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Power transformer related information
Introduction to Power Transformer
The function of the power transformer is power transmission, voltage conversion and insulation isolation. As a main soft magnetic electromagnetic component, it is widely used in power technology and power electronics technology. According to the size of the transmission power, the power transformer can be divided into several levels: 10kVA or more for high power, 10kVA to 0.5kVA for medium power, 0.5kVA to 25VA for low power, and 25VA or lower for micro power. The design of the power transformer is different for different transmission power, which should be self-evident. According to its main function is power transmission, the English name "Power Transformers" is translated into "power transformer", and it is still used in many literatures. Should it be called "power transformer" or "power transformer"? It is up to the authority in scientific terminology to decide.
It is used in almost all electronic products. Its principle is simple but the winding process of the transformer will have different requirements according to different applications (different uses). The main functions of transformers are: voltage conversion; impedance conversion; isolation; voltage stabilization (magnetic saturation transformer), etc. The core shapes commonly used in transformers are generally E-type and C-type cores.
1. Transformer ---- Static electromagnetic device. The transformer can convert AC voltage of one voltage into AC voltage of another voltage at the same frequency. The main components of the voltage transformer are an iron core and two windings on the core.
Transformer principle The coil connected to the power source receives AC power and is called a primary winding, also called a primary winding.
The coil connected to the load sends AC power, which is called the secondary winding, also called the secondary winding.
Voltage phasor U1 Voltage phasor U2
Current phasor I1 current phasor I2
EMF phasor E1 EMF phasor E2
Number of turns N1 Number of turns N2
2. The ideal transformer does not count the resistance and iron loss of the primary and secondary windings,
The transformer with the coupling coefficient K = 1 is called an ideal transformer. The ideal EMF balance equation is
e1 (t) = -N1 d φ / dt
e2 (t) = -N2 d φ / dt
If the instantaneous values of the voltage and electromotive force of the primary and secondary windings change according to the sine law,
There is no core loss. According to the principle of energy conservation, the relationship between the voltage of the primary and secondary windings and the effective value of the current is K = N1 / N2, which is called the turn ratio (also called the voltage ratio). Then the structure of the transformer Introduction
1. Iron core The iron core is the main magnetic circuit part in the transformer. Usually it has a high silicon content, the thickness is 0.35 \ 0.3 \ 0.27 mm,
Hot-rolled or cold-rolled silicon steel sheets coated with insulating varnish on the surface are stacked or wound. The core is divided into two parts, the core post and the cross-section. The core post is covered with windings; the cross-section is the basic form of the core structure used to close the magnetic circuit. Heart-type and shell-type
2. Winding winding is the circuit part of the transformer,
It is a double-wire wrapped (paper-wrapped) insulated flat wire or enameled round wire wound into a transformer. The basic principle is the principle of electromagnetic induction. Now a single-phase double-winding transformer is used as an example to illustrate its basic working principle: When the voltage & Uacute; 1, the current & Iacute; 1 flows, and an alternating magnetic flux & Oslash; 1 is generated in the iron core. These magnetic fluxes are called main magnetic fluxes. Under its effect, the windings on both sides respectively induce electric potential & Eacute; 1, & Eacute; 2, the formula of the induced potential is: E = 4.44fN & Oslash; m
In the formula: E--effective value of induced potential
& Oslash; m--The maximum value of the main magnetic flux is different because the number of turns of the secondary winding and the primary winding is different. The magnitudes of the induced potentials E1 and E2 are also different. When the internal impedance voltage drop is omitted, the voltage & Uacute; 1 and & Uacute; 2 are also the same. different.
When the secondary side of the Satons transformer is unloaded, only the primary magnetic flux current (& Iacute; 0) flows on the primary side. This current is called the exciting current. When the load current & Iacute; 2 flows through the load on the secondary side, a magnetic flux is also generated in the iron core in an attempt to change the main magnetic flux, but when the primary voltage is constant, the main magnetic flux is constant, and the primary side will flow. After passing through two parts of current, one is the exciting current & Iacute; 0 and the other is used to balance & Iacute; 2, so this part of the current changes with the change of & Iacute; 2. When the current is multiplied by the number of turns, it is the magnetic potential.
The above-mentioned balance effect is essentially a magnetic potential balance effect, and the transformer realizes energy transfer on the primary and secondary sides through the magnetic potential balance effect.
Power transformer parameters
First, transformer technical parameters have corresponding technical requirements for different types of transformers, which can be expressed by corresponding technical parameters. For example, the main technical parameters of power transformers are: rated power, rated voltage and voltage ratio, rated frequency, operating temperature level, The main technical parameters of general low-frequency transformers for temperature rise, voltage regulation, insulation performance and moisture resistance are: transformer ratio, frequency characteristics, nonlinear distortion, magnetic and electrostatic shielding, efficiency, etc.
A. Voltage ratio:
The two coils of the transformer are N1 and N2, N1 is the primary and N2 is the secondary. When an AC voltage is applied to the primary coil, induced electromotive force will be generated across the secondary coil. When N2> N1, its induction The electromotive force is higher than the voltage applied to the primary. This transformer is called a boost transformer: when N2 <N1, the induced electromotive force is lower than the primary voltage. This transformer is called a step-down transformer. Primary secondary voltage and coil The number has the following relationship: In the formula, n is called the voltage ratio (turn ratio). When n <1, then N1> N2, V1> V2, the transformer is a step-down transformer. Otherwise, it is a boost transformer.
B. Efficiency of the transformer:
At rated power, the ratio of the output power of the transformer to the input power is called the efficiency of the transformer, that is, η = (P2 ÷ P1) × 100% where η is the efficiency of the transformer; P1 is the input power and P2 is the output power.
When the output power P2 of the transformer is equal to the input power P1, the efficiency η is equal to 100%, and the transformer will not generate any losses. But in fact, there is no such transformer. The transformer always generates losses when transmitting electrical energy. This loss is mainly copper Damage and iron damage.
Copper loss refers to the loss caused by the resistance of the transformer coil. When the current is heated through the coil resistance, a part of the electrical energy is converted into thermal energy and lost. Because the coil is generally wound by insulated copper wire, it is called copper loss.
The iron loss of a transformer includes two aspects. One is the hysteresis loss. When AC current passes through the transformer, the direction and size of the magnetic lines of force passing through the silicon steel sheet of the transformer change accordingly, which causes the molecules inside the silicon steel sheet to rub against each other and release heat energy, which is lost Part of the electric energy, this is the hysteresis loss. The other is the eddy current loss. When the transformer is working, there are magnetic lines of force passing through the iron core, and an induced current will be generated on the plane perpendicular to the magnetic lines of force. Circulation and vortex, so it is called eddy current. The existence of eddy current makes the core heat and consume energy. This kind of loss is called eddy current loss.
