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Reasons for asymmetric transformer operation

Asymmetric transformer caused by the operation, on the larger side there are three types of reasons:
1, As the three-phase load is not the same, resulting in asymmetric operation
For example, a small transformer power supply lighting, welding and other loads, or some transformer-powered electric locomotives and steelmaking furnace with single-phase nature of the load. This load asymmetry makes the three-phase current flowing through the transformer asymmetric, asymmetrical three-phase current asymmetry caused by three-phase current impedance asymmetry, resulting in secondary side three-phase voltage asymmetry. Voltage asymmetry is detrimental to the operation of three-phase induction motors and lighting equipment. However, in general, the voltage asymmetry caused by the impedance of the transformer itself is not large. This asymmetric operation, the main thing to consider is that in operation, according to Y / Y0-12 wiring of the transformer neutral current does not exceed the low-voltage coil rated current of 25%.
2, composed of three single-phase transformers composed of three-phase transformer
When one phase in a three-phase transformer group is damaged and replaced by one phase with different parameters (eg with different impedance voltages or different transformation ratios), current and voltage asymmetries can result.
This transformer group, in operation, not only the current asymmetry in this group, but also make it connected to the neutral grounding transformer will generate ground current, and its own available capacity is less than three single-phase transformer capacity Sum. The available capacity and the degree of asymmetry depend on how well the transformer parameters fit together.
3, Asymmetric wiring caused by the asymmetric operation of the transformer
At this time can be divided into the following situations:
(1) Two lines and one system:
Transformer is still three-phase wiring, there is a phase in the circuit to act as a wire called two-wire system, the rural power grid in China is still a small part of this wiring method.
With this wiring method, the transformer capacity can not be reduced. Just because the resistance of the circuit and the earth is not the same, so that the voltage asymmetry. However, when the voltage loss in the circuit is less than 10%, the voltage asymmetry does not exceed 1% ~ 2%, which is allowed. If you exceed this range, as long as the line power to reduce the transmission, you can improve some.
There are several issues with this asymmetric operation:
If one of the wires in the ungrounded phase is short-circuited to ground, the grounding device will have dangerous contact voltage and step voltage due to the short-circuit current, therefore, wear insulated shoes and insulated gloves during the accident handling.
adjacent to the communication lines through the induction of dangerous voltage and interference.
(2) two-phase transformer operation:
In some cases, the need to transformer two-phase operation also caused the transformer asymmetric operation. Such as: neutral grounding system, when a phase line failure, the zero line to replace the temporary operation of the phase; three-phase transformer in a transformer transient fault to the two-phase transformer temporarily; three-phase transformer one-phase coil fault temporarily With two-phase operation.
The common feature of this asymmetric operation is that the capacity of the transformer is reduced, that is, the available capacity is less than the sum of the rated capacities of the two-phase transformers that are still operating. The size of the available capacity is related to the degree of asymmetry of the current.
Cause two-phase operation of the transformer there are reasons: such as one side of the transformer circuit breaker disconnect one phase and the transformer tap poor contact.
Transformer asymmetric operation, the transformer itself, the harm is not too large, so do not need to generator as the size of three-phase asymmetric current restrictions (Y / Y0-12 wiring transformers except), the main consideration is Due to the asymmetry of the current and voltage on the user's impact, as well as interference along the line of communication lines and relay protection of power system conditions.