I. Power transformers
Power transformer is one of the main equipments in power plants and substations. The function of transformer is various, which can not only increase the voltage to transmit electric energy to the power-consuming areas, but also reduce the voltage to various use voltage levels to meet the power demand. In a word, both step-up and step-down must be completed by transformer. In the process of power transmission in power system, there will inevitably be losses of voltage and power. When transmitting the same power, the voltage loss is inversely proportional to the voltage, and the power loss is inversely proportional to the square of the voltage. Transformers are used to increase voltage and reduce power transmission loss.
A transformer consists of two or more coil windings wound on the same iron core. The windings are connected by alternating magnetic field and work according to the principle of electromagnetic induction. The installation position of the transformer should be convenient for operation, maintenance and transportation, and a safe and reliable place should be selected. When using the transformer, the rated capacity of the transformer must be reasonably selected. When the transformer is in no-load operation, it needs large reactive power. These reactive powers should be provided by the power supply system. If the transformer capacity is too large, it will not only increase the initial investment, but also make the transformer run under no-load or light load for a long time, increase the proportion of no-load loss, reduce the power factor and increase the network loss. This operation is neither economical nor reasonable. If the transformer capacity is too small, it will overload the transformer for a long time and damage the equipment easily. Therefore, the rated capacity of transformer should be selected according to the needs of power load, and should not be too large or too small.
Second, the instrument transformer
Accurately speaking, the transformer is a voltage transformer. Transformer includes: current transformer and voltage transformer, which are special transformers. Their two main uses are: one is to measure the range of electrical instruments, and the other is the relay protection device of measuring machines, which can be used to isolate and protect large current, voltage and current, so that the current can be changed into low voltage and small current to continue working and play the role of small-scale power conversion, and can be used as signal automatic devices, control loops and relay protection.
The function of the voltage transformer is to convert the high voltage into the standard secondary voltage of 100V or lower for protection, measurement and instrumentation. At the same time, the use of voltage transformer can isolate high voltage from electrical workers. Although voltage transformer is also a device that works according to the principle of electromagnetic induction, its electromagnetic structure relationship is just the opposite compared with current transformer. The secondary circuit of voltage transformer is a high impedance circuit, and the secondary current is determined by the impedance of the circuit. When the impedance of the secondary load decreases, the secondary current increases, which makes the primary current automatically increase by one component to meet the electromagnetic balance relationship between the primary side and the secondary side. It can be said that voltage transformer is a special transformer with limited structure and use form. Simply put, it is "detection element".
Function of current transformer: The function of current transformer is to convert the primary current with a large value into the secondary current with a small value through a certain ratio, which is used for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5 A. ..
When measuring the large current of alternating current, it needs to be converted into relatively uniform current (the secondary rating of current transformer in China is 5A or 1A) in order to facilitate the secondary instrument measurement. In addition, the voltage on the line is relatively high, so direct measurement is very dangerous.
It is dangerous for current transformer to play the role of current conversion and electrical isolation. It is a sensor used for measuring instruments, relay protection and other secondary equipment in power system to obtain current information of electrical primary circuit. The current transformer converts a large current into a small current in proportion. The primary side of the current transformer is connected to the primary system, and the secondary side is connected to the measuring instrument and relay protection.
Third, the test transformer
It is especially suitable for the insulation strength test of high-voltage electrical equipment, electrical components and insulation materials in power system, industrial and mining enterprises, scientific research departments and so on under power frequency or DC high voltage. It is an indispensable and important equipment in high voltage test. The test transformer is called a booster. According to different media, there are five types of test transformers:
1. Insulated cylindrical test transformer: the complete set of equipment has strong compatibility, complete voltage capacity series and complete functions; The impedance voltage is low, and the system impedance is not more than 5%, which meets the requirements of AC pollution test; Adopt fast electronic protection device, high reliability. It is mainly used to test the insulation level of various insulation materials, insulation structures and electrical products that can withstand power frequency voltage, and also used as the power supply for non-local discharge frequency test of transformers, transformers, lightning arresters and other samples. Widely used in electrical manufacturing departments, power operation departments, scientific research units and universities. The insulation cylinder test transformer adopts epoxy casting cylinder shell, which is completely insulated, improves the insulation strength and has high output voltage. The highest voltage level of a single device can reach 750kV. The insulation cylinder test transformer has strong moisture resistance, which effectively weakens the magnetic leakage and greatly increases the impact resistance of the test short-circuit current. 2. Oil-immersed test transformer: large volume, heavy weight, large capacity, convenient maintenance and high later maintenance cost; Mainstream products. Suitable for power system, industrial and mining enterprises, transportation, post and telecommunications departments, scientific research units, etc. Conduct insulation strength tests on various high-voltage electrical equipment, electrical components and insulation materials under power frequency or DC high voltage.
3. Inflatable test transformer: it needs post-maintenance, has good arc extinguishing property, light weight and is dangerous. 4. Dry test transformer: maintenance-free, light in weight, small in size, reversible and high in cost.
5. Series high-voltage test transformer: small capacity, low voltage, light weight, convenient transportation, many connections and high cost.
Four, special transformer
In addition to AC voltage conversion, transformers have other uses, such as changing power frequency, rectifying equipment, welding equipment, electric furnace, voltage transformer and current transformer. These transformers with special purposes are generally called special transformers. Because the working conditions and loads of these transformers are different from those of ordinary transformers, they cannot be calculated by the calculation method of ordinary transformers.
1. energy-saving transformer:
This series of energy-saving transformers is suitable for AC 50HZ and the working voltage is below 500V V V. The insulation and heat resistance level of this series of transformers is class F. The iron core of this series of energy-saving transformers adopts cold-rolled oriented high-quality silicon steel sheets, and the coil structure adopts cylinders, elliptical tubes and rectangular tubes. Small size, excellent performance, safety and reliability.
2.ZSG series rectifier transformers: widely used in industrial charging, DC lighting, DC excitation, electroplating, electric traction, etc. ZSG series rectifier transformers have the advantages of small size, light weight, simple maintenance and reliable operation.
3. Special transformers for EPS and UPS: The special transformers for EPS and UPS have the characteristics of high permeability, low loss, low noise, low temperature, radiation resistance, non-toxicity, maintenance-free and high environmental protection. The special transformer has high impedance, high space efficiency and high power conversion efficiency.
4.QZB series autotransformer for step-down starting: the starting current is reduced by reducing the power supply voltage of autotransformer, and the starting current is changed by selecting different taps of autotransformer to achieve the purpose of changing the starting torque. This starting method has little influence on the power supply voltage, less mechanical impact and shorter starting time of the motor, and is usually used for infrequent starting and stopping of three-phase squirrel-cage asynchronous motors below 320KW.
5.BK series control transformer: used as power supply for control, lighting and indicator lights of machine tools and mechanical equipment in various industries.
Transformer plays a vital role in the circuit. To ensure the normal operation of transformer, improve the working life and performance of transformer, choose the type of transformer according to the actual situation.