Introduction to Measurement
Measuring instruments, such as ammeters, voltmeters,
kilowatt-hour meters, etc , whether electromechanical or
electronic, meet insuperable design problems if faced with the high
voltages or high currents commonly used in power
Current transformers are therefore used with the measuring
instruments to: (a) Isolate the instruments from the power
circuits. (b) Standardise the instruments, usually at 5 amps or 1
amp. The scale of the instrument (according to the C T ratio), then
becomes the only non-standard feature of the instrument.
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.