###### Abstract
Additionally, at present, there are few practical applications of AC-AC converters in underground mining environments, mainly due to the complexity of control and concerns over reliability [7]. The AC-DC-AC-type frequency converter, with its advantages such as excellent static and dynamic performance, has been widely applied [8; 9; 10]. In recent years, controlled PWM has been adopted on the rectifier side, and a new AC-DC-AC control technology known as double PWM frequency conversion and speed regulation technology has been widely applied [11; 12; 13]. Figure 2 illustrates the traditional basic topology of three-phase dual PWM voltage and frequency control. The power frequency transformer steps down the high-voltage power before delivering it to the frequency converter. This method, which combines the power frequency transformer with the frequency converter, occupies excessive operational space. Power frequency transformers are increasingly being replaced by medium- and high-frequency transformers, which can reduce both the weight and size of the transformer and the entire system [14; 15]. Revision opinions on the electrical part of the _Coal Mine Safety Regulations_ and the corresponding electrical equipment safety standards explicitly state that effective isolation must be ensured between high-voltage and low-voltage equipment to prevent electrical accidents [16]. High-frequency transformers not only minimize volume, weight, and material costs but also provide key functions such as voltage conversion and electrical isolation [17; 18; 19]. The high-frequency isolated DC-DC converter is a well-known topology for high-power DC-DC conversion, featuring electrical isolation and transformer capabilities and the ability to change the switching frequency [20; 21]. Therefore, the transformer DC link of the three-level high-frequency isolated inverter can be replaced by a high-frequency isolated bidirectional full-bridge DC-DC (Dual Active Bridge, DAB) converter. Figure 1: The operating conditions of the frequency converter in underground coal mine environments. In recent years, some scholars have conducted research on the topology of three-stage high-frequency isolated frequency converters comprising rectification, high-frequency isolated DC-DC conversion, and inversion. Ref. [22] proposed an improved structure based on the three-stage topology, featuring a dual-star interleaved connection for the rectification-inversion stage and a multi-winding common-core transformer, achieving isolation between the rectification and inversion stages. Ref. [23] presented a novel hybrid distribution transformer topology with an industrial frequency isolation transformer on the high-voltage side, broadening the compensation depth of the common DC bus and enabling energy quality management and flow regulation [24; 25], but increasing the volume of the hybrid distribution transformer. Ref. [26] proposed a high-frequency isolated hybrid distribution transformer structure using an AC/DC/AC converter with comprehensive energy quality management capability, reducing investment in energy quality management and providing a DC port for friendly integration of new sources/loads. Designing a high-frequency isolated converter effectively reduces current stress and system volume. Ref. [27] proposed a high-frequency isolated dual PWM frequency conversion and speed regulation structure, discussing a new method that combines high-frequency isolation and frequency control and addressing the issue of large volume in traditional frequency converters. However, the above-mentioned studies employed two-level output for the inverter stage. This paper presents a three-stage topology for high-frequency isolated NPC three-level inverter frequency conversion and speed regulation. The input stage employs a three-phase uncontrolled rectification control strategy, which is simple, utilizes small diodes, and saves space. The intermediate stage adopts an Equal Pulse-Width Modulation (EPWM) strategy.