Emulated Inertia and Frequency Support from Fast Acting Reserves

Abstract

Environmental concerns are increasingly driving the evolution of the energy sector. A high share of renewable energy sources (RES) into the power system will potentially support the reduction of CO2 emissions; however, this presents considerable challenges for the power system operators. Currently, transmission system operators (TSOs) and distribution system operators (DSOs) are facing substantial change in the power system architecture, moving from centralized architecture, where electricity is generated from large power plants by conventional synchronous generators, centrally located and connected to the transmission network into a distributed architecture, where electricity is generated by distributed energy resources (DER) converter connected to distribution networks. The transition towards a higher share of DER replacing conventional power plants, brings to the DSO new responsibilities and also new challenges to TSOs. The TSOs are no longer able to monitor the total generation because the majority of DERs are connected to the distribution network. Therefore, new requirements on control and supervision functionality to be integrated into distribution management systems (DMS) need to be identified. Moreover, the displacement of conventional generation by inertia-less resources (i.e. converter connected) entails an upsurge in the requirement for balancing and system stabilization services. The reduction of system inertia is leading to faster rate of change of frequency (RoCoF), resulting in possible cascade tripping of embedded renewable generation and also higher frequency deviations (nadirs/zeniths), which can potentially lead to load shedding and, in the worst case, system collapse. This thesis deals with the requirements and challenges that TSOs and DSOs are facing due to the high integration of distributed and inertia-less resources, and it sets out the various solutions. Some of the new requirements and related functions to be integrated into the DMS platform to cope with these challenges are summarized. This thesis focuses on the challenges and solutions of the reduction of system inertia and the potential benefits of specialized control schemes to utilize fast acting reserves (FAR) to compensate for the lack of inertia. State-of-the-art of control schemes and technologies that could potentially compensate for reduction in system inertia and assist the transition toward a RES based power system are presented and analyzed, emphasizing the benefits and drawbacks of each solution. The two main control schemes that will be analyzed during the course of the thesis are synthetic inertia control (SIC) (i.e. RoCoF based control) and fast frequency control (FFC) (i.e. frequency deviation based control). A trade-off analysis between SIC and FFC in a converter dominated network employing energy storage systems is presented, using the power system simulation software PowerFactory. This aims to define the constraints and abilities of the two controllers to improve the frequency performance, in general, and limiting the RoCoF, in particular. Moreover, the potential benefits and drawbacks of providing such services from electric vehicles (EVs) are investigated and analyzed. Experimental activities were conducted to validate the technical feasibility of series produced EVs to provide these frequency services, namely, SIC and FFC. Emphasis was placed on assessing the two controllers’ ability to mitigate the RoCoF and improve the frequency performance, including assessing several EVs’ parameters, such as EVs’response time and EVs’ accuracy in following the desired set-point. Adopting the All Island Power System (AIPS) of Ireland as a test case, and employing the TSO’s data and models, the ability of SIC and FFC to mitigate the RoCoF and compensate for the reduction in system inertia is investigated. Exploiting more than 700 simulated dispatches of the all island power system, a model based approach is proposed to quantify the relationship between FAR (employing SIC or FFC) and synchronous inertia. The application of this methodology will assist the system operator in increasing the penetration of converter connected resources while respecting the grid code constraint of maximum RoCoF. Overall it is shown that SIC or FFC can help to mitigate the RoCoF. It was demonstrated that SIC does provide a slightly better performance in terms of RoCoF compared to FFC. Meanwhile, the FFC performed better in terms of frequency nadir and steady state values. Moreover, it was shown that SIC was more sensitive to time delays in low inertia systems, which can easily lead to system oscillation.