Gene-based Approach to Protection Measures of Folding Open Core Transformer against Sudden Short Circuit
Guide reading
Recently, a symposium on anti-short-circuit technology of distribution transformers was held in Shenyang Transformer Research Institute. Technical discussion and exchange of solutions were conducted on the phenomenon of low passing rate of short-circuit capability test of distribution transformers. The Symposium on Technical Measures of Short Circuit Resistance of Distribution Transformers by State Grid Corporation has been round after round, and the Technical Measures for Improving the Dynamic Stability of 10kV Distribution Transformers withstanding Short Circuits has also been conveyed in the industry. Previously, some insiders said that 90% of the 10 kV distribution transformers in China were not qualified. In March 2016, the Ministry of Material and Materials of the State Grid held a report meeting on improving the short-circuit strength of distribution transformers in Beijing. The purpose is to enhance the short-circuit resistance of distribution transformers. EPTC Substation Technical Committee held "Seminar on Measures to Improve Short Circuit Capability of Amorphous Alloy Distribution Transformers" and so on. Behind so many intensive industry symposiums, there was a "Notice of State Grid Corporation on the Treatment of Supplier's Bad Behavior". In the random sampling inspection of special quality supervision for distribution transformers in 2017, the short circuit endurance test of distribution transformers was not qualified. Taking up the absolute majority of all problems of distribution and transformer equipment, this phenomenon has attracted great attention of industry experts and power departments. In addition to the explosion and injury of distribution transformers in Hunan and Guangdong, the problem of transformer anti-sudden short-circuit has not been a simple bidding, spot-checking, punishment, warning, meeting and then bidding, spot-checking, punishment, warning, meeting, and finally No. The cycle after cycle.
Is it difficult to resist sudden short circuit?
The main items of transformer sampling inspection are unqualified burst short circuit test, unqualified empty load loss and unqualified partial discharge. Sudden short-circuit test is a type of test project, destructive test. It tests the strength of coils, cores and assembling machinery in the process of transformer design and manufacture, and the ability of distribution transformer to resist sudden short-circuit. It is a big problem for transformer production design, technology and manufacturing cost that enterprises should bear to meet the requirements of anti-sudden short-circuit test for the whole line products. On the one hand, we need to face the market competition, that is, the winning price is not high, on the other hand, we also have to face the manufacturing costs from various aspects (such as: the price rise of main materials, especially the price rise after winning the bid, labor cost rise, transportation cost rise, financial cost rise, etc.). In 2016, we have experienced a big impact of cost rise.
Is it difficult for distribution transformer to resist sudden short circuit? The answer is No. Why do sudden short-circuit problems occur frequently? What's wrong with transformer genes? Transformer manufacturing cost problem? Is there a problem with low price competition in transformer industry? Or is it technology updating? Transformer industry is in an awkward situation of "good goods are not good prices" and "good goods at low prices".
Genes of decoding transformer
Animals, plants and even all organisms have genes, and our common products also have genes, transformers are no exception. The main purpose of distribution transformer anti-sudden short circuit test is to investigate the mechanical force of coils, so the protection of coils is the most important. In the manufacturing process of various transformers, different types of core structure and different materials form different transformer genes, which determine the advantages and disadvantages of various transformers in the ability to resist sudden short circuit. However, the advantages and disadvantages of Transformers'own genes do not represent the needs of the market, because there is a stronger external factor besides genes called "lineage". "
Genes of core-stacked transformers
At present, the cross-section of core column of laminated transformer is designed as a long circle, and the cross-section of upper and lower yokes is designed as a new type of transformer core with D shape. It is mainly composed of core pillar, upper yoke plate and lower yoke plate. The cross section of core pillar decreases in trapezoidal form from the middle layer to the upper and lower two sides respectively. The cross section of upper yoke plate and lower yoke plate decreases in right angle trapezoidal form from the middle layer to the upper and lower two sides respectively. The utility model has the advantages of short magnetic circuit and high space utilization ratio, effectively reduces the weight of the core, reduces the no-load loss and reduces the manufacturing cost. Because the section is long and circular structure, the force is not uniform enough, the anti-mechanical ability and the anti-sudden short circuit ability are weak. Especially in the broken line section, sometimes the protection of sudden short circuit is not in place or the force is not uniform. The large deviation of conductor displacement in the broken line section results in the impedance exceeding the standard and the anti-sudden short circuit is not qualified.
The gene of laminated core transformer is that the planar structure and lamination technology of core have some defects, such as unbalanced three-phase magnetic circuit, inconsistent local magnetic flux direction and magnetic conduction direction of silicon steel sheet, and many air joints, which restrict the improvement of energy efficiency level of transformer. Due to the factors of structure design, the material used is wasted, and it is also inferior to the three-dimensional coil core transformer in resisting sudden short circuit.
