Power transformer is a static electrical equipment, which is used to change the AC voltage (current) of a certain value into another voltage (current) with the same frequency or several different values.
Transformer is a static electrical equipment used to transform AC voltage and current to transmit AC energy. It realizes electric energy transmission according to the principle of electromagnetic induction. Transformers can be divided into power transformers, test transformers, instrument transformers and transformers for special purposes: power transformers are necessary equipment for power transmission and distribution and power user distribution; The test transformer is used to conduct withstand voltage (step-up) test on electrical equipment; Instrument transformer is used for electrical measurement and relay protection of distribution system (PT, CT); Transformers for special purposes include furnace transformers for smelting, welding transformers, rectifier transformers for electrolysis, small voltage regulating transformers, etc.
Power transformer is a static electrical equipment, which is used to change the AC voltage (current) of a certain value into another voltage (current) with the same frequency or several different values. When the primary winding is connected with AC, alternating magnetic flux is generated. The alternating magnetic flux induces AC electromotive force in the secondary winding through the magnetic conduction of the iron core. The secondary induced electromotive force is related to the number of turns of the primary and secondary windings, that is, the voltage is directly proportional to the number of turns.
The main function is to transmit electric energy. Therefore, the rated capacity is its main parameter. Rated capacity is a customary value of power, which represents the size of transmitted electric energy, expressed in KVA or MVA. When the rated voltage is applied to the transformer, the rated current that does not exceed the temperature rise limit under specified conditions shall be determined according to it. The more energy-saving power transformer is amorphous alloy core distribution transformer. Its biggest advantage is that the no-load loss value is very low. Whether the no-load loss can be ensured is the core problem to be considered in the whole design process. When arranging the product structure, in addition to considering that the amorphous alloy core itself is not affected by external forces, the characteristic parameters of the amorphous alloy must be accurately and reasonably selected in the calculation.
A transformer is a static electric energy converter that transmits electric energy without changing its frequency.
Transformer is a large number of electrical equipment in the power system and plays a very important role. The total capacity of transformer is about 9 times that of generator. Its function is to increase or decrease the voltage of electric energy in the power system, so as to facilitate the rational transmission, distribution and use of electric energy.
In the power system, the higher the voltage, the smaller the current and the smaller the power loss on the transmission line; The cross-sectional area of the transmission line can also be reduced, which can reduce the metal consumption of the conductor.
Figure 1
Due to manufacturing difficulties, the generator voltage cannot be very high (below 20kV at present). Therefore, in the power plant, it is necessary to use step-up transformer to raise the generator voltage very high, so as to send a large amount of electric energy to distant power consumption areas, such as 35kV, 66kV, 110kV, 220kV, 330kV, 500kV, etc.
At the power load, the step-down transformer is used to reduce the voltage to an appropriate value for the user's electrical equipment. When the power transformer transmits electric energy, it also has some active power loss, but the number is small, so the transmission efficiency is very high. The efficiency of small and medium-sized transformers shall not be less than 95%, and the efficiency of large transformers can reach more than 98%.
2 Classification of power transformers
1. By function:
Power transformers are divided into step-up transformers and step-down transformers according to their functions. Step down transformers are used in plant substations. Step down transformer of terminal substation, also known as distribution transformer.
2. By capacity:
According to the capacity series, power transformers are divided into R8 capacity series and R10 capacity series.
R8 capacity series refers to that the capacity grade increases by R8 1.33 times. This series is adopted for the old transformer capacity grade in China, such as 100kVA, 135kva, 180KVA, 240kVA, 320kva, 420kva, 560kva, 750kva, 1000KVA, etc.
R10 capacity series means that the capacity grade is increased by multiple of R10 1.26. R10 series has dense capacity grade, which is convenient for reasonable selection and is recommended by IEC (International Electrotechnical Commission). This series is adopted for the new transformer capacity grade in China, such as 100kVA, 125kva, 160KVA, 200KVA, 250kVA, 315KVA, 400KVA, 500KVA, 630kVA, 800kVA and 1000KVA.
3. Divided by phase number:
Power transformers are divided into single-phase and three-phase according to the number of phases. Factory substations usually use three-phase power transformers.
4. According to voltage regulation mode:
According to the voltage regulation mode, power transformers are divided into on load voltage regulation (also known as off excitation voltage regulation) and on load voltage regulation. Most factory substations use no-load voltage regulating transformers.
5. According to winding structure:
According to the winding structure, power transformers include single winding autotransformer, double winding transformer and three winding transformer. Most factory substations use double winding transformers.
6. Classification by winding insulation and cooling mode:
Power transformers are divided into oil immersed, dry-type and inflatable (SF6) according to winding insulation and cooling mode. Oil immersed transformers include oil immersed self cooling type, oil immersed air cooling type, oil immersed water cooling type and forced oil circulating cooling type. Most factory substations use oil immersed self cooling transformers.
Figure 2
The so-called inflatable transformer refers to the transformer whose magnetic circuit (core) and winding are located in a shell filled with insulating gas. In the past, SF6 gas was generally used, so it is also called gas insulated transformer
7. By winding conductor material:
Power transformers are divided into copper winding transformers and aluminum winding transformers according to the material of winding conductors. In the past, aluminum winding transformers were mostly used in factory substations, but copper winding transformers with low loss have been more and more widely used.

