When you turn on the light, the current instantly lights up the bulb and fills the room with light; when you charge your phone, the current flows from the socket into the phone to recharge it. Behind these seemingly ordinary electricity usage scenarios, there is a key device – the current transformer (CT). Maybe you are unfamiliar with it, but it plays an indispensable role in the power system. Simply put, a current transformer is a special transformer that can convert a large current into a small current in a certain proportion, which is convenient for measurement, protection, and control. It is like a “current translator” that “translates” a large current that is difficult to handle directly into a small current that is easy for us to operate so that various power equipment and systems can operate safely and efficiently.
The working principle of the current transformer
The working principle of a current transformer is based on the principle of electromagnetic induction, which is also the basic working principle of a transformer. Simply put, when a changing current passes through a conductor, a changing magnetic field will be generated around the conductor; and another conductor in this changing magnetic field will induce a current. It is like two swings close together. When one of the swings (current changes produce a magnetic field), it will cause the surrounding air to flow (magnetic field changes), and this flow will drive the other swing to swing as well (magnetic field changes induce current).
The current transformer is mainly composed of a primary winding, a secondary winding, and an iron core. The primary winding has few turns and is directly connected in series in the main circuit where the current needs to be measured, like the main road of a large river; the secondary winding has more turns and is connected in series in the measuring instrument and the protection circuit, like a tributary branching off from the main road. When the large current (primary current) in the main circuit passes through the primary winding, an alternating magnetic flux is generated in the iron core. This magnetic flux is like an invisible “bond” that connects the primary winding and the secondary winding. According to the law of electromagnetic induction, the alternating magnetic flux will induce a proportionally reduced small current (secondary current) in the secondary winding.
We can use an example from life to understand it more vividly. Suppose you are draining water into a large swimming pool (large current in the main circuit). It is difficult to directly measure the water flow of the swimming pool, but you can connect a small-diameter branch water pipe (secondary winding) to the drain pipe of the swimming pool. By measuring the water flow of the branch water pipe (secondary current), and then according to the diameter ratio of the branch water pipe and the main drain pipe (the ratio of the current transformer), you can accurately know the drain flow of the swimming pool (large current in the main circuit). Generally speaking, the secondary rated current of the current transformer is usually designed to be 5A or 1A, and such a small current is easy to measure and process. By measuring the secondary current and multiplying it by the ratio of the current transformer (the ratio of the primary rated current to the secondary rated current), the actual large current value on the primary side can be obtained. For example, a current transformer with a ratio of 100:5, when measuring a secondary current of 3A, can calculate the primary side current as 3A×(100÷5)=60A.
When the current transformer is working, the secondary circuit is always closed. Because the impedance of the series coil of the measuring instrument and the protection circuit is very small, the working state of the current transformer is close to the short circuit. This is very important. If the secondary side is open, the magnetic flux generated by the large current on the primary side cannot be balanced by the current on the secondary side, which will cause the magnetic flux of the iron core to increase sharply and the iron core to be severely saturated, and then induce an extremely high voltage in the secondary winding, which may damage the equipment and even endanger personal safety.
Different types of current transformers
In the field of electric power, current transformers can be divided into many types according to their working principles, structural characteristics, and application scenarios. Common ones include electromagnetic, Hall, photoelectric, etc. Let’s take a closer look at their respective characteristics and applicable scenarios.
- Electromagnetic current transformer: This is the most common type of current transformer, which works based on the Faraday electromagnetic induction principle. The primary winding has a small number of turns and is directly connected in series in the main circuit; the secondary winding has a large number of turns and is connected to the current coil of the measuring instrument, relay, etc. Electromagnetic current transformers have simple structures, reliable performance, and relatively low prices. They are widely used in various power systems, especially in medium-high voltage and high current measurement and protection occasions, such as substations and power plants. They can stably convert large currents into small currents to meet the measurement and protection needs of subsequent equipment. However, it also has some disadvantages. For example, magnetic saturation may occur under high current conditions, resulting in reduced measurement accuracy; and its insulation structure is relatively complex, with large volume and weight.
- Hall current transformer: uses the Hall effect to measure current. When current passes through a conductor, a magnetic field is generated around the conductor. When the Hall element is placed in this magnetic field, a Hall voltage proportional to the magnetic field strength is generated. By detecting the Hall voltage, the current size can be indirectly measured. The outstanding advantages of the Hall current transformer are fast response speed and high precision, which can quickly and accurately reflect the changes in current; at the same time, it also has good linearity and anti-interference ability and can work stably in complex electromagnetic environments. In addition, it can measure AC and DC currents and has a wide range of applications. It is often used in electronic equipment, automatic control systems, and on some occasions with high requirements for measurement accuracy and response speed. For example, the battery management system of electric vehicles, can accurately measure the battery charging and discharging current. Its disadvantages are relatively high cost and relatively limited measurement range.
