The key to selecting a box transformer monitoring unit is making sure its "point counts" and "interfaces" exactly cover the requirements of the box transformer single-line diagram and the monitoring system. Below are six points, organized around the questions design institutes and EPCs most often face when preparing drawings.
1. AC circuits: how many windings need measuring
First determine whether the box transformer is two-winding or double-split (one HV winding, two LV windings). Each side to be measured (HV side, LV side 1, LV side 2) takes one set of three-phase current and voltage inputs. A double-split transformer measured on both HV and LV sides therefore needs 3 sets of AC inputs. The NSA3 and NSA2 accept up to 3 sets; the NSA6000 up to 2.
2. Digital input (status) and digital output (control) counts
List every status to be acquired in the box transformer: LV circuit breaker position and fault trip, load-break switch position, fuse blown, enclosure door, transformer light/heavy gas, pressure relief, oil level, temperature controller alarm, inverter room smoke detector, and so on. Then list the switches that need remote control. We recommend a margin of 10% to 20% above the actual point count. The NSA series provides up to 40 digital inputs and 10 digital outputs.
3. DC analog inputs: temperature and transmitter signals
Transformer oil and winding temperatures can be wired directly as PT100 or converted to 4-20mA by a transmitter. The NSA series offers 6 DC analog inputs in total across PT100 and 4-20mA, so simply add the two signal types together when checking.
4. Communication interfaces and networking
- Uplink: site distances are long, so fiber is the usual choice. For a self-healing ring, choose a model with dual optical ports (NSA3, NSA2); Ethernet or RS485 uplink can also be chosen to suit the site (optional on the NSA6000).
- Downlink: if inverters, smart combiner boxes, meters and environmental monitors in the box transformer are to report through the monitoring unit, it needs enough RS485 master ports and Ethernet ports. The NSA3 and NSA2 can be fitted with 8 RS485 master ports and 4 Ethernet ports.
- Cable monitoring: if cable sheath grounding current and cable-joint temperature need to be monitored, choose the NSA3.
5. Communication protocols
First confirm the protocols required by the monitoring system and grid dispatch. Common ones are MODBUS-RTU, IEC 60870-5-103/104 and IEC 61850, all supported by the NSA series. For the differences between them, see Common Communication Protocols for Box Transformer Monitoring.
6. Mounting and operating environment
Space inside a box transformer is tight, so check the panel cut-out size, body depth and rear wiring clearance (NSA3 body depth 235mm, NSA2 225mm, plus at least 80mm clearance at the rear). For operating temperature, the NSA series is rated for −40°C to +70°C, suitable for both extreme-cold and high-heat regions; electromagnetic compatibility is tested to the GB/T 14598 series of standards.
Appendix: checklist of information to send the supplier
| Document | Purpose |
|---|---|
| Box transformer single-line diagram (including winding configuration and CT/PT ratios) | Determines the number of AC circuits and input ratings (5A/1A, voltage level) |
| Tele-signalling, remote control and telemetry point lists | Determines digital input, digital output and DC analog input counts |
| Site communication network diagram | Determines the number of optical, Ethernet and RS485 ports and the ring configuration |
| Protocol requirements of the monitoring system and dispatch | Determines the upstream protocol and point list format |
| Box transformer enclosure installation drawing | Verifies cut-out size, depth and wiring clearance |
| Climate conditions at the project site | Verifies operating temperature, humidity and ingress protection requirements |
All NSA series configurations are optional and can be customized per project; the final drawings govern. Send the documents above to sales@aoyipower.com or call +86 10 8646 1328, and we will recommend a configuration.