Three-phase EHV measurements during switching transients with open air sensors
In mixed overhead line and underground cable connections transients upon
switching actions occur. Knowledge of their magnitudes, in particular at the transition
points between lines and cables, is important information for safe operation of the
connection. This paper discusses measurements, performed for the Dutch transmission
system operator, based on open air sensors picking up capacitively the electric fields
from the line. Near each phase a sensor is placed, which can be installed in a short time
also when the connection is in operation. Options to minimize the effect of cross-coupling
between phases and sensors near other phases are discussed. This includes aspects of
positioning of the sensors in relation to methods to restore single-phase waveforms from
switching events by decoupling.
The differentiating-integrating measurement approach is adopted, meaning that the
sensor detects the time derivative of the line voltage and the original waveform is restored
by means of integration. This approach allows using long measurement cables between
sensor and integrator, as they are terminated characteristically, and mobile recording
equipment can be installed on a safe distance without loss of signal integrity. The method
has excellent electromagnetic compatibility aspects allowing for a straight response over
a bandwidth from 10 Hz to 5 MHz.
Results from measurements at a 380 kV mixed overhead line underground power cable
connection obtained at a transition point and at a substation are presented. Switching
transients recorded at the transition point were accurately restored as the main crosscouplings
could be determined from a fitting procedure assuming perfect symmetric
three-phase voltage after energization. For the substation measurements, due to the
close distance between different phase conductors, stronger cross-coupling occurs.
Though not all main coupling coefficients can be retrieved uniquely here, it is shown that
the individual phase waveforms can still be reliably restored.
It was observed that the switching overvoltages remained well below the standard limits
under all performed switching sequences. Depending on the line configuration switching
excite high-frequency oscillations, which potentially can disturb nearby equipment.
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