As per IEC, the Knee Point Voltage of a Current Transformer is defined as the voltage at which 10 % increase in voltage of CT secondary results in 50 % increase in secondary current.
How do you calculate CT knee voltage?
Importance of Knee Point Voltage for PS class CT:
- Differential protection Scheme:
- CT Knee Point Voltage can be calculated using the formula;
- Related Article:
- Example:
- If the system fault current is 30 KA, CTR-600/5, RCT+RL+RR=0.5 Ohm.
- Vkp = K * If/CTR * (RCT + RL + RR)
- Vkp = 2*30000/120*0.5 =250 Volt.
- Test Set-up:
What is knee point test in CT?
The knee-point voltage is the magnitude of the CT’s secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy limits.
What is CT voltage?
A Current Transformer (CT) is used to measure the current of another circuit. CTs are used worldwide to monitor high-voltage lines across national power grids. A CT is designed to produce an alternating current in its secondary winding that is proportional to the current that it is measuring in its primary.
What is VK of CT?
≡ 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 does 5P10 mean in CT?
If you see the specification or name plate of a protection class CT, you will find that it is given like 5P10. This CT can be interpreted as a protection class CT having an accuracy of 5% over a current range of 10 times of normal primary current rating.
What is knee voltage and what is its significance?
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 do you calculate knee voltage from a graph?
In the forward region of the graph, the voltage at which the current starts to increase quickly is called the knee voltage of the diode. Study of a diode circuit usually comes down to determining if the diode voltage is higher or lower than the knee voltage.
How do you read a CT nameplate?
CT Ratio: 100/5-5 A
The ratio 100/5 means If the CT primary carries 100 Amps then the output of the CT secondary will be 5 Amps. The dual 5-5 A means the first core used for protection and second core used for metering purpose.
Why is it important to know the knee point voltage of a current transformer?
Knee point voltage is nothing but a magnitude of the saturation limit of the current transformer. Each protection class CT has to be operated in its Non saturation region. So that we have to calculate the maximum allowable voltage limit, that can be applied to the secondary winding of the current transformer.
Which is the correct CT ratio?
CTs typically have a standard secondary output rating of 1 or 5 amps. A CT ratio is simply the primary and secondary currents being expressed as a ratio where the secondary is either 1 or 5 amps. A 100/5 ratio CT means that the primary current is 20 times greater than the secondary current.
Why CT secondary is 5A?
Also, with reduced secondary turns, the open circuit voltage will also reduce five-fold, as also the insulation thickness and size. Less number of turns also means that reduced core size. Hence, for a given CT ratio and burden, a CT with a 5A secondary will be cheaper, smaller and lighter.
How do you calculate CT ratio?
Calculate the CT ratio. The CT ratio is the inverse of the voltage ratio. In this example, the voltage ratio is 1:5, so the CT ratio is 5:1. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the CT output is 40 amps.
How do you choose CT polarity?
CT Polarity
The polarity of a current transformer is determined by the direction in which the coils are wound around the core of the CT (clockwise or counter clockwise), and by which way the secondary leads are brought out of the transformer case.
What is 0.2 S Class CT?
An 0.2s accuracy class means that the measurement has an error rate of 0.2% over a range of 20 to 120% of the nominal current (In) and at a specific accuracy above 1% of In.
Which is better 5P10 or 5P20?
Its application which differetiate the use of 5P10 and 5P20. If the breaker is used for backup protection say incommer etc you can use 5P20, but if it is say an incommer ACB/ MCCBof PMCC, you should use 5P10 only.
What does 5P10 for a CT of 100 5 mean?
Similarly 5P10 can be explained as CT will sense the current with the specified accuracy of 5% even with 10 times of its secondary current flows in the secondary.
What is ALF in CT?
The measure of this characteristic is known as the “Accuracy Limit Factor” (A.L.F.). A protection type CT with an A.L.F. of 10 will produce a proportional current in the secondary winding [subject to the allowable current error] with primary currents up to a maximum of 10 times the rated current.
What is the value of knee voltage?
The knee voltages for germanium and Silicon are 0.2 and 0.7volts. The I–V graph shows the behavior of a Semiconductor Diode.
What causes knee voltage?
The knee voltage for silicon diodes is about 0.7 volts, which is due to the properties of silicon when doped to form P-N junctions.Likewise, at the knee voltage, current starts to flow easily, with only a little resistance.
Is knee voltage same as barrier potential?
The forward voltage at which the current through PN junction starts increasing rapidly is known as knee voltage. The Knee voltage of a crystal diode is approximately equal to barrier potential.
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