Protection Transformers

Introduction to Protection Transformers

Protection 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 operation.

Prot CT

Protection CTs from ITL

Our protection current transformers are available in 5P or 10P accuracy class for each transformer ratio. This makes them usable in a number of applications ranging from magnetic trips or thermal overloads to IDMT overcurrent or earth fault relays and differential protection. For specific applications we offer special 'Class PX' Current transformer protection class.

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If you require more information on Protection class, please refer to our guide -> PDF

To help explain some of the terminology we use for our range of current transformers, we have produced a terminology section for your assistance.

Defining an instrument transformer

What is 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.

What is the aim of an instrument transformer

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.

pdfProtection Class - explained (575 KB) Download