Vacuum Electron-beam Valves in the Inverter for a Static Compensator

Abstract

The article presents the results from studying the possibility of designing inverters for power industry applications with a capacity of 1...2 MW with transformerless connection to a network with a frequency of 50 Hz and voltage up to 35 kV, with the use of high-voltage switching apparatuses on the basis of electron-beam valves for currents up to 50 A and switched voltages up to 100 kV. If we wish to make further progress in the technology of power converters for electric power industry applications, it is necessary to improve the components in terms of increasing the operating voltages and frequency range, the resistance to the effect of external influencing factors, robustness to changes in power operating conditions, and reliability enhancement. Electron-beam valve based switches can be considered as an alternative to power semiconductor devices as components for making inverters. The effectiveness of using electron-beam valves should be estimated by comparing their electrical and operating characteristics with those of power semiconductor devices under the conditions of their possible use in the converters of a power supply system with the voltage level of 35 kV and above. With the approximately equal levels of power switched by the valves, power semiconductor devices are able to pass currents equal to around a few kA at voltages lower than 5...6 kV. As to electron-beam valves, in passing the same power levels, they can switch a constant current of around 50 A at a voltage of 50 to 200 kV. A significant advantage of electron-beam valves is their ability to limit short-circuit current in the load and simultaneously withstand short-term dissipation of a large power on the anode. In principle, electron-beam valves can be used to construct inverters with a PWM quantization frequency of more than 10 kHz, which is not attainable for power semiconductor devices on the basis of IGCT and IGBT (up to 2 kHz). Electron-beam valves operating in the mode of switching a voltage ranging from 20 to 200 kV can have losses in the conducting period at a level of 2...3% of the switched power. With the switched power at a level of 300...400 kW, it becomes possible to construct PWM inverters for static reactive power compensators. The main disadvantage of electron-beam valves is a relatively low maximum value of their electrical efficiency (0.9) and durability (the service life of usual barium cathodes operating with a current density of up to 1 A/cm 2 and temperature of about 700...800 o C is no more than 8000 h). Decreasing the current density results in a proportional increase of the durability. A further increase in the electrical efficiency of an electron-beam valve is possible by optimizing the design of its electronic-optical system with the aid of computer modeling, and the durability of the cathode can be increased by optimizing its design, the application technology, and chemical composition. For example, increasing the durability up to 100 000 h at a current density of up to 30 A/cm 2 and a temperature of about 1100 о C is possible by using dispenser cathodes on the basis of a tungsten sponge with WBaOs coating and longitudinal grooves on the substrate.