What is a transformer

The English definition of mutual inductor: instrument transformer

Definition of Transformer

A transformer is a device that converts voltage or current in proportion. The function of a transformer is to convert high voltage or large current into standard low voltage (100V) or standard small current (5A or 10A, both refer to rated values) in proportion, so as to achieve standardization and miniaturization of measuring instruments, protection equipment and automatic control equipment. Transformers can also be used to isolate high voltage systems to ensure the safety of people and equipment.

The principle of mutual inductor

In the power supply lines, the current and voltage vary greatly, ranging from a few amperes to tens of thousands of amperes. In order to facilitate the measurement of secondary instruments, it is necessary to convert them into a relatively uniform current. In addition, the voltage on the line is relatively high. It is very dangerous to measure directly. The current transformer plays the role of current conversion and electrical isolation.
Earlier, most of the display instruments were pointer-type current and voltage meters, so the secondary current of the current transformer was mostly ampere-level (such as 5A, etc.). Now most of the power measurements are digital, and the sampled signals of the computer are generally milliampere-level (0-5V, 4-20mA, etc.). The secondary current of the micro current transformer is milliampere-level, and it mainly acts as a bridge between the large transformer and the sampling. The
micro current transformer is called "instrument current transformer". ("Instrument current transformer" has a meaning of multi-current ratio precision current transformer used in the laboratory, generally used to expand the instrument range.)
Current transformer principle circuit diagram Micro current transformer is similar to the transformer and works according to the principle of electromagnetic induction. The transformer transforms the voltage while the micro current transformer transforms the current. As shown in the figure, winding N1 is connected to the measured current, which is called the primary winding (or primary winding, primary winding); winding N2 is connected to the measuring instrument, which is called the secondary winding (or secondary winding, secondary winding).
The current ratio of the primary winding current I1 of the micro current transformer to the secondary winding I2 is called the actual current ratio K. The current ratio of the micro current transformer when working at the rated working current is called the rated current ratio of the current transformer, which is represented by Kn.
Kn=I1n/I2n
Micro current transformers can be roughly divided into two categories, measuring current transformers and protective current transformers.
Measuring current transformers
Measuring current transformers are mainly used in conjunction with measuring instruments. Under normal working conditions of the line, they are used to measure current, voltage, power, etc.
The main requirements for measuring micro current transformers are:
1. Reliable insulation, 2. Sufficiently high measurement accuracy, 3. When a large current occurs in a fault in the measured line, the transformer should be saturated within an appropriate range (such as 500% of the rated current) to protect the measuring instrument.
Protective current transformer
Protective current transformers are mainly used in conjunction with relay devices. When short circuit, overload and other faults occur in the line, they provide signals to the relay devices to cut off the fault circuit to protect the safety of the power supply system. The working conditions of protective micro current transformers are completely different from those of measuring transformers. Protective transformers only start to work effectively when the current is several or dozens of times larger than the normal current.
The main requirements for protective transformers are: 1. Reliable insulation, 2. Sufficiently large accuracy limit coefficient, 3. Sufficient thermal stability and dynamic stability. The
maximum primary current that a protective transformer can meet the accuracy requirements under rated load is called the rated accuracy limit primary current. The accuracy limit coefficient is the ratio of the rated accuracy limit primary current to the rated primary current. When the primary current is large enough, the iron core will be saturated and will not reflect the primary current. The accuracy limit coefficient represents this characteristic. The accuracy level of protective transformers is 5P and 10P.

The role of mutual inductor

The mutual inductor used in the power system is a special transformer that transmits the information of high voltage and high current of the power grid to the metering, measuring instruments, relay protection and automatic devices on the secondary side of low voltage and low current. It is a contact element between the primary system and the secondary system. Its primary winding is connected to the power grid, and the secondary winding is connected to the measuring instruments, protection devices, etc. The mutual inductor can measure the voltage, current and electric energy of the primary system in conjunction with the measuring instruments and metering devices; it can form electrical protection and automatic control of various faults in the power grid in conjunction with the relay protection and automatic devices. The performance of the mutual inductor directly affects the accuracy of the measurement and metering of the power system and the reliability of the action of the relay protection device.
Transformers are divided into two categories: voltage transformers and current transformers. Their main functions are: to accurately transmit the voltage and current information of the primary system to related equipment on the secondary side; to convert the high voltage and large current of the primary system into low voltage (standard value) and small current (standard value) on the secondary side, so as to standardize and miniaturize measuring and metering instruments, relays and other devices, and reduce the insulation requirements for secondary equipment; to electrically isolate the secondary side equipment and the secondary system from the high-voltage equipment of the primary system, thereby ensuring the safety of secondary equipment and personnel.

