## 2 Analysis of Topological Structure and Operational Principles  
The parameters of the high-frequency transformer used in this study were taken from [33,34]. The magnetic core material was ferrite, with a magnetic flux density of 0.261 T and an operating frequency of 10 kHz. Its maximum magnetic induction was 0.5 T. Under the same transmitted power and with the same current in each winding, the core size of the high-frequency transformer was reduced by more than 90% compared to that of a power-frequency transformer. Due to lower iron losses and copper losses, the efficiency of the high-frequency transformer was between approximately 95% and 98%.  
The topology of the high-frequency isolated NPC three-level inverter frequency conversion and speed regulation system is shown in Figure 3. The inverter topology consisted of a three-phase diode rectifier input stage, an intermediate isolated transformer transmission stage, and an NPC three-level inverter output stage. The input stage consisted of grid-side filtering inductor \(L_{1}\), a three-phase diode rectifier structure, and DC-side stabilizing capacitor \(C_{1}\). Due to the limited space in coal mines, smaller diodes were used, which were simpler to control and less expensive compared to IGBTs. The intermediate transmission stage included a high-frequency transformer, a single-phase full-bridge power switch on the primary side, a single-phase full-bridge diode rectifier on the secondary side, and the combined inductance of the external series inductor and the transformer leakage inductance \(L_{2}\). Since energy could be transferred in a single phase, a diode full-bridge on the secondary side was sufficient. The output stage consisted of DC bus capacitors \(C_{2}\) and \(C_{3}\) and three-phase bridge arms (each phase bridge arm comprising four IGBT power devices and two clamping diodes), with an LC filter installed between the inverter output and the load for filtering. The NPC three-level inverter circuit was used as the output stage because the three-level system could further reduce the output voltage harmonics compared with the two-level system, which was conducive to realizing the sinusoidal voltage waveform.  
The working principles of the high-frequency isolated NPC three-level inverter frequency conversion and speed regulation system topology are as follows:  
1. The three-phase industrial AC input is converted into a DC power source via a three-phase diode rectifier.
2. The DC voltage \(U_{\text{dc1}}\) is transmitted to the intermediate stage, where it is converted into a stable DC voltage \(U_{\text{dc2}}\) by the high-frequency isolated DC-DC stage, and then sent to the output stage with the NPC three-level inverter.
3. After the energy is delivered to the output stage, under the control of the DSP, the power devices in the output stage circuit are controlled by the dual closed-loop control SVPWM modulation control strategy, which can convert the DC power into AC power  
Figure 3: The topological structure of the high-frequency isolated NPC three-level inverter frequency conversion and speed regulation system.  
with the frequency and amplitude required for load operation, driving the load for speed regulation.