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What is the knee point voltage of CT?
Release Time : 2020-11-08 View Count : 次The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. For voltages greater than the knee point voltage, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.
Definition of KPV according to IEC : What is the knee point?
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. The knee point indicates the CT saturation point on the magnetization curve or B-H curve.
Above knee point, the CT gets saturated. The knee point is a important parameter for CT selection for a particular electrical installation. The knee point indicates the CT saturation point on the magnetization curve or B-H curve.
Importance of Knee Point Voltage for PS class CT:
Differential protection Scheme:
The magnetizing characteristics of both LV & HV sides CTs used for differential protection must have the same knee point voltage Vk and the same excitation current Ie at Vk/2. If there is variation in knee point voltage of CTs, the CT having less knee point voltage(KPV) will get saturated in case of the external fault and the differential protection relay will get tripped with nuisance tripping. Therefore, both the set of CTs used at the primary and secondary side of the equipment must have the same knee point voltage and the excitation current(Ie) at Vk/2.
The Knee point voltage is selected for PS class(Differential protection) CTs at half of the knee point voltage so that the point lies in between the ankle and the knee point of the magnetization curve of the CT. The point K on the below-given graph is the 'Knee Point'. The initial point of the graph is known as an ankle point of the magnetization curve of CT. The Knee Point Voltage has a significance when the CT is used for differential or REF protection.
Knee Point Voltage Calculation:
The CT knee point calculation can simply be done using the Ohm's Law.
According to Ohm's law
V=I*R
R is the total resistance of the secondary of the CT.
Knee Point Voltage Formula:
CT Knee Point Voltage can be calculated using formula;
Vkp = K * If/CTR * (RCT + RL + RR)
Where,
CT Knee Point Voltage can be calculated using formula;
Vkp = K * If/CTR * (RCT + RL + RR)
Where,
K = Constant, conventionally taken as 2.0
Vkp = The minimum Knee Point Voltage
If = Maximum Fault Current at the location, in Amperes
CTR = CT Ratio
RCT = CT Secondary Winding Resistance, in Ohms
RL = 2-way Lead Resistance, in Ohms
RR = Relay Burden, in Ohms
RCT + RL + RR = CT burden
Example:
If the system fault current is 30 KA, CTR-600/5, RCT+RL+RR=0.5 Ohm
Knee point voltage of the current transformer is;
Vkp = K * If/CTR * (RCT + RL + RR)
Vkp = 2*30000/120*0.5 = 250 Volt
Based on the above calculation, check the rated knee point voltage of CT. If the knee point voltage calculated on the basis of the power network is higher than the specified knee point voltage of CT, then there is a chance of CT saturation during a fault in the power network, and probably protection relay will not operate.