The efficiency of a transformer is closely related to the power level of the transformer. Generally, the larger the power, the smaller the loss and output power, and the higher the efficiency. On the contrary, the smaller the power, the lower the efficiency.
The core magnetic flux of the power transformer of the C transformer is related to the applied voltage. The excitation current in the current does not increase as the load increases. Although the load core will not saturate when the load is increased, the resistance loss of the coil will increase. The heat generated by the coil will not be released in time if it exceeds the rated capacity, and the coil will be damaged. If the coil you use is made of superconducting material, the current will not increase. It will cause heat, but there is also impedance caused by magnetic leakage inside the transformer, but the output voltage will decrease as the current increases, and the higher the current, the lower the output voltage, so the output power of the transformer cannot be infinite. If you say again that the transformer has no impedance, then when the transformer flows a current, a particularly large electric force will be generated, which will easily damage the transformer coil. Although you have a transformer with unlimited power, it cannot be used. It can only be said that with the development of superconducting materials and core materials, the output power of transformers of the same volume or weight will increase, but not infinitely!
The marking of the power transformer's nominal power, voltage, current and other parameters will fall off or disappear over time. Some commercially available transformers are not marked with any parameters at all. This causes great inconvenience to use. The method of judging the parameters of the unmarked power transformer is introduced below. This method is also useful for purchasing power transformers.
Second, the estimation of power The size of the power transmitted by the power transformer depends on the core material and cross-sectional area. The so-called cross-sectional area, whether it is an E-shaped shell structure or an E-shaped core structure (including a C-shaped structure), refers to the cross-sectional (rectangular) area of the section of the core post wrapped by the winding. After measuring the core cross-sectional area S, the power P of the transformer can be estimated according to P = S2 / 1.5. The unit of S in the formula is cm2.
For example: Measure the core cross-sectional area of a power transformer S = 7cm2, estimate its power, and get P = S2 / 1.5 = 72 / 1.5 = 33W? Excluding various errors, the actual nominal power is 30W.
Third, the measurement of the voltage of each winding is to make use of an unmarked power transformer, find the primary winding resistance, and distinguish the output voltage of the secondary winding is the most basic task. An example is used to illustrate the judgment method.
Example: A power transformer is known with a total of 10 terminals. Try to judge the voltage of each winding.
Step 1: Identify the number of windings and draw the circuit diagram.
Measure with the multimeter R × 1 gear, where all connected terminals are a winding. It is found that there are 3 groups connected by two, 1 group by the three connected, and one terminal is not connected with any other terminal. According to the above measurement results, draw a circuit diagram and number it.
It can be known from the measurement that the transformer has 4 windings, of which the windings with the taps ⑤, ⑥, and ⑦ are tapped windings. The ⑩ terminal is not connected to any of the windings, and it is the shield terminal.
Step 2: Determine the primary winding.
For a step-down power transformer, the primary winding has a smaller wire diameter and more turns than the secondary winding. Therefore, in a step-down transformer like Figure 4, the largest resistance is the primary winding.
Step 3: Determine the voltage of all secondary windings.
Connect AC power through the voltage regulator on the primary winding and slowly increase the voltage to 220V. Measure the no-load voltage of each winding in turn and mark it at each output. If the transformer does not heat up for a long time under no-load condition, the transformer performance is basically intact, and it is further verified that the primary winding determined is correct.
4. Determination of the maximum current of each secondary winding The output current of the secondary winding of the transformer depends on the diameter D of the enameled wire of the winding. The diameter of the enameled wire can be measured directly from the lead terminal. After measuring the diameter, the maximum output current of the winding can be obtained according to the formula I = 2D2. The unit of D in the formula is mm.
Power transformer series and parallel
The power transformer is the same as the general device. During emergency work, multiple transformers can be used in series and parallel under certain conditions. For example, commercially available power transformers can fully meet the requirements. When the transformer power meets the requirements without a suitable voltage, two or more transformers can be used in series.
1. The primary series of the power transformer has a constant N called the turns ratio in the calculation formula of the transformer. It is the ratio of the number of primary turns to the number of secondary turns. The relationship between the primary voltage ratio is N and the primary current ratio is 1 / N. For example: two transformers with 220V primary and 18V secondary, N is 13. If the primary of two transformers are connected in series, the output voltage will drop below 9V on a single secondary. In this case, when the secondary voltage of a single transformer is higher than the use of the power supply of the doubled electrical appliance, two or more transformers can be used in series. And if you connect two sub-series in series, there is not much use value. In this case, as long as the power requirements of a single transformer are guaranteed, the secondary output voltage is not necessarily the same, and its output voltage is calculated as: V single = (V1 times + V2 times + ... Vn times) / Vn.
2. Secondary series connection of the power transformer The secondary series connection of the power transformer is a combination of two or more transformers when the single power is satisfied and the secondary output voltage is not satisfied. If the primary input of the two transformers is 220V and the secondary output is 18V, if you want to supply 33V voltage to the load, you can apply the secondary of the two transformers in series. The secondary series connection of the power transformer is also very easy. Different secondary outputs can be used in the secondary series as long as the power of a single transformer is guaranteed. Under ideal conditions, when the primary input voltages of multiple transformers are the same, the total output is calculated as: V total = V initial order / (V1 times + V2 times + ... Vn times).
3. Primary paralleling of transformers This situation is a common example in our lives. Remote control transformers and main transformers (power switching transformers) in multiple old-fashioned color TV sets are all in the primary paralleling of transformers. 
4. Secondary parallel connection of transformers The secondary parallel connection of transformers is an application where the secondary output voltage of a single transformer is the same and a single power cannot be met. Its application is to superimpose the secondary currents of multiple transformers to meet the power requirements of the load. The secondary of the power transformer is connected in parallel, so that the output power can be the sum of the power of multiple transformers.
Pay attention to the following points in series application of power transformers:
(1) When the power transformer is connected in series and parallel, pay attention to the same end of the transformer. When applied in series, it must be serially connected instead of reversed:
(2) The above calculation is just an ideal algorithm, but in fact the loss of a single transformer after they are connected in series is very large. The secondary output voltage of each power transformer will be lower than the above calculation result.
(3) Different secondary outputs. When the secondary series is used, the secondary can be directly connected in series, or it can be connected in series after voltage stabilization.
(4) The common ground in the power circuit is necessary. Potential comparisons and voltage calculations can only be performed with a reference point.
Types and characteristics of power transformers
The classification of general commonly used power transformers can be summarized as follows:
(1) Divided by number of phases:
(1) Single-phase power transformer: used for single-phase load and three-phase power transformer group.
(2) Three-phase power transformer: used for raising and lowering voltage of three-phase system.
(2) According to the cooling method:
(1) Dry-type power transformer: Relying on air convection for cooling, it is generally used for small-capacity power transformers such as local lighting and electronic circuits.
(2) Oil-immersed power transformer: Rely on oil as cooling medium, such as oil-immersed self-cooling, oil-immersed air-cooled, oil-immersed water-cooled, forced oil circulation, etc.
