Analog Input - GE 745 Instruction Manual

TYPICAL WIRING
3.2.9

Analog Input

3.2.10 Tap Position Input
3.2.11 RTD Driver/Sensor
3.2.12 Output Relays
3–16
Terminals A1 (+) and A2 (–) are provided for the input of a current signal, from one of the
following: 0 to 1 mA, 0 to 5 mA, 0 to 20 mA, or 4 to 20 mA transducer outputs. This current
signal can represent any external quantity, such as temperature, current or voltage. Be
sure to observe polarity markings for correct operation. Both terminals are clamped to
within 36 volts of ground with surge protection. As such, common mode voltages should
not exceed this limit. Shielded wire, with only one end of the shield grounded, is
recommended to minimize noise effects. The A2 (–) terminal must be connected to the A5
(analog output common) terminal at the 745.
Terminals A3 (+) and A4 (–) are provided to monitor the position of an onload tap changer
from a stepped-resistance position indicator device. Terminal A3 is connected internally to
a 4.3 mA current source. This current is used to measure the value of the external
resistance. The 745 uses the measured resistance value to calculate the tap position. See
Dynamic CT Ratio Mismatch Correction on page 5–6 for more details on the tap position
input.
The maximum total resistance the tap changer input can measure is 5.1 kΩ. For example,
the maximum resistance increment per tap for a 33-position tap changer should not
exceed 151 Ω.
Terminals B10 (RTD hot), B11 (RTD compensation) and B12 (RTD return) provide for the
connection of various types of RTD devices. This connection may be made using two or
three wires to the RTD. Terminal B10 is connected internally to a 5 mA current source for
energizing the RTD. Terminal B11 is connected internally to a 5 mA current source for the
purpose of cancelling out the resistance of the wires connecting the RTD to the 745.
Terminal B12 is the return path for the two current sources.
In the three-wire connection scheme, the connection from terminal B11 to B12 is made at
the RTD. The three-wire connection scheme compensates for the resistance of the wiring
between the 745 and the RTD.
In the two-wire connection scheme, the connection from terminal B11 to B12 is made at
the terminal block on the rear of the 745. This connection must not be omitted. The two-
wire connection scheme does not compensate for the resistance of the wiring between the
745 and the RTD.
Eight output relays are provided with the 745. Output relays 2 through 5 have form-A
contacts while output relays 6 to 8 and the self-test relay, have form-C contacts. Since
output relays 2 to 5 are intended for operating a breaker trip coil, the form-A contacts have
higher current ratings than the form-C contacts. Note that the operating mode of the self-
test relay is fixed, while the other relays can be assigned through the Protection Elements
menu, or programmed by the user via the FlexLogic™ feature.
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 3: INSTALLATION