SunSpec Model 139 — LVRTX
SunSpec Model 139 (LVRTX) is the low-voltage ride-through extended function model in the SunSpec Alliance smart-inverter specification. It configures piece-wise linear voltage/time curves that govern how a grid-tied DER behaves during low-voltage grid disturbances, distinguishing trip from momentary-cessation regions. The template reads model identity, curve count, point count, and voltage and time scale factors, and exposes writable registers for curve selection, function enable, window time, reversion time, and ramp time. Used alongside Model 140 (HVRTX), it gives operators full Modbus TCP visibility and control over IEEE 1547-2018 disturbance-response settings.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 12, read and write
- Brand
- SunSpec
- Compatible models
- SunSpec Model 140 — HVRTX
- Last updated
Registers (12)
12 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 | ||||
| crvtype | 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 that the LVRTX function is enabled on every newly commissioned inverter before grid energisation.
- Monitor the active curve index to confirm the correct ride-through profile is loaded after a firmware update.
- Track reversion-timer values across a fleet to ensure automatic fallback to default curves after manual overrides.
- Alert when the active curve index changes unexpectedly, which may indicate an unauthorised configuration change.
- Compare window-time and ramp-time settings across multiple sites to enforce a uniform grid-code compliance baseline.
Installation
Enable Modbus TCP on the inverter or gateway under the communication or Modbus section of its web interface, and note the IP address and TCP port (default 502).
Connect the inverter or its communications gateway to the same LAN segment as the ModbusCloud Gateway, confirming the device is reachable on port 502.
In your ModbusCloud portal, add the ModbusCloud Gateway, import the SunSpec Model 139 LVRTX template, and assign the correct Modbus unit ID (commonly 1 or 126 for SunSpec devices).
After import, poll register address 0 (id) and verify the returned value equals 139; then check address 3 (modena) to confirm the function-enable state matches your grid-code requirement.
Things to watch for
Register address 0 returns 0xFFFF instead of 139. Why?
A value of 0xFFFF is the SunSpec not-implemented sentinel. The connected device does not expose Model 139 at the base address you have configured. Verify the SunSpec model map by scanning from the device base address (often 40001) and locating the model block that starts with 139 before assigning the template.
The gateway connects but tms_sf and v_sf read as unexpected large negative numbers.
tms_sf and v_sf are signed SunSpec scale-factor registers (int16). A value such as -2 means multiply the raw time or voltage reading by 10 to the power of -2. This is normal. Ensure your client applies the scale factor before displaying or logging curve point values; reading them raw without scaling will always appear incorrect.
Writing to modena or actcrv has no visible effect on the inverter.
Many inverters require the SunSpec ENABLED bit in modena to be set to 1 and a separate commit or write to the actcrv register in the same transaction. Also confirm the unit ID matches the inverter, not a gateway or logger. Some firmware versions ignore writes unless Modbus write-access is explicitly unlocked in the device settings.
Is it safe to write actcrv or modena on a live grid-connected inverter?
Use caution. Changing the active curve or disabling the LVRTX function on a live inverter can alter disturbance-response behaviour required by your grid-code or interconnection agreement, and may invalidate utility approval. Always schedule configuration changes during a maintenance window, verify the target curve meets local IEEE 1547 or EN 50549 requirements, and consult the inverter manufacturer before writing to these registers in production.
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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