(3) According to use:
(1) Power transformer: used for raising and lowering voltage of power transmission and distribution system.
(2) Instrument transformers: such as voltage transformers, current transformers, measuring instruments and relay protection devices.
(3) Test transformer: It can generate high voltage and conduct high voltage test on electrical equipment.
(4) Special transformers: such as electric furnace transformers, rectifier transformers, adjustment transformers, etc.
(4) Divided by winding form:
(1) Dual winding transformer: used to connect two voltage levels in the power system.
(2) Three-winding transformer: Generally used in a substation in a power system area to connect three voltage levels.
(3) Autotransformer: used to connect power systems with different voltages. Can also be used as a common step-up or step-down transformer.
(5) According to the form of iron core:
(1) Core transformer: Power transformer for high voltage.
(2) Amorphous alloy transformer: Amorphous alloy iron core transformer is a new type of magnetically conductive material, which reduces the no-load current by about 80%. It is a distribution transformer with ideal energy saving effect, especially suitable for loads in rural power grids and developing regions. Where rates are lower.
(3) Shell-type transformers: special transformers for large currents, such as electric furnace transformers, welding transformers; or power transformers for electronic instruments and televisions, radios, etc.
Working principle of power transformer
1  is a special transformer that shares a set of coils for output and input. Step-up and step-down are implemented with different taps. The voltage of the part with less than the common coil is reduced. The voltage of the part with more than the common coil is increased. .
2 In fact, the principle is the same as that of an ordinary transformer, except that its primary coil is its secondary coil. `` A general transformer is a primary coil on the left that generates electromagnetic voltage through the secondary coil on the right. The autotransformer affects itself. .
3 An autotransformer is a transformer with only one winding. When used as a step-down transformer, a part of the turns are drawn from the windings as a secondary winding. When used as a step-up transformer, the applied voltage is only applied to the windings-part of the line. Turn on. Generally, the part of the winding that belongs to both the primary and secondary is called the common winding, and the rest of the autotransformer is called the series winding. Compared with ordinary transformers, the autotransformer with the same capacity is not only small in size but also highly efficient. The larger the capacity, the higher the voltage. This advantage is even more prominent. Therefore, with the development of the power system, the improvement of the voltage level and the increase of the transmission capacity, the self-propelled transformer is widely used because of its large capacity, small loss, and low cost.
It can be known from the principle of electromagnetic induction that transformers should not have separate primary windings and secondary windings, and only one coil can achieve the purpose of voltage conversion. In Figure 1, when the primary winding W1 of the transformer is connected to the AC power U1, the voltage drop of the primary winding of the transformer is evenly distributed between the primary winding 1,2 of the transformer, and the voltage of the secondary winding W2 of the transformer is equal to the voltage of each primary winding. With 3,4 turns. With U1 constant, change the ratio of W1 and W2 to get different U2 values. This kind of primary and secondary windings are directly connected in series, and a self-coupled transformer is called an autotransformer or a single-turn transformer.
The primary and secondary windings of ordinary transformers are insulated from each other, using only magnetic and no electrical connections. Depending on the number of coil groups, this type of transformer can be divided into two-turn transformers or multi-turn transformers. The principle of electromagnetic induction It can be known that there is no separate primary winding and secondary winding, and only one coil can also achieve the purpose of voltage conversion. In Figure 1, when the primary winding W1 is connected to the AC power supply U1, the voltage drop per turn of the primary winding is averaged. The voltage distributed on the primary winding 1,2, and the secondary winding W2 is equal to the voltage per turn of the primary winding multiplied by 3,4. With U1 constant, changing the ratio of W1 and W2 will get different values of U2. In the former, the secondary winding is directly connected in series, and the self-coupled transformer is called an autotransformer, also known as a single-turn transformer.
The relationship between voltage, current, and turns in an autotransformer and the transformer are: U1 / U2 = W1 / W2 = I2 / I1 = K
The biggest feature of an autotransformer is that the secondary winding is part of the primary winding (as shown in the autotransformer step-down transformer in Figure 1), or the primary winding is part of the secondary winding (such as the autotransformer step-up transformer in Fig. 2).
Since the original transformer, the current direction of the secondary winding is the same as that of the ordinary transformer.
With the excitation current and losses of the transformer ignored, the voltage can be reduced as follows: I2 = I1 + I, I = I2-I1
Boost: I2 = I1-I, I = I1-I2
P1 = U1I1, P2 = U2I2
I1 is the primary winding current and I2 is the secondary winding current
U1 is the primary winding voltage and U2 is the secondary winding voltage
P1 is the power of the primary winding and P2 is the power of the secondary winding
Power transformer function
The most basic type of power transformer includes two sets of coils wound with wires, and they are weighed together inductively. When an AC current (having a certain frequency) flows in one of the coils, an AC voltage with the same frequency will be induced in the other coil. Degree of chain.
Generally, the coil connected to the AC power source is called "primary coil"; and the voltage across this coil is called "primary coil." The induced voltage in the secondary coil may be greater or less than the primary voltage, which is determined by the "turn ratio" between the primary coil and the secondary coil. Therefore, power transformers are divided into two types: step-up and step-down transformers. Most power transformers have fixed iron cores with primary and secondary coils wound around them. Due to the high magnetic permeability of iron materials, most of the magnetic flux is confined in the iron core. Therefore, the two sets of coils can obtain a relatively high degree of magnetic coupling. In some transformers, the coil and the iron core are tightly coupled, and the ratio of the primary to secondary voltage is almost the same as the ratio of the turns of the two. Therefore, the turns ratio of the transformer can generally be used as a reference index for the step-up or step-down of the transformer. Due to this step-up and step-down function, the transformer has become an important appendix of modern power systems. Increasing the transmission voltage makes it more economical to transmit power over long distances. As for the step-down transformer, it makes the use of power more diversified. It can be said that if there is no transformer, the industry will not be able to achieve the status quo of development.
In addition to the small size of power transformers, there is no clear dividing line between power transformers and electronic transformers. Generally, the power supply of a 60Hz power network is very large. It may cover a capacity as large as half a continent. The power limitation of electronic devices is usually limited by the ability to rectify, amplify, and other components of the system. Some of them are those that amplify power. However, compared with the power generation capacity of power systems, it still belongs to the scope of small power.
Transformers are commonly used in various electronic equipment, for the following reasons: to provide various voltage levels to ensure the normal operation of the system; to provide electrical isolation for parts operating at different potentials in the system; to provide high impedance for AC currents, but low impedance for DC; Maintain or modify the waveform and frequency response at different potentials. One of the important concepts of "impedance", that is, one of the electronic characteristics, is a preset device, that is, when the impedance of a circuit component changes from one level to another, a device is used in the meantime. -transformer.
