Transformer basics

[bejoyengineer]

I am a bit confused on the relation between the primary and secondary currents of a transformer. As I am into instrumentation, I would like to get an understanding on basics of electrical engineering. Kindly do take some time and answer these question ?Please
Now say we got a transformer and it's MVA rating is 50 MVA, Delta ?star and steps down from 115 KV to 13.8 KV.Now the primary side has a CT ratio of 2000/5 Amps and secondary has got a CT ratio as 3600/5 amps and these are connected to the differential relay . There is NO ?" ICT" connected and the CT's are star connected. Now my first question would be
1 What would be the value of current in the primary side and what would be the value at the secondary side. I assume that ? if we have a full load of 50 MVA being used ?then the currents values at primary would be 600 and at secondary-3600.Am I right??
2 so if the above is right as per my thought- then say the load is 25 MVA, so that would mean that the primary current would now be 1000 A and secondary would be 1800 A. so depending on the load the primary and secondary currents also changes while the voltage remains the same. Hope I am right??
3. On what basis or using which formula do we decide that the primary current should be 2000 and sec should be 3600? Is it the formula:::: Power=root 3 x voltage x current or is it only ::::: Power= voltage x current. When I used both these formulas to find current in the primary side, I was not able to get the current as 2000 A(I used P=50 MVA ,V=115000)but 3600 Amps was the answer when I used the formula::::: Power= voltage x current for the secondary side (I used P=50 MVA ,V=13.8KV).
4. Is 3600 Amps and 2000 Amps know as the Line current (ILine) of the primary and secondary ?

 

[glene77is]

BE,
You have a proclivity to profuse terms.
Your terms do not appear to be consistent.

"Kuphaldt" has a "iBiblio" sponsored website on Electrical theory,
very encyclopaedic. You should download, per their suggestion.

"Square-D" has a "Dry Type Transformer Study Course",
see their website.

"General Electric" has a "The Whys of Wyes" transformer white paper,
which has been sited in this forum.
Must do a search, as I don't have this website URL.

In fact, searching Mike Holt's Forums will probably turn up some good leads.
This is an incredible forum for study material and links.

Good reading! :smile:

 

[drbond24]

1 What would be the value of current in the primary side and what would be the value at the secondary side. I assume that ? if we have a full load of 50 MVA being used ?then the currents values at primary would be 600 and at secondary-3600.Am I right??
2 so if the above is right as per my thought- then say the load is 25 MVA, so that would mean that the primary current would now be 1000 A and secondary would be 1800 A. so depending on the load the primary and secondary currents also changes while the voltage remains the same. Hope I am right??
3. On what basis or using which formula do we decide that the primary current should be 2000 and sec should be 3600? Is it the formula:::: Power=root 3 x voltage x current or is it only ::::: Power= voltage x current. When I used both these formulas to find current in the primary side, I was not able to get the current as 2000 A(I used P=50 MVA ,V=115000)but 3600 Amps was the answer when I used the formula::::: Power= voltage x current for the secondary side (I used P=50 MVA ,V=13.8KV).
4. Is 3600 Amps and 2000 Amps know as the Line current (ILine) of the primary and secondary ?

1. (Power in VA) = (current)*(voltage)*(sqrt(3))
50 MVA = (current)*(115 kV)*(sqrt(3))
current = 251 amps on primary side
current = 2092 amps on secondary side (using same formula)

2. If load is halved, current is halved. Therefore @ 25 MVA the primary and secondary currents are 125.5 and 1046 amps respectively.

3. Use the formula I typed in #1 above.

4. Yes.

 

[72.5kv]

High Side 50MVA = 115kV*1.73*251.31

Typical high side CT ration is 500:5

Low Side 50MVA = 13.8kV*1.73*2094

Typical Low side CT ratio is 2500:5

 

[bejoyengineer]

transformer basics

transformer basics

Thanks for the links and replies. I was just waitin for the replies so that i could get to the next questions. as i had mentioned -"There is NO ?" ICT" connected and the CT's are star connected"
so if current = 251 amps on primary side and current = 2092 amps on secondary side ,then the line current passing the CT of the primary side is going to be 2.09 Amps (600/5 = 251/x,say x is for finding the secondary line current) and the line current passing the CT of the primary side is going to be 2.90 Amps.
1. so is this when we need an ICT to be placed (i.e, between the CT and differential relay) so that the difference of 0.81 (2.90 - 2.09) can be adjusted so that the differential relay does not initiate a trip due to the difference.Hope i am right??
2. the substation that i work does not have an ICT. does this mean that the value or difference (0.81) can be set in the relay itself (siemens make)?

there any good sites for better understaning on basics of electrical- say something like presentation sites. thanks

 

[jghrist]

Modern transformer protection relays will adjust for different CT ratios and connections with interposing CTs.