When purchasing high-power UPS, users often face confusion when choosing between low-frequency and high-frequency machines. From the perspective of UPS manufacturers, they naturally claim their product is better. "One says theirs is good, the other says theirs is better." Manufacturers providing low-frequency machines emphasize their higher stability and reliability, while those offering high-frequency machines tout their space-saving design and relatively lower cost. In reality, it's hard to definitively say which is better as both have their pros and cons. Users should objectively evaluate their applications and needs based on a comprehensive understanding of these two types of UPS machines and select the one that suits their requirements.

　　Principle Analysis of Low-Frequency and High-Frequency Machines

　　Low-frequency and high-frequency machines are distinguished by the operating frequency of their UPS circuit design. Low-frequency machines are designed based on traditional analog circuit principles, consisting of a thyristor (SCR) rectifier, IGBT inverter, bypass, and low-frequency step-up isolation transformer. Since both the rectifier and transformer operate at the low frequency of 50Hz, it is called a low-frequency UPS. High-frequency machines, on the other hand, typically consist of an IGBT high-frequency rectifier, battery converter, inverter, and bypass. The IGBT can be controlled to turn on and off via gate drive signals. The switching frequency of the IGBT rectifier usually ranges from several kilohertz to tens of kilohertz, or even up to hundreds of kilohertz, which is far higher than that of low-frequency machines, hence the name high-frequency UPS.

　　In a low-frequency UPS circuit, the main three-phase AC input passes through phase-shifting inductors and is then converted into DC voltage by a rectifier composed of three SCR bridge arms. The output DC voltage is adjusted by controlling the conduction angle of the SCR bridge. Since SCRs are semi-controlled devices, 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 removed. It will only naturally turn off when its current reaches zero. Therefore, its turn-on and turn-off operations are based on a low-frequency cycle, without high-frequency control. As SCR rectifiers are step-down rectifiers, the DC bus voltage is lower than the input AC voltage. To ensure the output phase voltage remains at a constant 220V, a step-up isolation transformer must be added to the inverter output.

　　In comparison, high-frequency UPS rectifiers are step-up rectifiers, with their output DC bus voltage typically higher than the peak of the input line voltage, generally around 800V. If the battery is directly connected to the bus, the standard number of batteries required is 67, which poses significant challenges for practical applications. Therefore, high-frequency UPS systems are usually equipped with a separate battery voltage converter. When the utility power is normal, the battery converter lowers the 800V bus voltage to the battery pack voltage. When the utility power fails or exceeds limits, the battery converter raises the battery pack voltage to the 800V bus voltage. Since the high-frequency machine's bus voltage is around 800V, the inverter output phase voltage can directly reach 220V, eliminating the need for a step-up transformer after the inverter. Therefore, whether there is an isolation transformer is the main structural difference between low-frequency and high-frequency machines.

　　The Role of UPS Output Isolation Transformers

　　Isolation transformers utilize the principle of electromagnetic induction to electrically isolate power distribution or signals. In UPS systems, isolation transformers are typically designed at the inverter output to enhance UPS performance and improve power quality at the load end. Generally, UPS output isolation transformers have the following four advantages:

　　Reduce Zero-Ground Voltage and Optimize the UPS Power Supply Network

　　Installing an isolation transformer on the UPS inverter output can isolate the electrical connection between the input and output, effectively reducing the output zero-ground voltage. Since the secondary winding of the isolation transformer adopts a Y-connection method, grounding the neutral point creates a new zero line, thereby reducing the zero-ground voltage. In fact, small machines from HP, IBM, and SUN, which require precise computing capabilities and high-reliability data processing and transmission, have very high requirements for zero-ground voltage. Adding an isolation transformer can completely resolve issues caused by high zero-ground voltage.

　　Filter Load-Side Harmonics and Improve Power Quality

　　Isolation transformers inherently have inductive characteristics. Output isolation transformers can filter a large amount of low-order harmonics on the load side, reduce high-frequency noise, and significantly attenuate high-order harmonics. Using power isolation transformers can effectively suppress noise interference in AC power supplies and improve the electromagnetic compatibility of equipment.

　　Enhance Overload and Short-Circuit Protection, Protect Load and UPS Host

　　Due to their inherent characteristics, isolation transformers are the most stable components in UPS systems. During normal UPS operation, if a large short-circuit current occurs, the transformer will generate a reverse electromotive force, delaying the impact and damage of the short-circuit current on the load and inverter, thereby protecting both the load and the UPS host.

　　Block DC While Allowing AC, Protect Load During UPS Failures

　　High-capacity UPS systems use high-frequency designs for AC/DC conversion, improving the UPS input power factor (above 0.98) and input voltage range. The high-frequency DC/AC inverter reduces the size of output filter inductors and increases power density. However, without an output isolation transformer, if an IGBT in the inverter bridge is damaged and short-circuited, the DC high voltage on the BUS will be applied to the load, endangering its safety. Output isolation transformers can block DC while allowing AC, solving such issues and ensuring the load operates safely during UPS failures.

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