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Frequency Control of Distributed Energy Resources Integrated into Low Voltage Distribution Network
2020, Ibrahim, Ismail, 0000-0002-6744-8227
The growth in the integration of converter interfaced renewable energy has reduced the system inertia, which threatens system stability due to high rate of change of frequency and frequency nadir issues unless steps are taken to mitigate it. There is a need to provide sufficient fast frequency response to maintain adequate inertia in the system. This thesis investigates the capabilities of a large population of distributed energy resources such as residential energy storage and heating loads to provide an emulated inertial response. There is a need to find improved ways to analyze, test and evaluate the new control strategies for the provision of inertial response. Real-time simulation with hardware in the loop provides a very promising approach for this which enables the coupling of real hardware with power system simulations thus enabling study of the real-time interactions between the hardware and control and the wider system. This thesis presents a real-time simulation platform with hardware in the loop aimed at providing a platform for the test and evaluation of new technologies and new control strategies for the integration of distributed energy resources. The distribution grid model used for the real-time simulation is based on distribution grid in the Manchester area. This network contains three feeders with 330 houses, 6350 nodes and three-phase, four-wire configuration distribution cables. The conventional on/off controlled demand response, when installed in a large number of loads require some coordination for stable operation of the power system. Also, it is not possible to incorporate inertia emulation techniques into the on-off controlled demand response. This thesis proposes an inertial emulation technique for heating loads, both resistive loads and heat pumps. The thesis presents the small-signal transfer functions, stability analysis, and aggregated response of a large population of virtual inertia controlled resistive heaters and heat pumps. Finally, the thesis presents an investigation of the control based on grid-following and grid-forming approach for a single-phase residential battery storage system for the provision of frequency support. The thesis presents the full switched, reduced order model and small-signal transfer function for single-phase grid following and grid forming energy storage converters with virtual inertia control. Then the thesis presents stability analysis and simulations of a large population of energy storage converters in the real-time simulation platform. There are no stability issues with the grid-following approach, but the grid forming approach has more limited stability range. The simulation results shows the interaction between frequency support with voltage, and the effect of controller settings on voltage and frequency support.
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Neutral Current Optimization Control for Smart Transformer-fed Distribution System under Unbalanced Loads
2020-05-11, Chen, Junru, Zhu, Rongwu, Ibrahim, Ismail, O'Donnell, Terence, Liserre, Marco
EU In a 3-phase 4-wire LV distribution system, unbalanced loads lead to neutral current (NC) looping resulting in increase of power losses and variation of neutral potential. Compared to the conventional power transformer, Smart Transformer (ST) has strict current limitations to avoid overcurrent. However, its advantages on the downstream LV grid voltage regulation can provides the capability to regulate excessive NC. This paper proposes a closed-loop NC optimization control in order to, on the one hand, minimize the NC current in the normal operation satisfying the standard EN 50160 requirement, on the other hand, suppress the NC current in extreme cases to avoid the overcurrent damage of the ST. The proposed control strategies are validated by experimental tests via the hardware-in-the-loop setup and a case study based on a 350kVA, 10kV/400V, ST-fed distribution network under unbalanced loading profile according to the 3-phase 4-wire distribution grid in Manchester area. The results clearly prove the effectiveness and flexibility of the proposed NC optimization control strategies on the NC suppression and minimization.