Transformer loading and solar headroom

Free tool for plant electrical engineers

Transformer loading and solar headroom calculator

Enter your transformer rating and present maximum demand, and the solar or storage you plan to add. See the loading today, the full-load current and incomer rating at 415 V, how much solar the transformer can take without reverse-flow problems, and whether an upgrade is needed. Runs in your browser; nothing is uploaded.

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Your numbers

Fill in the required figures to begin.

What limits solar on an existing transformer

Loading

Loading is maximum demand divided by rating. Above about 80 per cent there is little room for growth, and above 100 per cent the transformer is being overloaded for part of the day. Storage that shaves the peak is one way to buy headroom without a new transformer.

Reverse flow

At midday solar output minus your minimum load flows back through the transformer towards the grid. That reverse flow must stay within the transformer rating and within what the DISCOM allows for your connection. If the plant is designed for zero export, the EMS curtails the inverters instead, and the reverse-flow figure tells you how much curtailment to expect.

Incomer rating

Full-load current at 415 V is rating divided by root three times voltage. The incomer air circuit breaker is the next standard rating above it. Adding solar on the same bus does not raise the incomer current, because solar reduces what the incomer carries, but the bus and its short-circuit rating still have to be checked for the added infeed.

Can I add solar to a transformer that is already at 90 per cent?

Yes, because solar reduces the transformer loading in daylight rather than adding to it. The checks are reverse flow at minimum load and the short-circuit contribution of the inverters on the bus, both of which the feasibility study covers.

What does the DISCOM allow for reverse flow?

It depends on the connection agreement and the current GERC order. We design for self-consumption with a zero-export relay unless export is explicitly permitted, so reverse flow at the meter is controlled.