Rapidly Accelerated Synchronous Generators in CAES Systems for Frequency Support in Power Grids
Authors:   , Sturov Efim 
Published: December 2021

<p>Modern electrical networks are transformed through the use of intermittent sources of energy, such as small-scale photovoltaic installations and wind turbines. By reducing the carbon footprints associated with centralised power grids, they are made more vulnerable to contingent under-frequency events. The renewable energy sources can't provide the required rotational inertia to make the power grid's frequency stable and to be able to assist in restoring the frequency. In New Zealand, Transpower (system operator) is responsible for normalising the frequency in case of contingent events to avoid blackouts in the networks.    In case of contingent events in power grids, additional power must be delivered to the networks with the use of primary frequency support systems. Internationally these systems are represented by under loaded power plants, where power output can be adjusted by controlling the primary governor output. This approach incurs no-load running costs and to avoid these costs generation units should be maintained at rest. The most efficient and technically feasible solution is to use synchronous generators that are already present in the power grids or can be additionally delivered to the grids as stand-alone units. However, with the use of the traditional synchronisation method, the generators cannot be synchronised with power grids in a short timeframe (up to 10 s in some countries).   To overcome this disadvantage, a novel synchronisation approach should be designed to synchronise synchronous generators from rest of the electrical networks. This thesis proves that it can be achieved by a ballistic synchronisation approach (and then the improved 2-stage ballistic approach), which computes and follows an acceleration trajectory which simultaneously synchronises both phase and frequency. To achieve this fast acceleration a novel environmentally friendly small-scale compressed air energy storage (ss-CAES) system has been designed. This system utilises a hydraulic drivetrain which transmits very high torque directly to the shaft of a synchronous generator, thus enabling its rapid acceleration.  The hydraulic drivetrain is composed of a proportional throttle valve and a variable-displacement hydraulic motor. The central controller from National Instruments outputs a voltage that controls the opening of the proportional valve. It changes the flowrate in the main hydraulic circuit, meaning that it is possible to control the output torque and velocity of the hydraulic motor. Since it is coupled to a synchronous generator, the control system can control the dynamics of the drivetrain by changing its voltage output.   Computer simulations indicate that this approach enables very rapid synchronisation of a model system to the grid in < 1.5 s at a 100-kW scale. The modelling of the prototype helped to verify the control parameters of the system before the implementation of the algorithm built into the hardware. It should be noted that this model was simulated with the use of the corresponding manufacturer's data. To increase the accuracy of the mathematical model and verify the control parameters, the system components were experimentally characterised with the use of a ubiquitous high-speed data acquisition system.  It resulted in a realistic and accurate mathematical model of the complex electro-hydraulic system, despite the well-known challenges of modelling the hydraulic domain. This model was utilised for the tuning of the control parameters of the system before its experimental testing. Experimental runs confirmed the feasibility of the proposed acceleration and synchronisation approach for synchronisation from the rest of the generator in < 4 s.</p>