Manufacturer of substation equipment, demiks believes that the,The use of partial discharge methods to evaluate the insulation condition of AC high-voltage equipment has received widespread attention and made great progress. Some research results have been put into practical application in engineering. Partial discharge measurement under AC conditions has become an effective method for evaluating the insulation state of dielectrics. However, the measurement and analysis of partial discharge under DC conditions are not as mature as those under AC conditions.

In terms of mechanism, under DC conditions, the magnitude and polarity of the voltage remain unchanged. Once the internal air gap discharges, the space charge will establish a reverse internal electric field in the air gap. After the discharge is extinguished, the space charge will be neutralized by the internal conductivity of the medium and the reverse electric field will be reduced to a certain extent. Only then will the second discharge occur. It can be seen that there is no discharge repetition cycle under DC voltage, and the number of discharges per unit time is much less than that under AC voltage.

With the widespread application of high-voltage DC equipment, a large number of equipment work under DC conditions, and the partial discharge of insulation is a phenomenon that cannot be ignored. The evaluation of DC partial discharge signals is mainly based on the statistical analysis of basic parameters such as discharge amount, discharge number, time parameter curve, the first test voltage greater than the specified partial discharge amount, the number of discharges within a specified time under a specified DC voltage, and the number of discharges within the self-discharge time.

1. No periodicity

In AC PD, the system processes the statistical parameters of multiple discharge pulses, including the discharge amount Q, phase Ф, and the number of discharges N, and forms various curves and spectra to study the PD discharge signals of different parts or different properties in electrical equipment; since DC PD does not have a repetitive cycle, that is, there is no phase Ф in AC statistical parameters, the characteristic quantity of DC PD needs to be re-determined.

2. Randomness of discharge time

Under DC environment, the discharge time of the test sample is random, and the number of discharges per unit time is much less than that of AC voltage. For different discharge types, the time interval between two discharges has certain characteristics, which can be used for defect analysis.

3. High starting discharge voltage

During DC PD, the distribution of the insulating field strength is proportional to the resistivity of the medium. Generally, the resistivity of the insulating medium is greater than the resistivity of the infiltrate. Therefore, the breakdown field strength of the medium under DC conditions is generally greater than the breakdown field strength of the infiltrate, so the starting discharge voltage of DC PD is higher than that under AC voltage.

4. Fewer discharge times

Under DC voltage, when the voltage on the air gap inside the insulation is greater than the breakdown voltage of the air gap, the positive and negative ions generated by partial discharge in the air gap attach to the air gap wall to form an electric field opposite to the external electric field, so that the air gap can no longer continue to partially discharge. Only after a considerable period of time, when these charges leak due to the surface conductivity of the air gap and the volume conductivity of the dielectric, will the next partial discharge begin, so the number of discharges is much smaller than that under AC voltage.

5. Statistical analysis of DC partial discharge data

1). Q-t analysis chart

The Q-t analysis chart shows the information of each discharge in the time dimension. Due to the long test time, only the waveform within a certain time width can be displayed. (Or sampling data)

2). Qn-Δt analysis chart

The Qn-Δt analysis chart is used to show the characteristics between the discharge interval and the number of discharges.

3). Qn-Q analysis chart

The Qn-Q analysis chart reflects the relationship between the discharge amount and the number of discharges.

The main products of Shanghai Demiks Power Technology Co., Ltd. include: "Series resonance test equipment, partial discharge test system, high voltage insulation withstand voltage, transformer test, circuit breaker test, relay protection, secondary circuit test, cable line test, lightning arrester insulator tester, SF6 test, oil test, DC system battery test, reactive power compensation generator detection, ground insulation resistance test, power metering test, etc." More than 200 products in 20 categories".

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