Difference between
The current sensor can detect both AC and DC. Measuring AC and DC flows out of AC and DC. External power supply is required.
The current transformer can only measure AC. when the output signal is AC 5A or ac1a, there is no need for external power supply.
Ordinary current transformer can only measure AC current. If the AC current transformer is open circuit, it will produce high voltage. In serious cases, it will break through the line insulation and produce electric shock, threatening personnel safety. Therefore, the secondary side cannot be open circuit during use, and the secondary side must be short circuited during disassembly.
Hall current sensor, DC and AC current can be measured. The secondary side does not have to be short circuited when the Hall current sensor is removed.
Ordinary current transformer can be measured directly without power supply. The Hall current sensor needs plus or minus 15V or plus or minus 12V working power supply.
Current transformers generally output AC 5 A or AC 1 A. The Hall current sensor generally outputs a small signal of 50mA or 100mA or 5V, and can also output 4-20mA, which is convenient for automatic control.
In terms of measurement accuracy, Hall current sensor is also better than current transformer. The measured current distortion, multiple harmonics and non sine wave have a great impact on the accuracy of current transformer measurement results. The influence on Hall current sensor is much smaller.
In the measurement frequency range, the Hall current sensor is much wider than the current transformer.
In addition, the response time of Hall current sensor is faster than that of current transformer.
The common current sensor / transformer converts a large current into a small current with the same frequency and phase for measurement or isolation. According to different transformation principles, there are current sensors / Transformers Based on electromagnetic induction principle, Hall effect and fluxgate.
Current transformer is similar to a transformer with few primary turns and more secondary turns. Ideally, the current ratio of the primary stage is inversely proportional to the turns ratio, and the current conversion ratio is marked with the rated current of the primary stage, such as "300A / 5A", indicating that the output current is 5A when the measured current is rated at 300A. Due to the leakage inductance and resistance of the primary coil, as well as the nonlinear excitation current and core magnetization curve, the ratio error and phase error of the transformer will be caused. The accuracy of transformer used for metering and billing is generally level 0.1 1. According to the principle of transformer, it can not measure DC current. It is usually designed for power frequency measurement. The accuracy is the parameter under power frequency. The bandwidth is narrow, so it is not suitable for harmonic analysis and non sinusoidal measurement. When using current transformer, be sure not to open the secondary circuit, otherwise high voltage will be generated, endangering personal and equipment safety.
Current transformer
The iron core in the current clamp is divided into two parts to avoid disconnecting the measured circuit. It is very easy to measure and widely used. There are two types based on the principle of electromagnetic induction and Hall effect.
The current clamp based on the principle of electromagnetic induction is the same as the transformer. The iron core is divided into two parts. When closed, the two parts of the iron core need to be closely combined. Some current clamps are connected to the secondary resistance, and the output is voltage signal, and the output without internal resistance is current signal. Affected by the closure degree of two iron cores, the accuracy of current clamp is usually worse than that of transformer. Similarly, the current clamp based on electromagnetic induction can only measure AC.
The current clamp based on Hall effect processes an air gap in the iron core to place the hall element. Hall element is used to measure the magnetic induction intensity in the air gap. According to different control modes, there are two types: open-loop and closed-loop. The open-loop hall type uses a Hall element with good linearity, and the output voltage of the hall element is directly proportional to the measured current. The closed-loop hall type uses zero flux technology, and there is a compensation coil on the iron core. When the primary measured current generates magnetic flux in the iron core, the hall element detects the magnetic induction intensity in the iron core and converts the error voltage into current to drive the compensation coil through negative feedback to offset the magnetic flux in the iron core. Finally, the measured current is consistent with the magnetic flux generated by the compensation coil in the opposite direction, By measuring the current of the compensation coil, the measured current can be converted according to the turns ratio.
Both open-loop and closed-loop Hall current clamp can measure DC and AC. The open-loop hall is affected by the nonlinearity of the iron core and the temperature characteristics of the hall element, the accuracy and linearity are poor, but the cost is low. The closed-loop hall has less dependence on the linearity of the hall element, and the iron core works under zero flux, so the accuracy is higher than that of the open-loop. However, the current clamp has the problem that the closing degree of the movable iron core is not ideal. There is almost no one equal to or better than 0.1%, and being able to achieve 1% is a very high index. The hall element needs to provide working voltage, so both current clamps need to supply power, and the closed-loop hall needs to drive the compensation coil, which consumes more power.
Current clamp
Generally, like Hall current clamp, there are two types of open-loop and closed-loop hall, and the output is current or voltage signal. Due to the closed form, the accuracy is higher than that of the same type of current clamp.
In addition, there are current sensors using fluxgate technology, whose accuracy is better than 0.05% or even 12ppm, but this type of sensor is very expensive and fragile. Once the sensor is not powered in use, the measured current will cause damage to the sensor.
Closed mouth current sensor
4 Connection between current sensor / transformer and power analyzer
The current measurement of pa5000 power analyzer has two groups of ports: direct input and sensor input. When the output of the used sensor / transformer is current signal, it is connected to the current direct input port of pa5000. Correctly set the "CT" proportion coefficient of PA according to the used sensor / transformer, such as 300A / 5A, then CT = 300A / 5A = 60.
When the output of the current sensor / current transformer used is a voltage signal, connect it to the external sensor port (BNC interface) of pa5000, and correctly set the "sensor ratio" of PA according to the sensor / transformer used, with the unit of MV / A. if the current clamp parameter is converted from 1000A to 1V, the sensor ratio is set to 1mV / A.

