2022 Ultimate 35kv Class Power Distribution Transformer Guide

2022 Ultimate 35kv Class Power Distribution Transformer Guide1

35kv distribution transformers include three-phase epoxy resin dry-type transformers and three-phase oil-immersed distribution transformers.

The rated capacity range of 35kv three-phase epoxy resin dry-type transformer is: 50~2500kva, high voltage 38.5kv, low voltage 0.4kv, using DYN11, Yyno connection group.

The short-circuit impedance of the 35kv three-phase epoxy resin dry-type transformer is 6%.

The rated capacity range of 35kv three-phase oil-immersed distribution transformer is: 800~31500kva, high voltage: 35~46kv, low voltage 6.3~13.8kv, using Yd11, Ynd11 connection group.

This article will tell you in detail about the 35kv three-phase oil-immersed distribution transformer.

Daelim can provide you with dry or oil immersed 35kv distribution transformers. They have all obtained industry standards such as IEEE, ASNI, CSA. These standards can be used to ensure the quality of the transformers you purchase. At the same time, daelim also provides you with a two-year after-sales warranty. Daelim’s installation team in North America can also provide you with professional installation services according to your specific installation location.

Table of Contents

What is a 35kv Power Distribution Transformer?

35kv three-phase distribution transformer

35kV distribution transformer refers to the oil-immersed transformer with high voltage 35kV, low voltage 0.4kV and capacity range of 50~1600kVA.

The 35kv distribution Transformer directly supplies power to the distribution network, and is a product with a large amount of applications and a wide range of applications. The traditional 35kV class 9 product with a capacity of 50-1600kVA adopts a single-segment multi-layer cylindrical structure for its high-voltage winding, a 50-500kVA low-voltage winding adopts a cylindrical structure, and a 630-1600kVA low-voltage winding adopts a new spiral structure. .

There are many parallel winding wires in the spiral winding, the lead wire is difficult to make, the transposition of the wires may be incomplete, and the leakage magnetic potential between the wire groups is large.

The end of the transformer coil using this winding needs to use a laminated wood end ring to enhance the short-circuit resistance of the coil. stability.

The high-voltage winding of the 35kV distribution transformer adopts a segmented multi-layer cylindrical structure.

This structure has stable electrical performance and impact resistance; the voltage between winding layers is low, which can reduce the amount of partial discharge; there is a radial oil channel in the middle, which can increase the heat dissipation area of ​​the winding.

The low-voltage winding adopts foil winding, which improves the filling factor of conductive material in the iron window and reduces the volume of the transformer;

Increase the thermal conductivity of the winding radial direction, low temperature gradient, good thermal stability; high mechanical strength, strong short-circuit electric power, strong impact resistance, and high electrical strength.

What's inside a 35kv transformer?

Inside of 35kv transformer: iron core

Once the core diameter of the 35kv transformer is determined, the potential size of each turn of the winding of the 35kv transformer mainly depends on the magnetic flux density.

Since the voltage level of the winding is fixed, the number of turns of the winding is inversely proportional to the magnitude of the magnetic flux density, and the impedance voltage is proportional to the square of the number of turns of the winding, so the value of the impedance voltage has a certain relationship with the magnitude of the magnetic flux density.

Therefore, when selecting the magnetic flux density, it is necessary to ensure that the impedance voltage, no-load loss and no-load current meet certain technical requirements.

The winding type is determined by the capacity of the 35kv transformer. The high-voltage and low-voltage windings with a transformer capacity of 50kVA to 315kVA are all double-layer or multi-layer single-section cylindrical windings.

The high-voltage winding with a capacity of 400k VA to 1600k VA is a segmented multi-layer cylindrical winding, and the low-voltage winding is a foil winding.

Copper foil is used for low-voltage windings of 400k VA and above, and the resistivity is 0.02097Ω.mm2/m at 75℃ and 0.01725Ω.mm2/m at 20℃;

The low-voltage winding of 315k VA and below adopts paper-clad copper flat wire (ordinary oxygen-free copper wire), the insulation thickness is 0.3mm, and the width-thickness ratio is between 2.5 and 7;

High-voltage windings of 1000k VA and below use acetal enameled round copper wire, 1250k VA~1600k VA use acetal enameled flat copper wire or paper-coated flat copper wire, the resistivity of the enameled wire at 75 ℃ is 0.02135Ω.mm2/m, At 20℃, it is 0.01756Ω.mm2/m, and the width-thickness ratio of enameled flat wire is 2.5~6;

The electrical density of the winding wire is between 2.5 and 3.5A/mm2.

Inside of 35kv transformer : winding

35kv three-phase epoxy resin dry-type transformers

Once the core diameter of the 35kv transformer is determined, the potential size of each turn of the winding of the 35kv transformer mainly depends on the magnetic flux density.

