# Testing and Commissioning Testing and commissioning from string level to energisation and acceptance. Source: https://crestwatt.com/services/testing-commissioning/ Publisher: CrestWatt — Electrical contracting for utility-scale solar, storage and grid connections. Contact: info@crestwatt.com Language: en-GB Commissioning on a utility-scale plant is a programme of several thousand individual tests, each with a method, an acceptance criterion and a record. CrestWatt runs commissioning as a primary discipline, with dedicated test teams, calibrated instruments and a records system that compiles the acceptance pack as the work proceeds. **Standards and methods cited:** IEC 62446-1, BS 7671, IEEE 400.2, ENA EREC G99, IEC 61850 ## DC commissioning to IEC 62446-1 ### Category 1 tests, every string - Continuity of protective earthing and equipotential bonding - Polarity - String open-circuit voltage and short-circuit current, corrected for irradiance and temperature - Insulation resistance of DC circuits - Functional test of switchgear, isolation and monitoring ### Category 2 tests, where specified - IV curve tracing, compared against expected curve - Infrared thermographic survey of modules, connectors and combiner boxes under load ### Combiner box and string monitoring - Fuse and disconnector verification - String monitor channel mapping and communications verification against the SCADA register map ### Inverter and transformer station - Pre-energisation checks to OEM procedure - Energisation support and functional verification ## AC, LV and HV commissioning - LV distribution inspection and testing to BS 7671, with certification - Transformer pre-energisation checks: IR, ratio, vector group, winding resistance, Buchholz and protection device function - HV cable testing: IR, VLF withstand to IEEE 400.2, sheath test, phasing - Switchgear: IR, contact resistance (micro-ohmmeter), circuit breaker timing and travel, trip and close coil operation, interlock verification, HV pressure test where specified - Earthing: earth electrode resistance, continuity, and step and touch potential measurement where specified - Protection: secondary injection, CT ratio and polarity by primary injection, CT magnetisation curve, scheme functional and stability tests - DNO and owner’s engineer witness testing ## Communications and control - Fibre optic OTDR and insertion loss certification, per link - Structured cabling certification, per link - Modbus loop testing, addressing and end to end communications verification - SCADA point to point verification from field device to HMI, with signal list sign-off - Power plant controller functional testing against G99 requirements, including active and reactive power control, voltage and frequency response, and ramp rate - Monitoring platform data validation ## Close-out - Snag identification, classification and tracking to closure - Test record compilation with instrument calibration certificates - Handover documentation structured to the client’s FAC checklist from the outset - Final Acceptance Certificate support and witness ## Programme management Test plan, record format and witness schedule agreed before mobilisation. Weekly progress report against the test plan with a live snag register. All issues raised in writing on the day identified. ## Recent scope - 72 MW solar PV: data cable testing and termination across 13 transformer stations and 213 string combiner boxes, to the client’s splicing plan and SCADA drawings - 40 MW solar farm (54 MWp): fibre optic and CAT7 testing and certification; witness testing of earthing, inverter, LV and transformer station works ## Next step Send the scope and the programme. We will confirm the test team and the earliest mobilisation date. ## Common questions **What does IEC 62446-1 Category 1 testing cover?** Every string is tested for continuity of protective earthing and equipotential bonding, polarity, open-circuit voltage and short-circuit current corrected for irradiance and temperature, and insulation resistance of the DC circuits, together with a functional test of switchgear, isolation and monitoring. **What is the difference between Category 1 and Category 2 PV tests?** Category 1 tests are applied to every string as the baseline commissioning set. Category 2 tests — IV curve tracing against the expected curve, and infrared thermographic survey of modules, connectors and combiner boxes under load — are carried out where the client or owner’s engineer specifies them. **Does the scope include protection and HV commissioning?** Yes. Transformer pre-energisation checks, HV cable testing, switchgear contact resistance and timing, earthing measurements, and protection commissioning by secondary injection, primary injection for CT ratio and polarity, CT magnetisation and scheme stability testing, with DNO and owner’s engineer witness. **How is the handover pack produced?** Records are compiled continuously against the client’s FAC checklist as the work proceeds, with instrument calibration certificates attached and a live snag register tracked to closure, rather than assembled after completion. ## Related scopes - [Cable Termination and Testing](https://crestwatt.com/services/cable-termination-testing/): LV, MV and HV terminations to 132 kV; DC crimping and MC4 assembly; fibre fusion splicing; Modbus RS-485 termination. IR, VLF, sheath, TDR, OTDR and certification. - [Protection, Control and Switchgear](https://crestwatt.com/services/protection-control-switchgear/): Relay, CT and VT supply; panel design, build and wiring; IEC 61850 configuration; settings; secondary and primary injection; site acceptance. - [Electrical Site Supervision and QA](https://crestwatt.com/services/site-supervision-qa/): Embedded electrical supervisors and QA engineers providing inspection, witness testing and structured digital records.