3.3 APPLICATION NOTES
3.3.1 SETTING GUIDELINES
The positive sequence current in at least two current inputs exceeds the CTS I1 setting. The CTS I1 setting should
be below the minimum load curr
ent of the protected object. Therefore, 10% of the rated current might be used.
The high set ratio of negative to positive sequence current, CTS I2/I1>2, should be set below the ratio of negative
sequence to positive sequence current for the minimum unbalanced fault current. When the balanced full load
current is flowing and the secondary of Phase A CT1 is disconnected, the currents measured by the device are:
I
A
= 0
I
B
= 1
Ð
-120°
I
C
= 1
Ð
-240°
The positive and negative sequence currents are calculated as:
I I aI a I
A B C1
2
1
3
2
3
= + +
( )
=
I I a I aI
A B C1
2
1
3
1
3
= + +
( )
=
The ratio of negative to positive sequence current is 50%, therefore a typical setting of 40% might be used.
The low set ratio of negativ
e to positive sequence current, CTS I2/I1>1, should be set above the maximum load
unbalance. In practise, the levels of standing negative phase sequence current present on the system govern this
minimum setting. This can be determined from a system study, or by making use of the device's measurement
facilities at the commissioning stage. If the latter method is adopted, it is important to take the measurements
during maximum system load conditions, to ensure that all single-phase loads are accounted for. A 20% setting
might be used.
If the following information is recorded by the relay during commissioning:
I
full load
= 500 A
I
2
= 50 A
Therefore I2/I1 ratio is given by I2/I1 = 50/500 = 0.1
To allow for tolerances and load variations a setting of 20% of this value may be typical. Therefore set CTS I2/I1>1
= 20%.
Due to the sensitive settings suggested above, a long time delay is necessary to ensure a true CT failure. We
recommend using the default setting for this time time delay. After the CTS Time Delay expires (CTS Time Delay),
the CTS Fail Alarm is asserted.
P64x Chapter 14 - Supervision
P64x-TM-EN-1.3 293
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