Introduction to Protection
Protective Current Transformers are designed to measure the
actual currents in power systems and to produce proportional
currents in their secondary windings which are isolated from the
main power circuit. These replica currents are used as inputs
to protective relays which will automatically isolate part of a
power circuit In the event of an abnormal or fault condition
therein, yet permit other parts of the plant to continue in
To help explain some of the terminology we use for our range of
current transformers, we have produced a terminology section for your
Defining an instrument transformer
What is an instrument transformer
What is the aim of an instrument transformer
An instrument transformer is an electrical device intended to supply measuring instruments such as meters, relays and other similar apparatus.
There are two types of instrument transformer:
- Current Transformer ( CT ), in which the secondary current is under normal working conditions, practically proportional to the primary current and phase shifted from it by an angle close to zero in the appropriate direction for connections.
- Voltage Transformer ( VT ) also known as a Potential Transformer ( PT ). In which the secondary voltage is under normal operating conditions, practically proportional to the primary voltage and phase shifted from it by an angle close to zero in the appropriate direction for connections.
The basic purpose of an instrument transformer is to reduce the voltage and current of an electrical network to a standardised, non hazardous level.
They prevent any direct connection between instruments and high voltage circiuts which would be dangerous to operators and would need instrument panels with special insulation. They also do away with the need for expensive special instruments when high currents and voltages have to be measured.