Why do high voltages develop across the leads of an open CT secondary?

Current Transformers are transformers that produce proportional current in the secondary. The open-circuit voltage of a CT has absolutely nothing to do with the system voltage of the primary. For a given core secondary winding design, the secondary open-circuit voltage is the same, whether the CT is on a 400V circuit or an 110kV system. The voltage developed on the secondary of an open-circuited CT is a function of the current flowing in the primary and the inductance properties of the secondary windings. If the secondary is open-circuited, that is seen as infinite impedance, so all of the primary current is forced through the magnetizing branch. The voltage seen on the secondary is this primary current times the magnetizing impedance.

INSTRUMENT TRANSFORMERS

Instrument transformers , like conventional transformers, work on the principle of mutual induction. Instrument transformers, unlike conventional transformers: are used for measuring high voltage and current parameters in electrical circuits, are also used for obtaining electrical feed backs to protection devices in the electrical circuits.

There are two types of instrument Transformers.

1.Current transformers ( IS 2705 , IEC 60044-1)

2.voltage transformers (IS 3156, IEC 60044-2)

CURRENT TRANSFORMERS

Current transformers are used to step down high current to very low current that can be handled comfortably.

WHY CURRENT TRANSFORMERS?

• High currents can not be measured directly using ammeters due to limitations of current carrying capacity of the measuring instruments.
• Also, high currents can not be used in protective relays. To allow high current to pass, larger size of conductor is required .
• This leads to practical difficulties in handling the devices
• The practical solution to this difficulty is to step down the current to a very low value that can be as low as 1 amp or 5 amp.
• Unlike a regular transformer, a current transformer steps down only current.
• Voltage developed in the secondary will be negligibly less
• (ie, current in the secondary x resistance of the winding)
• The basic principle of CT functioning is ampere-turn matching
• AT primary = AT secondary