#Transformer testing

 #Transformer testing.

Ist of all you  should keep in mind Tx should be Energized within one month after testing has done.

If you wait too long ,moisture, or insulation deterioration can occurs.

#Example 

you tested a transformer on Ist Jun you should Energized it before Ist July. 

if delay say three months may need to repeat test.

1. Turn ratio test .TTR.

check if the voltage ratio between Pri and Secondary correct. 

 Limite Allowed mistake 0.5 % 

if input 100V output should be 10V (example)

expected 10V.

measured 10.4 ....OK 

measure 11 V not okay 

Deviation |10.4-10/10×100=0.4% okay.

2. Winding Resistance test .

check wire inside Transformer like checking wire is demage or loose. checking the insulation strength of insulation 

■ Detect loose Connections 

■Detect demage Winding. 

#Example 

expected Resistance 2M ohm 

measured Resistance =2.0 M ohm 

If Resistance =3M ohm problem possibl fault.

3. Current Diviation in Multiple Transformer 

when Transformer operate together their current should be balanced. 

Rule. Deviation should not exceed 0.5 test leavel or 10% parallel operations. 

#example 

you have two Transformer in parallel 

                   

T1= 100KV ....current 100A

T2= 100 KV.....current 108A 

Difference 8% ok within 10% 

if T2 =120A  20% not acceptable. 

3.PI test .

measure insulation health .

formula = IR10mint/ IR 1mint .

PI value should not less then 2 

●IR at 1 minute =100Mohm 

●IR at 10 minute = 250 Mohm 

●PI= 250/100= 2.5 okay .

4. parallel operations 

two or more Transformer working together they Shere the same load like team work .

example. 

instead of one big Transformer use 2 small ones together.

●why are parallel ?

more power capacity. 

backup if one fail

easy maintenance. 

☆condition for parallel operations 

■same voltage ratio : both must give same out put voltage 

T1= 11k/415

T2= 11kv/415....OK 

if T=415v  and T2= 400v not okay circulating current. 

■Same polarity 

■Same phase .order of phase must be same .

■same tap settings.

what is Tap settings. 

a Tx has a tap changer like a Switch. it is used to increase or decrease out put voltage.

 ■why do we need 

input voltage is not always constant. 

#example 

sometime supply 11kv 

some time it drop to 10.5 kv 

but we always want output 415v .

so we adjusts Tap settings. 

#example 

Voltage. 

input voltage low 10.5 kv (low)

output become low .move tap to higher position (+Tap)

out put becomes normal 

●Case 2.

Voltage high 

move tap lower position(-Tap)

output becomes normal