Transformer neutral point, not the difference between grounding

Update date:2018-04-25 Source:MAXGE

Transformer neutral point, not the difference between grounding

Transformer components include body (core, windings, insulation, leads), transformer oil, tanks and cooling devices, pressure regulators, protection devices (hygroscopicifiers, airbags, gas relays, oil conservators, and temperature measuring devices, etc.) ) and outlet bushings.


1. Advantages and Disadvantages of Transformer Neutral Grounding System

(1) Advantages: For the neutral point grounding system of the power supply, if a single-phase grounding occurs, the voltage of the other two phases does not rise, which can reduce the insulation level of the entire system; in addition, the single-phase grounding will generate a large short-circuit current Is This allows the protective device (relay, fuse, etc.) to operate quickly and accurately, improving the reliability of the protection.

(2) Disadvantages: As for the neutral point grounding system of the power supply, since the single-phase short-circuit current Is is large, the switch and the electrical equipment must be selected to have a larger capacity, and can also cause system instability and interference with the communication line; The advantages and disadvantages of ungrounded systems:

(1) Advantages: For ungrounded systems with neutral transformers, due to the limitation of single-phase grounding current, the interference to communications is small; in addition, single-phase grounding can be run for a period of time, which improves the reliability of power supply.

(2) Disadvantages: When the transformer neutral point is not grounded, when the phase is grounded, the voltages of the other two grounds are increased by a factor of 2, which easily causes breakdown in the weak insulation place, thus causing a short-circuit in the two phases.


2. Grounding methods for various voltage grade power supply lines:

(1) Neutral grounding systems are generally used in high-voltage or ultra-high voltage systems of 110 kV and above. The purpose is to reduce the insulation level of electrical equipment and to avoid the asymmetry caused by continuous operation after single-phase grounding.

(2) The factory power supply system uses a voltage of 1kv - 35kv, generally neutral point ungrounded system, because the factory power supply distance is short, the capacitance to the ground is small (Xc is large), single-phase ground current is small, this can run for a period of time, Improve the stability of the system and power supply reliability, the advantages of small communications interference. In coal mines, China, West Germany and other countries prohibit the neutral grounding. The main purpose is to reduce the single-phase grounding current for safety, but even a small single-phase grounding current is not allowed in coal mines, so it is in the coal mines. Downhole, a leak detection relay is installed, which means that when the grid-to-ground insulation resistance is reduced to a dangerous value or a person touches a phase conductor or a phase of the power grid is grounded, the power can be cut off quickly to prevent electric shock or electric leakage accidents. .

(3) For power supply systems (380/220 V) below 1kv, except for some special cases (downhole, swimming pools), most of them are neutral point grounding systems, which are mainly for the purpose of preventing insulation damage and subject to the risk of electric shock.

Related content: Causes of transformer losses

Transformer loss is a concept in the field of modern physics and refers to the sum of no-load loss P and short-circuit loss Pk. No-load loss P refers to the active power absorbed by the transformer when the rated voltage is applied to one winding of the transformer and the rest of the windings are open-circuit. Short-circuit loss Pk, for a two-winding transformer, the active power absorbed by the transformer when it is passed through one winding of the transformer at its rated current and the other winding is shorted is called the short-circuit loss of the transformer; for multiple-winding transformers, the short-circuit loss is specified A pair of windings prevail. The loss of transformers can be attributed to two major reasons:


First, a large amount of copper wire is needed to wind the transformer. The copper wire has a resistance. When the current flows, the resistance will consume a certain amount of power. This part of the loss is often consumed as heat, and this loss is called “copper loss”.

Second, when the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the core because the core itself is also a conductor. In the plane perpendicular to the magnetic force line, the potential is induced. This potential is formed on the cross section of the core. Closes the circuit and produces a current called "vortex." This "eddy current" increases the loss of the transformer, causing the core of the transformer to heat up and the temperature rise of the transformer to increase. The loss caused by "eddy currents" is what we call "iron loss."

Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Because the transformer has iron losses and copper losses, its output power is always smaller than the input power, and the efficiency η represents the relationship between the output power and the input power: η = output power / input power.

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