Classification and application of transformer material iron core

Structure and characteristics of transformer core

Core and purpose classification:

The iron core is both the magnetic circuit of the transformer and its mechanical skeleton. The iron core consists of two parts: an iron core and an iron yoke. The core column is equipped with winding, and the iron yoke connects the core to form a closed magnetic circuit. Iron yoke is divided into upper iron yoke, lower iron yoke and side iron yoke.

In order to reduce the hysteresis loss and eddy current loss in the iron core, the iron core is generally stacked with silicon steel sheets with high permeability. The silicon steel sheet is divided into two kinds of hot rolled and cold rolled, and its thickness is 0.35MM and 0.5MM. Both sides of the silicon steel sheet are coated with paint film with thickness of 0.01-0.13, so that the insulation between the sheet and the sheet is made.

According to the structural form and process characteristics, the transformer core can be divided into laminated and involute two types, laminated core can be divided into core and shell type two types.

Laminated core classification:

1. Shell structure

The shell type structure is the top, bottom and side of the iron yoke surrounding the winding, that is, the shell type transformer is characterized by both an upper iron yoke, a lower iron yoke, and a side iron yoke.

Due to the limitations of transportation, the large-capacity three-phase transformer needs to reduce the height of the iron core, and move part of the upper and lower iron yoke of the ordinary three-phase chip transformer to the outside of the two side columns, which is the three-phase five-column core structure.

2. Core structure

The core structure is the iron yoke against the top and bottom of the winding, but does not surround the side of the winding, the shell structure has better mechanical strength, but the manufacturing is complex, and the iron core is more material. The core structure is relatively simple, and the winding assembly and insulation treatment are also easier. Therefore, all domestic Power Transformers adopt core structure.

Involute core:

The involute type iron core is composed of two parts of the iron core column and the iron yoke, the iron core column is the same specification of the involute shape of the silicon steel sheet piece by piece into a cylindrical core column. The ratio of the outer diameter of the core column to the diameter of the inner hole is 4.5-6. This kind of involute silicon steel sheet is rolled one piece by one piece on a special forming machine using the principle of cold extrusion plastic deformation. For three-phase transformers, the advantage of the involute core over the laminated type is that the three-phase magnetic circuit is completely symmetrical, and it can also save silicon steel.

Transformer material classification of iron cores

Iron cores can be divided into high, low frequency and COIL three kinds:

1, high frequency class: iron core FerritecoreFerritecore for high frequency transformer It is a ceramic magnet with spinel crystal structure, this spinel for iron oxide and other bivalent metal compounds. Such as kFe2O4 (k represents other metals), the metals commonly used at present are manganese (Mn), zinc (Zn), nickel (Ni), magnesium (Ng), copper (Cu).

Its common combinations such as manganese zinc (MnZn) series, nickel zinc (NiZn) series and magnesium zinc (MgZn) series. This material has high permeability and impedance physical properties, and its frequency range is from 1kHz to more than 200kHz.

2, low frequency: silicon steel (LAMINATION)

Silicon steel sheet used in low-frequency transformers, there are many kinds, according to its production process can be divided into different

A: Calcining (black)

N: No forging (white) two kinds.

According to its shape can be divided into: EI type, UI type, C type, mouth.

Mouth silicon steel sheet is often used in transformers with large power, it has good insulation performance, easy heat dissipation, and magnetic short circuit, mainly used in power greater than 500~1000W and high-power transformers.

A set of silicon steel sheets composed of two C-type silicon steel sheets is called CD type silicon steel sheets. The power transformer made of CD type silicon steel sheets has the same cross-sectional area, and the higher the window, the greater the power of the transformer. Coils can be installed separately on both sides of the core, so the number of coil turns of the transformer can be distributed on two wire packages, so that the average turn length of each wire package is shorter, and the copper consumption of the coil is reduced. In addition, if the two coils required to be symmetrical are wound on the two wire packages, the effect of complete symmetry can be achieved.

A set of silicon steel sheets composed of four C-type silicon steel sheets is called ED type silicon steel sheets. The shape of the transformer made of ED silicon steel sheet is flat and wide, under the same power conditions, ED transformer is shorter than CD transformer, and the width is larger, and because the coil is installed in the middle of the silicon steel sheet, there is an external magnetic circuit, so the magnetic leakage is small, and the overall interference is small. However, all the coils are wound on a wire package, which is thicker, so the average turn length is longer and the copper consumption is larger.

The transformer made of C-type iron core with excellent performance is small in size, light in weight and high in efficiency. From the point of view of assembly, C-type silicon steel sheet parts are few and have strong versatility, so the production efficiency is high. However, C-type silicon steel sheet processing processes are more complicated and require special equipment manufacturing, so the current cost is still higher.

