The knee point voltage of the current transformers should be at least twice the resistance, and the maximum lead loop resistance between the CT and the. For devices burden expressed in VA at specified current or voltage, the burden CT winding resistance = ohms QUIZ: READ THE KNEE POINT VOLTAGE. 10 % increase in voltage gives you 50 % increase in excitation current iscalled knee point voltage. Knee point voltage i.e. saturation of CT means even if current in primary circuit increases, CT secondary current will not increase proportionately. It is the maximum, fixed.


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Apply the voltage to the secondary winding with the primary and other windings being open-circuited. Increase the voltage until you reach a point where a small increase in voltage results in a big increase in current.

Knee Point Voltage of Current Transformer PS Class

Now slowly decrease the voltage to a few measuring points, while you measure the current. It is clear from the curve that, linear relation between V and Ie is maintained from point A and K.

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.

Beyond the knee point, for slight increase in ct knee point voltage calculation emf a large increasing in excitation current is required. This phenomena may cause unexpected tripping of power transformer.

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 ct knee point voltage calculation groups of CTs and transformer trips.

CT Knee point voltage calculation - Entries - Forum - Industry Support - Siemens

So for choosing CT for differential protection of transformerone should consider current transformer PS class rather its convectional protection class.

The electrical power system load current always flows through current transformer primary; irrespective of whether ct knee point voltage calculation current transformer is open circuited or connected to burden at its secondary.

If CT secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage.

Every current transformer has its won non-linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core. If one can measure the rms ct knee point voltage calculation across the secondary terminals, he or she will get the value which may not appear to be dangerous.

As the CT primary current is sinusoidal in nature, it zero times per second.

Knee Point Voltage of Current Transformer PS Class

As frequency of the current is 50 Hz. The rate of change of flux at every current zero is not limited by saturation and is high indeed.


This develops extremely high peaks or pulses of voltage. This high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the CT insulation, and may case accident to personnel.

The actual open-circuit voltage peak is difficult to measure accurately because of its very short peaks.