I. Preparation and preliminary verification before testing

Before conducting the test, ensure that the harmonic detector has sufficient battery power and that the function settings are intact. Verify that the equipment calibration label is within its validity period. Record the basic information of the circuit under test, including the supply voltage level, load type, transformer capacity, and current operating load value. Inspect the on-site environment, avoiding strong interference sources such as high-power variable frequency equipment and electric welding devices, and ensure that the distance between the instrument and live lines complies with safety regulations. Organize the testing objectives in advance, distinguishing between measuring only voltage harmonics, measuring only current harmonics, or conducting simultaneous voltage and current measurements. Prepare the corresponding specifications of current clamps and voltage test leads, and confirm that the accessories have no visible damage and the insulation layer is intact.

II. Standardize wiring operation steps

Voltage wiring: The voltage terminals of the instrument are connected to the phase line and neutral line under test through test leads, with each phase line corresponding to a neutral line. The wiring terminals must be securely fastened without any looseness, and it is prohibited to entwine or squeeze the leads together. The three-phase circuit must be connected in the order of A, B, C, and N phase sequence. A wrong phase sequence can cause data distortion.

Current wiring: Open the clamp buckle of the current clamp, pass a single phase wire through the center of the clamp jaw, close the buckle to ensure a tight fit, and ensure that the contact surface of the clamp jaw is free of debris and oil stains; distinguish the direction of incoming and outgoing current indicated by the clamp meter, and configure a current clamp for each phase of the three-phase load separately, correspondingly connecting to each current channel of the instrument.

Wiring sequence: First connect the terminals on the instrument side, then connect the secondary circuit on site or the low-voltage outlet terminal; when removing after the test is complete, reverse the operation, first disconnect the on-site wiring, and then remove the leads from the instrument terminals.

III. On-site reading data collection specifications

After wiring is completed, turn on the instrument, select the harmonic measurement mode, set the matching voltage transformation ratio and current transformation ratio parameters, and wait for the instrument data to stabilize before recording. The duration of a single sampling should be no less than 10 seconds, and the peak and average values of the continuously loaded equipment should be recorded in sections. For intermittently fluctuating loads, the sampling period should be extended, and data should be retained in three groups: peak load, stable load, and low load. The total distortion rate and the amplitude of each harmonic from 2nd to 50th order should be fully recorded, and the original voltage and current waveforms should be saved simultaneously. Collect two sets of data from the same circuit. If there is a significant deviation between the two sets of readings, check the wiring and clamp position, and recollect and compare the data. The data should be synchronized with the acquisition time and on-site load conditions for subsequent comparison and analysis.

IV. Stratified data analysis method

Basic numerical assessment: Compare with the power quality standards to determine if the total harmonic distortion rate of voltage and current are within the specified range. If the amplitude of a single harmonic exceeds the standard, focus on analyzing the harmonic at the corresponding frequency.

Load correlation analysis: The high-frequency harmonic values are relatively high, mostly caused by nonlinear loads such as variable frequency and switching power supplies; the low-frequency harmonics are abnormal, mostly due to uneven distribution of single-phase loads and the concentrated operation of rectification equipment. By comparing the harmonic data of each phase of the three-phase system, if the value of a certain phase is significantly higher, it can be inferred that the nonlinear load in that phase is concentrated.

Longitudinal comparative analysis: Data collected during different load periods was retrieved. The harmonic wave increased synchronously with the rise in load, indicating that the harmonic source is the equipment in use on site. There was no significant fluctuation in harmonic waves during load changes, suggesting interference from the upstream power supply line or the transformer side in the substation area.

Comprehensive judgment: Based on the auxiliary judgment of waveform curve, it is confirmed that there is persistent harmonic interference on site due to severe waveform distortion and excessive harmonic values. The transient peak values exceed the standard, but the mean values are normal, which is caused by short-term impact loads. Therefore, there is no need to determine long-term power quality abnormalities.