UPS (Uninterruptible Power Supply) is an indispensable part of the modern power supply system, which can provide continuous power in the event of a power grid failure, ensuring the normal operation of critical loads. UPS can be divided into power frequency UPS and high-frequency UPS according to their operating frequencies, and these two types of UPS have significant differences in design, performance, and application scenarios.
Let’s understand the definition and working principle of power frequency UPS. The power frequency UPS mainly consists of a Silicon Controlled Rectifier (SCR) rectifier, an Insulated Gate Bipolar Transistor (IGBT) inverter, a bypass, and a power frequency boost isolation transformer. SCR rectifier and transformer both operate at 50Hz power frequency, hence they are called power frequency UPS. During the rectification process, three-phase AC power is converted into DC voltage through a commutation inductor, and the DC output voltage is adjusted by controlling the conduction angle of the SCR. The step-down characteristic of SCR rectifier means that the DC bus voltage is lower than the input AC voltage. Therefore, in order to obtain a constant 220V AC output, a step-up isolation transformer is needed to increase the voltage. The bus voltage of a power frequency UPS is usually between 300V and 500V, allowing for direct connection of a certain number of batteries, typically 6-pulse or 12 pulse rectification, depending on the number and configuration of thyristors.
In contrast, high-frequency UPS mainly consists of IGBT high-frequency rectifier, battery converter, inverter, and bypass. Its core is IGBT rectifier, which can be switched and controlled at high frequencies of several thousand to several hundred KHz. Compared with power frequency UPS, high-frequency UPS has advantages in power density, volume, and weight. Due to the boost characteristics of high-frequency rectification, the DC bus voltage of high-frequency UPS is higher than the peak value of the input line voltage, usually around 800V. Therefore, the AC voltage output by the inverter can directly reach 220V without the need for a step-up transformer. The battery converter of high-frequency UPS is responsible for reducing and increasing the voltage of the battery pack during normal and faulty mains power supply.
In terms of performance comparison, both power frequency UPS and high-frequency UPS have their own advantages and disadvantages. For example, in terms of zero bias faults, high-frequency UPS has inherent defects. If the zero line is disconnected under unbalanced load conditions, it may lead to UPS output zero bias faults, and in severe cases, it may cause damage to the load equipment. In contrast, power frequency UPS can still maintain normal power supply even when the neutral line is disconnected.
In addition, the issues of zero ground voltage rise and battery rack charging are more prominent in high-frequency UPS. The rectifier and inverter of high-frequency UPS may experience a rise in zero ground voltage due to high-frequency harmonic coupling, making it difficult to meet the requirements of some servers for zero ground voltage. However, due to the use of isolation transformers, this issue is relatively minor for power frequency UPS.
In terms of selection principles, it is necessary to comprehensively consider factors such as actual load requirements, environmental conditions, and cost budgets.Power frequency UPS is more favored in applications that require long-term power supply and high stability requirements due to its stability and ability to withstand unbalanced loads. High frequency UPS, due to its small size, light weight, and relatively low cost, has more advantages in environments that are sensitive to space and weight and require high transient response. Both power frequency UPS and high-frequency UPS play an important role in ensuring the stability of power supply. But when choosing, users need to carefully weigh the performance characteristics of both and their own needs in order to achieve the best economic and application results. With the continuous advancement of power electronics technology, we can foresee that future UPS designs will be more efficient, intelligent, and environmentally friendly, providing users with more reliable power solutions.