Communication Interface Defects Revealed In UL 1741 SB Certified Inverters

As the SunSpec Alliance predicted during development of the IEEE 1547-2018 standard, testing the electrical functionality of devices conforming to IEEE 1547-2018 would require approximately ten times more effort than the predecessor standard (IEEE 1547-2003) and that communication capabilities would both help to relieve the testing burden through automation and add to it because protocol compliance would also need to be validated. We noted that communication technology was not generally well understood in most power conversion engineering teams at that time. Research conducted by Sandia National Laboratories, SunSpec Alliance, and the University of California San Diego during the period of 2016 to 2018 largely confirmed those assessments.

With this in mind, SunSpec updated its SunSpec Modbus certification program, defined a specific communication interface certification for DER devices that must comply with the IEEE 1547-2018 standard, and encouraged member organizations to have their products SunSpec Certified.

By June 2023, very few vendors had completed SunSpec Modbus Certification for 1547. We naturally assumed they had achieved compliance using IEEE 2030.5 or DNP3. By September 2023, SunSpec began receiving reports of non-interoperable UL 1741 SB certified devices with SunSpec Modbus interfaces, so we investigated further.

A person we contacted had evaluated a variety of UL 1741 SB certified devices with SunSpec Modbus interfaces and found that many of these devices had at least one of the following SunSpec Modbus defects, and that several had multiple defects:

  • Inconsistent use of Modbus registers
  • No support for Modbus broadcast
  • Non-compliant data bit formats (big endian, little endian, with & without byte swap)
  • Non-compliant Power Factor settings
  • Incorrect use of Reactive Power sign convention
  • Missing mandatory values (e.g. monitoring point for reactive power missing)
  • Non-compliant time precision for time series data
  • Inoperable Volt/VAR settings (e.g. Volt/VAR registers would automatically change when toggling Volt/VAR mode on and off)
  • And the most egregious defect: no communication interface at all
While a certain number of defects should be expected with any new technology, the nature of the defects reported are alarming. If these issues are pervasive, one should assume that every new DER installation must be studied to determine the feasibility of connecting the DER to a control system. This would be a huge cost multiplier and is not what we want of the industry. If your company is interested in avoiding this type of pain, please contact membership@sunspec.org andconsider applying for SunSpec Modbus Certification for 1547.

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