Alright, so you're staring at a spec sheet for a GE Multilin 850, or maybe you're trying to figure out why the under voltage relay keeps nuisance-tripping your feeder. You've got questions. I've got answers, mostly from learning the hard way during commissioning gigs and emergency call-outs.
FAQ: Common Questions on GE Transformer Protection Relays
What's the difference between the GE Multilin 850, 750, and 489 for transformer protection?
Good question. These are the three workhorses you'll find on medium-voltage transformers, and people mix them up constantly.
GE Multilin 850 (and the 750): This is the flagship transformer differential relay. The 850 is the full-featured version with integrated breaker control and metering. The 750 is the “stripped” version—same protection core, but fewer bells and whistles. You use the 850 for main tie-main or complex switchgear. You use the 750 when you just need the protection and don't want to pay for the touchscreen.
GE Multilin 489: This is a different animal. The 489 is a motor management relay. People accidentally specify it for transformers all the time because they see “protection” and “current inputs.” Don't do it. The 489 has motor-specific thermal modeling and start-up algorithms. It will not provide proper transformer differential protection (87T). It's for pumps, fans, and compressors, not power transformers.
Stick to the 850 or 750 for the 87T function. I've seen a site try to use a 489 for a 5 MVA transformer. It didn't end well—they had to re-pull the control wiring.
How do I correctly set the Transformer Differential (87T) on a GE Multilin 850?
This is where most commissioning errors happen. It's not just about entering the CT ratios.
Step 1: Enter Winding Data. You need the transformer nameplate: MVA, voltage ratio (e.g., 13.8 kV Delta to 4.16 kV Wye), and vector group (e.g., Dyn1). The 850 has a wizard that handles the phase shift compensation—usually Dyn1 or YNd1. Get this wrong and you'll have a differential trip at 50% load.
Step 2: Confirm CT Config. The relay automatically calculates the “matching factor” to balance the primary and secondary CT outputs. You must verify the CTs are wired correctly (e.g., all polarities with ‘H1’ toward the transformer). In March 2024, I was on a site where a contractor wired one CT polarity reversed. The relay showed a steady-state differential current of 0.8 per unit. We spent six hours chasing a phantom fault. It was a wire swap.
Step 3: Set the Slope and Minimum Pickup. I typically start with:
- Minimum Pickup (87P): 0.2 – 0.3 per unit (to ride through inrush)
- Slope 1: 25%
- Breakpoint: 2.0 per unit
- Slope 2: 60%
What is an Under Voltage Relay (ANSI 27) and why does my GE relay keep tripping?
The ANSI device number 27 is an under voltage relay. Its job is to detect when the voltage drops below a set threshold for a set time. This protects motors from stalling and sensitive electronics from brownouts.
Why it nuisance trips:
Most common cause: Incorrect time delay. Motors starting can dip voltage temporarily. If your 27 pickup is 90% of nominal with a 1-second timer, every motor start will trip it. You need to coordinate the under voltage relay with the motor acceleration curves. Set the time delay to at least 5-10 seconds for industrial feeders.
Second common cause: Voltage transformer (VT) fuse failure. If a VT fuse blows, the relay sees a 0V signal and will trip the whole bus. Ensure you have a VT fuse failure alarm (ANSI 60) and that your 27 element is blocked during fuse failure detection.
I had a client who couldn't understand why their feeder breaker kept opening after a lunch break. Turns out, a power quality event from a neighboring factory was causing a 15% voltage sag for 2.5 seconds. They had no time delay. We added a 7-second delay. Problem solved.
Are GE Multilin 850 relays compatible with older GE protection relays?
Yes, but with caveats. The 850 speaks modern protocols: DNP3, Modbus RTU/TCP, IEC 61850 (on newer models). Older relays like the old electromechanical IAC or early SR489's used hardwired analog signals (4-20 mA) or discrete I/O. You can't just plug an Ethernet cable into a 1970's relay.
You'll need a protocol gateway or discrete I/O interface to talk to old recloser controls or regulator controls. The 850 has plenty of digital inputs and outputs—you wire the old relay's “Trip” contact into the 850's digital input. Then the 850 can send the alarm to the SCADA system. It's a common retrofit strategy: keep the old relay as a backup, but use the 850 for modern data logging and communication.
What is a Power Conditioner vs. a Surge Protector? Do I need one for my relay cabinet?
People confuse these all the time. Let me clear it up.
Surge Protector (SPD): This is your first line of defense. Clamps down on voltage spikes (lightning, switching surges). It shunts the surge to ground. It's a sacrificial device—it absorbs energy and may self-destruct protecting your equipment. You absolutely need one on the AC supply to your relay panel. I recommend a Type 2 SPD at the panel feeder.
Power Conditioner: This is a broader category. It can include voltage regulation (boost/buck), noise filtering (EMI/RFI), and sometimes isolation. A power conditioner handles steady-state problems—voltage sags, swells, and electrical noise.
For a GE Multilin 850 in a substation, you generally need a surge protector for transient protection. You might need a power conditioner if your control power is noisy (e.g., from a nearby variable frequency drive). If your relay keeps having communication drop-outs during a motor start, a power conditioner with noise filtering may fix it.
Don't just buy a cheap power strip. Get a proper industrial SPD and, if needed, an isolation transformer for the control panel. I've seen a $200 SPD save a $10,000 relay panel from a lightning strike. It's cheap insurance.
What about Electrical Outlet Box Types for my control panel?
This sounds mundane, but it's a frequent point of failure. In an IEC or NEMA panel, you're not using residential outlets. You're using industrial pin-and-sleeve devices or NEMA locking configurations (L5-30, L6-30, etc.).
For control power to a relay cabinet, you typically use a NEMA 4X or 12 enclosure with a receptacle (outlet) for temporary service or laptop charging. The box itself is a sheet metal enclosure with cutouts. Use a mud ring if you're mounting a standard device box inside the panel.
The key specification: Environmental rating. Inside a metal enclosure, you can use a standard cast iron or fiberglass box with a gasketed cover. Outside, you need a NEMA 4X (stainless steel or fiberglass) to handle rain and corrosion. I've pulled out corroded cast-iron boxes from coastal substations every 3 years. Spend the extra $20 on fiberglass.
Quick Troubleshooting Table for Common GE Relay Issues
Based on my last 12 months of field service calls for GE Multilin 850 relays:
| Symptom | Most Likely Cause | Quick Fix (30 min check) |
|---|---|---|
| 87T trips at 30% load | CT polarity reversed or wrong VT compensation | Verify CT wiring per schematic; check vector group |
| Under voltage (27) random trips | VT fuse blown or time delay too short | Check VT secondary voltage; increase 27 time delay to 5s |
| Relay won't connect to SCADA | IP conflict or cable damaged | Ping the relay IP; verify cable pinout (straight vs. cross) |
| Ground fault (50G/51G) tripping | Zero-sequence CT not matching relay setting | Verify CT ratio and relay pickup setting match |
Note: These are my personal troubleshooting steps. Always refer to the GE Multilin 850 manual (GEK-xxxxx) for the final word on settings.
Hope this helps you get your transformer protection up and running. If you're facing a specific issue, the answer is almost always in the CT wiring or the coordination setting. Not in the relay itself. Good luck.
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