Problem-1
A 3-phase delta-star connected, 30-MVA,
33/11kV transformer is protected by a
differential relay. Calculate the relay setting for
faults drawing up to 200% of rated current. The
CT current ration on the primary side is 500:5,
and that on the secondary side is 2000:5
The primary line current is
MVA
30 106
IP 
 524.88 A
3
3KV
3  33 10
The sec ondary line current is
Is  3 IP
3  524.88  1574.64 A
The primary CT current is
 5 
I1  524.88 
  5.249 A
 500 
The sec ondary CT current is
 5 
I1  1574.64 
 3  6.818 A
 2000 
The relay current at 200% of the rated current
I r  2  I 2  I1 
2   6.818  5.249   3.3138 A
Problem-2
A 3-phase delta-star connected, 15-MVA,
33/11kV transformer is protected by CTs.
Calculate the CT ratios
for differential
protections such that the circulating current does
not exceed 5A.
The primary line current is
MVA
15 106
IP 
 262.44 A
3
3KV
3  33  10
The sec ondary line current is
Is  3 IP
3  262.44  787.30 A
The CT ratio on high voltage side
 787.3 
CTratio  
  157.46 A
 5 
The CT ratio on low voltage side
CTratio
 262.44 

 (5)  757.6 A
3 

Problem-3
If the fault current is 2000A, the relay setting is
50% and CT ratio is 400:5, then determine the
plug setting multiplier (PSM).


Fault Current
PSM  

Re
lay
setting

(
CT
sec
ondary
current
)

CT
ratio


2000
 10
0.5  5  80
Problem-4
A 3-phase 345/34.5kV transformer is rated at
50MVA and its short term emergency rating is
60 MVA. Using standard CT ratios available,
determine the CT ratios, and the currents in the
power transformer and the CTs. The 345kV is
star connected, while the 34.5kV side is delta
connected.
The primary line current is
MVA
IP 
3KV
60 10
 100.4 A
3
3  345 10
6
The sec ondary line current is
60 10
 1004.1A
3
3  34.5 10
6
The CT ratio of 1000 / 5 on 34.5kV voltage side
 5 
I r  1004  
  5A
 1000 
The CT ratio on 345kV voltage side
 5 
Ir  
  2.9 A
 3
 100.4 
CTratio  
  34.64
 2.9 
The CT ratio of 200 / 5 on 345kV voltage side
 5 
I r  100.4  
  2.51A
 200 
The CT ratio on 34.5kV voltage side


I r  5  3  4.35 A