Genes of Stereo Coil Core Transformer
Stereo coil core is closed structure, coils need to be wound on the core. Due to the limitation of space structure in the process of coil winding, some technological safeguards can not be implemented, or even the operation is difficult, and the protection is not comprehensive. For the coil structure with small capacity and low voltage as a single conductor, these problems have emerged in operation; once the low voltage coil conductor exceeds two products, the protection of coil transposition and lifting is more obvious; after the coil winding is completed, the core is turned up, and the gap between the core and the inner barrel of the low voltage coil can not be sustained, and so on; we know that the gap between the core and the inner barrel of the low voltage coil can not be sustained. The electric performance and short-circuit resistance of the foil winding coil are improved a lot. The foil winding of low-voltage coil will greatly enhance both the electrical performance and the short-circuit resistance. If the closed structure is large-capacity products, the foil winding structure will require more special foil winding machine. The winding process is very complex and inefficient. Many technological measures (such as the increase of copper wire, the tension of foil) are adopted. Compactness enhancement, foil edge protection, etc.) can not be guaranteed.
The design cannot vary with the price of raw materials. Because once the design is determined and the size of the coil-wound gear disc is determined, the future design can only be changed around the size of the made gear disc, otherwise it is necessary to re-produce the gear disc. There are many tools needed, such as gear discs and core blocks. For transformers of the same capacity, there are three kinds: full copper, full aluminium, half copper and half aluminium. Gear discs also need three kinds accordingly. Core needs stress relief annealing. The high energy consumption annealing process not only increases the cost but also does not protect the environment. Core pellets are a huge expense.
So there are many shortcomings in the gene of three-dimensional coil transformer, such as endless equipment input, difficult annealing process, difficult processing of large-capacity core, difficult control of three-phase coil balance, and high energy consumption annealing equipment in the manufacturing process of energy-saving transformer.
Genes of amorphous alloy transformers
Amorphous alloys have the greatest advantages of high saturated magnetic induction, low coercivity and low loss.
In the manufacture of distribution transformer, amorphous alloy cores adopt a lap winding structure of approximately rectangular shape. The internal stress of the formed cores is very large, and the magnetic properties are seriously damaged. These stresses can only be eliminated by annealing through heat treatment. Heat treatment annealing process is the most critical and difficult process to control in the whole manufacturing process of amorphous alloy core. The loss of amorphous alloy strip is related to the force on the surface of alloy material. The loss will rise rapidly with the increase of force. After the amorphous alloy core is stressed, the no-load loss of transformer will increase by 50%-70%. Therefore, the amorphous alloy core can not be used as the framework of transformer, but the winding is used as the main support, the core should be suspended on the winding, and the winding pressure should be set up with another frame to make the core and core. The force of windings is not related to each other. No external force factor can be produced in the process of assembly and transportation of core after annealing. Otherwise, the no-load loss will increase because of its sensitivity to stress.
The poor toughness and stress sensitivity of amorphous alloy ribbon make the gene of amorphous alloy transformer noisy, brittle and easy to fall off and slag. In the process of transformer manufacturing, the core assembly and winding assembly can not be pressurized, and the core can not be clamped, which makes it difficult to implement many measures needed to resist sudden short circuit. Therefore, the gene makes amorphous junction. Gold Transformer Resist Sudden Short Circuit in Distribution Transformer Disaster Areas. At the same time, the gene of amorphous alloy strip is also the fundamental reason that transformer can not be maintained and core can not be recycled.
Gene of foldable open core transformer
Folding open three-dimensional core transformer perfectly integrates the traditional laminated core transformer with the closed three-dimensional triangular coiled core transformer. Each single core frame of the closed three-dimensional triangular transformer is truncated and separated into upper and lower parts. The core of each part is 1 piece and 1 stack, which can be combined with multiple seams. This not only improves the mechanical strength of the core, but also reduces the noise of the core. Sound and no-load current, and a set of multistage inserts improve the insertion efficiency of the upper yoke.
Excellent genes compared with traditional iron core
1. The foldable open-ended three-phase solid core transformer is balanced, has a fully symmetrical magnetic circuit, low no-load current, reduces the third harmonic and greatly reduces the noise.
2. The power-saving effect is remarkable. It can easily meet the standard of S13 transformer. It has compact structure, small volume, low cost, strong heat dissipation and overload capacity.
3. Silicon steel strip is processed into trapezoidal material strip, which is cut by nesting method. There is no waste and no waste of material.
