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The relationship between type selection and UPS unit

1. Influence of UPS on generating units:
UPS as the mains power failure to the standby diesel generator set of power supply in the middle of the power equipment can be battery power without delay to the important load inverter power supply, thus ensuring the diesel generator set start power supply load before electricity, when put into use diesel generator set, the UPS ACTS as the load of unit, for the battery to charge. Rectifier filter is one of the main components of UPS, will this nonlinear load to the diesel generator set reflection of higher harmonic, of which five times and seven times harmonic wave is most serious, especially in nonlinear load is bigger and generator capacity is small the harm is more apparent, mainly manifested in the following several kinds of failure phenomenon:
1) the output voltage of the generating set suddenly increases to over 440V, which will result in UPS damage.
2) the output frequency of the generating set reaches 50-60hz, resulting in the protection action of UPS.
3) generating set of abnormal frequency or voltage in severe mechanical resonance phenomenon at the same time, the diesel engine sounds rhythmic rocking and present ups and downs, serious when still appear damage generator excitation circuit and AVR.
4) UPS will automatically shut off the rectifier due to detection of overvoltage or overfrequency, and the backup battery will discharge to the load or supply power directly to the load from the bypass.

2. General selection of generating units
1) the permanent magnet excitation method is adopted to improve the transient characteristics of the unit and enhance the capacity of the unit to withstand non-linear load
The permanent magnet excitation system consists of five parts: generator main excitation winding (rotor), permanent magnet generator (PMG), automatic voltage regulator (AVR), exciter and rotating diode. Excitation system working principle is: when the generator, the permanent magnet generator (PMG) of permanent magnet rotor coaxial rotation, cutting PMG three-phase sine voltage of stator winding, introducing generator automatic voltage regulator (AVR) as the input power of AVR, AVR by detecting the generator of three-phase output voltage automatically adjust the excitation current of exciter magnetic field, which regulates the exciter rotor three-phase output voltage, after rotating diode rectifier to generator rotor excitation circuit. It can be seen from this that the adjustment of output voltage is actually achieved through the adjustment of AVR to the magnetic field of the exciter (as shown in the attached figure). The system has the following features:
(1) because the PMG system provide a has nothing to do with the output voltage waveform distortion of the stator and the size of a constant excitation source, thus can provide higher motor starting to bear ability, and the host of the output voltage waveform distortion of the stator has anti-jamming, generator can be improved with nonlinear load capacity.
Parliamentary AVR measurement of three-phase output voltage (three-phase square root detection), with high precision of steady-state voltage adjustment rate (usually up to + / -0.5%).
Greater resistance to radio interference.
Higher resistance to short-circuit current (usually up to 3 times rated current, lasting 10S).
2) select the capacity of the unit
When the diesel generator set start supply power to the load, because the UPS has been in power from the battery to the number of seconds time load discharge, therefore the unit power supply when the access will be the first UPS rectifier components, but also influence the size of the battery charging current. For the UPS with 12 pulse rectifier, the generator capacity needs to be above 2.5 capacity to ensure the normal operation of the unit: take 400KVA UPS as an example
2.5 times X400 = 1000 kva
Therefore, as a routine choice, the actual capacity of the unit needed should not be less than 1000kVA to ensure the stable operation of the unit under any circumstances.

3. Cost saving techniques for selecting generators
If above unit selection method, there is a big problem, that the generator rated capacity equal rated power output of the generator set, and according to this selection, resulting in waste of investment, users and generating set long-term work under light load conditions will also adversely in diesel engines, the engine is generally not allowed to work long hours to under 30% load, can appear otherwise will exhaust manifold leak, the moving parts wear, fuel consumption increase, the phenomenon such as engine life short. The output of the generator set size of active power depends on the output of the diesel engine, the apparent power is mainly depends on the capacity of generator, generator set in the capacity of the UPS just increase the capacity of generator, the transient characteristics will be greatly enhanced, and the output of the whole sets of active power actually does not increase, and therefore may wish to adopt the way of "small horse-drawn cart" to solve, namely the power of diesel engine according to the sum of all active power load slightly larger to select, generator power is in accordance with the above method to configuration. Let's take a case study and again take the UPS of 400KVA as an example
1) generator: in the brushless permanent magnet excitation form of Stanford, LVI634D, its apparent power is 1000kVA(the generator has a large capacity and can withstand stronger harmonic impact)
2) the active power of the engine needs to be about 1.2-1.5 times that of UPS to ensure the normal operation of the unit: Swedish tycoon TAD1631GE is selected and its active power is 440kW, as calculated below
400 x0. 8 = 320 kwx1. 2 x = 384 kw
Due to engine occupies more than 70% of the unit cost of the whole, only increase the capacity of generator and reduce the capacity of the engine, the first maximum save user cost, also can ensure the normal use of user equipment, and according to the above calculation, engine working at this time the best working state, the load value is about 87% of the engine capacity (384/440 = 51%), and choose 1000 kva units, such as load value of about 384 / (1000 * 0.8) = 48%).
Therefore, reasonable analysis of our UPS and generator after a reasonable match will save a lot of costs.