THE STORY OF TRANSFORMER STABILITY
The transformer stability test used to conform the CT connections for Differential and REF protection. This test also used as a primary injection test for HV & LV side BCT's.
Example Transformer Data
Voltage Ratio : 230/ 34.5/13.8 kV
MVA Rating : 100/133 MVA
Vector Group: YNyn0(d5)
% Impedence : 12.24 @ 100 MVA BASE.
CALCULATION TO FIND THE PRIMARY AND SECONDARY CURRENT
PRIMARY SIDE : -
HV side full load current = Power / √3 x Voltage = 100×10^3 = 100000000 = 251 Amps
√3 × 230× 1000 399050
HV side Impedence = HV side Voltage = 230× 1000 = 916 Ohms
HV side full load current 251
HV side Impedence/ Phase = HV side Impedence X % Impedence = 916 X 12.24 = 112 Ohms
100
HV side current / Phase = Applied Voltage / HV side Impedence/ Phase
= 380 / 112 = 3.38 Amps
LV side SIDE : -
Transformer Ratio = 230/ 34.5 = 6.66
LV side current = 6.6666* 3.38 = 22.53 Amps
TEST -1 Three Phase Balanced Injection
Test connection:
Three phase short circuit outside protected zone on 34.5kV side. Three phase
voltage injected on HV side = 380V
The balanced current can measured in the relay as follows
Relay - Primary
Current (A)
|
|
HV
|
|
3.326<0°
|
22.201<181.141°
|
3.205<241.5°
|
22.935<59.28°
|
3.288<121.5°
|
23.573<299.34°
|
Note : -
1. LV current 180 dec phase sifted from HV current.
2. No return current in Neutral
TEST -2 Three Phase Unbalanced condition
To make this condition open any one of CT FT fingers and short by insert the FT jack.
The Unbalanced current can measured in the relay as follows
Relay- Primary
Current (A)
|
|
HV
|
|
0.00
|
21.985
|
0.00
|
22.226
|
0.00
|
22.265
|
TEST -3 Three Phase REF balanced condition
Test connection:
Three phase short circuit outside protected zone on LV side. Single Phase
Injection was done one by one on the
primary to check the stability and sensitivity.
1. No return current in Neutral
2. During single phase injection voltage will developed in the one phase Tertiary winding. If winding is connected with ground a circulating current will flow through ground during stability.
TEST -4 Three Phase REF Unbalanced condition
Polarity of the neutral BCT was reversed at the protection panel to simulate the unstable condition.
TEST -5 BCT primary injection
PRIMARY INJECTION HV & LV WINDING :
Test connection: Three phase short circuit outside protected zone on LV side. Three phase voltage injected on HV side = 380V
PRIMARY INJECTION TERTIARY WINDING :
Test connection : The shorting and grounding bar was removed from the Delta winding and
current was injected with Severker Test Kit and secondary currents were
measured in the Transformer LCP panel.
