Woodward MRM4, Manual

Page: 321 / 460

Protective Elements
•
For testing the threshold value, a current has to be fed to phase A which is lower than three times the
adjusted threshold value » Threshold« (I2).
•
Feeding only phase A results in » %I2/I1 = 100%« , so the first condition » %I2/I1 >= 2%« is always
fulfilled.
•
Now increase the phase L1 current until the relay is activated.
Testing the dropout ratio of the threshold values
Having tripped the relay in the previous test, now decrease the phase A current. The dropout ratio must not be
higher than 0.97 times the threshold value.
Testing %I2/I1
•
Configure minimum threshold value » Threshold« (I2) (0.01 x In) and set » %I2/I1« greater or equal to
10%.
•
Apply a symmetrical three-phase current system (nominal currents). The measuring value of » %I2/I1«
should be 0%.
•
Now increase the phase L1 current. With this configuration, the threshold value » Threshold« (I2) should
be reached before the value » %I2/I1« reaches the set » %I2/I1« ratio threshold.
•
Continue increasing the phase 1 current until the relay is activated.
Testing the dropout ratio of %I2/I1
Having tripped the relay in the previous test, now decrease the phase L1 current. The dropout of » %I2/I1« has to
be 1% below the » %I2/I1« setting.
Successful test result:
The measured trip delays, threshold values, and dropout ratios are within the permitted deviations/tolerances,
specified under Technical Data.
Special Notes on Earth Fault Current Transformers
Elements:
Earth Fault Protection
Functional Description
Earth current measurement is usually been done with a summation current transformer (core-balance
transformer). This CT has a large primary window through which all three-phase conductors can pass.
Note that the earth fault current settings are based on the earth CT rated primary current (In), not on FLA or
the phase CT ratio. For example, a pick-up setting of 0.10 gives a trip or alarm for an actual earth leakage
current of 5 A on the primary side of the sensor with a 50:5 CT.
This function is only useful for a earthed power system. The earth return is normally made from the neutral of
the secondary wire winding of the supply power transformer. Resistance earthing is acceptable as long as the
resulting fault current is at a level the protective device can be set to detect.
The earth CT, which provides sensitive protection for high-resistance earth faults, may saturate for a robust
EN MRM4 01/11 Page 321