High frequency modeling of the earth system using new qualitative concepts in electromagnetics

Master's thesis in the field of electrical-power engineering

Abstract:

Ground system refers to a set of conductors with different structures that are placed in the ground to protect equipment, human safety against electrocution, prevent unwanted overvoltages and damage to HV equipment, and on the other hand, to be the basis for zero voltage In transient phenomena, such as lightning and switching, the ground system is responsible.

The importance of ground resistance is because it plays an important role in protecting the power system, so its value is measured so that it does not exceed a certain standard value in any system. While the lightning current, according to the type of its waveform, includes different frequencies that show a different impedance from what was said, and knowing the value of this impedance is very important for network protection.

Different methods have been provided to calculate the impedance of the earth, and the last mentioned methods include circuit, transmission line and electromagnetic methods. In the first two methods, the problem is solved by using simplifying assumptions and by approximation, while in the electromagnetic method, the calculation is done with an exact method. It should be noted that analytical methods and direct solutions are very difficult and time-consuming, and numerical methods also face many problems. Thus, the impedance of the earth system, which has a circular motion in the frequency domain in polar coordinates, is modeled using fuzzy theory. At the same time, without using incorrect approximations and by using the accurate primary data obtained from the electromagnetic method, the effect of various parameters such as the radius of the ground rod and the specific resistance of the ground has also been taken into account. Also, by maintaining the accuracy, the simulation time has also been reduced, and by using the capabilities of fuzzy logic, important analyzes and interpretations have been obtained from the effective parameters. Formation:

Equipment protection by fault damping

Human safety against electrocution

Prevention of unwanted overvoltages and damage to HV equipment

Being a basis for zero voltage

Types of earth systems They are used in electrical networks in the form of plate electrode, rod electrode, mesh and ground well.

One ??of the most common ground systems is the use of a vertical conductive rod (rod) to connect the ground; In this type of earth system, various parameters are involved in its dynamic behavior. These parameters include ground conductivity, ground permeability, soil ionization, type and radius of conductors buried in the ground. .

About the effect of different parameters of the ground on the behavior of the earthing system[1] and as a result of lightning overvoltage, it can be briefly said that from conductivity and permeability[2], the conductivity of the ground has the most effect on the transient response of the bare conductor buried in the ground. And the effect of increasing ground resistance on the earthing system will eventually increase lightning overvoltage. Until now, studies of the transient states of power networks, including substations, power plants, and transmission and distribution lines, have been carried out without considering the exact behavior of the ground networks. Although there are solutions for considering the effect of ground networks in transient state studies, these solutions are approximate and have many limitations.  These limitations are such that a model for ground networks has not yet been provided in power network transient analysis software. This is while the exact behavior of the earth networks requires the modeling of these networks in the frequency domain, which is a double limitation in the analysis of the time domain of the modes.This is the fact that the accurate behavior of ground networks requires the modeling of these networks in the frequency domain, which is a double limitation in the time domain analysis of transient states. [1]

One ??of the important cases is the transient states of lightning. In order to properly assess the overvoltage caused by lightning, a proper understanding of the electromagnetic field along the path of the lightning wave is required. [2] One of the most important parts in the path of the lightning wave is the earth system, and therefore measuring or calculating the impedance of the earth is very important. On the other hand, there are different methods to solve this type of electromagnetic problems. Maxwell's relations are the main model and the general rule governing electromagnetic problems, and many other methods are started from these relations or derived from them. For example, parallel to the common methods of solving electromagnetic problems, numerical methods are also considered as a valuable tool in solving such problems and have been developed in recent years and have found wide applications. The following flowchart shows a simple representation of classical electromagnetic problem solving methods: [3]

Abstract:

The set of conductors with various configurations those are used to protect the station and human from electrical shocks and prevent the undesirable over voltages on the station, are called grounding system that is base for zero voltage in the transient phenomena like switching and lightning.

Nowadays the practical method is to meager and record the resistance or ohmic impedance of grounding system that differs from the original impedance of it because of various frequency contents of lightening's current and the value of grounding impedance is very important in the network protection.

More approaches are proposed to calculate the earthing impedance. The last of them are: circuit approach, transmission line approach and electromagnetic approach. In the circuit and transmission line approaches by using simplifying assumptions can solve the problem but calculations in the electromagnetic approach is done by accurate method. And analytical and also numerical solutions have many limitations.

In this thesis, the behavior of grounding system in frequency domain has been modeled. It means that without any assumption and with accurate input data, by using the fuzzy inference method the input impedance of vertical and horizontal rod in frequency domain - which has circular movement in polar plane - has been modeled. And also some interpretation about the effects of some important parameters have been extracted by means of fuzzy method. Simulation time is considerably reduced and great accuracy is achieved.

Beside the impedance, the induced voltage of lightening in the earthing electrodes have been examined too.

At last it is shown that this method can easily model the same problems in engineering.