Siemens 7SR242 Duobias - Figure 2-2 Secondary Injection Using a Variac

7SR242 Duobias Commissioning & Maintenance Guide
©2010 Siemens Protection Devices Limited Chapter 6 Page 15 of 57
Secondary testing of the bias characteristic will be greatly simplified by the use of automated numeric protection
test equipment such as the Omicron CMC256. This equipment can be programmed using setting which match
those of the relay to test for accuracy over the whole operating range and give a clear easy to use graphical
display of relay performance against the specified characteristic.
The relay characteristic can however be tested manually by recording a sequence of operating points for
increasing levels of Restrain current. This can be achieved phase by phase using a single current source such as
a Variac with two independently variable current limiting resistors as shown in figure 2-2 or from two independent
single or three phase current sources. When two separate sources are used the phase of the two sinusoidal
supplies must be the same and the Restrain and Operate currents must be calculated from the sum and
difference of the two currents.
During manual testing the Operate and Restrain currents can be monitored on the relay in the Differential Meters
in the Instruments menu.
For manual testing, the bias slope is usually checked for Restrain current up to 250% of nominal current. For
testing above this level the continuous current rating of the relay inputs is likely to be exceeded, equipment or test
procedure should be arranged in such a way that the short term thermal withstand of the relay current inputs is
not exceeded during testing.
Figure 2-2 Secondary Injection using a Variac
2.1.1.1 Results for testing 87BD with a Variac
B
IAS CURRENT (X I
N
)
M
EASURED ON AMMETER A1
0.00 1.00 1.50 2.00 2.50
87BD INITIAL
SETTING
87BD
1ST BIAS SLOPE
SETTING
Operate Current Measured on Ammeter A2
0.10 0.10 0.10 0.11 0.16 0.21 0.26
0.20 0.20 0.20 0.22 0.33 0.44 0.56
0.30 0.30 0.30 0.35 0.53 0.71 0.88
0.40 0.40 0.40 0.50 0.75 1.00 1.25
0.50 0.50 0.50 0.67 1.00 1.33 1.67
0.50 0.60 0.50 0.86 1.29 1.71 2.14
0.50 0.70 0.50 1.08 1.62 2.15 2.69
Selected Settings Test Results
0.00 1.00 1.50 2.00 2.50
Phase A Pickup
Phase B Pickup
Phase C Pickup