7SG14 Duobias-M-200 Commissioning
The relay has a built feature to avoid a false operation when energising a transformer. This uses the presence of
even harmonic in the operate signals to distinguish between an inrush and an internal fault. Three different
methods are included in the relay.
The test method will depend upon the Inrush Restraint Method selected. As previously mentioned the SUM
method has an advantage over the CROSS method in terms of operating speed if an internal fault occurs on
energising the transformer.
Only the Inrush Restraint method selected need be tested.
Sum - Restraint Method
This method uses one Inrush Sum some with which to compare the operate current in each phase. The square
roots of the even harmonic content in each phase differential current is summed and then divided by the Inrush
Setting to arrive at an overall threshold.
Cross – Inhibit Method
The magnetising inrush restraint feature can be checked by injecting the relay with 2
nd
harmonic current into one
set of inputs while fundamental is injected into another set of inputs. If possible the test should be done three
phase as all phases are blocked if one phase exceeds the Inrush Inhibit Setting.
Check the
87 Inrush Inhibit
setting is set to [Enabled].
Inject W1 inputs with a balanced three phase current of nominal amplitude and frequency. This will operate all
three phases of the biased differential function. The 87BD and phase LED’s will be lit.
Inject about 5% of nominal 2
nd
harmonic current into W2 inputs. Slowly raise the 2
nd
Harmonic Current until the
biased differential resets. The approximate levels of 2
nd
harmonic to block operation are set out in the table below.
Digital Test sets such a Omicron 256-6 use a ratio injection to test the relay inhibit, the Duobias M does not work
like this as its setting is a percentage of the operate current.
2
ND
H
ARMONIC
C
URRENT INTO
W2
REQUIRED
TO BLOCK
87BD
FOR NOMINAL
FUNDAMENTAL CURRENT INJECTION OF
W1
INPUTS
S
ETTING AT RECOMMENDED TEST POINT IN
TERMS OF
2
ND
/
OPERATE
I
NRUSH
S
ETTING
(
FRACTION
OF
I
OP
)
50 Hz Relay(x I
n
)
60 Hz Relay(x I
n
)
50Hz Relay(x I
n
)
60Hz Relay(x I
n
)
0.10 0.108
0.120 0.100 0.132
0.12 0.128
0.144 0.121 0.159
0.14 0.150
0.168 0.141 0.186
0.16 0.172
0.192 0.161 0.213
0.18 0.194
0.219 0.181 0.241
0.20 0.216
0.245 0.200 0.268
0.22 0.239
0.268 0.220 0.296
0.24 0.262
0.293 0.240 0.325
The relay will filter out some of the second harmonic content in currents due to the anti-aliasing filter roll off.
The filter response attenuates the 2
nd
(100Hz) harmonic current by approximately x 0.94 for a 50Hz Relay, and by
x 0.84 for a 60Hz relay.
The root mean square value of operate current must be calculated, as this is what is used to set the Inrush Inhibit
level. Typically digital test sets such as the Omicron uses 2
nd
Harmonic / Fundamental to check the relay
accuracy. However the relay uses the percentage of second harmonic to the root mean square of operate current.
As the operate current includes the injected 2
nd
harmonic as well as the fundamental this must be taken into
account.
9.5 Checking the Differential Highset (87HS)
Connect the test current source to relay or test plug. The test can be done single phase or three phase. This tests
requires the injection of current in excess of the relay rating, so ensure that the duration of the test does not
exceed the relay overload withstand rating.
Use the LCD display to check that the LV input settings are as follows:
LV Interposing CT Multiplier 1.00
LV Interposing CT Connection Yy0
©2010 Siemens Protection Devices Limited
Chapter 7 Page 9 of 20
