Cause analysis and preventive measures of burst of

2022-08-08
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Cause analysis and preventive measures of voltage transformer burst

in the 6 ~ 35kV neutral point ineffective grounding system, due to the magnetic circuit saturation effect of the core inductance of transformers, voltage transformers, arc suppression coils and other equipment, continuous ferromagnetic resonance overvoltage with high amplitude is excited. Ferromagnetic resonance can be fundamental resonance, high-order harmonic resonance and sub harmonic resonance. Although the amplitude of the overvoltage generated by this resonance is not high, because the overvoltage frequency is often far lower than the rated frequency, and the iron core is in a highly saturated state, its manifestation may be that the phase to ground voltage rises, the excitation current is too large, or swing at low frequency, causing insulation flashover, lightning arrester burst, high zero sequence voltage hydraulic servo UTM experimental machine can carry out dynamic test and fatigue test components False grounding phenomenon and incorrect grounding indication. In serious cases, it may also induce protection maloperation or over-current in the voltage transformer, causing Pt burnout

1 fault phenomenon and related data

the 6kV system has eight sections. The electrical combination cabinet produced by Shanghai Huatong Switchgear Factory is used. Since the equipment of this factory was put into operation, there has been no major defects in the main components, but its auxiliary measurement Pt has been damaged for eight times. The phenomenon is manifested in the body burst and internal insulation material spray failure, resulting in the relevant protection of the 6kV system can not be put into operation, and some automatic functions cannot be realized. This brings great hidden dangers to the safe and stable operation of the plant system

2 preliminary study on the cause of the fault

according to the fault phenomenon, after preliminary judgment, it is estimated that it is caused by the following reasons

1) poor product quality: if the insulation of the product itself, core lamination and winding process are not qualified, it may lead to excessive heating of the voltage transformer and long-term operation of the insulation at high temperature, resulting in accelerated aging of the insulation and breakdown. The primary winding of this type of voltage transformer has an inter turn short circuit, so the current will increase rapidly, and the ferromagnetism will also saturate rapidly, resulting in resonant overvoltage, insulation breakdown, and high-voltage fuse fusing

2) the secondary load of the voltage transformer is heavy, and the primary and secondary currents are large, which makes the sum of the load currents on the secondary side exceed the rated value, resulting in increased heating of the internal winding of Pt, especially when the voltage is higher than the rated voltage (6kV) of Pt, the internal heating of Pt is more serious; Moreover, the system belongs to the neutral point non effective grounding system, so the primary side voltage is easy to deflect during operation. When a phase has high voltage, the phase PT is more prone to thermal plastic mechanical adjustment structure to promote the expansion and explosion of the development space of the extruder industry

No. voltage transformer model phenomenon remarks

601vki7.2c phase burst, causing inter turn short circuit. After being replaced with jdzx voltage transformer, less than two days of operation, phase B burst again, causing inter turn short circuit

603vki7.2a phase burst, causing inter turn short circuit

604vki7.2a phase burst, causing inter turn short circuit

606vki to extend service life 7.2A, C phase burst, Cause inter turn short circuit

607vki7.2a, c-phase burst, cause inter turn short circuit

main technical parameters: transformation ratio 600/root number 3/100, root number 3/100/3, rated capacity 90/100va, Shanghai transformer factory

note: 1) vki7.2 voltage transformer is imported, and the corresponding domestic product model is jdzx; There is a group (3 sets) of mutual inductors with a transformation ratio of 6000/3/100/3/100/3 in 6kV section I to VIII. 2) the process is resin casting semi insulation, and the primary neutral point withstand 3KV (factory value)

3) the voltage is caused by ferromagnetic resonance, resulting in error. The transformer is broken down, because: the load of the busbar where the broken down voltage transformer is located is more inductive, especially in Section III and IV, with large capacity deep well pump, its inductive reactance is greater than the capacitive reactance in load distribution. For some reason, the system voltage fluctuates (such as frequent startup and shutdown of deep well pump, etc.), causing sudden changes in current and voltage in the circuit, It may lead to the rapid saturation of the core of the voltage transformer and the reduction of the inductive reactance. When the inductive reactance is less than the capacitive reactance, ferromagnetic resonance will occur, resulting in the excitation current of the voltage transformer increasing dozens of times, and the overvoltage amplitude will reach nearly 2.5ue, or even more than 3.5ue, and the duration is long. The voltage transformer operates under such a large voltage and current, which will rapidly increase its temperature and cause damage. According to the above analysis, the QC team was organized to analyze the causes and sent out for investigation. The investigation results showed that: (1) there should be no product quality problems, because the transformer is widely used in East China. From the collected data, the performance of the products of the factory is good. Although there have been some problems in some places, there is no such a large amount of damage as Ertan Power Plant. (2) The reason why it is suspected that PT damage is caused by heavy secondary load of voltage transformer is not tenable, because the rated capacity of pt0.5 secondary winding is 90va. On November 16, 1998, 604pt secondary side star connection load was measured, 100V three-phase AC power supply was applied to the secondary connection circuit (stop standby automatic switching), ia=0.61a, ib=1.05a, ic=0.605a, and the star connection load capacity was measured: sa=35.2va, sb=60.6va, sc=35va, The total output capacity of Pt is 105.6va; The next day, measurement

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