title: ENA Engineering Recommendation G5/5 - Harmonic Voltage Distortion tags: [harmonics, power-quality, network-connection, ENA, technical-standard, DNO] instrument_row: 106 source_file: sources/energy-networks/ena-er-g5-5.md
ENA ER G5/5 - Harmonic Voltage Distortion
What it is: The GB technical standard governing harmonic distortion limits and the connection of non-linear or resonant plant to distribution and transmission networks. Issued by the Energy Networks Association (ENA). Effective 17 June 2020. Supersedes G5/4-1 (2005).
Full document (free PDF): https://www.dcode.org.uk/assets/uploads/ENA_EREC_G5_Issue_5__2020_.pdf
The core problem it addresses
Connecting modern power electronics (inverters, drives, rectifiers) to the grid introduces harmonic currents - distortions at multiples of the 50 Hz fundamental frequency. If too many such connections are made without coordination, cumulative harmonic voltage distortion can exceed limits that damage equipment and interfere with metering and protection systems. G5/5 is the mechanism by which network operators and connecting parties manage that cumulative risk.
Three-stage assessment structure
| Stage | Applies to | Method |
|---|---|---|
| Stage 1 | LV connections | Linear substage pass/fail; measurements required from substage 1D |
| Stage 2 | Below 33 kV (or Stage 1 failures) | Screening (2A), refined measurement (2B), full harmonic load flow (2C) |
| Stage 3 | 33 kV and above | Headroom allocation against planning levels; DNO must issue formal harmonic specification first |
The decision to approve connection rests with the network operator. In practice, if a connection passes G5/5 (with or without mitigation), it is usually approved.
Key provisions
- Planning and compatibility levels set for harmonics up to the 100th order (5,000 Hz)
- Some individual harmonic order limits increased versus G5/4; total harmonic distortion (THD) limits unchanged
- Interharmonic distortion now defined and limited, aligned to IEC 61000 series
- Stage 3 changed from first-come-first-served to allocation from available harmonic headroom
- Network operators must produce formal harmonic specifications for HV connections
- Connecting parties must submit compliance reports with standardised content
Practical relevance
G5/5 is a gating requirement for a wide range of GB energy projects: - Grid-scale solar, wind, and battery storage (all use power electronics) - Large industrial loads and data centres connecting at MV or HV - EV charging hubs at MV level - Any generation or load connecting at 33 kV and above triggers a Stage 3 assessment, which requires the DNO to produce a harmonic specification - a process that adds time and complexity to connection applications
Relationship to other instruments
- G5/5 sits alongside ER G98 (row 107) and ER G99 (row 108), which govern the technical and protection requirements for generation connections at Type A (small) and Type B/C/D (larger) scale respectively
- G5/5 compliance is typically a condition of the connection agreement issued under those recommendations
- The harmonic planning levels in G5/5 feed into distribution licence conditions and connection offer terms set by DNOs
Version history
| Version | Year | Key change |
|---|---|---|
| G5/4 | 2001 | First major version |
| G5/4-1 | 2005 | Amendment adding Appendix B |
| G5/5 | 2020 | Full rewrite - extended to 100th order, Stage 3 headroom allocation, interharmonics added |