Basler BE1-CDS220, Instruction Manual

Page: 83 / 476

BE1-CDS220 Protection and Control 4-17
To use this function, the relay must always be installed with the optional independent ground sensing
input
I
G
that is the zero-sequence current entering the zone of protection. Figure 4-14 shows the
configuration of this protective function. The zero-sequence current exiting the zone of protection is the
calculated neutral for the three-phase CT input circuit designated by the BESTlogic mode setting.
D2840-20.vsd
02-08-99
3I
O
3I
o
Z
0
I
R
I
OP
I
R
I
O
I
O
I
O
I
O
I
O
I
O
-3I
o
Figure 4-14. 87ND Polarity Configuration
These paragraphs discuss the details of how the function works and how to set it up. Figure 4-15 shows a
detailed functional block diagram of the neutral differential protection function. The measured ground
current and the neutral current are tap adjusted to eliminate magnitude mismatch. The Restraint Current
function determines the magnitude of the restraint current as the maximum of the compensated currents
in multiples of tap. The Operating Current function determines the magnitude of the differential current as
the phasor sum of the compensated currents.
D2843-01
02-08-99
Restraint
Current
Operating
Current
Restrained
Element
Fund
I
op
Minimum
Pickup
Slope
87ND Restrained Pickup
Tap
Adjustmen
t
Fundamenta
l Phaso
r
Fun
d per
unit
Ground
Phasor
87ND
Restrained
Trip
TM
Settable
Delay
Figure 4-15. 87 Neutral Differential Protection Functional Block Diagram
NOTE
The CTs for this three-phase input circuit must be connected in wye with zero-
sequence compensation for the grounded winding provided by internal delta
compensation. If the CTs are connected in delta to provide external zero-
sequence compensation, the calculated 3I0 (neutral) current exiting the zone will
always be zero since it is filtered out by the CTs delta connection.