High current/high voltage/high frequency measurement
2 stage Current Transformer 1000:1

Typical thru-hole torroidal CT's use a burden of 1 Ohm per turn.
The 2nd core has 100 turn and uses 100 Ohms which appears as 10 Ohms burden to the 1st core so no additional R is needed.

The frequency response "can" be quite limited, depending on design and cost.
e.g. one frequency decade  20kHz ~ 200kHz 


  That may contribute to your error.








simulate this circuit &ndash; Schematic created using CircuitLab

This is my attempt to draw a 1 wire CT  insulated from secondary (not shunted as shown)

Since voltage drop on primary ~ 0 the voltage is not amplified.  This is because the transformer is in series , not parallel with the load.

The burden resistor ( here 100 Ohms)  is per Mfg suggested value to create Volts per Amp.