2–2 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
INTRODUCTION CHAPTER 2: OVERVIEW
• Dynamic CT ratio mismatch correction which monitors the on-load tap position and
automatically corrects for CT ratio mismatch
• FlexLogic™ which allows PLC style equations based on logic inputs and protection
elements to be assigned to any of the 745 outputs.
The 745 also includes a powerful testing and simulation feature. This allows the protection
engineer the ability to test the relay operation based on captured or computer generated
waveform data which can be converted to a digitized format and downloaded into the
745’s simulation buffer for “playback”. A waveform capture function that records waveform
data for fault, inrush, or alarm conditions is also provided.
The auto-configuration function eliminates the need for any special CT connections by
having all CTs connected in wye.
2.1.2 Protection Features
The following table outlines the protection features available for windings 1, 2, and 3, as
well as the common protection elements.
Symbol Common protection element Symbol Winding 1 protection elements
59/81-1 Volts-per-hertz 1 150/46 Negative sequence instantaneous overcurrent
59/81-2 Volts-per-hertz 2 151/46 Negative sequence time overcurrent
81U-1 Underfrequency 1 150P1 Phase instantaneous overcurrent 1
81U-2 Underfrequency 2 150P2 Phase instantaneous overcurrent 2
81U-R1 Frequency decay rate 1 150N1 Neutral (3I
0
) instantaneous overcurrent 1
81U-R2 Frequency decay rate 2 150N2 Neutral (3I
0
) instantaneous overcurrent 2
81U-R3 Frequency decay rate 3 150G1 Ground instantaneous overcurrent 1
81U-R4 Frequency decay rate 4 150G2 Ground instantaneous overcurrent 2
81-H5 5th harmonic Level 151P Phase time overcurrent
81O Overfrequency 151N Neutral (3I
0
) time overcurrent
87 Differential (percent) 151G Ground time overcurrent
50/87 Instantaneous differential 187TG Ground differential (restricted ground fault)
AN-1 Analog input level 1 1THD Total harmonic distortion level
AN-2 Analog input level 2 1AD Current demand
--- Insulation aging: aging factor, hottest spot limit,
and total accumulated life
--- Tap changer monitor
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