The main parameters of the transformer

1. Rated voltage.The rated voltage of the transformer includes the primary rated voltage and the secondary rated voltage. The primary rated voltage refers to the rated voltage value connected to the end of the primary winding of the transformer. The secondary rated voltage refers to the voltage of the secondary winding when the voltage connected to the primary winding is the rated value, the tap changer is placed at the rated tap position, and the transformer is unloaded.
2. Rated current. The rated current of the transformer includes the primary rated current and the secondary rated current, which respectively refers to the long-term allowable current of the primary and secondary windings that do not exceed the allowable temperature of each part under the rated voltage and the specified ambient temperature.
3. Rated capacity.Rated capacity refers to the capacity transmitted by continuous operation of the transformer at rated voltage and rated current. For a double-winding transformer, its rated capacity is expressed as the capacity of the winding (the two windings of a double-winding transformer have the same rated capacity). For a three-winding transformer, the rated capacity of each winding should be given. The rated capacity of each winding of the three-winding transformer is the same, and some are different, and there are three types according to the capacity ratio of the three windings: 100%/100%/100%, 100%/100%/50%, 100%/50%/100%.
4. Rated frequency. China’s standard industrial frequency is 50Hz.
5. No load loss.No-load loss refers to the loss of the transformer when the rated voltage is applied to the terminal of one winding and the remaining winding is open. The loss of the transformer in the no-load state is mainly the hysteresis loss and eddy current loss in the iron core, so the no-load loss is also called the iron loss.
6. Short-circuit loss. For a double-winding transformer, short-circuit loss refers to the power consumed by the transformer when one winding of the transformer is shorted and the current flowing through the other winding is rated current. For the three-winding transformer, the short-circuit loss measured by the three-winding pin-two short-circuit test should be provided, and when the capacity ratio of the three windings is 100%/100%/50% or 100%/50%/100%, the short-circuit loss data is the value when the smaller capacity side of the pair of windings reaches its rated current. Short-circuit loss is mainly caused by the resistance of the winding, so it is also called copper loss.
7. Short-circuit voltage, also known as impedance voltage. For a double-winding transformer, the short-circuit voltage refers to the voltage applied when one winding is short-circuited and the voltage at the rated frequency is applied to the other winding, and the rated current flows through the short-circuited winding. There are three short-circuit voltages for the three-winding transformer, which are represented by three short-circuit voltages: high-medium, high-low and medium-low. Since the impedance of the high-capacity transformer is mainly reactance, the reactance of the transformer winding can be approximated by the short-circuit voltage, so the short-circuit voltage is a very important parameter, which is of great significance for the voltage variation and parallel operation of the transformer, for example, one of the conditions for the parallel operation of two transformers is to equal the short-circuit voltage. Unless otherwise specified, the short-circuit voltage is expressed as a percentage of the rated voltage of the applied voltage winding.
8. No load current. No-load current refers to the current value of the transformer under no-load operation at rated voltage. The no-load current of a winding is usually expressed as a percentage of the rated current of that winding. For a three-winding transformer, this percentage is based on the winding with the highest rated capacity.
9. Temperature rise.Temperature rise refers to the difference between the temperature of each part of the transformer and the temperature of the surrounding cooling medium.
   Core design and calculation
10. The role of the core:the transformer is manufactured according to the principle of electromagnetic induction, the magnetic circuit is the medium of electric energy conversion, because the core is made of silicon steel sheet with high permeability, as long as the small excitation current is passed, the required magnetic flux can be obtained.
11. The material of the iron core:the grade and lamination coefficient of commonly used cold-rolled silicon steel sheet are as follows. Because the surface of the silicon steel sheet has a good adhesion insulation film, it is not painted. The lamination coefficient depends on the thickness of the insulation film, the waviness, the difference of the same plate and the size of the burr.
12. Core section shape:the cross-section shape of the core column is a circular stepped ladder, and the series of core diameter φ70~φ1600 is 6~26 (in 1/4 circle). When the core diameter is φ70~φ395, the cross section shape of the yoke is the same as that of the core column. When the core diameter is φ340~φ1600, the cross section shape of the yoke is D-shaped.
13. Core diameter:D0=KDPzh0.25

