Types of Current Transformer

Instrument Transformer and Power Management (P1) Course
Chapter (6) : Current Transformers
6.4 Types of Current Transformer
       In general, there are two types of current transformers , the wound-type , and the through type.
6.4.1 Wound–type current transformer  Single ratio current transformer 
       The wound type has a separate primary and secondary winding mounted on a laminated iron core as shown in Fig. (1)

       This type of current transformer is designed so that the primary winding consists of one or more turns of heavy wire connected in series in the circuit to be measured as shown in fig. (2). The secondary winding consists of a larger number of turns of relatively smaller wires as shown in fig. (3 ).
       And is connected to instruments or control devices, which are usually designed to operate at five amperes for full scale deflection, Therefore instrument current transformers are usually designed so that, the secondary winding has a current rating of five amperes or one ampere.
       The current ratings of the primary winding of an instrument current transformer are determined by the maximum value of the load current to be measured.

       Assume that the current rating of the primary winding of a current transformer is 250 amperes, and the secondary winding has a current rating of five amperes , This means that the ratio between the primary and secondary currents is  250 to 5),or ( 50 to 1).

       This means that the secondary winding will have 50 times as many turns as the primary winding, if the primary winding has four turns; the secondary winding will has 200 turns.

       Therefore, the ratio of the primary to the secondary currents is inversely proportional to the ratio of primary to secondary turns.

Example :
       Using the 250 to 5 instrument current transformer mentioned previously, to measuring the following current an ammeter would  indicate :
  •  Load current 200 amperes meter indicates four amperes.
  • Load current 150 amperes meter indicates three amperes.
  • Load current 100 amperes meter indicates two amperes.
       The ratio of instrument current transformer will vary depending on the maximum line current it is expected to carry. Ratios may range from one to one to 400 and higher to 1 ampere. Dual ratio current transformer  :
        Wound type current transformer may be constructed so they have a dual ratio, this is done by having two primary windings as shown in fig. (4) Which may be connected in series or parallel by links.

       The lines are connected to terminal l and 4. For the low ratio, the windings are connected in series by connected 2 to 3 but for high ratio, the windings are connected in parallel by connecting 1 to 2 and 3 to 4.
6.4.2 Through type current transformers Bushing current transformer 
        Fig. ( 5 – a ) shows a typical assembly of a bushing current transformers as it would be mounted in a transformer or circuit breaker. This consists of a cylindrical ring core built up of thin iron laminations. Around the core is wound copper wire which forms the secondary winding. The primary winding is formed by the bushing conductor .The relative instantaneous direction of the primary and secondary currents are usually marked on the current transformer coil with colored strip or large dots as shown in figures ( 5 - a) and ( 5 - b).

       The white strips on the secondary coil indicate the polarity of the primary conductor with reference to the secondary terminal. When the current in the primary conductor flows toward the white side of the secondary , current in the secondary coil flows out from the XI terminal. Since the through type current transformer must be capable of operating under over -saturated conditions (short circuits), and their accuracy at lower ratings is poor, most of the bushing transformer are used at high voltages and low currents. Further, they are used in relaying circuits rather than metering circuits.

       Schematic diagram of multi -ratio bushing -type C. T showing method of marking leads and instantaneous direction of current.
  Similarly : X1 –X3 = 120 turns and     XI -  X4  = 160 turns.

        Let us assume that the transformer in fig. ( 6 ) has ratios of 800 / 600 / 400 –5 amperes. Since transformer ratio is proportional to turns and there is only one turn on the primary , the number of turns on the secondary may be calculated.
Ratio = ( Secondary turns / Primary turns) = ( Secondary turns / 1 )
Therefore, XI -X2 = 400 / 5 = 80 turns. Window current transformer :
        If we enclose the current transformer in fig. (5) In a molded case, we obtain another type of current transformer called the window type (fig. 7). Through the core and secondary winding, there is an insulated hole through which the user can place his own conductor.

       This conductor then becomes the primary of the transformer. The ratio given on the nameplate of the transformer is the ratio using one primary turn. This ratio may be decreased by increasing the number of primary turns. The transformer shown in fig.(7) has a ratio of 400 / 5 amperes, If two turns were placed through the hole, the ratio would become 200 / 5 amperes. Bar Primary current transformer :
        This style uses a solid bar permanently mounted incorporated through the hole in the window -type transformer. This transformer as shown in fig. (8) Is known as the bar -primary type, construction of this type is particularly suited to withstand the stresses of heavy over current.
       In order to avoid magnetic stresses that could distort the bus and damage the transformer, care must be taken to properly mount these transformers with respect to adjacent conductors. This type of current transformer is generally used on circuits carrying in excess of 1,000 amperes .

6.5 Classification of current transformers :
       CT's may be classified as dry, oil gas or compound filled for indoor or outdoor use. The application will determine the type to be used.
       Usually, either the dry or compound filled type is used for voltage below 22,000 and either the compound or oil-filled or voltage above 22,000. As the voltage is increased, insulation becomes of greater importance and more complicated equipment results.

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