This document contains calculations for protection settings for a 7.5MVA transformer connecting a 33kV feeder to an 11kV bus. It provides the base MVA, impedance values, fault levels, and resulting fault currents. It then calculates the operating times for overcurrent and earth fault protection and ensures adequate margins between devices. A table summarizes the selected settings for overcurrent and earth fault protection at each point.

1. April
2016
4
Protection Settings
Calculations
PROTECTION SETTINGS CALCULATIONS
Useful information
- Base MVA = 100MVA
- Rating of the TX 2 = 7.5MVA
- Percentage Impedance of the TX 2 at Tap 9 and Tap 17 is 7.70% and 7.47%
respectively
- Fault MVA at Chemosit 33kV bus = 0
62.10426.258 −∠
- Conductor size of the 33kV Kericho feeder and length from Chemosit is 2
75mm
ACSR and 20KM respectively
- Settings for 5L5 at Chemosit (Back up O/C & E/F);
 O/C : PSM = 300A, & TMS = 0.3
 E/F: PSM = 50A, & TMS = 0.4
Source Impedance`Calculation
0
62.104
26.258
100
)33( ∠=
MVA
MVA
kVZs
.3747.0098.0 puj−−=
Total Z p.u of the 33kV Kericho feeder upto the 33kV bus at Kericho 33/11kV s/s
)0365.00375.0(20)33(2
75
jxkVZ mm
+=
puj 73.075.0 +=
Impedance p.u. of the 7.5MVA TX 2 as expressed to the new MVA base (Tap 17)
= upj .996.0
The resultant Total impedance p.u. upto 11kV bus
)2()33()( 5.77511 2 TXZkVZZbusZ MVAmmskV ++=
= puj 3513.16523.0 +
Resultant fault MVA
= MVA0
23.6464.66 −∠
Fault Current at the 11kV bus
7.5MVA TX 2 – Kericho 33/11kV s/s
Calculations by: afanda rodgers
)3513.16523.0(
100
j
MVA
MVAf
+
=
1005.7
10047.7
)2(5.7
xMVA
MVAx
jTXZ MVA =

2. April
2016
4
Protection Settings
Calculations
A
kVx
MVAx
I f
113
100064.66
=
= A89.3497
Protection relays/equipment operating times calculations on the 11kV fault current of
3497.89A (ph – ph)
1. 5L5 O/C relay
sec
1)
3003
89.3497(
3.014.0
)/( 02.055
−
=
x
x
cot L
=1.5260 sec
2. New 7.5MVA TX 2 HV O/C relay
sec
1)
1503
89.3497(
35.014.0
)/( 02.0
−
=
x
x
cotHV
=1.1704 sec
Operating margin between the two units above is 0.357 sec
3. New 7.5MVA TX 2 LV O/C relay
sec
1)
396
89.3497(
25.014.0
)/( 02.0
−
=
x
cotLV
=0.7860 sec
Operating margin between LV and HV over current is 0.385 sec
4. All 11kV feeder reclosers
sec
1)
300
89.3497(
14.014.0
)/( 02.011
−
=
x
cot recloserskv
7.5MVA TX 2 – Kericho 33/11kV s/s
Calculations by: afanda rodgers

3. April
2016
4
Protection Settings
Calculations
=0.3893 sec
Operating Margin between TX 2 LV O/C and 11kV reclosers is 0.397 sec
Operating times of protection units on 11kV Earthfaults of 1165.96A
1. 5L5 E/F relay
sec
1)
50
6.388(
3.014.0
)/( 02.055
−
=
x
FEt L
=1.3376 sec
2. 7.5MVA TX 2 SBEF relay
sec
1)
80
96.1165(
38.014.0
)/( 02.0
−
=
x
FEtSBEF
= 0.9664 sec
Operating margin between 5L5 and the 7.5MVA TX 2 SBEF is 0.3712 sec
3. TX 2 LV E/F
sec
1)
45
96.1165(
3.014.0
)/( 02.0
−
=
x
FEtTX
=0.6245 sec
Operating margin between the TX 2 SBEF and TX 2 LV E/F is 0.3420 sec
4. 11kV feeder reclosers
7.5MVA TX 2 – Kericho 33/11kV s/s
Calculations by: afanda rodgers

4. April
2016
4
Protection Settings
Calculations
sec
1)
15
96.1165(
15.014.0
)/( 02.011
−
=
x
FEt recloserskV
= 0.2310 sec
Operating Margin between the TX 2 LV E/F and the 11kV reclosers is 0.3936 sec
Table
Protection Relay/Equipment Over Current Settings Earthfault settings
1 33kV Feeder 5L5 PSM =300A, TMS = 0.3 PSM = 50A, TMS = 0.4
2 7.5MVA TX 2 HV PSM = 150A, TMS = 0.35 Not Enabled
3 7.5MVA TX 2 LV PSM = 396A, TMS = 0.25 PSM = 45A, TMS =0.3
4 TX 2 SBEF ------------------------- PSM = 80A, TMS = 0.38
5 11kV Feeder Reclosers PSM = 300A, TMS = 0.14 PSM = 15A, TMS = 0.15
7.5MVA TX 2 – Kericho 33/11kV s/s
Calculations by: afanda rodgers