The power frequency transformer is wound by two wires, one is the secondary coil (output) and the other is the primary coil (input). However, most of the transformers in the circuit board are wound into three windings by three wires, which are the primary winding (main winding, feedback winding). The feedback winding is generally used for circuit starting and secondary winding.
A two-wire parallel winding transformer is typically used in the secondary of a full-wave rectified transformer. The primary of the transistor push-pull output transformer.
The characteristics of the transformer double wire winding:
The two windings have symmetrical DC resistance and the AC impedance is symmetrical. Satisfying the phase relationship required by the circuit (when the first end of the two-wire winding is connected, the current directions of the two coils are opposite to the junction point, and the phase sequence of the two coils is opposite at the same time), so that the two coils are completely balanced.
The two ends are connected together, and the two head ends are connected together as a center tap. After the two ends are used as output heads (for the center tap), the two sides are symmetric to the center tap voltage. How to judge the wiring mode of the low frequency transformer
There are only two types of windings, Y and Δ, and the combination of double-ring transformers is four: Y/Y, Δ/Y, Y/Δ, Δ/Δ.
The Y/Y junction group has the same phase of input and output, and there are 5 different phases, which are represented by clock, which are 0 points and 2, 4, 6, 8, and 10 points respectively. The hour point indicates the phase relationship between input and output. Further description, please ask the landlord to use the drawing to illustrate.
The Δ/Δ junction group is the same input and output, and there are 5 different phases, which are represented by clock. It is also 0 and 2, 4, 6, 8, 10, and the hour is the same between input and output. Phase relationship, further description, please ask the landlord to use the drawing to explain it.
The Δ/Y and Y/Δ junction groups are different phases of input and output. They are 1 point and 3, 5, 7, 9, 11 points by clock. They are also the phase relationship between input and output. Vector is also needed. Figure to describe.
The question of why you choose depends on the advantages and disadvantages of their various knots.
It can be seen from the Δ junction that each phase winding is connected to the other two-phase winding head and tail. The advantage is that the third harmonic will cancel out in the Δ-shaped winding. The disadvantage is that there is no neutral point and no grounding can be used. The method controls the ground potential.
The advantages and disadvantages of the Y-junction are exactly opposite to the Δ junction. The induced third harmonic cannot be cancelled, which will affect the next stage or electrical equipment, but it has a neutral point. You can use the neutral point to select a grounding method. , control system ground voltage and protection measures.
The grounding of the neutral point is called working grounding, and the power system is indispensable for working grounding. It has 4 functions:
1. Meet the needs of system operation. The neutral point grounding can make the relay protection operate accurately and eliminate the single-phase grounding overvoltage; the neutral point grounding can prevent the zero-sequence voltage offset and maintain the basic balance of the three-phase voltage.
2. Reduce the contact voltage of the human body. If the neutral point is not grounded, when one phase of the system has a ground fault, when the person stands on the ground and touches another phase, the contact voltage that the human body will receive will approach the line voltage. When the neutral point is grounded, the neutral point grounding resistance is small, and the potential difference between the neutral point and the ground is close to zero. If one phase is grounded, when the person stands on the ground and touches another phase, the human body receives The contact voltage is only close to the phase voltage, thus reducing the contact voltage of the human body.
3. Ensure that the faulty equipment is quickly cut off. In a neutral point ungrounded system, when one phase is grounded, the grounding current is small, and the protection device cannot quickly act to cut off the current, and the fault will continue for a long time.
4. It can reduce the design insulation level of electrical equipment and power lines. In the neutral point grounding system, when one phase is grounded, the voltage of the other two phases is still close to or equal to the phase voltage. Therefore, the insulation design can only be considered according to the phase voltage, which can reduce the investment of the power equipment. China's power system is generally divided into the following voltage levels: ultra-high pressure 500KV is the main grid across inter-provincial areas, high-voltage 220KV is the regional transmission network in the province (now developed areas are developing 500KV), high-voltage 110KV is county-level network Main supply system (developed area has 200KV and 110KV coexisting at the same time), medium voltage 35KV is a small network frame at the county level, or a main grid of a town, medium voltage 10KV (some large enterprises use 10KV or 6KV inside) for the street And rural main distribution lines
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