Research on Mine Wide Voltage Adaptive Power Supply

To solve the problem of multiple voltage levels and wide range of variations of underground power supply in domestic coal mines, this paper putting forward four wide power supply of adaptive design options with wide input voltage (AC 80-850 V), including phase shifted rectification based on fully controlled bridge, direct voltage adjustment based on AC power supply, voltage adjustment based on AC power supply rectification followed by chopping control, and a power supply voltage adjustment based on magnetic coupling transformer and thyristor automatic switching. After experimental comparison, the preferred scheme for automatic switching between magnetic coupling transformer and thyristors is preferred. The main circuit of this scheme adopts magnetic coupling transformer and thyristor control method, and the control circuit adopts a three way drive circuit based on MCU control, while adopting the strategy of instantaneous voltage tracking control. This scheme achieves stable operation of the output DC power supply voltage at 24V when the input AC power supply voltage varies from 80 to 850 V. The experimental results showed programme was stable and reliable, and it’s worth applying and has practical significance.


Introduction
The input voltage levels of domestic mine power supply suitable for monitoring system include 127 V, 220 V, 380 V, 660 V, and so on.The primary winding of a power frequency transformer designed multiple sets of taps, each matching to a voltage class of the power supply [1].This method has drawbacks such as narrow adaptability to voltage variations and inconvenient use.Among them, the first issue is particularly prominent.The existing power supply designed within ± 25% for each voltage class, but it is still difficult to meet use needs.Especially difficult to adapt to the instantaneous changes in grid voltage caused by the start and stop of heavy load equipment.
In view of the above problems, considering safety and cost, the existing power supply at home and abroad have not been successfully solved [2][3][4][5][6].The author has designed a reliable and efficient power supply that can solve problems such as high voltage levels, poor power supply quality, and large voltage fluctuations in mine power grids.

Adaptive Power System
The adaptive wide power supply system consists of an adaptive power supply, a switch regulated power supply, a safety barrier circuit, and an intelligent charging and discharging manager.Among them, Adaptive power supply is a key component of this system.The overall structure is shown in figure 1.The adaptive power supply is AC voltage of 80-850 V, which can automatically adapt to the voltage required by the mine monitoring system.The processed adaptive power supply meets the input voltage range requirements of the switching power supply.The output of the switch mode power supply is divided into two channels, one directly passes through the safety barrier circuit for output, and the other supplies power to the intelligent charging and discharging manager.During a power outage in a mine, the intelligent charging and discharging manager controls the backup battery to supply power to the load.This article discusses the adaptive power supply part in the system.

Design of Adaptive Power supply
The following introduces four design schemes for wide voltage adaptive power supply, including Phase-Shifting Control Based on Full-Bridge Rectifier, AC Regulation, Rectification and Chopper Regulation, and Switch Based on Thyristors and Auto-Transformer.

Phase-Shifting Control Based on Full-Bridge Rectifier
It used the technology of phase-shifting, changing the AC voltage (80-850 V) into DC voltage (120-370 V), then exporting 24 V (DC) by switching power supply.Block diagram is shown in figure 2. It used full-bridge rectifier, by shifting phase control to achieve the first stepping down of voltage and filtering to achieve smooth voltage by the large capacitance [7].Voltage stabilization control achieved output voltage regulation and stabilization, by input voltage sampling, zero crossing detection, output voltage sampling, adjustment trigger pulse, and so on.(1) Design of the main circuit The main circuit is shown in figure 3. Considering the high-voltage input and the cost, we can use the bolt-thyristor.Full-controlled rectifier circuit with four thyristors can be effectively improved the ability of resisting to shock.However, in the experiment, it was found that the waveform's distortion of input voltage would occur and the system might not work reliably when the input voltage was higher 700 V AC and the load was beyond 150 W, as shown in figure 6(b).

AC Regulation (1) Design of the main circuit
This strategy used IGBT to achieve the control of AC/AC directly conversion, the main circuit topology of AC regulator is shown in figure 7. Z1 is the IGBT, and D1-D4 is the fast recovery diodes.The IGBT chopper adjusts the output voltage by switch on and off. 1) PWM control Controlling convenient, easy to optimize the parameters; but the ripple is greater, while the value does not conform to the theoretical value when the duty cycle is smaller.
2) FM control With the frequency increase, ripple is becoming very small, but it is difficult to be controlled because of two variables, in additional to demanding the filter circuit.Based on the above design, the author created experimental devices and the actual test results showed that power electronic devices serious heat and easily damaged when the input voltage in excess of 450 V.

Rectification and Chopper Regulation (1) Design of the main circuit
The main circuit topology is shown in figure 9.The circuit changes AC into DC by the full-bridge controlled rectifier circuit in the first step, and there is a BUCK circuit in the second-step [8].It is achieved step-down output according to feeding back the output voltage to regulate the duty cycle of the switch.The disadvantage of the BUCK circuit is wide scope of changes, and can not ensure the switch always works in Discontinuous Current Mode making it easy to damage the switching transistor.

Switching
Power Supply (2) Design of the control circuit The strategy of basically design is similar to the first strategy, so does not need too much introduction.
(3) Results Based on the above design, we can see that two steps can be achieved in theory, but it showed the system requirements need high value of permissible voltage in the actual circuit.Furthermore, the heat of device is serious.

