What is the purpose of calculating knee point voltage for Current transformers?

Questions by rohitasruti   answers by rohitasruti

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s kumar

  • Mar 31st, 2007

The main purpose of calculating the knee point voltage is to identify the core saturation characteristics .This is mainly done on protection CT's

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  • Oct 5th, 2007

After knee point, saturation of the CT occurs. Hence the CT has to work below the knee point. By selecting a CT suitably for operating point below the knee point, proper operation of the protection (like differential, REF where protection class CTs are used) can be ensured.


  • Oct 5th, 2007

The purpose of calculating knee point is to select CT with operating point below knee point as above knee point CT tends to saturate and cause mal operation of protection relay. Knee point calculation is done for CTs used for differential protection and REF.


  • Oct 3rd, 2009

Purpose of collecting Knee voltage is to use the circuit as an operating or as an metering circuit.

Operating circuit works far below knee voltage.

Metering circuit works at the verge of saturation.

Shona Balaji Santharam

  • Jan 1st, 2018

Knee Point Voltage of Current Transformer is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%.

In differential and restricted earth fault (REF) protection scheme, accuracy class and ALF of the CT may not ensure the reliability of the operation. It is desired that, differential and REF relays should not be operated when fault occurs outside the protected transformer. When any fault occurs outside the differential protection zone, the faulty current flows through the CTs of both sides of electrical power transformer. The both LV and HV CTs have magnetizing characteristics. Beyond the knee point, for slight increase in secondary emf a large increasing in excitation current is required. So after this knee point excitation current of both current transformers will be extremely high, which may cause mismatch between secondary current of LV & HV current transformers. This phenomena may cause unexpected tripping of power transformer. So the magnetizing characteristics of both LV & HV sides CTs, should be same that means they have same knee point voltage Vk as well as same excitation current Ie at Vk/2. It can be again said that, if both knee point voltage of current transformer and magnetizing characteristic of CTs of both sides of power transformer differ, there must be a mismatch in high excitation currents of the CTs during fault which ultimately causes the unbalancing between secondary current of both groups of CTs and transformer trips. So for choosing CT for differential protection of transformer, one should consider current transformer PS class rather its convectional protection class. PS stands for protection special which is defined by knee point voltage of current transformer Vk and excitation current Ie at Vk/2.

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