An Automatic Voltage Regulator (AVR) requires a droop CT in order to operate in Voltage Droop mode.
Voltage droop is load depended (mostly linear); the more load, the less voltage.
The rating is normally specified by the AVR manufacturer.
Furthermore it's common use for generator manufacturers to lead only one pole winding through the primary side of the CT. Thanks to this a physical smaller CT can be selected.
A generator doesn't need a droop CT if it has to run in island operation only.
For a detailed explanation you just have to read the replies on the thread "Droop Setting for Governors"
Small encore: The Basler *.pdf file is:
parallel_op.pdf
Doc Title: "Parallel Operation with a Network System."
If you post your email, I will email it to you (1,167KB).
One small correction, GOG - a generator will need a droop CT if it has to run in parallel with other generators, whether islanded or not. The droop function is used to set up reactive power sharing between paralleled generators.
CT primary is usually chosen as the next standard rating above generator full load current, secondary to match AVR (1A or 5A). It is normally a metering class CT, placed in the generator B phase (or S, depending on practice)lead.
Suggestion: Regarding the location of CT on a specific phase, one is supposed to use the manufacturer connection/schematic diagram for the AVR since there are also PTs and their connections involved.
A normal configuration is for the AVR to employ the droop circuit for quadrature compensation, where one phase current is converted to a proportional voltage and added vectorially to the sensing voltage across the other two phases. This is usually translated to be the B phase current and the C-A voltage, but it can of course be any combination that has the same phasor relationship.
It goes without saying that the manufacturer's schematic diagram must be consulted in designing the installation.
The CT ratio should on the order of 125% of the full load of the machine. The purpose of the CT is to cause the reactive power to droop off as load increases. This allows the machine to share load with other machines or the utility.