Transformer --- An electrical appliance that uses the principle of electromagnetic induction to transfer power or transmit signals from one circuit to another is an important component of power transmission or signal transmission
1. Power Transformer ---- Static Electromagnetic Device Power Transformer can convert AC voltage of one voltage to AC voltage of another voltage at the same frequency. The main components of the power transformer are an iron core and two sheathed on the iron core. Winding.
Principle of Power Transformer The coil connected to the power source receives AC energy, which is called the coil connected to the primary winding and the load, and sends out AC energy, which is called the secondary winding's primary winding. The voltage phasor U1 and the voltage phasor U2.
Current phasor I1 current phasor I2
EMF phasor E1 EMF phasor E2
Number of turns N1 Number of turns N2
At the same time, once linked, the phasor of the magnetic flux of the secondary winding is φm. This magnetic flux is called the main magnetic flux.
2. The ideal transformer does not count the resistance and iron loss of the primary and secondary windings,
The transformer with the coupling coefficient K = 1 is called an ideal transformer. The ideal EMF balance equation is
e1 (t) = -N1 d φ / dt
e2 (t) = -N2 d φ / dt
If the instantaneous values of the voltage and electromotive force of the primary and secondary windings change according to the sine law,
There is no core loss. According to the principle of energy conservation, it can be obtained that the relationship between the voltage of the primary and secondary windings and the effective value of the current is K = N1 / N2, which is called the turn ratio (also called the voltage ratio).
Power transformer loss
When the primary winding of the power transformer is energized, the magnetic flux generated by the coil flows in the iron core, because the iron core itself is also a conductor, and a potential is induced on a plane perpendicular to the magnetic field lines. This potential forms a closed loop on the cross section of the core. And produce a current, as if a vortex p so called "eddy current". This "eddy current" increases the loss of the transformer and increases the temperature rise of the transformer's core heating power transformer. The losses caused by "eddy currents" are called "iron losses". In addition, winding a power transformer requires a large number of copper wires. These copper wires have resistance. This resistance consumes a certain amount of power when current flows. This part of the loss is often turned into heat and consumed. We call this loss "copper loss." ". Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Because the power transformer has iron loss and copper loss, its output power is always less than the input power. For this reason, we introduce an efficiency parameter to describe this, η = output power / input power.
Power transformer material
To wind a power transformer, we must have a certain understanding of the materials related to the power transformer, so I will introduce this knowledge here.
1. Iron core materials The core materials used in power transformers are mainly iron plates, low silicon wafers, high silicon wafers, and the addition of silicon to steel sheets can reduce the conductivity of the steel sheet and increase the resistivity. It can reduce eddy currents and make it Loss reduction. We usually refer to the silicon steel sheet as silicon steel sheet. The quality of the silicon steel sheet used for the quality of the power transformer has a great relationship. The quality of the silicon steel sheet is usually expressed by the magnetic flux density B. The B value of the general black iron sheet It is 6000-8000, low silicon wafer is 9000-11000, and high silicon wafer is 12000-16000.
2. Materials commonly used for winding power transformers are enameled wire, gauze covered wire, silk covered wire, the most commonly used enameled wire. For the requirements of the wire, the conductive property is good, the insulating varnish layer has sufficient heat resistance, and it must have certain corrosion resistance. Generally, it is better to use QZ type high strength polyester enameled wire.
3. Insulating materials In the winding transformer, the insulation between the layers of the coil frame and the isolation between the winding resistances must use insulating materials. The general power transformer frame material can be made of phenolic paperboard, and the layers can be made of polyester film or telephone paper. Isolation, yellow wax cloth can be used for isolation.
4. After the winding of the impregnated material power transformer is completed, the last step is to impregnate the insulating paint, which can enhance the mechanical strength of the power transformer, improve the insulation performance, and extend the service life. In general, cresol varnish can be used as Impregnation material.
Comparison of power transformers
I. What are the advantages and disadvantages of machine winding and manual winding of the coil in the production of power transformers?
The advantages of machine-wound power transformers are high efficiency and beautiful appearance, but it is more troublesome to wind a tall, small hole-shaped transformer, and it is not as good as manual winding in terms of reliability in insulation processing. Manual winding can make the magnetic leakage of the transformer very small, and it can adjust the layout of the coil turns at any time during the winding process, so the real Hi-END transformer must be purely manual winding, purely manual winding The only drawbacks are low efficiency and slow speed.
Second, which type of ring, EI, R, C type power transformer is the best?
They each have their own advantages and disadvantages and there is no one who is best to say, so strictly speaking, any type of power transformer can do the best. In terms of structure, the ring type can minimize the magnetic leakage, but the EI type can make the IF density better. As far as magnetic saturation is concerned, the EI type is stronger than the ring type, but the ring type is better than the EI type in terms of efficiency. However, the key to the problem is whether you can make good use of their respective strengths while avoiding weaknesses, and this is the most fundamental of a good power transformer.
In imported amplifiers, the application of toroidal power transformers is still mainstream, which basically illustrates a problem. Enthusiasts' evaluation of power transformers must be objective and fair. You can't take a reference to something that is not done well and say that it is not good. Some people say that toroidal power transformers are susceptible to magnetic saturation, so why don't you find a way to make them not easy to saturate? And this can be achieved through technical means originally. If you don't work hard or blindly to save costs, then it will of course be magnetically saturated. In the same way, as long as you make it carefully, the efficiency of the EI power transformer can also be very high.
The quality of the power transformer has a great impact on the sound, because the transmission energy of the power transformer is closely related to the iron core and the coil, and its transmission rate plays a decisive role on the sound. Like the EI type power transformer, people usually think that its intermediate frequency is relatively thick, and the high frequency is relatively thin. Why? Because its transmission speed is relatively slow. What about the ring type? The low frequency is more fierce, and the middle and high frequencies are slightly weaker. Why? Because it has a fast transmission speed, but if you change the structure effectively, you can make both the ring type and the EI type perfect, The key depends on how you do it.
But at least it is certain that the R-type power transformer is not too easy to make. It can also be used as a small current pre-amplifier and CD player power supply. If it is used as a power supply for the post-amplifier, it has more serious defects. Because the structure of the R-type power transformer itself is not easy to change, and the ring and EI types are relatively easy to achieve beautiful sound by changing the structure. Power amplifier power supplies made with R-type power transformers usually have a stiff and lacking aura, and they tend to be stiff at low frequencies without bouncing power.
Third, the silicon steel sheet of the core of the power transformer contains more silicon, the better?
It has not been seen that the magnitude of the silicon content of the silicon steel sheet does not greatly affect the quality of the transformer, and the orientation and non-orientation are related to the model of the iron core. Secondly, even if the cores of the same model are not processed well, the quality difference is very large, and the difference is sometimes as high as 40-50%.
