Papers by Mehrdad Mokhtari
The behaviour of a grounding electrode can be predicted by using either the electrical circuit mo... more The behaviour of a grounding electrode can be predicted by using either the electrical circuit model or electromagnetic computation. Despite its advantages over the latter, the grounding circuit model fails to accurately predict the behaviour under transient conditions due to the absence of two key factors, namely the soil ionization, and the current rate–of–rise. A new equivalent circuit model of a grounding electrode with dynamic circuit elements (Rd, Cd, and Ld) was developed to consider both soil ionization and current rate–of–rise factors. A generalized formula was derived to calculate the dynamic inductance, Ld, for all standard current wave shapes such as Conseil International des Grands Reseaux Electriques (CIGRE), double–exponential, and IEC 62305–1 (International Electrotechnical Commission). The computed inductance, Ld, dynamically changes with the change in the lightning current parameters, thus improving its accuracy for all current rate–of–rise conditions. The consider...
This paper describes a study on the characteristics of preliminary breakdown pulses (PBPs) preced... more This paper describes a study on the characteristics of preliminary breakdown pulses (PBPs) preceding negative return strokes. Such characterization is needed due to the lack of available data, especially for negative lightning, in Malaysia. Measurements were carried out in Johor Bahru, Malaysia in October 2014 using broadband antennas and a high resolution Picoscope. It was found that 84% of the negative return strokes (RS) were preceded by PBPs. 70% of the PBPs fit the BIL model and another 30% the BL model. The arithmetic mean of the PBP-RS interval is 13.2ms, which is about four times smaller compared to a similar measurement made during April to June, 2009. The difference may probably be due to a variation in the thundercloud formation which is dependent on the monsoon period in Malaysia. Further studies are required to characterize the PBPs and their relations with the monsoon period and with geographical region.
International Transactions on Electrical Energy Systems, Oct 9, 2015
The soil ionization phenomenon effectively reduces the grounding electrode resistance. The existi... more The soil ionization phenomenon effectively reduces the grounding electrode resistance. The existing methods to determine the soil ionization effect on the transient resistance of a grounding electrode are valid only for homogeneous soils. However, most soil structures are non-homogeneous. Realizing the importance of the issue, this study proposes a hybrid method to incorporate the soil ionization effect on the transient resistance of a grounding electrode located in non-homogeneous soil condition. The electromagnetic field approach is utilized to compute the ionized soil volume around the electrode to determine the equivalent radius of the electrode. A circuit-based method of computing the grounding resistance of the electrode in non-homogeneous soil conditions is proposed based on the per-unit-length distributed parameter method. The proposed method is validated by using an electromagnetic field-based numerical solution for the non-homogeneity effect of the soil on grounding electrode resistance.
Archives of Electrical Engineering, Sep 1, 2016
Grounding electrode resistance non-linearly changes under impulse conditions due to soil ionisati... more Grounding electrode resistance non-linearly changes under impulse conditions due to soil ionisation phenomenon. Several models have been proposed to model soil ionisation for grounding electrodes applications. However, to date, there is yet an attempt made to compile all these works into a comprehensive review article. Therefore, this paper is written with the objective of summarizing all related works in this field as a onestop reference. With reference to the literature, this paper is written to summarize the working principles of the soil ionisation models as well as the accuracy and performance analysis of the models. This paper, particularly highlights the deficiencies of the available models in terms of accuracy and performance. This knowledge will contribute to the development of a new accurate and efficient soil ionisation model.
International Transactions on Electrical Energy Systems, Sep 12, 2016
This paper proposes a new model of soil ionization, in the form of an equivalent circuit containi... more This paper proposes a new model of soil ionization, in the form of an equivalent circuit containing capacitive and resistive elements representing a network of soil particles and air voids. In previously published models, the effect of soil ionization is only indirectly considered by changing the soil resistivity and the electrode radius. This has resulted in inferior performance of those models in terms of voltage response accuracy. In this work, the energy balance concept was applied to compute the arc resistance. The soil structure is modeled by using two-layer capacitor elements. The soil ionization process is incorporated inside the two-layer capacitor elements using specially derived differential equations. Unlike previous models, a discrete form of soil ionization is used. The accuracy of the proposed equivalent circuit model was confirmed by the small differences between the computed and experimental results.
