Teaching and research.
In 1999, the University of Texas MKang and others proposed to pay the power electronic transformer, as shown in -2. The circuit is composed of two primary and secondary power conversion circuits and a high-frequency transformer. In the two-level power conversion circuit, each bridge arm is connected by two back-to-back power devices. This connection can achieve bidirectional energy flow.
The flyback power electronic transformer-3 is a power electronic transformer using a flyback converter. This structure avoids too many intermediate stages and simplifies the structure. It can also be seen from the figure that the entire device has only six switching devices, which is much less than the previous two structures.
The obvious advantage of this circuit is that it requires fewer switching devices; the inductance and capacitance of the power supply side constitute the LC filter circuit, which can solve the power quality problem to a certain extent. However, because the main switch of the flyback conversion circuit has a large voltage stress, it is difficult to apply to the occasion of high input voltage. In addition, the transformer of the flyback conversion circuit needs to be added with an air gap, so the leakage inductance is large, which will cause a lot of The high peak voltage further increases the voltage stress on the transformer and makes the electromagnetic interference more serious.
3. Analysis structure block diagram and circuit structure of power electronics transformer with double PWM conversion The basic idea of ​​double PWM conversion is to perform three-phase PWM rectification conversion on the primary side of the transformer, and then modulate the DC voltage into a high-frequency voltage, and then couple it to the auxiliary through the high-frequency transformer. After the side, high-frequency rectification is performed on the secondary side, and finally through a three-phase PWM inverter circuit, a three-phase power frequency AC voltage is obtained. The structural block diagram is shown in -1.
The specific circuit structure of power electronic transformer with double PWM conversion is shown in -2. The circuit consists of voltage-type three-phase PWM rectifier circuit, single-phase bridge inverter circuit, high-frequency power transformer, single-phase bridge rectifier circuit, three-phase bridge Inverter circuit composition. The working process of the circuit is: on the primary side of the transformer, the power frequency high-voltage alternating current is converted into direct current through a three-phase voltage-type PWM rectifier circuit, thereby realizing the phase control of the grid current, and then inverting it to high through a single-phase inverter circuit Frequency single-phase alternating current is coupled to the secondary side through the transformer, then through the single-phase high-frequency rectifier circuit, the high-frequency alternating current of the secondary side of the transformer is changed into direct current, and finally through the three-phase inverter circuit and the filter circuit, the power frequency alternating current is obtained.
It can be seen from -2 that the primary and secondary power electronic transformers based on double PWM conversion contain a single full-bridge rectifier circuit and a three-phase full-bridge rectifier circuit, and are all composed of full-control devices. The rectifier circuit is an AC / DC converter circuit applied earlier. The development of the rectifier circuit has experienced the development process from uncontrolled rectifier (diode rectifier), phase controlled rectifier (crystal tube rectifier) ​​to PWM rectifier (gate-off power switch tube). Although the traditional phase-controlled rectifier has a long application time, mature technology and is widely used, the following problems still exist: the voltage waveform distortion of the grid side caused by the commutation of the crystal tube.
Harmonic currents on the grid side cause harmonic "pollution" to the grid.
The grid-side power factor decreases during deep control.
The dynamic response is relatively slow in closed-loop control.
The diode rectifier circuit will generate grid-side harmonic currents and "pollute" the power grid. In addition, the disadvantage of diode rectification lies in the uncontrollability of its DC voltage. In response to the above deficiencies, PWM rectification has made comprehensive improvements to traditional phase control and diode rectification. The key improvement is to replace the semi-controlled power switch tube or diode with a fully controlled power switch tube, and replace the phase controlled rectifier or uncontrolled rectifier with PWM chopper controlled rectifier. Therefore, PWM rectification can achieve the following excellent performance: the grid-side current is a sine wave.
Grid side power factor control (such as unit power factor control).
Bidirectional power transmission.
Faster dynamic control response.
