Curtailment and DNO export limitation

Solar or battery output can be held below the equipment’s available power for several different reasons. The monitoring graph may look similar in each case, but the cause, responsibility and remedy are not the same.

The main possibilities are:

• a DNO-agreed export limit, normally controlled under G100
• an inverter disconnecting on high or low voltage under its connection protection
• a manufacturer or site-control function reducing output before a protection trip
• the mandatory reduction in active power when grid frequency is high
• ordinary inverter clipping, which is a design limit rather than network curtailment

Do not diagnose the cause from a flat or interrupted generation graph alone. The export reading at the connection point, the inverter event log and measured voltage are more useful than the shape of one app chart.

G100 limitation controls net export

A G100 Customer Limitation Scheme measures current at the property’s connection point and controls selected generation, storage or loads. Its job is to keep the net flow within the Maximum Export Limit agreed with the DNO.

Net export is generation and battery discharge minus demand in the property. If solar is producing 5kW and the home is using 2kW, export is 3kW. A 3kW export limit would not need to reduce generation in that moment. If household demand then fell to 1kW, the scheme would need to control another 1kW unless a battery or other managed load absorbed it.

This cap is independent of voltage. It can operate while voltage is normal, and a voltage problem can occur on a system with no G100 export limit. The agreed limit and protected settings should appear in the DNO and commissioning records. The owner must not raise or disable them without DNO agreement.

The statutory supply range is not an inverter trip setting

Regulation 27 of the Electricity Safety, Quality and Continuity Regulations sets the declared low-voltage supply at 230V between phase and neutral, with permitted variation of 10% above and 6% below. This gives an upper value of 253V and a lower value of 216.2V at the supply terminals.

Those figures describe supply quality. They are not the current G99 inverter protection thresholds. The regulation itself also does not set out the ten-minute averaging method sometimes quoted alongside 253V.

National Grid Electricity Distribution publishes its own customer guidance for its licence areas. It describes an out-of-limits supply as one with a ten-minute mean above 253V during a seven-day measurement period, and says it will seek to resolve a confirmed problem at no cost to the customer. That is useful evidence of one DNO’s published process, not wording that should be attributed to regulation 27 or assumed to be every DNO’s identical investigation method.

Why the inverter can see a higher voltage

Export current flowing through the impedance of the installation and local network produces voltage rise. The inverter terminals may therefore be at a higher voltage than the property’s supply terminals while the system is exporting.

The rise tends to increase with export current and circuit impedance. Long or undersized AC cable runs, poor terminations and a weak local network can all contribute. Neighbouring generation can also lift the voltage on a local feeder. The ENA’s distributed generation guide identifies voltage rise, thermal limits, reverse power flow and power quality as network issues that a DNO may have to assess.

This distinction matters because the remedy depends on where the excessive rise occurs. A large difference between the inverter and supply terminals points the installer towards the customer’s AC circuit. A confirmed supply-terminal problem belongs with the DNO.

Current G99 voltage protection

For a Type A generating module connected to a 230V low-voltage network, G99 Issue 2 gives these overvoltage protection settings:

• stage 1: 262.2V with a 1.0-second delay
• stage 2: 273.7V with a 0.5-second delay

An inverter operating under those settings should not be described as tripping merely because an instantaneous display passes 253V. Its event log, actual protection settings and measured voltage still need checking. Legacy equipment, a site-specific protection arrangement or a different connection category may not behave in exactly the same way.

Some equipment or site controllers can reduce active power before a protection trip. The trigger and control curve are product- and design-specific. G99’s voltage protection figures do not by themselves prove that every domestic inverter must follow a universal voltage-versus-power reduction curve.

Frequency response is a separate function

G99 requires generating modules to reduce active power when system frequency rises above 50.4Hz. This Limited Frequency Sensitive Mode Overfrequency response is a grid-support function, not evidence of an inverter fault.

It is normally straightforward to separate from an export cap or voltage event because the inverter log should record frequency, and the reduction follows system frequency rather than the property’s export level.

Clipping is not network curtailment

Clipping occurs when the available DC power from the solar array is greater than the inverter can convert to AC. Output then reaches the inverter’s AC rating and stays there. It can happen with normal grid voltage and no export limit.

A flat top at the inverter’s rated AC output suggests clipping. A flat net-export line at the DNO-agreed limit, while household demand and generation move around it, suggests export limitation. Neither pattern is conclusive without the underlying power-flow and event data.

A practical diagnosis sequence

Start by finding the DNO acceptance and commissioning documents. Confirm the Maximum Export Limit, the equipment included in the limitation scheme and the approved settings.

Then compare data from the same period:

1. Record solar production, battery charge or discharge, household demand and net grid flow.
2. Export the inverter’s voltage, frequency and event logs rather than relying on a daily graph.
3. Check whether net export is being held close to the agreed limit while demand changes.
4. Check whether reductions line up with overvoltage, frequency or protection events.
5. Have a competent installer measure voltage at the inverter and at the supply terminals under representative export conditions.
6. Check the inverter’s AC circuit, cable size, terminations, meter or current-transformer position and commissioned settings.

A battery can reduce lost solar only while it has charge power and empty capacity available. Once it is full, any continuing surplus still has to be exported, diverted or reduced. It is therefore not a guaranteed cure for an export cap or a voltage problem.

Do not alter G99 protection or G100 export settings as a diagnostic test. Incorrect settings can breach the connection agreement and remove required network protection. If the customer’s wiring and settings are sound but the supply-terminal voltage is outside the applicable limits, give the DNO the dated measurements and event records. If the aim is to increase an agreed export limit, submit a connection-variation application rather than changing the limiter.

Related entries

• G100 export limitation
• Export limitation with batteries
• Solar export limitation
• Inverter clipping
• G99 application pathway