4-10 Protection
and
Control BE1-CDS220
The measured currents are phase, zero-sequence, and tap compensated. Section 3, Input and Output
Functions, Input Current, describes the setup of the relay for phase and zero-sequence compensation.
Setup of the tap adjustment compensation is described later in this section. The restraint current function
uses the compensated current to calculate the restraint current magnitude (in multiples of tap). Depending
on the setting, it calculates the maximum or average restraint current. The Operating Current Function
determines the magnitude of the fundamental, second and fifth harmonic differential current as the phasor
sum of those components of the compensated currents.
Figure 4-10 shows the characteristic of the Restrained Element function. This comparator has a slope
setting and a minimum pickup setting. The slope setting is the ratio of operate current to restraint current.
The slope setting should be set above the maximum mismatch caused by excitation losses, tap
mismatch, and load tap changers. The minimum pickup setting determines the minimum sensitivity of the
restrained element. If the ratio of operate current to restraint current is above the slope setting and the
operate current is above the minimum pickup setting for any of the three phases, the 87RPU (87
restrained element picked up) logic output is set.
D2840-24
02-08-99
Restraint
Current
Operating
Current
Restrained
Element
2nd
5th
Harmonic
Unrestrained
Element
Fund Iop
2nd Iop
5th Iop
Minimum
Pickup
Slope
2nd
harmonic
inhibit
5th
Harmonic
Inhibit
2X Unrestrained
setting
87 Unrestrained
Trip
87 Restrained
Trip
TM
87 Restrained
Pickup
5th Harmonic
Status
2nd Harmonic
Status
Phase
Compensation
and Tap
Adjustment
Fundamental
Phasor
2nd harmonic
Phasor
5th harmonic
Phasor
2nd
per unit
5th
per unit
Fund
per unit
Transient
Monitor
Restraint
Current
Fund
I
op
Unrestrained
setting
Harmonic
2 Cycle
Delay
TM
Figure 4-9. 87 Phase Differential Protection Functional Block Diagram
Summary of Contents for BE1-CDS220
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