scheme enables individual DFIG generators to contribute an effective inertial response which helps to stabilise the rate of change of frequency and minimize the maximum frequency deviations in particular under severe disturbance conditions. To achieve the proposed SIC, lithium-ion supercapacitors are required in the dc link of the DFIG power electronic conversion system to increase extra energy storage capability. The DFIGs dc voltage operates at various levels following the derived SIC algorithms for energy exchange with the connected ac grid, and the conventional control system for DFIG is modified to implement the algorithms. The functions and system benefits of SIC scheme are verified using a DFIG model connected to a power system and compared with the simulations of conventional DFIG control scheme. Practical issues such as size, cost, weight, connection configuration, and charging/discharging rate of lithium-ion capacitors for the SIC are discussed and relevant recommendations are presented.