title: ENA Engineering Recommendation P28 - Voltage Fluctuations (Flicker) tags: [power-quality, flicker, voltage-fluctuations, DNO, distribution, transmission, ENA, technical-standard, wind, arc-furnace, EV-charging, BESS, p28] instrument_row: 111 source_file: sources/energy-networks/ena-er-p28.md

ENA ER P28 - Voltage Fluctuations (Flicker)

What it is: The GB technical standard setting limits for voltage fluctuations (flicker) and rapid voltage changes (RVCs) caused by loads and generators connecting to distribution networks and transmission systems. Issued by the Energy Networks Association (ENA). Issue 2 effective 23 May 2019.

Full document (free PDF - Issue 2): https://dcode.org.uk/assets/files/Qualifying%20Standards/ENA_EREC_P28_Issue%202_(2019).pdf

The core problem it addresses

When equipment causes rapid, repetitive voltage changes on the network, customers connected nearby experience visible fluctuation in their lighting - this is "flicker". At high severity it is uncomfortable and can damage sensitive equipment. Without a common standard, there would be no consistent basis for DNOs to assess whether a new arc furnace, wind farm, or EV charging hub would create unacceptable conditions for existing customers.

P28 sets planning limits (the total flicker a network node may carry) and emission limits (the share allocated to each new connection). It defines the assessment methods for different equipment types. DNOs use it as the basis for connection conditions and, where necessary, requirements for mitigation equipment such as a STATCOM or Static VAR Compensator.

Who it affects

P28 is required reading for anyone developing or connecting:

• Wind turbines: blade passing and tower shadow create periodic voltage fluctuations. P28 flicker assessment (using IEC 61400-21 manufacturer coefficients) is almost always required for a G99 connection application.
• Arc furnaces and welding equipment: the original motivation for P28. Large steel plant creates highly irregular flicker. Connection conditions typically require a STATCOM.
• Large motor loads: frequent-start compressors, crushers, and pump stations cause voltage steps on weak rural networks.
• EV rapid charging: high-power chargers and aggregated smart charging can cause rapid voltage changes on LV networks. P28 Issue 2's RVC limits increasingly appear in DNO EV guidance.
• BESS: battery systems cycling rapidly for frequency response can cause repeated voltage steps. Northern Powergrid NIA research (2024) found most BESS configurations remain within P28 limits but high-cycle frequency services require monitoring.

Two categories of disturbance

Flicker: Continuous rapid voltage variations, measured using the IEC 61000-4-15 flickermeter standard. Quantified as: - Pst: short-term severity (10-minute window) - Plt: long-term severity (2-hour window, cube root mean of 12 Pst values)

Rapid Voltage Change (RVC): A discrete voltage step event from a single switching action. Issue 2 introduced explicit RVC limits alongside flicker limits. The key threshold is the maximum permissible d% voltage change per event.

Planning levels

P28 Issue 2 aligns with IEC 61000-3-7. Limits tighten at higher voltage levels (transmission) and relax at lower voltage (LV), reflecting different sensitivity and aggregation effects.

Individual emission limits for a new connection are a fraction of these, calculated from the ratio of the installation's agreed capacity to the available fault level at the point of common coupling.

Issue 1 to Issue 2: what changed

Issue 1 (1989) covered only loads on distribution networks. Issue 2 (2019) was a full technical revision: - Extended to transmission system connections - Added generator connections (wind turbines, reciprocating engines) as disturbing equipment - Introduced Rapid Voltage Changes as a separate category - Aligned planning levels with IEC 61000-3-7 - Added annexes for equipment-specific assessment

Relationship to other instruments