Recently, I’ve found an instrument transformer through internet, and in my word it is excellent. Now, I’m sharing the features of this transformer here and obviously on my own way as well as based on my study about it.

It is a combined instrument transformer (Type- JUK 123a), consists of two parts: current element and voltage element placed in one hermetic housing, filled with transformer oil. The JUK 123a has been designed for revenue metering, control, indication and relay protection in almost all climatic conditions. It conforms to IEC 60044-3 standard but can be type tested according to other international standards to meet specific requirements.


The JUK 123a serves for measurement and circuit protection of high voltage line substations with the highest voltage for equipment of 123 kV at 50 Hz frequency, in effectively earthed and compensated systems. The device is for outdoor application, with ambient temperatures from – 40oC to + 40oC, humidity up to 100% at altitudes up to 1000 m above the sea level.

Technical data

Current transformer

– Rated primary currents: from 50 A up to 3000A

– Rated secondary currents: either 1 A or 5 A. 1 A is especially chosen for low measuring and protection burdens; it also gives an overall lower burden requirements through lower cable burden

– Rated 1s thermal current: from 20 kA to 63 kA

– Rated continuous thermal current: we offer extension up to 200% for 1 A and up to 120% for 5 A

– Reconnection: 1 to 2 or 1 to 2 to 4

– Burden and accuracy classes: normally for measuring cores we offer 0.1, 0.2 or 0.5 and for protection cores 5P or 10P. Other classes are quoted on request.

Voltage transformer

– Rated primary voltage: 110:√3 kV

– Rated secondary voltage: 100:√3 V or 110:√3 V

– voltage factor and time: generally according to IEC standard; 1.2 Un continuously or 1.5 Un/30 s for systems with effective earthing with automatic fault tripping and 1.9 Un/8 h for systems with insulated neutral point without automatic earth fault system.

– Accuracy classes: 0.1 or 0.1 and 3P; 0.2 or 0.2 and 3P; 0.5 or 0.5 and 3P for measuring windings and 3P or 6P for residual windings.

Current part of JUK123a

The current transformer is placed in the head (head-type). The magnetic cores and secondary windings are housed in the protective aluminum cover supported by an aluminum tube. Inside the tube there are finishing leads of the secondary windings. The cover and tube are insulated with an insulating paper impregnated with transformer oil. The distribution of electric field is controlled by metal shields. The magnetic cores, secondary windings and the protective cover are fixed inside the head of transformer. Inside the head, the external metal shield is connected to the primary terminal. The aluminum cover and the aluminum tube are connected to the earthing terminal inside the terminal box. The primary current winding is running through the hole in the protective cover to the insulated terminals. The finishing leads of the secondary winding and the internal shield are running to the terminal box.

Voltage part of JUK123a

The primary, secondary and residual windings are coiled on the coiled and cut jacket-type magnetic core. The multi-layer primary winding and the finishing lead of the primary winding are insulated with an insulating paper impregnated with transformer oil. The distribution of electric field is controlled by metal shields. The core and the windings are housed in the bottom tank. The high voltage terminal of the primary winding (A) is connected to the P2 terminal of the current part inside the head. The earthing terminal (N) of the primary winding, external shield, the finishing leads of the secondary and additional winding are running to the terminal box.

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