Higher values of currents and voltages in electrical transmission and distribution systems cannot be measured by ordinary voltmeters and ammeters. Such high values would damage those measuring devices. Instrument transformers transform these high values of current and voltages to low values that can be measured by the low voltage metering devices. The combination of current transformers (CT) and voltage transformers (VT) constitutes an instrument transformer.
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| Fig 1. Different current transformers and its basic symbolical representation. |
A current transformer is an alternating current sensing device. It has a core, a primary winding, and a secondary winding. The primary winding is connected in series with the power line whose current to be measured. The secondary winding can be connected to an ammeter.
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| Fig 2. Parts of a CT |
As shown in Fig 2, the secondary winding is wounded on a ring type hollow core which is made up of steel. The power line whose current to be measured is passed through the ring. The conductor passed through the ring is treated as the primary winding of the CT (single turn primary). Assume 'N' as the number of turns on the secondary. Larger the current flowing through the primary, more the number of turns on the secondary.
Let us assume 'Ip' as the high current flowing through the power line; namely the primary current. When this current starts flowing a magnetic flux is developed in the core. This magnetic flux, in turn, induces an alternating current in the secondary winding. The current in the primary winding divided by the number of turns on the secondary gives the current that is measured by the ammeter (secondary current, 'Is').
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| Fig 3. Connection of CTs for measuring the currents of power lines |
BURDEN OF A CURRENT TRANSFORMER
The burden of CT is defined as the load connected at the secondary of a CT. It can be the ammeter or a relay etc. Mathematically it is the product of voltage and current at the secondary of a CT. It is expressed in Volt-Amperes (VA).
Why the secondary of a transformer should always be short circuited?
Unless a burden is connected to the CT, the secondary terminals should always be short-circuited. This is because when a high current passes through the primary, a very high voltage is likely to generate at the secondary due to its larger number of turns. This high voltage with a range of kilovolts can damage the CT. Once The secondary is short circuited or connected to a burden, the high voltage across the secondary can be nullified.
How does a CT saturate?
Every CT has a rated burden or load. When the load exceeds its burden capacity, the core of the CT will get saturated. Due to this saturation, the magnetic property of the core will be lost. Thereby the current measured by the CT will not be the accurate value.
