There is no single answer to "which transformer monitoring unit is best": grid procurement projects, large solar and wind bases, energy-storage plants and retrofits of existing plants all value different things. This article takes the AOYI NSA3.5 and the transformer monitoring units of the major protection-relay manufacturers NR Electric and XJ Electric as examples, and compares them side by side based on product information published on the manufacturers' official websites and official channels (checked in October 2026). Items not found in published information are marked "not published", which does not mean the product lacks the function; please rely on each manufacturer's latest technical documents when selecting.
Some ranking articles online list transformer monitoring unit models that do not match the manufacturers' official information; this article relies only on the manufacturers' official websites and channels.
1. Two kinds of manufacturer
- Major protection-relay manufacturers, such as NR Electric and XJ Electric. Their product lines cover integrated automation for step-up substations, the compact substation unit is often supplied under the same brand as the substation system, and they are widely used in grid and group procurement projects.
- Specialist transformer monitoring unit manufacturers, such as AOYI. Their products are built around compact substations for solar, wind and energy storage, and their features evolve quickly.
Both kinds now offer "3-in-1" products, which combine the transformer monitoring unit, communication gateway and fiber switch in one device. The 3-in-1 was first introduced by AOYI with the NSA2 in 2012; see Origin of the 3-in-1 Transformer Monitoring Unit.
2. Comparison table
| Item | AOYI NSA3.5 | NR Electric PCS-9726A | XJ Electric WGC-613 / 614 (solar), WFC-611 (wind) |
|---|---|---|---|
| 3-in-1 | Yes: measurement & protection + communication gateway + fiber-ring switching | Yes: protocol conversion, data acquisition, ring switching | WGC-613 / 614 include a communication gateway and 2 fiber channels for a self-healing ring |
| Communication interfaces | 2 single-fiber ports (SC, 20 km); 8 RS485; 4 Ethernet ports | Ethernet, RS485; port counts not published | 2 fiber channels; others not published |
| Cable-joint temperature | Built-in passive wireless temperature sensing (induction-powered, battery-free), 9 points as standard | Not published | Not published |
| Fault recording | Yes, 8 records, 2 cycles before and 8 cycles after the fault, exportable in COMTRADE | Yes, 64 records | Yes, number of records not published |
| Fiber-ring monitoring | Yes: ring status, fiber-port links, 12 alarm types; fiber breaks located to the port | Not published (the website mentions communication status monitoring of downstream devices) | Not published |
| Differential protection | Optional | Not in the protection list on the website | WFC-611 includes differential |
| Operating temperature | −40°C to +70°C | Not published | −40°C to +70°C |
Sources: product information published on each manufacturer's official website and official channels, checked in October 2026. If it has since been updated, the manufacturer's latest information prevails.
3. What the table shows
- 3-in-1 is now standard: all three combine measurement and control, communication gateway and fiber ring in one unit; the differences lie in port counts and additional functions.
- Cable-joint temperature: the NSA3.5 integrates temperature sensing in the unit, with sensors drawing power by induction from the cable current and needing no batteries. The published information from NR Electric and XJ Electric shows no integrated temperature sensing; the common practice on site is to add a separate fiber-optic temperature host and connect it to the monitoring unit over RS485.
- Fiber-ring monitoring: all three support a self-healing ring. In the published information, only the NSA3.5 reports ring status and the link state of each fiber port and locates a fiber break to the specific compact substation and port (using AOYI's patented industrial ring fault detection technology).
- Fault recording: all three have it. NR Electric stores more records; for the NSA3.5, the recorded cycles and COMTRADE export are published.
- Differential protection: where energy-storage three-winding transformers or double-split compact substations need differential protection, XJ Electric's WFC-611 includes it, and the NSA3.5 offers it as an option.
4. Choosing by scenario
| Scenario | Focus on | Options to consider |
|---|---|---|
| Grid or group procurement, where the compact substation units must match the step-up substation's integrated automation brand | Compatibility with the substation system, procurement shortlisting | NR Electric, XJ Electric |
| Solar and wind compact substations, aiming to eliminate the communication cabinet and switch | Whether it is 3-in-1; RS485 and fiber port counts; protocols | All three offer 3-in-1 products; choose by interfaces and additional functions |
| Cable-joint temperature monitoring without a separate fiber-optic temperature system | Whether temperature sensing is integrated and battery-free; number of sensing points | AOYI NSA3.5 (passive, 9 points as standard) |
| Large sites with long fiber runs, where a break may go unnoticed | Whether ring status and fiber-port links are reported, and whether breaks can be located | AOYI NSA3.5 |
| Analyzing the cause of trips after the event | Number of fault records, cycles recorded, COMTRADE export | All three have fault recording |
| Energy storage or three-winding transformers needing differential protection | Whether differential is standard; three-side current acquisition | XJ Electric WFC-611; AOYI NSA3.5 (optional) |
5. Checklist of questions for manufacturers
- Is it a true 3-in-1: can the RS485 master ports acquire data directly from inverters, combiner boxes and meters, and convert protocols for upstream reporting?
- How many fiber ports, single-fiber or dual-fiber, which connector, and what transmission distance?
- Cable-joint temperature: is it integrated, battery-powered or passive, how many sensing points, and is there an alarm when a sensor drops offline?
- Fault recording: how many records are stored, how many cycles are recorded, and can they be exported in COMTRADE?
- Fiber ring: beyond indicator LEDs, can it report ring status and fiber-port link alarms?
- Protection: can the overvoltage and undervoltage settings be set clear of the high / low voltage ride-through zones? Is differential protection needed?
- Operating temperature, electromagnetic compatibility level, and the type test report.
For selection details, see How to Select a Transformer Monitoring Unit; for a full introduction to the AOYI NSA3.5, see What Is the NSA3.5? and Products. For a configuration recommendation, email sales@aoyipower.com or call +86 10 8646 1328.