Design of intelligent overvoltage protection module for large current converter

introduction

High-current direct current is an indispensable power supply for the metallurgical, chemical and nonferrous processing industries. Nowadays, the more commonly used schemes in China are saturated reactor voltage regulation, rectifier transformer step-down rectifier tube rectification and rectifier transformer direct step-down and thyristor rectification. Since both schemes belong to the scheme of power frequency rectification, there are many parallel power electronic devices in use and the system is huge, and its fast and effective protection has always been difficult. The current decentralized protection scheme is not easy to integrate, and the anti-interference performance is poor, and it is difficult to debug and adjust on site. For this reason, this paper mainly designs an intelligent protection module that integrates overvoltage detection and overvoltage action for the representative overvoltage protection in the protection module of high-current direct current power electronic converter. It has the advantages of simple structure, rapid response, wide protection threshold and easy integration.

Overvoltage analysis

In high-current DC power electronic converters, power electronic devices are very sensitive to voltage. Once the applied voltage exceeds the maximum rating allowed by the device, the device will be damaged immediately. There are many reasons for overvoltage in the rectifier, which can be divided into two situations: external and internal. General external factors include common natural disasters, such as overvoltage caused by lightning, which can be avoided by good grounding. However, overvoltage caused by internal factors is inevitable, which is divided into four categories: operating overvoltage when closing the switch, operating overvoltage when opening the switch, device commutation overvoltage, and inverter commutation failure overvoltage.

The overvoltage of closing operation is caused by the distributed capacitance between the primary and secondary windings. At the moment of closing the primary side, the high voltage is coupled to the secondary side through the distributed capacitance, causing overvoltage on the secondary side. The overvoltage of opening operation is caused by the magnetizing inductance of the rectifier transformer when it is working. If the transformer is unloaded or the load impedance is high, the primary side switch is disconnected, and the stored energy cannot be released through the load, but can only be released through the distributed capacitance on the secondary side. The stored energy is exhausted during the oscillation process, and the overvoltage of opening is the most serious at the peak of the excitation current. The overvoltage of device commutation mainly refers to the overvoltage of thyristor commutation. When the thyristor commutates, the residual carriers in each PN junction layer of the device recombine to generate reverse current. If this current drops to a value close to zero in a very short time, there will be a large Ldi/dt voltage that is enough to burn out the thyristor. The overvoltage of inverter commutation failure means that the converter fails to achieve the expected goal in AC-DC, resulting in local or overall overvoltage.

These factors that cause overvoltage in the rectifier device cannot be eliminated fundamentally, but the overvoltage protection function can be achieved by effectively utilizing fast sensing devices and reasonable conditioning circuits through artificial intelligence modules.

Design of overvoltage protection module

Traditional large current converter overvoltage protection mainly adopts the protection of separation devices, such as exhaust arrester, valve arrester or metal fuse, etc. Semiconductor fuse has been developed abroad, which is recoverable but expensive. Through comprehensive consideration, this paper selects the 51 series single-chip microcomputer with high cost performance at home and abroad as the main control core, combined with the voltage transformer with fast response and related conditioning circuit to form the overvoltage protection module of the entire converter.

Overvoltage detection circuit

Overvoltage detection mainly involves connecting a voltage transformer in parallel in the rectifier device, and processing the detected voltage through a rectifier filter circuit. The circuit diagram is shown in Part A of Figure 1. The voltage transformer is a transformer with an iron core, which is mainly composed of primary and secondary coils, an iron core and insulation. When a variable voltage U1 is applied to the primary winding, a magnetic flux φ is generated in the iron core. According to the law of electromagnetic induction, a voltage U2 is induced in the secondary winding. Changing the number of turns of the primary or secondary winding can change the voltage ratio between the primary and secondary sides, so that voltage transformers with different ratios can be formed. The voltage transformer in this article reduces the input high voltage to a low voltage output according to the turns ratio. Its primary side is connected to the primary system, and the secondary side is connected to the signal processing circuit. Its working principle is the same as that of the transformer, and its basic structure is also an iron core and primary and secondary windings. Its characteristics are that the capacity is very small and relatively constant, and it is close to the no-load state during normal operation, and the primary and secondary currents are relatively small. According to the design, the voltage U1 at the input of the voltage transformer is the detection voltage of the entire protection device, and the output voltage U3 after signal processing is the detected voltage. U3 will be compared with the protection threshold voltage UREF. If U3 < UREF, the output is high level; if U3 > UREF, the output is low level; this is convenient for subsequent single-chip microcomputers to make judgments. The protection threshold voltage UREF is the reference voltage, which is a DC voltage. Therefore, in the rectification and filtering process of U3, combined with the input and output voltage parameters, and the maximum output current that can be tolerated in the circuit is less than 10mA, a rectifier bridge below 0.3W and a filter capacitor C1 ≥ 0.144/(f×R1×r) that needs to filter low-frequency harmonics can be selected, where r is generally 0.002, and the capacitor C2 that filters high-frequency harmonics is a low-value ceramic dielectric capacitor.

