Intrument Transformers

This topic discusses about instrument transformers. We will try to understand what is a instrument transformer and what is basically its purpose in the system. There are 2 types on instrument transformer; one is current transformer (CT) and the other one is potential/ or voltage transformer (PT or VT).

Current Transformer

A current transformer transforms line currents into values suitable for standard protective relays and isolates the relays from line voltages. A current transformer has two windings, designated as primary and secondary, which are insulated from each other. The secondary is wound on an iron core. The primary winding is connected to protective devices, instrument meters, or control devices. The secondary winding supplies a current in direct proportion and at a fixed relationship to the primary current. The various types of primary windings are discussed in the following section.

Common types of construction of CT’s:

• Wound type current transformer – one that has a primary winding consisting of one or more turns mechanically encircling the core or cores. The primary and secondary windings are insulated from each other and from the core(s) and are assembled as an integral structure.
• Bar type current transformers – one that has a fixed, insulated, straight conductor in the form of a bar, rod or tube that is single primary turn passing through the magnetic circuit and is assembled to the secondary, core and winding.
• Window type current transformer – one that has a secondary winding insulated from and permanently assembles on the core, but has no primary winding as an integral part of the structure. Complete or partial insulation is provided for a primary winding in the window through which one or more turns of the line conductor can be threaded to provide the primary winding.
• Bushing type current transformer – one that has an annular core and a secondary winding insulated from and permanently assembled on the core, but has no primary winding or insulation for a primary winding. This type of current transformer is for use with a fully insulated conductor as the primary winding. A bushing type current transformer usually is used in equipment where the primary conductor is a component part of other apparatus.

Voltage Transformer

A voltage transformer (potential transformer) is basically a conventional transformer with primary and secondary windings on a common core. Standard voltage transformers are single-phase units designed and constructed so that the secondary voltage maintains a fixed relationship with primary voltage. The required rated primary voltage of a voltage transformer is determined by the voltage of the system to which it is to be connected and by the way in which it is to be connected. Most voltage transformers are designed to provide 120V at the secondary terminals and when nameplate rated voltage is applied to the primary.

Voltage transformers are capable of continuous and accurate operation when the voltage applied across the primary is within 107% (plus or minus) of rated primary voltage.

Standard accuracy classifications of voltage transformers range from 0.3-1.2, representing ratio corrections to obtain a true ratio. These accuracies are high enough so that any standard transformer will be adequate for protective relaying purposes as long as it is applied within its open-air thermal voltage limits. Standard burdens for voltage transformers with a secondary voltage of 120V are shown below.