When a power transformer is working normally, the transformer core and clamps are usually grounded at one point. If there are two or more grounding points on the iron core or clamping components, a closed circuit is formed between the two points. Under the action of the transformer leakage magnetic field, a circulating current is generated between the two points, causing local overheating of the transformer. When the circulating current is too large, it increases the iron core loss, and in severe cases, it can cause iron core burnout, resulting in non-stop accidents of the transformer. For the grounding current of transformers in operation, DL/T 596-1996 "Preventive Test Code for Power Equipment" stipulates that the grounding current of transformer iron cores during operation is generally not greater than 100mA.

1. External factors and their hazards

1) The insulation of the down conductor is damaged

The insulation of the top bushing of the transformer iron core and clamp lead out line is damaged, and the insulation of the grounding lead of the iron core and clamp is also damaged. As a result, the grounding of the lead down line overlaps with the transformer box, forming an external closed circuit between the overlap point and the grounding point of the lead down line. The closed circuit induces current in the leakage magnetic field of the transformer, causing abnormal current in the iron core and clamp lead down line.

2) Metal foreign object overlapping downline

During the overhaul of the transformer, metal objects such as iron wires were accidentally dropped from the box and overlapped with the iron core down lead and the transformer shell to form an external closed circuit. The closed circuit induced current in the leakage magnetic field of the transformer, resulting in abnormal current in the iron core and clamp down lead.

2. External fault cases

1) Case 1: Insulation damage of down conductor

The iron core and clamp lead wires of the transformer are led out from the top of the transformer through the top bushing. The top bushing is damaged due to external forces and other reasons, resulting in a decrease in insulation capacity to zero, causing the iron core lead wires to be connected to the shell, forming a temporary grounding point, as shown in Figure 4. The down conductor of the transformer iron core is wrapped by an insulation sheath to achieve insulation with the transformer shell. Due to external force damage, natural weathering, and other reasons, the insulation sheath is damaged. After the damage, the copper bar of the down conductor of the iron core overlaps with the transformer shell, forming a temporary grounding point, as shown in Figure 5.

Both of the above situations can lead to the formation of a closed circuit between the temporary grounding point and the grounding point under the lead wire of the transformer's iron core and clamp grounding wire. The closed circuit induces current in the leakage magnetic field of the transformer, resulting in abnormal current in the iron core and clamp grounding wire, exceeding the 100mA standard and requiring evaluation.

The necessity of one point grounding of iron core and the hazards of multi-point grounding

During normal operation of a transformer, there is an alternating magnetic field around the live winding. Due to electromagnetic induction and parasitic capacitance, the live winding generates a floating potential in the iron core and its metal components through the coupling effect of parasitic capacitance. If the distance between the iron core and the surrounding metal components is not equal, the potential of the iron core and the surrounding conductor will be different. When the potential difference between the two points reaches the point where the insulation can be broken down, continuous partial discharge will occur. During the continuous discharge process, transformer oil will gradually deteriorate and its insulation performance will decrease. If it continues to develop, it will evolve into iron core overheating, burning out the iron core and causing accidents.

To solve this discharge phenomenon, the iron core and its metal components must be reliably grounded. The parasitic capacitance between the iron core and its metal components and the ground is short circuited, keeping the iron core and its metal components always at ground potential. In theory, the grounding current should be zero, but in practical situations, due to reasons such as the phase symmetry of the three-phase voltage and the different capacitance of each phase, there may be a certain amount of current in the grounding down conductor, usually around ten milliamps.

When the iron core is grounded at multiple points, the uneven potential between the iron cores will form a circulating current between the grounding points, causing local overheating of the iron core. In severe cases, the gas generated by overheating will cause light gas protection to act, and in severe cases, it will cause heavy gas protection to act, leading to tripping accidents. The formation of short circuit faults between iron core pieces due to local iron core melting increases transformer losses and seriously affects the performance and normal operation of the transformer.

3. Causes of multi-point grounding faults in transformer iron cores

1) The iron core is connected to the outer shell. Loose displacement of the iron core and metal components left at the lower part of the iron core (such as screw caps, metal powder and wires from the grinding of submersible pump bearings) can all cause the iron core to be connected to the shell, resulting in multi-point grounding faults of the iron core and abnormal grounding current of the iron core.

2) The iron core is connected to the clamp. The warping of the iron core, metal foreign objects, impurities, and other reasons can all cause direct or indirect short circuits between the iron core and the clamp, resulting in multi-point grounding faults of the iron core and abnormal grounding current of the iron core.

4. Identification method for abnormal grounding current of iron core

1) Grounding current testing method

The transformer iron core is grounded at a single point through a grounding lead, with a grounding current of milliampere level. When the iron core is grounded at multiple points, the current flowing through the grounding lead wire increases sharply. The magnitude of this current depends on the relative position between the fault point and the normal grounding point, that is, the amount of magnetic flux contained in the short-circuit turn. The maximum current can reach several hundred amperes. The common method is to use a specialized clamp ammeter to measure the current value on the grounding lead of the transformer iron core, and determine whether there is a multi-point grounding problem in the transformer.

2) Chromatographic analysis method

When the iron core of a transformer has multiple grounding points, the transformer will exhibit high or medium temperature overheating phenomena. Gas chromatography analysis of characteristic gas content in transformer oil shows that the total hydrocarbon content exceeds the attention value (150 µ L/L), with ethylene (C2H4) and methane (CH4) accounting for a large proportion, and acetylene content being low or unchanged. The chromatographic three ratio is usually "022" or "021".

3) Insulation resistance testing method

Measuring insulation resistance is a simple and feasible method for detecting whether there are multiple grounding points in the iron core. During the detection, the grounding lead wire needs to be disconnected first, and the insulation resistance of the iron core and clamp to ground should be measured separately to determine whether there are multiple grounding faults in the iron core and clamp. Measure the insulation resistance of the iron core to the clamp to determine if there is overlap between the two. This method cannot detect short circuits between transformer core pieces.

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