- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Seven. 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.
- 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.
- 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.