Where: KD – diameter experience coefficient, cold rolled silicon steel sheet, copper wire KD=52~57

Pzh – Capacity per column (kVA)

5. Magnetic flux density selection principle:

Core magnetic density general hot-rolled silicon steel sheet 1.4~1.47T; Cold rolled silicon steel sheet 1.6~1.75T

6. No-load loss:

The loss measured by the transformer during no-load, no-load loss mainly includes the hysteresis loss (proportional to frequency) and eddy current loss (proportional to frequency square) in the core silicon steel sheet.

7. No-load current:

The current measured when the transformer is no-load, no-load current is mainly the reactive component of the excitation current (proportional to the frequency) and the active component of no-load loss.

8. Factors affecting no-load performance

8.1 Core material: the no-load loss and current of hot-rolled silicon steel sheet is larger than that of cold-rolled silicon steel sheet; The thicker the thickness of each sheet, the greater the no-load loss and current, but too thin will increase the additional coefficient of the process; Generally, the thickness of each piece is 0.23, 0.27, 0.3mm;

8.2 Core magnetic density: If the core magnetic density is too high, no-load loss and no-load current will increase.

8.3 Laminated form: If the number of each laminated is large, the no-load loss and no-load current will increase, generally 2 pieces are used in a stack.

8.4 Joint form: oriented cold rolled silicon steel sheet, generally adopt full oblique joints, such as the use of semi-straight semi-oblique joints, each increase in a direct joint will increase the no-load loss of about 3.5%;

8.5 Burr size: large burr, no-load loss and no-load current will increase, generally ≤0.03mm;

8.6 Clamping mode: The core screw is used than the adhesive tape and binding tape, and the no-load loss and no-load current are increased.

8.7 Manufacturing process: such as shear, handling, wrestling will produce stress, so that no-load loss and no-load current increase;

8.8 Degree of cleanliness: Keep the core clean without dust and foreign matter, otherwise it will also increase no-load loss and no-load current.

Winding design and calculation

1. Wire material:The wire of the transformer winding is often made of round copper wire and copper flat wire drawn by electrolytic copper or oxygen free copper rod (resistivity is about 1%~1.5%) to make acetal enamelled wire, paper covered wire, combined wire and transposed wire. Aluminum wire has also been used, but it is rarely used because of its high resistivity, poor mechanical strength and difficulty in welding.
2. Winding form:cylinder type (layer type) : single, double, multi-layer cylinder type and segmented cylinder type. It is often used in small and medium high voltage or low voltage windings.

Screw type: single, single half, double, double half, four, four half screw type; It is often used in medium and large low-voltage windings.

Continuous type: often used in medium and large high voltage and low voltage windings.

Tangle type: often used in 66kV and above large high voltage winding.

Inner screen type: commonly used in 66kV and above large high voltage winding.

3. Winding arrangement:Double winding: high – low arrangement. Three winding: step-down transformer for high – medium – low; The booster transformer is arranged in high – low – medium.

4. Voltage ratio deviation:

The rated voltage ratio is the ratio of the rated voltage of one winding to the rated voltage of another winding with a lower or equal rated voltage.

The deviation of the voltage ratio (variable ratio or turn ratio) is the difference between the measured no-load voltage ratio of the product and the specified standard voltage ratio, often expressed as a percentage of the specified standard voltage ratio.

The allowable deviation of no-load voltage ratio stipulated in the national standard GB1094.1 is shown in the table. In order to consider the deviation of manufacturing and measurement, in the calculation, generally should not exceed half of the allowable deviation value specified in the following table, that is, the calculation value of the allowable deviation of the no-load voltage ratio, usually V%≤±0.25%

When the high voltage winding voltage is low, and the capacity of the product, the voltage ratio (especially the tap voltage ratio) of the allowable deviation, such as can not meet the requirements, should be timely consultation with the user.

