I. Core Advantages: Why Choose Three-Channel?
High measurement efficiency: Three-phase windings are charged and measured simultaneously, reducing the traditional several-hour work to just a few minutes.
Higher measurement accuracy: Due to synchronous measurement, the temperatures of the three-phase windings are almost completely consistent, completely avoiding the errors caused by inconsistent temperature rise due to sequential measurements, making the results more accurate.
Automated data processing: The instrument will automatically calculate and directly display the imbalance rate of the three-phase resistances, eliminating the need for manual recording and calculation, and reducing human errors.
Easier discharge process: Although the total energy of simultaneous discharge of three phases is greater, the instrument usually integrates an efficient discharge circuit, which can safely and quickly complete the discharge, significantly reducing the waiting time.
II. Typical Application Scenarios
Factory testing: As a mandatory inspection item for each product before leaving the factory.
Handover testing: Before new transformers are installed and put into operation, used to verify whether the equipment meets the standards.
Preventive testing (pre-test): Conducted at regular intervals (such as 1-3 years) as an important basis for equipment condition maintenance, timely detecting potential hazards.
Fault diagnosis: When suspecting problems such as inter-turn short circuits, poor contact of leads, or switch faults in the windings, perform more precise measurements to assist in diagnosis.
III. Standard Testing Process (SOP)
Safety Preparation: The first step is to ensure that the transformer has been completely powered off, tested for electricity, and grounded. Before the test, all external connection lines must be removed or disconnected to ensure complete isolation on all sides.
Test Wiring (Example: YN Connection): Connect the instrument’s current output line (I+) and voltage measurement line (V+) to the top of the A, B, and C phase bushings of the transformer. Then, connect the corresponding current return line (I-) and voltage measurement line (V-) to the neutral point (O) of the transformer. Please use the four-wire method and ensure all connections are firm and well contacted.
Parameter Settings: Turn on the instrument power supply, select the appropriate test current (most instruments can automatically recommend) and the correct winding connection method (such as YN or D).
Start Testing: Start the test program. The instrument will automatically charge the three-phase windings and display the changing curves of charging current and resistance values on the screen in real time.
Read Results: When the rate of change in resistance values is less than the threshold set by the instrument and tends to stabilize, the instrument will automatically lock and save the data, ultimately displaying the resistance values of A, B, and C phases and the calculated three-phase imbalance rate.
Discharge and Line Removal: After the test, do not remove the lines immediately. Wait for the instrument to complete the internal automatic discharge program. Wait for the screen to indicate “Discharge Complete” or the discharge indicator light to go out before using a dedicated discharge rod to perform multiple, thorough discharges to ground the windings. Confirm there is no residual voltage before removing all test lines.
IV. Key Precautions and Result Judgments
On-site Safety and Operating Points
Safety, Complete Discharge: This is the most important precaution. Transformer windings are huge inductors that can store a large amount of energy. Even if the instrument indicates discharge completion, the windings themselves may still have residual voltage. Therefore, it is necessary to use a dedicated discharge rod for manual discharge to ensure safety.
Correct Wiring is the Foundation: Strictly follow the four-wire method of wiring. The voltage measurement line (V line) should be connected to the inner side of the current output line (I line), and directly connected to the terminals of the windings to avoid contact resistance from clamps and leads, ensuring measurement accuracy.
Eliminate Remnant Magnetic Field Influence: After multiple consecutive tests or changing the tap position, a remnant magnetic field will be generated in the core, which will affect the charging speed and measurement stability of the subsequent tests. Usually, the influence can be eliminated or reduced by using the “demagnetization” function built into the instrument or by manually swapping the test polarity.
Accurately record the environmental temperature: During the test, the upper oil temperature or winding temperature of the transformer should be accurately recorded. This is because the resistance value is closely related to temperature, and only by converting the measurement results of each time to the same reference temperature can effective comparison and trend analysis be conducted.
Judgment criteria for results
Core indicators: Three-phase resistance imbalance rate
General standards: For power transformers with a rated capacity of 1600 kVA or above, the difference between the resistance values of each phase winding should not exceed 2% of the average value of the three phases.
Special cases: For transformers without a neutral point lead-out, the measured resistance is the inter-line resistance, and the difference between them should not exceed 1%.
Important method: Longitudinal historical comparison
Compare the current measurement result (converted to the same temperature) with the factory test report value of the transformer and the preventive test data of each time. If significant and beyond the normal range changes are found in the values, it indicates that there may be potential faults in the windings, and further inspection is required.