Abstract: when the user buys high-power UPS power supply, they often face the confusion of choosing between power frequency machine and high frequency machine. As far as UPS manufacturers are concerned, they are certainly considered as their own good, "public said public is reasonable, and grandma says that women have reason". The manufacturer who provides power frequency machine says that the power frequency machine is stable and reliable, and the manufacturer who provides high frequency machine will say that the high frequency machine saves space and the cost is relatively low. In fact, it is difficult to generalize which one is superior or inferior between power frequency machine and high frequency machine. It can be said that each has its advantages and disadvantages. Users should objectively examine their own applications and requirements on the basis of comprehensive understanding of the two UPS models and choose the products suitable for their own needs.
Principle analysis of 1 power frequency machine and high frequency machine
The power frequency machine and the high frequency machine are distinguished according to the design circuit frequency of ups. The power frequency machine is designed by the traditional analog circuit principle, which consists of SCR rectifier, IGBT inverter, bypass and power frequency step-up isolation transformer. Because the working frequency of rectifier and transformer is 50Hz, it is called power frequency ups as its name implies. High frequency machine is usually composed of IGBT high frequency rectifier, battery converter, inverter and bypass. IGBT can control its opening and closing by controlling the driving of gate pole. IGBT rectifier switch frequency is usually from thousands to dozens of kHz, even hundreds of kHz, far higher than power frequency machine, so it is called high frequency ups.
In the power frequency UPS circuit, the main three-phase AC input is converted into DC voltage after receiving the rectifier composed of three SCR bridge arms through commutation inductance. The DC voltage value is adjusted by controlling the conduction angle of SCR of rectifier bridge. Since SCR belongs to semi-control device, the control system can only control the opening point. Once SCR is on, even if the gate pole drive is withdrawn, it can not be closed. Only when the current is zero, can it be turned off naturally. Therefore, both the opening and closing are based on a power frequency period, and there is no high frequency on-off control. Since SCR rectifier belongs to step-down rectifier, the AC voltage of DC bus voltage output through inverter is lower than input AC voltage. In order to make the output phase voltage get constant 220V voltage, it is necessary to add step-up isolation transformer in inverter output.
In contrast, high frequency UPS rectifier belongs to step-up rectifier, and the output DC bus voltage is higher than the peak value of input line voltage. The typical value is about 800V. If the battery is directly connected to the bus, 67 batteries are required for standard distribution, which will bring great limitation to the practical application. Therefore, the high frequency ups will be equipped with a battery voltage converter separately. When the power is normal, the battery converter will reduce the 800V bus voltage to the battery bank voltage; in case of mains power failure or over limit, the battery converter will raise the battery voltage to 800V bus voltage. Since the bus voltage of high frequency machine is about 800V, the output phase voltage of inverter can reach 220V directly, and no step-up transformer is needed after inverter. Therefore, whether there is an isolation transformer is the main difference between power frequency machine and high frequency machine in structure.
2. Function of UPS output isolation transformer
Isolation transformer is a device which uses electromagnetic induction principle to isolate power distribution or signal. Isolation transformer is usually designed in the output end of inverter in UPS, which can increase UPS performance and improve the power supply quality at load end. Generally, the output isolation transformer of UPS has four advantages:
2.1 reduce zero ground voltage and optimize the ups terminal power supply network
The inverter output of UPS can be installed with isolation transformer to isolate the electrical connection between input and output, thus effectively reducing the output zero ground voltage. Because Y-type connection is adopted for the auxiliary winding of the isolation transformer, a new zero line is generated after the neutral point is grounded, so as to reduce the zero ground voltage. In fact, the small-scale machines of HP, IBM and sun have high requirements for zero ground voltage because of their precision calculation ability and high reliable data processing and transmission capability. The installation of isolation transformer can solve some problems caused by the high zero ground voltage.
2.2 filter the harmonic at the load end and improve the power supply quality
The isolation transformer has inductance characteristics. The output isolation transformer can filter out a large number of low harmonic at the load end, reduce high frequency interference and make the high-order harmonic greatly attenuate. The power isolation transformer can effectively suppress the noise interference in the AC power supply and improve the electromagnetic compatibility of the equipment.
2.3 enhance the protection capability of overload short circuit, protect load and UPS host
Because of its own characteristics, the isolation transformer is the most stable device in UPS. In the normal operation process of UPS, if a large short-circuit current transformer is encountered, it will produce reverse electromotive force, delay the impact damage of short circuit current on load and inverter, and it can protect the load and UPS host.
2.4 "AC resistance and DC" and protection load in case of UPS failure
The ac/dc conversion part of high-level UPS adopts high frequency design, which improves the input power factor (above 0.98) and the input voltage range of ups. The high frequency of dc/ac inverter reduces the volume of output filter inductor and has high power density. As there is no output isolation transformer, once IGBT of inverter bridge arm is broken down and short circuit, bus bus DC high voltage will be added to load, which endangers the safety of load. The output isolation transformer has the capability of "AC resistance and DC", which can solve such problems and make the load run safely in case of UPS failure.
3. Comparison of performance between power frequency machine and high frequency machine
3.1 in terms of reliability, power frequency machine is better than high frequency machine
SCR rectifier is used in power frequency machine. After more than half century development and innovation, the technology has been very mature and has strong resistance to current shock. SCR is a semi-control device, and no direct or false triggering faults will occur. Compared with the high frequency rectifier, the IGBT high frequency rectifier is of high switching frequency, but IGBT has strict voltage, current working area and low impact resistance. Therefore, IGBT rectifier is lower than SCR rectifier in terms of overall reliability.
3.2 in terms of environmental adaptability, high frequency machine is better than power frequency machine
High frequency machine takes microprocessor as the processing control center, burns complex hardware analog circuit into microprocessor, and controls UPS operation by software program. Therefore, the volume, weight and other aspects have obvious reduction, and the noise is also small, and the impact on space and environment is small, so it is more suitable for office places with less stringent reliability requirements. Because of this, many manufacturers of small and medium-power ups have generally launched high frequency machines.
3.3 in terms of load requirements for zero ground voltage, power frequency machine is better than high frequency machine
The zero line of high-power three-phase high frequency machine will introduce rectifier and be the neutral point of positive and negative bus. This structure will inevitably cause the high frequency harmonic coupling between rectifier and inverter on the zero line, raise the zero ground voltage, and cause the zero ground voltage rise at the load end. It is difficult to meet the site demand of IBM, HP and other manufacturers for the zero ground voltage less than 1V. In addition, when switching between the municipal power and generator, the high frequency machine often has to bypass due to the lack of zero line, which may cause the major fault of load flash under specific conditions. Power frequency machine does not need zero line to participate in the work because rectifier, UPS can keep normal power supply when the zero line is disconnected.
4. Closing remarks
In terms of structure, the difference between ups and ups of power frequency machine is mainly reflected in the isolation transformer, and the use of power frequency machine to isolation transformer greatly improves the reliability of ups. From the aspect of comprehensive performance, the power frequency machine and the high frequency machine have their own advantages and disadvantages. For example, if users want to build medium and large IDC, the requirements for reliability and stability should be placed first, and high-power power frequency UPS should be the first choice; if it is a general office application or mainly considering the space occupied by equipment, high frequency UPS can be used. National hotline: 400-808-1931