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Topic Title: calculation of line impedance
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Created On: 13 October 2016 04:37 PM
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 13 October 2016 04:37 PM
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I need to calculate the line impedance for a three phase installation so as to ensure that X-ray type medical equipment does not receive a supply that exceed 220milli ohms. (manufacturers' limit criteria)

In essence the distribution will originate from a 11kv/400v 1500kVA transformer possibly at 5.5% impedance (all yet to be confirmed) via a Main ACB/MCCB switchboard where a sub main cable will be run direct to the medical machine

I can calculate the cable line impedance element but am struggling with the impedance the derives from the transformer element. Any help /advise would be much appreciated.
 14 October 2016 01:23 AM
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well, to get an accurate answer, you need to knw something about the source impedance /fault level on the H~V side and some oproperties of the transformer that may be best obtained by calling the maker, as there are a number of possible internals.

Generally however 1.5MVA is 500kVA per phase (say 2000A full load between easy numbers friends)pretending that they were 3 single phase loads, and at that load it droops by 5.5%

If at 2000 amps it drops 5% of 230V, or ~ 12V the resistance per phase line is 12/2000, say 6milliohms - so low compared your quarter ohm limits that being wrong by a factor of 2 does not affect the conclusion.

regards Mike
 18 October 2016 09:29 AM
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The idea of Percent Impedance is a strategy for expressing the inductance estimation of a line

reactor. The evaluated percent impedance esteem for a given inductor depends on the percent of

ostensible voltage that would be dropped over the reactor with the evaluated current streaming and at

evaluated major recurrence. A question that surfaces is: How would I compute the percent

impedance on the off chance that I know the estimation of the inductance?

Ascertaining Percent Impedance:

From Ohm's Law the voltage drop over the inductor would be:

E = I XL

(XL = 2 F L)

Where: XL

Inductive Impedance

E Voltage (Volts)

I Fundamental Current (Amps)

F Fundamental Frequency (Hertz)

L Inductance (Henries)

The percent impedance is characterized as the voltage drop over the reactor as a rate of ostensible

voltage. To compute percent impedance you would require the inductance per stage estimation of the reactor

(given) and the line to nonpartisan estimation of the voltage. The Line to Neutral Voltage can be found by

partitioning the Line to Line Voltage by 1.73. (VL-N = VL-L/1.732)

So the recipe for Percent Impedance would be as per the following:

%Z = L (in Henries)/[(VL-L/1.732)/(I x 2? x F)] x 100%


Ascertain the percent impedance for a reactor that has a for each stage inductance of .001475 Henries that

is utilized on a drive with 16 An of info current streaming at 480 V and at a 60 Hz key recurrence.

%Z = .001475/[(480/1.732)/(16 x 2 x 3.14 x60)] x 100%

.001475/(277.14/6031.86) x 100%

.001475/.04595 x 100%

3.2 %

Note likewise that the percent impedance is specifically relative to the present stream and conversely

relative to the voltage. This implies for instance that if just 50% of the current was streaming, the

compelling impedance esteem would be one portion of the full load esteem and that the percent impedance

would twofold if the voltage was divided

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