Definition, performance comparison and characteristics of high frequency UPS and industrial frequency UPS
People are accustomed to call the third and fourth generation UPSs as power frequency UPS and high frequency UPS respectively. However, the definition of these two UPSs is not yet unified. Let’s talk about the definition of industrial frequency UPS and high frequency UPS.
The main difference between the so-called power frequency and high frequency UPS lies in the different forms of rectifiers used. There are two commonly used rectifiers for UPS: one is SCR multi-phase phase-controlled rectifier. Since this rectifier works at power frequency, it is defined as power frequency rectifier; the other is SPWM Boost PFC high frequency with IGBT as the switch. Switching rectifier, because the operating frequency of this rectifier is F times higher than the power frequency (50Hz) [F is the carrier ratio of SPWM control, that is, F=(ωc/ωs)≥50], it is defined as a high-frequency rectifier. The power frequency rectifier is a step-down rectifier, and the high-frequency rectifier is usually a boost rectifier (such as Boost SPWM switching rectifier). The high frequency rectifier has a higher mains input power factor than the industrial frequency rectifier, and the mains input power factor is less affected by the load rate. The reason is that the high frequency rectifier has the PFC function, which eliminates the influence of the displacement factor. The so-called PFC function is to make the mains input current track the change of the mains input voltage. This function can only be realized in high-frequency SPWM control. Since the power frequency rectifier is a power frequency phase-controlled rectifier with time lag and cannot realize the PFC function, the high frequency rectifier has a higher input power factor than the power frequency rectifier and is less affected by the load rate.
Usually the UPS that uses a power frequency rectifier is called a power frequency UPS, and the UPS that uses a high frequency rectifier is called a high frequency UPS.
Some performance comparisons of power frequency rectifiers and high frequency rectifiers are shown in Table 1. Some performance comparisons of industrial frequency UPS and high frequency UPS are shown in Table 2. It can be seen from the two tables that the performance of high-frequency UPS is better than that of industrial frequency UPS.
| / Rectifier type Compare items | Power frequency rectifier | High frequency rectifier |
| Mains input power factor PF | It is related to the number of phases of the rectifier and is related to the control angle α of the SCR. For 24-phase SCR phase-controlled rectifier, PF=0.9971×cosα, when a=30°, PF=0.864; for 12-phase SCR phase-controlled rectifier, when a=30°, PF=0.988×cos30°=0.856 PF is greatly affected by load rate | It is related to the switching frequency and the SPWM modulation degree M. When the switching frequency is ≥20kHz and M=0.8, PF=0.99. Because of the PFC function, the mains input power factor is relatively high, and it is less affected by the load rate |
| Volume and mass | Because of the input rectifier transformer, the volume and mass are relatively large | Because there is no need to input a rectifier transformer, the volume and mass are small |
| Adaptability to mains voltage fluctuations | Adjusting the control angle a of the SCR can adjust the output DC voltage of the rectifier, but the adjustment is non-linear, and the adaptability is less than that of the high-frequency rectifier | Adjusting M can adjust the DC output voltage of the switching rectifier, and the adjustment is linear, with strong adaptability |
| Can it be isolated from the mains | Because there is a mains input rectifier transformer, it can be isolated | Because there is no mains input rectifier transformer, it cannot be isolated |
| failure rate | Because of fewer components, low switching frequency, and low DC voltage, SCR has stronger impact resistance than IGBT, so the failure rate is low | Because there are many components, high switching frequency, and high DC voltage, IGBT has strict voltage and current working area, and its anti-impact ability is not as good as SCR, so the failure rate is high. |
| Same bridge arm switch collusion failure | without | Have |
| Switching loss | Because it works at power frequency and low switching frequency, the switching loss is small | Because it works at high frequency (=20kHz), the switching frequency is high, so the switching loss is large |
| Whether there is the influence of UPS load nature on mains input power factor | Because the power frequency rectifier has a one-way output, the pulsating current twice the operating frequency generated by the asymmetric load or nonlinear load of the inverter on the DC power supply cannot be transmitted to the mains, and it has no effect on the input power factor of the mains. | Because the high-frequency rectifier can work in two-way four-quadrants, the pulsating current that is twice the operating frequency generated by the asymmetric load or non-sexual load of the inverter on the DC power supply will all flow into the mains through the high-frequency rectifier, and input current to the mains. The THDi has an impact, which will reduce the power factor of the mains input |
| Transformer loss | For high-power industrial frequency rectifiers, the loss of the mains input rectifier transformer itself is 2.0%~2.5% | No transformer loss |
| Note: | 1. Power frequency rectifier refers to the SCR multi-phase phase-controlled rectifier that transfers the mains power to the phase-shifting transformer; | 2.The high-frequency rectifier refers to the IGBT SPWM three-phase half-bridge high-frequency switch Boost PFC rectifier |
| / UPS type Compare items | Work UPS | High frequency UPS |
