18-pulse H-class insulation dry-type rectifier transformer

1 Introduction

Most of the DC power supplies used in industry are obtained from the AC power grid through the rectifier equipment composed of rectifier transformers and rectifiers, and are widely used in metallurgy, chemical industry, traction and other fields, such as urban rail transit, DC transmission of steel rolling motors, DC excitation of synchronous motors, etc. The function of the rectifier transformer is to convert the AC power grid voltage into the voltage required by the rectifier device, and improve the operating characteristics of the AC and DC sides by changing the number of phases and phase angles. The rectifier transformer can isolate the rectifier equipment from the power grid circuit to ensure the safety of the equipment, limit the short-circuit current, reduce the electromagnetic interference of the rectifier equipment to the power grid and other parallel rectifier equipment, and suppress the current rise rate of rectifier components such as thyristors.

Due to the oil-free, moisture-proof, heat-resistant, flame-retardant, and corrosion-resistant properties of dry-type transformers, they are widely used in various aspects of industry and life. At present, there are two main types of dry-type transformers: one is the resin-cast dry-type transformer (ORDT) represented by Europe, and the other is the varnish-impregnated dry-type transformer (OVDT) represented by the United States. As a dry-type rectifier transformer with H-class insulation, using C-class insulation material Nomex paper as the insulation medium, it has higher reliability and environmental protection characteristics, and has better economy, and is widely welcomed.

The heat resistance level of H-class dry-type rectifier transformer is 180℃. The main insulating material is Nomex paper, which is a patented product of DuPont in the United States. It is a synthetic insulating material based on aromatic amide fiber. It is C-class and has a heat resistance level of 220℃. Nomex paper has many advantages. It is an excellent electrical insulating material. The transformer made of it can be moisture-proof, flame-retardant, and has good environmental adaptability. In addition, the transformer is compact in size and occupies a small space. The transformer's resistance to cold and heat shock, short circuit, and overvoltage are better than other types of transformers. In particular, the manufacturing process and product structure characteristics of H-class rectifier transformers have very obvious advantages for rectifier transformers that require multiple taps and complex structures with multiple phase-shifting windings, which shortens the manufacturing and processing cycle and reduces the cost. After vacuum pressure impregnation (VIP), the winding has good rigidity and guaranteed mechanical strength. At the same time, due to the paint film covering the surface of the insulating material, the moisture resistance of the transformer is improved.

2 Formation and working principle of phase shift

The dry-type phase-shifting rectifier transformer is a device that is specially used to provide multi-phase rectifier power for medium and high voltage inverters. It adopts the Yanbian triangle phase-shifting principle and can form rectifier transformers with equivalent phase numbers of 9, 12, 15, 18, 24 and 27 through multiple secondary windings with different phase-shifting angles. The primary side of the transformer is directly connected to the high voltage grid, and its secondary side has multiple three-phase windings. It represents the low-voltage three-phase windings on the secondary side of the Yanbian triangle connected transformer according to 0°, θ°, ..., (60-θ)°, etc., and also represents the phase-shifting angle of the line voltage of each low-voltage three-phase winding relative to the corresponding winding. When each phase is connected in series by n H-bridge units, θ=60°/n, which realizes the multiple input and forms 6n pulse rectification. In this way, if the power of each H-bridge unit is balanced and the current amplitude is the same, theoretically, the primary input current does not contain harmonics below 6n±1, and the power factor can be improved. Generally, there is no need to equip reactive compensation and harmonic filtering devices. It is most suitable for use in environments with high fire protection requirements and large load fluctuations, such as offshore oil platforms, thermal power plants, water plants, metallurgy and chemical industry, mining and building materials and other special working environments.

The multi-winding dry-type phase-shifting rectifier transformer is designed according to different users, with a capacity ranging from 200kva to 10000kva, a large primary impedance, a transformer efficiency of >98%, an H-class insulation system, and a winding temperature rise limit of 120k. In order to improve the quality of power, the output waveform of the rectifier transformer is not like the power transformer, which has only three sinusoidal pulses in one cycle. Instead, the number of pulses of each transformer in one cycle is determined based on the primary side voltage and installed capacity. High-voltage variable frequency speed regulation technology is currently diversified. The cascade multiplexing technology represented by Siemens technology can basically achieve perfect harmonic-free. It uses a rectifier transformer to superimpose (connect in series) multiple low-voltage modules to form a high-voltage output. The power device uses IGBT. At present, most domestic high-voltage inverter manufacturers use this technology. ABB's ACS5000 series inverters are three-level topology structures. The 36-pulse rectifier transformer has a total of 6 phase shift groups, and each two phase shift groups supply power to a frequency conversion unit. The power device is IGCTS. ABB also has an inverter that uses 12-pulse rectifier inverter technology, and its transformer uses a three-winding form. The 18-pulse rectifier inverter technology represented by AB (Rockwell) requires a rectifier transformer that uses a three-split form.

