evaluation of the corona phenomenon and grounding system...

Vol. 5(18) Special Issue, Dec. 2015, PP. 2585-2599

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Article History:

IJMEC DOI: 649123/10176 Received Date: Jul. 03, 2015 Accepted Date: Nov. 18, 2015 Available Online: Dec. 29, 2015

Evaluation of the Corona Phenomenon and Grounding System Impact on

the Lightning Waves Propagation by Using EMTP-RV

Ebadollah Amouzad Mahdiraji and Nabiollah Ramezani

Islamic Azad University of Sari Branch, University of Science and Technology of Mazandaran

*Corresponding Author's E-mail: [email protected], [email protected]

Abstract

n this paper, the behavior of the grounding system at different frequencies with considering soil

ionization effect has been investigated under lightning impulse with and without the corona

phenomenon. Also using EMTP-RV software, a model of corona with two different grounding

systems was constructed at the UHV alternating current transmission line and the effect of soil ionization

changes and corona pulses were shown in the voltage signals and it was found that the corona

phenomenon causes voltage signal attenuation and damping and increasing the wave front time. Also it

has been shown that the high-frequency grounding system has a higher validity than the low-frequency

grounding system. The induced voltage and charged current on the transmission line of power system

has been studied. This study is performed to show the positive impact of corona and grounding system

with soil ionization on induced voltage.

Keywords: Transient Modeling, corona phenomenon, lightning impulse, grounding system, soil

ionization.

1. Introduction

According to statistics, lightning is the most important cause of transmission lines Trip in power system [19]. Studies show that lightning has many impacts on resistance level of transmission lines and waveforms which are created by lightning strike caused a high voltage level. Due to the frequency dependence parameters of the transmission line, corona phenomenon can play an important role in the process of electromagnetic transition [20]. The impact on the wave’s process is increasing more and more by increasing the voltage level [7]. Electromagnetic environment is associated with the features of transmission line corona such as electric discharges of Corona, Corona losses, magnetic field, electric field, noise and distortion [18]. Additionally, the effects of the lack of corona consideration were shown in increasing the price of electrical equipment. Due to high slope and domain, Lightning causes a sharp increase in grounding potential in drain or hitting the Earth. The increase in ground potential could have heavy human and financial risks. The increase of the grounding potential is a function of the behavior of lightning transient waves and transient characteristics of the grounding systems. Lightning grounding system consists of three main parts: metal conductors (Middle conductors) that guide lightning current into the grounding electrode, metal electrodes buried in the soil, and grounds around the electrodes [5, 13, 16]. A proper grounding system is needed in power systems for better protection against lightning strikes and its electromagnetic effects and it is necessary to identify the transient behavior of the grounding system in order to reduce the harmful effects caused by the direct hit of lightning strikes and electromagnetic effects caused by indirect lightning strike. Firstly in this paper, methods of modeling the grounding system were introduced in

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Ebadollah Amouzad Mahdiraji et al. / Vol. 5(18) Special Issue, Dec. 2015, pp. 2585-2559 IJMEC DOI: 649123/10176

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International Journal of Mechatronics, Electrical and Computer Technology (IJMEC)

Universal Scientific Organization, www.aeuso.org PISSN: 2411-6173, EISSN: 2305-0543

high frequency and transient behavior of the grounding system is described in the transient state for different electrodes. In the simulation part of grounding system models, power transmission tower has been implemented and the grounding terminal voltage difference was shown in different models [2, 9, 10]. In this paper, Corona impulse is not only studied issue during lightning impulse to transmission line but also the grounding system design with considering soil ionization and its impact on the existing signal functions of UHV transmission line is intended to improve the current and voltage level.

2. THE EFFECTS OF THE CORONA AND CONDUCTED RESEARCH 2.1. Q-V curve profile

If the waves in the UHV transmission line to face increasing voltage amplitude, the field surrounding the air around the transmission line will be deflected and ionization will occur around transmission line. Due to this, corona phenomenon will occur and charging increase will result in increasing the radius of the conductor which is shown in Figure 1. Increasing the radius due to corona will cause wave impedance reduce but reciprocal wave impedance in conductors will be remained unchanged. Increasing the radius of the conductor increases the capacitance and line-to-ground conductance as well as reducing the coupling coefficient. Corona has no effect on the line conduction and inductance parameters will be remained unchanged [12].

Figure 1: Capacitance equivalent circuit.

