Oil-immersed transformer core poor grounding (mostly multi-point grounding) poses risks of core overheating, insulating oil decomposition, and equipment damage. CHH Power adopts standardized detection methods and targeted handling measures—divided into non-stop temporary solutions and shutdown thorough repairs—to ensure safe and stable transformer operation. Below is a detailed breakdown of core detection techniques and operational protocols.
1. Core Detection Methods for Core Poor Grounding
CHH Power’s maintenance team uses two complementary methods to accurately diagnose core multi-point grounding, covering both gas analysis and electrical measurement:
(1) Chromatographic Analysis of Insulating Oil
By testing dissolved gas in transformer oil, hidden core overheating caused by multi-point grounding can be identified:
- Key Gas Indexes: Focus on methane (CH₄) and ethylene (C₂H₄) content. If these two gases are significantly higher (exceeding GB/T 7252 standard thresholds) while carbon monoxide (CO) and carbon dioxide (CO₂) content remain stable, it indicates local overheating of the core (caused by eddy current in short-circuited silicon steel sheets), suggesting a high probability of multi-point grounding.
- CHH Power’s Operation: Use a gas chromatograph to analyze oil samples, comparing results with historical data to judge fault severity (e.g., sharp increase in ethylene indicates severe overheating).
(2) Core Ground Wire Current Measurement
This is a direct and rapid on-site detection method, suitable for real-time fault judgment:
- Equipment & Operation: Use a clamp ammeter to measure current on the core grounding tube’s ground wire (CHH Power recommends using a precision clamp ammeter with milliamp-level resolution).
- Judgment Standards:
- Normal Grounding: No closed current loop exists; the ground wire current is at the milliamp level (≤0.3A).
- Multi-Point Grounding: The core forms a short-circuit coil around the magnetic flux, generating annular circulation current. The current value depends on the fault point’s position relative to the normal ground point (i.e., the magnetic flux enclosed by the short-circuit coil), usually reaching tens of amperes—directly confirming multi-point grounding.
2. Temporary Handling Solutions (Without Stopping the Transformer)
For transformers that cannot be shut down immediately (e.g., critical power supply loads), CHH Power implements temporary measures to limit fault expansion, while strengthening monitoring:
(1) Temporarily Disconnect the Ground Wire (For Large Fault Current)
- Operation: If the measured ground wire current is large (e.g., >10A), temporarily disconnect the core ground wire.
- Safety Note: Disconnection may cause floating potential on the core, so real-time monitoring must be strengthened (e.g., continuous chromatographic analysis of oil gas, core insulation resistance measurement) to prevent arc discharge or insulation breakdown.
(2) Series Sliding Resistor (For Unstable Multi-Point Grounding)
- Purpose: Limit the ground current to ≤1A, avoiding excessive eddy current and overheating.
- Resistor Selection: Calculate resistance value using the formula: R = U/I (U = measured voltage of the normal working ground wire; I = target current ≤1A).
- Operation: Connect the sliding resistor in series with the core working ground wire, adjust the resistance to stabilize the current within the safe range.
(3) Move the Normal Grounding Plate
- Application: When the fault point is confirmed but cannot be handled immediately.
- Operation: Move the core’s normal grounding plate to a position close to the fault point. This reduces the magnetic flux enclosed by the short-circuit loop, significantly lowering the circulation current and mitigating core overheating.
(4) Continuous Gas Monitoring
- Use on-line oil chromatographic monitoring equipment to track the content of methane and ethylene in real time. If gas generation accelerates, immediately arrange for shutdown to avoid irreversible damage.
3. Thorough Fault Repair (After Stopping the Transformer)
Once the transformer can be shut down, CHH Power implements thorough repairs to eliminate multi-point grounding completely, including fault point location and targeted treatment:
(1) Accurate Fault Point Location Methods
If the fault point is not visible, use the following on-site techniques to locate it:
- DC Method:
- Disconnect the connection between the core and the clamp (grounding terminal).
- Inject 6V DC voltage into the silicon steel sheets on both sides of the iron yoke.
- Use a DC voltmeter to measure the voltage between adjacent silicon steel sheets in sequence. A voltage of 0 or reversed meter indication indicates a faulty connection point (short-circuited sheets).
- AC Method:
- Apply 220–380V AC voltage to the transformer’s low-voltage winding, generating magnetic flux in the core.
- Disconnect the core-clamp connection, and use a milliampere meter to measure the current between silicon steel sheets. A current of 0 indicates the fault point (no current flow through the short-circuited loop).
(2) Fault Treatment Measures
- Visible Fault Points: If the fault is caused by foreign objects (e.g., metal shavings) or damaged insulation pads, remove the foreign objects and replace the insulation materials (complying with IEC 60641 standards).
- Invisible/Irreparable Fault Points: If the core silicon steel sheets are short-circuited over a large area and the fault point cannot be located, cut off the annular core (break the short-circuit loop) to eliminate the circulation current.
- Post-Repair Verification: After repair, measure the core ground wire current (should return to ≤0.3A) and perform oil chromatographic analysis (methane/ethylene content back to normal) to confirm fault elimination.
By combining chromatographic analysis and current measurement for accurate diagnosis, and adopting graded handling measures (temporary control + thorough repair), CHH Power effectively resolves core poor grounding faults, extending the transformer’s service life and ensuring power grid safety.
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