As an important component of this technology, the rectifier transformer emerged and developed rapidly along with the technology of high-voltage inverter. Depending on the number of inverter units and voltage levels, the number of output windings and voltage of the phase-shifting rectifier transformer are also different. 3kv mostly uses 3 levels, the phase shift is divided into 0° and ±20°, and the voltage of each phase-shift group is 630v; 6kv mostly uses 6 levels, the phase shift is divided into ±5°, ±15°, ±25°, and the voltage of each phase-shift group is 630v. There are also 5 or 7 levels. When the 5-level phase shift angle is 0°, ±12°, ±24°, the voltage is 710v, and when the 7-level phase shift angle is 0°, ±8.57°, ±17.14°, ±25.71°, the voltage is 490v; 10kv mostly uses 8 levels, the phase shift is divided into ±3.75°, ±11.25°, ±18.75°, ±26.25°, and the voltage of each phase-shift group is 720v. There are also 9 and 10 levels. Theoretically, the more stages there are, the fewer harmonics there are on the transformer input side, and the less pollution there is to the power grid. However, the more stages there are, the more power units there are in the inverter, which increases the manufacturing cost. Therefore, the above stages are generally adopted by inverter manufacturers. The transformer required for ABB's ACS5000 inverter is simpler in structure than the above ones; 12-pulse and 18-pulse rectifier transformers are mostly split, which is suitable for ABB and AB inverters. It is used to improve the impact of the high-order harmonics of the rectifier on the power grid and communication equipment.

On the basis of the three-phase voltage of the power grid, in order to obtain a uniformly distributed multi-pulse secondary voltage, the secondary voltage of each phase needs to be evenly distributed within 120°. For this purpose, the two connection groups y, d11 and yd1 are used to achieve a mutual phase shift of 60°. Then the required phase angle is obtained by using the secondary side extended triangle phase shift. According to the definition of the connection group, the clockwise phase shift is (+) and the counterclockwise phase shift is (-). For example: the interval of the 18-pulse phase-shifting transformer is: 360°/18=20°. The phase shift angles of the connection groups are as follows: y, d11-20°; y, d11; y, d11+20°.

3 18-pulse H-class insulation dry-type rectifier transformer design overview

3.1 Determination of capacity

The choice of core is related to voltage, while the choice of wire is related to current, that is, the thickness of the wire is directly related to the heat generation. In other words, the capacity of the transformer is only related to the heat generation. For a well-designed transformer, if it works in a poor heat dissipation environment, if it is 1000kva, if the heat dissipation capacity is enhanced, it may work at 1250kva. In addition, the nominal capacity of the transformer is also related to the allowable temperature rise. For example, if a 1000kva transformer allows a temperature rise of 100k, if it can be allowed to work at 120k under special circumstances, its capacity is more than 1000kva. It can also be seen from this that if the heat dissipation conditions of the transformer are improved, its nominal capacity can be increased. Conversely, for the same capacity inverter, the volume of the transformer cabinet can be reduced.

The selection of transformer capacity is generally based on comprehensive considerations of voltage, current and environmental conditions. The required load should be determined based on the capacity, nature and usage time of the user's electrical equipment, and the transformer capacity should be selected accordingly. During normal operation, the power load borne by the transformer should be around 75%-90% of the rated capacity of the transformer.

The transformer is composed of two or more coil windings wound on the same iron core. The windings are connected by an alternating magnetic field and work according to the principle of electromagnetic induction. The installation location of the transformer should be convenient for operation, maintenance and transportation, and a safe and reliable place should be selected. When using the transformer, the rated capacity of the transformer must be selected reasonably. When the transformer is running at no load, a large reactive power is required. This reactive power must be supplied by the power supply system. If the capacity of the transformer is too large, it will not only increase the initial investment, but also cause the transformer to be in no-load or light-load operation for a long time, increase the proportion of no-load loss, reduce the power factor, and increase the network loss. Such operation is neither economical nor reasonable; if the transformer capacity is too small, the transformer will be overloaded for a long time and easily damage the equipment. Therefore, the rated capacity of the transformer should be selected according to the needs of the power load, and should not be too large or too small.