Corona Charge-voltage curves study according to the study done on wave attenuation. Charge-Voltage wave features means transient waves’ process and its relationship with the inception voltage, total electrical charging is by the corona in conductors. Charge-voltage curve is shown in Figure 2. OA is the part of the curve which shows linear behavior and the slope of this part of the curve is named conductor geometric capacitance .conductor to be addressed. AB is part in which the corona phenomenon spreads and curve to its non-linear behavior and shows nonlinear behavior curve and will be called the slope of the conductor’s dynamic capacitance curve. BC section shows the end of the wave and is parallel to OA [12].

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Figure 2: Charge-voltage curve profile with Corona.

2.2. Corona strike equivalent circuit Studies show that conductance can be ignored under lightning voltage in the corona atmosphere [3]. Corona equivalent circuit of UHV transmission line is shown in Figure 3.

Figure 3: Corona equivalent circuit of the three-phase transmission line.

C22, C23 and C24 in figure (3) are coupling capacitance between phases in the corona model.

In the proposed model C14, C17 and C26 are the capacitance between the line and the Corona model and C15, C18 and C21 are the capacitance between Corona model and the ground. In simulation, L and R are supposed to be series. DC5, DC6 and DC7 are the corona inception voltages. Before transmission line voltage reaches to corona inception voltage, diode is turned off and does not conduct but when transmission line voltage is higher than inception voltage, diode is turns on and conducts. In this simulation Corona model is added to the transmission line each 100 meters. Figure 4 shows a section of transmission line with corona model. According to corona calculation



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parameters were shown and Corona constant loses and also L = 0.064 mH and R = 16 Ω were considered [12].

Figure 4: Calculation model for transmission line of corona.

3. GROUNDING SYSTEM MODELS AT LOW AND HIGH FREQUENCIES 3.1. Grounding system model in low-frequency In the study of the behavior of the electrical grounding system in low frequency, usually only static analysis is done that led to the known resistance formulation for Earth resistance. For a vertical electrode, equation (1) and equation (2), have been proposed to obtain the nonlinear resistance of the ground rod and Ig [1, 4, 6, 14, 17]:

RT (t) = 𝑅0

√1+𝑖(𝑡)

𝐼𝑔

(1)

Ig = 𝐸𝑜 𝜌

2𝜋𝑅02 (2)

In these relationships, RT(t) is the nonlinear resistance of the ground rod, R0 is the tower footing resistance at low current and low frequency, i(t) is the current through the rod and E0 is the critical electric field intensity (about 300KV/m) [1]. Table 1 is the soil and Ground resistance according to [3]:

Table 1: Ground and soil resistance at UHV tower.

> 2000 ≥100 Soil resistance /Ω) m)

30 10 Ground Resistance (Ω)

3.2. Grounding system model in high-frequency In general, the analyzing methods of high-frequency model of the grounding system and its dynamic behavior in the discharge of lightning current is divided into three categories of Circuit theory based method , transmission line theory method and electromagnetic field theory method. Circuit Theory based method with vertical electrode and considering soil ionization are used in this paper. In [1, 15] a simple method as shown in figure .5 have been proposed for modeling the Grounding system at high frequencies. In this method, it is assumed that the longitudinal flow is distributed uniformly in the electrode. In this study, rod diameter is 16 mm and the length of copper rod is 2 m as well as the burial depth is 0.5 meters.



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Figure 5: Ground electrode impedance model with nonlinear controlled resistance.

In this circuit, is determined with the same equations (1) and (2) and equation (3) is used to calculate the vertical electrode capacitance:

(3)

So that is the soil permeability coefficient. Also, by increasing the depth of the vertical electrode, inductance value slightly decreases. So for vertical electrodes at different depths, inductance value is approximately equal. To calculate the inductance of a vertical electrode, equation (4) is proposed:

(4)

4. DESIGN MODEL COMPONENTS

In this simulation, a 1600 meters transmission line, 5 towers with 400 meters span and 1000 KV power supply at a distance of 1.5 km is considered.

4.1. UHV transmission line model Different Frequency parameters of transmission line is mostly relevant to the process of magnetic transients state caused by lightning strike, therefore FD LINE model is used in this simulation. Transmission line features consists of three two-bundle conductors and the bundle space is 400mm and conductor radius is 30 mm. The coefficient of temperature relative density is (δ=1 ) and conductor surface roughness coefficient is (m=0.82) and also guard wire have a radius of 16mm. Bends height in the line is 17m and 15m in wire guard [8, 11]. 4.2. Lightning current model The double exponential function 2.6/50µs was used in this lightning current simulation. The modeled lightening current source was considered 200 KA.

1)4

ln(

2)(

a

l

lFC

1

4ln

2)(

a

llHL



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Figure 6: Lightning strike model overhead guard wire.

