A Comprehensive Guide to Core-Passing Current Transformers: Avoiding Errors for Accurate Metering, Protection, and Safety

 Abstract:

The core-passing current transformer is a common electrical device widely used in metering, detection, and protection circuits due to its simple wiring and easy installation. However, slight inattention during use can cause significant errors, leading to inaccurate metering, malfunction of protection devices, and even electrical accidents. This is related to the ampere-turns capacity of the current transformer.

1. Accident Phenomenon

An electroplating factory in Berlin, Germany, has three motors with specifications of Y180M-4 22kW. They are equipped with LMZ1-0.5, 100/5, 300 ampere-turns current transformers, and ammeters with a range of 0-100A. During actual operation, the current value was always found to be around 27A, while the actual working current on the primary side measured by a clamp-on ammeter was 82A. These two values were significantly inconsistent, and the situation was similar for all three motors. We replaced the current transformer, secondary wiring, and ammeter for one of the motors, but the issue persisted.

2. Accident Analysis

Upon careful analysis, we discovered a common pattern: the detected and metered currents on the primary and secondary sides were nearly three times different, which alerted us. After carefully examining the transformer nameplate, we realized that an important issue had been overlooked: the ampere-turns capacity. It was noted as 300 ampere-turns, so it should be wound three times in a 100/5 circuit, rather than the conventional one-turn core-passing.

3. Accident Handling

We wound the primary circuit three times around the transformer, and the detected current was 81A, which was basically consistent with the 82A measured by the clamp-on ammeter on the primary circuit. This indicates that we should not overlook this issue.

The core-passing current transformer is a common electrical device widely used in metering, detection, and protection circuits due to its simple wiring and easy installation. However, slight inattention during use can cause significant errors, leading to inaccurate metering, malfunction of protection devices, and even electrical accidents. This is related to the ampere-turns capacity of the current transformer. The so-called ampere-turns capacity refers to the maximum rated current value when the primary side of the current transformer has a single-core wire passing through it, i.e., the product of the rated current and the number of turns passing through the core. For example, for a model LMZJ-0.5 with 400 ampere-turns, the maximum current is 400A when the primary side has a single-turn core-passing. If two turns are used, the primary rated current is 200A. It is often used in conjunction with the detection current, indicating both the rated current operating range of the primary side of the current transformer and implying the wiring method. Ignoring this issue can lead to unpredictable problems.

Common Questions about Current Transformers:

22. How to Determine the Number of Turns for Core-Passing in a Current Transformer?

Answer:
(1) Calculate the designed ampere-turns of the current transformer based on the amperes and number of turns on its nameplate.
(2) Divide the designed ampere-turns by the required primary amperes, and the result must be an integer, which is the number of turns for core-passing.
(3) The number of times the primary wire passes through the center hole of the current transformer is the number of turns.

5. How to Determine the Polarity of a Current Transformer Using the DC Method?

Answer:
(1) Connect the positive terminal of the battery to "L1" of the current transformer and the negative terminal to "L2".
(2) Connect the positive terminal of the DC milliammeter to "K1" of the current transformer and the negative terminal to "K2".
(3) When the battery switch is closed or directly connected, the DC milliammeter should indicate positively. When the battery is disconnected, the milliammeter should indicate negatively. This indicates that the polarity of the current transformer is correct.

10. What are the Wiring Methods for Measurement Current Transformers?

Answer:
The wiring methods for measurement current transformers include:
(1) Incomplete star connection.
(2) Three-phase complete star connection.

24. How to Correctly Select the Transformation Ratio of a Current Transformer?

Answer:
When selecting a current transformer, its primary rated current I1n should be chosen based on its long-term maximum secondary working current I2, such that I1n ≥ I2. However, it is not advisable to let the current transformer operate frequently below its rated primary current and should try to operate above its rated primary current as much as possible.

25. Briefly Describe the Working Principle of a Current Transformer.

Answer:
When the primary winding of a current transformer is connected to a circuit with a load current I1, it generates an alternating magnetic flux with the same frequency as I1. This flux passes through the secondary winding, generating an induced electromotive force E2. Since the secondary winding is a closed circuit, a current I2 flows through it, generating an alternating magnetic flux Φ2. Φ1 and Φ2 pass through the same closed iron core, combining to form a composite magnetic flux Φ0. The role of Φ0 is to transfer the energy of the primary winding to the secondary winding during the current exchange process.

28. What are the Main Factors Affecting the Error of a Current Transformer?

Answer:
The main factors affecting the error of a current transformer include:
(1) When the primary current exceeds the rated value by several times, the current transformer operates in the nonlinear part of the magnetization curve, increasing both the current ratio error and angle error.
(2) An increase in the secondary circuit impedance Z2 increases the ratio error; a decrease in the power factor cosΦ increases the ratio error but decreases the angle error.
(3) The frequency of the power supply generally has a minor impact on the error. When the frequency increases, the error decreases slightly initially but then continues to increase.

31. What are the Main Parameters to Consider When Selecting a Current Transformer?

Answer:
The following main parameters should be considered when selecting a current transformer:
(1) Rated voltage.
(2) Accuracy class.
(3) Rated primary current and transformation ratio.
(4) Secondary rated capacity.

32. Briefly Describe the Basic Structure of a Current Transformer.

Answer:
The basic structure of a current transformer consists of two mutually insulated windings and a common iron core. The winding connected to the power supply is called the primary winding, which has a small number of turns. The winding connected to measurement instruments, relays, etc., is called the secondary winding, which has a larger number of turns.

