Maintenance Process Guide: Inverter Reactive Power for the CN→UK Corridor

1. Process summary

This guide is a maintenance reference for keeping inverter reactive power behaviour, documentation and dispatch commands in a defensible state after first commissioning. It is written for the CN→UK export lane because, in our observation, Chinese-origin inverters frequently ship with factory reactive defaults set for CN grid conditions rather than GB conditions (a model assumption, not a universal rule); reactive power is one of the items that drifts after commissioning and surfaces during DNO witness testing.^[3][4]

The workflow below assumes a Responsible Person has already completed the G98/G99 commissioning sign-off.^[3] Each annual cycle should re-confirm that firmware, setpoints, measured response and the dispatch reference table still match the DNO-accepted connection agreement.

For example equipment on this page we use the Sungrow SG125HV string inverter family (PV), with reactive-mode behaviour per the Sungrow SG125HV User Manual (current published revision, OEM manual title; revision number confirmed at RFQ).^[1] The workflow is DNO-agnostic: the applicable DNO is the one named in the site's connection offer letter.

2. Step-sequence table

Equipment-family assumption: where steps above reference an OEM datasheet or Modbus/Sunspec map, the working assumption is the Sungrow SG125HV family datasheet and the Sungrow SG125HV User Manual (current revision, confirmed at RFQ).^[1] Replace with the equivalent datasheet/manual for any other inverter make used on site.

3. Document checklist

Use literal filenames so the compliance pack stays auditable. TradVolt recommends storing all files in a single folder per site.

• site-id_inverter_qcapability_datasheet.pdf — Sungrow SG125HV datasheet (or site-specific OEM datasheet)^[1]
• site-id_inverter_firmware_release_notes_vX.Y.pdf — manual rev named at RFQ^[1]
• site-id_dno_connection_offer_acceptance.pdf — DNO-issued per site^[5]
• site-id_commissioning_test_certificate_A3-2.pdf — G99 form pack^[3]
• site-id_baseline_reactive_setpoints_v1.xlsx
• site-id_reactive_profile_matching_note_v1.pdf
• site-id_verification_visit_plan.pdf
• site-id_reactive_response_test_report_YYYY-MM-DD.pdf
• site-id_modbus_sunspec_register_map_vX.Y.pdf^[4]
• site-id_dispatch_reference_table_v1.xlsx
• site-id_compliance_pack_index_v1.xlsx
• site-id_oem_liaison_note_YYYY-MM-DD.eml

4. Timeline table

5. Cost table (indicative)

All slots below are labelled indicative. TradVolt does not name fixed prices. Where a slot cannot be sourced from a regulator, DNO schedule or OEM tariff, it is marked UNCERTAIN.

6. DNO & standards evidence

Reactive power requirements for inverters connecting to the GB distribution system are set out in Engineering Recommendation G99 (for installations above the G98 threshold) and Engineering Recommendation G98 (for installations up to and including 16 A per phase).^[2][3] Both documents and the associated G99 commissioning form pack (form A3-2 and related) are hosted by the Energy Networks Association (ENA) — see the ENA engineering document library for the current issue/amendment.^[3] The DNO connection offer is the legally binding per-site document that determines whether the site must operate at unity power factor or follow a DNO-specific Q(P) or cosφ(P) curve.^[5]

The relevant clauses within G99 cover the full operating range of reactive capability and the voltage-related reactive modes. The DNO connection offer is treated on this page as the controlling site-specific document. No specific Type-Test certificate number, G99 issue/amendment number beyond the published scope clauses cited, or named DNO is asserted on this page.

7. FAQ

7.1 Why does reactive power drift after commissioning?

Firmware updates, CT or VT replacement, and aggregator dispatch changes can shift the reactive setpoints away from the DNO-accepted values; this is a commonly observed cause in O&M post-commissioning records (a model assumption for the purposes of this guide).^[3][4] A periodic re-test catches drift before the next witness visit.

7.2 How often should reactive response be re-verified?

Many UK O&M contracts specify an annual re-test or a re-test triggered by any firmware change that touches reactive modes (model assumption / market convention; the binding interval for any specific site is set out in the connection offer).^[5][6] The DNO can also request a re-test if it suspects non-conformance.

7.3 Is G98 or G99 relevant to my site?

G98 applies to installations up to and including 16 A per phase. G99 applies above that threshold. The DNO confirms the applicable engineering recommendation in the connection offer.^[2][3][5]

7.4 Do Chinese-origin inverters ship with GB defaults?

Not necessarily. In our observation, some ship with factory defaults aligned to CN grid conditions rather than GB grid conditions (a model assumption, not a universal rule). The UK commissioning engineer must re-set reactive parameters before first parallel and must record the settings on the commissioning test certificate (form A3-2 or equivalent).^[3]

7.5 Can reactive power be controlled remotely?

Yes, via Modbus or Sunspec registers exposed by the inverter.^[4] The aggregator or DNO SCADA system writes to those registers using the dispatch reference table held by the Responsible Person. For the Sungrow SG125HV family, the register map is documented in the Sungrow SG125HV User Manual (current revision, confirmed at RFQ).^[1]

8. Claim-row index

Every numbered footnote below corresponds to a sup-citation used in body prose.

Request a Quote for reactive-power maintenance support

Evidence-date stamp: 2026-09-22. No specific Type-Test certificate, G99 issue/amendment number beyond the cited scope clauses, or named DNO is asserted on this page. Torque, SOC and DC voltage limits are not applicable to this reactive-power workflow and are not stated here.