Switch Based on Thyristors and Auto-Transformer
It used Auto-Transformer and thyristors to achieve adaptive automatic switching, by changing input voltage (AC: 80-850 V) into a specific voltage range (AC: 80-264 V) firstly, then exporting DC 24 V through the switching power supply.
(1) Design of the main circuit The output voltage changes following the input voltage in the range of 80-264 V, and regulates to in the range of 80-264 V by an Auto-Transformer in input voltage in the range of 265-850 V.
The MCU controls the thyristor switch in the main circuit, enabling it to turn on and off as required, while also ensuring a voltage output within the range of 80-264 V through voltage sampling.Please refer to figure 10 for its structural representation.When operating at high voltages (AC: 265-850 V), thyristor TR1 allows current flow through Auto-Transformer T1, with both thyristors TR2 and TR3 turned off simultaneously.Conversely, when operating at low voltages (AC: 80-275 V), thyristor TR1 turns off while both thyristors TR2 and TR3 are activated.The switch power module outputs a 5V power supply to supply power to MCU.The secondary side of transformer T2 sampled by a sampling resistor and divided into two paths.One path enters the MCU, which outputs a signal to the hardware logic judgment circuit; The other path is directly output to the hardware logic judgment circuit.The hardware logic judgment circuit judges the output power to drive the chip and drive the pulse transformer, achieving control of two sets of thyristors.
It can achieve software and hardware judgment in the logic judgment circuit.The logic judgment of hardware started faster than the software's when the voltage instantaneous great changes.For example, when the input voltage momentarily drops from the high voltage range to the low voltage range, MCU controls rectification sampling, compares logic judgment, and TR2 and TR3 to quickly conduct, while also controlling TR1 to turn off.
(3) Design of the software MCU uses A/D sampling to monitor input voltage parameters in real time and adjust them in real time, ensuring the power system operates in the best state.Basic procedures process is shown in figure 12 [9].The experimental results of this scheme shows that it can meet the following design requirements well within the entire input voltage range (AC: 85-825 V): 1) The system can reliably start repeatedly and work stably when the time exceeds 1 second; 2) Optional load ranges from 0 to 200 W; 3) The temperature rise shall not exceed 40℃ and shall not cause thermal interference; 4) AC input and output withstand voltage greater than 3000 V; 5) Strong shock resistance, low input and output harmonics.
The main circuit of this scheme requires a power switching transformer, so its volume and weight are larger than other schemes.But when the system requires a smaller power capacity or input voltage range, the volume and weight of the transformer will decrease.The circuit board does not require any changes, and its system flexibility much more better.

The Comparison of Several Strategies
The advantages and disadvantages of the four adaptive power supply schemes are shown in table 1.The author believes that power supply should ideally meet the requirements of low cost, low thermal interference, high voltage resistance, strong resistance to frequent impacts, and adaptability to underground environmental conditions.Under current technological conditions, it is difficult to design a wide range adaptive power supply entirely using power electronics technology.After analysis, we prefer automatic switching mode based in autotransformer and thyristors.System is safe and reliable

Disadvantage
The requirements for the withstand voltage value of power electronic devices are high, and the capacitance value of filtering capacitors is relatively high.
The system is larger in use of step-down transformer The harmonics increase and the power factor decreases when the input voltage exceeds 700 V, leading to decreased system reliability.
The impact resistance of the system is substandard.When the input voltage exceeds 400 V, a substantial surge current occurs, compromising the reliability of the system.
the impact-resistance capability is inadequate.the heat is severe, and the system is unreliable.

Conclusions
The adaptive power supply system preferentially selects automatic switching mode based on autotransformer and thyristors.It solves the problems of multiple voltage levels, poor power supply quality, and large variations in the mine power grid, and provides a reliable intrinsic safety power supply for the mine monitoring system.As a key part of the adaptive power supply system, this adaptive power supply can be widely used in mine safety power supply equipment.With slight adjustments, this scheme can also be used in other industrial situations with large power voltage variations, and has good application prospects.

Figure 1 .
Figure 1.Overall structure of system.

Figure 3 .
Figure 3. Main schematic circuit diagram of phase-shift rectification.

Figure 5 .
Figure 5. Simulation wave of phase shift rectification.

Figure 8 .
Figure 8. Simulation wave of AC regulation.

Figure 9 .
Figure 9. Main circuit's topology of rectification and chopper.

Figure 10 .( 2 )Figure 11 .
Figure 10.Structure of the main circuit of switching based on thyristors.(2)Design of the control circuit The control circuit is the open-loop design, and hardware structure is shown in figure11.It is designed that control circuit based on 32-bit MCU including the transformer T2, signal rectifier sampling circuit, switching power supply module, MCU, logic judge and driver circuit and so on.

NFigure 12 .
Figure 12.Software flow chart of MCU.(4)The experimental results The experimental results of this scheme shows that it can meet the following design requirements well within the entire input voltage range (AC: 85-825 V):1) The system can reliably start repeatedly and work stably when the time exceeds 1 second; 2) Optional load ranges from 0 to 200 W;3) The temperature rise shall not exceed 40℃ and shall not cause thermal interference; 4) AC input and output withstand voltage greater than 3000 V; 5) Strong shock resistance, low input and output harmonics.The main circuit of this scheme requires a power switching transformer, so its volume and weight are larger than other schemes.But when the system requires a smaller power capacity or input voltage range, the volume and weight of the transformer will decrease.The circuit board does not require any changes, and its system flexibility much more better.

Table 1 .
The comparison of the adaptive power.