Applied Mechanics and Materials, 2016
Lightning has been the major cause of power interruptions in Malaysia. Malaysia is located near t... more Lightning has been the major cause of power interruptions in Malaysia. Malaysia is located near the equator having approximately 200 thunder days per year. It is known to be among the highest lightning activity country worldwide. TNB as the main power supply utility in Malaysia has made an effort to reduce the power interruptions via installation of Lightning Detection Network (LDN) in 1994. This paper presents the history leading to modern LDN that capture lightning radiation fields at multiple remote sensors. The network has undergone a series of improvement between 2004 and 2009 in response to evolving detection technologies and requirements. Recent analyses on actual performance of the current LDN are also summarized. The paper includes short and long term requirements for improving the lightning measurements needed to address some open scientific questions and fill the needs of emerging applications.
International Journal of Electrical and Computer Engineering, Apr 1, 2016
The effect of frequency dependent soil properties on the impedance and transient response of the ... more The effect of frequency dependent soil properties on the impedance and transient response of the grounding electrode was investigated. The frequency dependent soil models as proposed by Scott, Smith-Longmire, and Visacro-Alipio were critically reviewed. A novel method was proposed to integrate the frequency dependent soil electrical properties in the circuit model of grounding electrode. To validate the application of the method in circuit model, the voltage responses of the grounding electrode obtained by the circuitand electromagnetic models were compared. The voltage responses obtained by the circuit and electromagnetic models were in excellent agreement in terms of voltage peaks and wave shapes. The differences between voltage peaks obtained by the circuit and electromagnetic models were found less than 1%.
International Journal of Electrical and Computer Engineering, Apr 1, 2016
This aim of this paper is to study the preliminary breakdown pulses (PBP) of positive cloud-to-gr... more This aim of this paper is to study the preliminary breakdown pulses (PBP) of positive cloud-to-ground (+CG) lightning flashes in Malaysia. There is a difference in PBP characteristics when a comparison is made between positive and negative flashes. To the best of the authors' knowledge, this is the first time the PBP characteristics preceding positive lightning in Malaysia are reported. This study was carried out in Johor Bahru, Malaysia using a broadband antenna system during a monsoon period (Oct-Dec, 2012). It is found that the PBP for +CG lightning is much more complicated compared to the positive cloud-to-ground (-CG) lightning since only negative PBP trains are usually present in the -CG lightning. Out of 21 analyzed positive flashes, four types of PBP trains were identified according to their initial polarities. These are negative PBP (20%), positive PBP (15%), PBP with polarity reversal (10%), and PBP with irregular polarity (15%). The balance 40% of flashes is without any PBP. The effect of geographical region may also cause the observed differences in the cloud discharge processes including the types of PBP. More studies are needed to determine the relationship between the return stroke and PBP.
IEEE Transactions on Electromagnetic Compatibility, 2017
Thispaper proposes an improved CIGRE grounding electrode resistance model by integrating the ener... more Thispaper proposes an improved CIGRE grounding electrode resistance model by integrating the energy balance of soil ionization. The main contribution of the improved CIGRE model is its accuracy to compute the time-dependent grounding electrode resistance under impulse condition. In addition, it can obtain correct resistance-current relation (hysteresis characteristic) of the grounding electrode. The accuracy of the improved CIGRE model is achieved by taking into account the effect of the arc resistance in soil, considering the energy balance of the soil ionization. The improved CIGRE model was validated for low resistivity soils ranging from 40 to 370 Ω•m. The comparison of the computed grounding electrode resistance obtained from the improved CIGRE model and experiment confirms the increase in the accuracy of the resistance and voltage response of the grounding electrode. The grounding electrode resistances obtained from the improved CIGRE model and experiments at current peak and current half value at decaying period (tail) have shown maximum differences of 3.5 and 2%, respectively. Moreover, precise voltage responses of grounding electrodes compared with the experimental results were obtained under different impulse currents.
