Thyristor Application in Power Regulator

Most current power regulators use servo motors to drive the carbon brush to move the adjustment voltage. It has the advantages of high efficiency, good output waveform, and simple circuit. However, due to carbon brush and mechanical transmission, it has short working life and slow response speed. Replacing "carbon brushes" with "non-contact" is the current development direction of high-power power regulators. The term "non-contact" means that the AC switch in the main circuit uses a device such as a thyristor and uses a microcomputer logic control to regulate the voltage. Use non-contact control instead of servo motor and carbon brush to adjust the pressure, thereby prolonging the service life, accelerating the response speed and improving the reliability.

The basic principle of the system

The voltage regulator is composed of a main circuit, an 8031 ​​microcontroller, a detection circuit, a control circuit, a drive and an alarm interface circuit. As shown in Figure 1.

The voltage phasors in the figure have the following relationship:

i=â–³+o

As can be seen from FIG. 1, the voltage ΔU needed to be compensated is detected by a computer-controlled voltage sampling circuit, and the control system changes the conduction combination of the triac thyristors S1 to S6 to compensate for the input voltage (Ui) or load variation, so that the voltage regulator The output voltage (Uo) remains stable to achieve regulation. In order to make operation and maintenance easy and easy to use, full use of the powerful features of the microcomputer, set up a complete self-test function.

Thyristor application points are noteworthy problems <br> <br> trigger thyristor as a switching device drive issues, when a shorter duration of the trigger pulse, the pulse amplitude must be increased, but also depends on the pulse width of the anode current reaches Latching Current time. In this system, due to the presence of inductive loads, the rate of rise of the anode current is low, and if a wide pulse trigger is not applied, the thyristors often cannot maintain the on state. Considering that the load is strongly inductive, the system uses high-level triggering. The disadvantage is that the thyristor loss is too large.

Thyristor Blocking Problem Thyristor is a kind of switching device. During the application process, the factors affecting turn-off time include junction temperature, on-state current and its decreasing rate, reverse recovery current decreasing rate, reverse voltage and positive dv/dt value. Wait. Among them, the junction temperature and the reverse voltage have the greatest influence. The higher the junction temperature, the longer the turn-off time. The higher the back pressure, the shorter the turn-off time.

In the system, due to the presence of inductive loads, there will be a large back-EMF across the inductor during commutation. This abnormal voltage is applied to both ends of the thyristor and can easily cause damage to the thyristor. To prevent this, surge voltage absorbing circuits are commonly used.

Dv/dtdi/dt effect problem

When the critical rate of rise of the off-state voltage dv/dt of the thyristor is large, there is a possibility that the thyristor will conduct at a voltage much lower than its forward voltage. If the dv/dt on the circuit exceeds the dv/dt allowed by the device, the thyristor will turn on and lose its blocking capability. In the application circuit, the gate of the thyristor is connected to the cathode through a resistor, and the displacement current is bypassed from the outside to prevent erroneous conduction caused by dv/dt.

Excessive di/dt can easily cause breakdown of the thyristor, which uses a sharp high-level trigger at the front to increase the initial conduction area, thereby improving the di/dt capacity.

The consequences of thyristor breakdown due to erroneous conduction and di/dt caused by excessive dv/dt are very serious. It can be seen through the principle circuit that the occurrence of such a situation will cause the transformer to be short-circuited to generate a "circulating current", causing damage to the thyristor and even the transformer. In the circuit design, reliable thyristor on-off detection measures are adopted to avoid this phenomenon.

Over-voltage and over-current protection measures

Overvoltage generation mainly has the following reasons:

(1) Surge voltage when transformer is put into operation;

(2) Surge voltage generated when transformer tap changes;

(3) Surge voltage when lightning strikes;

(4) Surge voltage generated when the DC link is disconnected.

In the circuit, a surge absorber is added to absorb the surge voltage invaded by a transformer's electromagnetic transfer and absorb the magnetic energy generated when the transformer is turned off. To avoid surge voltage generated by lightning strikes, semiconductor surge arresters can be used.

Elimination of the circulation is a key issue for the regulator. To solve this problem, we have adopted the following technical measures:

(1) Ensure that the thyristor trigger signal is reliable. Using a software filter program to make the output trigger control signal have only one valid in each group, and then use the 74LS273 and the developed anti-circulation logic circuit (PAL16V8) to ensure that no false trigger occurs even if the microcontroller is out of control. In addition, the trigger signal leads are shielded and other measures to prevent interference.

(2) Ensure reliable conversion. In normal operation, often to change the size of the compensation voltage, that is to adjust the thyristor conduction combination, such as: the S1 conduction for S2 conduction, you must turn off S1 at the same time to S2 trigger control signal to achieve thyristor Conversion. If the timing or combination of the conversion is improper, a circulation will be formed and the thyristor will be damaged. In the design, a zero-point switching technique is adopted. That is, S1 is naturally turned off when the current passes through zero, and S2 is triggered to turn on. It can be seen that the most critical thing in the conversion process is accurately detecting the current zero-crossing signal. To this end, software and hardware combinations and interlocking techniques are adopted to ensure the accuracy of zero-crossing signals. The actual operation shows that the above technique successfully solves the circulation problem.

Influence of inductive load

Due to the presence of inductive loads, the trigger pulse width should be increased. Otherwise, the thyristor will weaken the trigger signal before the anode current reaches the hold-up current, which may cause the thyristor to fail to conduct normally. When turned off, inductive loads can also cause problems for thyristors.

In the actual system, high-level triggering is used to ensure reliable conduction of the thyristors. In order to ensure that no shutdown failure occurs in the thyristor conversion process, a reliable interlocking technique is used to ensure that the thyristor is not damaged.

Zero-crossing switch technology

In the process of voltage regulation, the zero-trigger detection technology is used to control the trigger signal of the thyristor gate to ensure that it is turned on and off at zero-crossing, so as to avoid pollution of the power grid due to thyristor switch during voltage regulation.