Since the voltage level of the winding is fixed, the number of turns of the winding is inversely proportional to the magnitude of the magnetic flux density, and the impedance voltage is proportional to the square of the number of turns of the winding, so the value of the impedance voltage has a certain relationship with the magnitude of the magnetic flux density.

Therefore, when selecting the magnetic flux density, it is necessary to ensure that the impedance voltage, no-load loss and no-load current meet certain technical requirements.

The winding type is determined by the capacity of the 35kv transformer. The high-voltage and low-voltage windings with a transformer capacity of 50kVA to 315kVA are all double-layer or multi-layer single-section cylindrical windings.

The high-voltage winding with a capacity of 400k VA to 1600k VA is a segmented multi-layer cylindrical winding, and the low-voltage winding is a foil winding.

Copper foil is used for low-voltage windings of 400k VA and above, and the resistivity is 0.02097Ω.mm2/m at 75℃ and 0.01725Ω.mm2/m at 20℃;

The low-voltage winding of 315k VA and below adopts paper-clad copper flat wire (ordinary oxygen-free copper wire), the insulation thickness is 0.3mm, and the width-thickness ratio is between 2.5 and 7;

High-voltage windings of 1000k VA and below use acetal enameled round copper wire, 1250k VA~1600k VA use acetal enameled flat copper wire or paper-coated flat copper wire, the resistivity of the enameled wire at 75 ℃ is 0.02135Ω.mm2/m, At 20℃, it is 0.01756Ω.mm2/m, and the width-thickness ratio of enameled flat wire is 2.5~6;

The electrical density of the winding wire is between 2.5 and 3.5A/mm2.

Inside of 35kv transformer: no-load loss

Inside of 35kv transformer winding

The no-load characteristics of a 35kv transformer mainly refer to no-load current and no-load loss, which are one of the main performance indicators of the transformer.
The no-load characteristics of the transformer mainly depend on the core structure, silicon steel sheet grade and magnetic flux density.

The magnetic potential (I0N) and magnetic flux (Φ) of the 35kv transformer are generated by the no-load current (I0), and the no-load loss is due to the magnetic flux.

It mainly passes through the iron core, and a certain loss will be generated in the iron core silicon steel sheet. No-load loss is mainly core loss, includingIt includes hysteresis loss and eddy current loss.

The no-load loss does not change with the load. Generally, only by adjusting the magnetic density and changing the insulation structure can the purpose of reducing the no-load loss be achieved.

How much does a 35kv transformer weigh?

The weight of a 35kv transformer consists of the silicon steel sheet, the weight of the windings, and the weight of the transformer oil.

The main material of the transformer core is silicon steel sheet, which is one of the main materials of the transformer.

The main material of 35kv transformer high and low voltage windings is copper wire.

Four types of copper wires are mainly used in the series products studied in this paper, namely enameled copper flat wire, enameled copper round wire, paper-coated copper flat wire and copper foil.

According to the unit price, it can be divided into enameled copper wire, paper-coated copper flat wire and copper foil.

What is the winding structure of a 35kV distribution transformer?

Segmented Layer Windings for 35kV Distribution Transformers

The power transformer of the core structure adopts the concentric winding of circular section.

The winding of circular section is very stable under the action of tensile force and compression force, the wire length is the smallest under the condition of a given effective section of the iron core, the operation is reliable, and the manufacture is simple.

Concentric windings are cylindrical. The heights of the high voltage and low voltage windings are approximately the same.

The high-voltage winding is arranged outside, and the low-voltage winding is directly sleeved on the core column.

35kV distribution transformer windings usually take the following forms: single-layer type, segmented-layer type, spiral type, continuous type, foil type.

Continuous windings for 35kV distribution transformers

Continuous windings for 35kV distribution transformers

The continuous winding is generally used as the outer winding of the 35kV distribution transformer and has the following characteristics:

The winding is composed of several round pie-shaped wire pie. The bus pie is generally between 30 and 100, and it is an even number. The number of parallel wires is generally not more than 4, and the maximum is 6.

Under normal circumstances, the odd-numbered pie is called “reverse pie (or reverse segment)”, starting from the winding end, and the wire is wound from the outside to the inside. Even-numbered cakes are “positive cakes (positive segment)”, and the wires are wound from the inside to the outside. A reverse pie and an adjacent positive pie form a unit, called a “double pie unit”. The oil passages inside the unit are called “outward oil passages”, and the oil passages outside the unit are called “inward oil passages”; in special cases (such as the outgoing wires of the windings are drawn from the inner diameter side), count from the winding end. , odd-numbered pie is positive pie, even-numbered pie is negative pie.

A positive cake and an adjacent reverse cake form a unit, the oil passages in the unit are called “inward oil passages”, and the oil passages between the units are called “outward oil passages”.

