When to use OLTC? 1

The capacity of a long transmission line may be limited by the ability to compensate for line voltage drops under load.

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Ohm's law
Not just a good idea;
It's the LAW!

The need for a tap changer depends on the voltage variations required on the secondary side, the voltage fluctuations expected on the primary side, and the characteristics of the grid. The purpose of a tap changer is to keep the voltage at the consumer (load) end constant.

The secondary terminal voltage will drop significantly if the transformer impedance is high and the load power factor is relatively low. The incoming voltage will fluctuate with load if the grid is weak, small, and less interconnected.

With a solid interconnected grid in India, we are now avoiding on-load tap-changers in 765/400 kV and 400/220 kV auto-transformers. Tap changing on the HV side has no effect, as the incoming grid voltage is constant. Line-end OLTC is a weak link in auto-transformers, as the tap-changer will be at high potential. The secondary voltage dip, if any, is taken care of by the downstream transformer primary neutral-mounted OLTC.

DETC is used when individual tap changing in a large population of units is complex (e.g. Distribution transformers) or when tap changing is required very rarely ( large GSUs).
The above are views from a transformer manufacturer's angle.

Where are you? North America or IEC territory? What is the primary and secondary voltage?

If this were in NA, for a step-down from HV to MV in a distribution substation, some form of automatic voltage regulation would normally be used. It could be OLTC on the power transformer, a separate bus regulator, or regulators on each MV feeder.

Is this for a substation transformer or a generating station transformer?
- in substations the OLTC is common in order to regulate the low voltage system, particularly distribution substations (at least at my company). Almost every distribution substation has a OLTC or a separate regulator to regulate the distribution sub voltage and ensure customers are getting good voltage at all load levels.
- In generating stations, At least in the US OLTCs are rarely applied to conventional machines and the de-energized tap changer is used. The generators perform the voltage control on the low voltage side.
- in older type I and type II wind farms an OLTC was used to regulate the voltage on the 34.5kV bus as the generators did not provide voltage control.
- for new inverter based distributed generation, it’s common to specify an OLTC and I have no idea why. The IBR plants all perform voltage control like a synchronous machine. At our current IBR wind farm the OLTC is in manual and the taps are never changed (and there’s no issues operationally) so it seems pointless.

I was searching for OLTC topics as I'm currently looking at a inverter based project where we didn't show an OLTC but the client insists that they want it. From speaking to a few people at work, and from what wcaseyharman said above, it sounds like the inverters should be able to handle the voltage control. I was also told that the inverters and the OLTC could potentially fight with each other. Not a lot of inverter based generation OLTC info that I found online so any other insight on this would be much appreciated.

A Cigre report found that 41% of transformer failures were due to OLTC. If you don't need it, don't spend the extra money or add extra risk.

If the OLTCs are used at the supply end to compensate for downstream line loss, they will be controlled by current rather than voltage.
As the current increases, the tap changer increases the voltage to compensate for downstream voltage drop.
You may be able to do this with inverters by using a current derived signal to vary the output voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Most of the reported OLTC failures were in auto-transformers where the line end tap-changer is at a high voltage of 132 or 220 kV. OLTC is now being eliminated in India in 400 kV and 765 kV auto-transformers.