Discussion on Several Issues in New Power Voltage Stabilizer

Most of the power voltage stabilizers on the market currently use servo motors to drive carbon brushes to adjust the output voltage. It has the advantages of high overall efficiency, good output waveform and simple circuit, but due to the carbon brushes and mechanical transmission, its service life is short and the response speed is slow. Replacing "carbon brushes" with "contactless" is the current development direction of high-power power voltage stabilizers. The "contactless" here refers to the use of power devices such as bidirectional thyristors to replace carbon brushes, and the use of microcomputers to achieve logic control and voltage regulation through compensation transformers, thereby achieving the purpose of extending service life, accelerating response speed and improving reliability.

2 Basic principles of the system

The voltage stabilizer is mainly composed of main circuit, single chip microcomputer, detection circuit, control circuit, drive and alarm interface circuit, etc. As shown in Figure 1.

Figure 1 Voltage stabilization principle diagram

The voltages in the figure have the following relationship: As shown in Figure 1, the microcomputer detects and calculates the voltage △U that needs to be compensated through the voltage sampling circuit, and the drive system changes the bidirectional thyristor conduction combination to compensate the input voltage (Ui), so that the output voltage (Uo) of the voltage regulator remains stable, thereby achieving the purpose of voltage stabilization (see reference 1 for the working principle). In order to make the operation simple and easy and make full use of the functions of the microcomputer, a complete self-test system is set up.

3 Discussion of Several Issues

3.1 Transformer input problem

Compared with Reference 2, the surge current caused by the saturation of the transformer when it is put into operation is larger. In order to reduce the core volume, the working magnetic induction intensity Bm of the general transformer is selected near the inflection point of the B-H curve. The worst case at the moment of closing is that the maximum magnetic induction intensity is 2Bm+Br (residual magnetism). Obviously, the transformer will be saturated, causing damage to the thyristor and distortion of the voltage waveform. To prevent saturation at the moment of closing, Bm≤Bs/3 can be selected.

3.2 Trigger driver problem

As a current control device, when the duration of the trigger pulse is short, the pulse amplitude must be increased accordingly. At the same time, the pulse width also depends on the time it takes for the anode current to reach the holding current. In this system, due to the presence of inductive loads, the anode current rise rate is low. If a strong wide pulse trigger is not applied, the thyristor often cannot maintain the on state. Considering that the load is inductive, this system uses level triggering, and its disadvantage is that the thyristor loss is large.

3.3 Thyristor blocking problem

Thyristor is an avalanche device. The conduction of this device is caused by the multiplication of carriers on the middle collector junction. In the application process, the factors affecting the turn-off time include junction temperature, on-state current and its drop rate, reverse recovery current drop rate, reverse voltage and forward dv/dt value. Among them, junction temperature and reverse voltage have the greatest influence. The higher the junction temperature, the longer the turn-off time; the higher the reverse voltage, the shorter the turn-off time.

In the system, due to the presence of inductive load, a very high back electromotive force will be generated at both ends of the inductor during the commutation process. This abnormal voltage is applied to both ends of the thyristor, which can easily cause damage to the thyristor. To prevent this, a surge voltage absorption circuit should usually be used.

3.4dv/dt and di/dt effect issues

When the off-state voltage rise rate dv/dt of the thyristor is large, it is possible to conduct at a voltage much lower than its forward transition voltage. If the dv/dt in the circuit exceeds the dv/dt value allowed by the device, the thyristor will be mis-conducted and lose its blocking ability. 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 mis-conduction caused by dv/dt.

Too large di/dt can easily cause thyristor breakdown. In the circuit, a strong level trigger with a steep leading edge is used to increase the initial conduction area, thereby improving the di/dt capacity.

The consequences of false triggering caused by large dv/dt and thyristor breakdown caused by excessive di/dt are very serious. As can be seen from Figure 1, the occurrence of this situation will lead to shoot-through, causing damage to the thyristor and even the transformer. In circuit design, reliable thyristor on-off detection and current limiting measures can be used to avoid the occurrence of this fault.

3.5 Overvoltage and overcurrent protection measures

The main reasons for overvoltage are as follows:

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

(2) Surge voltage generated when the transformer tap is switched;

(3) Surge voltage when lightning strikes;

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

In the circuit, adding a surge absorber can absorb the surge voltage caused by the electromagnetic transfer of the transformer primary system, and can also absorb the transformer magnetic energy generated when the transformer is turned on and off. In order to avoid the surge voltage generated by lightning intrusion, a semiconductor lightning arrester can be used.

In the system, in order to avoid the shoot-through phenomenon caused by false triggering, the following technical measures can be adopted:

(1) Ensure that the trigger signal of the thyristor is reliable. Use software filtering program to make only one output trigger signal in each group valid, and then use 74LS273 and anti-circulation logic circuit (PAL16V8) to ensure that there will be no false triggering even if the microcontroller is out of control. In addition, the trigger signal lead uses shielded wires and other anti-interference measures to prevent false triggering.

(2) Ensure reliable commutation. In normal operation, the compensation voltage is often changed, that is, the conduction combination of the thyristor is adjusted. If the timing or combination of commutation is improper, a direct conduction will occur and damage the thyristor. In the design, the zero-crossing switching technology is used. The most important thing is to accurately detect the working state of the thyristor. To this end, the combination of software and hardware and interlocking technology are adopted to ensure that the switch works accurately. The actual operation results show that the above technology is feasible.

3.6 Effect of inductive load

Due to the presence of inductive load, the trigger pulse width should be increased, otherwise the trigger signal will weaken before the anode current of the thyristor reaches the holding current, causing the thyristor to fail to conduct normally. When turning off, inductive load will also cause some problems for the thyristor.

In actual systems, level triggering is used to ensure that the thyristor is reliably turned on.

3.7 Zero-Crossing Switching Technology

During the voltage regulation process, the signal generated by zero-crossing detection is used to control the thyristor gate trigger signal to ensure its zero-crossing switching, thereby avoiding pollution to the power grid caused by the switching of the thyristor during the voltage regulation process.

Due to the adoption of the above series of measures, this power supply is technically very mature.