34. What are the Errors of a Current Transformer and How are They Defined?

Answer:
The errors of a current transformer are divided into current ratio error and angle error.
Ratio Error:
Where Ke is the rated current ratio;
I1 is the primary current value;
I2 is the secondary current value.
Angle Error: The angle between the secondary current vector, rotated counterclockwise by 180°, and the primary current vector. It is stipulated that when the rotated vector leads, the error is positive; otherwise, it is negative.

37. What are the Requirements for the Material and Cross-Sectional Area of the Secondary Conductor from the Measurement Current Transformer to the Electric Energy Meter?

Answer:
(1) The secondary conductor should be made of single-core copper insulation wire.
(2) The cross-sectional area of the secondary conductor should be determined based on the rated secondary load of the current transformer and should be no less than 4mm².

44. What is the Rated Capacity of a Current Transformer?

Answer:
The rated capacity of a current transformer is the apparent power Sn consumed when the secondary rated current In passes through the secondary rated load Z2. I.e.,
Sn = I2n × |Z2|

51. How to Calculate the Secondary Impedance that a Current Transformer Can Handle Based on its Rated Secondary Capacity?

Answer:
The secondary impedance that a current transformer can handle should be calculated using the following formula:
Secondary Impedance
Where Sn is the secondary rated capacity (V•A);
In is the secondary rated current (typically 5A).

52. What Operating Conditions are Related to Changes in the Error of a Current Transformer During Operation?

Answer:
The error of a current transformer during operation is related to the primary current, frequency, waveform, changes in ambient temperature, and the magnitude and power factor of the secondary load.

53. What is the Meaning of the Rated Voltage of a Current Transformer?

Answer:
(1) The current transformer can only be installed in power circuits with a voltage rating less than or equal to its rated voltage.
(2) It indicates the insulation strength of the primary winding of the current transformer.3

55.How is the Replacement Cycle for Transformers Regulated?
Answer: The replacement cycle for transformers is as follows:
(1) High-voltage transformers should be replaced at least once every 10 years (current inspection can be used as an alternative to replacement).
(2) Low-voltage current transformers should be replaced at least once every 20 years.

Correct Method of Winding

Firstly, we should determine the transformer's multiplication factor based on the load size. Then, the primary wire should be wound through the center of the transformer as required. Note that the number of turns on the outer circle should not be considered as the winding turns; instead, the number of turns that pass through the interior of the current transformer should be used.

For example, a current transformer with a maximum current transformation ratio of 150/5 has a maximum primary rated current of 150A. If it needs to be used as a 50/5 transformer, the wire should be wound 150/50=3 times, i.e., 3 turns through the inner circle, while the outer circle will only have 2 turns (regardless of the number of turns in the inner circle, if the wire is passed from inside to outside, the number of turns in the outer circle will always be one less than that in the inner circle; of course, if the wire is passed from outside to inside, the opposite applies). If the outer circle turns are counted, with 3 outer turns, the actual number of turns through the core will be 4, and the transformed primary current will be 150/4=37.5A, making it a 37.5/5 current transformer with a multiplication factor of 7.5. However, during meter reading, the staff calculates the electricity based on a 50/5 current transformer with a multiplication factor of 10. The error is: (10-7.5)/7.5=0.33, which means 33% more electricity is billed.

Conversion Between Transformation Ratio and Number of Turns

Sometimes the nameplate of a current transformer may be lost during use. When the user's load changes and the transformation ratio of the current transformer needs to be changed, the transformer should first be verified to determine its maximum primary rated current. Then, the conversion between the transformation ratio and the number of turns can be performed as needed.

For example, if a current transformer with a maximum primary rated current of 150A is to be used as a 50/5 transformer, the conversion formula is:

Number of Primary Turns Through Core = Maximum Primary Rated Current of Existing Current Transformer / Primary Current of Transformer to be Transformed = 150/5 = 3 turns

That is, to convert it into a 50/5 current transformer, the number of primary turns through the core is 3.

The maximum primary rated current can be deduced from this. For example, if the original current transformer has a transformation ratio of 50/5 and 3 turns through the core, and it needs to be changed to a 75/5 transformer, we first calculate the maximum primary rated current: Maximum Primary Rated Current = Original Primary Current × Original Number of Turns Through Core = 50 × 3 = 150A. The number of turns through the core after conversion to 75/5 is 150/75 = 2 turns.

That is, when the original 50/5 current transformer with 3 turns through the core is changed to a 75/5 current transformer, the number of turns through the core should be changed to 2.

As another example, if the original 50/5 current transformer has 4 turns through the core and needs to be changed to a 75/5 current transformer, we first find the maximum primary rated current to be 50 × 4 = 200A. The number of turns through the core after conversion should be 200/75 ≈ 2.66 turns. When actually winding, the number of turns can only be an integer, either 2 turns or 3 turns. When we use 2 turns, the primary current becomes 200/2 = 100A, forming a 100/5 transformer, which introduces an error. The error is (Original Transformation Ratio - Current Transformation Ratio) / Current Transformation Ratio = (15 - 20) / 20 = -0.25, or -25%. This means that if we still calculate the electricity based on the 75/5 transformation ratio, we will underbill by 25% of the electricity. When we use 3 turns, we will overbill the user's electricity. Because the primary current becomes 200/3 = 66.66A, forming a 66.6/5 transformer, the error is (15 - 13.33) / 13.33 = 0.125, or 12.5% overbilling when calculating the electricity based on the 75/5 transformation ratio. Therefore, when we do not know the maximum primary rated current of the current transformer, we cannot change the transformation ratio arbitrarily, as it is likely to cause metering errors.