Obviously, PWM rectification is no longer a traditional AC / DC converter. Due to the bidirectional transmission of electrical energy, when the PWM rectifier circuit draws electrical energy from the power grid, it operates in the rectifying working state; when the PWM rectifying circuit transmits electrical energy to the power grid, it operates in the active inverter working state. The so-called unit power factor means that when the PWM rectifier circuit operates in the rectified state, the grid-side voltage and current are in phase (positive resistance characteristics); when the PWM rectifier circuit operates in the active inverter state, its grid-side voltage and current are inverse ( Negative resistance characteristics). Since the grid-side current and power factor of the PWM rectifier circuit can be controlled, it can be widely applied to non-rectifier applications such as reactive power compensation. This article makes use of the advantages of the power factor controllable on the grid side of the PWM rectifier circuit and the two-way transmission of electric energy. Simultaneously with the light-weight design, Kang active controls the power of the primary and secondary sides of the transformer so that it operates at a unit power factor on the grid side in steady state.
It can be seen from -2 that the primary and secondary power electronic transformers based on double PWM conversion include a primary single-phase inverter circuit and a secondary three-phase inverter circuit. Among them, the role of the primary single-phase inverter circuit is to convert the DC output voltage of the primary rectifier circuit to the working voltage of the high-frequency transformer; the secondary three-phase inverter circuit is to convert the DC voltage of the secondary rectifier circuit to power frequency AC voltage for users. The main characteristics of the voltage-type inverter circuit are that the DC side parallel large capacitor is equivalent to a voltage source; due to the characteristics of the DC side voltage source, the AC side output voltage is a rectangular wave, and its waveform is independent of the load impedance, while the AC side current waveform And the phase will change due to the load impedance; when the load is inductive, reactive power needs to be provided, the DC side capacitor plays a role in buffering the reactive energy, and the feedback diodes connected in parallel to each arm of the inverter bridge are AC side to DC side Feedback of non-functional quantities provides channels.
The power electronic transformer can be used in the transmission link of the power system. The power electronic transformer is relatively easy to combine with the flexible transmission technology. In order to improve the stability of the power system; in the distribution link, due to the support of modern power electronic technology and modern control technology, the power electronic transformer has the function of suppressing harmonic bidirectional flow, preventing the failure of the load side from affecting the power supply voltage, eliminating The effect of voltage sag, rise, and interference of power supply side voltage such as overvoltage and undervoltage on the load, so it can improve power supply reliability and power quality. The theory and implementation of power electronic transformers is a new research field, and its research results will provide new ideas and methods for the manufacture of transformers and their application in power systems. After they become practical, they will be used in transformer manufacturing and power generation. There are huge benefits in the system.
Wang Zhaoan, Huang Jun. Power Electronics Technology. Beijing: Mechanical Industry Press, 2002. Very high, and soon suffocated the flame.
For substances that cannot be extinguished with water, if they catch fire, they can be extinguished with dry powder. Generally, sodium krypton carbonate is added with 4540% fine sand, diatomaceous earth or talc. The fire extinguishing powder sprayed from the dry powder fire extinguisher is covered on the solid burning material, forming an isolation layer that hinders combustion, and this solid powder fire extinguishing agent emits steam and carbon dioxide when it encounters a fire, using his heat absorption to cool down and isolate the air Extinguish the flame.
Carbon tetrachloride is a colorless transparent liquid, which does not support combustion, does not conduct electricity, and has a low boiling point (76.8C). Carbon tetrachloride fire extinguisher is to use these properties. When carbon tetrachloride falls into the fire zone, it quickly evaporates. Because of its heavy vapor (5 to 5 times that of air), it is densely concentrated around the source of the fire and surrounds the burning material, which plays a role in insulating the air. Then the burning flame is extinguished quickly, which is especially suitable for the fire extinguishing of live equipment. But it has a big disadvantage, that is, when it is heated above 250C, it can interact with water vapor to generate hydrochloric acid and phosgene. Phosgene is a very toxic gas. Carbon tetrachloride itself is also toxic. Therefore, when using this fire extinguisher, wear a gas mask.
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