The continuous winding must be transposed, and the radial direction of the wire cake at the transposed position is higher than that of the normal part. To prevent increasing the radial size of the winding, fractional turns are used. The numerator value is generally the total number of winding struts minus 1.

The line segments of the continuous winding are all wound on the struts to form vertical oil passages on the inner surface of the winding, and spacers are pierced on the struts to form the oil passages between segments. Each turn of the winding can consist of one or several parallel wires. The coil of this structure can reduce the eddy current loss of the coil and facilitate winding.

The continuous winding end support surface is large, which is stable to axial force during short circuit, and has a large heat dissipation surface.

The capacitance of the continuous winding to ground is much larger than the longitudinal capacitance of the winding, so under the lightning overvoltage, the potential gradient between the line segments at the inlet end of the winding is large.

In view of the above characteristics, continuous coils are widely used in transformers in terms of voltage and capacity.

Segmented Layer Windings for 35kV Distribution Transformers

Under high voltage, even if a multi-layer cylindrical coil is used, the interlayer voltage is still very high, and it is difficult to ensure the dielectric strength required for the insulation between adjacent layers. In order to reduce the interlayer voltage, the coil is divided into several sections in the axial direction, so that the segmented cylindrical coil is composed of several multi-layer line segments. Its structural features are:

(1) Two multi-layer cylindrical windings are connected in series to form a segmented multi-layer cylindrical winding.

A soft angle ring and an end ring are arranged in the series connection to form inter-section insulation.

A spacer block with a certain thickness is stuck on the corresponding position of the strut of the oil passage between layers to form the oil passage between sections.

(2) The wire width should be as narrow as possible on the premise of ensuring a certain cross-sectional area and aspect ratio.

(3) Since the ground potential of the two multi-layer cylindrical windings differs by half during operation, only an electrostatic screen (high-voltage outlet end) is placed on the inner diameter side of one multi-layer cylindrical winding.

On these structures, various electrical shields are used so that when the coil is subjected to an impulse voltage, the voltage is evenly distributed across the layers of the coil. Some constructions only use a “line” shield at the beginning of the coil, while others also have an inner shield connected to the “neutral” end of the coil.

Some cylindrical coils are also equipped with capacitor rings at the end of the layer.

(4) The tap line is generally arranged at the outermost layer of the multi-layer cylindrical winding at the high-voltage outlet end, and the tap line adopts the lead-out method of the bow head.

Foil windings for 35kV distribution transformers

Foil windings for 35kV distribution transformers

The copper foil or aluminum foil is wound on a special foil winding machine, each layer is one turn, and the interlayer insulation is the interturn insulation.

The interlayer insulation and end insulation are wound simultaneously when the winding is wound. Foil windings are often used as low-voltage windings.

Compared with low-voltage windings wound with multiple wires, such windings have high space utilization, are convenient for automatic winding, and have high productivity.

Its characteristics are:

(1) Weld the copper bar on the metal foil as the head and end of the foil winding.

(2) The axial air passage of the foil winding is formed by the drawing bar or short glass fiber board of the corresponding heat resistance grade, which is wound in the winding winding process to form the air passage.

(3) The temperature distribution of all sections of the line segment is relatively uniform, which improves the cooling of the coil.

(4) The inter-turn capacitance is evenly distributed along the coil, and the coil increases the stability to impulse voltage.

(5) The magnetic potential of the coil is evenly distributed along the height of the high and low voltage coils, and the axial force is the smallest during short circuit.

(6) Since the ampere-turn distribution of the foil winding is easy to control, its radial leakage magnetic component is small, and the axial electromotive force caused by it is not large, so the axial compression of the transformer winding is relatively easy to deal with.

(7) The interlayer capacitance of the foil winding is much larger than the ground capacitance, so it has a good impact distribution under the action of the impulse voltage.

To sum up, combined with the characteristics of the high and low voltage windings of 35kV distribution transformers, the selection types of the windings are as follows:

(1) The high-voltage winding has high voltage, small current, and many turns, so the interlayer voltage is high.

The layered winding is selected. In order to reduce the interlayer voltage, the segmented layered winding is selected for the capacity of 400k VA and above;

(2) The low-voltage winding has low voltage and large current. Double-layer or multi-layer cylindrical windings are used for 315kVA and below, and foil windings are used for 400k VA and above.

(3)There are at most 2 longitudinal oil passages in the high voltage winding, and at most 2 longitudinal oil passages in the low voltage winding.

Conclusion

35kV distribution transformer is a widely used electrical equipment.

The reduction of production cost and the improvement of product performance are of great significance to manufacturers and users.

The design of a 35kV distribution transformer is the first step in the manufacture of the entire transformer.

The quality of the design directly affects the manufacturing cost and economic benefits of the product. The high-quality design of the transformer can not only improve the operating performance of the product, reduce the design cost, but also reduce the design workload and design time.

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