Power electronics technology
(1) The single-unit capacity of the electric load is getting larger and larger, and the capacity of the power electronic device is required to be larger and larger. The requirements for the voltage capacity and current capacity of the power electronic device are becoming larger and larger. A difficult problem with technology. At present, people only rely on series-parallel technology to solve this problem, but reliability has been plagued by a large number of applications of series-parallel technology.
If the voltage and current capacity of power electronics is solved, it will be a very promising prospect: the uncontrollable problem of the AC grid will be solved, freeing people from complicated calculations and panic about accidents; All electrical equipment will work in an orderly manner: no impact, no negative sequence, no harmonics...
(2) Series and parallel application
For larger power electronics, when the voltage or current rating of a single power electronic device does not meet the requirements, it is often necessary to operate the power electronics in series or in parallel.
2.1 thyristor series
When the rated voltage of the thyristor is less than the actual requirement, more than two devices of the same type can be connected in series. Ideal series requires that each device withstands equal voltage, but in reality, due to the difference between device characteristics, there is generally a problem of uneven voltage distribution. The leakage current flowing through the devices in series is always the same, but due to the dispersion of static volt-ampere characteristics, the voltages experienced by the devices are not equal. When the two thyristors are connected in series, the forward voltage experienced under the same leakage current IR is different. If the applied voltage continues to rise, the device with the high voltage will first turn to the turning voltage and turn on, so that the other device assumes that all the voltage is also turned on, and both devices lose control. Similarly, in the reverse direction, due to the different volt-ampere characteristics and uneven pressure, one of the devices may be reversed first and the other may be broken down. This problem of equalization due to the different static characteristics of the device is called the static uneven pressure problem.
In order to achieve static voltage equalization, first select the device with the same parameters and characteristics as possible, and also use the resistor equalization. The resistance of RP should be much smaller than the forward and reverse resistance of any device blockage, so that the voltage shared by each thyristor depends on the voltage division of the resistor.
Similarly, the problem of uneven pressure due to differences in device dynamic parameters and characteristics becomes a dynamic non-uniform pressure problem. In order to achieve dynamic voltage equalization, the device with the same dynamic parameters and characteristics should be selected firstly. In addition, the RC parallel branch can be used for dynamic voltage equalization, as shown in Figure 2b. For thyristors, the use of gate-strong pulse triggering can significantly reduce the difference in device turn-on time.
2.2 Parallel connection of thyristors
In high-power thyristor devices, multiple devices are often used in parallel to carry large currents. When the thyristors are connected in parallel, there is a problem of uneven current distribution due to the difference between static and dynamic characteristic parameters. The current sharing is not good, and some devices have insufficient current and some overloads, which may hinder the output of the entire device and even cause damage to devices and devices. The first measure of current sharing is to pick the devices whose characteristic parameters are as consistent as possible. In addition, a current sharing reactor can also be used. Similarly, the use of gate-strong pulse triggering also contributes to dynamic current sharing. When thyristors are required to be connected in series and in parallel at the same time, they are usually connected by a combination of the first and the last. In comparison, even if redundant design is adopted, the series connection is more risky than parallel connection. Therefore, when circuit design is carried out, if both methods can meet the requirements through circuit conversion, parallel connection should be preferred, and the implementation of current sharing is relatively easy. .