Figure 1 Circuit diagram of overvoltage intelligent protection module

Overvoltage Comparator Circuit

The voltage signal detected by the voltage transformer must be converted into a signal that can be recognized by the microcontroller through a comparison circuit. Moreover, the rapid and accurate judgment of overvoltage is related to the actual necessity of the entire protection module and is the key to the overvoltage protection module.

Here we choose LM339 voltage comparator, and its specific circuit is shown in part B of Figure 1. In the figure, the reference voltage UREF is connected in the same phase, and the voltage U3 is detected in the opposite phase. R2 and R3 are connected in series to provide the UREF voltage, and R3 is a variable potentiometer that can change the size of UREF. A positive feedback loop is formed by transistor M5 at the output end of 1/4LM339, thus forming an inverting input hysteresis comparator, whose upper threshold voltage UT+ and lower threshold voltage UT- are shown in Formula 1-1 and Formula 1-2 respectively:

and  (1-2)

Therefore, the hysteresis voltage is: (1-2)

Among them, r is the internal resistance of the transistor in the amplification area, which can be ignored. UOH is the high level 5V output by LM339, and UOL is the low level 0V output. According to the design requirements, R3 is adjusted to 3KΩ, so =5V. Assume that the input AC effective voltage U1 of the rectifier is 209V, and the fluctuation of 10% is a safe voltage. At this time, the voltage U3 detected by the voltage transformer is less than 2.6V, and less than the reference voltage UREF. The output is open, and the overvoltage protection module does not work. As a positive feedback, the emitter follower transistor M5 is turned on UREF=2.8V. When U1 increases to 230V, U3 is greater than 2.8V, the output is low level, the overvoltage protection module is activated, the microcontroller outputs a blocking pulse, and M5 is cut off UREF=2.7V, which makes U3 greater than UREF. At this time, the state after flipping is extremely stable, avoiding the instability caused by voltage fluctuations near the overvoltage. Due to the generation of a certain hysteresis, after the overvoltage protection is activated, the input voltage U1 of the rectifier device must drop to 230-5=225V, that is, U3＜UREF, before the rectifier device can return to normal.

Pulse output control

The high current rectifier is mainly rectified by thyristors, and the conduction of thyristors requires a trigger pulse. The control of the trigger pulse here is based on the generation of thyristor trigger pulses by the single chip microcomputer. Therefore, controlling the generation of thyristor trigger pulses controls the output of the entire rectifier, thus achieving the purpose of overvoltage protection.

Figure 2 Pulse output control program flow chart

A single-chip microcomputer is a microcomputer that can realize corresponding functions through programming. In this paper, the P0 port of the thyristor trigger pulse output is controlled by the P1.0 port of the 51 series single-chip microcomputer. The output terminal U4 of the voltage comparator can be connected to the P1.0 port of the single-chip microcomputer, and a PNP transistor is used in the middle for level conversion as shown in part C of Figure 1. When U4 is at a high level, the P1.0 port of the single-chip microcomputer has a built-in high-resistance pull-up resistor. After reset, the P1.0 port is at a high level and the transistor M6 is cut off; when U4 is at a low level, the transistor M6 is turned on and pulls the P1.0 port to a low level. At this time, the P0 port is forcibly reset. The pulse output control program flow chart is shown in Figure 2, which ensures that the thyristor trigger pulse can be turned off in time when the rectifier device is over-voltage, and it can automatically operate when the voltage returns to a safe value.