Views: 7 Author: Site Editor Publish Time: 2020-12-10 Origin: Site
The reason why the UPS power supply and the diesel generator are incompatible is mainly caused by the low input power factor caused by the input harmonic current of the UPS, and another is the large internal impedance of the generator. The traditional solution is to derate the generator, so that the generator has enough capacity to compensate for the reactive power caused by the input harmonic current of the UPS. The power consumption of the load carried by the generator is about 30% of its rated capacity. Obviously, this is a kind of "large horse-drawn cart" phenomenon, which is uneconomical, and the diesel generator works in a small load state, which makes the diesel generator set more prone to failure and reduces the working reliability of the diesel generator set. The reason is that the diesel generator works for a long time under a small load, the temperature in the cylinder is low, and the lubricating oil that normally enters the cylinder cannot be completely burned; and the fuel cannot be fully burned, causing carbon deposits at the piston ring and fuel injection nozzle severe, cylinder wear is increasing. As a result, the above-mentioned parts accelerate the occurrence of faults, reduce the performance of the diesel engine, and emit black smoke in the exhaust. Good condition diesel engine generator set requires that the load must work under 60% of the rated load, which is more beneficial to the diesel generator. It can be seen that using the diesel generator derating scheme to solve the problem is not a fundamental solution to the problem. The fundamental solution to the problem should be to correct the power factor (PFC) at the input end of the UPS so that the UPS is close to a linear load, producing a small harmonic current to the grid or generator.
Power factor correction is divided into passive correction and active correction. Active power factor correction is usually followed by a boost converter after the rectifier. After correction, the input current is close to a sine wave, the power factor can reach 0.99, and the harmonic current can be reduced to within 5%. However, because this method uses more primary converters, the reliability of the UPS will decrease, which is more prominent in high-power UPS. Therefore, active power factor correction is generally used in single-phase input low-power UPS (below 25KVA). For three-phase input large and medium-power UPS, passive correction is usually used.
Because this filter only uses LC components, connecting it in parallel with the input of the rectifier has no effect on the reliability of the UPS power supply. For the three-phase 6-pulse rectifier, the harmonic current is mainly the 5th and 7th harmonics .
The filter is designed so that the impedance to the 5th harmonic current with the largest amplitude is zero, and the impedance to the 7th harmonic current is very low. Therefore, the 5th and 7th harmonic currents basically flow into the filter, and will not be sent back to the diesel generator, causing the generator output voltage to be distorted. This method is simple, and the filtering effect is also very good. The total THD of the harmonic current can be reduced to within 10%, and the power factor can reach 0.95. But the disadvantage is that the size and weight of the UPS are increased due to the addition of the filter, but the larger size and weight of the UPS does not have much relationship. The key is to require high reliability, so this LC filter corrects the power factor. The method has been widely used in three-phase input large and medium power UPS.
Since the input harmonic current of the UPS at light load has little effect on the AC power system, it can even be ignored. The LC filter we designed mainly considers the suppression of the input harmonic current and the performance of improving the input power factor when the UPS is fully loaded. Therefore, UPS with passive filters often exhibit a particularly low leading power factor at no-load and light-load, which is a capacitive load. This situation has no effect on the transformer of the commercial power supply. However, when the diesel generator supplies power to the capacitive load, the output voltage may be too high or it may shut down without excitation, resulting in a serious failure of the power supply system. Let's analyze the reasons for this phenomenon.
Because the electric potential of the generator must be equal to the sum of the voltage drop of the internal impedance of the generator and the impedance of the external load. Therefore, the voltage regulator can be adjusted to change the generator potential U to control the output voltage of the generator.
In engineering design, as long as the technical requirements for UPS and diesel generator selection are carefully considered, the mismatch between them can be avoided. For a three-phase input UPS, the input power factor is not less than 0.95, and the THD should be less than 10%. If this requirement is not met, a filter needs to be installed. The frequency conversion range of the generator output should be less than ±10%, and the output power frequency change rate should be less than 5Hz/s. The voltage regulator (AVR) of the selected diesel generator should not be affected by the distortion of the output voltage waveform of the unit, and the internal resistance should be small. Don't choose the capacity of the UPS battery too large, usually it can guarantee that the UPS power supply can work for 10-30 minutes, and the main power supply is the generator during the power failure. 4 stroke 6 cylinder diesel engine and powerful 12 cylinder diesel engine are more popular.
In addition to UPS, you can also buy a diesel generator as emergency power supply. If you plan to buy a diesel generator, it is recommended that you choose these two first: KAI-PU KP206 6 cylinder 4 stroke electric starting water cooled diesel engine and KAI-PU KPV970 12 cylinder electric start 4 stroke diesel engine generator set.