Power frequency electromagnetic field measurement was done using PMM8053A type electromagnetic field measurement instrument or NARDA NBM550 electromagnetic field measurement instrument. Instrument range: electric field 0.01V/m ∼ 100 kV/m; magnetic field 1 nT–10 mT; frequency range: 5 Hz–100 kHz; measurement equipment should be in the calibration period.

The measuring instrument probe is installed in a well-insulated and adjustable height to the ground tripod, the fixed probe height from the ground is 1.5 m, and the probe is connected to the host with an optical fiber longer than 5 m. The probe is placed at the selected measurement point, and the measurement personnel holds the host for measurement and reads the electric field strength and magnetic induction strength value. Testers should be far enough away from the probe of the measuring instrument, at least 2.5 m in general, to avoid large electric field distortion at the instrument.

During the measurement process, when the instrument is turned on or switched to the measurement mode, we wait for the instrument to stabilize before taking readings; each measurement point readings are not less than three groups, and the measurement results are taken as the arithmetic mean of multiple measurements.

Using the German ROHDE & SCHWARZ company, radio interference measurement produced the R&S FSH3 electromagnetic interference test receiver, supporting the model HFH2-Z2 electrically shielded ring-type active antenna. The performance of the instrument conforms to the technical specifications of the CISPR radio interference measurement instrument. All measurements are made using the quasi-peak detection method. The measurement equipment should be in the calibration validity period.

Measurement was based on "GB 7349–1987 high-voltage overhead transmission lines and substations radio interference measurement methods.” The test should select 0.5 MHz as the measurement reference frequency, and the frequency value with the smallest amplitude within ±10% of the reference frequency as the measurement frequency point and the radio interference value of the frequency measurement after statistical evaluation of the radio interference level value.

To avoid errors due to possible standing waves in the line when measuring at a single frequency, multiple frequencies should be measured within the 0.15–30 MHz band, such as 0.15, 0.25, 0.50, 1.0, 1.5, 3.0, 6.0, 10.0, 15.0, and 30.0 MHz. The measurement position is the measurement reference distance. The measurement must be well selected for each frequency to avoid interference that is not generated from the transmission line or converter station.

The background interference field strength can be measured at selected measurement points during line or converter station outages. If the line or converter station equipment cannot be blacked out, it can be measured at 400 m from the line, 1 km from the converter station, or one to two environments similar to the measurement point location; the measured environmental interference field strength value is the measurement point of the background interference field strength value.

In the direction of the 500 kV side of the bridge arm reactor, two sections are selected for measurement along the direction of the vertical bridge arm reactor to the inlet pipe busbar interval. The measurement point is the inspection walkway between the bridge arm reactor and the valve hall, as shown in the red part in Figure 1.

During large loads, 500 kV bridge arm reactor frequency electric field measurement is carried out. From the distribution curve, it can be seen that the electric field intensity is maximum in the middle position of the inlet and outlet lines of the upper and lower bridge arms of the same phase of the bridge arm reactor (for example, the middle position of the upper and lower bridge arms of phase A). In the position of the middle of the junction of the two phases, the electric field strength is the smallest.

The maximum value is 12.14 kV/m at 14 m in unit and 11.56 kV/m at 54 m in unit. The maximum value is 12.21 kV/m at 56 m in unit Ⅱ and 12.23 kV/m at 56 m in unit Ⅱ.

(1) Bridge arm reactor inspection walkway electric field strength tested during unit Ⅰ operation

Starting position (0 m): 10 m outside of the upper bridge arm of phase A (10 m is the position of the upper bridge arm of phase A). The electric field intensity distribution curve is shown in Figure 2.

The starting point (0 m) for the upper bridge arm A phase outside the 10 m position and the electric field intensity distribution curve is shown in Figure 3.

From the distribution curve, we can see that the electric field strength is greatest in the middle of the inlet and outlet lines of the upper and lower bridge arms of the same phase of the bridge arm reactor (for example, the middle of the upper and lower bridge arms of phase A). The electric field strength is the smallest in the middle of the junction of the two phases.

The maximum value is 11.56 kV/m at 54 m when unit Ⅰ is operating.

The maximum value is 12.23 kV/m at 56 m in unit Ⅱ.

The 500 kV bridge arm reactor work frequency magnetic field measurement is carried out during the large load. The measurement content is: unit Ⅰ unit Ⅱ bridge main linkage change the north side of the inspection walkway work frequency magnetic field distribution. The maximum value of the magnetic field of unit Ⅰ is 0.424 mT at 52 m from the measurement point. The maximum value of the magnetic field of unit Ⅰ is 0.375 mT at 12 m from the measurement point. The maximum value of the magnetic field of unit I is 0.487 mT at 17 m from the measurement point. The maximum value of the magnetic field of the unit I E side is 0.411 mT at 52 m from the measurement point.

(1) Unit Ⅰ operation E side bridge arm reactor inspection walkway power frequency magnetic field test

The starting point of the test (0 m) is 10 m outside of phase A of the upper bridge arm, and the test results are shown in Figure 4.

The starting point (at 0 m) is 10 m outside the C-phase line, and the measurement results are shown in Figure 5.

Since the converter station is mountainous and forested on the other three sides except for the doorway side, it cannot be tested. So, radio interference measurements were carried out at 20 m outside the doorway in the north direction of the back-to-back project. From the measurement results, it can be seen that the radio interference outside the doorway does not exceed 50 dB (μV/m), which meets the requirements of the relevant environmental protection limits [≤55 dB (μV/m)].

A comparison of the EMD (electromagnetic disturbance) level between the back-to-back converter stations and the traditional flexible and straight converter station projects is shown in Table 2, 3. The measured values of the power frequency electric field of the conventional flex-station are 1.21 kV/m ∼ 5.86 kV/. The measured values of the back-to-back converter station’s electric field are 2.97 kV/m ∼ 12.23 kV/m. The analysis showed that the EMF characteristic parameter values of the conventional flex-straight converter station were smaller than those of the back-to-back converter station. This was due to the more complex operating conditions of back-to-back converter stations where DC and AC equipment coexist and the structure is more compact.