Determining the thermal limits for transformers is more complicated. The rating of a transformer is generally given in kVA or MVA, but there may be three different figures quoted, corresponding to natural (convective) cooling, fan cooling and cooling by pumped oil. Furthermore, transformers have long thermal time constants and thus can be overloaded for short periods of time without causing overheating or significant damage. Calculation of the damaging effect of such transient temperatures can be difficult.

Transformers are generally chosen to match the expected maximum demand and are normally operated at a significant proportion of their thermal limit, partly because standing (no load) losses are significant and so the efficiency of a lightly loaded transformer is poor. Thus, in areas with very high penetrations of embedded generation, thermal limits of transformers can restrict further installation. In round figures, this only occurs when the rating of an embedded generator connected below a particular transformer, less the minimum load in the same area, exceeds the maximum load in that area.

Most transformers can accommodate reverse power flow, up to the normal forward rating, without a problem. However, there are some (not many) on-load tap-changers that have very limited reverse-power capability. Furthermore, some automatic voltage regulators (control relays) associated with on-load tap-changers can be affected by reverse power flow.

Thus, the thermal limits of existing transformers are not normally a limiting factor in the installation of distributed generators, unless the installed capacity of generators exceeds the maximum demand in the area served. Such high penetrations are not common, at least in the UK, although there is one example in Wales where two wind farms are connected below one substation. Power flow through this substation is frequently reversed and occasionally generation has to be curtailed when the reverse power flow reaches the transformers ' thermal limits.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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