A good iron core and the same material are critical to the heat treatment and wire winding process. Good heat treatment requires only a small 10mA excitation current to reach 15,000 Gauss, and poor heat treatment may require a 50mA excitation current to achieve The corresponding 15,000 Gauss is very different. From a professional perspective, to judge the good or bad of the core, the comprehensive evaluation is mainly carried out through several indicators of the exciting current, iron loss, and saturation parameters.
Fourth, if the band silicon steel sheet of the ring-shaped power transformer adopts the splicing process, does it mean that the quality is definitely not good?
It can't be generalized, but the splicing break head is not easy, because one more break will have one more magnetic leakage point, so it is best not to exceed 2–3 joint points. In the production process, all broken ends are spliced before being pickled. However, to manufacture high-end audio equipment toroidal transformers, strictly speaking, it is better to use silicon steel sheets without splicing, and the process quality will be more guaranteed.
5.What are the particulars of silicon steel sheet materials in power transformers?
Because the loss of silicon steel in the alternating magnetic field is very small, silicon steel sheets are mainly used as magnetic materials for power transformers. Silicon steel sheets can be divided into two types: hot rolled and cold rolled. Cold rolled silicon steel strips have the advantages of small size, light weight and high efficiency due to their high magnetic permeability and low loss. The performance of hot-rolled silicon steel strip is slightly inferior to that of cold-rolled silicon steel strip.
The ordinary EI type power transformer is made of silicon steel plate into 0.35–0.5mm thick E-type and I-type pieces, which are heat-treated and then inserted into the winding package. Most of these iron cores use hot-rolled silicon steel sheets (containing silicon Models of high-quality high-quality silicon steel sheets are D41, D42, D43, D301). The cores of toroidal and C-type power transformers are formed by winding cold-rolled silicon steel strips. The C-type power transformers are heat-treated and dipped and then cut.
The leakage inductance of the power transformer is generated by the magnetic flux that does not pass through the primary and secondary coils. These magnetic fluxes pass through the air to form a closed magnetic circuit. Increasing the coupling density between the primary and secondary of the power transformer can reduce leakage inductance. The leakage inductance of a good power transformer should not exceed 1/100 of the inductance of the primary coil, and the output transformer of a hi-fi amplifier should not exceed 1/500.
One of the important parameters for judging the quality of silicon steel sheet for audio power transformers is the maximum magnetic line density of silicon steel sheet. The commonly used types of high-quality silicon steel sheets are as follows: D41–D42, maximum magnetic line density (unit – GS Gauss) 10000–12000GS; D43, maximum magnetic line density 11000–12000GS; D301, maximum magnetic line density 12000–14000GS.
Power transformer detection
First, the detection of Zhongzhou power transformer: ??
A. Set the multimeter to R × 1 position, and check the on-off condition of each winding one by one according to the arrangement of the winding pins of the mid-cycle transformer, and then determine whether it is normal.
B, testing insulation performance: put the multimeter in R × 10k, and do the following state tests:
(1) the resistance between the primary winding and the secondary winding;
(2) the resistance between the primary winding and the case;
(3) The resistance between the secondary winding and the case.
The above test results are divided into three situations:
(1) The resistance value is infinite: normal; ??
(2) Resistance value is zero: there is a short-circuit fault; ??
(3) Resistance is less than infinity, but greater than zero: leakage fault. ??
Second, the detection of power transformer: ??
A. Check whether there is obvious abnormal phenomenon by observing the appearance of the transformer. Such as whether the coil leads are broken, de-soldering, whether the insulation material has burnt marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.
B. Insulation test. Use the multimeter R × 10k gear to measure the resistance between the iron core and the primary, the primary and each secondary, the iron core and each secondary, the electrostatic shielding layer and the 衩 secondary, and the windings of the secondary. The pointer of the multimeter should point to infinity. The position does not move. Otherwise, it indicates that the transformer has poor insulation performance.
C. Detection of coil on / off. Set the multimeter to R × 1. In the test, if the resistance value of a winding is infinite, it means that the winding has an open circuit fault.
D. Identify the primary and secondary coils. The primary and secondary pins of a power transformer are generally drawn from both sides, and the primary winding is usually marked with 220V, and the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, and so on. These tags are used for identification.
E. Detection of no-load current.
(1) Direct measurement method. Open all secondary windings and place the multimeter in the AC current block (500mA, string it into the primary winding. When the plug of the primary winding is plugged into 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is greater than 10% ~ 20% of the transformer full load current. Generally, the normal no-load current of common electronic equipment power transformers should be about 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.
(2) Indirect measurement method. A 10? / 5W resistor is connected in series in the primary winding of the transformer, and the secondary is still completely unloaded. Set the multimeter to AC voltage. After power-on, use two test leads to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = U / R.
F. Detection of no-load voltage. Connect the primary of the power transformer to 220V mains, and then use the AC voltage of a multimeter to measure the no-load voltage values (U21, U22, U23, U24) of each winding in sequence. The allowable error range is generally: high voltage winding ≤ ± 10 %, Low voltage winding ≤ ± 5%, voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%.
G. Generally low temperature power transformers allow temperature rise of 40 ℃ ～ 50 ℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
H. Detect and identify the same-named end of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the series method to use a power transformer, the same-named ends of the windings participating in the series must be connected correctly and no mistakes can be made. Otherwise, the transformer will not work properly. I. Comprehensive detection and discrimination of short-circuit faults in power transformers. The main symptoms after a short-circuit fault of the power transformer are severe heat generation and abnormal output voltage of the secondary winding. Generally, the more inter-turn short-circuit points inside the coil, the larger the short-circuit current, and the more severe the transformer heats up. The simple way to detect whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced earlier). With a short-circuit fault, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will quickly heat up within tens of seconds after no-load power-on. Touching the iron core with your hands will feel hot. At this time, it is not necessary to measure the no-load current to determine that the transformer has a short-circuit point.
The four domestic transformer manufacturers are: Shenyang Transformer Factory (acquired by TBEA Co., Ltd. in 2004), Xi'an Transformer Factory, Baoding Transformer Factory, TBEA Co., Ltd., and famous foreign companies include Siemens and ABB.
Power transformer magnetically shielded satellites are thousands to tens of thousands of kilometers away from the ground. In order to send all kinds of information back to the earth accurately, mutual interference between various instruments on the satellite and the influence of the cosmic magnetic field should be avoided. In telecommunication technology, The coils of some communication equipment can cause mutual inductance; in order to maintain accuracy, the influence of stray magnetic fields and geomagnetic fields must be avoided, all of which must use magnetic shielding. How to magnetically shield? You can do a simple experiment first.
Take a copper plate (or a thick cardboard) at a certain distance below a permanent magnet, put an iron pin on the table, and make the permanent magnet and copper plate (or thick cardboard) slowly move down together. When the permanent magnet When it is a certain height from the table, the iron needle is sucked onto the copper plate (or cardboard), and the height is recorded.
