Transformer DC Resistance Tester

A: When selecting the test current, refer to the range specified in the technical specifications section; avoid operating outside this range. The general guidelines are as follows:

• High-voltage side of a 10kV/1000kVA transformer: With a resistance value of approximately 1–5 Ω, a current range of 1–10 A is typically selected.

A: The auxiliary magnetization method connects two high-voltage coils in parallel and one in series, introducing approximately 1.5 times the resistance of the high-voltage coils into the entire test circuit. This factor must be taken into account when selecting the measurement range. If the range is exceeded, the output current may fail to reach the set value or become unstable. Additionally, when disconnecting the short-circuit points of the three wires used in the auxiliary magnetization method after discharge is complete, residual current may remain. This may cause arcing or discharge during disconnection, which is a normal phenomenon; appropriate protective measures must be taken during operation.

A: “Charging”: This is usually because the selected current is too high, exceeding the range, and the current cannot reach the preset value. It may also indicate an issue with the transformer’s magnetic circuit.
“Current too low”: The selected current is too low, which can cause unstable readings for high-capacity transformers.
Solution: Verify the range and select an appropriate current to retest. If the current remains unchanged and stays at zero for an extended period, check the circuit for open circuits.

A: First, check if the power cord is properly plugged in, confirm that the power supply is functioning normally, and verify that the instrument’s power switch is in the “ON” position. Then, check if the fuse has blown. If using battery power, check if the battery has sufficient charge. If all of the above are normal, try replacing the power cord or plug; if the issue persists, contact the manufacturer for repair.

A: First, check if the power supply is normal; then check if the fuse has blown. If the fuse is confirmed to be blown, replacing it with one of the same specification will restore functionality. If the fuse is intact, check if the display connection cable is loose or damaged, and try restarting the device.

A: Unstable data is typically caused by poor contact in the test leads, loose fixtures, insufficient discharge of the test winding, or the presence of induced voltage. Solutions:
Fully discharge the winding before testing (using a dedicated discharge rod for at least 2 minutes).
Check the test leads for loose or intermittent connections.
Clean any oxidation from the terminals and secure the connection firmly using C-clamps or specialized pliers.
If the issue persists, check the test specimen for corrosion.

A: 5A–10A DC resistance testers: This is typically caused by the built-in transformer being damaged by high voltage.
DC resistance testers rated at 20A or higher: This may be due to a switching power supply failure.
Solution: Replace the damaged transformer or switching power supply promptly. Repairs must be performed by a professional engineer; if on-site repair is not possible, return the unit to the factory for servicing.

A: First, check if the temperature protection has been triggered, and prioritize checking the fan’s operating status. If the fan is operating normally, power on the unit without performing any test operations. Wait until the fan has completed cooling the unit, then try the test again.

A: Possible causes include an open circuit in the test leads, improper range selection, an internal break in the winding, or an instrument output anomaly. Solution:
Check the continuity of the test leads and replace any damaged wires.
Estimate the resistance value based on the transformer’s capacity and select the appropriate current range.
If the winding is confirmed to be normal but the error persists, restart the instrument or switch to manual mode to troubleshoot.

A: According to standards such as GB 50150 and IEC 60076, the limits for three-phase DC resistance imbalance are as follows:
Transformer Capacity Phase Resistance (with neutral point tap) Line Resistance (delta connection)
1600 kVA and above Not more than 2% of the three-phase average Not more than 1% of the three-phase average
1600 kVA and below Not more than 4% of the three-phase average Not more than 2% of the three-phase average
Formula for calculating imbalance: Imbalance (%) = (Maximum value – Minimum value) ÷ Average value × 100%
In addition, when compared to values measured at the same location previously (converted to the same temperature), the variation should not exceed 2%.

A: Measurement is too high: Usually caused by loose connections or poor soldering; it may also be due to dirty or oxidized tap changer contacts. Inspect and retighten connection points, or clean the tap changer contacts.
Measurement is too low: May indicate an inter-turn short circuit, creating an additional parallel current path. It is recommended to verify this by conducting other tests, such as an induced voltage withstand test.
Temperature not corrected: The instrument’s temperature compensation function has not been enabled.
Residual magnetism effect: For large transformers, performing a demagnetization procedure beforehand can eliminate the influence of residual magnetism on inductance charging and discharging.
Incorrect tap switch position: Confirm that the tap switch is set to the specified position and locked to prevent slippage during testing.

A: When the DC resistance reading of one phase in a three-phase transformer fluctuates repeatedly while the other two phases remain stable, this strongly indicates an intermittent contact issue in that phase. The most common causes are:
Loose terminal connections: Poor contact at bolt connections or bushing connection points.
Tap changer contact issues: Oxidation, contamination, or misalignment of contacts leading to unstable contact resistance.

