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Why it needs- CT short Circuited while the PT open circuited?

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Question added by Raja Soomro , Protection Engineer , Saudi Electricity Company
Date Posted: 2017/08/16
Rahil Shaikh
by Rahil Shaikh , Project Engineer , Saudi Electricity Company (SEC)

Level of high voltage increased in secondary of CT and no current in PT.

hamed tahaa
by hamed tahaa , Electrical design,Maintenance, protection and control engineer , UPPER EGYPT FOR ELECTRIC DISTRIBUTION company

CT secondary will develop a very high voltage when secondary is open circuited

CT primary energised also secondary energised, but open circuited . it will burn

pt open circuit the voltage stable and not shorted because it give  small current and need higher secondary resistance

 

RAJAN KUMAR
by RAJAN KUMAR , Senior Electrical Engineer , Samsung Engineering India

Because high voltage develop in secondary of CT and in PT there is no such current develop

Abdullah Zahid
by Abdullah Zahid , Testing and Commissioning Engineer , Mabran Al-Arabia Contracting

I assume you meant the secondaries and would like to divide the Answer in two portions.First, Why CT secondary is short circuited (if no burden is connected)?The current in the primary of CT is independent of secondary current. The primary is always in series with the load. Hence all load current, let's say Ip, must pass through the primary of CT. When CT is short circuited the current in the secondary (Is) will be in relation with the primary current(Ip). Now assume the secondary being opened, the secondary current will be zero but current in primary is still equal to the load current. Now keeping in mind the equivalent circuit of a transformer all this current will become magnetizing current. This magnetizing current(which in normal case is negligible) will increase the flux and hence there will be a great voltage induced on the secondary which will be in the order of kV. Since the terminals of secondary are very near in CT and are not insulated for that much voltage, a breakdown will occur between the terminals and will destroy CT.Second, Why PT secondary should be kept opened?when the secondaries are opened for a PT a very minor current is present in primary that is only magnetization current(or current due to other losses in reality). No suppose the secondary is short-circuited, the voltage presence on the secondary will make a huge amount of current in the order of kA to pass through the secondary of the PT. This current will damage the windings of the transformers. 

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