Transformer Classification

Transformers are divided into two categories: voltage transformers and current transformers. Voltage transformers can be used in high-voltage and near-high-voltage power systems for voltage and power measurement, etc. Current transformers can be used for the measurement of exchange current, exchange electricity, and protection in power traction lines.
Classification of voltage transformers
According to use
Measuring voltage transformers (or measuring windings of voltage transformers. Within the normal voltage range, they provide grid voltage information to measuring and metering devices.
Protective voltage transformers (or protective windings of voltage transformers. Under grid fault conditions, they provide grid fault voltage information to relay protection and other devices.
According to insulating
medium Dry-type voltage transformer. Made of ordinary insulating material impregnated with insulating paint as insulation, mostly used in low voltage levels and below.
Cast insulated voltage transformer. Made of epoxy resin or other resin mixed materials, mostly used in voltage levels and below.
Oil-immersed voltage transformer. Made of insulating paper and insulating oil as insulation, it is the most common structural type in China and is often used in voltage levels and below.
Gas-insulated voltage transformer. Made of gas as the main insulation, mostly used in higher voltage levels.
Usually, low-voltage transformers used exclusively for measurement are dry-type, and high-voltage or ultra-high-voltage sealed gas-insulated (such as sulfur hexafluoride) transformers are also dry-type. Casting type is suitable for voltage transformers of 35kV and below, and products above 35kV are all oil-immersed.
According to the number of phases
Most products are single-phase, because the voltage transformer has a small capacity and a small body size, and the internal and external insulation requirements between three-phase high-voltage bushings are difficult to meet, so only 3-15kV products sometimes use a three-phase structure.
According to the voltage conversion
principle Electromagnetic voltage transformer. It transforms voltage based on the principle of electromagnetic induction. The principle and basic structure are completely similar to transformers. It is mostly used in China at voltage levels of 1000A and below. Capacitive
voltage transformer. It consists of a capacitive voltage divider, a compensating reactor, an intermediate transformer, a damper, and a carrier device protection gap. It is used in a neutral point grounding system for voltage measurement, power measurement, relay protection, and carrier communication.
Photoelectric voltage transformer. It uses the principle of photoelectric conversion to achieve voltage conversion. It is still under development.
According to the conditions of use, it is divided
into indoor voltage transformers. It is installed in indoor power distribution devices and is generally used at voltage levels of 1000A and below.
Outdoor voltage transformer. It is installed in outdoor power distribution devices and is mostly used at voltage levels of 1000A and above.
According to the operating status of the primary winding to the ground,
it is divided into Voltage transformer with grounded primary winding. The end of the primary winding of a single-phase voltage transformer or the neutral point of the primary winding of a three-phase voltage transformer is directly grounded. Voltage
transformer with ungrounded primary winding. Both ends of the primary winding of a single-phase voltage transformer are insulated from the ground; all parts of the primary winding of a three-phase voltage transformer, including the wiring terminals, are insulated from the ground, and the insulation level is consistent with the rated insulation level.
According to the magnetic circuit structure
Single-stage voltage transformer. The primary winding and the secondary winding (multiple secondary windings can be set as needed) are wound on the same iron core, and the iron core is at ground potential. China uses single-stage at and below voltage levels.
Serial voltage transformer. The primary winding is divided into several units with the same number of turns and connected in series between the phase and the ground. Each unit has its own independent iron core, with multiple iron cores, and the iron core carries a high voltage. The secondary winding (multiple secondary windings can be set as needed) is in the last unit connected to the ground. This type of structure is currently commonly used in China's voltage levels.
Combined transformer
A transformer that is a combination of a voltage transformer and a current transformer is called a combined transformer. There are also transformers that are produced in conjunction with combined electrical appliances and are called combined transformers.
Classification of current transformers
According to the purpose
Measuring current transformer (or measuring winding of current transformer. Within the normal working current range, it provides the current information of the power grid to the measuring and metering devices.
Protective current transformer (or protective winding of current transformer. Under the power grid fault state, it provides the power grid fault current information to the relay protection and other devices.
According to the insulation medium
Dry current transformer. It is made of ordinary insulating materials after varnishing as insulation.
Casting current transformer. Current transformer cast with epoxy resin or other resin mixed materials. Oil-
immersed current transformer. It is made of insulating paper and insulating oil as insulation, generally outdoor type. At present, it is commonly used in various voltage levels in China.
Gas insulated current transformer. The main insulation is composed of gas.
According to the current conversion principle
Electromagnetic current transformer. A current transformer that realizes current conversion based on the principle of electromagnetic induction.
Photoelectric current transformer. A current transformer that realizes current conversion through the principle of photoelectric conversion is still under development. According
to the installation method
Through-type current transformer. A current transformer used to pass through a screen or wall.
Pillar-type current transformer. A current transformer installed on a plane or pillar and used as a primary circuit conductor pillar.
Bushing-type current transformer. A current transformer that has no primary conductor and primary insulation and is directly sheathed on an insulating bushing. Busbar current transformer
. A current transformer that has no primary conductor but has primary insulation and is directly sheathed on a busbar.