Design and example of low power transformerTaking the transformer used in the article of Weiner electronic power supply technology using TL431 as high-power adjustable regulated power supply (hereinafter referred to as "Li") as an example, we will lead you to complete the whole process of transformer design step by step.In this article, we do not deduce the relevant application formulas, but hope the readers can draw inferences from this design example.

1. Calculate secondary output power (P2)The maximum output voltage in Li is 24V. Assuming that the rated output current is 1a, the voltage drop of K790 tube is 3V, and the power consumption of voltage doubling rectifier circuit is ignored, then:P2=(243)x1=27W2. Calculate primary power (P1)Assumed transformer efficiency = 0.75, then P1 = 27W / 0.75 = 36WNote: the efficiency of transformer varies slightly according to the output power. Generally, for transformers with capacity below 100W Between 0.7-0.8, below 100W to 1000W, between 0.8-0.9. In practical application, the lower output power shall be taken as the smaller value.3. Calculate the wire diameter of primary and secondary coils (d)Where, I - winding working current, J - current density (usually J is 3-3.5a/mm2)Primary winding current I1 = 36 / 220 = 0.164 (a)3.1 wire diameter of primary winding3.2 wire diameter of secondary winding:Secondary winding current I2 = 1x1.17 = 1.17a

Where 1.17 is the rectification coefficient of secondary ac current of transformer.Since there is no 0.67mm in the specification of enamelled wire, it is taken as 0.7mm.Usually, we take the current density of the secondary coil as a smaller value to obtain a small internal resistance of the power supply and reduce the temperature rise.4. Calculate the cross-sectional area of iron coreWe use the following formula to calculate the cross-sectional area of iron core (I think it is a relatively simple empirical formula)Where s - sectional area of iron core K - coefficient P2 - secondary powerThe value of K is related to the output power of the transformer. For K below 100W, take 1.25-1.1 (the lower value is taken for the higher power), and take 1 directly for 100W-1000W. In this example, take 1.15, then:Theoretically, under the condition that the cross-sectional area of the iron core remains unchanged, the tongue width and stack thickness of the transformer iron core can be in any proportion. However, in the actual design, the manufacturing process of the coil, the symmetry of the shape, leakage reactance and other factors should be considered. Generally, the tongue width is about 1.5 2 times the stack thickness. In this example, the EI sheet with a width of 66mm is selected, and the stack thickness is 2.7cm.5. Turns of primary and secondary windings5.1 calculation of turns per volt (W0)Where, F -- alternating current frequency (Hz), B -- magnetic flux density (T), s -- sectional area of iron core (cm2)The value of B will vary depending on the iron core material. See the following table for general silicon steel sheet materials and their values:In this example, H23 tablets are selected, and B is 1.42, then5.2 number of primary turns (W1)W1=220W0=1160(T)5.2 secondary turns (W2)Where, 1.2 is the coefficient of adding filter capacitor after rectification, and 1.12 is to compensate the secondary voltage drop caused by winding impedance after load. Generally, the number of secondary turns shall be increased by 5% 25% (the larger value shall be taken if the output power is small).So far, the design of the transformer has been basically completed.

