# transformer impedance

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#### sahafi

##### Member
I need information about transformer impedance. How it is calculated and the benefit when it low or high?
Thanks

#### charlie b

##### Moderator
Staff member
Re: transformer impedance

It is not so much ?calculated,? as it is a design choice made by the manufacturer. It is also a design decision made by the engineer or contractor, to choose the standard value offered by the manufacturer, or to special-order a different value. It is expressed as a percentage, based on the rated KVA and the rated voltage of the transformer. The formula S = V*2/R can be used to correlate the three numbers.

A low value of impedance will give you lower energy loses internal to the transformer, and will require a lower value of air conditioning to cool the room in which it is installed. It also gives you a lower value of voltage drop to downstream components, as it gets loaded higher and higher towards its rated power.

A high value of impedance will limit the fault current available at the secondary terminals of the transformer, and therefore to all components downstream. This is sometimes needed, if the switchboards or panels selected for the job have a small value for their rated fault current.

That should give you a start. But is there something specific you were looking for?

#### wanderer20001us

##### Senior Member
Re: transformer impedance

I understand that the impedence value stamped on transformers, especially larger units (+1000kVA), is from a test.

The secondary is bolted together. Variable voltage is applied on the primary and increased until rated current flows through the bolted secondary. The ratio of the applied primary voltage verse the rated primary voltage is the impedence value as a percent.

#### sahafi

##### Member
Re: transformer impedance

I have a power transformer with unknown impediance so how can i calculate that.

#### steve66

##### Senior Member
Re: transformer impedance

sahafi:

Depending on what you need to know the impedence for, you may just want to estimate the impedence using tables. I have a reprint (I'm not sure of the source) from the IEEE standard 242-1986 (R1991), IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. For example, for a 500KVA three phase transformer, the nominal range of Z is 2.2-5.4%. Use the number that gives you the worst case (2.2% for fault studys).

Steve

#### ron

##### Senior Member
Re: transformer impedance

#### jcormack

##### Member
Re: transformer impedance

Short the primary - raise the voltage on the secondary until full rated current flows - measure the voltage at this point then divide by the rated secondary voltage- result x 100 - this will be % Z.

[ April 12, 2004, 01:56 PM: Message edited by: jcormack ]

#### pwhite

##### Senior Member
Re: transformer impedance

i will have to dig up my old classwork in transformers & circuits.

some of the information i will need is the primary/secondary voltage size, stacking factor,
type of transformer (step up, step down....).

if you have the manufacturer brand and model #, i can give you generalized information on that unit. i have several friends with that information.

#### scott thompson

##### Senior Member
Re: transformer impedance

Just FYI;

To determine (field test) a Transformer's %Z, do the following:

</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Short Circuit the Secondary,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Place an Ammeter on the Secondary side,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Connect the Primary side to a variable AC voltage source - like a "Variac" Variable Autotransformer,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Starting at zero volts on the Primary, increase the Primary voltage until the Ammeter on the Secondary side shows the full-load current rating of the Secondary side,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Measure the voltage on the Primary side,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Divide the measured Primary voltage /vs the rated Primary voltage,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Ratio between the Rated / VS / Measured Primary Voltage is the Transformer's %Z.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">
Scott35

#### wyedelta

##### Member
Re: transformer impedance

what mentioned above is the simplest short circuit test to determine transformer impedance,in addition to that we may connect wattmeter so as to determine other loses and exact kva ratings.

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