Distributed generation can reverse feeder voltage gradients and make a transformer OLTC react to conditions it was not originally designed to regulate. Load-flow studies should include high-load/low-generation and low-load/high-generation cases, relevant reactive-power controls and tap limits. The goal is not merely keeping one bus in band but avoiding unacceptable voltage elsewhere and unnecessary tap activity.
Study independent demand and generation extremes
Annual peak demand does not identify the most difficult export-voltage case. Include low demand with high generation, high demand with low generation, and the credible upstream-voltage range. Retain the inverter's reactive-power capability at the relevant active-power output rather than assuming unrestricted kvar support. Identify which nodes experience voltage rise or drop and whether the OLTC has usable taps remaining. A satisfactory average operating point can conceal a limit reached for only a few hours.
Coordinate transformer and inverter voltage objectives
An inverter responding rapidly to local voltage can alter the reactive current that the transformer controller observes. The OLTC may then respond on a different timescale and shift the inverter's operating point again. Model the actual control functions, targets, limits and delays, including any fixed-power-factor mode. Do not assume that two devices each described as voltage-regulating will cooperate automatically. The design should assign their respective roles and test the combined response under changes in generation and feeder topology.
Use time series to estimate tap activity
A set of static load flows finds limiting voltage states but does not determine the number of tap operations. Apply a representative sequence of demand, generation and source voltage with the controller's deadband and timing behavior. Report voltage excursions, time at tap limits and operation counts, explaining the period represented. These are planning estimates, not a guaranteed maintenance interval. Retain the assumptions so a later increase in generation can be assessed against the same baseline.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEC 60214-1:2014 — Tap-changers, Performance requirements and test methods.
[3] IEEE C57.131-2024 — Tap-changers, Performance requirements and test methods.