Current Transformers; Output Relays; Communications - GE 735 Instruction Manual

2.2 ELECTRICAL 2 INSTALLATION

2.2.2 CURRENT TRANSFORMERS

Conventional 1 or 5 A current transformers are used for current sensing. A relaying class CT of the appropriate ratio with
enough output to not saturate under short circuit conditions should be selected. For backup protection schemes, these CTs
are wired in series with the primary protection relays and test switches, if installed. Typical primary/backup CT wiring is
shown in the previous section.
Normally the 735/737 will be connected for residual ground fault sensing as shown in the ALTERNATIVE CT WIRING sec-
2
tion of Figure 2–8: TYPICAL WIRING DIAGRAM on page 2–6. When the drawout chassis is removed, the CT secondaries
are automatically connected together by the internal shorting fingers to prevent dangerous high voltages from open CTs.
More sensitive ground fault detection can be achieved using a core balance (zero sequence) detection method as shown in
the TYPICAL WIRING DIAGRAM. For this configuration the three phase cables (plus neutral if 4-wire system) pass through
the window of a separate CT which senses the zero sequence component of the 3 currents. If a ground shield is present in
the 3 phase cable, it must also pass inside the window of the ground fault sensing CT.

2.2.3 OUTPUT RELAYS

Three separate dry contact relays are provided: TRIP, AUX TRIP and SERVICE. TRIP and AUX TRIP are identical non-fail-
safe Form A contacts which both close whenever the relay trips. These contacts remain closed until the current in all three
phases and ground drops to zero signifying that the breaker has opened. The contacts remain latched for an additional
100 ms then open. The AUX TRIP relay can be programmed as a trip follower (main trip), as an 86 lockout relay, or as a
separate Ground Fault relay. Figure 2–8: TYPICAL WIRING DIAGRAM on page 2–6 shows the relay contact state as
untripped with no control power applied. Typically the breaker 52a contact is wired in series with the TRIP relay contact to
break the trip coil current. For large trip coils an auxiliary relay may be required.
The SERVICE relay is failsafe; that is, the contacts are normally picked up and drop out whenever the 735/737 detects an
internal fault or control power is lost. These contacts are Form C. Contact ratings are shown in Section 1.3.3: OUTPUTS on
page 1–7. Connect the SERVICE relay output to a warning device such as a SCADA monitor.
For more complex control schemes or for status signalling to a SCADA system, the 737 has 8 additional Form C relays.
These can be programmed with option switches 6 and 7 to select the operating mode as: energize on trip (pulsed), latched
cause of trip, phase/ground pickup or both pickup and cause of trip. See Section 3.4.2: OPTION SWITCHES [14] on page
3–8 for details.

2.2.4 COMMUNICATIONS

Continuous monitoring and control of the 735/737 from a remote computer, SCADA system or PLC is possible using the
serial communications port terminals.
Two-wire RS485 is the preferred standard. Four-wire RS422 is also available as an option. RS485 data trans-mission and
reception are accomplished on a single twisted pair with transmit and receive data alternating over the same two wires.
When the RS422 option is installed, separate twisted pairs are required for transmission and reception. The serial port pro-
tocol is a subset of the Modicon Modbus protocol as described in Chapter 4: COMMUNICATIONS.
Lightning strikes and ground surge currents can cause large momentary voltage differences between remote ends of the
communication link. For this reason, surge protection devices are internally installed across the relay RS485/RS422 com-
munication port terminals. A separate power supply with an optocoupled data interface is used internally to prevent noise
coupling to the circuitry. The source computer/ PLC/SCADA system should have similar transient protection devices
installed either internally or externally to ensure maximum reliability under fault conditions. Use shielded, twisted pair con-
necting wire to minimize communication errors from noise. A suitable type of wire is Belden #9841 which is shielded 24
AWG, stranded twisted pair having a characteristic impedance of 120 Ω. Ground the shield at one point only as shown in
the following diagram to prevent ground loops.
Correct polarity is essential. Each relay must be connected with terminal H9 (labelled A+) connected to the same wire and
terminal H10 (labelled B–) connected to the other wire. Terminal H8 (labelled shield) should be connected to the ground
wire inside the shield. Each relay must be daisy chained to the next one. Avoid star or stub connected configurations.
Observing these guidelines will result in a reliable communication system that is immune to system transients.
2-8 735/737 Feeder Protection Relay GE Power Management