What is VK in current transformer?
≡ Special accuracy class This class specifies the minimum value of the knee point voltage Vk of the CT. If we consider the magnetising curve V(Io) of the CT, the knee point voltage Vk is defined as the point on this curve from which a 10% increase in voltage causes a 50% increase in the magnetising current Io.
What is RCT in CT?
INSTRUMENT TRANSFORMERS. RCT. Indoor current transformer. The RCT indoor current transformer is used for metering and relaying applications in medium voltage (5 – 15 kV) switchgear.
What is the use of CT and PT?
CT is used to measure current while PT is used to measure voltage. CT is connected in series while PT is connected in parallel. CT ratio range is from 1 to 5A while the PT range is from 110V. We connect the output parameter from CT to the ampere meter while we connect the PT output to the voltmeter.
What happens at knee voltage?
Knee voltage is the forward voltage at which the flow of the current through the PN junction of the diode increases rapidly. The knee voltage is generally observed in Zener Diodes.
How to calculate knee point voltage of current transformer?
According to IEC, Knee Point Voltage of a Current Transformer is defined as the voltage at which a 10 % increase in voltage of CT secondary results in a 50 % increase in secondary current. Above knee point, the CT gets saturated. The knee point is a important parameter for CT selection for a particular electrical installation.
What is the significance of knee voltage for Protection CT?
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.
What should the knee point voltage be for a PS-class core?
For this CT the knee point voltage for PS-Class core is V k >= 120V for 3S1-3S3 winding and V k >=60V for 3S1-3S2 winding. Also the excitation current should less than or equal to 100mA at V K /4 for anyone of Core-3 windings. Current Transformers magnetization curve helps to understand the knee point voltage.
When does the current increase at the knee point?
In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the knee point is reached. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%.