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EnpulseExport-limit metering when a CT won't fit: Rogowski coils for commercial solar
Commercial-solar export limiting needs accurate current metering at the main supply, but busbars, parallel cables and live retrofits often rule out a standard CT. Here's how the Algodue MFC150/F Rogowski coil, with its built-in integrator, solves it, and the inverter-matched kits for Fronius, SolarEdge and Sungrow.
Export limiting needs a meter where a CT often won't go
Most commercial solar connections now come with an export limit, sometimes zero export. The inverter holds the site under that limit by reading a meter at the point of supply that measures net import and export across all three phases. No accurate current measurement on the main, no compliant export control.
On a house that's easy: a split-core CT clips straight onto the consumer mains. A commercial main switchboard is a different story. The incoming supply is often a busbar, or two to four parallel cables per phase, or conductors fatter than any clip-on CT window. And the board is live, feeding a working site, so threading a solid-core CT during a planned outage usually isn't on the table either.
Where current transformers run out of room
A solid-core CT needs the conductor passed through its window, so it belongs to the build, not a retrofit. A split-core CT opens up to clip on, which helps, but the window is still a fixed size: it won't close around a busbar or a bundle of parallel cables, and the oversized versions get bulky and expensive fast.
There's also the safety catch every metering hand knows. A CT secondary must never sit open-circuit while the primary is energised, or it can develop a dangerous voltage across the open terminals. On a busy commercial install that's one more thing to get wrong.
What the Rogowski coil changes
A Rogowski coil is a flexible winding on a non-magnetic core. You loop it around the conductor and close it. With no iron core there's nothing to saturate, so it stays linear across a wide range, it isn't damaged by overloads, and it reads AC accurately even when there's a large DC component riding on top. It also draws no power from the main it's measuring.
The MFC150/F is the version with the integrator built in. A bare Rogowski coil outputs a voltage proportional to the rate of change of current, not the current itself, so that signal has to be integrated before a meter can use it. The /F does that internally and gives a clean 333 mV output at full scale (RMS), delivered already calibrated. It needs a small DC supply to run the integrator, anywhere from 4 to 26 V DC at under 5 mA, and the kits below include a 12 V supply for it.
Two things make it the right fit for a commercial main:
- The cord is only 8 mm thick and fully flexible, so it wraps around a busbar, several parallel cables, or an awkward run that no clip-on CT will close around, and it goes onto a live, in-service board without re-terminating anything, worked safely to the usual rules.
- The output is a low 333 mV signal with no open-circuited-secondary danger, so the hazard you manage with a CT simply isn't there.
One more thing that matters on a busbar or a bundle: the reading doesn't depend on where the conductor sits inside the loop, and the coil rejects current from nearby conductors. You get a consistent measurement even when the cable isn't centred and there's other live copper close by.
Matched to your inverter's smart meter
Export limiting is only as good as the meter the inverter reads. Enpulse builds Rogowski-coil kits matched to the smart meters the major inverter brands use for import and export metering:
- Fronius Smart Meter
- SolarEdge SE-MTR-3Y-400V-A
- Sungrow DTSU666-20
Each kit pairs the MFC150/F coils (333 mV full-scale output) with the 12 V supply the built-in integrator needs, so you fit a complete, matched set instead of sourcing and verifying the pieces yourself.
Pick the full-scale rating to suit the main: 1 kA, 2 kA or 3 kA, all on the same 60 cm coil with about a 17.5 cm opening. That loop wraps a wide busbar or a thick bundle of parallel cables, which is often the deciding factor on a commercial main where a clip-on CT won't close.
Need a different range, or a particular opening for an odd geometry? We build custom versions too, so the current range and the loop size aren't locked together. Ask us if the standard three don't fit your board.
The coils are built to UL 61010-1 and IEC standards, rated CAT III 1000 V / CAT IV 600 V with a 7400 V insulation test, so they hold up to scrutiny on a commercial board.
Made for the retrofit
This is where it earns its keep. Adding compliant export limiting to an existing site almost always means working on a board that's already running. Clip the coils around the existing busbar or cables, land the millivolt output on the meter, power the integrator, and you have accurate three-phase metering with no outage and nothing re-terminated.
Spec the right kit
Tell us the inverter and meter you're using and what the main looks like, busbar or cables and the rating, and we'll point you to the right kit and full-scale range.