Zero-export solar monitoring
Measure the point of common coupling with an independent electricity monitor, retain the controller state beside it, and prove whether low PV output was curtailment or a fault.
- Measure
- Import and export at the point of common coupling, PV output, active export limit and controller state
- Calculate
- Export-limit margin, curtailed intervals and the residual between connection flow, generation and site demand
- Sensors
- ZEM-65, for independent measurement on a suitable low-voltage connection Wireless 3-Phase Electricity Monitor datasheet
- On site
- ZGW-20 Gateway, keeps the owner record in Edge without joining the export-control loop Gateway datasheet
Choose an evidence point outside the control loop
Place the owner meter at the same electrical boundary as the agreed import and export limit, while keeping it independent of the controller's own sensor. If the point of common coupling is medium voltage or outside the ZEM-65 rating, acquire a documented read-only value from an approved plant meter instead.
Export limit monitoring hardware
Use one independent connection measurement, read-only controller context and one Gateway. Meter PV feeders separately only when the site record does not already establish generation at the required boundary.
Keeps connection evidence and controller context together in the owner's local Edge record
1 per site up to 250 wireless devices
Use an approved plant meter interface where the connection is outside this rated boundary
Independent owner measurement of import and export at the point of common coupling
1 per suitable LV connection three-phase, 3 or 4 wire
- Choose the CT rating for a suitable low-voltage connection: 120 A split-core, or 300 A, 1 kA and 3 kA Rogowski coils
Class 0.5S to IEC 62053-22 with its CTs, 110 to 480 V line to line, CAT III at 480 V
Only with a documented register map and an available single-master RS-485 bus
Read-only acquisition of documented export limit, curtailment state, PV output and alarms
1 per available bus up to 30 registers
Modbus RTU over RS-485; register availability depends on the exact controller and firmware
Measurements for solar curtailment monitoring
Declare one sign convention before comparing systems. This guide uses import as positive and export as negative. Keep the raw directional quantities as well as any signed net value so a changed convention cannot silently reverse the result.
Keep monitoring beside the certified scheme
This is an owner-side, read-only record. It observes the point of common coupling and the plant controller without becoming a controller, protection function, DSO channel or settlement path.
- Wired
- Zigbee
- Edge
- Platforms
| Position | What it tells you | Reference | Sensor |
|---|---|---|---|
| Point of common coupling | Independent import and export evidence against the agreed connection limit | Confirm CT direction during a known import and an approved export test ENA G100 | ZEM-65 (on this page), owner evidence meter |
| Export-limiting controller | Active limit, operating state, requested reduction, sensor health and alarms | Read only documented values; the certified scheme keeps its own sensor and authority NGED | ZMB-31 (on this page), documented registers |
| PV plant output | Whether all inverters reduced together, one unit failed or generation stayed below the active limit | Use aligned inverter or feeder totals and retain the operating state and alarms IEC 61724-1 | ZMB-31 (on this page), plant total and equipment state |
Zero export is a limit, not a zero reading. Measurement uncertainty, controller deadband, response time and changing site load can leave a small signed exchange. Compare excursions with the approved scheme tolerance and commissioning record.
A flat export trace can be healthy curtailment. If connection export holds at the active limit while irradiance remains available and all inverter groups reduce together, the pattern supports curtailment. One inverter falling against its peers, an alarm or export well inside the limit points towards a fault or another constraint.
Residuals locate missing evidence. Compare connection flow, PV output and measured site demand over aligned intervals. A residual can come from unmetered loads, storage, transformer loss, timing, CT ratios or different electrical boundaries; investigate it before assigning lost energy.
Diagnose curtailment without joining the control loop
Use aligned connection, plant and controller data to classify events. Preserve the source readings, data age and active limit beside every conclusion.
- Test the export-limit margin
- Compare fresh connection power with the active limit using the declared sign convention. Record the limit source and effective time; a configured value that arrived late cannot explain an earlier event.
- Separate curtailment from equipment faults
- Classify an interval as supported curtailment only when the connection is at the limit, available solar remains, plant groups reduce coherently and the controller state agrees. Otherwise retain the event as unexplained or fault-related.
- Suppress conclusions from stale inputs
- Expose source age and expected-input completeness. If the connection meter or controller data is stale, withhold the curtailment classification instead of carrying the previous state forward as current.
- Trend the balance residual
- Calculate the aligned difference between connection flow, generation and measured site demand. Alert on a persistent change in the residual, then check timing, scale, direction and missing assets before blaming the controller.
Commissioning checks
Commission the owner record with the site's authorised electrical and controls teams, using the approved export-limitation test plan.
-
The PCC boundary is documented
Trace the independent meter on the single-line diagram and compare its location with the connection agreement and controller sensor.
Pass when the owner meter observes the agreed boundary without sharing or altering the controller's measurement path.
-
Direction and scale are proven
Compare each phase during a known import condition and an approved export condition, then verify CT ratios, units and signed totals.
Pass when import and export signs, phase order, ratios and energy registers agree with the declared convention.
-
Limit response is time-aligned
During the approved scheme commissioning test, compare active limit, controller state, plant output and independent PCC power on one clock.
Pass when the record preserves request, plant response and connection result with enough timing detail for the approved test.
-
Stale data fails visibly
In an approved test environment, interrupt one monitoring input without disturbing the certified control path.
Pass when the source becomes stale, the classification is withheld and no old value appears as fresh evidence.
-
Curtailment and faults remain distinct
Review a limit event and an inverter alarm or outage against irradiance, peer output and the residual.
Pass when the record explains the evidence for each classification and leaves ambiguous intervals unresolved.
What owner monitoring does not replace
This design supplies independent operational evidence around an approved export-limitation scheme. It has no authority over that scheme.
- EpiSensor is not the certified export-limiting controller, its fail-safe sensor, interface protection or loss-of-mains protection.
- The independent owner record does not replace the DSO-approved commissioning procedure, connection agreement, locked settings or required witness tests.
- It does not become DSO or TSO operational telemetry, a revenue or settlement meter, or proof of market-service delivery.
- Monitoring alone cannot calculate curtailed energy without a declared counterfactual method and fresh irradiance, availability, limit and plant-state evidence.
- Any write path to an inverter, EMS or power plant controller requires a separate approved controls design. This guide establishes no EpiSensor write path.
Sources
- Engineering Recommendation G100 Issue 2 Amendment 2 (opens in a new tab) Energy Networks Association
- Customer Export Limitation Schemes (opens in a new tab) (opens in a new tab) National Grid Electricity Distribution
- IEC 61724-1:2021 Photovoltaic system performance, Part 1: Monitoring (opens in a new tab) (opens in a new tab) International Electrotechnical Commission
- VDE-AR-N 4105 generation plants on the low-voltage network (opens in a new tab) (opens in a new tab) VDE FNN
- Wireless 3-Phase Electricity Monitor datasheet (opens in a new tab) EpiSensor. Specifications, ranges and ordering codes.
Related
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- GuideRenewable energy grid integration and stability
- GuideStale, missing and invalid telemetry
Third-party devices for this application
More in the Device Directory: Energy meters (62), Current transformers (7)
Bring the connection agreement and commissioning record
An engineer can map the independent evidence points, check the available read-only interfaces and keep the monitoring path outside the approved control scheme.