The design of transformers generally only considers the rated capacity, not the rated power, because the current is only related to the rated capacity. For voltage source inverters, since their input power factor is close to 1, the rated capacity is almost equal to the rated power. This is not the case with current source inverters. The power factor of the input side transformer is at most equal to the power factor of the load asynchronous motor. Therefore, for the same load motor, its rated capacity is larger than that of the transformer of the voltage source inverter.

3.2 Selection of core magnetic flux density

The basic issues of transformer design are magnetic flux and current density. The current of the transformer is proportional to the capacity, and the current density (i.e. the thickness of the wire) is considered according to the heat generated by the conductor. For magnetic flux, the basic relationship in electromagnetism is:

u=4.44fwφ,

in:

u is voltage;

f is the frequency, which is 50 Hz here, a fixed value;

w is the number of turns of the coil;

φ is the magnetic flux.

Since the magnetic flux density b of silicon steel sheets is limited by the material, it can generally only be designed to 1.4-1.8 Tesla, and φ=bs, so to increase φ, generally only the cross-sectional area of ​​the core can be increased. The core of the transformer is generally a three-phase column type. The cross-sectional area of ​​the core can be determined according to the above formula, and the size of the core window should be considered in principle to put the coil in. The larger the capacity of the transformer, the thicker the wire, and the larger the core window needs to be. In the design of the transformer, the amount of copper and iron can be considered in a balanced manner. Because once the capacity of the transformer is determined, the current is determined, and the thickness of the wire is determined. If the number of turns w is increased, the magnetic flux φ can be smaller, and the cross-sectional area of ​​the core can be smaller, but to wind these turns in, the core window must be larger; on the contrary, if the number of turns w is reduced, the magnetic flux φ will be larger, the cross-sectional area of ​​the core will be larger, but the core window can be smaller.

3.3 Harmonic current problem

Since the unidirectional blocking effect of the rectifier element will cause the waveform of the alternating magnetic field of the rectifier transformer to be distorted, even if the grid voltage is an ideal sine wave, the current taken from the AC grid by the rectifier is non-sinusoidal. Another reason for the generation of harmonics is due to non-linear loads. When current flows through a linear load, the current on the load is linearly related to the applied voltage; when current flows through a non-linear load, the current on the load is a non-sinusoidal wave, which generates harmonics.

Harmonic voltage and current generated by rectifier and inverter: The function of rectifier is to convert AC into DC, while the function of inverter is to convert DC into AC. The diode in its circuit is regarded as an ideal diode, that is, the forward impedance is close to zero and the reverse impedance is infinite. Therefore, the current is only allowed to flow in one direction. From the output end of the rectifier, the current waveform of each phase is a rectangular wave, not a sine wave. Using the Fourier series expansion to expand the rectangular waveform of the cycle, it can be seen that in addition to the industrial frequency sine wave (50hz fundamental wave), a series of high-order waveforms - harmonics are superimposed. It should be said that the motor uses a frequency converter for speed regulation, which can not only complete the speed regulation at a high level, but also save a lot of electric energy (nearly 30%). However, as analyzed above, high-order harmonics will be generated during the frequency conversion speed regulation process, that is, high-order harmonic pollution will be formed, causing the TV and audio systems in the factory area to not work properly, and also interfering with the normal operation of secondary instruments - pressure, flow, programmable controllers and intelligent controllers. Harmonics will also cause transformers, motors, capacitors and reactors to overheat. Increasing the number of phases or pulses of the commutation device is a very effective measure to reduce the harmonic current generated by the commutation device.

3.4 Problems with operation beyond nameplate capacity

Determining the nameplate capacity of a transformer requires comprehensive consideration of other factors, such as the impact of ambient temperature. Lowering the temperature can increase the output power of the transformer and reduce the loss of the transformer. Reasonable selection of the number of transformers and technical and economic comparisons are all factors that affect the selection of transformer capacity.

