In any power network, transformers are one of the most critical and expensive assets, directly impacting the stability of the entire power grid. But we cannot directly assess the health of a transformer’s internal windings and connection points through external observation. Therefore, it is necessary to rely on precise electrical testing to diagnose its internal condition. Among these, the DC resistance measurement of transformer windings is a fundamental yet crucial diagnostic method.
This test is not merely a simple procedural task but a critical quality control and condition assessment method throughout the entire lifecycle of a transformer (from manufacturing and installation to operation and maintenance). According to international and national standards such as GB/T 1094.4, DC resistance measurement is a mandatory routine test for transformers.
This article will delve into why DC resistance testing is an indispensable part of transformer maintenance, how it reveals potentially serious defects, and what key performance and safety information can be interpreted from its data.
To gain a deeper understanding of mainstream testing methods (such as the constant current source method) and their key operational points, please read our technical article: [Transformer DC Resistance Testing: An Analysis of Key Methods and Precautions].
The Core Purpose of DC Resistance Testing
Measuring DC resistance essentially involves applying a stable direct current to the transformer winding and accurately measuring the resulting voltage drop, thereby calculating the resistance value according to Ohm’s Law.
This seemingly simple physical principle can reveal the integrity and health of the transformer’s internal conductive circuit. Its primary objectives can be summarized in the following key aspects:
1. Verifying Transformer Manufacturing Quality
The long-term reliability of a transformer stems from its excellent manufacturing quality. The DC resistance test is the first critical line of defense before the transformer leaves the factory.
Checking the integrity of internal connections: A transformer contains numerous soldered joints and mechanical connections, such as those between winding conductors, between winding leads and the tap changer, and between leads and bushings.
Any poor soldering, poor connections, or loose bolts will result in abnormally high or unstable readings. These defects can accelerate insulation aging and may ultimately lead to severe internal faults.
Verifying conductor specifications and materials: The DC resistance value is directly related to the conductor’s length, cross-sectional area, and conductivity. Through measurement, it is possible to verify that the conductor specifications meet the design requirements and to check for any corner-cutting or use of substandard materials.
2. Diagnosing Potential Risks in In-Service Transformers
For transformers already in operation, periodic DC resistance testing is crucial for condition assessment and early fault detection.
Detecting damage during transportation and installation: During long-distance transport and hoisting, a transformer may experience bumps and vibrations, leading to loose internal connections or winding deformation. Performing a DC resistance measurement before commissioning and comparing it with the factory data is a vital step to confirm that the transformer is in good condition after installation.
Assessing the contact condition of the tap changer: The On-Load Tap Changer (OLTC) is the only moving component in a transformer and is also a common point of failure. Frequent operations can lead to contact wear, corrosion, or poor contact. By measuring DC resistance at different tap positions, the health of each contact can be accurately determined, effectively preventing power outages caused by tap changer faults.
Checking the phase balance of windings: For a three-phase transformer, the DC resistance values of the windings for each phase should be very close (unless specified otherwise by design). If the measurement results show a significant imbalance, it could indicate an inter-turn short circuit, a connection defect, or localized damage in one of the phases. This will lead to an asymmetrical magnetic field, increased losses, and localized overheating during transformer operation.
Extended Applications of DC Resistance Data: The Foundation for Performance Assessment and Trend Analysis
DC resistance measurement data is not only used for fault diagnosis but also serves as an indispensable foundation for other critical performance assessments.
1. The Basis for Accurate Temperature Rise Test Calculations
Accurate DC resistance measurement is a prerequisite for performing effective temperature rise calculations. The temperature rise test is a core test for evaluating a transformer’s thermal performance and load capacity. Its principle involves calculating the average temperature rise of the winding by comparing its cold-state resistance (R1) before loading with its hot-state resistance (R2) after loading.
2. A Key Parameter for Load Loss (Copper Loss) Calculation
A transformer’s load loss (also called copper loss) is primarily generated by the current (I) flowing through the winding resistance (R), and its magnitude is directly proportional to the resistance value. Accurate DC resistance data is fundamental for calculating and verifying the transformer’s energy efficiency parameters and for assessing its operational economy.
3. The Cornerstone for Predictive Maintenance and Condition Trend Analysis
Every DC resistance measurement should be properly recorded. From factory acceptance and handover to each scheduled maintenance, these data points form a historical trend line reflecting the transformer’s health.
For a transformer in stable condition, its resistance value should remain constant at the same temperature. Any slight deviation from the historical trend can be an early warning signal, enabling intervention before a fault occurs and facilitating true predictive maintenance.
Key Milestones for DC Resistance Testing: Throughout the Transformer’s Lifecycle
To maximize the value of the testing, DC resistance measurements should be performed at the following key milestones:
Factory Acceptance Test (FAT): As the final verification of product quality, to obtain authoritative original baseline data.
Site Acceptance Test (SAT): To confirm that the equipment is undamaged after transportation and installation, serving as a crucial part of the asset handover process.
After Major Repairs or Modifications: To verify the quality of the repair work and the correctness of all connections.
Periodic Preventive Maintenance: Depending on the equipment’s criticality and operating environment, it is typically recommended every 1-3 years as a routine condition check.
After an Abnormal Event: This test should be conducted immediately after a transformer has experienced an external short circuit, a nearby fault, or severe mechanical shock to assess whether the windings have been deformed or damaged by significant electrodynamic forces.
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In summary, DC resistance testing is not just a routine procedure but a core strategy for proactively identifying risks, safeguarding transformer assets, and extending their service life. Accurate and repeatable measurement data is the foundation for all of this.
However, the quality of the data is directly dependent on the test instruments used. Outdated or low-precision equipment might cause you to miss crucial early warning signs.
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