We mainly use EI type silicon steel sheet. E type silicon steel sheet is also known as shell type or day type silicon steel sheet, its main advantage is that the primary and secondary coils have a common wire frame, and a high window duty coefficient (duty coefficient Km: copper wire net cross-sectional area and window area ratio); The silicon steel sheet forms a protective shell for the winding, so that the winding is not susceptible to mechanical damage; At the same time, the cooling area of the silicon steel sheet is larger, and the magnetic field of the transformer is less divergent. However, its primary leakage is larger, the external magnetic field interference is also larger, in addition, because the winding average perimeter is longer, under the same number of turns and core cross-sectional area conditions, EI type iron core transformer used more copper wire.

The thickness of the silicon steel sheet is commonly used in 0.35mm and 0.5mm.

The assembly method of silicon steel sheet has two kinds of overlapping method and overlapping method. The overlapping method is to distribute the openings of the silicon steel sheet one to one alternately on both sides, this overlapping method is more troublesome, but the silicon steel sheet gap is small, the magnetic resistance is small, and it is conducive to increasing the magnetic flux, so the power transformer adopts this method. Stacking method is often used in the case of DC current, in order to avoid DC current caused saturation, there needs to be a gap between the silicon steel sheet, so the stacking method will be E and I pieces on the side, the gap between the two can be adjusted by paper

3, COIL class: divided into three types

A.TOROID ring core: made of O-shaped sheets, or rolled up from silicon steel sheets. This kind of core is very difficult to wind.

B. ODCORE Rod core.

C. RUMCORE: Drum core

Transformer material core action

The role of the transformer core is mainly in two aspects: hysteresis loss and eddy current loss:

1, hysteresis loss, it refers to the transformer working in the AC state because of the hysteresis phenomenon of iron loss, and the use of transformer core can change this phenomenon to a certain extent. It can also alleviate the rise of the transformer surface during operation.

2. Eddy current loss refers to the alternating current generated by the transformer when it is working, and the induced current generated by the magnetic flux in the iron core will change, and this change will be called eddy current. When the eddy current is lost, the temperature of the surface of the core will rise, but because of the material of the core (silicon), our resistivity will increase during this process, which can reduce the effect of the eddy current.

Transformer core troubleshooting

The winding and iron core of the transformer are the main components of transferring and transforming electromagnetic energy. It is a matter of concern to ensure their maximum operation. Statistics show that the fault caused by the iron core problem accounts for the third place in the total transformer accidents. The manufacturing department has paid attention to the defects of the transformer core, and has made technical improvements in the aspects of the core can be grounded, the core grounding monitoring, and the guarantee of a little grounding. The operation department also takes the detection and discovery of core faults to a considerable degree. However, transformer core failure still occurs frequently, which is mainly due to the multi-point grounding and poor grounding of the core. This article introduces two kinds of fault diagnosis and treatment methods.

1, the reason why the iron core needs to be grounded when the transformer is in normal operation, there is an electric field between the charged winding and the fuel tank, and the iron core and other metal components are in the electric field. Due to the uneven distribution of capacitors and different field strengths, if the core can not be grounded, it will produce charging and discharging phenomenon, destroying the insulation strength of solid insulation and oil, so the core must have a point of grounding.

The iron core is composed of silicon steel sheets, in order to reduce the eddy current, there is a certain insulation resistance between the sheets (generally only a few ohms to dozens of ohms), because the capacitance between the sheets is very large, can be regarded as a path in the alternating electric field, so the iron core only needs a little ground to clamp the entire stack of iron core laminated potential to the ground potential.

When the iron core or its metal components are grounded at two or more points (multiple points), a closed loop will be caused between the ground points, and it will have a partial magnetic flux, induced electromotive force, and form a loop, resulting in local overheating, or even burning the core.

The transformer core is only a little ground, which is the normal ground. That is, the iron core must be grounded, and it must be a little grounded.

The core failure is mainly caused by two reasons, one is short circuit caused by poor construction technology, and the other is multi-point grounding caused by accessories and external factors.

2, iron core multi-point grounding type

(1) After the installation of the transformer is completed, the positioning pin transported on the tank top cover is not turned over or removed, constituting a multi-point grounding.

(2) Because the core sandwich limb plate is too close to the core column, the core lamination is tilted for some reason, touching the sandwich limb plate, forming a multi-point grounding.

(3) The bushing of the yoke screw is too long, and the yoke laminates collide to form a new ground point.

(4) The insulating paperboard between the foot of the clamp under the iron core and the iron yoke falls off or is damaged, so that the laminated sheet at the iron yoke of the foot of the cushion is grounded.