As for the overload capacity of the transformer, it is related to factors such as the initial load rate, ambient temperature, and ventilation and heat dissipation conditions, and can only be emergency and short-term. When overloaded, the first requirement is not to damage the insulation of the transformer and reduce the efficiency. During the peak power consumption in the four seasons, it is possible to overload, and light load operation will occur during the valley. The amount and time between the "overload" and "light" loads are basically equal, and they will complement each other, but it is best not to overload.

Calculation formula for overload percentage (n):

n = (i - le) / ie × 100

Where:

i — actual load current of transformer;

ie—transformer rated current.

Of course, for transformers equipped with forced air cooling, the emergency overload capacity can reach 40-50, and the overload discontinuity time can be appropriately extended (but long-term operation under overload conditions is never allowed), which can be determined by the technical conditions of the product.

Taking into account the influence of the above factors on the selection of transformer capacity, from the perspective of energy saving, economy, practicality, safety and reliability, it is generally appropriate to select a transformer load rate of 0.65-0.8.

3.5 Suppressing circulation issues

For 18-pulse and above rectification transformation, the winding of the rectifier transformer is realized by zigzag connection (z connection), and each rectifier unit is connected in parallel (or in series) to supply power to the load together. As long as the voltage u (n) (n = 1, 2, ..., m) on the AC side of the m groups of 6-pulse rectification is sequentially phase-shifted by α = 60°/m, a multi-phase rectification with p = 6m pulses can be obtained. For the 18-pulse phase shift, theoretically, it does not contain 17th, 19th and lower harmonics, thus greatly reducing the influence of low-order harmonic circulating currents; usually, as long as the total number of turns of the two sides of the transformer is equal to 1.732, the occurrence of circulating current problems can be well avoided, but this is only a theoretical calculation. In practice, the number of turns cannot be a decimal, so it can only be reasonably selected and distributed during design to make the ratio as close as possible.

3.6 Requirements for impedance calculation

The smaller the value of the transformer secondary reactance, the greater the difference in load distribution. Theoretical calculations show that increasing the secondary reactance of the rectifier transformer can partially reduce the problem of uneven load current distribution. Since the only adjustable secondary reactance of the rectifier transformer is the transformer internal lead reactance and the secondary bus reactance, the adjustable range is very limited. Moreover, the load rate of the rectifier unit often changes with the production process and the switching of the standby unit. Therefore, such an idea has great limitations and is actually impossible. Designing the rectifier transformer winding according to the split transformer structure (such as axial splitting) and increasing the impedance between the windings is also conducive to improving the problem of uneven load current distribution. However, for thyristor rectifiers, there may be other factors that are not conducive to the safe operation of thyristors.

Although the use of thyristor rectifiers can properly adjust the currents of the two sets of secondary windings to achieve balance, there are other factors that are not conducive to the safe operation of thyristor rectifiers.

The use of saturated reactors for fine adjustment can better solve the problem of uneven load current distribution between the two. However, it also has a price. The space occupied by the saturated reactor, the increased manufacturing cost, its own power consumption and the impact on the power factor cannot be ignored.

In the 18-pulse rectifier circuit, the main rectifier circuit is composed of three groups of 6-pulse thyristor rectifier bridges, which are powered by three groups of completely independent secondary windings.

During operation, the rectifier will cause the voltage waveform at each point of the power grid to be distorted, interfering with the normal operation of other electrical equipment on the power grid. Similarly, when the disturbance of the power grid exceeds a certain limit, it will also cause the specified performance of the rectifier to decrease, causing its operation to be interrupted or even damaged. This is the compatibility issue between the rectifier and the power grid. According to the provisions of the national standard gb10236-88, compatibility means: first, the interference of the rectifier to the power grid is within the allowable range of the power grid; second, after the rectifier is connected to the power grid, the disturbance of the voltage fluctuation, frequency, waveform and other parameters on the primary side of the rectifier transformer (including the disturbance caused by itself after connection) should be lower than the anti-interference limit value of the selected rectifier.

According to the national standard GB10236-88, the maximum value of the phase change gap allowed for a rectifier with a Class B immunity level is 120°. If the phase change gap is too large, it will cause trigger failure, false triggering or unstable operation of the inverter. If the two-phase AC terminals involved in the phase change of the transformer are short-circuited instantly during the phase change, the secondary line voltage of the transformer will drop to nearly zero, resulting in a gap in the voltage waveform.

