In solar and wind plants, power from the inverters or wind turbines is first stepped up by a box-type transformer (pad-mounted substation), then fed through the collector lines to the step-up substation. A plant of several hundred MW often has more than a hundred box transformers spread across several square kilometers. The box transformer monitoring unit (box-type transformer measurement, control and protection unit) is the "eyes and hands" installed in each one: it sends the transformer's operating data to the monitoring system in real time and opens or closes switchgear on remote command. It is also called a box transformer smart monitoring unit or a box transformer measurement and protection device.
1. What the unit does: telemetry, tele-signalling, remote control and protection
| Function | Meaning | Typical objects in a box transformer |
|---|---|---|
| Telemetry | Acquires and reports analog quantities | Three-phase currents and voltages on the HV and LV sides, with calculated active and reactive power, power factor and frequency; transformer oil temperature, winding temperature, etc. (PT100 or 4-20mA transmitters) |
| Tele-signalling (status) | Acquires switch and contact states | Open/closed position of the LV circuit breaker and HV load-break switch, fuse blown, enclosure door open/closed, light/heavy gas (Buchholz), pressure relief, oil level and similar signals |
| Remote control | Executes remote commands | Remote opening and closing of the LV circuit breaker and any switch fitted with a motor-operated mechanism |
| Protection | Makes local decisions and alarms or trips | Overcurrent stages I/II/III, earth fault, overvoltage, undervoltage; non-electrical protection such as gas and over-temperature |
The unit also records SOE (sequence of events) and fault waveforms, so the order in which a fault unfolded can be reconstructed afterwards.
2. Why a box transformer needs its own monitoring unit
- Unattended operation: box transformers are scattered across open sites, and manual patrols are slow and costly. The monitoring unit lets operators see the status of every transformer from the central control room.
- Fast fault location: which transformer tripped and why is immediately visible on the monitoring system, so repair crews can go straight to the right spot.
- Dispatch requirements: grid dispatch and the plant monitoring system need real-time box transformer data, and the monitoring unit reports it using the agreed communication protocol.
3. How it relates to the communication gateway and fiber switch
The traditional approach puts three devices in each box transformer: the monitoring unit collects data, a communication gateway aggregates RS485 data from inverters, combiner boxes, meters and other devices and converts protocols, and a fiber switch carries the signals back to the control room over optical cable. Each device is installed and commissioned separately, taking up space and adding points of failure.
AOYI's NSA series Smart Station Monitoring Unit combines all three in one device: transformer measurement and protection, communication gateway (multiple RS485 master ports for inverters and other devices) and fiber switching (dual optical ports forming a self-healing fiber ring) are integrated in a single unit, reducing the quantity of and investment in generation-side secondary equipment. The NSA3 adds cable sheath grounding current monitoring and wireless cable-joint temperature sensing.
4. Typical network topology
In renewable plants, the monitoring units on the same collector line are usually daisy-chained with optical cable into a ring, with both ends of the ring connected to switches in the control room. If the cable breaks at any point, data is rerouted the other way around the ring and communication continues. The monitoring system in the control room then reads data from each unit via protocols such as IEC 60870-5-104 and IEC 61850 and forwards it to dispatch.
In short: box transformer monitoring unit = data acquisition + remote control + local protection + upstream communication for the box transformer. When selecting one, focus on whether the signal point counts, communication interfaces and protocols match the project drawings and the monitoring system.
Further reading: How to Select a Box Transformer Monitoring Unit: Six Key Points and a Checklist, Self-Healing Fiber Rings in Renewable Plants, or go straight to the NSA3, NSA2 and NSA6000 selection comparison.