A: Analysis is primarily conducted through the following aspects:

• Compare with standard values: Compare the measured values with the standard values in the transformer technical manual to determine if there are any abnormalities in the windings. Note that the measured values must be temperature-compensated to the standard temperature (20°C) before comparison.
• Calculate three-phase imbalance: Check whether the resistances of the three phases are balanced; an imbalance exceeding the limit indicates a possible internal defect.
• Assessing winding condition: Abnormally low resistance values may indicate a turn-to-turn short circuit, while abnormally high values may indicate poor contact or broken strands.
• Checking connection quality: Abnormal resistance values may indicate loose or oxidized connection terminals.
• Analyzing historical data: By comparing historical data from multiple measurements, trends in resistance values can be identified, aiding in the early detection of potential issues.
• Combine with other test results: Integrating results from insulation resistance tests, turns ratio tests, and gas chromatography analysis of transformer oil allows for a more accurate assessment of the transformer’s overall condition.

A: Transformer windings are large inductive loads that store a significant amount of magnetic energy after testing. If the test leads are disconnected immediately after testing, the magnetic energy stored in the core will instantly convert into an extremely high reverse electromotive force, which may cause severe electric shock injuries to operators and damage to equipment. Therefore:
After testing is complete, you must wait until the discharge alarm stops before turning off the power and disconnecting the test leads.
It is recommended to wait at least 10 seconds after the discharge alarm stops to ensure that the energy has been fully released before disconnecting the leads.

A: Before switching tap positions on a no-load tap-changing transformer, you must press the reset button to initiate the discharge process. Only after the discharge is complete and the alarm sound has stopped should you switch tap positions. It is strictly prohibited to switch tap positions during the test or while the discharge process is still in progress.

A: If an external power outage occurs during testing, the instrument will automatically initiate the discharge procedure. Do not disconnect the test leads immediately; wait until the instrument has fully discharged (the alarm stops) before proceeding. It is recommended to wait at least 30 seconds to several minutes to ensure complete discharge.

A: When the battery level indicator shows low power, recharge the battery promptly. A red light on the charger indicates charging is in progress; a green light indicates charging is complete.
If the instrument is not used for an extended period, it is recommended to charge it once a month for maintenance to prevent the battery from being damaged due to self-discharge.
Never use a non-dedicated power adapter to charge the instrument, as this may cause an explosion.
If the battery runs low during testing, you may connect the charger to perform an emergency test.

A: Yes. The instrument housing must be securely grounded to ensure the safety of both the equipment and the operator, as well as to prevent the buildup of static electricity and electromagnetic interference.

A: Use a four-wire (Kelvin) connection method for the test leads and ensure that all connections are secure and reliable.
If the transformer terminals are exposed to air for an extended period, a layer of oxidation may form on their surface, potentially causing unstable or inaccurate measurement results. When connecting the leads, be sure to remove this oxidation layer, or after connecting the test clamps to the terminals, twist them firmly a few times to break through the oxidation layer and ensure good contact.

A: Yes. Prolonged use may lead to a decline in measurement accuracy. It is recommended to calibrate the instrument once a year to ensure the accuracy and stability of measurement values. Regular calibration also helps identify potential internal issues with the instrument and prevent systematic errors.

A: Storage Environment: Store the instrument in a dry, well-ventilated environment free of corrosive gases, and avoid locations exposed to direct sunlight, rain, or excessive dust.
Avoid Severe Vibration: The instrument should be protected from severe vibration.
Battery Maintenance: Charge and discharge the battery every three months to extend its service life; if the battery is aging, replace it promptly.
Keep Dry and Clean: Clean the instrument regularly and keep it dry.
Professional Maintenance: Instrument maintenance and calibration should be performed by qualified personnel.

A: Cause 1: Poor contact in internal cables—Turn off the power, check if the cables are loose, and secure them.

A: The transformer DC resistance tester is primarily used to measure the DC resistance of power transformer windings, but it can also be used to test the DC resistance of inductive equipment such as motors, current transformers, reactors, and switchgear.

A: Many modern DC resistance testers have a built-in transformer demagnetization function, which can effectively reduce residual magnetism in the core after testing and prevent excessive inrush current when the transformer is put into operation. For large transformers, it is recommended to perform demagnetization before testing.

A: Check whether the memory or data interface is functioning properly.
Check if the memory is full; if so, delete some old data to free up space.
Ensure that the options and parameters for data storage or export are set correctly.
If the problem persists, you may need to contact the manufacturer for repair.

A: During calibration, if the test leads are coiled like an inductive coil rather than spread out, this may cause significant fluctuations in the displayed values. The solution is to pull the test leads apart to reduce their mutual inductance.

A: Yes, it is normal. This unit uses an automatic current selection output mode. To ensure the stability of measurement results, the actual measured current may be lower than the charging current; this is a normal operating phenomenon and requires no special action.