As the basic equipment for voltage conversion and electrical isolation, power transformers are the core of network connection in power systems. They are used in power transmission and distribution. At present, many iron core oil-immersed transformers are used in the system, which are large in volume and weight. The level and power capacity increase increase the no-load loss of the transformer. The fluctuation of the primary side voltage amplitude will affect the secondary side output voltage. The load change will bring the instability of the secondary side output voltage, and the harmonic generated by the core saturation will cause the excitation. Inrush current, harmonic intrusion on the primary side and secondary side induces new faults, etc. New energy needs to be converted and adjusted before grid connection, and flexible control of power flow, with fault diagnosis and isolation. The limitations of traditional transformers are obvious. Can not meet the above requirements.
First, the introduction of solid state transformer (SST) technology
Solid-state transformer (SST), also known as power electronic transformer, is a combination of power electronic conversion technology and high-frequency energy conversion technology based on electromagnetic induction principle to realize the conversion of one electric characteristic electric energy into another electric characteristic electric energy. Static electrical equipment. Compared with traditional transformers, it has the advantages of small size, light weight, etc., and has many advantages that traditional transformers do not have, including high power supply quality, high power factor, automatic current limiting, and reactive power compensation capability. Frequency conversion, output phase number conversion and easy automatic monitoring. Due to the above advantages, the use of solid state transformers in power systems can improve system reliability, improve power quality, facilitate new energy generation and grid connection, and promote smart grid construction. And development.
The basic principle block diagram is shown in Figure 1: First, the power frequency AC signal is converted into a high frequency square wave signal by the power electronic converter, the signal is transmitted through the high frequency isolation transformer, and the high frequency square wave signal is restored to the power frequency by the power electronic converter. AC signal. This process can be done by the controller with appropriate control of the power electronic converter.
01 The first step AC/DC rectifier converts 7KV single-phase AC power into 10KV DC. 02 The second step high-frequency DC/DC converter converts DC 10K into DC 400V. It includes high-voltage H-bridge circuit, low-voltage H-bridge circuit. And intermediate high-frequency transformer. 03 third-phase DC / AC inverter, DC 400V converted to 60Hz, ± 120V AC.
By increasing the operating frequency of the DC/DC converter and the DC/AC inverter, it is possible to achieve a smaller volume and lighter weight than conventional transformers, thereby greatly reducing the size and weight of passive components.
Second, the application of SiC silicon carbide devices in solid state transformers
An ideal semiconductor power device should have such static and dynamic characteristics: it can withstand high voltages in the blocking state; it has a high current density and a low conduction voltage drop in the on state; in the switching state and switching, It has short on/off time, can withstand high di/dt and dv/dt, has low switching loss, and has full control function. However, due to limitations in voltage, power tolerance, etc., these Si silicon-based high power In solid-state transformer applications, devices have to use device strings, parallel technology and complex circuit topology to meet the requirements of practical applications, resulting in greatly increased failure rate and cost of the device, which restricts the further development of solid-state transformers in smart grid applications.
In recent years, as a new wide-bandgap semiconductor material, silicon carbide (SiC), due to its excellent physical and electrical properties, it is receiving more and more attention from the industry. The important advantage of silicon carbide power devices is their high voltage resistance. (tens of thousands of volts), high temperature (greater than 500 ° C) characteristics, breaking through the serious system limitations caused by silicon-based power device voltage (several kV) and temperature (less than 180 ° C) limits. With the silicon carbide material technology Progress, various silicon carbide power devices have been developed, such as silicon carbide power diodes SBD, MOSFETs, etc. Due to cost, output and reliability, silicon carbide power devices are the first to be industrialized in the low-voltage field, current commercial products The voltage level is 650-1700V, and the solution adopted in solid-state transformers is still mostly series. In the next few years, with the advancement of technology, high-voltage silicon carbide devices, especially the appearance of silicon carbide devices above 15kV, and corresponding driving technology and anti-interference The development of technology will be conducive to the structural simplification and reliability improvement of solid state transformers.
Third, summary
Future power distribution systems may consist of many distributed renewable energy sources and power grids. Both new power distribution facilities and energy storage systems can be interconnected with the grid or microgrid. The energy Internet has the ability to control energy in both directions, enabling it to provide important Plug-and-play functionality, and can isolate the system from the user's faulty end. Energy Internet requires advanced high-voltage, high-frequency and high-temperature power semiconductor devices. Practice results show that SiC devices can significantly simplify the circuit structure of solid-state transformers. Reduce the heat sink space and increase the unit power density by increasing the switching frequency. It can be seen that the new high-voltage large-capacity silicon carbide power device will open up new applications in solid-state transformer applications, and continue to develop and change the smart grid. Significant impact.