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CURRENT C I RC U I T S
The maximum current burden imposed on each current transformer at 5 amperes is listed in Table
AMPS
5
This data is for the 3. 0 ohm basic reach tap setting for 60 cycles.
or 5 0 cycles will be lower.
POTENT IAL C I RC U I T S
The maximum potential burden imposed o n each potential transformer a t 120 volts i s listed in Table VI .
C I RCU IT
P olarizing
Restraint
P olarizing
Restraint
The potential burden of the mho unit is maximum when the restraint tap is set for 100 percent.
The restraint circuit burden and hence the total relay burden will decrease when the restraint tap
setting is less than 100 percent.
The potential burden at tap settings less than 100 percent , can be calculated from the following
formula.
The terms ( a+ jb) ( c + jd) , etc. represent the burdens of the mho unit potential circuit expressed in
watts and vars with their taps on 100 percent.
( watts + j vars) for restraint circuit and ( c + jd)
C onsider a 230kV transmission line 5 0 miles long having a phase-to-neutral impedance of:
GE K-45495
R
CYCLES
. 043
60
FREQ
R
60
1534
60
1060
50
2200
50
1160
VA
( a + jb)
=
CALC U LAT I ON OF SETT I N GS
0. 14 + j 0. 80 ohms per mile ,
z
50 (0. 14 + j0. 80)
=
pri
PT Ratio
230 , 000/115
=
CT Ratio
600/5
120/1
=
=
C T Ratio
z
z
0
sec
pn
PT Ratio
z
( 7. 0 + j40. 0)
sec
Z
2. 43
80. 50
=
sec
BURDENS
TABLE V
X
P . F .
. 026
0. 86
TABLE VI
X
p . F .
10
1. 00
1580
0. 5 5
135
1. 00
1600
0. 73
2
+ ( c + jd)
100
The values of these terms are given in Table VI.
( watts + j vars) for polarizing circuit.
=
7 + j40 ohm total
=
2000/1
=
�
0. 42 + j2. 4 ohms
=
2000
ohms
7
WATTS
VA
1. 08
1. 25
The burden for the lower tap setting
WATTS
VARS
9. 4
0. 1
4. 2
6. 3
6. 5
0. 4
4. 3
5. 9
v.
VA
9. 4
7. 6
6. 5
7. 2
(a + jb)
=
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