This detailed analysis will explore the common causes for fluctuating DC resistance readings in one phase of a three-phase transformer, combining theoretical principles with practical examples and illustrative data patterns.
The DC resistance test measures the ohmic resistance of a transformer winding by injecting a known, regulated DC current (typically between 1A and 50A, depending on the transformer rating) and measuring the resulting voltage drop across the winding. According to Ohm's Law:
R = V/I
Where:
R is the DC resistance in ohms (Ω).
V is the measured voltage drop in volts (V).
I is the injected DC current in amperes (A).
For the reading to be stable, both the injected current (I) and the measured voltage (V) must stabilize rapidly after the test begins. Fluctuations indicate that one or both of these values are unstable over the measurement period.
Fluctuations in a single phase, while the other two phases provide stable readings, strongly localize the problem to that specific winding, its connections, or the associated tap changer mechanism. The most common causes are categorized as follows:
This is the most frequent and often the most critical cause. A fluctuating reading typically points to an intermittent, high-resistance connection.
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Mechanism: At the location of a loose connection (e.g., a bolted terminal, a bushing connection, or a lead-to-winding crimp), the current path is unstable. The injected DC current creates localized heating (P = I2R). As the loose joint heats up, the contact pressure may temporarily expand or shift, momentarily improving the connection (resistance drops). As the current is maintained, micro-vibrations or minor movements can cause the contact area to shift again, leading to an abrupt increase in resistance and thus a drop in the measured current/increase in measured voltage (or vice-versa, depending on the test set's regulation method).
Data Example:
| Time (s) | Phase A (Ω) | Phase B (Ω) | Phase C (Ω) |
| :------: | :----------------: | :----------------: | :----------------: |
| 5 | 0.125 Ω | 0.124 Ω | 0.128 Ω (Stable) |
| 10 | 0.125 Ω | 0.124 Ω | 0.135 Ω (Jump) |
| 15 | 0.125 Ω | 0.124 Ω | 0.126 Ω (Drop) |
| 20 | 0.125 Ω | 0.124 Ω | 0.141 Ω (Spike) |
For windings connected to a tap changer, the movement and seating of the contact mechanism are crucial for stable resistance.
Mechanism: If the selector contacts, diverter switch contacts (in OLTCs), or the moving contacts (in DETCs) are pitted, dirty, or misaligned, they may not establish a firm, stable electrical bridge. When the DC current is applied, the contact resistance might intermittently change due to slight physical movement caused by the magnetic forces of the current or internal temperature changes within the oil. This is especially true if the test is performed immediately after a tap change operation.
Specific OLTC Issue - Transition Resistors: During the brief tap-change operation, the transition resistors are temporarily in the circuit. If a test is attempted during the transition, or if the mechanism fails to fully seat and leaves a transition resistor partially in the circuit due to a mechanical fault, the measured resistance will be erratic and abnormally high.
Diagnostic Action: Perform the test on all tap positions. If the fluctuation only occurs on one or two taps, the issue is almost certainly within the tap changer contacts for those positions.
While less common than connection issues, a fault within the winding itself can cause fluctuations, particularly in windings made of numerous parallel strands (e.g., continuously transposed cable - CTC).
Mechanism: A broken or severely compromised strand or transposition joint within the winding bundle may intermittently contact the adjacent strand or the main conductor body. The slight magnetic forces from the DC test current can cause the broken strand end to move, making and breaking contact, resulting in a fluctuating, but typically higher, resistance value.
Data Example: The fluctuation here might be smaller and more rapid than a connection issue, manifesting as an unstable "fuzz" around the expected reading, rather than distinct jumps. This fault is extremely serious and indicates imminent winding failure.
Although the problem is isolated to one phase, the possibility of external or procedural error must be eliminated first.
Mechanism: The test leads themselves, particularly the voltage-sensing leads (V), are highly sensitive. If the voltage lead for the problematic phase is poorly clamped, has a faulty connector, or is physically moved during the test, the reading will fluctuate wildly. The current leads (I) are less sensitive to movement but a loose connection can cause the current injection to be unstable, leading to fluctuating V and thus R.
Diagnostic Action:
1. Check Lead Integrity: Turn off the test set, inspect and clean the clamps/jaws, and re-establish the connection for the affected phase (both I and V leads).
2. Grounding Loop: Ensure the test set is properly grounded and that no extraneous grounding loops are interacting with the measurement circuit of the affected phase.
When faced with a fluctuating reading, a systematic approach is mandatory.
Re-Test Setup: Immediately halt the test, clean all connection points on the transformer bushing/terminal, and re-clamp the current and voltage leads for the affected phase, ensuring a four-wire Kelvin connection is correctly used. Repeat the test.
Temperature Stability: Ensure the oil temperature is stable. Rapid changes in temperature can cause genuine, non-fluctuating drift, but if the temperature is stable and the reading is erratic, temperature is not the direct cause. All readings must be corrected to a standard reference temperature (e.g., 75℃) using the formula:
Where: k is 234.5 for copper or 225 for aluminum; Tstd is the standard temperature; Tm is the measured temperature; Rm is the measured resistance. Note: The fluctuation itself is an indication of a fault, regardless of the temperature correction.
Wiggle Test: If the winding is connected to an OLTC, try to gently rock the tap changer drive mechanism or motor while the current is applied and monitored. If the reading changes or becomes stable during this movement, the issue is definitively inside the tap changer (e.g., a seating issue or a contact misalignment).
Measure on All Taps: As noted, test every tap position. A fluctuation that is present only on certain taps points directly to the contacts associated with those taps.
If external and tap changer diagnostics fail to resolve the issue, the transformer must be taken out of service and de-tanked, or the winding compartment opened for internal visual inspection.
Focus Areas: The inspection must target the transition between the winding and the terminal lead, the soldered/crimped connections to the bushing, and the entire exposed current path for the affected phase. Signs of arcing, localized discoloration (hot spots), or loose bolts are confirmation of a severe high-resistance connection.
Future Impact: A fluctuating reading is a pre-failure condition. The repeated arcing and high I^2R losses at the faulty connection will generate copious amounts of fault gas (acetylene, hydrogen, etc.), leading to a thermal runaway and catastrophic failure if not addressed.
Conclusion
A repeated, erratic fluctuation in the DC winding resistance of a single phase is an unequivocal indicator of an intermittent contact problem, most commonly a loose connection at a terminal or a fault within the tap changer contacts. Unlike a stable, incorrect reading (which suggests a winding short or incorrect tap setting), a fluctuating value signals a physical, non-continuous change in the current path resistance over the test period. The data patterns, characterized by abrupt jumps or spikes in resistance, must be treated with the utmost seriousness, prompting a systematic diagnostic process to avoid an imminent and costly in-service failure.
Kingrun Transformer Instrument Co.,Ltd.
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