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Tính toán công suất máy phát điện khi hòa vào UPS

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If the power source of the static UPS includes a generator within the installation, when determining the sizing ratio (RD), account must be taken of the voltage drop on the serial impedance of the latter due to the various harmonics and the over-temperature compatible with the insulation class of the generator.
The parameter best suited to this calculation is the sub-transient reactance (X”d%) of the alternator calculated for each frequency in play.

UPS rectifier technology THDU%
6 impulses 2.3 X"d%
12 impulses 1.6 X"d%
IGBT 0.2 X"d%

Table 1 - THDU% of generator set as a function of type of UPS

Where it can be seen that the harmonic content of the voltage as a percentage (THDU%) depends on the drop on the sub-transient reactance of the various current harmonics (In%).
At industrial frequency, the values of sub-transient reactance range from 8% to 20%, but for a correct value, reference must be made to the data plate rating or, in the absence of this, to the manufacturer.

Figure 1 - Output THDU% Vs. Genset load rate (12%X’’d - 2/3 Pitch)

For the spectra of the common electronic converters the calculations are as shown in Table 1.

The common practice combined with the above mentioned considerations, lead to consider that the sizing ratios RD:
1.3 < RD< 2.5

are sufficient in the majority of cases to deliver THDU% of less than 8% and meet over-temperature requirements. Note that in this respect, a generator is more susceptible than a transformer. The deformation of the waveform could trigger instability in the voltage regulation, causing switch-off of the generator and power failure once the back-up time of the UPS has expired.

It should also be emphasised that synchronous generators have limited capacity in supplying capacitive loads. This is because the latter absorb “magnetising” current, i.e. with sufficient phase as to be added to energisation, with a consequent increase in voltage.
Power factors of less than 0.97 can seriously compromise the regulation and hence the stability of the generator set. In this case, the static system would disconnect from the power supply but would no longer have the energy reserve of the generator set at its disposal. This situation is indicative of incorrect sizing of the installation at the design stage.

In installations with a generator set, it is often advisable to assess carefully the correctarchitecture of the UPS and the sizing ratio, rather than oversize the alternator unduly.
However, since the final result depends on the typical parameters of the power source as well as on the characteristics of the UPS, it should ideally be the task of the generator set's manufacturer  to propose an ideal sizing ratio (RD) of the power necessary for the source with respect to the power necessary for a static UPS.

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