It is necessary for threelevel explosionproof inverters to have high thermal stability and good output characteristics avoiding problems caused by power devices, such as IGBT, so it becomes a hot and difficult research point using only one control algorithm to guarantee both output characteristics and high thermal stability. Firstly, the simplified SVPWM (Space Vector Pulse Width Modulation) algorithm was illustrated based on the NPC (neutralpointclamped) threelevel inverter, and then the quasisquare wave control was brought in and made into a novel holographic equivalent dualmode modulation algorithm together with the simplified SVPWM. The holographic equivalent model was established to analyze the relative advantages comparing with the two single algorithms. Finally, the dynamic output and steady power device losses were analyzed, based on which the power loss calculation and system simulation were conducted as well. The experiment proved that the highpower threelevel explosionproof inverter has good output characteristics and thermal stability.
The highpower threelevel inverter has been applied widely in the field of mine hoist, and it shows high performance in application. However, with the diversification and complexity of applications, inverters used in explosionproof environments need increasingly high requirements of system stability. There are many factors, like internal faults, overload, transients, and so on, that affect the system stability. But for explosionproof inverters, feedback from the applications of this stage shows that the primary factor affecting the stability of the system is the thermal fault; that is, the power loss of power device is too large, and the cooling system with certain cooling capacity can not meet the cooling needs and results in damage of power devices due to accumulated heat. Fault tolerant control for thermal fault of inverters has received attention paid by scholars at home and abroad, and some progress has been made already.
The power losses of internal power devices during operation are the main heat sources of inverters. So the modeling analysis of power devices is the major research of thermal analysis, and the topology is shown in Figure
Topology of neutralpointclamped threelevel inverter.
The threelevel NPC inverter compared with traditional twolevel topology achieves higher transfer density of energy and lower harmonic content of voltage and current. It has been widely used in the field of highvoltage power inverter and now is the main topology of power circuit in the mainstream products from many worldrenowned electric manufacturers at this stage, and, therefore, the thermal fault tolerant control based on highpower explosionproof NPC threelevel inverter is the main content of this paper. Several experts and scholars have conducted research in this area with some practical results. Power loss, heat models, and life prediction of power devices were researched in [
Hence, considering modulation strategy, a new modulation algorithm based on holographic equivalent dual mode is proposed in this paper. The calculation can not only adjust dynamic power device losses according to output property, but also achieve optimal thermal management of power devices in different speed zones of hoister. Therefore, heat fault tolerant control for highpower threelevel explosionproof inverter can be realized.
Firstly, the principle of SVPWM is introduced simply based on the inverter topology shown in Figure
Space voltage vector diagram of three level inverter.
The space voltage vectors depicted in Figure
Space voltage sectors and corresponding output voltage states.
Space voltage vectors  Output voltage state 







































The diagram of threelevel voltage space vectors shown in Figure
Diagram of transforming between threelevel space vector to twolevel space vector.
Due to the symmetry of the six small hexagons, the algorithm of SVPWM is analyzed based on reference voltage vector through the small hexagon marked with
Vector translations of reference voltages under different






















Vector translation of reference voltage under
Based on the principle of voltsecond balance, the relationship can be expressed as follows:
Introducing translation vector
On the basis of Figure
As shown in Figure
Through the above analysis, it can be found out that the first step is to judge the reference voltage vector, located in twolevel space vector label
In Table
Fault tolerant control of thermal stability based on holographic equivalent dual mode is a method to switch two holographic equivalent modes to adjust the losses of power devices. The first mode, improved on the basis of traditional PWM modulation, is a simplified SVPWM algorithm based on threelevel reference voltage vector transformation. This algorithm can simplify the threelevel power device control, reduce the computation time, and improve the quality of the output power quality. But the power loss under this modulation is too large to maintain the thermal stability under continuous working. The second mode is quasisquare wave modulation algorithm, which needs the quasisquare wave voltage in each cycle to switch the power devices on and off twice. As shown in Figure
Quasisquarewave voltage and counter electromotive force corresponding to the angle of the basic waveform and turnoff position.
Temperaturefrequency regional distribution with runtime of dualmode modulation.
Two kinds of modulation mechanisms of the holographic equivalent modes are as follows:
(
(
Benchmark effective power/torque and reactive power are decided by the value of
The experiment platform is shown in Figure
Experiment platform. (a) Motor part; (b) inverter part.
The parameters of winding asynchronous motor and component parameters of inverter main circuit are shown in Tables
Parameters of winding asynchronous motor.
Rated power 
475 
Stator voltage 
6000 
Stator current 
59 
Rotor voltage 
640 
Rotor current 
435 
Rated speed (r/min)  735 
Power factor  0.85 
Component parameters of inverter main circuit.

1100 V 
DClink capacitor 

Power device parameters  Infineon, FF1400R17IE4 
Switching frequency  2000 Hz 
The well depth is 348 m. Meanwhile, the lifting conditions of one cycle are shown in Figure
Lifting conditions of mine hoist in one cycle.
Simulation analysis of power losses in the same bridge
Power loss comparison between two equivalent modes. (a) Power loss of
It can be seen by analyzing the simulation results in Figures
Through the thermal analysis in Figure
Thermal analysis comparison between the two equivalent modes. (a) Temperature rise of heatsink based on simplified SVPWM; (b) temperature rise of heatsink based on quasisquarewave modulation; (c) temperature rise of substrate based on simplified SVPWM; (d) temperature rise of substrate based on quasisquarewave modulation.
The electrical performance of the threelevel explosionproof inverter under normal condition in one cycle is shown in Figure
Electrical performance of inverter.
Harmonic characteristic comparison.
Harmonic characteristic based on simplified SVPWM
Harmonic characteristic based on dualmode modulation
Surface and the substrate temperature rises of IGBT based on holographic equivalent dual mode. (a) Surface temperature rise of IGBT based on simplified SVPWM; (b) surface temperature rise of IGBT based on quasisquarewave modulation; (c) substrate temperature rise of IGBT based on simplified SVPWM; (d) substrate temperature rise of IGBT based on quasisquarewave modulation.
Based on highpower threelevel explosionproof inverter output characteristic and the special requirements of thermal stability, in this paper, we put forward a holographic equivalent dualmode control algorithm based on the simplified SVPWM and quasisquarewave modulation and calculate the power losses of power devices in threelevel explosionproof inverter on this basis and establish thermal models of power devices and cooling system for thermal simulation. The effectiveness of holographic equivalent dualmode control on both the system output characteristics and thermal stability is verified by experimental analysis, so this mode plays an important role in realizing power device thermal fault tolerant control and improving the thermal stability of highpower explosionproof inverter.
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors would like to thank the Doctoral Scientific Research StartUp Foundation of Henan Normal University, Grant no. 5101239170001.