Replace the copper plate with an iron plate and repeat the above experiment. At this time, the permanent magnet must be placed closer to the iron pin to attract the iron pin to the iron plate, which indicates that the iron plate is blocking some magnetic lines. If you use a pure iron plate, the permanent magnet must be placed closer to attract the iron pin. This shows that the pure iron plate is blocking more magnetic lines.
If you use a pure iron cover to completely surround the permanent magnets and not touch each other, even if the iron needle is close to some pure iron covers, it cannot be sucked up. This is because copper plates or cardboard are non-magnetic materials, and magnetic lines can pass through them without obstruction, so iron needles can easily suck up. The iron plate is a magnetic material, which has a large magnetic permeability and a good magnetic permeability. Most of the magnetic induction lines entering the iron plate are concentrated in the iron plate. Pure iron is made into a shield, and the permanent magnet is closed. Most of the magnetic induction lines of the permanent magnet are concentrated in the pure iron shield. The thicker the shield, the better the shielding effect. If permanent magnets or other objects that can generate magnetic fields are placed outside the pure iron shield, the magnetic lines outside the shield can not basically enter the shield, and the objects inside the shield can also be protected from the magnetic field outside the shield. For shielding purposes.
For high frequency alternating magnetic fields, the situation is quite different. Metals such as copper and aluminum with good conductivity are ideal magnetic shielding materials. The reason why the copper cover can shield the high-frequency alternating magnetic field is because the high-frequency alternating magnetic field can cause a large eddy current on the copper cover. Due to the demagnetization of the eddy current, the magnetic field at the copper cover is greatly weakened, so that the High-frequency alternating magnetic fields cannot penetrate outside the cover. For the same reason, the high-frequency alternating magnetic field outside the cover cannot penetrate into the cover, so as to achieve the purpose of magnetic shielding. Generally, the smaller the resistivity of a metal, the larger the eddy current caused, and the better the shielding effect of a shield made of this metal. The resistivity of magnetic materials such as iron is generally large, causing less eddy currents and less demagnetization. On the other hand, the high-frequency power loss of magnetic materials is large, and the shielding effect is poor, so when shielding high-frequency alternating magnetic fields, Made of magnetic material.
The principle of shielding is the same. However, at high frequencies, no material with high permeability has been used for shielding. Materials with high magnetic permeability at low frequencies become very low at high frequencies. Even the special high-frequency ferrite is difficult to exceed 100, which is much worse than the magnetic permeability of thousands of thousands of silicon steel sheets or pure iron at low frequencies, which cannot effectively gather magnetic fields. At the same time, these materials are all disposable molding materials. After firing, they cannot be processed twice to meet different needs. Therefore, it is necessary to use eddy current loss and back-EMF to generate a reverse magnetic field to achieve shielding. The best materials for eddy currents are low resistivity materials such as pure copper and pure aluminum.
National Standard for Power Transformers
GB 1094.3-2003 Power Transformers Part 3: Insulation Level, Insulation Test and Air Gap of External Insulation -2003 Standard voltage · GB 19212.1-2003 Safety of power transformers, power supply units and similar products Part 1: General requirements and tests · GB / T 10760.1-2003 Off-grid wind turbine generators Part 1: Technical conditions · GB / T 10760.2-2003 Off-grid wind turbine generators Part 2: Test methods · GB / T 1094.10-2003 Power transformers Part 10: Sound level determination GB / T 12325-2003 Allowable deviation of power quality supply voltage GB / T 14099.1-2004 Gas Turbine Procurement Part 1: General Principles and DefinitionsGB / T 14099.2-2004 Gas Turbine Procurement Part 2: Standard Reference Conditions and RatingsGB / T 15146.11-2004 Nuclear Criticality for Fissile Materials Outside the Reactor Safety based on nuclear criticality limiting and controlling moderators. GB / T 17625.6-2003 Electromagnetic compatibility limits Harmonic currents generated in low-voltage power supply systems for equipment with a rated current greater than 16A GB / T 17680.10-2003 Criteria for emergency planning and preparation for nuclear power plants Criteria for emergency field radiation monitoring, sampling and analysis of nuclear power plant operating units · GB / T 17680.6-2003 Criteria for emergency planning and preparation for nuclear power plants On-site emergency response functions and organizations · GB / T 17680.7-2003 Criteria for emergency planning and preparation of nuclear power plants Functions and characteristics of on-site emergency facilities · GB / T 17680.8-2003 Criteria for emergency planning and preparation for nuclear power plants On-site emergency planning and implementation procedures · GB / T 17680.9-2003 Nuclear power plant emergency Planning and preparation guidelines Maintenance of on-site emergency response capabilities GB / T 18039.3-2003 Low-frequency conducted disturbances and signal transmission compatibility levels for public low-voltage power supply systems in electromagnetic compatibility environmentsGB / T 18039.5-2003 Low-frequency conducted disturbances for public power supply systems in electromagnetic compatibility environments And electromagnetic environment for signal transmission · GB / T 18451.2-2003 Power characteristics test of wind turbines · GB / T 19068.1-2003 Off-grid wind turbines Part 1: Technical requirements · GB / T 19068.2-2003 Off-grid wind turbines Units Part 2: Test methods · GB / T 19068.3-2003 Off-grid wind turbines Part 3: Wind tunnel test methods · GB / T 19069-20 03 Technical conditions for wind turbine controllers GB / T 19070-2003 Test methods for wind turbine controllers GB / T 19071.1-2003 Asynchronous generators for wind turbines Part 1: Technical conditions GB / T 19071.2-2003 Wind power Unit asynchronous generator Part 2: Test method · GB / T 19072-2003 Wind turbine tower · GB / T 19073-2003 Wind turbine gearbox · GB / T 19115.1-2003 Off-grid household wind-solar hybrid power generation system Part 1: Technical conditions · GB / T 19115.2-2003 Off-grid household wind-solar hybrid power generation system Part 2: Test method · GB / T 19184-2003 Cavitation erosion assessment of water bucket turbines · GB / T 19519-2004 Standard Composite insulators for AC overhead lines with a voltage higher than 1000V-Definitions, test methods and specifications GB / T 19568-2004 Code for assembly and installation of wind turbines GB / T 2694-2003 Technical conditions for manufacturing transmission line towers GB / T 2893.1 -2004 Graphical symbols Safety colors and safety signs Part 1: Safety signs in workplaces and public areas · GB / T 2900.33-2004 Electrical and electronic terminology power electronics · GB / T 2900.36-2003 Electrical and electronic terminology electric traction · GB / T 2900.4 9-2004 Electrotechnical term Power system protection · GB / T 4585-2004 Artificial pollution test for high-voltage insulation of AC systems · GB / T 7267-2003 Power system secondary circuit control, protective screen and cabinet basic size series · GB / T 8564-2003 Technical Specifications for Installation of Hydrogenerator Units ·
· JB / T 10317-2002 Technical parameters and requirements for single-phase oil-immersed distribution transformers
Load analysis of power transformer
Overload analysis of common windings of power autotransformers Compared with ordinary transformers, power autotransformers have obvious economic benefits. Therefore, in ultra-high voltage power grids with a voltage of 330? KV and above, autotransformers have been widely application.
