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Transformers are used to achieve a change in the supply voltage, for both medium and low voltage supplies. The choice of the protection devices must take into account transient insertion phenomena, during which the current may reach values higher than the rated full load current; the phenomenon decays in a few seconds.

The curve which represents these transient phenomena in the time-current diagram, termed “inrush current I0”, depends on the size of the transformer and can be evaluated with the following formula (the short-circuit power of the network is assumed to be equal to infinity)

where:

K  ratio between the maximum peak inrush current value ( I0 ) and the rated current of the transformer (I1r): (K= I0 / I1r);

τ  time constant of the inrush current;

_Ir1  rated current of the primary;

t  time.

The table below shows the indicative values for t and K parameters referred to rated power Sr for oil transformers.

Further to the above consideration, the follwing diagram shows the inrush current curve for a 20/0.4kV of 400kVA transformer. This transformer has an inrush current during the very first moments equal to about 8 times the rated current; this transient phenomenon stops after a few tenths of a second.

If someone looks for Power Factor (p.f.) value in a UPS data-sheet, he will find two values: input and output.

What do they mean? Isn’t the load that defines the p.f.?

Focusing on the IT loads even if the concept is general, the first thing to understand is that the server is the load for the UPS and the UPS is the load for the utility. Therefore the input p.f. represents the the way the UPS affect the utility and allows to chose properly cables, breakers, PDU, etc.
An example can clarify the concept. Let’s compare two three-phase UPSs with input p.f. 0.99 (1.) (IGBT technology) and p.f. 0,68 (2.) (six pulse SCR rectifier) both in the worst condition for the mains, discharged batteries, supplying a 180 kW p.f. 0.9 load. Neglecting for a moment the efficiency it gets:

1. Pn=180kW, Sn=180kW/0.99=182kVA, I=182kVA/(400Vx√3)= 262A;
2. Pn=180kW, Sn=180kW/0.68=265kVA, I=265kVA/(400Vx√3)= 382A.

In the second case a p.f. compensator banks is needed increasing the plant cost.
Please, notice the the use of the UPS 1. gives benefits even comparing with the load directly connected to the facility:

Load: Pn=180kW, Sn=180kW/0.9=200kVA, I=200kVA/(400Vx√3)=288A.

Then, what does the UPS output p.f. mean?
It is the the p.f. of the load that the UPS can supply without derating. Consider that both the UPS output limits kVA and kW can not be exceeded and that the output p.f. is given by the ratio kW/kVA.

In this case two examples are necessary.

Example 1

Load 180kW p.f. 0.9 (Sn=200kVA)

Output p.f. 0.9 for UPS 1. and 0.8 for UPS 2.

1. Pn=180kW, Sn=180kW/0.9=200kVA
2. Pn=180kW, Sn=180kW/0.8=225kVA

In case 2. a bigger UPS is requested to supply with consequent cost increase.

Example 2

Load 180kW p.f. 0.8 (Sn=225kVA)

Output p.f. 0.9 for UPS 1. and 0.8 for UPS 2.

1. Pn=180kW, Sn=180kW/0.9=200kVA
2. Pn=180kW, Sn=180kW/0.8=225kVA

In case 1. the UPS is overload and after few minutes it would switch to bypass load supply. This happens because the UPS Sn limit is exceeded.

Therefore there is no the perfect output p.f. but the proper one.

It is important to say that in the IT application the servers’ p.f. is typically 0.9 leading.

Conclusion: choose the UPS with the highest input p.f. and the proper output p.f.

Tags: leading, load, p.f., Power Factor, UPS

The need to guarantee an ever greater continuity of service has led to an increase in the use of emergency supply generators, either as an alternative to, or in parallel with the public utility supply network. Typical configurations include:

• “Island supply” (independent functioning) of the priority loads in the case of a lack of energy supply through the public network;
• supply to the user installation in parallel with the public supply network. Unlike the public supply network, which has a constant contribution, in case of a short-circuit, the current supplied by the generator is a function of the parameters of the machine itself, and decreases with time; it is possible to identify the following successive phases:1. a subtransient phase: with a brief duration (10÷50 ms), characterized by the subtransient reactance X”d (5÷20% of the rated impedance value), and by the subtransient time constant T”d (5÷30 ms);2. a transitory phase: may last up to some seconds (0.5÷2.5 s), and is characterized by the transitory reactance X’d (15÷40% of the rated impedance value), and by the transitory time constant T’d (0.03÷2.5 s);3. a synchronous phase: may persist until the tripping of external protection, and is characterized by the synchronous reactance Xd (80÷300% of therated impedance value).

Tags: generators, supply