The protection current transformer (CT) is a critical component that works in conjunction with relay protection systems to signal the device to disconnect the faulty circuit during short circuits or overloads, ensuring the safety of the power system. Unlike measuring transformers, its operating conditions are entirely different. Under normal primary current ranges, it maintains acceptable accuracy. However, when subjected to fault currents, it is designed to saturate early to protect the measuring instruments from damage. The CT starts to operate at several times the normal current level, and the error—both current and phase—must remain within the specified error curve. When assessing the current error and phase difference, the combined error is considered.
When the primary current (i1) is small, the secondary current (i2) changes linearly. As i1 increases, the magnetic flux density in the core rises. Due to the non-linear properties of ferromagnetic materials, the excitation current (i0) contains high harmonic components, causing the waveform to become distorted, often appearing pointed rather than sinusoidal. Even if i1 is a perfect sine wave, i2 will not be sinusoidal.
Non-sinusoidal waveforms cannot be analyzed using phasor diagrams, which necessitates more complex error analysis. This leads to a rapid increase in i0, effectively reducing the amount of i1 that is converted into i2. As a result, i2 and i1 no longer change proportionally, increasing the overall CT error.
During a short-circuit fault, the current can reach up to 10 times the rated current, significantly increasing the error and potentially affecting the sensitivity and selectivity of the protection system. Additionally, as a specialized type of transformer, the CT must operate under its rated load. If the secondary load exceeds this value, the error will also increase.
As discussed, CT errors are unavoidable and depend on the core's excitation characteristics and the secondary load. To manage these errors, it is essential to consider the maximum fault current at the CT location, the ratio of the actual current to the rated current, the rated current ratio, and the rated secondary load. Understanding the accuracy level, accuracy limit factor, rated current ratio, and rated load is crucial for proper selection.
For protection CTs, the accuracy level is defined by the maximum allowable composite error at the nominal accuracy limit. The accuracy limit factor indicates the ratio of the maximum primary current (i1max) to the rated primary current (i1n) that meets the composite error requirement. The rated current ratio is the ratio of the primary to secondary rated currents, while the rated load is the secondary load used to determine the accuracy level.
In the past, the national standard GB/T 1208-75 classified protection CTs into B and D accuracy levels. Today, the updated standards include 5P and 10P, indicating that the composite error at the rated accuracy limit is limited to 5% and 10%, respectively. The accuracy limit factor series includes values like 5, 10, 15, 20, 30, etc., meaning that if the short-circuit current is less than this multiple of the rated current, the error remains within the acceptable range.
Two key conclusions can be drawn: First, when selecting the accuracy level for a protection CT, the exact limit factor must be chosen simultaneously. For example, 5P20 and 1200/5A means that the current should not exceed 20 times the rated primary current (i.e., 24 kA), and the composite error should not exceed 5%. Second, the selection process involves verifying the 10% error curve based on the maximum primary current, secondary load, and accuracy limit factor, as well as determining the rated current ratio and secondary load accordingly.
In steady-state operation, the CT’s secondary load must meet the 10% error curve requirements. If the actual load is less than what the curve allows, the measurement error will stay within 10%. A higher secondary load increases the likelihood of core saturation.
Consider an example of a 110kV substation where the maximum primary current (i1max) is 25 kA, and the secondary load is 40 VA. Based on the relay protection requirements, the CT should have a steady-state error of no more than 10%, so a 10P accuracy level is selected. According to the 10% error curve, the accuracy limit factor is 24. Dividing the 25kA by 24 gives approximately 1042A. To ensure sufficient margin, the accuracy limit factor should be set to at least 1250A.
When selecting the rated primary current of the CT, different current ratings should be used for metering, measurement, and protection. Common accuracy levels for measurement include 0.2, 0.5, 0.2S, and 0.5S. For the first two, the accuracy requirements are 5%, 20%, 100%, and 120% of the rated current, respectively. For the latter two, the requirements are 1%, 5%, 20%, 100%, and 120% of the rated current. The selection principle should take into account the minimum and maximum current values over time, and choose an appropriate primary current accordingly.
For the protection coil, the rated current can be selected to be larger since the output at rated current is not a major concern. What matters is how well it responds to the required protection current. A higher-rated CT may be more cost-effective and easier to implement. For instance, a 100/5, 5P30, 30VA CT may be difficult to manufacture, but a 300/5, 5P10, 30VA CT would be much simpler and more practical.
For differential protection coils, it is important that the parameters match those on the other side. If both sides are new, using the same manufacturer is ideal. Otherwise, the volt-ampere characteristics of the other side should be provided to ensure consistency.
In practice, microcomputer protection devices consume very little power. The main load on the CT comes from the resistance and contact resistance of the signal cable. For example, a 5A CT with a 100m signal cable using 4mm² wire (about 4.5 ohms per km) would have a line resistance of 0.9 ohms, resulting in a line loss of 22.5VA, which is significant.
The accuracy limit factor primarily reflects the CT's ability to handle current. It indicates how many times the rated primary current the CT can handle before the composite error exceeds 10% (for 10P) or 5% (for 5P). When selecting the capacity, it's not necessary to focus too much on it.
There are cases where two CTs are used in series, but this should be done with caution. The performance after series connection must be verified. If there is a lack of capacity, a simple solution is to use two 4mm² wires in parallel, effectively doubling the cross-section and halving the loop resistance, thus reducing the actual load significantly.
The CT capacity must be calculated based on the rated current. During design, the overcurrent should not exceed the accuracy limit factor (ALF). At that point, the CT should theoretically output ALF times the rated secondary current, and the actual output should fall within the 10% (10P) or 5% (5P) error range. For example, a 300/5, 5P30, 30VA CT has a rated secondary impedance of 1.2Ω. With a power factor of 0.8, the actual load should not exceed the rated value; otherwise, the performance may be affected.
It is important to note that during large overcurrent conditions, the secondary output is not purely sinusoidal, and the composite error is considered. In real applications, even if the core hasn't reached full saturation, the waveform distortion is acceptable. Keeping the CT in a linear region would require excessive size, weight, and cost, making it impractical.
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