Compared with ordinary transformers, the structure and working principle of autotransformers are fundamentally different. They have the characteristics of easy power transmission and small size. In different operating modes of the autotransformer, the current flowing in the common winding is different from the series winding of the same core. This article starts with the analysis of the current flow of the autotransformer and derives the characteristics of the common winding overload. The overload protection and the relationship between the third-side reactive capacity and the common winding capacity are discussed for reference by design and operating personnel. ?
1Current direction of autotransformer in different operating modes ??
1.1 Several common use forms of autotransformer?
(1) According to the voltage level, there are 35kV and 10kV on the third side;
(2) According to the connection with the system, the third side includes:
① Supply power directly to users;
② Supply power directly to users and install reactive power compensation devices;
③ Do not directly supply power to users, only connect reactive power compensation devices;
④ It does not directly supply power to the user or connect to the reactive power compensation device. It is only used as a balanced winding. ?
1.2 Analysis of the current flow and overload of the autotransformer under various operating modes?
The operation mode of an autotransformer used in a step-down substation can be summarized into two types, one is high voltage to medium voltage (or low voltage) or low voltage to medium and low voltage at the same time, such as a and b in the above method of connecting to the system Two types; the other is high voltage and low voltage power supply to the medium voltage at the same time, such as b, c in the above system access method  For the sake of intuition, for example, to analyze, assuming a transformer variable is 120MVA, the voltage The ratio is 220/110 / 10kV? And the capacity ratio is 100/100/50. Usually, the capacity of the common winding is designed to be equal to the calculated capacity of the autotransformer, so the transformer's common winding capacity is: MVA (K12 is the high-voltage side and the medium-voltage (Transformation ratio on the side)
 It can be seen that the rated current of the high-voltage side is that the rated current of the high-voltage side is equal to the rated current ICe of the series winding;
The rated current on the medium voltage side is I2e = 120? 000 / (31/2 × 110) = 630A;
The rated current on the low side is I3e = 60? 000 / (31/2 × 10) = 3? 464A;
The rated current of the common winding is IGe = calculated capacity / (31/2 × 110) = 60? 000 (31/2 × 110) = 315A.
?? The first type of operation mode of the autotransformer used in the step-down substation can be divided into three cases, as shown in Figures 1-3. ?
A. The high-voltage side supplies power to the medium-voltage side separately (Figure 1)?
I3 = 0 at this time. This operation mode is the auto-transform operation mode of the auto-transformer. The high-voltage side supplies power to the medium-voltage side in an auto-coupled manner, with S1 = S2. According to the principle of magnetic potential balance in the core, there are:?
Among them: I1, I2, I3 are the currents of high-voltage side, medium-voltage side, and low-voltage side; IAB and IDB are the currents of series winding and common winding when operating in auto-couple mode; Transformer mode (electromagnetic induction) current on the high-voltage side; WAB, WCD, and W3 are the number of turns of the series winding, common winding, and low-voltage winding, respectively. ?
When the autotransformer runs under the rated load, that is, S2 = 120MVA, U1 = 220kV, K12 = 2, we can get: IC = IDB = 315A ??
It can be seen that in this operation mode, if the transformer is not overloaded, the common winding will not be overloaded, so the overload protection of the autotransformer at this time can be installed as a normal transformer. ?
B. The high-voltage side supplies power to the low-voltage side separately (Figure 2)?
I2 = 0 at this time. This operation mode is the working mode of the double-winding ordinary transformer. The high-voltage side supplies power to the low-voltage side by the ordinary transformer method, with S1 = S3. ?
When the autotransformer runs under the rated load, that is, S3 = 60MVA, U1 = 220kV, we can get: IG = IB = 157.5A ??
It can be seen that in this operation mode, even if the low-voltage side of the transformer is fully loaded, the current in the common winding does not reach the rated value. Therefore, at this time, the overload protection of the autotransformer can be installed as a common transformer. ?
C. Current flow of the high-voltage side to the medium- and low-voltage side at the same time (Figure 3)?
This method can be seen as the superposition of the above two methods. The input capacity on the high-voltage side is divided into two parts:. ?
The capacity transferred from the high-voltage side to the medium-voltage side in an auto-coupled manner is equal to the output capacity of the medium-voltage side, = S1, which is equivalent to the high-voltage side supplying power to the medium-voltage side alone, and the high-to-medium voltage windings are self-coupled. IAB and IDB are the currents flowing in the series winding and the common winding.
For the high-voltage side, the capacity transferred by the transformer (electromagnetic induction) between the high and low voltage windings is equal to the output capacity of the low-voltage side, = S3, which is equivalent to the high-voltage side supplying power to the low-voltage side alone, and the high-low voltage windings are powered by electromagnetic induction. , IB is the high-side current. ?
As can be seen from the figure, there are two currents in the common winding: IDB and IB, and the two currents are in opposite directions, so the current in the common winding is: IG = IDB-IB?
When the low-voltage side is running at full load, that is, S3 = 60MVA in this example, then S2 = 60MVA, and there are U1 = 220kV and K12 = 2, which are substituted into equations (1-1 ′) and (1-1 ″). Can be obtained:?
Therefore, the current in the common winding is: IG = IDB-IB = 0?
When the medium voltage side is running at full load, that is, S2 = 120MVA, then S3 = 0MVA ?, and substituting it into formula (1-1) or (1-2), the same can be obtained: IDB = 315A; IB = 0A, Therefore, the current of the common winding is: IG = IDB-IB = 315A ???
It can be known from the above analysis that under this operation mode, if the transformer is not overloaded, the current in the common winding will be in the range of 0 to 315A, and will not exceed the rated value. Therefore, at this time, the common winding of the autotransformer No overload, no overload protection required. ?
As shown in Figure 4, when the high and low voltage sides supply power to the medium voltage side at the same time, the output capacity of the medium voltage is composed of two parts. ?
The capacity of the high-voltage side is transferred to the medium-voltage side in an auto-coupled manner, which is equal to the output capacity of the medium-voltage side, = S2, which is equivalent to the high-voltage side supplying power to the medium-voltage side alone. , IAB, IDB is the current flowing in the series winding and the common winding. ?
For the high-voltage side, the capacity transferred by the transformer method (electromagnetic induction) is equal to the output capacity of the low-voltage side, = S3, which is equivalent to the high-voltage side supplying power to the low-voltage side alone, and IB is the current flowing through the high-voltage side. ?
It can be seen from the figure that in this operation mode, the current in the common winding is: IG = IDB + IB, where IDB can be obtained by formula (1-1 ″).