3.7 Body structure

The inverter uses multiple phase shifting groups of the rectifier transformer to form a phase difference between the secondary windings. Each phase shifting group supplies power to the corresponding power unit to achieve input multiplexing, complete rectification and inversion in these units, and then superimpose. With multiple phase shifting, the pollution of the harmonics generated by each unit to the power grid can be eliminated. This is the basic working principle of the perfect harmonic-free inverter. A group of phase shifting units of the transformer supplies power to one phase of the inverter. Three phases require three groups of phase shifting units. Therefore, a winding consisting of three groups of phase shifting units on the secondary side is used.

The secondary side of the 18-pulse dry-type rectifier transformer has many outlets, which are placed evenly on the outer pole for convenience. Therefore, the primary winding is placed on the inner pole, and it is difficult to tap the primary winding, and there is generally no tapping. Since the three-phase transformer has three sets of windings, the inter-group working voltage of the three sets of windings is the phase-to-phase voltage of the inverter, and the insulation distance between them belongs to the creepage distance, so the insulation distance should meet the national standard requirements and be tested according to the end-to-ground requirements (one set of power-on withstand voltage, and the other two sets of grounding). Each secondary winding is connected to the inverter in series, so the phase shift angle should be gradually changed in sequence and consistent with the inverter side to reduce the unit (inter-segment) voltage gradient. The same-name end of the winding is wound. If the inner triangle conductor is a single wire, double wires can be used along the edge, and taps are welded at the transition between single and double wires.

3.8 Temperature rise calculation

Temperature rise reflects the rationality of the entire transformer design from one aspect and verifies the quality of insulation and heat dissipation. Different design schemes are adopted according to the maximum temperature rise requirements and different heat dissipation requirements. In actual temperature rise tests, the resistance method is often used to test the winding temperature rise, because in comparison, the thermocouple method measures the temperature rise of the outer layer of the transformer coil, while the resistance method measures the average temperature rise of the transformer coil. The formula for measuring the winding temperature rise using the resistance method is:

△t=（k+t1）（r2-r1）/r1-（t2-t1）

Where:

△t—winding temperature rise;

r1 – resistance at the beginning of the experiment;

r2—resistance at the end of the experiment;

k—for copper windings, equal to 234.5; for aluminum windings: 225;

t1—room temperature at the beginning of the experiment;

t2—room temperature at the end of the experiment.

Problems in the operation of 4h-class insulation dry-type rectifier transformers

Since the rectifier transformer winding current is non-sinusoidal and contains many high-order harmonics, in order to reduce the harmonic pollution to the power grid and improve the power factor, the pulse number of the rectifier equipment must be increased. This can be solved by the phase shifting method. The purpose of phase shifting is to create a phase shift between the same-name end line voltages of the secondary winding of the rectifier transformer.

Dry-type transformers have excellent performance and strong overload capacity. As long as the design is correct, they can operate smoothly under general industrial site conditions. Some problems are caused by short-circuit failures caused by damage to the insulation system. Therefore, the design of dry-type rectifier transformers mainly lies in the design of the insulation system, sufficient insulation margin, and avoiding burrs on copper wires and damage to insulating paper caused by winding, as well as burns and cracks on the windings caused by assembly welding, etc., so as to avoid operating failures.

5 Conclusion

The primary winding of the H-class open-type transformer adopts a layered structure, with Nomex cardboard as the interlayer insulation barrier. The low-voltage winding adopts foil or pancake coils. The high and low voltage coils of this structure have direct large-area contact with the air, so their heat dissipation is very good, and the windings are particularly strong in short-circuit resistance and have strong over-nameplate operation capabilities. A layer of adhesive is brushed on the core with multiple joints, and suspended noise isolation measures are taken in the structure to greatly reduce the noise. The manufacturing process does not use epoxy resin vacuum casting or winding, but can use traditional manufacturing processes similar to oil-immersed transformers. The manufacturing equipment and mold investment is small, the product is easy to modify, and the risk of the manufacturer is small, which can improve the qualified rate of finished products and reduce manufacturing costs. Even if the winding is partially damaged during manufacturing, transportation or operation, it can still be repaired. The H-class open dry-type transformer adopts VPI

The system vacuum pressure impregnation treatment process effectively protects the coil from various external pollutants. With the wide application of high-voltage inverters in energy-saving technology transformation and the continuous maturity of the manufacturing process of the matching dry-type rectifier transformers, the H-class dry-type rectifier transformers will have a greater application prospect.