- Photoelectric current transformer: current measurement is achieved through optical principles. It uses the characteristics of light to convert current signals into optical signals for transmission and processing and then restores the optical signals to electrical signals for output. Photoelectric current transformers have excellent insulation properties. Because their signal transmission uses optical fiber, they are not affected by electromagnetic interference and can work reliably in high voltage and strong electromagnetic interference environments. Moreover, it has a wide dynamic range and high measurement accuracy, which can meet the high-precision requirements of metering and relay protection at the same time. In addition, it is small in size, light in weight, easy to install and maintain, and adapts to the development trend of miniaturization and intelligence of modern power systems. It has gradually been widely used in smart grids, high-voltage substations, and other fields. However, it also has some technical difficulties, such as being sensitive to ambient temperature and vibration, high processing requirements, and relatively expensive prices.
Widely applied fields: the all-rounder in the world of electricity
With its unique functions, current transformers play an indispensable role in many fields and can be called the “all-rounder” in the world of electricity.
- Power system: In key parts such as power plants, substations, and transmission lines, current transformers are used to measure high voltage or high current and convert them into low current signals that can be measured by instruments, to measure, monitor, and protect the safe and stable operation of power systems. For example, in substations, current transformers are used to measure and protect equipment such as transformers and generators. When the transformer has a fault such as an overload or short circuit, the current change sensed by the current transformer will be transmitted to the relay protection device in time, quickly cutting off the fault circuit to avoid the expansion of the accident. In some large power plants, the output current of the generator is accurately measured and monitored by the current transformer to ensure that the generator operates in the best condition and provides a stable power supply to the power grid.
- Industrial automation: In the industrial production process, current transformers are widely used in the current measurement and protection of motors, transformers, and other equipment. By real-time monitoring of the current changes in the equipment, equipment failures can be discovered in time to avoid production accidents. For example, in steel mills, when large motors are running, current transformers monitor the motor current in real-time. Once the current rises abnormally, it indicates that the motor may be overloaded or the winding short-circuited. The system will immediately sound an alarm and take corresponding protective measures. In addition, current transformers are also used in current feedback control systems to achieve precise control of motor speed and torque and improve production efficiency and product quality. On automated assembly lines, current transformers are used to accurately control the operation of motors to ensure accurate coordination of all links in the production line.
- New energy field: In new energy systems such as photovoltaic power generation and wind power generation, current transformers are used to monitor and control the current of power generation equipment. Since the current fluctuations of new energy power generation systems are large, current transformers can convert them into stable low-current signals, which are convenient for measuring, monitoring, and protecting the operation of equipment. In solar photovoltaic power stations, current transformers monitor the output current of photovoltaic panel arrays and feedback data to the control system to achieve maximum power point tracking control of photovoltaic panels and improve power generation efficiency; in wind farms, current transformers monitor the current of wind turbines to detect wind turbine failures promptly and ensure the stable operation of wind power generation.
- Transportation: In rail transit systems, current transformers are used to monitor and control the contact network of trains. Since the voltage of the contact network of trains is very high, current transformers can convert it into a measurable or controllable low current, which facilitates the access and operation of monitoring and control equipment and improves the safety and reliability of rail transit. For example, when the train is running, the current transformer monitors the contact network current in real-time. Once there is an abnormal current, such as a short circuit or overload, the system will take timely measures to ensure the safety of the train operation.
- Communication system: In communication systems, current transformers are mainly used for lightning protection and anti-interference. By reducing high current to a measurable or controllable low current, current transformers can protect communication equipment from weather factors such as lightning and improve the reliability and stability of the communication system. In some outdoor communication base stations, installing current transformers can effectively prevent the instantaneous high current generated by lightning from damaging communication equipment.
- Computer room and precision instrument testing: In computer rooms, data centers, and precision electronic instrument testing, current transformers are used for power supply monitoring and fault detection. They can monitor the current changes in power supply equipment in real-time, ensure the safe and stable operation of the equipment, and issue an alarm or cut off the power supply in time when the equipment fails. In large data centers, the current transformer monitors the power supply current of the server, timely discovers the hidden dangers of power failure, and ensures the normal operation of the data center.
Use and maintenance: Make the current transformer run efficiently
The correct use and maintenance of the current transformer is the key to ensuring its stable operation and accurate measurement, which is not only related to the normal operation of the power system but also affects the service life and safety of the equipment.
Choose a suitable current transformer
When choosing a current transformer, multiple factors should be considered comprehensively. The first is the current size. According to the maximum load current of the measured circuit, the appropriate rated primary current is selected, which is generally 1.0 to 1.3 times the load current when the line is operating normally. This can ensure that the transformer can provide accurate current conversion under most working conditions and avoid saturation problems caused by excessive current. For example, if the normal load current of a line is 200A, it is more appropriate to choose a current transformer with a rated primary current of about 250A.
