When purchasing high-power UPS power supplies, users often face the confusion of choosing between industrial frequency units and high-frequency units. As far as UPS manufacturers are concerned, of course they all think it is their own good. Manufacturers that provide power frequency machines say that power frequency machines have high stability and reliability, and manufacturers who provide high frequency machines will say that high frequency machines save space and are relatively low in cost. In fact, it is difficult to generalize whether a power frequency machine or a high frequency machine is better. It can be said that each has its advantages and disadvantages. On the basis of a comprehensive understanding of these two UPS models, users should objectively examine their own applications and needs, and choose products that suit their needs.

1 The principle analysis of industrial frequency machine and high frequency machine

The power frequency machine and the high frequency machine are distinguished according to the working frequency of the UPS design circuit. The power frequency machine is designed based on the traditional analog circuit principle and consists of a thyristor (SCR) rectifier, an IGBT inverter, a bypass and a power frequency step-up isolation transformer. Because the working frequency of its rectifier and transformer is 50Hz, it is called a power frequency UPS as the name suggests. The high frequency machine is usually composed of IGBT high frequency rectifier, battery converter, inverter and bypass. The IGBT can be turned on and off by controlling the drive added to the gate. The switching frequency of the IGBT rectifier is usually from a few kilohertz to tens of kilohertz, or even as high as hundreds of kilohertz, which is much higher than that of a power frequency machine. It is a high-frequency UPS.

In the industrial frequency UPS circuit, the three-phase AC input of the main circuit is converted into a DC voltage after being connected to a rectifier composed of three SCR bridge arms through a commutation inductor, and the output DC is adjusted by controlling the conduction angle of the rectifier bridge SCR Voltage value. Since the SCR is a semi-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 its 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 is a step-down rectifier, the AC voltage output by the DC bus voltage through the inverter is lower than the input AC voltage. If the output phase voltage can obtain a constant 220V voltage, a step-up isolation transformer must be added to the inverter output.

In contrast, high-frequency UPS rectification is a step-up rectification, and the output DC bus voltage is higher than the peak value 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 standard battery cells needs to be 67, which brings great limitations to practical applications. Therefore, generally high-frequency UPS will be equipped with a separate battery voltage converter. When the mains power is normal, the battery converter will reduce the 800V bus voltage to the battery pack voltage; when the mains fails or exceeds the limit, the battery converter will increase the battery pack voltage to 800V bus voltage. Since the bus voltage of the high-frequency machine is about 800V, the output phase voltage of the inverter can reach 220V directly, 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 structure between a power frequency machine and a high frequency machine.

2 The role of UPS output isolation transformer

Isolation transformer is a device that uses the principle of electromagnetic induction to electrically isolate power distribution or signals. The isolation transformer is usually designed at the output end of the inverter in the UPS, which can increase the performance of the UPS and improve the quality of the power supply at the load end. Generally, the output isolation transformer of UPS has the following four advantages:

2.1 Reduce the zero-ground voltage and optimize the UPS terminal power supply network

The installation of an isolation transformer on the inverter output of the UPS can isolate the electrical connection between the input and the output, thereby effectively reducing the zero-ground voltage of the output. Since the secondary winding of the isolation transformer adopts the Y-type connection, a new neutral line is generated after the neutral point is grounded, so as to achieve the purpose of reducing the zero-ground voltage. In fact, HP, IBM, and SUN minicomputers have extremely high requirements on the zero-ground voltage because they have to ensure precise computing capabilities and high-reliability data processing and transmission capabilities. The installation of an isolation transformer can completely solve the problem of zero-ground voltage deviation. Some problems caused by high.

2.2 Filter out harmonics on the load side and improve the quality of power supply

The isolation transformer itself has inductance characteristics. The output isolation transformer can filter out a large number of low-order harmonics at the load end, reduce high-frequency interference, and can greatly attenuate high-order harmonics. The use of power isolation transformers can effectively suppress the noise interference entering the AC power supply and improve the electromagnetic compatibility of the equipment.

2.3 Enhance the overload and short-circuit protection capability to protect the load and the UPS host

Due to its own characteristics, the isolation transformer is the most stable device in UPS. During the normal operation of the UPS, if it encounters a large short-circuit current, the transformer will generate a reverse electromotive force, delay the impact of the short-circuit current on the load and the inverter, and protect the load and the UPS host.

2.4 "Passing AC and blocking DC", protects the load when the UPS fails

The AC/DC conversion part of the high-rated UPS adopts high frequency design, which improves the input power factor (above 0.98) and input voltage range of the UPS. The high frequency of the DC/AC inverter part reduces the volume of the output filter inductor , The power density is large. Since there is no output isolation transformer, once the IGBT of the inverter bridge arm is broken down and short-circuited, the high DC voltage of the BUS bus will be added to the load, endangering the safety of the load. The output isolation transformer has the ability to "pass AC and block DC", which can solve such problems and make the load run safely when the UPS fails.

3 Performance comparison between industrial frequency machine and high frequency machine

3.1 In terms of reliability, industrial frequency machines are better than high frequency machines

The power frequency machine adopts the thyristor (SCR) rectifier. After more than half a century of development and innovation, this technology has been very mature, and its ability to resist current impact is very strong. Since the SCR is a semi-controlled device, there will be no faults such as pass-through and false triggering. In contrast, although the IGBT high-frequency rectifier used in the high-frequency machine has a higher switching frequency, the IGBT has a strict voltage and current working area and has low impact resistance. Therefore, in terms of overall reliability, IGBT rectifiers are lower than SCR rectifiers.

3.2 In terms of environmental adaptability, high-frequency machines are better than industrial frequency machines

The high-frequency machine uses a microprocessor as the processing control center, burns complicated hardware analog circuits into the microprocessor, and controls the operation of the UPS in the form of software programs. Therefore, the volume and weight are significantly reduced, the noise is also small, and the impact on space and environment is small, so it is more suitable for office places that have less stringent requirements on reliability. Because of this, many manufacturers of small and medium power UPS generally introduced high-frequency machines.

3.3 In terms of load requirements for zero-ground voltage, industrial frequency machines are better than high frequency machines

The zero line of the high-power three-phase high frequency machine will introduce the rectifier and serve as the neutral point of the positive and negative bus bars. This structure inevitably causes the high frequency harmonics of the rectifier and the inverter to be coupled to the zero line, raising the zero The ground voltage causes the zero-ground voltage of the load terminal to rise, and it is difficult to meet the site requirements of manufacturers such as IBM and HP that the zero-ground voltage is less than 1V. In addition, when the mains and generators are switched, the high-frequency generators often have to switch to bypass due to the missing of the neutral line, which may cause major failures such as load flashing under certain operating conditions. The power frequency machine does not need the neutral line to participate in the work because of the rectifier. When the neutral line is disconnected, the UPS can maintain normal power supply.