4.3. UHV tower model Figure 7 depicts the Multi-surge impedance model in UHV alternating current transmission line with attenuation constant of (γ=0.7 ) and Damping coefficient (∅=1 ) according to [8, 11] which is used to this research.

Figure 7: Transmission system Multi-surge impedance model.

4.4. Insulator string flashover model Insulator string flashover phenomena are simulated with a switch in EMTP/RV software that consists of input and output signal. Electric discharge voltage of insulation string is considered 5MV.



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Figure 8: Insulator string flashover model.

5. ANALYSIS AND SIMULATION RESULTS According to Corona and grounding system models, simulation was carried out in 1000 KV Ultra high voltage alternating current transmission line and the impacts on emissions of the induced voltage and charged current considering soil ionization on transmission line was evaluated. In this simulation attenuation and damping analysis of the signal voltage caused by a 200 KA lightning strike to the guard wire was done with and without corona phenomenon and soil ionization in the low frequency grounding system and high frequency grounding system are done.

5.1.1. The analysis of induced voltage on the UHV transmission line considering the grounding system and soil ionization in the absence of corona Lightning impulse is shown from 0.01ms because the latency in Lightning data is considered 0.01ms. The evaluation of induced voltage was investigated in transmission line. Figure 9 shows the voltage of power system transmission line with low-frequency grounding system and soil ionization in 10Ω resistance in the absence of corona. For example, at a distance of 100 m from the point of lightning strike, the voltage amplitude in low-frequency grounding system with 10Ω resistance is about 2.5 MV. The voltage amplitude in high-frequency grounding system with soil ionization at the distance of 100 m from the point of lightning strike is about 2.59 MV.

Figure 9: Induced voltage on transmission line with low-frequency grounding system and soil ionization in

R=10Ω without corona.



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Figure 10: Induced voltage on transmission line with high-frequency grounding system and soil ionization

without corona.

5.1.2. The analysis of induced voltage on UHV transmission line considering corona, grounding system and soil ionization In the second part of the analysis of transmission line induced voltage, the corona phenomenon was considered. The results obtained of the simulation show this phenomenon has a significant impact on transient waves damping, and this will save the purchase of electrical equipment. Figure 11 shows the voltage of UHV transmission line in low-frequency grounding system with soil ionization in resistance of 10Ω and figure 12 shows the inducing voltage of transmission line in high-frequency grounding system with corona and soil ionization. In low-frequency grounding system, voltage amplitude at a distance of 100 m from the lightning strike is about 2.07 MV and in high-frequency grounding system, voltage amplitude is about 2.12 MV.

Figure 11: Induced voltage on transmission line with low-frequency grounding system, soil ionization in R=10Ω

and corona.

Figure 12: Induced voltage on transmission line with high-frequency grounding system, soil ionization and

corona.



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5.1.3. The analysis of induced voltage on the UHV transmission line considering the grounding system and in the absence of corona and soil ionization In the third section of the analysis of UHV transmission line induced voltage, the soil ionization and corona was not considered. Figure 13 shows the voltage of UHV transmission line in low-frequency grounding system without soil ionization and corona phenomenon in resistance of 10Ω. Figure 14 shows the voltage of UHV transmission line in high-frequency grounding system without corona and soil ionization. For example at the distance of 100 m from the point of lightning strike, the voltage amplitude in low-frequency grounding system with 10Ω resistance is about 2.60 MV and in high-frequency grounding system is about 2.64 MV.

Figure 13: Induced voltage on transmission line with low-frequency grounding system in R=10Ω without corona

and soil ionization.

Figure 14: Induced voltage on transmission line in high-frequency grounding system without corona and soil

ionization.

5.1.4. The analysis of induced voltage on UHV transmission line considering corona, grounding system without soil ionization In the next part of the induced voltage UHV transmission line analysis, Figure 15 shows the voltage of transmission line with low-frequency grounding system and corona in the absence of soil ionization and figure 16 shows the voltage of transmission line in high-frequency grounding system and corona phenomenon without soil ionization. Voltage amplitude in figure 15 at the distance of 100 m from the point of lightning strike is about 2.13 MV and voltage amplitude in figure 16 is about 2.16 MV.



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Figure 15: Induced voltage on transmission line with low-frequency grounding system and corona in R=10Ω

without soil ionization.

Figure 16: Induced voltage on transmission line in high-frequency grounding system and corona without soil

ionization

5.2.1. The analysis of UHV transmission line current in grounding system and soil ionization in the absence of corona

In another analysis of power system components analysis, lightning strike is shown at different distances from the point of collision in the absence of corona. For example, at a distance of 100 m from the point of lightning strike, in the low-frequency grounding system and soil ionization with resistance 10Ω, provides a charging about 500 A and in high-frequency grounding system with soil ionization charging is about 527 A.

