Hazards of power transformer short circuits, cause analysis and improvement measures

Power transformers are fundamental components in power systems, providing energy consumption and serving as crucial inductive devices ensuring safe power operation. Their structure comprises a primary coil, a secondary coil, and an iron core, utilizing the principle of electromagnetic induction to alter AC voltage. While long-term technological improvements have enhanced the reliability and stability of power supply, various prominent hidden dangers remain. Some transformer units, due to insufficient short-circuit withstand capacity, are prone to short circuits. To effectively diagnose fault causes and locations, research into transformer faults and diagnostic techniques must be intensified, enabling the adoption of corresponding technologies to efficiently resolve transformer fault diagnosis.

I. Hazards of Power Transformer Short Circuits

1. Impact of Surge Current: A sudden short circuit in a transformer generates a large short-circuit current. Although the duration is short, the risk may already have formed before the transformer's main circuit is cut off, potentially leading to internal damage to the transformer and reduced insulation level.

2. Impact of Electromagnetic Force: During a short circuit, excessive current generates significant electromagnetic force, affecting stability and potentially damaging the transformer's windings (e.g., deformation, insulation breakdown). Other components may also be damaged, potentially leading to power transformer fires and other power safety accidents.

II. Causes of Power Transformer Short Circuits

1. Current calculation programs are based on idealized models assuming uniform leakage field distribution, identical coil diameters, and equal-phase forces. In reality, the transformer's leakage field is not uniformly distributed, being relatively concentrated in the yoke area, where the electromagnetic wires experience greater mechanical force. Transposition wires, due to ramping at transposition points, alter the force transmission direction, generating torque. Unevenly spaced axial shims, due to the shim's elastic modulus factor, cause the alternating force generated by the alternating leakage field to exhibit delayed resonance. This is the fundamental reason why coils in the core yoke, transposition points, and tap-changing areas deform first.

2. The use of ordinary transposition wires with poor mechanical strength leads to deformation, stray strands, and exposed copper when subjected to short-circuit forces. With ordinary transposition wires, the high current and steep transposition ramps generate significant torque at this point. Simultaneously, coils at both ends of the winding, due to the combined action of radial and axial leakage fields, also experience significant torque, causing torsional deformation. For example, the A-phase common winding of the Yanggao 500kV transformer has 71 transposition points, and 66 of these showed varying degrees of deformation due to the use of thicker ordinary transposition wires. Similarly, the Wujing No.11 main transformer showed different degrees of wire reversal and exposure at the ends of the high-voltage winding in the core yoke area because of the use of ordinary transposition wires.

3. Short-circuit capacity calculations do not consider the impact of temperature on the bending and tensile strength of the electromagnetic wire. The short-circuit capacity designed at room temperature does not reflect the actual operating conditions. Test results show that temperature significantly affects the yield strength (0.2%) of the electromagnetic wire. As the temperature increases, the bending and tensile strength and elongation decrease. At 250℃, the bending and tensile strength is lower than at 50℃, and the elongation decreases by over 40%. In actual operation, the average winding temperature of a transformer under rated load can reach 105℃, and the hottest point can reach 118℃. Transformers generally undergo reclosing procedures during operation. Therefore, if the short circuit point cannot be cleared immediately, a second short-circuit impact will follow in a very short time (0.8s). However, due to the rapid temperature increase of the windings after the first short-circuit current impact (GB1094 specifies a maximum of 250℃), the short-circuit withstand capacity significantly decreases, explaining why short circuits frequently occur after transformer reclosing.

4. Loose winding, improper transposition, excessive thinness, causing electromagnetic wire suspension. Deformation is frequently observed at transposition points, especially at the transposition points of transposition wires.

5. The use of soft wires is also a major cause of poor short-circuit capacity. Due to a lack of early understanding or difficulties in winding equipment and processes, manufacturers were reluctant to use semi-hard wires or did not have such requirements during design. Transformers that experienced failures all used soft wires.

6. Excessive spacing between coil layers leads to insufficient support for the electromagnetic wire, increasing the risk of poor short-circuit capacity.

7. Uneven pre-tightening force on each winding or each layer causes coil jumping during short-circuit impact, resulting in excessive bending stress on the electromagnetic wire and deformation.

8. Poor short-circuit capacity due to lack of curing between windings or wires. Early windings treated with varnish showed no damage.

9. Improper pre-tightening force of windings causes misalignment of ordinary transposition wires.

10. Frequent external short-circuit accidents and accumulated effects of multiple short-circuit current impacts lead to electromagnetic wire softening or internal relative displacement, eventually causing insulation breakdown.

III. Improvement Measures to Enhance Power Transformer Short-Circuit Withstand Capacity

1. Conduct short-circuit tests on transformers as a preventive measure.

The operational reliability of large transformers depends primarily on their structural design and manufacturing processes and, secondly, on various tests conducted during operation to monitor the equipment's condition. Short-circuit tests can assess the mechanical stability of the transformer, allowing for improvements in weak areas and ensuring the structural strength design is well-informed.

2. Standardized design and emphasis on axial compression processes in coil manufacturing.

Manufacturers, during design, must consider not only reducing transformer losses and improving insulation levels but also enhancing the mechanical strength and short-circuit resistance. In manufacturing, many transformers utilize insulation plates, with high- and low-voltage coils sharing a single plate. This structure requires high manufacturing precision, requiring densification of shims and constant-pressure drying of individual coils after processing, measuring the coil height after compression.

After the above processes, coils on the same plate are adjusted to the same height, and a hydraulic device is used during assembly to apply the specified pressure to the coils, ultimately achieving the design and process requirements. During assembly, in addition to paying attention to the compression of high-voltage coils, special attention must be paid to controlling the compression of low-voltage coils.