SunSpec Model 135 — LFRT
SunSpec Model 135 (LFRT) is the Low Frequency Ride-Through array model defined by the SunSpec Alliance for grid-connected inverters and distributed energy resources. Used alongside Model 136 (HFRT) and the curve models 141 and 142, it configures how an inverter responds to under-frequency grid events. The template communicates over Modbus TCP and exposes the active curve selector, the module enable flag, windowed activation and revert timers, ramp time, plus the curve-count and scale-factor registers. Pairing this template with the ModbusCloud Gateway lets operators verify and audit frequency protection settings across a fleet of SunSpec-compliant inverters from a single portal.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 11, read and write
- Brand
- SunSpec
- Compatible models
- SunSpec Model 136 — HFRT · SunSpec Model 141 — LFRTC · SunSpec Model 142 — HFRTC
- 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 | ||||
| hz_sf | Read |
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Use cases
- Verify that the LFRT module-enable flag is active on every inverter after a grid-code change.
- Monitor the active curve index across a multi-inverter site to confirm uniform frequency protection settings.
- Alert on unexpected revert-timer expiry that may indicate a polling interruption or communication loss.
- Track ramp-time and window-timer values over time to detect unauthorized configuration changes.
- Report the supported curve count and point capacity to validate firmware compliance before grid-code audits.
Installation
Enable Modbus TCP on the inverter or gateway device under its communication or SunSpec settings menu, noting the assigned IP address and TCP port (default 502).
Connect the inverter to the same LAN segment as the ModbusCloud Gateway and confirm IP reachability with a ping; SunSpec devices typically use unit ID 1 or 126 depending on the manufacturer.
In the ModbusCloud portal, add the Gateway, import the SunSpec Model 135 LFRT template, and assign the correct unit ID and base register offset matching the device's SunSpec map start address.
Trigger a single register read of the id register (address 0) and confirm the returned value is 135; then read ncrv and npt to verify the device reports its expected curve capacity.
Things to watch for
Why does the id register return an unexpected value?
SunSpec models are chained sequentially in memory; the absolute Modbus address of model 135 depends on all preceding models. If you poll address 0 and get a wrong ID, you are likely pointing at a different model in the chain. Use SunSpec device discovery (scan from the base address) to find the correct offset for model 135.
The modena flag reads false after I write true. What is wrong?
Several SunSpec-compliant inverters require a separate grid-guard or write-enable unlock before control registers accept writes. Check the manufacturer's documentation for any prerequisite enable sequence. Also confirm you are using function code 16 and that the unit ID matches the inverter, not the communication gateway.
hz_sf and tms_sf return negative numbers. Is that correct?
Yes. SunSpec scale factors are signed exponents of 10. A tms_sf value of -2 means divide the raw curve time values by 100 to get seconds. Always apply these scale factors before interpreting any curve-point data; ignoring them will produce values that are off by one or more orders of magnitude.
How should I handle writing to actcrv and modena safely?
Writing to actcrv selects a different protection curve, which directly affects how the inverter responds to low-frequency grid events. Always verify the target curve is fully programmed before activating it. Test changes during a maintenance window; an incorrect LFRT curve can cause the inverter to trip unnecessarily or fail to trip when required, potentially violating grid-code obligations.
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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