[RE-wrenches] AC Coupling Standard?

Harry Mahon Michael.Mahon at sma-america.com
Thu Dec 1 07:55:58 PST 2022

Jeff - THAT is a bunch of loaded questions! My thoughts..

Let's start at the end.  NO, SMA did not create any "standard".  Sunny Islands have been utilizing FSPC (now commonly called Frequency Watt control) for more than 15 years, and someone decided that the window of frequency to control Sunny Boy PV output would be 61-62Hz for 60Hz nominal SIs. Others on the list may have personal knowledge of the actual decision making process (Sam?); that is before my time at SMA.

The SI FSPC was only used in off-grid or micro-grid mode (no utility grid present) when the SI was free to set AC grid frequency where it wanted. If the SI & SB were set up in grid-tied battery backup mode (where the Sunny Boy normally operated in grid-tied mode with no FSPC operational, 60.5 Hz seen as "bad grid (0 W output)" according to "old" UL 1741 testing), the SI would dynamically, via communications signal, change the SB into offgrid mode with "Freq-Watt" enabled, operating from 61Hz (no curtailment of output) to 62Hz (0 W output from SB). SB was kept in off-grid mode as long as SI was forming microgrid, and when the grid came back, the SB would be put back into grid-tied mode when the SI closed the connection to the grid 5 min later.

This required two sets of operating parameters in the Sunny Boy and a method to switch between the two as Freq-Watt was not required for any grid interactive inverters in US until around 2017. Ensuring a normally grid tied inverter can recognize an external signal to switch to microgrid mode with new operating parameters is challenging, especially if AC coupling equipment from different manufacturers.

But now some utilities want Freq-Watt operation in grid-interactive inverters, so it is conceptually possible to set Freq-Watt on the grid-interactive inverters, and have this function work BOTH in grid-tied and microgrid mode without needing any signal/mode/parameter change if the grid is lost.

I believe pre-SA versions of UL 1741 had testing for grid-interactive inverters to support Freq-Watt (and this would be when grid-tied) but this function was not enabled. UL 1741 SA definitely tested inverters to ensure they supported this function.

Buuuutttttt, it depends on which Source Requirement Document (SRD) the UTILITY (HECO, CA ISOs like PG&E, SCE and SDG&E, and those in ISO NE territory) decides to implement whether or not the Freq-Watt function is enabled when connecting to their grid AND what setpoints determine the operation (e.g. what freq the ramp starts at and how fast power goes down with increased frequency).

So, there is not currently any "standard" for implementing this for grid-interactive inverters. Has anyone on this list activated Freq-Watt on an inverter where the utility does not require it (with the utility's blessing of course)?

Even with all the noise around the impending requirement of UL 1741 SB, I don't think utilities will require Freq-Watt be enabled until PV penetration is noticeable. And then they still can choose their own setpoints.

I think it will remain the discretion of manufacturers how their equipment operates in microgrid mode, but without a standard set of parameters for Freq-Watt operation required for grid-tied operation I don't see any "common standard" for microgrid Freq-Watt settings.

Michael Mahon
Senior Technical Trainer, Solar Academy
SMA America

From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> On Behalf Of Jeff Clearwater via RE-wrenches
Sent: Thursday, December 1, 2022 7:33 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Cc: Jeff Clearwater <jeffc at villagepowerdesign.com>
Subject: [RE-wrenches] AC Coupling Standard?

Hey Esteemed Wrenchies,

Does anyone know if manufacturer's are using any common standard for AC Coupling frequency shift algorithms?

Is it part of any UL standard?  Seems like since it's a battery charge control issue it needs some rigor?  Are manufacturer's just copying each other?  Did SMA's Sunny Island/Sunny Boy set that standard?



Jeff C.

Jeff Clearwater

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cell - 413-559-9763
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