Transformer fault localization

• core objective: Transformer fault location aims to accurately determine the nature of the fault (e.g., short-circuit, open-circuit, discharge), the specific components (e.g., windings, cores, on-load tap-changers, bushings) in which it occurs, and its physical location by means of systematic monitoring, testing, and analysis.

• positioning logic:: Follow the principle of "from the surface to the inside, from macro to micro". The first step is to analyze online monitoring data and relay protection action information for initial investigation; secondly, to conduct a series of non-destructive electrical diagnostic tests after a power outage; and lastly, to incorporate advanced acoustic/vibration techniques or internal inspections, if necessary.

• Key Online Methods:

  • Dissolved Gas Analysis (DGA) in Oil: Determine the energy density and type of fault (e.g., overheating, arcing, partial discharges) by analyzing the type and content of gases characteristic of the fault.

  • Relay protection action analysis: Quickly determine the area (internal/external) and severity of a fault based on the type of protection that is acting (e.g., differential, gas, overcurrent).

• Core offline testing:

  • Electrical Routine Testing: Includes winding DC resistance, ratio, insulation resistance, dielectric loss factor, etc. It is used to determine the conductivity of the winding, turn-to-turn short-circuiting and the health of the insulation system.

  • Mechanical Condition Diagnostics: Frequency response analysis (SFRA) is the most effective means of diagnosing mechanical damage such as deformation and displacement of windings.

• ultimate goal: Accurate fault localization is the basis for formulating a repair plan (on-site repair, return to the factory for overhaul or scrapping), assessing the damage of the accident, and analyzing the cause of the fault to prevent the recurrence of similar accidents.

I. Initial diagnosis of faults: online monitoring and analysis of accident information

The first step in fault localization after a transformer trip is not an immediate field test, but a detailed analysis of the pre-trip online data and protection action information.

1. Analysis of relay protection action information

• Gas Relay(Gas) protection action: This is the most direct evidence that clearly indicates that the fault is inside the fuel tank.

  • Light gas alarm: Usually corresponds to initial latent faults, such as localized overheating and minor discharges, suggesting a low fault energy.

  • Heavy gas tripping: Indicates that a severe internal short-circuit failure has occurred, producing a violent oil flow shock.

• Differential protection action: Indicates that a severe phase-to-phase or ground short-circuit fault has occurred within the protective zone formed by the differential CT (i.e., transformer body and bushings).

• Overcurrent protection/impedance protection action: If only the backup protection operates, the fault may be external or internal to the transformer and needs to be further judged in conjunction with other information.

2. DGA - Dissolved Gas Analysis in Oil
DGA is the most effective means of diagnosing latent faults within a transformer and analyzing the nature of faults that have occurred. Different types of faults cause insulating oil and paper to decompose at different temperatures, producing specific combinations of gases.

• Low temperature overheating (<300°C):: Produces mainly methane (CH₄), ethane (C₂H₆).

• Medium to high temperature superheat (300°C - 700°C): Mainly produces ethylene (C₂H₄).

• Partial Discharge (PD):: Produces mainly hydrogen (H₂) and a small amount of methane.

• Arc discharge (>700°C): Produces large quantities of hydrogen (H₂) and acetylene (C₂H₂). Acetylene is the only effective characteristic gas for arc faults.

By analyzing the changes in DGA data before and after the failure and using theDuval Triangle Equivalent graphical methods can accurately determine whether the fault is overheating, partial discharge or energetic arcing.

II. Electrical diagnostic testing and localization after a power outage

After completing the initial diagnosis and ensuring safety measures are in place, a series of offline electrical tests are used to further localize the fault.

III. Advanced fault location technology

For some complex faults, which may be difficult to pinpoint with traditional electrical testing, more advanced techniques are required.

1. SFRA - Sweep Frequency Response Analysis

• principle:: SFRA is known as the "fingerprint" test for transformers. It measures the response of a winding by injecting a swept signal (Hz to MHz) into the winding and plots a unique frequency response curve. The inductance (winding) and capacitance (turn-to-turn, layer-to-layer, and ground) inside the transformer form a complex RLC network, and any change in mechanical structure (e.g., deformation of the winding, displacement of the core, loosening of the clamps) will change this network, which will result in a change in the "fingerprint" curve.

• appliance: By comparing the current test curve with the factory or historical curve, it is possible to determine with extreme sensitivity whether the transformer has suffered mechanical damage, as well as the severity and approximate area of damage.

2. Acoustic Partial Discharge Location

• principle: When partial discharges occur inside a transformer, ultrasonic signals are generated. These sound waves propagate through the insulating oil to the tank wall.

• localization: Arranging several ultrasonic sensors outside the tank wall, by measuring the time difference (time delay) of the discharge signal reaching different sensors, utilizing thetriangulation algorithmIn addition, it is possible to calculate the three-dimensional spatial coordinates of the discharge source within the tank, enabling precise physical localization of the discharge point.

Frequently Asked Questions (FAQ)

1. What should be the first step in fault localization after a transformer trip?
The first step is definitely not to test immediately. First.ensuring safety(isolation, power testing, grounding), followed by theCollecting information: Record and analyze in detail which protection operated, the time of operation, the load and weather conditions prior to tripping, and sample the transformer for Dissolved Gas Analysis (DGA) in oil in the first instance. The combination of this information can have a basic judgment on the nature and scope of the fault, guiding the subsequent testing program.

2. What is the difference between DGA and SFRA in troubleshooting?
They are complementary.The DGA diagnoses theelectrochemical failure, which tells you if an event such as overheating, discharge, etc. is occurring or has occurred inside the transformer. And SFRA diagnoses theStructural mechanical failureIt tells you whether the "skeleton" of the transformer (windings, core, etc.) has been deformed or displaced because it is unable to withstand electrical forces or external impacts. A serious short-circuit fault can produce both an electric arc (DGA can detect acetylene) and a deformation of the windings (SFRA can detect a curve change).

3. To what extent can the fault be localized without lifting the transformer bell jar (opening the case)?
With the suite of non-invasive diagnostic techniques described above (DGA, electrical testing, SFRA, acoustic localization, etc.), the vast majority of faults (more than 90%) can be localized to specific components (e.g., high-voltage A-phase windings) and to specific types of faults (e.g., inter-turn short circuits) without opening the box. This precise localization is critical to assessing the feasibility of a repair and developing an efficient repair plan, avoiding blind, costly out-of-the-box inspections.