The power frequency machine and the high frequency machine are distinguished according to the operating frequency of the design circuit of the UPS. The power frequency machine is designed by the traditional analog circuit principle and consists of a thyristor (SCR) rectifier, an IGBT inverter, a bypass and a power frequency boosting isolation transformer. Because its rectifier and transformer operating frequency are 50Hz, the power frequency UPS is called as the name suggests. High frequency machines are usually composed of IGBT high frequency rectifiers, battery converters, inverters and bypasses. The IGBT can control its turn-on and turn-off by controlling the driving applied to the gate. The switching frequency of the IGBT rectifier is usually several kilohertz to several tens of kilohertz, or even hundreds of kilohertz, which is much higher than the power frequency machine. For high frequency UPS.
In the power frequency UPS circuit, the main circuit three-phase AC input is converted into a DC voltage by a commutating inductor, connected to a rectifier composed of three SCR bridge arms, and the output DC voltage value is adjusted by controlling the conduction angle of the rectifier bridge SCR. Since the SCR is a half-controlled device, the control system can only control the turn-on point. Once the SCR is turned on, it cannot be turned off even if the gate drive is cancelled. It can only be turned off naturally after the current is zero. Therefore, its turn-on and turn-off are based on a power frequency cycle, and there is no high-frequency turn-on and turn-off control. Since the SCR rectifier belongs to the step-down rectification, the AC bus voltage of the DC bus voltage is lower than the input AC voltage. To make the output phase voltage obtain a constant 220V voltage, the boost isolation transformer must be added to the inverter output.
In contrast, high frequency UPS rectification is a boost rectification, and the output DC bus voltage is higher than the peak of the input line voltage. Generally, the typical value is about 800V. If the battery is directly connected to the busbar, the required number of battery segments is required to be 67 knots, which brings great limitations to practical applications. Therefore, the general high-frequency UPS will be equipped with a battery voltage converter separately. When the mains is normal, the battery converter will reduce the bus voltage of 800V to the battery voltage; when the mains fails or exceeds the limit, the battery converter raises the battery voltage to 800V bus voltage. Since the high-frequency bus voltage is about 800V, the output phase voltage of the inverter can directly reach 220V, and the step-up transformer is no longer needed after the inverter. Therefore, the presence or absence of an isolation transformer is the main difference in the structure between the power frequency machine and the high frequency machine.
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