What is Current Transformer Knee Point?

What is Current Transformer?

Current transformers (CTs) are devices that are used to measure the current flowing in an electrical circuit. They work by producing a secondary current that is proportional to the primary current flowing through the CT's core. This secondary current can then be measured by a meter or other monitoring device, allowing for accurate measurement and monitoring of the electrical system.

Related Article: Instrument Transformers in Power System Protection

Uses of Current Transformers

Current transformers (CTs) are widely used in the electrical power industry for a variety of applications. Some of the main uses of CTs include:

  1. Measurement - CTs are primarily used for measuring electrical currents in high-voltage and high-current power systems. They are used to step down the current flowing through a power line to a level that can be accurately measured by a meter or monitoring device.
  2. Protection - CTs are also used for protection purposes, such as detecting overcurrent, undercurrent, or ground fault conditions in a power system. In these applications, the CTs are connected to protective relays that monitor the current flowing through the power system and trip circuit breakers or disconnect switches in case of a fault.
  3. Metering - CTs are commonly used for revenue metering in the utility industry, where accurate measurement of power consumption is essential for billing purposes. The CTs are used to measure the current flowing through the power lines, which is then used to calculate the power consumption.
  4. Monitoring - CTs are used for monitoring the performance and efficiency of electrical equipment, such as motors, generators, and transformers. The CTs can be used to measure the current flowing through the equipment, which can be used to diagnose any problems or issues that may arise.
  5. Power Quality Analysis - CTs are used for power quality analysis, which involves monitoring and analyzing various parameters of the electrical power system, such as voltage, current, and frequency. The CTs are used to measure the current flowing through the power system, which can be used to identify issues such as harmonics, voltage sags, and power factor problems.

The CT Knee Point

The accuracy of the CT's measurement depends on its linearity, which is the degree to which the secondary current is proportional to the primary current. Ideally, a CT would provide a linear output for any primary current, but in practice, CTs have a limit to their linearity, known as the knee point.

The knee point is the point at which the CT core begins to saturate due to the magnetic flux density becoming too high. As the core saturates, the output of the CT becomes non-linear, which means that the ratio of primary current to secondary current is no longer linear. This can result in inaccurate measurements and can lead to errors in the monitoring and protection of the electrical system.

Figure 1. CT Knee Point

How to Determine CT Knee Point? 

To avoid operating a CT above its knee point, it is important to select a CT with a suitable rating for the expected primary current. CT manufacturers typically provide a knee point specification for their CTs, which can be used to determine the maximum primary current that can be measured accurately. 

For example, if a CT has a knee point of 50A, it can accurately measure up to 50A, any primary current above 50A will cause the CT to operate non-linearly, leading to inaccurate measurements.

It is also important to note that the knee point can be affected by the burden impedance connected to the CT secondary, as well as the frequency and waveform of the primary current. Therefore, it is recommended to test the CT under actual operating conditions to verify its performance and accuracy. The testing should include measurements of the CT's output at various primary currents to ensure that it remains linear up to its rated current and knee point.

To value of knee point depends on the following factors: 
  1. Short Circuit Current
  2. CT secondary resistance
  3. The resistance of the wire that connects the CT and the relay.
  4. Burden of the relay
Click this article for sample calculation.

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