Autotransformers and booster transformers, because of their small impedance, should have greater deviations. The calculation deviation (V%) of the voltage ratio of each tap position of high voltage and medium voltage is calculated according to the following formula:

Where: et – potential per turn (V); et=U2/W2W2 – the number of turns per phase of the low voltage winding;

U2 – Phase voltage of low voltage winding (v)

W – The number of turns per phase at each tap position of the high or medium voltage winding;

U – Phase voltage (v) at each tap position of the high or medium voltage winding.

5. Current density selection principle:

The current density of the winding wire is based on the load loss (Pk); Temperature rise of long-term working current; Temperature rise of sudden short circuit; Withstand electrical power (mechanical force) in case of sudden short circuit; Economy and so on to choose. The current density of copper wire is generally about 3.0A2.

6. Main longitudinal insulation selection: the main consideration of withstand voltage: long-term operating voltage; Induction test voltage; Short-time power frequency withstand voltage; Impulse withstand voltage (full covered, truncated, operating wave), etc.

7. Load loss

7.1 Resistance loss of winding wire: I2R note should be converted to the reference temperature (generally 75C).

7.2 Eddy current loss of the winding wire: Eddy current is generated due to the leakage flux passing through the wire, resulting in eddy current loss, which is proportional to the square of the frequency and the thickness of the wire perpendicular to the leakage flux, often expressed as a percentage of the resistance loss.

Note: When the three-winding transformer calculates the load loss of the out-inside (generally high-low voltage) winding, the middle (generally medium voltage) winding, although there is no current through it, but it is in the position of the maximum leakage magnetic field, so it is necessary to add 3 times the eddy current loss of the middle (generally medium voltage) winding.

7.3 Circulation loss of winding loss: the position of the wire in the leakage magnetic field is not the same, or the length of the wire is not the same, and the transposition is not complete, and the circulation between the wires is generated, resulting in circulation loss, often expressed as a percentage of the resistance loss.

7.4 lead loss: including lead resistance loss and additional loss (eddy current loss).

7.5 stray loss: leakage flux through the clamp, pull plate, oil tank and other steel parts and produce eddy current, resulting in stray loss.

8. A common electrical failure of a winding

8.1 three-phase resistance imbalance: due to material, welding, structure (B phase lead is short) will cause three-phase resistance imbalance, attention, lead preparation and welding quality, so that the three-phase resistance imbalance rate, generally not more than 2%.

8.2 Inter-turn short circuit: due to the burr or improper transposition of the wire, the turn insulation is damaged, resulting in inter-turn short circuit. The pad should be deburred and the manufacturing process strengthened.

8.3 Induction or impact breakdown: due to material, design, process and other reasons, resulting in inter-turn, inter-segment, inter-layer breakdown. Select a reasonable insulation structure (such as partial capacitance compensation for high voltage windings). Strengthen the manufacturing process and pay attention to cleanliness.

8.4 ground discharge: due to material, design, process and other reasons, resulting in high voltage winding or ground discharge.

Choose reasonable insulation structure (such as using thin paper tube small oil gap and Angle ring structure), use electrostatic plate to improve the end electric field. Strengthen the manufacturing process and pay attention to cleanliness.

9. Measures to improve the mechanical strength of winding

9.1 winding wire: generally use good mechanical strength semi-hard copper wire. Transposition wire should be used sticky transposition wire (its bending strength is more than 3 times that of ordinary transposition wire).

9.2 ampere-turns balance: high and low voltage winding should try to achieve ampere-turns balance, and the unbalance ampere-turns of medium and large transformers generally does not exceed 5%.

9.3 Winding: Pay attention to calculation and manufacturing tolerances.

9.4 Compaction: the pad is densified; The winding compaction force is generally 2.5MPa; It is best to use constant pressure or with pressure drying and whole set.

9.5 Tightening: The low-voltage winding is equipped with a secondary brace, and all the windings are rolled on the cardboard tube.