⑴ Transformerless UPS
Transformerless UPS is a high-frequency UPS without isolation function. It can completely use no transformer. Its typical circuit is shown in Figure 1. The circuit is composed of a three-phase IGBT Boost SPWM high-frequency switching rectifier and a three-phase half-bridge IGBTSPWM high-frequency inverter through a battery (consisting of 67 batteries in series, with a DC voltage of 800V). In fact, it is composed of two three-phase Half-bridge inverters are connected in series (because this inverter is a two-way four-quadrant converter that can both rectify and reverse), the previous inverter works in Boost PFC rectification state, DC The output voltage is 800V; the next inverter works in the Buck inverter state, and the AC output voltage is 220/380V. This kind of UPS not only has the two advantages of high-frequency UPS (namely, small size and quality, high input power factor of the mains), but also has the advantage of high conversion efficiency. There are two reasons for this advantage: one reason is that the loss of the transformer is saved. For transformers with a power greater than 50kV, the loss is 2%~3%; the other reason is that the DC voltage of the DC bus is increased to 800V, which makes The conversion current of the UPS is reduced, and the conversion loss and line loss of the UPS can be reduced by 0.5%~0.6%. Together, the conversion efficiency of the UPS can be increased by 2.5%~3.5%. As shown in Table 1 is a set of typical test data.
|Load factor||The efficiency of a transformerless UPS||Efficiency of UPS with transformer|
|UPS internal temperature||35°||40°|
|Note: Calculated according to the load power factor of 0.9.|
The improvement of UPS conversion efficiency means that the loss of UPS itself is reduced. For example, for the full load efficiency of a 500kV UPS, an increase of 2.5% in efficiency is equivalent to a reduction of 12.5kW loss, which is beneficial to reducing the heat generation of the UPS, It is very beneficial to improve UPS reliability and reduce environmental requirements.
In actual use, such as in the “1+1” redundant parallel and dual-bus configuration system, the actual load rate of the UPS is only 30% to 40%. At this time, it is more practical to improve the conversion efficiency of the UPS. As shown in Figure 2, in the load change range of 25% to 100%, the conversion efficiency of UPS is basically stable above 94%.
⑵High frequency of UPS
Since high-frequency UPS is set as the development direction of today’s UPS, it will naturally be proposed that “high-frequency is the development direction of UPS.” In fact, high frequency UPS and high frequency are two concepts. High-frequency UPS refers to the name of a UPS that is different from the use of SCR phase-controlled rectifiers and uses SPWM high-frequency rectifiers, and high-frequency refers to increasing the operating frequency of the high-frequency UPS to more than 20kHz. It seems that the higher the frequency, the better the performance of the UPS, but in fact this is a misunderstanding. There are several reasons.
① When the operating frequency of UPS is increased to a certain level, high-frequency effects will be generated, such as collection effect, proximity effect, and increase the distribution parameters of the circuit, which will cause qualitative changes in inductance and capacitance, which is not conducive to the electromagnetic compatibility design of the circuit. It is not conducive to the work of ZVS or ZCS soft switch.
② The high frequency will increase the switching loss proportionally and reduce the UPS efficiency. Switching loss AP=(EIon/6)∫C(tr﹣tf), where E is the DC voltage applied to the switch; Ion is the current flowing through the switch; ∫C is the switching frequency. It can be seen that the higher the switching frequency, the greater the switching loss.
The purpose of high frequency for the SPWM converter is to increase the harmonic frequency, reduce the harmonic content, increase the UPS mains input power factor, and reduce the harmonic content of the UPS output voltage. For the IGBT SPWM high-frequency switching rectifier and inverter in the high-frequency UPS, the double Fourier series expression of the AC voltage is
In the formula, the harmonic frequency ∫h=50mF or ∫h=50 (mF+n); F=(carrier frequency/mains frequency) (called carrier ratio).
From this we can find two ways to achieve high frequency of UPS: One is to increase the frequency of the carrier triangle wave and increase the carrier ratio F to make the UPS achieve high frequency. The disadvantage of this method is that it may produce high frequency effects and make The switching loss increases proportionally; the second is the parallel cascade superposition method of N SPWM UPS controlled by the carrier triangle wave phase shifting’SPWM, and the method of increasing F by N times to increase the frequency of the UPS. The advantages of using this method are as follows.
① The operating frequency of a single UPS can be limited to the extent that it does not produce high effects, and the equivalent synthesized frequency of the UPS can be increased by N times.
②No increase in switching loss. Although the parallel cascade superposition method increases the number of switching tubes of the UPS by N times, it also reduces the current flowing through the switches by N times, so the switching loss is not increased.
③Generally speaking, the switching speed of small current IGBT is faster than that of high current IGBT, and its supply is sufficient and the price is also cheaper. Therefore, the parallel cascade superposition method is helpful to improve the conversion efficiency of UPS and reduce the cost.
④ When the UPS is high-frequency, the frequency is increased greatly, and increasing F by N times is faster than increasing F only.
To sum up, it is better to use the parallel cascade superposition method to make the UPS high frequency, which is the development direction of the UPS high frequency.
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