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Current transformer:Key components of power systems

source：织梦技术论坛 time：2024-10-24 09:14nbsp; click：

1. Basic concepts and working principles of current transformers

(I) Definition and function Current transformer, abbreviated as CT (Current Transformer), is an electrical device that converts large current into small current in a certain proportion. Its main function is to provide small current signals proportional to the primary current for measuring instruments, relay protection devices, etc., so as to realize the monitoring, control and protection of the power system.

For example, in a high-voltage power system, the primary current may be as high as thousands of amperes or even tens of thousands of amperes. It is very difficult and dangerous to directly measure such a large current. Through the current transformer, the large current can be converted into a standard 5A or 1A small current, so that the measuring instrument and protection device can work safely and accurately.

(II) Working principle The working principle of the current transformer is based on the law of electromagnetic induction. When the primary current passes through the primary winding of the current transformer, an alternating magnetic field will be generated in the iron core. This alternating magnetic field will induce an electromotive force in the secondary winding, thereby generating a secondary current. Since the number of turns of the secondary winding is much greater than that of the primary winding, according to the principle of the transformer, the secondary current is proportional to the primary current and its value is much smaller than the primary current.

For example, the transformation ratio of a current transformer is 1000:5, that is, when the primary current is 1000A, the secondary current is 5A. In this way, by measuring the secondary current, the magnitude of the primary current can be indirectly known.

2. Types and characteristics of current transformers

(I) Classification by use

Measurement current transformers are mainly used for metering and measurement of power systems, requiring high accuracy and good stability. The accuracy levels of measurement current transformers are usually divided into 0.1, 0.2, 0.5, 1.0, etc. The smaller the number, the higher the accuracy. For example, the measurement error of a 0.2-level measurement current transformer at rated current does not exceed ±0.2%.

Protection current transformers are mainly used in relay protection devices of power systems, requiring that they can accurately reflect the changes in primary current under fault conditions so that the protection device can operate in time. The characteristics of protection current transformers are that they have good saturation characteristics and anti-interference capabilities under fault current. For example, in the event of a short-circuit fault, the primary current will increase sharply, and the protection current transformer can maintain a certain output characteristic under large current to ensure reliable operation of the relay protection device.

(II) Classification by installation method

The through-type current transformer is installed in the hole through which the bushing or busbar of the power equipment passes, and the primary winding is the bushing or busbar of the power equipment. Through-type current transformers have the advantages of simple structure and easy installation, and are suitable for busbar measurement and protection in high-voltage power systems.

Pillar-type current transformers are installed on pillars, and the primary winding is an independent winding. Pillar-type current transformers have the advantages of small size, light weight, and easy maintenance, and are suitable for measurement and protection of outdoor high-voltage power equipment.

Bushing-type current transformers are installed on the bushing of power equipment, and the primary winding is the conductive part of the bushing. Bushing-type current transformers have the advantages of good insulation performance and high reliability, and are suitable for internal measurement and protection of high-voltage power equipment.

(III) Classification by insulating medium

Dry-type current transformers use solid insulating materials, such as epoxy resin, silicone rubber, etc., as insulating media. Dry-type current transformers have the advantages of small size, light weight, oil-free, fireproof and explosion-proof, and are suitable for places with high fire protection requirements, such as high-rise buildings, subways, etc.

Oil-immersed current transformers use insulating oil as the insulating medium. Oil-immersed current transformers have the advantages of good insulation performance and good heat dissipation performance, and are suitable for the measurement and protection of important equipment in high-voltage power systems. However, oil-immersed current transformers have disadvantages such as oil leakage and flammability, and require regular maintenance and inspection.

SF6 gas-insulated current transformers use SF6 gas as the insulating medium. SF6 gas-insulated current transformers have the advantages of good insulation performance, small size, and light weight, and are suitable for the measurement and protection of important equipment in high-voltage and ultra-high-voltage power systems. However, the manufacturing process of SF6 gas-insulated current transformers is complex and the cost is high.

3. Main parameters and performance indicators of current transformers

(I) Transformation ratio The transformation ratio of a current transformer refers to the ratio of the primary current to the secondary current. For example, a current transformer with a transformation ratio of 1000:5 means that when the primary current is 1000A, the secondary current is 5A. The selection of the transformation ratio should be determined according to the actual measurement and protection needs, which should not only meet the measurement accuracy requirements, but also ensure that the changes in the primary current can be accurately reflected in the event of a fault.

(II) Accuracy level The accuracy level is an indicator to measure the measurement accuracy of the current transformer. As mentioned above, the accuracy level of the current transformer for measurement is usually divided into 0.1, 0.2, 0.5, 1.0, etc., and the accuracy requirements of the current transformer for protection are relatively low. The selection of the accuracy level should be determined according to the actual measurement and protection requirements. For occasions requiring high-precision measurement, a current transformer with a higher accuracy level should be selected.

(III) Rated capacity The rated capacity refers to the apparent power allowed by the current transformer under the rated current and rated load on the secondary side. For example, a current transformer with a rated capacity of 5VA means that when the rated current on the secondary side is 5A and the rated load is 1Ω, the apparent power allowed by the current transformer is 5VA. The selection of rated capacity should be determined according to the actual secondary load size to ensure that the current transformer will not be overloaded during operation.

(IV) Saturation multiple The saturation multiple refers to the ratio of the maximum primary current that the current transformer can withstand under fault conditions to the rated primary current. For example, a current transformer with a saturation multiple of 20 means that under fault conditions, the maximum primary current that can be sustained is 20 times the rated primary current. The selection of saturation multiple should be determined according to the actual requirements of relay protection to ensure that the current transformer can accurately reflect the changes in the primary current under fault conditions.

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