Research On Measurement System Of Corona-Generated Space Charge Under HVDC Transmission Lines

With the increasing of voltages applied to high voltage direct current (HVDC) transmission lines, the problem of electromagnetic environment caused by corona discharge is becoming increasingly prominent. Especially the existence of corona-generated space charges make characteristcs of ion flow field under the line change so much. Therefore, the space charge density has got more and more attention as one of the important quantities that help describe the electrical environment in the vicinity of lines. We can see that if the measurement of space charge density is implemented, on the one hand, the numerical basis can be provided for building ion flow field model. On the other hand, it can provide theoretical guidance for the design of lines, which is necessary to construct an economical, reasonable and environment-friendly HVDC transmission system.In this paper, the measurement system of space charge density based on the ion counter has been realized. Through selecting the suspended reference potential, the space charge density in the air can be achieved in theory. This method can overcome the shortcomings of other methods which can only measure space charge density near the earth surface. And the measurement device has the advantages of small volume and easy to carry. The system is composed of a measurement unit and a signal acquisition unit. The measurement unit includes a charge collecting element, a test element and a bias voltage element. Its main function is to collect the space charge and to realize the transformation of measurement signal from space charge density to current. The signal acquisition unit is built based on the data acquisiton card and the Labview software and it can measure voltage signal of millivolt level. The measurement of the charge density can be realized by converting the current signal into a voltage signal through the sampling resistor.After the measurement system is manufactured, a calibration device, which has five layers of parallel plates, is designed and developed. The calibration device is divided into three regions—the corona region, the injection control region and the calibration region. By controlling the applied voltages to plates, the space charge with known density can be produced in the calibration area uniformly. Use the measurement system to achieve the charge density. Then fit the measured value and the theoretical value, and the calibration result will be obtained.In the end, a scale wire model of the HVDC transmisson line is used to set up a test platform in the laboratory environment. And the distribution characteristics of space charges in positive polarity under the model are studied. The research results show that the space charge distribution is symmetrical on both sides of the wire in the plane parallel to the earth and the farther the distance away from the wire is, the smaller the density will be. The measured value right under the wire is the maximum.It is also be found that the space charge density in the plane which is vertical to the earth and parallel to the wire, increases rapidly along with the increase of relative height to the earth. The density and the relative height have a liner relationship.