Current Transformer.

Experiment No.: -14

Title of the Experiment:

            To perform the testing of polarity and magnetizing characteristics of current transformer.

Theory:

CT provides the isolation between protective relays from the high-voltage power system. It also provides the scaled version of high current (1/5A) in the secondary winding. It is used for metering and protection applications.

Polarity:

 The relative polarities of CT primary and secondary terminals are identified either by painted polarity marks or symbols. P1 and P2 for the primary terminals. S1 and S2 for the secondary terminals.  Its significance is in showing the direction of current flow relative to another current or to a voltage as well as to aid in making proper connections.

Figure 1. Polarity markings of CT.

CT magnetizing characteristic:

CT is used for metering as well as protection. A metering CT produces proportioned current till full load current and saturates during fault thereby, thereby preventing meter damage. On the other hand, a protective CT is designed to faithfully reproduce the largest fault current. The protective CT is designed to operate much below the knee point so that it maintains its transformation ratio during high-magnitude short-circuit currents.

When the primary current goes on increasing, secondary induced voltage increases and demands high flux which push CT to enter saturation region. Further, CT draws a higher magnetizing current requirement as given in non-linear nature of B-H curve. Any further increase in flux demand causes a disproportionately large increase in the magnetizing current requirement of the CT. Due to the flatness of the excitation curve in this region, a very large amount of exciting current is demanded by the CT. The waveform of the exciting current becomes distorted and is, in fact, very peaky in nature. The secondary voltage consists of only sharp pulses near the zero-crossings of the primary current. Generally, CT saturation is specified in terms of knee point voltage and corresponding magnetizing current. The knee point is defined as that point on the excitation characteristic at which a 10% increase in excitation voltage results in a 50% increase in the excitation current.

Figure 2. CT magnetizing characteristics.

CIRCUIT DIAGRAM:

Figure 3. Experimental setup for over-current relay testing.

                                               

PRECAUTION:

  1. Main supply is in OFF condition.
  2. Make sure that varic is in initial condition.

PROCEDURE:

Polarity test:

  1. Apply the impulse DC voltage at primary winding of CT
  2. Observe the needle movement of analog voltmeter that connected at CT secondary

Magnetizing characteristic test:

  1. Make the connection as per the diagram.
  2. Vary the variac from initial position in steps of 50 volt till 150 V and then vary the voltage in steps of 10 volt till getting 50% increment of current while increasing 10% voltage (It’s known as Knee Point Voltage).
  3. Observe the current and voltage from ammeter and voltmeter, respectively.
  4. Draw the magnetizing characteristics in graph sheet.

TABULATION:

Polarity test:

PolarityVoltmeter needle movement
Positive to P1 and Negative to P2 
Positive to P2 and Negative to P1 

Magnetizing characteristic:

Sl. No.VoltageCurrent
1
2  
3  
4  
5  
6  
7  
8  
9  
10  

CONCLUSION:

To be written by student.

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