1. Basic working principle of gas protection

When a fault occurs inside the power transformer oil tank (including the grounding short circuit through arc resistance caused by slight turn to turn short circuit and insulation damage), due to the action of fault point current and arc, transformer oil (usually 25# insulating oil) and other insulating materials will decompose due to local heating to produce gas. Because the gas is relatively light, they will flow from the oil tank to the upper part of the conservator. When the fault is serious, the oil will expand rapidly and produce a large amount of gas. At this time, violent gas mixed with oil flow will rush to the upper part of the conservator to make the contact of the relay act, connect the specified control circuit, and send a signal in time or automatically cut off the transformer to form a protection device that acts in response to the above gas, which is called gas protection.Gas relay is the main component of gas protection, such as fj3-80 and QJ series. It is installed on the connecting pipe between the oil tank body and the conservator. In order to increase the sensitivity and reliability of gas protection, all the gas generated by faults in the transformer must pass through the gas relay smoothly, Therefore, when installing the transformer, the oil tank body shall be inclined to the conservator by 1 1.5%, and the oil pipe shall be inclined to the conservator by 2 4%.The operation of the old gas relay (float mercury contact relay) is not stable enough, and its anti vibration ability is poor, which is easy to cause misoperation due to mercury flow due to vibration. The open cup baffle gas relay has high vibration resistance and reliable action. Therefore, the open cup baffle gas relay is widely used in China's power system.During normal operation, the relay is filled with transformer oil. In case of slight internal fault of the transformer, a small amount of gas generated by decomposition of the transformer rises and accumulates in the air chamber at the upper part of the gas relay, forcing the oil level to rotate and fall, so that the upper magnetic steel is close to a pair of dry spring contacts and sends a light gas fault signal after action. If it is a serious fault, a large amount of gas will be generated. At the same time, the oil temperature will rise, the hot oil will expand, and the pressure in the tank will increase sharply, resulting in the rapid impulse of oil and air flow to the lower baffle of the relay, resulting in the lower magnetic steel close to another pair of dry spring contacts and acting on tripping.2. Setting of gas relay

The setting of gas relay can be carried out through differential pressure flow rate test or oil pump oil speed test equipment. The starting value is set according to the oil flow speed. Generally, 0.5 1.5m/s is used for heavy gas. When the transformer has forced oil circulation device, the range of 1 1.5m/s is used; when there is no forced oil circulation device, the range of 0.6 1m / S is used; For light gas, the volume of the space occupied by the gas in the relay is set, and the signal is sent when the range is 250 300CM3.

Precautions for use of current transformerPay attention to the following points when using current transformer(1) The configuration of current transformer shall meet the requirements of measuring meter, relay protection and automatic device, and shall be powered by separate secondary winding respectively. Only in this way can it meet the requirements of different equipment without mutual influence.

(2) When selecting current transformer, the rated voltage shall be equal to the voltage of the tested circuit, its primary rated current shall be greater than the maximum continuous working current of the tested circuit, and its structure, model and capacity shall meet the accuracy requirements.(3) Open circuit is not allowed for the secondary winding of current transformer in operation.(4) The secondary winding, iron core and shell of current transformer shall be reliably grounded, and only one grounding point is allowed.(5) According to the development of the power grid, check whether the dynamic thermal stability current and rated continuous thermal current of the current transformer meet the requirements under the actual operating current ratio. If the short-circuit current of the substation exceeds the specified value on the nameplate of the current transformer, change the current ratio or replace the current transformer in time.(6) If there is doubt about the rated continuous thermal current, the secondary winding shall be short circuited for temperature rise test. If it does not meet the requirements, the current transformer shall be replaced.What is the harm of open circuit at the secondary side of current transformerIn normal operation, the impedance of the current transformer is very small and close to the short-circuit state. Most of the magnetic potential generated by the primary current is offset by the magnetic potential generated by the secondary current, so that the total magnetic flux in the iron core and the induced potential of the secondary winding are small. However, when the secondary side of the current transformer is open circuit, it is equivalent that the load impedance Z is infinite and the secondary current is zero. The secondary current cannot generate magnetic potential to offset the magnetic potential on the primary side. The total magnetic flux is equal to the magnetic flux potential of the primary winding, that is, the primary current completely becomes the excitation current. As a result, the magnetic flux density of the iron core increases, resulting in iron core saturation and iron loss increase, resulting in serious heating of TA iron core, It may even burn out the insulation; At the same time, the increase of the magnetic flux density of the iron core will also distort the magnetic flux waveform into a flat top wave, which will induce a high voltage on the secondary side when the magnetic flux passes through zero, and its peak value can reach thousands of volts, which not only endangers the insulation of the current transformer itself, but also is extremely unsafe for the insulation of human body and secondary equipment. Therefore, the open circuit on the secondary side of the current transformer is never allowed in operation. If the secondary side of the current transformer in operation is open circuit, relevant protection and automatic devices can be disabled during treatment to reduce the secondary load to zero.