Question

What is the kVA load on the distribution transformer if the secondary of the 100/5 A CT connected in primary of 11 kV/433 V is 5Amps?  
1. 866 VA
2. 760 VA
3. 1900 kVA
4. 760 kVA

Answer 1

Correct Answer - Option 3 : 1900 kVA

Concept:

• A Current Transformer (CT) is used to measure the current of another circuit.
• CTs are used worldwide to monitor high-voltage lines across national power grids.
• A CT is designed to produce an alternating current in its secondary winding that is proportional to the current that it is measuring in its primary.
• In doing so, the current transformer reduces a high voltage current to a lower value and therefore provides a safe way of monitoring electrical current flowing in an AC transmission line.

 

Calculation: 

Given:

CT Primary current, I_CT,p = 100 A 

CT Secondary current, I_CT,s = 5 A

Primary Line voltage of Transformer, V_R = 11 KV

Distribution Transformer ⇒ star - star connected

Now,

CT's are single phase, therefore I_CT,p = I_R

Transformer primary phase current, I_R = 100 A

KVA load on distribution transformer = √3 V_R I_R

= √3 × 11 × 100

≈ 1900 KVA