To reduce CO2 emissions produced by electricity generation, conventional fossil fuel power plants are being decommissioned. Renewable energy sources (RES) and interconnectors are replacing these old power plants but have different operating characteristics. A key concern as the conventional fossil fuel power plants are displaced is the reduction in system inertia. A reduction in system inertia will require faster control schemes to be implemented to prevent frequency and rate of change of frequency (ROCOF) limits from being exceeded. Frequency response schemes that can emulate inertia, often called synthetic inertia, are required and this will necessitate the need for fast controlled devices. VSC-HVDC is a desirable interface option for the power levels and controllability required to provide synthetic inertia. This paper reviews supplementary frequency control schemes applied to VSC-HVDC and co-ordinated control strategies applicable to HVDC connected systems and islanded systems.
Retaining frequency stability is becoming increasingly challenging as the power system incorporates more non-synchronous generation. Assessing the frequency stability in a system has been predominantly completed by focusing on the quantity of connected synchronous kinetic energy in the system, or inertia. This traditionally was considered a function of generator constructionnetwork factors typically were not considered. The research in this paper investigates how network topology, power flow, droop gain distribution, and inertia distribution all impact frequency stability. A generic four-area model has been created that allows discrete system setups. This research has shown that certain topologies lead to a more severe rate of change of frequency. A key finding is that the frequency drop is further increased when there is greater power flow into the area that experiences the disturbance. The extent to which the rate of change of frequency and frequency drop are influenced differently is emphasized, highlighting the need to procure different services depending on which metric is of primary significance at a specific location.
Future frequency control methods will be required to operate in faster timeframes than traditional forms of frequency control. Key to providing fast frequency response within low inertia systems will be power electronic (PE) interfaced energy sources. Along with improving the timeframe and quantity of response, they will also need to be scalable without causing detrimental impacts to other forms of dynamic system behavior. Simply allowing the PE-based fast frequency controls to respond quickly under small disturbances may lead to unwanted oscillatory behavior. To overcome this potential behavior, this paper proposes the use of an adaptive frequency control implementation that uses fast Fourier Transform (FFT) along with an adaptive algorithm to detect oscillatory behavior and adjust the time constant of a first order filter. The use of this adaptive frequency control is shown to significantly reduce oscillatory frequency control action for small disturbances, without reducing the timeframe of operation under large disturbances.
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