The GE Multilin 745 is a management relay for power transformers with two or three windings. It combines dual-slope differential with harmonic restraint, per-winding overcurrent, frequency and overexcitation, and stores events, trends and waveform captures. It communicates over Modbus and DNP 3.0, with optional Ethernet.
What the 745 is
Short answer. The Multilin 745 is a protection and management relay for power transformers in GE’s Multilin family. It covers small, medium and large transformers, with two or three windings. Its core is the transformer differential (87T); to it are added per-winding overcurrent, frequency and overexcitation.
This sheet describes what the manufacturer specifies for the device. It contains no settings or procedures.
Protection functions
| Group | Functions (ANSI) |
|---|---|
| Differential | Three-phase with dual percentage slope and harmonic restraint (87); unrestrained (87/50); optional restricted earth fault (87R) |
| Per winding | Instantaneous phase, calculated neutral and ground overcurrent (50P, 50N, 50G); time phase, neutral and ground overcurrent (51P, 51N, 51G); instantaneous and time negative sequence (50_2, 51_2) |
| Frequency | Underfrequency (81U), rate of change of frequency for load shedding (81R) and overfrequency (81O) |
| Voltage and flux | Overexcitation (24), when the relay is connected to a single-phase voltage transformer |
The relay includes adaptive harmonic restraint, to tell inrush current from a fault, and adaptive time overcurrent elements, which adjust the pickup when the load carries harmonics. For function 24, see overexcitation protection.
The specifications also mention a harmonic derating factor and optional transformer loss-of-life monitoring. These are measurement and monitoring functions, not tripping functions.
Current transformer connection
The specification states that the relay automatically configures the compensation of the current transformers, so all of them are connected in wye. This avoids the special delta connections that older relays required. For the general criterion see vector group compensation in the 87T differential and CTs for transformer differential protection.
Records and analysis
- Events: the last 256, with 1 ms time stamp.
- Waveform capture: sampling at 64 times per cycle, up to 128 cycles, for phase currents, differential current, ground, voltages and digital inputs and outputs. Waveforms can be replayed with a simulation buffer.
- Trends: parameter recording between 1 second and 1 hour, up to eight values.
- Setting groups: up to four.
Communications
- Front: RS232 with Modbus RTU.
- Remote: two RS485 ports, one configurable as RS422, with Modbus RTU or DNP 3.0 level 2, up to 19 200 bit/s.
- Optional Ethernet: 10BaseT with Modbus TCP.
- Time synchronization: IRIG-B input.
The specification does not list IEC 61850. To integrate the relay into a SCADA, see gateway and redundant links to the control center.
Setting software
The manufacturer provides a Windows setting program with a graphical interface. The specification consulted says nothing more; that is why this sheet does not describe its menus. For an overview of GE tools see GE EnerVista software.
Services for the 745
At Tiza Group we review and set transformer relays remotely through our remote relay configuration and test the differential with secondary injection.
Related guides
- Transformer differential protection 87T
- 87T trip without an internal fault
- Overexcitation protection 24
- GE Multilin T60 and T35 relay
- GE Multilin 750 and 760 relay
- GE EnerVista software
References
- GE Multilin, “745 Guideform Specifications” (manufacturer specification, 2006).
- ANSI/IEEE C37.2: device function numbers.
Frequently asked questions
Is the 745 suitable for two- and three-winding transformers?
Yes. The manufacturer's specification calls for options for two or three restraint windings. The number of windings is defined when ordering the relay.
What does the 745 differential function protect?
The three-phase differential with dual percentage slope and harmonic restraint, which prevents trips due to inrush current, and an unrestrained differential for high-magnitude internal faults. Restricted earth fault (87R) is optional.
What communications does the 745 have?
A front RS232 port with Modbus RTU, two remote RS485 ports (one configurable as RS422) with Modbus RTU or DNP 3.0 level 2, and an optional 10BaseT Ethernet port with Modbus TCP. It includes an IRIG-B synchronization input.