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Calculate knee point voltage easily using our free online Knee Point Voltage Calculator. Get accurate results, formulas, examples, and step-by-step guidance.
In electrical power systems, protection and accuracy are very important. One small mistake in measurement can cause big damage to equipment. This is why engineers always focus on current transformers and potential transformers. One key parameter of these devices is the knee point voltage.
Many students, technicians, and engineers find it difficult to calculate knee point voltage manually. It involves several values like fault current, CT ratio, resistance, and burden. To make this process simple, we have developed an easy and reliable Knee Point Voltage Calculator. With this online tool, you can calculate knee voltage in seconds without any confusion.
This calculator is designed for beginners and professionals. It gives accurate results and shows clear calculation steps.
Knee point voltage is the voltage level at which a current transformer core starts to saturate rapidly. After this point, a small increase in voltage causes a very large increase in magnetizing current.
In simple words, it is the limit after which the CT can no longer give accurate output.
For protection current transformers, knee point voltage is very important. If the CT saturates during a fault, protective relays may not work correctly. This can lead to system failure.
Knee point voltage is mainly used in protection CTs and sometimes in PTs for special applications.
Knee point voltage helps engineers to understand the performance of a transformer during fault conditions.
It ensures that:
Without proper knee point voltage, protection systems may fail when they are needed most.
To calculate knee point voltage, you need three main things.
These values are combined using a standard formula.
The standard formula for knee point voltage is:
Vkp = K × Isec × Rtotal
Where:
Secondary fault current is calculated as:
Isec = IF ÷ CTR
CT ratio is calculated as:
CTR = Iprimary ÷ Isecondary
Total resistance is calculated as:
Rtotal = RCT + RL + RB
Where:
Our online knee point voltage calculator is designed to be simple and fast.
After clicking, the calculator will instantly show the knee point voltage. It will also display step-by-step calculations for better understanding.
No manual math is required. Everything is done automatically.
Let us understand this with a simple example.
Suppose we have the following values.
Fault current = 25,000 A
CT ratio = 500/5 A
CT resistance = 0.15 Ω
Lead resistance = 0.10 Ω
Relay burden = 0.25 Ω
Safety factor = 2
Step 1: Calculate CT ratio.
CTR = 500 ÷ 5 = 100
Step 2: Calculate total resistance.
Rtotal = 0.15 + 0.10 + 0.25 = 0.50 Ω
Step 3: Calculate secondary fault current.
Isec = 25,000 ÷ 100 = 250 A
Step 4: Calculate knee point voltage.
Vkp = 2 × 250 × 0.50
Vkp = 250 V
So, the required knee point voltage is 250 volts.
This means the CT should have a knee point voltage of at least 250 V to avoid saturation.
In most cases, knee point voltage is mainly used for current transformers. However, in some special protection and metering systems, potential transformers also have saturation characteristics.
For PTs, knee voltage refers to the voltage at which the core starts saturating and the output becomes distorted.
The concept is similar, but the calculation method is different and usually provided by the manufacturer.
For accurate PT knee voltage, datasheets are recommended.
In practical testing, knee point voltage is found by applying increasing AC voltage to the CT secondary and measuring the excitation current.
When a small increase in voltage causes a large increase in current, that point is marked as the knee voltage.
This method is mostly used in laboratories and commissioning tests.
Our online calculator provides a theoretical and design-based value for engineering purposes.
The knee point voltage is one of the most important parameters for protection transformers. It ensures that CTs work correctly during fault conditions and provide reliable signals to relays.
Manual calculation is time-consuming and confusing. That is why our Knee Point Voltage Calculator is the best solution for students, engineers, and technicians.
It is fast, accurate, easy to use, and shows complete steps. With this tool, you can design safer and more reliable electrical systems.
If you work with power systems, this calculator is a must-have tool for you.
You can calculate it using the formula:
Vkp = K × Isec × Rtotal
Or simply use our online knee point voltage calculator for instant results.
It prevents CT saturation during faults and ensures proper relay operation.
Most engineers use a safety factor of 2.0 for reliable performance.
No, it is mainly required for protection CTs. Metering CTs usually work at lower currents.
Yes, you can use it for most protection CTs used in substations and industrial systems.