Excellent Genes Compared with Closed Stereo Coil Core
In addition to all the advantages of the closed three-dimensional coil core, the product has the following advantages:
1. Because of its open structure, its coil can be wound independently. Traditional winding machines can be used without introducing new winding equipment. Moreover, its winding process is much simpler than closed winding, and the winding difficulty is much lower.
2. Easy to repair, the original closed-ended three-dimensional coil core transformer must have special winding machine and die to repair. It is very difficult to repair, and basically needs to return to the original factory for repair. Because of its structural characteristics, foldable open-ended three-dimensional core transformer only needs to remove the upper yoke and stand in sequence, and replace the coil separately by pulling it out. It is easy to repair and does not need to return. Repair in the original factory.
3. The difficulty of producing large-capacity transformers is reduced. Because the original closed-ended three-dimensional coil transformer has to winding coils on the core, when it needs to produce large-capacity transformers, its core die, core winding machine and winding machine need special design, and the requirements for equipment are very high. The actual production of transformers is also very difficult, so it can only make prototype, not suitable. For mass production, it is not conducive to the improvement of transformer industrialization ability, and foldable open core transformer is more suitable for large capacity transformer production because its coils can be wound independently, and the core production does not need dies.
4. In the production process, because the silicon steel sheet is only subjected to partial shear stress and bending stress, the stress is small and the large capacity can be neglected. It can avoid annealing, save the purchase of annealing furnace, save the annealing electricity, effectively achieve energy saving and consumption reduction, avoid the damage to the environment caused by annealing process, and greatly reduce the production process and cost.
Anti-sudden Short Circuit Technology Guarantee
Because of the special form of the opening structure, the coil can be wound independently and the cross section is circular. The bearing force of the circular coil is the most uniform. Therefore, more perfect technological safeguard measures can be taken to control the sudden short circuit.
1. Internal bracing;
2. Pressure up and down;
3. The process is tightened.
4. External wrapping.
Photo Provided: Yawei, Jiangsu
Specific measures:
1. Design, this is the first process. When designing impedance, the impedance value should be designed as positive as possible, but it should not be too large (after short circuit test, the change of impedance does not exceed the national standard value of finished products), too large will also affect the load loss, in addition, the design of various margins is the lowest.
2. When the product capacity exceeds 315 kVA, the foil-wound structure can be adopted for low-voltage coils, and the axial force of the foil-wound structure will have little influence.
3. Low-voltage coil head, this is the absolute focus! Individual suggestion is to adopt "8" word weaving pattern.
4. Upgrading, end insulation and adjacent turns must be firmly tied, while overlapping with adjacent turns when inserting shrinkage bands;
5. The inner part of the low-voltage coil must be tightened by braces, and the gap from the step of the core can be filled.
6. The brace between the high and low voltage coils should be 8 or 4 widths, and be placed cross-wise with a distance of 10-15 mm.
7. If the low-voltage coil is foil-wound structure, it is necessary to cut and increase the processing of welding wire at the welding points of copper bars and foil at the head and end.
8. When the low-voltage coil is wound in transposition, if four or six coils are wound side by side, according to the number of turns, it is guaranteed to change one position per half circle.
9. After winding the high-voltage coil, tie it with braces and shrinkage bands outside.
10. The insulation of high voltage coil end and its adjacent line turns are fixed by tape braiding and glue brushing.
11. Internal bracing: When winding the coil, it must be in accordance with the relevant dimensions of the drawings, and the layers are compact. Especially, the epoxy liner should be placed inside the low-voltage coil (the forming part can increase the mechanical force, the thickness should not be less than 3 mm), and the gap between the liner and the core parts should be tightened by braces.
12. Up and down pressing: try to use laminated wood, and up and down to its, and the upper and lower clamps to set up reinforcement plate to ensure mechanical force;
13. External wrapping: Place a circle of braces evenly on the outer layer of the high-voltage coil and tighten it with weft-free tape; the coil needs to be dipped (once).
Photo Provided: Baotou Branch Electricity
Concluding remarks
No matter which way to increase the short-circuit resistance of products, it will inevitably result in increased costs and decreased production efficiency. If each transformer manufacturer continues to fight against each other's price, the result will be very dangerous. The real product is to win by quality. The gene of transformer can basically see the problem in resisting sudden short-circuit, while the national grid. The competent authorities and experts and scholars in the industry can also make contributions to the promotion of new materials, new technologies and new technologies of transformers. There is still a wider space to improve the industrialization ability of distribution transformers. I hope this article can help the distribution transformer industry develop healthily and steadily. Thank you! (Wen/Wu Yinchuan)
