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The modern electrical power system is changing rapidly, as the global climate change forces us to implement new solutions, so that the present technological advancement can be maintained without catastrophic damage to the globe. In terms of power generation, this can be observed by the increasing amount of renewable energy sources, like wind and solar power. As the penetration of these technologies increases, the control of a power system changes, and new challenges are introduced. One of these challenges is transient frequency stability, which is more evident as more and more renewables replace the conventional generation methods. This is because conventional generation utilizes large rotating masses that introduce inertia to the system, which in turn improves the stability. Renewable technologies on the other hand are grid-connected via power electronics and thus the inertia is decoupled from the system. Additionally, low inertia is also present in the so-called microgrid applications, where the system consists of small resources. Typically, these systems can operate in islanded mode, meaning that the stability and inertia of the utility grid is disconnected. In these systems, frequency deviations due to power imbalances are much faster and more severe, which may cause damage to grid-connected devices. Power electronics and energy storage solutions can be utilized to mitigate the challenges that low inertia introduces. With proper control methods, these devices can be used to provide “virtual inertia” to the system, which enhances the stability. This thesis focuses on utilizing Uninterruptible Power Supplies (UPSs) to improve transient frequency stability, specifically in an extremely low-inertia islanded system, in which the generation is provided by a single diesel generating set. A control algorithm for UPS manufacturer Eaton’s 93PM 200 kW three-phase UPS is developed that aims to control the device’s power based on the present input frequency (and rate of change of frequency). The development is first done in a simulation environment by comparing different control methods, after which two of the methods (linear droop and ROCOF-control) are implemented into a real device. The control is tested in a small-scale islanded system, where the UPS is fed by a diesel generating set. The performance is evaluated by analyzing the frequency deviations during load steps. The simulation and the laboratory testing results showed that the developed control methods were successful, and the studied UPS device was able to compensate the applied load steps effectively. For example, the frequency nadir improved by approximately 8 Hz during a 160 kW load step with the linear droop and ROCOF-control methods. Additionally, the control methods enabled the system to survive a 200 kW load step without the generator tripping to underspeed. Both control methods showed promising results, but all in all the linear droop method was superior due to a more straightforward parametrization process. |