Distributed Generation Systems, 2017
This chapter is assigned to define a distributed generation (DG) resource. The features and the o... more This chapter is assigned to define a distributed generation (DG) resource. The features and the operation of DG are also explained. The technologies used in DG are also explained. The advantages and disadvantages of DG are given.
Archives of Electrical Engineering, 2016
Grounding electrode resistance non-linearly changes under impulse conditions due to soil ionisati... more Grounding electrode resistance non-linearly changes under impulse conditions due to soil ionisation phenomenon. Several models have been proposed to model soil ionisation for grounding electrodes applications. However, to date, there is yet an attempt made to compile all these works into a comprehensive review article. Therefore, this paper is written with the objective of summarizing all related works in this field as a one- stop reference. With reference to the literature, this paper is written to summarize the working principles of the soil ionisation models as well as the accuracy and performance analysis of the models. This paper, particularly highlights the deficiencies of the available models in terms of accuracy and performance. This knowledge will contribute to the development of a new accurate and efficient soil ionisation model.
International Transactions on Electrical Energy Systems, 2016
This paper proposes a new model of soil ionization, in the form of an equivalent circuit containi... more This paper proposes a new model of soil ionization, in the form of an equivalent circuit containing capacitive and resistive elements representing a network of soil particles and air voids. In previously published models, the effect of soil ionization is only indirectly considered by changing the soil resistivity and the electrode radius. This has resulted in inferior performance of those models in terms of voltage response accuracy. In this work, the energy balance concept was applied to compute the arc resistance. The soil structure is modeled by using two-layer capacitor elements. The soil ionization process is incorporated inside the two-layer capacitor elements using specially derived differential equations. Unlike previous models, a discrete form of soil ionization is used. The accuracy of the proposed equivalent circuit model was confirmed by the small differences between the computed and experimental results.
International Journal of Electrical and Computer Engineering (IJECE), 2016
The effect of frequency dependent soil properties on the impedance and transient response of the ... more The effect of frequency dependent soil properties on the impedance and transient response of the grounding electrode was investigated. The frequency dependent soil models as proposed by Scott, Smith-Longmire, and Visacro-Alipio were critically reviewed. A novel method was proposed to integrate the frequency dependent soil electrical properties in the circuit model of grounding electrode. To validate the application of the method in circuit model, the voltage responses of the grounding electrode obtained by the circuitand electromagnetic models were compared. The voltage responses obtained by the circuit and electromagnetic models were in excellent agreement in terms of voltage peaks and wave shapes. The differences between voltage peaks obtained by the circuit and electromagnetic models were found less than 1%.
International Journal of Electrical and Computer Engineering (IJECE), 2016
This aim of this paper is to study the preliminary breakdown pulses (PBP) of positive cloud-to-gr... more This aim of this paper is to study the preliminary breakdown pulses (PBP) of positive cloud-to-ground (+CG) lightning flashes in Malaysia. There is a difference in PBP characteristics when a comparison is made between positive and negative flashes. To the best of the authors' knowledge, this is the first time the PBP characteristics preceding positive lightning in Malaysia are reported. This study was carried out in Johor Bahru, Malaysia using a broadband antenna system during a monsoon period (Oct-Dec, 2012). It is found that the PBP for +CG lightning is much more complicated compared to the positive cloud-to-ground (-CG) lightning since only negative PBP trains are usually present in the -CG lightning. Out of 21 analyzed positive flashes, four types of PBP trains were identified according to their initial polarities. These are negative PBP (20%), positive PBP (15%), PBP with polarity reversal (10%), and PBP with irregular polarity (15%). The balance 40% of flashes is without any PBP. The effect of geographical region may also cause the observed differences in the cloud discharge processes including the types of PBP. More studies are needed to determine the relationship between the return stroke and PBP.