(5) The large and medium-sized transformer with the submersible oil pump device, due to the wear of the submersible oil pump bearing, the metal powder enters the tank, silts the bottom of the tank, forms a bridge under the action of electromagnetic force, connects the lower iron yoke and the foot or the bottom of the box, and forms a multi-point grounding.

(6) The thermometer seat cover on the oil tank cover of the oil-immersed transformer is too long, and the upper clamp or iron yoke, and the side edge of the column meet to form a new ground point.

(7) The oil immersed transformer tank has fallen into a metal foreign body, which makes the core lamination and the box through to form a ground.

(8) The wood pad between the lower clamp and the iron yoke step is damp or the surface is not clean, and there is more sludge, so that the insulation resistance value is reduced to zero, which constitutes a multi-point grounding.

3, the abnormal phenomenon that occurs when the multi-point grounding (1) generates eddy currents in the iron core, the iron loss increases, and the local overheating of the iron core.

(2) When the multi-point grounding is serious, and it is not treated for a long time, the continuous operation of the transformer will cause the oil and winding to overheat, so that the oil paper insulation gradually aging. It will cause the aging of the two insulation layers of the iron core lamination and fall off, which will cause the larger iron core to overheat and the iron core will burn. (3) Multi-point grounding for a long time causes the oil-immersed transformer oil to deteriorate and produce flammable gas, so that the gas relay operates.

(4) Due to overheating of the iron core, the wooden pad and clamp in the body are carbonized.

(5) Serious multi-point grounding will cause the ground line to burn off, so that the transformer loses the normal point of grounding, the consequences are unimaginable.

(6) Multi-point grounding can also cause discharge. 4, multi-point grounding fault detection

The determination method of iron core multi-point grounding fault is usually detected from two aspects:

(1) Gas chromatography was performed. In the chromatographic analysis, if the content of methane and olefin components in the gas is high, and the content of carbon monoxide and carbon dioxide gas is little changed compared with the past, or the content is normal, it indicates that the iron core is overheating, and the iron core overheating may be caused by multi-point grounding.

When acetylene gas appears in the chromatography, it indicates that intermittent multi-point grounding has occurred in the iron core.

(2) Measure whether the grounding wire has current. On the ground lead of the ground bushing outside the transformer core, use a clamp gauge to measure whether there is current on the lead. When the transformer core is grounded normally, no current loop is formed. The grounding line current is very small, MA level (generally less than 0.3A). When there is a multi-point grounding, there is equivalent to a short circuit turn around the main magnetic flux of the iron core, and the circulation in the turn is determined by the relative position of the fault point and the normal ground point, that is, the amount of magnetic flux surrounded in the short circuit turn. Generally up to tens of amperes. By measuring whether there is current in the ground lead, the multi-point ground fault of the iron core can be accurately determined.

5, multi-point grounding fault removal

(1) Temporary exclusion method when the transformer cannot be shut down:

① There is an external grounding wire. If the fault current is large, the ground wire can be temporarily opened to run. However, the monitoring must be strengthened to prevent the suspension potential of the core after the fault point disappears.

If the multi-point grounding fault is unstable, a slip wire resistor can be inserted into the working grounding wire to limit the current below 1A. The slip wire resistance is selected by dividing the voltage measured by opening the normal working ground wire by the current on the ground wire.

③ The gas production rate of the fault point should be monitored by chromatographic analysis.

④ After the exact fault point is found by measurement, if it cannot be handled, the normal working ground lug of the iron core can be moved to the same position of the fault point to reduce the circulation to a greater extent.

(2) Thorough overhaul measures. After the transformer is found to have a multi-point grounding fault, the transformer that can be stopped should be stopped in time, and the multi-point grounding fault should be completely eliminated after exit. To troubleshoot such faults, take appropriate maintenance measures according to the type and cause of multipoint grounding. However, in some cases, the failure point cannot be found after the power failure, in order to find the exact ground point, the following methods can be used on site.

① Direct current method. The connecting sheet of the iron core and the clamp are opened, and 6V DC is passed into the silicon steel sheet on both sides of the yoke, and then the voltage between the silicon steel sheets at all levels is measured by a DC voltmeter in turn. When the voltage is zero or the indicator is reversed, it can be considered as the fault ground point.

② AC method. The transformer low-voltage winding is connected to the AC voltage of 220 ~ 380V, and there is a magnetic flux in the iron core. If there is a multi-point ground fault, a current will occur when measured with a milliammeter (the connection of the core and the clamp should be open). A milliammeter is used to measure each level along the yoke point by point. When the current in the milliammeter is zero, the fault point is there.