| Mains input power factor PF | It is related to the number of phases of the rectifier and the control angle a of the SCR. For 12-phase SCR phase-controlled rectifier, when a=30°, PF=0.856 Because there is no PFC function, it is greatly affected by the load rate | It is related to the switching frequency and the degree of modulation. When the switching frequency=20kHz, M=0.8, THDi=5%, PF=0.9987. Because of the PFC function, it is less affected by the load rate |
| Volume and mass | Because of the transformer, the volume and mass are large | Because the transformer is not used when isolation is not required, the volume and mass are small, and the mass is about 60% of the industrial frequency UPS |
| Adaptability to the pollution of mains electricity | Stronger | powerful |
| Can it be isolated from the mains grid | Because of the input rectifier transformer, it can be isolated from the mains grid | Because it does not use a transformer, it cannot be isolated from the mains grid |
| Collusion failure of switch tubes of the same bridge arm | without | Have |
| failure rate | Because there are fewer components, low switching frequency, low DC voltage, and the impact resistance of SCR is better than that of IGBT, the failure rate is low | Because there are many components, high switching frequency, high DC voltage, and IGBT has strict voltage and current working area, the impact resistance is not as good as SCR, so the failure rate is high |
| Switching loss | Because it works at power frequency, the switching frequency is low, so the switching loss is small | Because it works at high frequency and the switching frequency is high, the switching loss is large |
| Connection mode of rectifier and inverter | direct connection | Connected by a comprehensive filter circuit composed of passive and active filters |
| Affected by grid disturbance | Basically no effect | Because the switching rectifier transmits power in both directions, it is greatly affected by grid disturbances |
| reliability | Slightly higher | Slightly lower |
| noise | Because it works at power frequency, there is noise | Because the working frequency is ≥20kHz, there is no noise |
| Line loss | Because the DC bus voltage is low and the line current is large, the line loss is about 0.5% larger than that of a high-frequency UPS | Because the DC bus voltage is high and the line current is small, the line loss is about 0.5% smaller than that of the industrial frequency UPS |
| Transformer loss | The increased transformer loss due to the transformer is 2.0%~2.5% | Because there is no transformer, it does not increase the loss of the transformer, so it is 2.0%~2.5% smaller than the power frequency UPS |
| Note: | ① Compared with industrial frequency UPS, the main advantages of high-frequency UPS are that the volume and quality are about 40% smaller, and the input power factor of the mains is also higher, which represents the development direction of the current UPS. But the industrial frequency UPS also has some unique advantages, so it is still necessary to continue to use; | ②At present, it is still difficult for the working frequency of high-frequency UPS to reach 20kHz, most of which work at 15kHz, and the noise has not been completely eliminated |
Power frequency UPS and high frequency UPS are determined by the form of the rectifier used, rather than distinguished by whether a transformer is used or not. Whether to use a transformer in a UPS is determined according to needs, not according to the type of UPS.
The transformer in UPS has the following 6 purposes.
① Transform the number of phases: that is, use the phase shifting effect of the input transformer to transform the three-phase mains voltage into 12-phase, 18-phase, 24-phase, … for rectification, so as to improve the input power factor of the mains.
②Transformation: SCR phase-controlled rectifier cuts some electric energy when it is working, so it is a step-down converter; SPWM inverter also cuts some electric energy when performing SPWM modulation, so it is also a kind of reduction Pressure type converter. The series connection of the two will make the output voltage of UPS not reach the original voltage level of the mains (230V), and the transformer must be used to boost the voltage to reach the original voltage level of the mains.
③Used for electrical isolation between the load and the mains power grid to prevent high-voltage spike voltage (peak value of 6000V, duration of 0.1~10ms) and transient overvoltage (peak value of 20000V, duration of 1~ 100μs) has an impact on the load and the output neutral line contains a DC component due to the unequal amplitude of the positive and negative half-cycle voltages. This component is unfavorable to the load and the transformer in the load.
④Convert three-phase three-wire system voltage into three-phase four-wire system voltage.
⑤It is used to reduce the harmonics of the output voltage. For example, a three-phase △/Y transformer can be used to transform the two-level phase voltage into a three-level line voltage, so that the harmonic content of the output voltage is reduced.
⑥Used as a three-phase isolation transformer interface (IFT), which can transform the input current iN into 1/3 of the output current, which is suitable for three-phase Buck or three-phase Boost switching rectifier circuits.
In addition, when the transformer is used as an isolation transformer at the output of the UPS, it also has the functions of reducing zero voltage, enhancing overload and short-circuit protection, and preventing zero current from flowing.
From the function of the transformer in the above UPS, it can be known that whether it is a power frequency UPS or a high frequency UPS, it is possible to use a transformer or not to use a transformer. For example, for a power frequency UPS, when the Boost DC/DC conversion is used in its DC link It is also not necessary to use an output isolation transformer to boost the voltage when the inverter is used. For high-frequency UPS, when the load needs to be electrically isolated from the mains grid, an output isolation transformer must be used.