Abnormal Phenomena of Transformer Core Grounding

(1) Eddy currents are generated in the core, increasing iron losses and causing local overheating of the core.

(2) In severe cases of multi-point grounding, if left unaddressed for a prolonged period during continuous operation, it will cause the oil and windings to overheat, gradually aging the oil-paper insulation. This can cause the insulation layer between core laminations to age and fall off, leading to even greater core overheating and potentially burning out the core.

(3) Prolonged multi-point grounding degrades the oil in oil-immersed transformers and generates combustible gases, which triggers the gas relay (Buchholz relay).

(4) Core overheating causes the carbonization of wooden blocks and clamps within the active part.

(5) Severe multi-point grounding can burn out the grounding lead, causing the transformer to lose its normal single-point grounding, leading to disastrous consequences.

(6) Multi-point grounding can also cause discharge phenomena.

3. Reasons why the core requires a single-point ground during normal operation:

During normal transformer operation, an electric field exists between the energized windings and the oil tank, and the core and other metal components are situated within this field. Due to uneven capacitance distribution and varying field strengths, if the core is not reliably grounded, charging and discharging phenomena will occur, damaging the insulation strength of both the solid insulation and the oil. Therefore, the core must have a reliable single-point ground.

The core is composed of silicon steel laminations. To reduce eddy currents, there is a certain insulation resistance between the laminations (typically only a few ohms to tens of ohms). Since the inter-lamination capacitance is extremely large, it can be considered a conductive path in an alternating electric field. Therefore, a single-point ground is sufficient to clamp the potential of the entire stack of core laminations to the ground potential.

If the core or its metal components have two or more (multi-point) grounds, a closed loop will be formed between the grounding points. This loop links with part of the magnetic flux, inducing an electromotive force (EMF) and forming a circulating current, which causes local overheating and can even burn out the core.

A transformer core having only a single-point ground is the only acceptable normal grounding method. That is, the core must be grounded, and it must be a single-point ground.

Core faults are mainly caused by two aspects: poor construction workmanship leading to short circuits, and multi-point grounding caused by accessories and external factors.