GB 39220-2020Limits and monitoring methods of total electric field of DC transmission project (English PDF)

3.1 DC transmission project

The project which transfers direct current from a power supply location to a power demand location. DC transmission project includes DC transmission lines, converter stations and grounding electrode systems.

3.2 Total electric field

The combined electric field OF the electric field generated by the charge on the DC charged conductor AND the electric field generated by the space charge caused by the conductor corona. The physical quantity, which measures the intensity of the total electric field, is the electric field intensity. Its unit is volts per meter (V/m); kilovolts per meter (kV/m) is commonly used in engineering.

3.3 Percentile level of total electric field (En)

SORT the continuous measurement data (equal time interval sampling values)

of the total electric field at the monitoring point according to the absolute value from small to large. The n% data is called the percentile level of total electric field, En. Its meaning is that, the absolute value of n% of the measured data during the measurement period is less than or equal to En. For example, E95 and E80 respectively indicate that, the absolute value of the measured data within 95% and 80% of the measurement period is less than or equal to E95 and E80.

5.1 Monitoring instrument

The monitoring instrument of the total electric field shall be able to measure the size and polarity of the total electric field at the same time; and, have the function of automatic continuous measurement and recording.

The field mill is generally used to monitor the total electric field. The field mill shall use a square metal plate with an area of 1 mx1 m and good conductivity as the ground reference plane. It must be grounded reliably.

The field mill shall be within the validity period of calibration. See Appendix A for its calibration method.

5.2 Environmental conditions

The monitoring of the total electric field shall be carried out under the weather with wind speed (2m above the ground) less than 2m/s, no rain, no fog, and no snow.

5.3 Monitoring method and frequency

The monitoring point shall be selected at a place where the terrain is flat and there is no obstruction. The field mill shall be placed directly on the ground. The distance between the upper surface and the ground shall be less than 200mm.

PLACE a square metal plate with an area of 1 mx1 m and good conductivity on the wall of the converter station; is arranged in a direction perpendicular to the wall. The distance between two adjacent monitoring points may be 5m.

Generally, monitor to a distance of 50m from the wall.

5.4.3 Building

When monitoring outside the building, the total electric field monitoring point shall be arranged on the side of the building close to the DC transmission project; and not less than 1m away from the building.

When monitoring on the balcony of the building or the platform used for living, working or studying, the points shall be arranged in an area not less than 1m away from walls or other fixed objects (such as guardrails). However, the points should not be arranged in a location, which needs to be reached with the help of tools (such as ladders) or by special methods (such as climbing).

5.5 Data recording and processing

5.5.1 Data recording

During monitoring, it shall record meteorological conditions such as wind speed, wind direction, temperature, relative humidity, air pressure, and weather conditions during the monitoring period. In addition to recording the monitoring data of each monitoring point, it shall record the specific location of the monitoring point and the start and end time of each monitoring.

When monitoring DC overhead transmission lines, it shall also record the line parameters of the pole wire at the monitoring point or monitoring path, such as wire height, distance between poles, wire type, operating voltage, operating current, tower number, line direction, number of circuits on the same pole and line arrangement. When monitoring the converter station, it shall also record the operation mode of the converter station, converter valve power, DC voltage, etc.

For each monitoring point, it shall monitor for at least 30min; sample at equal intervals during the monitoring period; record at least 100 data.

5.5.2 Data processing

In the continuous monitoring of the total electric field, the monitoring data is highly dispersed. The cumulative probability method shall be used for data processing.