Figure 17: Transmission line current with low-frequency grounding system and soil ionization in R=10Ω without corona.



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Figure 18: Transmission line current with high-frequency grounding system and soil ionization without corona.

5.2.2. The analysis of UHV transmission line current in grounding system, soil ionization and the corona In the second part of the analysis, power system transmission line current was studied with corona phenomenon and soil ionization. Figure 19 shows the provided current in power system transmission line with low-frequency grounding system and soil ionization in 10Ω resistance. In 10Ω resistance current amplitude is 530 A. Figure 20 show the current provided in Ultra high-voltage transmission line in high-frequency grounding system with corona and soil ionization. The current amplitude at a distance 100 m from lightning strike is 523 A. It is known that corona phenomenon increases the current and charging in the UHV power system transmission line.

Figure 19: Transmission line current with low-frequency grounding system, soil ionization in R=10Ω and

corona

Figure 20: Transmission line current with high-frequency grounding system, soil ionization and

corona.

5.2.3. The analysis of UHV transmission line current with grounding system in the absence of corona and soil ionization

In the third section of the UHV transmission line current analysis, figure 21 shows the transmission line current with low-frequency grounding system in 10Ω without corona and soil



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ionization. Figure 22 shows the transmission line current with high-frequency grounding system in the absence of corona and soil ionization. In the figure 21, current amplitude at a distance 100 m from the lightning strike is 527 A and in the figure 22, current amplitude is 543 A.

Figure 21: Transmission line current with low-frequency grounding system in 10Ω without corona and soil

ionization.

Figure 22: Transmission line current with high-frequency grounding system without corona and soil

ionization.

5.2.4. The analysis of UHV transmission line current in grounding system and corona in absence of soil ionization In this section, transmission line current without soil ionization has been analyzed. Figure 23 shows the transmission line current with low-frequency grounding system in R=10Ω and corona without soil ionization. Figure 24 shows the transmission line with high-frequency grounding system and corona without soil ionization. For example in the figure 23, current amplitude at a distance 100 m from the lightning strike is 543 A and current amplitude in the figure 24 is about 531 A.

Figure 23: Transmission line current with low-frequency grounding system in R=10 and corona

without soil ionization.



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Figure 24: Transmission line current with high-frequency grounding system and corona in the absence of soil ionization.

5.3. Evaluation of the simulation results According to the analysis of UHV transmission line electromagnetic transient modeling,

corona phenomenon causes induced voltage attenuation but this phenomenon increases the transmission line current. Soil ionization has a significant impact on the earth resistance and causes current and voltage are damped. Conducted analysis showed, the attenuation in low-frequency grounding system is more than high-frequency grounding system. In Tables 2 and 3, shows the simulation analysis in low and high frequency grounding systems at a distance 100 m from the lightning strike.

Table 2: Simulation analysis in low-frequency grounding system.

With Soil ionization

Without Soil ionization

2.50 MV 2.60 MV Voltage amplitude Without corona

2.07 MV 2.13 MV Voltage amplitude With corona

500 A 527 A Current amplitude Without corona

530 A 543 A Current amplitude With corona

Table 3: Simulation analysis in high-frequency grounding system

With Soil ionization

Without Soil ionization

2.59 MV 2.64 MV Voltage amplitude Without corona

2.12 MV 2.13 MV Voltage amplitude With corona

527 A 543 A Current amplitude Without corona

523 A 531 A Current amplitude With corona



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CONCLUSION

In this paper, lightning impulse transient modeling associated with corona model and the grounding system in soil ionization with high and low frequency were analyzed in EMTP-RV software and it was found that the corona phenomenon and soil ionization causes voltage signal attenuation and Damping and increasing wave front time. This is the positive aspect of the corona and grounding system with soil ionization in electromagnetic transient analysis and improving the cost-saving in electrical equipment. On the other hand, in this simulation, the grounding system was studied with two, low and high frequency model. It was found that the high frequency grounding system is more valid than low frequency grounding system.

This study showed that taking into account the corona phenomenon and grounding system with ionization in the analysis of electromagnetic reduce the amplitude of voltage signals, which causes the flashover and back flashover not happen also in the high-frequency grounding system model, soil ionization only be effective on the earth resistance and does not affect the rest, such as capacitors and inductors therefore the effect of the soil ionization in the high-frequency grounding system is less than low-frequency grounding system. Conducted analysis showed that corona phenomenon increases the charging on the power system transmission line but the grounding system with soil ionization causes current signal attenuation.

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