Magnetic fields from transmission lines: comparison of calculations and measurements

An experiment has been performed to compare the calculated and measured magnetic fields produced by a double-circuit 400 kV power transmission line. The phase currents were measured in the substation at one end of the line, taking particular care to measure the zero-sequence currents accurately, and the earth-wire current was measured at the span where the experiment was conducted. These currents were used to calculate the magnetic fields, using a number of computer programs based on Ampere's law. The magnetic field was measured at 22 positions ranging from 100 m on one side of the line to 500 m on the other side. Measured and calculated fields generally agreed well. The largest errors were ±7%±1 nT. These errors are attributed to a mixture of random errors in the calibration, resolution and synchronisation of the measuring instruments, and systematic errors stemming from the measurement of the zero-sequence currents.