Download STUDY OF TRANSIENT CHARACTERISTICS

Summary of Doctor Engineering Thesis
A Study on Transient Characteristics of Electric Grounding Systems
By : Mohamed Abdel Azim Ibrahim NAYEL
Doshisha University, Kyoto, Japan
Different grounding systems, such as buried vertical and horizontal electrodes and large grounding
grids, are often a part of lightning protection systems in industrial and electric power plants. Large impulse
currents during a lightning strike flow into the grounding system and induce high transient voltages before
these are dissipated into the ground. These current surges and induced voltages could couple with
equipments which are connected to the grounding system, and cause operation errors, malfunctions and even
destruction of the equipments. Consequently, electromagnetic compatibility (EMC) studies require the
knowledge of the spatial and temporal distribution of voltages and currents along a grounding system in the
case of lightning.
This thesis has studied transient characteristics of grounding systems, induced voltages to overhead
circuits from the grounding systems, and transient voltage distribution in an intelligent building due to
lightning.
The thesis has proposed a modeling method of a grounding-electrode impedance in a frequency
domain. Transient voltages of various electrodes were measured when applying a step-like current. The
frequency responses of the measured voltages and injected current are numerically evaluated by linearly
approximated Laplace transform to avoid an aliasing error and thus to improve the accuracy. Then, a
frequency response of the grounding impedance is simply given as a ratio of the voltage and the current. A
model circuit of a grounding electrode is determined based on the frequency responses. The model circuit
obtained by the proposed method is compared with that by a conventional time-domain approach. The
accuracy of the proposed method has been confirmed to be satisfactory in comparison with the measured
transient results, and to be higher than that of the time-domain approach.
The thesis has presented field measurements of surge propagation characteristics on a single straight
counterpoise and on a cross-shaped counterpoise composed of two straight counterpoises. It has been found
that the propagation velocity decreases significantly as the wave travels along the conductor. The attenuation
of a current is greater and its propagation velocity is smaller than those of a voltage on the straight
counterpoise when a small current from a pulse generator is applied. This has suggested a nonuniformity of
the wave propagation on the underground naked conductor. The grounding resistance defined by the current
and the voltage peaks shows to be current-dependent, and the dependence becomes less noticeable as the
counterpoise length becomes longer, when a large current more than 5 kA is applied from a current impulse
generator. The grounding resistance of the cross-shaped counterpoise is smaller than that of the single one,
and also the current dependence is less noticeable.
a
It has been carried out an experimental investigation of grounding resistances of rod electrodes with
rather simple configuration assuming the grounding of a gas tower. Various arrangements of the rods are
examined so as to find a low resistance with a small number of rods, small space and easy installation. The
total resistance of short-circuited parallel rods has been found to be nearly proportional to the inverse of the
number of rods, when the separation distance is nearly the same as the embedded length of the rods into the
ground and an applied current is small. A double rod, two rods together with separation 10cm, is not
effective to reduce the grounding resistance. Also, an experimental investigation has been carried out on
mutual impedances of grounding electrodes for various grounding arrangements and electrode shapes. A
transient induced voltage to an electrode is observed to be similar to the inducing one when a current is
applied individually. Correspondingly, the frequency characteristic of the mutual impedance evaluated by
numerical Laplace transform from the measured voltage and current is similar to that of the self impedance.
An equivalent model circuit of the mutual grounding impedance is derived from the frequency characteristic.
The thesis has investigated experimentally an induced voltage to a loop circuit due to a pulse current
injected into a grounding system. The maximum induced voltage was measured to be 1.5 to 2.8 V by a
current with the amplitude of 1.0 A. For the induced voltage was contributed significantly by an overhead
current lead wire and an underground wire connecting grounding electrodes, the arrangement has to be
carefully examined to reduce the induced voltage. A simulation model of the induced voltage proposed in
this thesis shows a satisfactory accuracy compared with the measured results.
Finally, the thesis has experimentally studied transient voltages on floors in an intelligent building due
to lightning with special reference to grounding of a lightning rod, the building structures and the floors by
using an 1/10 scaled-down building model. It has been found that the floor voltages are the smallest in the
case of the floors bonded to a building structure except the case of the structure being used as a lightning rod.
The common grounding of all the floors causes very high floor voltages to a reference voltage line when the
floor grounding is connected to the lightning rod grounding. Individual grounding of the floors, the building
structures and the lightning rod produces rather high voltages on the floors to the reference voltage. A
voltage difference between the floors is significantly reduced by bonding the floors to the building structure
or by the common grounding of the floors.
b