Power Transformers: Short Circuit Risks, Causes, and Improvement Measures

Power Transformers: Short Circuit Risks, Causes, and Improvement Measures

Power transformers are fundamental components in power systems that provide energy transmission and are crucial induction devices ensuring safe power operation. Their structure consists of primary coils, secondary coils, and an iron core, utilizing the principle of electromagnetic induction to alter AC voltage. Through long-term technological improvements, the reliability and stability of power supply have continuously enhanced. However, various prominent hidden dangers still exist. Some transformer units suffer from insufficient anti-short-circuit impact capability, making them prone to short circuit phenomena. To effectively determine fault causes and locations, research on transformer failures and diagnostic technologies must be intensified to adopt corresponding technologies that efficiently solve transformer fault diagnosis problems.

1.Hazards of Power Transformer Short Circuits

• Impact of Surge Current: A sudden short circuit in a transformer generates a large short-circuit current. Although its duration is brief, before the main circuit of the transformer is disconnected, this hidden danger may have already formed, potentially causing internal damage to the transformer and reduced insulation levels.

• Impact of Electromagnetic Forces: During a short circuit, overcurrent generates significant electromagnetic forces that affect stability. In severe cases, transformer windings can be affected to a certain extent, such as winding deformation, damage to winding insulation strength, and damage to other components. In extreme cases, this may lead to power safety accidents such as transformer combustion.

2.Causes of Power Transformer Short Circuits

(1) Current calculation programs are developed based on idealized models that assume uniform leakage magnetic field distribution, identical turn diameters, and in-phase forces. However, in reality, the leakage magnetic field in transformers is not uniformly distributed and is relatively concentrated in the yoke section, where electromagnetic wires experience greater mechanical forces. At transposition points of continuously transposed cables (CTC), the climbing slope changes the direction of force transmission, generating torque. Due to the elastic modulus factor of spacer blocks, uneven axial distribution of spacer blocks can cause alternating forces produced by alternating leakage magnetic fields to experience delayed resonance. This is the fundamental reason why winding discs at the iron core yoke section, transposition points, and corresponding positions with tap changers deform first.

(2) Using conventional transposed conductors with poor mechanical strength makes them prone to deformation, strand separation, and exposed copper when subjected to short-circuit mechanical forces. When using conventional transposed conductors, large currents and steep transposition climbs at these positions generate significant torque. Additionally, winding discs at both ends of windings experience considerable torque due to the combined effects of radial and axial leakage magnetic fields, leading to twisting deformation. 

For example, the phase A common winding of the 500kV Yanggao transformer had 71 transpositions, and due to using relatively thick conventional transposed conductors, 66 of these transpositions showed varying degrees of deformation. Similarly, the WuJing No. 11 main transformer also exhibited different degrees of wire flipping and exposure at the high-voltage winding ends in the iron core yoke section due to using conventional transposed conductors.

(3) Short-circuit resistance calculations fail to consider the impact of temperature on the bending and tensile strength of electromagnetic wires. Short-circuit resistance designed at room temperature cannot reflect actual operating conditions. According to test results, the temperature of electromagnetic wires significantly affects their yield limit (σ0.2). As the temperature of electromagnetic wires increases, their bending strength, tensile strength, and elongation all decrease. At 250°C, bending and tensile strength are considerably lower than at 50°C, while elongation decreases by more than 40%. In actual operation, transformers reach an average winding temperature of 105°C at rated load, with hot spot temperatures reaching 118°C. Most transformers undergo automatic reclosing processes during operation.

Therefore, if a short-circuit point doesn't immediately disappear, the transformer will experience a second short-circuit impact within a very short time (0.8 seconds). However, after the first short-circuit current impact, the winding temperature rises sharply. According to GB1094 standards, the maximum allowable temperature is 250°C, at which point the winding's short-circuit resistance has significantly decreased. This explains why most transformer short-circuit accidents occur after reclosing operations.

(4) Loose winding construction, improper transposition processing, and excessive thinness cause electromagnetic wires to become suspended. From the perspective of damage locations in accidents, deformation is most commonly found at transposition points, especially at transposition locations of transposed conductors.

(5) The use of soft conductors is one of the main reasons for poor short-circuit resistance in transformers. Due to insufficient early understanding of this issue or difficulties with winding equipment and processes, manufacturers were reluctant to use semi-rigid conductors or had no such requirements in their designs. Transformers that have failed all used soft conductors.

(6) Excessive assembly gaps result in insufficient support on electromagnetic wires, creating hidden dangers for transformer short-circuit resistance.

(7) Uneven pre-tightening forces applied to various windings or tap positions cause winding discs to jump during short-circuit impacts, resulting in excessive bending stress on electromagnetic wires and subsequent deformation.

(8) Lack of curing treatment between winding turns or wires leads to poor short-circuit resistance. Early windings treated with varnish immersion suffered no damage.

(9) Improper control of winding pre-tightening force causes misalignment of conductors in conventional transposed conductors.

(10) Frequent external short-circuit incidents cause cumulative effects of electromagnetic forces after multiple short-circuit current impacts, leading to softening of electromagnetic wires or internal relative displacement, ultimately resulting in insulation breakdown.

3.Improvement Measures to Enhance Power Transformer Short-Circuit Resistance

(1) Conduct Short-Circuit Testing to Prevent Problems Before They Occur

 The operational reliability of large transformers primarily depends on their structure and manufacturing process quality, followed by various tests conducted during operation to timely grasp equipment conditions. To understand a transformer's mechanical stability, short-circuit testing can be performed to identify weak points for improvement, ensuring confidence in the structural strength design of transformers.

(2) Standardize Design and Emphasize Axial Compression Process in Coil Manufacturing

When designing transformers, manufacturers should consider not only reducing losses and improving insulation levels but also enhancing mechanical strength and short-circuit fault resistance. In terms of manufacturing processes, since many transformers use insulated press plates with high and low-voltage coils sharing a single press plate, this structure requires high manufacturing process standards. Spacer blocks should undergo densification treatment, and after coil processing, individual coils should undergo constant-pressure drying with measurement of compressed coil height.

After the above processing, coils on the same press plate should be adjusted to the same height. During final assembly, specified pressure should be applied to coils using hydraulic devices to achieve the designed and process-required height. During final assembly, attention should be paid not only to the compression of high-voltage coils but also particularly to controlling the compression of low-voltage coils.