Transformer short-circuit fault causes and hazards

Power transformers will affect the safe operation of transformers. Through the statistics of the operation and accidents of power transformers throughout the country over the years of instrumentation World Net, it has been found that the number of failures caused by external short circuits increases year by year.

1, the cause of transformer short-circuit fault

The lack of short-circuit resistance of the transformer itself is the main cause of transformer short-circuit damage. In addition, improper management of operations is also a factor that cannot be ignored. Transformer short-circuit resistance is not enough, mainly in a part of the transformer under the impact of the short-circuit current under the intensity of the specified and the protection of the rapid movement, still occurs winding deformation, and even insulation breakdown. For example, a SFL6-31500/110 transformer at the Kaifeng Binhe station in Henan Province caused three-phase short-circuit explosion of the bus cable on the low-voltage side. This caused the main transformer to trip by the protective action of heavy gas, causing the A phase of the low-voltage side of the transformer to break and the iron core to shift. However, the short-circuit current is only 4.5 times the rated current, and the protection operation time is 0.08s.

Transformer dynamic stability is poor, can not withstand a variety of short circuits, the reasons for the following are roughly the following: (1) In the transformer structure design, the role of the electromotive force acting on the transformer winding, can not be accurately reflected only by the static calculation Transformer sudden burst of short-circuit current withstand various stresses. (2) The selection of the wire is not proper, and there is a phenomenon that the thermal performance is neglected and the mechanical stress is neglected. (3) The axial compression force of the winding is not enough. (4) The fixed fulcrum of the wire is not enough, the bracket is not firm, and the wire is badly soldered. (5) There is not enough support between the inner winding and the core column. (6) The process conditions in the manufacturing process are not standardized and the quality control is lax.

2. Secondary protection after transformer short-circuit fault
When a short-circuit fault occurs in the transformer, if the fault occurs on the external line, the differential protection action in the main protection trips the transformer and then exits the grid.

3, the transformer short-circuit fault manifestations and hazards

3.1 The main form of transformer winding short circuit fault

Under the action of the transformer short-circuit electromagnetic force, winding damage and lead displacement are the most likely consequences of transformer short-circuit faults. Both the experimental research and the operation practice show that the damage modes under the action of the short-circuit electromagnetic force of the transformer winding mainly have the following forms:

(1) Deformation of windings causes cracking of the crucible insulation and causes a short circuit in the turn. This type of failure is the main form of a small-sized distribution transformer with a small diameter, which is damaged by a stretched electromagnetic force in a short circuit accident.

(2) Winding deformation leads to a decrease in the strength of the main insulation which causes a breakdown of the main insulation. The deformation of the winding leads to a decrease in the strength of the main insulation, which results in a breakdown of the main insulation. This is a medium-sized power transformer winding subjected to a stretched short-circuit force. The main damage mode. Therefore, when designing the short circuit strength of the transformer winding, the cumulative effect of the residual deformation of the winding under the action of the short-circuit electromagnetic force must be considered.

(3) Amplitude instability of the windings, due to windings placed inside the transformer, or windings of intermediate windings of a multi-winding transformer, under short-circuit conditions, may be subjected to circumferential compression caused by short-circuit electromagnetic forces. The instability of the inner winding of the transformer in the medium-sized and above is the main form of damage of the transformer under the action of the electromagnetic force in the short-circuit direction.

(4) Axial instability of the winding, axial instability of the winding is the main form of winding damage caused by short-circuit axial electric force and short-circuit amplitude to electric force.

(5) The overall displacement and inclination of the three-phase transformer windings, and the overall displacement and tilt of the transformer windings occur in the case of three-phase symmetrical short-circuit faults.