SunSpec Model 709 — DER Trip LF

Installation

1. 1Enable Modbus TCP on the DER device under the communication or Modbus section of its web interface, and note the assigned TCP port (commonly 502).
2. 2Connect the DER device to the same LAN segment as the ModbusCloud Gateway, or verify IP reachability if the device is on a separate VLAN.
3. 3In the ModbusCloud portal, add the Gateway, import the SunSpec Model 709 DER Trip LF template, and set the device unit ID to match the value configured on the inverter (commonly 1 or 126 for SunSpec-compliant devices).
4. 4Spot-check the id register at offset 0: it must read 709; if it returns 0xFFFF the model is not present at that base address and you need to run SunSpec model discovery to find the correct offset.

Things to watch for

01

The id register returns 0xFFFF instead of 709. Why?

SunSpec models are placed dynamically after the Common Model at base address 40001. The offset of Model 709 depends on which models precede it. Run a SunSpec model discovery scan from the portal or use a tool like pysunspec2 to enumerate all model IDs and their base addresses, then update the template base address to match.

02

Hz and Tms values look wrong or are orders of magnitude off. What now?

All frequency and time point values in Model 709 are raw integers scaled by hz_sf and tms_sf respectively. For example, if hz_sf reads -2 and the Hz register reads 5950, the actual frequency is 59.50 Hz. Confirm the scale factor registers are being applied before interpreting curve point data.

03

Writing to adptcrvreq has no visible effect. What is wrong?

The adopt-curve operation requires that the target curve index be greater than 1 (index 1 is the read-only active curve). Write the desired curve index to adptcrvreq, then read adptcrvrslt: a COMPLETED result confirms the new curve was promoted; IN_PROGRESS means the device is still processing; FAILED indicates the index was out of range or the curve data was invalid.

04

Is it safe to write to the ena or adptcrvreq registers on a live grid-connected system?

Proceed with caution. Disabling the low-frequency trip function (ena = 0) removes IEEE 1547-2018 grid-protection behaviour and may violate interconnection agreements or utility requirements. Similarly, adopting an untested curve set can change the trip threshold immediately. Always coordinate with the site engineer and utility before writing to these registers on an energised installation.

Registers (9)

9 read / 2 write

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