Basler BE1-CDS220, Instruction Manual

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1-8 General Information BE1-CDS220
D2837-18.dwg
12-14-98
600:5
400A
600:5
400A
600:5
400A
CDS
1200:5
1200A
5.0 A
Secondary
10.0 A
Secondary
Input 1 Tap = 5
Input 2 Tap = 10
So 5A = 1 x Tap
10A = 1 x Tap
Current out balances current in
(in multiples of tap).
Figure 1-2. Tap Adjust For Mismatch
40A
D2837-17.vsd
12-14-98
2400/240 Volts
H
1
H
0
400A
X
1
X
0
Figure 1-3. Currents on Different Voltage Bases
BE1-CDS220 Solution:
The BE1-CDS220 relay applies a tap adjustment factor to the measured currents to cancel the effect of
dissimilar CT ratio and voltage bases by converting the currents to per unit quantities on a common base.
The tap adjusted currents are used by the percentage restrained differential protection functions to
determine the restraint and differential currents. Thus the mismatch in magnitudes under normal balanced
conditions is eliminated. The tap adjustment factor can be manually entered or automatically calculated
by the relay using parameters entered for each CT input circuit and for the differential tap settings.
The setup parameters for each of the current input circuits are described in Section 3, Input and Output
Functions, Power System Measurement Functions. The CT ratio is included to allow the currents to be
metered, displayed, and reported in primary values. The CT ratio is also used as a parameter for the
automatic tap calculation feature.
The setup parameters for the tap adjustment factor are described in Section 4, Protection and Control
Functions, 87 Differential Protection. The user can enter the mega-volt-ampere (MVA base for the
application and the kilovolt (kV) base for each current input, and the relay will automatically calculate the
taps using these settings and the settings for each current input circuit. Or, the user can enter the taps
directly for each CT input.
Problem 3: Mismatch Caused by Load Tap Changers
The voltage transformation ratio (and thus the current transformation ratio) of a transformer within the
zone of the differential can be adjusted in service by typically
±
10% with the use of load tap changers.
The tap adjustment factors can be used to eliminate the mismatch when the tap changer is at neutral; but,
when the tap changer is off of neutral, differential current will be measured.