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Why Temperature Matters in G99 Reactive Power Capability Studies

One of the most overlooked risks in G99 studies is the effect of temperature on both inverter capability and transformer rating.

PQ capability at 30 degrees Celsius and 50 degrees Celsius compared against the reactive power requirement

In many projects, reactive power capability is assessed primarily against the grid code requirement at the connection point, with the focus naturally placed on demonstrating compliance. However, what is sometimes underestimated during early project development is how strongly ambient temperature can influence the actual capability available from the plant. A recent G99 reactive power capability assessment highlighted that ambient temperature is not simply a background assumption. It can have a direct impact on both the reactive power capability of the inverters and the thermal rating of key plant equipment, particularly inverter transformers.

This is important because reactive power capability is not determined by one item of equipment in isolation. It depends on how the overall plant performs as a system, including the inverter capability, transformer thermal rating, internal network losses, voltage profile, and the margin available within the design. Where temperature has not been considered properly, a site may appear to have sufficient reactive capability on paper, but in practice the available headroom can reduce under more onerous operating conditions.

High ambient temperature can reduce the reactive power capability of the inverters, depending on the manufacturer data, while also reducing the thermal rating of key equipment such as inverter transformers. In practice, this can constrain the overall site PQ capability and, depending on the available design margins, may affect compliance, export capability, or the wider electrical design of the plant. An example illustrating the impact of temperature on inverter reactive power capability is shown in the figure below.

Why identifying this early matters

The benefit of identifying this early is that the project team still has practical options available. Depending on the project, mitigation could include:

  • selecting equipment with better thermal performance
  • increasing inverter numbers
  • increasing transformer MVA rating or adopting enhanced cooling arrangements
  • reviewing tap position strategy and site voltage profile early
  • reducing internal electrical losses so that less reactive capability is lost before the connection point
  • adding reactive compensation equipment where appropriate
  • in some cases, reassessing the TEC/export capacity of the plant

These are the kinds of decisions that are far better considered during project development, when there is still flexibility in the design, rather than later when procurement and detailed design have already progressed.

More than a compliance exercise

At RenSolv, we see G99 load flow and reactive power compliance studies as more than a compliance exercise. When carried out properly and early enough, they can identify technical limitations that may otherwise only become visible later in the design, when changes are more difficult and more costly to implement.

If you are developing a solar, BESS, or co-located project and would like to discuss reactive power capability, plant thermal limitations, or wider G99 compliance studies, please get in touch.