Accuracy requirements should not be ignored. For current transformers used for measurement purposes, it is recommended to use models with an accuracy level of 0.5 or 0.2. When the load current changes greatly, or when the load current is lower than 30% of the rated current of the primary side of the current transformer during normal operation, a transformer with an accuracy level of 0.5 should be selected to ensure the accuracy and stability of the measurement. For example, in power metering, to accurately calculate the amount of electricity, current transformers with a precision of 0.2 or even higher are usually selected.
The installation environment will also affect the selection of current transformers. Different environmental conditions, such as temperature, humidity, altitude, pollution level, etc., have different degrees of influence on the performance of the transformer. In an environment with high temperature, high humidity, or corrosive gas, a current transformer with a corresponding protection level should be selected, such as a dry-type current transformer with good sealing performance and corrosion resistance; in high-altitude areas, due to the thin air and poor heat dissipation conditions, a transformer that can adapt to the high-altitude environment needs to be selected. In addition, according to the installation location and space size, choose the current transformer with the appropriate installation method and external dimensions, such as pillar type, busbar type, bushing type, etc.
Precautions for use
The correct connection of the current transformer is crucial. When connecting, pay attention to the polarity of the primary and secondary coil terminals. In my country, L1 and L2 are generally used to indicate the primary coil end, and K1 and K2 are used to indicate the secondary coil end pin. L1 and K1 have the same polarity, and L2 and K2 have the same polarity. From the current direction, if the primary current flows from L1 to L2 at a certain moment, the secondary current flows from K2 to K1. If the polarity is reversed, the current passed by the instrument and relay connected to the secondary side is not a normal current, which may cause an accident in serious cases.
When using a current transformer, open circuits and short circuits on the secondary side must be strictly avoided. The secondary side is open-circuited, which is very dangerous because when the secondary side is open-circuited, the secondary current is zero, and no magnetic flux can be generated to offset the effect of the primary side’s magnetic flux. The primary current will be used for excitation, and the core magnetic density will increase sharply, resulting in overheating of the core and burning of the transformer; at the same time, due to the saturation of the core, high voltage will be generated on the secondary side of the current transformer, endangering the safety of personnel and equipment. Therefore, when disassembling the instrument, the secondary coil of the current transformer must be short-circuited first. Short-circuiting the secondary side is also undesirable.
Although the secondary side is similar to a short circuit when the current transformer is working normally, if an accidental short circuit occurs, it may cause excessive current, damage the transformer, or affect the measurement accuracy. Regular maintenance and calibration are important measures to ensure the performance of the current transformer. Clean the transformer regularly to remove accumulated dust and dirt to prevent it from affecting the insulation performance and heat dissipation effect of the transformer. Check whether the connection part is loose or corroded to ensure that the circuit connection is stable and reliable. According to the manufacturer’s recommendations, the transformer should be calibrated and inspected regularly, and the calibration results should be recorded. If deviations or abnormalities are found, corrective measures should be taken in time, such as adjusting or replacing the transformer. Generally speaking, for applications with high accuracy requirements, it is recommended to perform calibration at least once a year; for ordinary applications, the calibration cycle can be appropriately extended according to actual conditions, but it should not exceed 3 years.
The matching of the secondary load is also critical. The secondary rated capacity of the current transformer must be greater than the actual secondary load, and the actual secondary load should be maintained between 25% and 100% of the secondary rated capacity. The load impedance is not only affected by the resistance and reactance of the measuring instrument and relay coil but also by factors such as the wiring contact resistance and the resistance of the secondary connection wire. If the secondary load impedance is too large, the output voltage of the transformer will be reduced, thereby increasing the measurement error; if the load impedance is too small, the transformer may be overloaded, affecting its normal operation. When selecting measuring instruments and relays, ensure that their input impedance matches the secondary load impedance of the transformer. At the same time, the cross-sectional area and length of the secondary connection wire should be reasonably selected to minimize the wire resistance to ensure that the secondary side load is within an appropriate range. For example, in long-distance transmission, the wire cross-sectional area should be appropriately increased to reduce the impact of wire resistance on the load.
As a key device in the power system, the current transformer provides indispensable support for the measurement, protection, and control of electricity with its unique working principle. From the common electromagnetic type to the advanced Hall type and photoelectric type, different types of current transformers play their advantages in their respective fields and are widely used in many fields such as power systems, industrial automation, and new energy, ensuring the stable transmission of electricity and the safe operation of equipment. The correct use and maintenance of current transformers can ensure the stability of their performance and the accuracy of measurement, extend the service life of equipment, and escort the reliable operation of power systems.