Handheld Partial Discharge Detector Ultrasonic Measurement Method

Using built-in sensors for testing
There are two principles for the partial discharge detection of switch cabinets: ultrasonic principle and transient ground voltage (TEV) principle. The ultrasonic sensor and TEV sensor built into this product are specifically used to measure the partial discharge of high-voltage switch cabinets. These sensors are located at the front end of the host. During testing, this part needs to be brought close to (for ultrasonic measurement) or brought up close to (for TEV measurement) the high-voltage switch cabinet. Note that “close to” and “up close to” are different, depending on which principle is used to test the switch cabinet.
Handheld partial discharge detector – Ultrasonic measurement
If you choose to use the built-in ultrasonic sensor to measure the partial discharge in the switch cabinet, after turning on the device, you will hardly need to perform any other operations or settings. Because by default, this product selects the built-in ultrasonic sensor upon startup. The screen’s upper left corner will display which type of sensor is currently connected. This state represents the ultrasonic measurement mode.
In the ultrasonic measurement mode, the measurement data is in dBuV. Since dBuV is a logarithmic function value based on 1uV as the reference, the measurement data in the ultrasonic mode can be positive or negative. According to the processing capacity of the ultrasonic amplifier of this product, the test range can reach from -6dBuV to 68dBuV. The larger the negative value, the smaller the ultrasonic signal is and it gets closer to 0uV, but it is not a negative number! Usually, the measured data in an interference-free environment is between -6dBuV and 0dBuV.

In many cases, the environment where the tested cabinet is located is filled with various complex ultrasonic wave interferences, such as flickering fluorescent lights, ultrasonic rodent repellents, running fans, etc. Therefore, before testing the cabinet, it is necessary to measure the environmental values to determine the level of interference. When the ultrasonic environment value is too high (such as exceeding 10 dBuV or so), all sources of interference need to be eliminated; otherwise, the excessive interference signal will mask the true signal and affect the measurement results. It is recommended to use the TEV method to measure the cabinet value when the ultrasonic interference is too large and cannot be effectively eliminated.
The ultrasonic signal travels through air as the medium of transmission and will leak out from the gaps in the cabinet. Therefore, when using ultrasonic waves to measure the value of the switch cabinet’s cabinet body, the sensor needs to be placed close to the gap of the cabinet. At the same time, the discharge sound inside the cabinet can be listened to through the earphone (the ultrasonic signal can be converted into audible sound through digital filtering).
According to the regulations of the State Grid, the insulation condition of the switch cabinet can be judged by referring to Table 2:
Table 2
Ultrasonic reading
Explanation

-6 to 0 dBuV, no discharge sound
No localized discharge in the equipment.
0 to 6 dBuV, with a brief discharge sound
There is a slight discharge in the equipment. Attention should be paid to it in the later stage.
Above 6 dBuV, there is a discharge sound.
There is obvious discharge in the equipment. It should be determined based on the TEV test.
Note that the boundary point (6dBuV) varies slightly in different regions. In some areas (such as abroad and the Southern Grid), 6dBuV is used as the boundary point. Anything above 6dBuV is considered to indicate a significant discharge phenomenon. However, for users of the State Grid, the boundary point is generally set at 8dBuV. Regardless of whether it is 6dBuV or 8dBuV, the ultimate goal is to predict the insulation condition of the switch cabinet. Therefore, it is recommended to use 6dBuV as the boundary point, which can provide more timely warnings about the operating condition of the switch cabinet.