Export Limiting and Net-Metering Commissioning for a 100 kWp Commercial Rooftop
A 100 kWp commercial rooftop system uses two 50 kW three-phase inverters connected to a 415 V, 50 Hz network. At noon the inverters produce 96 kW AC and the site load is 10 kW, so the net meter shows 86 kW export. The sanctioned export capacity is 80 kW. The inverter export limit is disabled and the installers propose to fix the issue by setting each inverter to 40 kW. The AC cable is 95 mm² aluminium, 80 m one way, and the net meter is a three-phase four-wire meter with 200/5 A CTs. During a test, the meter showed 5 kW import while the inverters were clearly exporting. The utility witness test is scheduled for next week and the owner wants to avoid a failed inspection.
1. Calculate the actual export and assess the proposed 40 kW-per-inverter setting. Recommend a better export-control method.
Read the worked answer
Actual export = generation - site load = 96 - 10 = 86 kW.
This exceeds the 80 kW sanctioned export by 6 kW, or 7.5%.
If each inverter is fixed at 40 kW, total generation is 80 kW. With the 10 kW load, export becomes 70 kW, which complies at that moment. But if the load falls to zero, export becomes 80 kW, exactly at the limit; any measurement error or cloud-edge overshoot can cause an exceedance. The fixed setting also curtails generation unnecessarily whenever the site load is above zero.
A better method is closed-loop export control using a CT at the point of common coupling. The controller measures export and sends a limit to the inverters, commonly set to about 75-78 kW to leave metering and control margin. The inverters then generate the site load plus the allowed export, maximising self-consumption. If the utility requires a hard limit, configure the inverter export limit and verify it with a calibrated meter. Record the setting, firmware, and CT ratio, and confirm that loss of the CT or communication causes a safe reduction in output.
2. Explain why the meter showed 5 kW import while the inverters were exporting. What would you check on the CTs and meter?
Read the worked answer
A net meter can show import while the system is exporting if the CT polarity or phase association is wrong. The meter calculates power from voltage and CT secondary current with a defined direction. If a CT is reversed, the meter sees the export current as import. If the CTs are on the wrong side of the point of common coupling, or one phase is paired with the wrong voltage phase, the three-phase power calculation can also produce a false import reading.
Checks:
- Confirm the CT arrow points from the grid toward the installation, or follow the meter manufacturer’s marked direction.
- Verify that each CT is on the correct phase and that the voltage and current inputs are paired correctly.
- Check the 200/5 ratio setting and the meter multiplier; a wrong ratio gives a scaled error.
- Inspect CT secondary wiring for open circuits, shorting links, and correct burden.
- Compare the meter reading with a clamp meter and a known load, then reverse one CT at a time only under safe procedures to confirm the effect.
- Confirm the meter is configured for net metering and bidirectional energy, not just import.
Do not energise the witness test until the CT polarity and ratio are proven correct.
3. List the commissioning checks you would complete before the utility witness test, including the AC cable check.
Read the worked answer
AC cable check
For 95 mm² aluminium, 80 m one way, loop length 160 m, resistivity about 0.0282 Ω·mm²/m:
R = 0.0282 × 160/95 = 0.0475 Ω.
At about 140 A total, ΔV = √3 × 140 × 0.0475 = 11.5 V, or 2.8% of 415 V. This may exceed the project’s 2% limit. Check the actual current, power factor, and cable derating; if 2% is required, use 150 mm² or shorten the route. Verify lugs, torque, phase sequence, and insulation resistance.
Commissioning checks
- Array and string Voc, Isc, polarity, and insulation resistance.
- DC/AC SPD status, earth continuity, and bonding of frames and equipment.
- Inverter firmware, grid code, protection settings, and export limit.
- Anti-islanding test and trip times for over/under voltage and frequency.
- Phase sequence, AC voltage, frequency, and power quality at the point of common coupling.
- Net meter CT ratio, polarity, and communication.
- Verification that the export limit responds to load changes and fails safe.
- Labels, emergency shutdown, and documented witness test results.
Record all readings and retain the settings report for the utility.