How do you measure 11kv voltage?

By electrostatic moving iron type voltmeter, you can measure upto about 200kV directly.

In substations, high voltages, such as 11 kV, are measured using a relatively small step-down voltage transformer (UK terminology) or potential transformer (US terminology).

The secondary voltage of a voltage transformer is typically standardised at 110 V, which then matches the full-scale deflection of the voltmeter to which it is connected. So a typical transformation ration would be 11 kV/110 V, 33 kV/110 V, 132 kV/110 V, etc.

Voltage Transformers, together with current transformers, are classified as 'instrument transformers', whose outputs are used for measurements or for driving protective relays. They also have the important function of being able to electrically isolate their secondary (measurement) circuit from their high-voltage primary circuit.