**1. Introduction**

Because of the ongoing rise in the prices of fossil resources, which mostly have an exhausted impact on the world's economies, as well as the harmful influence of these resources on the environmental footprint, the world rapidly resorts to utilizing sustainable resources. Indeed, the wind energy conversion system (WECS) has emerged as one of the fastest-growing energy systems [1–4]. Doubly fed induction generator (DFIGs) introduced a main important advantage since these generators are based on using a partial converter which means lower cost than the generators that use a complete power converter, leading to a great spread of DFIGs in wind energy markets. Brushless doubly fed machines (BDFMs) are slip recovery machines that reduce the required power converter if the required speed control range is limited, resulting in a significant cost saving for these machines [5]. As a result, using this BDFM is a cost-effective option for using inside the variable-speed WECS. Another main advantage of the BDFMs over the traditional DFIGs is their reliability, as the brush gear and slip rings were removed. Nowadays, both the brushless doubly fed reluctance machine (BDFRM) and brushless doubly fed induction machine (BDFIM) attract the most attention from researchers, especially on the subjects of stability, maximizing the extracted power and power quality. The main significances of the BDFRM's rotor design are to give it; higher reliability, robustness, and operation free from maintenance.

**Citation:** Rihan, M.; Nasrallah, M.; Hasanin, B.; El-Shahat, A. A Proposed Controllable Crowbar for a Brushless Doubly-Fed Reluctance Generator, a Grid-Integrated Wind Turbine. *Energies* **2022**, *15*, 3894. https://doi.org/10.3390/en15113894

Academic Editor: Adrian Ilinca

Received: 1 April 2022 Accepted: 23 May 2022 Published: 25 May 2022

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Moreover, considering the lack of rotor copper losses, the BDFRM is expected to outperform the BDFIM in terms of efficiency [6–8]. As a result, the brushless doubly fed reluctance generator (BDFRG) is discovered to be the most appealing for WECS [9–11]. Moreover, the BDFRG has been studied in a lot of fields by researchers as mentioned in papers [12–15].

Some of the grid codes considered that WTs have to be able to remain connected under zero voltage for a duration time up to 150 ms [16–20]. As the stator windings of the BDFRG are tied directly to the network, any occurrence of grid disturbances especially voltage dip can easily lead to an abrupt absence of the BDFRG magnetization, raising the current values in the machine side converter (MSC) above the threshold value and also raising the voltage of the DC-link above the threshold value, which can easily lead to destroying the power converter [21]. So, there are many different technical challenges facing the ability of the BDFRGs to satisfy the grid codes' requirements of keeping these generators connected under faults, principally due to the lower ratings of the used partial power converters.

This study is the first one to boost the ability of the BDFRG of staying connected during faults by using the crowbar. Commonly, currently, there are two used solutions to promote the ability of the doubly fed generators to keep connected during faults: using the crowbar and applying the demagnetization method [20]. Usually, when the crowbar was applied to protect the doubly fed generators during the occurrence of faults, the Machine Side Converter (MSC) is short-circuited, absorbing from the grid large amounts of reactive power, meaning more voltage dropping and more instability for the grid [22]. While on the other side, applying the demagnetization method [23,24], based on controlling the output of the MSC for tracing and counteracting the stator flux oscillations to eliminate the occurred transients on the induced electromagnetic force in the rotor winding. However, the industrial realization of the demagnetization method is very complex [20].

This paper introduces a proposed solution for increasing the ability of the gridconnected BDFRG WT to remain in service under the occurrence of faults and subsequently increasing the ability of the BDFRG to satisfy the grid code requirements. The proposed solution is mainly depending on using a new automatically controllable crowbar protection technique. One of the main aims of the proposed solution is keeping the connecting of the BDFRG MSC during the faults, reducing reactive power absorption from the network. For assessing the proposed solution efficacy, the "BDFRG grid-connected wind farm" performance would be examined under the occurrence of heavy conditions of different faults with and without using the proposed solution.
