Abstract
Electricity generation from wind power plants has increased dramatically during recent decades because of their clean operation, low maintenance, and low running costs. However, integration of wind farms into power grids still has some problems, such as power fluctuations that affect grid stability.In addition, faults can occur in the power system as a result of a variety of sudden events, such as lightning striking the power line or utility poles shorting the power lines to ground. With higher penetration of wind farms into existing power grids, short circuits could increase to levels that will exceed the switchgear capacity to handle them. The use of fault-current limiters is an urgent requirement to avoid having to upgrade existing protection systems. Although superconducting fault-current limiters (SFCLs) are considered a promising solution to the increasing fault levels, they are still expensive. Integrating the current-limiting function into other superconducting devices will help in mitigating this cost issue. Two superconducting devices have been studied in this thesis and are proposed for limiting fault currents in addition to their primary functions.
The first device is the superconducting fault-current-limiting transformer (SFCLT), which is proposed as a replacement for normal power transformers and superconducting fault-current limiters with the added advantages of lowering losses and volume, and increasing safety compared to normal transformers. The performance of the SFCLT was investigated in relation to the thermal behaviour of the superconducting windings. It works as a low-impedance transformer in normal conditions and as a resistive-type fault-current limiter during fault periods. With incorporation of the SFCLT, the losses of the system decrease and the transformer could limit the high fault currents in the grid within a few milliseconds.
Superconducting magnetic energy storage (SMES) devices are very effective in smoothing wind farm output power because they exhibit large power densities and fast response times compared to other energy storage devices. However, increasing the fault-current level by adding new wind-generation units to existing power grids can be very harmful to wind power generators and other power system elements. High current levels may destroy the generators and other power system apparatus. Using a fault-current limiter device will be very useful in limiting the fault currents but it will add cost and complexity to the system. Thus, integration of the fault-current-limiting function into the SMES circuit will be a valuable addition to wind turbine systems, and so the second device studied in this research is the superconducting magnetic energy storage fault-current limiter (SMES-FCL). The SMES-FCL is designed to be connected to an AC system that contains a wind turbine generator with a squirrel-cage-type induction motor. The SMES-FCL is used to smooth the output power generated from the wind turbine and compensate the voltage drop to support the load. It is used to limit different types of faults to reduce the short-circuit level of a system and protect the system components from the first cycle.
With the rapid increase in offshore wind farms, there is a need to connect such farms to loads or the grid by DC line. To improve the stability of a DC system that has a wind turbine generator and reduce high fault currents, a DC SMES-FCL is developed and tested. Thus, a control for a wind turbine based on a doubly-fed induction generator (DFIG) is developed and simulated in MATLAB/Simulink software before connection to a DC line. This DC SMES-FCL is tested in different conditions, and the results demonstrate the benefits of using SMES-FCLs in such systems to improve system stability and reduce fault-current levels.
| Date of Award | 13 May 2020 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Egyptian government with the British council in Egypt |
| Supervisor | Xiaoze Pei (Supervisor), Francis Robinson (Supervisor) & Vincent Zeng (Supervisor) |