IB is the current induced by the low-voltage side to the medium-voltage side through the transformer, then:
When the high-voltage side is running at full load, the above example has S1 = 120MVA, and U1 = 220kV, K12 = 2, substituting into the formula (1-1 ″), we can get: IDB = IGe = 315A ?; In order not to overload the common winding, the output current IB = 0 on the low side must be set.
When the low-pressure side is running at full load, S2 = 60MVA, substituting into formula (1-3), we can get: IB = IGe = 315A ??
From the above formula, if the common winding is not overloaded at this time, the current IDB must be set to 0 ?. ?
From the above analysis, it can be seen that in this operation mode, if the high-voltage side of the transformer runs at full load, the low-voltage side cannot supply power to the medium-voltage side, otherwise the common winding will be overloaded, that is, when the high-voltage side has a large transmission capacity, it will limit the low The output of the side capacity; if the low voltage side of the transformer is running at full load, the high voltage side cannot supply power to the medium voltage side, otherwise the common winding will be overloaded. It should be noted that in the latter case, the output of the transformer has not yet reached the rated load, and its output is 60MVA ?, which is only half of the rated power . ?
2 The relationship between the capacity of the common winding and the capacity of the third-side access reactive power compensation device?
From the analysis above, it can be known that when the third side of the step-down transformer substation is connected to the reactive power compensation device, the high and low voltage sides will simultaneously supply power to the medium voltage side. If the transmission capacity of the low voltage side reaches the calculated capacity, in order not to overload the common winding When the reactive power loss of the transformer itself is not considered, the high-voltage side can no longer supply power to the medium-voltage side. ?
In the power system, the high-voltage side transmits power to the medium-voltage side, and reactive power compensation on the low-voltage side is a common operation mode. In order to not affect the supply of power from the high-voltage side to the medium-voltage side system with the rated capacity, and to fully utilize the reactive power compensation device connected to the third side, the capacity of the common winding and the reactive power compensation capacity connected to the third side must be clarified. relationship. ?
2.1 When the reactive power loss of the transformer is not considered, the capacity of the common winding must be increased?
Take Figure 4 as an example. At this time, if the output capacity of the medium voltage side is S2 = S1e + S3e = S1 + S3, the passing capacity of the common winding is SG = SJS + S3 (SJS is the calculated capacity of the autotransformer ). ?
Because the low-voltage side is connected to a reactive power compensation device, its input is only reactive power, that is, S3 = jQD, as shown in Figure 5. ??
In the complex power pie chart, S3 = OD is always drawn on the + jQ axis square. Make two circles with D as the center and DC and DG as the radius. DC = SJS, DG = S1, because SG = SJS + S3, S2 = S3 + S1, so OC = SG, OG = S2, that is, The “mandatory capacity” is the amplitude of the OC shown in the figure (mandatory capacity—the capacity requirement that the winding may pass through the maximum capacity). At this time, the output capacity on the medium voltage side is the amplitude defined by the vector OG in the figure, and The "mandatory capacity" of the common winding and the output capacity of the medium voltage side are closely related to the power factor of the high voltage side, which will increase as the power factor decreases. When the high and low voltage sides transmit power to the medium voltage side at the same time, the load calculation formula in the common winding is :?
For an autotransformer with a rated capacity of 120MVA, when the high-side power factor is assumed to be 0.9, when a 60MVAR reactive power compensation device needs to be connected to the third side, the common winding capacity can be obtained according to formula (1-3) :?
2.2 When the reactive power loss of the transformer itself is considered, and the third side requires a small reactive power compensation, can it not increase the common winding capacity?
According to formula (1-4), it can be calculated that, for an autotransformer with a rated capacity of 120? MVA, when the third side is connected to a reactive power compensation capacity of not more than 15? MVAR, the common winding may not increase the capacity, usually not. An overload has occurred. However, at this time, the common winding needs to be added with overload protection to prevent overload situations that may occur under special operating modes.
Power transformer identification
1. The cores of the common power transformers are E-shaped and C-shaped. The E-shaped iron core transformer has a shell structure (iron core wrapped coil) and uses D41 and D42 high quality silicon steel sheets as the iron core, which is widely used. C-shaped iron core transformer uses cold-rolled silicon steel strip as iron core, with small magnetic leakage and small volume, and has a core structure (coil wrapped iron core).
2. Identifying the number of terminals from the winding There are two windings common in power transformers, namely a primary and a secondary winding, so there are four terminals. In order to prevent AC noise and other interference, some power transformers often add a shielding layer between the primary and secondary windings, and the shielding layer is the ground terminal. Therefore, there are at least four power transformer terminals.
3. The silicon steel sheet of the E-shaped power transformer is identified by the laminated method of the silicon steel sheet. The air gap is not left between the E sheet and the I sheet, and the entire iron core is tightly connected. There is a certain air gap between the E and I slices of the audio input and output transformers. This is the most intuitive way to distinguish between a power supply and an audio transformer. As for C-shaped transformers, they are usually power transformers.
Power transformer conclusion
From the above analysis, it can be seen that the current flow of the autotransformer is different from that of ordinary three-winding transformers. On the common winding of the autotransformer, there will be an overload of the common winding when the transformer has not yet reached rated operation, resulting in The difference between an autotransformer and an ordinary transformer in terms of overload protection: When the third side of the autotransformer is connected to a power source (also a reactive power compensation device in a step-down substation), the autotransformer is installed on all three sides In addition to overload protection, overload protection must also be installed at the common winding. In addition, when the reactive power compensation device is connected on the third side, it must be studied whether the capacity of the common winding needs to be increased.
Power transformer characteristics
1 Working frequency Transformer core loss has a great relationship with frequency, so it should be designed and used according to the use frequency. This frequency is called the working frequency.
2 Rated power Under the specified frequency and voltage, the transformer can work for a long time without exceeding the output power of the specified temperature rise.
3 Rated voltage refers to the voltage allowed on the coil of the transformer. It must not exceed the specified value during operation.
4 Voltage ratio refers to the ratio of the primary voltage and the secondary voltage of the transformer. There is a difference between the no-load voltage ratio and the load voltage ratio.
5 No-load current When the secondary of the transformer is open, the primary still has a certain current. This part of the current is called no-load current. The no-load current is composed of magnetizing current (generating magnetic flux) and iron loss current (caused by core loss). For a 50Hz power transformer, the no-load current is basically equal to the magnetizing current.
6 No-load loss refers to the power loss measured at the primary when the transformer secondary is open. The main loss is the core loss, followed by the loss (copper loss) caused by the no-load current on the copper resistance of the primary coil, which is very small.
7 Efficiency refers to the percentage of the ratio of secondary power P2 to primary power P1. Generally, the higher the rated power of the transformer, the higher the efficiency.
8 Insulation resistance indicates the insulation performance between each coil of the transformer and between each coil and the iron core. The level of insulation resistance is related to the performance of the insulating material used, the temperature level and the degree of humidity .
Power transformer product information
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