2015 IEEE Conference on Energy Conversion (CENCON), 2015
This paper describes a study on the characteristics of preliminary breakdown pulses (PBPs) preced... more This paper describes a study on the characteristics of preliminary breakdown pulses (PBPs) preceding negative return strokes. Such characterization is needed due to the lack of available data, especially for negative lightning, in Malaysia. Measurements were carried out in Johor Bahru, Malaysia in October 2014 using broadband antennas and a high resolution Picoscope. It was found that 84% of the negative return strokes (RS) were preceded by PBPs. 70% of the PBPs fit the BIL model and another 30% the BL model. The arithmetic mean of the PBP-RS interval is 13.2ms, which is about four times smaller compared to a similar measurement made during April to June, 2009. The difference may probably be due to a variation in the thundercloud formation which is dependent on the monsoon period in Malaysia. Further studies are required to characterize the PBPs and their relations with the monsoon period and with geographical region.
Applied Mechanics and Materials, 2016
Lightning has been the major cause of power interruptions in Malaysia. Malaysia is located near t... more Lightning has been the major cause of power interruptions in Malaysia. Malaysia is located near the equator having approximately 200 thunder days per year. It is known to be among the highest lightning activity country worldwide. TNB as the main power supply utility in Malaysia has made an effort to reduce the power interruptions via installation of Lightning Detection Network (LDN) in 1994. This paper presents the history leading to modern LDN that capture lightning radiation fields at multiple remote sensors. The network has undergone a series of improvement between 2004 and 2009 in response to evolving detection technologies and requirements. Recent analyses on actual performance of the current LDN are also summarized. The paper includes short and long term requirements for improving the lightning measurements needed to address some open scientific questions and fill the needs of emerging applications.
International Transactions on Electrical Energy Systems, 2015
The soil ionization phenomenon effectively reduces the grounding electrode resistance. The existi... more The soil ionization phenomenon effectively reduces the grounding electrode resistance. The existing methods to determine the soil ionization effect on the transient resistance of a grounding electrode are valid only for homogeneous soils. However, most soil structures are non-homogeneous. Realizing the importance of the issue, this study proposes a hybrid method to incorporate the soil ionization effect on the transient resistance of a grounding electrode located in non-homogeneous soil condition. The electromagnetic field approach is utilized to compute the ionized soil volume around the electrode to determine the equivalent radius of the electrode. A circuit-based method of computing the grounding resistance of the electrode in non-homogeneous soil conditions is proposed based on the per-unit-length distributed parameter method. The proposed method is validated by using an electromagnetic field-based numerical solution for the non-homogeneity effect of the soil on grounding electrode resistance.
IEEE Transactions on Power Delivery, Feb 1, 2015
The behavior of a grounding system can be predicted by using either the electrical equivalent cir... more The behavior of a grounding system can be predicted by using either the electrical equivalent circuit models or electromagnetic computation. Despite its advantages over the latter, the equivalent circuit model fails to accurately predict the behavior under transient conditions due to the absence of two key factors, namely: 1) the current rate-of-rise and 2) soil ionization. This paper proposes a method to enhance the performance of the equivalent circuit model by taking into consideration of both mentioned factors. It is discovered that by using the proposed method, the estimated values of R and L of the equivalent circuit model are improved. The computed inductance dynamically changes with the change in the lightning current parameters, thus improving its accuracy for all current rate-of-rise conditions. The soil ionization effect is implemented as recommended by CIGRE, and this further improves the accuracy of the model. As a result, the voltage response of the model becomes more accurate and comparable to the electromagnetic computation results. Another important feature of the proposed model is that it can be directly applied or connected to power system equipment. Thus, an accurate grounding system effect on the transient performance of key power equipment, such as surge arresters, can be obtained.
Applied Mechanics and Materials, 2016
International Transactions on Electrical Energy Systems, 2015
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Papers by Mehrdad Mokhtari