SunSpec Model 129 — LVRTD
SunSpec Model 129 — LVRTD is a standardised SunSpec information model that configures the Low Voltage Ride-Through Disconnect behaviour of grid-tied inverters and other distributed energy resources. The template reads and controls the active curve index, module enable flag, and timing registers (wintms, rvrttms, rmptms), plus the read-only capability registers ncrv and npt and the scale factors tms_sf and v_sf. Using Modbus TCP, the ModbusCloud Gateway polls the device's fixed block at the model's base address, letting operators verify which LVRTD curve is active and confirm the reversion timeout in force. Supported companion models include SunSpec Model 130, Model 137, and Model 138.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 11, read and write
- Brand
- SunSpec
- Compatible models
- SunSpec Model 130 — HVRTD · SunSpec Model 137 — LVRTC · SunSpec Model 138 — HVRTC
- Last updated
Registers (11)
11 read, 5 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| actcrv | Read and write | ||||
| modena | Read and write | ||||
| wintms | Secs | Read and write | |||
| rvrttms | Secs | Read and write | |||
| rmptms | Secs | Read and write | |||
| ncrv | Read | ||||
| npt | Read | ||||
| tms_sf | Read | ||||
| v_sf | Read |
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If a device is not in the library yet, we build the template for free as soon as we have the register data.
Use cases
- Verify which LVRTD curve is active on each inverter after a grid-code change to confirm correct configuration.
- Monitor the modena enable flag across multiple sites to detect inverters where LVRTD has been inadvertently disabled.
- Track the rvrttms reversion timeout to ensure curve changes programmed via SCADA will not silently revert to default.
- Compare ncrv and npt across a fleet to identify devices with limited curve storage before pushing new grid-code settings.
- Alert on unexpected actcrv changes that may indicate a third-party controller overriding the site's approved LVRTD profile.
Installation
Enable Modbus TCP on the inverter or gateway device under the communication or Modbus section of the device web interface, then note the assigned TCP port (typically 502).
Connect the device to the same LAN segment as the ModbusCloud Gateway and confirm the device IP is reachable from the Gateway by pinging it from your network.
In the ModbusCloud portal, add the Gateway, then import the SunSpec Model 129 — LVRTD template and assign the correct unit ID matching the device's SunSpec Modbus profile.
Read back the id register (should return 129) and the ncrv register to confirm the model block is located correctly and live data is flowing.
Things to watch for
The id register returns NaN or 0xFFFF. What is wrong?
The model block has not been located at the correct base address. SunSpec devices chain models sequentially from a base address (commonly 40001). Scan the device's full SunSpec map to find where model 129 actually starts, then update the template's starting address in the portal accordingly.
modena reads 0 even though LVRTD should be active.
ModEna is a bitfield: bit 0 must be set (value 1) for the function to be enabled. A value of 0 means the function is off. If the device supports LVRTD but shows 0, write 1 to the register via a verified Modbus client, then confirm the device acknowledges the change before relying on cloud monitoring.
Why do actcrv, modena, and the timing registers show no change after writing?
Some inverters protect grid-code parameters with a Grid Guard or installer-level PIN. The write is silently ignored unless the correct access code has been unlocked in a separate register. Consult the device manufacturer's Modbus documentation for the unlock procedure before attempting writes.
Writing actcrv or modena — what safety precautions are required?
Both actcrv and modena are write registers that directly govern grid-protective behaviour. Writing incorrect values can cause an inverter to disconnect from or remain connected to the grid outside permitted voltage ranges, potentially breaching grid-code compliance or invalidating equipment warranties. Always verify the target curve definition is correct before activating it, and test during off-peak hours.
Knowledge base articles for this device
SunSpec Modbus: one register map for many inverters
SunSpec Modbus is the open register map SMA, Fronius, SolarEdge and Kostal share. The layout, int+SF vs float, the TCP port per brand and reading a mixed fleet.
Fronius Modbus: read a Symo GEN24 over Modbus TCP
Fronius Modbus explained: enable Modbus TCP on the Symo GEN24 on port 502, choose between int+SF and float, read the right SunSpec registers and scale them correctly. Practical guide for solar installers.
SMA Modbus: reading a Sunny Boy over Modbus TCP (installers)
SMA Modbus explained: enable the Modbus TCP server on port 502, choose between the SMA profile (unit ID 3) and SunSpec (unit ID 126), read the right registers, and handle the NaN values at night. Practical guide for installers.
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