Naderi_whole_thesis_ex_pub_mat.pdf (850.62 kB)
Improvement of fault ride through capability of wind turbines by fault current limiters
thesisposted on 2023-05-27, 09:27 authored by Seyedbehzad Naderi
Due to increasing penetration levels of renewable energy in the power system, it is essential to keep renewable energy resources connected to the power grid during a fault considering the grid code requirements. In this thesis, the Fault Ride Through (FRT) capability of wind turbines is taken into account. All types of wind turbines are considered and for each one, the proposed methods will be discussed and studied. In the first approach for Fixed Speed Wind Turbines (FSWTs), to achieve maximum FRT capability, this thesis proposes a single-phase Optimum Resistive Type Fault Current Limiter (OR-FCL) as an efficient solution during various grid faults. A dedicated control circuit is designed for the OR-FCL that enables it to insert an optimum value of resistance in the FSWT's fault current path for improving transient behaviour of the FSWT. The optimum resistance value depends on fault location and pre-fault active power. The control circuit of the proposed OR-FCL is capable of calculating the optimum resistance value for all the pre-fault conditions. By using the proposed control circuit, the FSWT can achieve its maximum FRT capability during symmetrical and asymmetrical faults, even at zero grid voltage. In the second approach for the FSWTs, to reduce the number of components, a three-phase Controllable Resistive Type Fault Current Limiter (CR-FCL), with the same operation of the OR-FCL, is investigated to improve the FRT capability of the FSWT. Fully and partially rated converter wind turbines and solar power systems utilise an inverter to connect to the grid. The inverters are vulnerable due to their damaging effects of voltage sags in their terminals on semi-conductor self-turnoff switches. In this thesis, a Bridge Type Fault Current Limiter (B-FCL) is proposed with simple control system for the FRT improvement of the inverters during different types of faults. The proposed B-FCL located, in Point of Common Coupling (PCC), is also capable of compensating for the voltage sag in the PCC and creates safe conditions not only for the inverter but also for any equipment which has been connected to the PCC in the power system. In the second approach, to analyse the FRT capability of inverter interfaced distributed generation, and to reduce the number of components, an FCL is placed in the DC link. This thesis proposes a DC Link Resistive Type Fault Current Limiter (DC-RFCL) based-voltage source inverter (VSI) for the FRT capability improvement, which is a new approach in the use of FCLs. Instead of using three-phase FCLs in the AC side of the VSI as in the first approach, just one single-phase proposed DC-RFCL is connected in series with the DC side of the VSI. Doubly fed induction generator (DFIG)-based wind turbines employ small-scale voltage sourced converters with a limited over-current withstand capability, which makes the DFIG-based wind turbines very vulnerable to grid faults. Often, modern DFIG systems employ a crowbar protection at the rotor circuit to protect the rotor side converter (RSC) during the grid faults. This method converts the DFIG to a squirrel cage induction generator, which does not comply with the new grid codes. The recent grid codes require wind turbines to stay connected to the utility grid during and after the power system faults, especially under high penetration levels of wind power. Furthermore, the crowbar switch is expensive. Therefore, in this thesis, the FRT capabilities of DFIG-based wind turbines are studied. To improve the FRT capability of the DFIG-based wind turbines, three approaches are proposed: a modified DC chopper, a non-controlled FCL and a DC link resistive type fault current limiter. In the first approach, the modified DC-link chopper is proposed in order to maintain both the DC link voltage and the high current level in the stator and the rotor sides in a permissible level, without incorporating any extra fault current limiting strategies. In the second approach for the FRT improvement of the DFIG-based wind turbine, a non-controlled fault current limiter is proposed. Co-operative operation of the chopper circuit and the non-controlled FCL, which is located in the rotor side of the DFIG, are studied. It is demonstrated that locating the proposed topology on the rotor side is effective from the leakage coefficient point of view, limiting transient over-currents rather than on the stator side. Furthermore, it is shown that, by obtaining an optimum non-superconducting inductance value, the rate of the fault current change is limited to lower than the maximum rates of current change in the semi-conductor switches of the DFIG's converters during the fault. In the third approach for the FRT improvement of the DFIG-based wind turbine, this thesis proposes the application of the DC Link Resistive Type Fault Current Limiter (DC-RFCL) to improve the FRT capability of the DFIG. The proposed DC-RFCL is employed on the DC side of the RSC. The DC-RFCL solves crowbar protection activation problems and eliminates subsequent complications in the DFIG system. The proposed DC-RFCL does not have any significant impact on the overall performance of the DFIG during normal operation. The proposed approach is compared with the crowbar-based protection method. Simulation studies are carried out using PSCAD/EMTDC software. In addition, a prototype is provided to demonstrate the main concept of the proposed approach.
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