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Generator set and uninterrupted power supply

The power supply system composed of diesel generating set and UPS is widely used in the Department of high demand for power supply safety. This system not only requires high automation of diesel generating set, but also requires the AC synchronous generator to adapt to the nonlinear load characteristic of UPS, so that it ensures the reliable supply of UPS to the load without city electricity. Electricity. This paper gives a brief introduction to the problems and solutions that often occur in the matching of the generator set and the UPS, so that the user can be used as a reference when choosing and supporting the diesel generator set.
1 present situation of modern synchronous alternator
Synchronous alternator matching with modern diesel generating set includes introduction and complete import: New Era (Stanley), Li Lei Ma, marathon, SIEMENS and so on. Because of the characteristics of performance and structure, self excited constant pressure type is generally adopted.
The fundamental difference between self excited synchronous alternator and exciting synchronous generator is that the self excited synchronous generator provides both power source and signal source from the armature winding of the generator to AVR, while the PMG AVR power source is obtained from the armature winding of the auxiliary exciter. The armature winding of the generator provides only the signal source, and the power of the AVR is lost. The output voltage of the generator is kept stable by excitation of the magnetic field winding of the exciter and controlling its current by excitation of the mechanical and mechanical armature winding, the rotating rectifier and the magnetic field winding. The power output of the former AVR is influenced by the EMF waveform of the armature winding, and the effect of the load character, especially the nonlinear load on the armature winding electric potential is more obvious; the power source of the latter AVR is derived from the armature winding of the PMG, and is independent of the potential wave distortion of the armature winding of the generator.
2 UPS and synchronous alternator influence each other
The influence of nonlinear load UPS on generators has been explained in theory in many articles, so it is no longer repeated here. In application, when the selection of diesel generators and UPS is unreasonable, the following phenomena often occur:
(1) generator output voltage oscillation: the oscillation range is as high as the rated voltage of + (10%-20%), and when the AVR is adjusted to the best, the oscillation is still greater than 2%.
(2) current oscillation: when the UPS load is stable, the output current of the generator oscillates within the range of + (20% - 50%), and the current oscillation can not be adjusted.
(3) frequency (speed) oscillation: in general, the frequency oscillation is smaller than the voltage and current oscillation, but the influence is larger, which leads to the frequent switching and abnormal working state of UPS. The frequency oscillation is generally within + 5%, and the diesel engine's work is too strong and weak because of the regular load. The vibration of the diesel generator set is aggravated, the mechanical wear is accelerated and even the machine parts are seriously damaged. One of the most obvious characteristics of frequency oscillations is that the working noise of diesel engine is regular and slight, so we must attach great importance to it.
(4) UPS work is not normal: one is frequent switching. When the frequency and voltage oscillations exceed the UPS transmission range, the battery is supplied by the battery, and the generator is returned to the normal when the UPS is not loaded, and then the UPS is automatically put into interlacing, and the two is easily caused by the UPS bypass work, which has a certain effect on the load.
3 the influence of the conventional configuration on the generator set
According to UPS capacity, the capacity of the general matching generator set is 2 to 2.5 times the capacity of UPS. In fact, U1) S generally works at rated capacity of 50% to 80%, and for generating units, the power output may be about 30% of the rated capacity. In this way, the capacity of the generating unit can not be fully utilized, the investment of the equipment is increased, the phenomenon of "big Mara small car" is caused, and the generator unit is easier to produce fault, increase the maintenance and reduce the working reliability of the generator set. The reason is that:
(1) according to the characteristics of the diesel engine, if the temperature in the cylinder is low under the small load for a long time, the lubricating oil in the normal cylinder can not be completely burned, and the fuel can not be fully burned, causing the serious carbon deposition at the piston ring and the nozzle and the aggravation of the cylinder wear. The performance of the oil machine decreased and the exhaust smoked.
(2) the diesel engine is below the rated load of 30%, and the economy becomes worse.
Combined with all kinds of factors, the generator set must load more than 60% of rated load, which is beneficial to diesel engine.
4 configuration of generating unit and UPS
In fact, the power ratio between UPS and generator can be matched between 1: (1 - 2.5). The key is to consider the excitation mode of modern synchronous generators in generating units.
When the generator in Figure 1 (a) is used, the power ratio should be at least 1: (2 - 2.5), but it can not be ruled out that the generator unit is not affected by the UPS. When the PMG generator of B (b) is used, the power ratio is preferable to 1:1, and it can ensure the normal operation of the generator set.
Common UPS is generally below 200kVA, and AC brushless generator is generally more than 500kVA for PMG type, but PMG can be assembled in any capacity model and needs to replace AVR, so it must be carefully considered when selecting AVR.
5 Conclusion
In the matching of the generator set and UPS, the power ratio can be completely 1:1, and in the type selection of the generator set, not only the suitable power matching should be taken into account, but also the excitation type of the generator should be understood, so that it can adapt to the normal work of the UPS (nonlinear load).