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In order for power to be usable (e.g. high-quality), its characteristics must be within the tolerances of the supplied load. Therefore it is not enough for power to be present. Let’s take this necessary condition as the starting point for the analysis.

Power distribution

Information on electrical power quality is somewhat scarse. The main independent sources of information considered here are:

• European Network of Transmission System Operators for Electricity (ENTSO-E);
• The Italian Regulatory Authority for Electricity and Gas;
• Leonardo Energy (LE).

The available statistics respectively refer to High Voltage lines, Low Voltage lines and power quality within the system used. In fact, high-quality power supplies are frequently compromised by consumers’ loads within their own plants, such as non-linear loads or loads with high inrush current. Unfortunately, the lack of information on Mdium Voltage lines, which is of extreme relevance to all consumer loads connected via transformer to the grid, does not allow for a complete analysis. The reason probably stems from the fact that individual quality standards are defined in Medium Voltage.
The ENTSO-E Statistical Yearbook 2008 provides an overview of electricity exchanges between European nations and lists the major periods of unavailability, classified according to the reason:

• Maintenance
• New construction
• Overload
• Failure in protection devices
• Outside impacts (animals, plants)
• Atmospheric agents
• Other reasons/unknown

The bottom line is that in 2008, the total downtime of international tie lines of 220 to 380 kV with rated power from 300 to 1745 MVA between Italy and neighbouring countries amounted to approximately 6500 hours.
However as regards low-voltage lines, the Regulatory Authority informs us that in 2007 in Italy, each user experienced 4.73 power outages of between 1 second and 3 minutes and 2.16 power outages of over 3 minutes’ duration, giving a total downtime of 58 minutes.

Tags: grid availability, high voltage, low voltage, mains availability, medium voltage

 Leonardo ENERGY is the premier web site delivering a range of virtual libraries relating to electrical energy.

It is an initiative dedicated to build information centres to serve designers, engineers, contractors, architects, general managers, teachers and students, professionally or otherwise involved with electrical power.

Leonardo ENERGY has developed a micro site dedicated to Data Centres complementary to Datacenter-Link.

Tags: data centre, Datacenter-Link, Leonardo ENERGY

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