Evaluation of a 132 KV Transmission Line Performance

European Journal of Scientific Research ISSN 1450-216X Vol.29 No.4 (2009), pp. 533-539 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm

Evaluation of a 132 kV Transmission Line Performance via

Transient Modelling Approach

M. Z. A. Ab Kadir Department of Electrical and Electronics Engineering, Faculty of Engineering

Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia E-mail: [email protected]

Tel: +60-389464362; Fax: +60-389466327

J. Sardi Department of Electrical and Electronics Engineering, Faculty of Engineering

Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

W. F. Wan Ahmad Department of Electrical and Electronics Engineering, Faculty of Engineering


H. Hizam Department of Electrical and Electronics Engineering, Faculty of Engineering


J. Jasni Department of Electrical and Electronics Engineering, Faculty of Engineering


Abstract

The paper presents the transient modelling technique used in PSCAD/EMTDC to model a 132 kV transmission line for analyzing the lightning performance in terms of critical backflashover current, backflashover rate (BFR) and probability of the transformer damage. For the purpose of simulation, a 132 kV Kuala Krai-Gua Musang line in Malaysia is chosen based on its poor performance and data for that line is obtained from the Tenaga Nasional Berhad (TNB). The effect and variations in results will be observed as the footing resistances and soil resistivities of the towers are varied and these results will later be compared to the actual data and other transient software. Keywords: Insulation coordination, backflashover, transient modelling, PSCAD/EMTDC,

lightning performance 1. Introduction Lightning interference occurs mainly on overhead lines and has been a problem since the earliest days of the electricity supply industry. Overvoltages which occur on the lines, travel toward the terminal or substation, and can cause damage, particularly to expensive equipment such as transformers. In view of

Evaluation of a 132 kV Transmission Line Performance via Transient Modelling Approach 534

their importance, cost and the difficulty of making internal repairs, the protection of large transformers against lightning overvoltages is usually given special consideration. In all cases the most protection is obtained by connecting the protective device as closely as possible to the transformer terminals and joining the earth to the transformer tank with the minimum impedance.

Backflashover occurs when lightning stroke terminates on the overhead ground wire or tower. A stroke that so terminates forces currents to flow down the tower and out on the ground wires. Thus voltages are built up across the line insulation. If these voltages equal or exceed the line critical flashover voltage (CFO), flashover occurs [1].

Study on backflashover is very important to evaluate lightning performance as majority of lightning strokes terminate on shield wire than phase conductor [2-5]. Backflashover analysis was done to 132 kV overhead transmission line from Kuala Krai to Gua Musang through rural area of Kelantan. This line was chosen as it is the worst line performance in peninsular Malaysia and has high ground flashes density. Lightning Detection System Lab (LDS), TNB Research records from Jan 2004 – July 2007 indicate that the average ground strokes densities of the area on which the line route range from 6 to 20 strokes/km2/year while the mean multiplicity of lightning strokes observed is 3 [6].

This paper presents the analysis of the line performance, in terms of backflashover rate (BFR), critical backflashover current and probability of the transformer damage at the substation. The effect and variations in results will be observed as the footing resistances and soil resistivities of the towers are varied from the actual recorded values to the average values taken by taking into account the area topography for Kuala Krai-Gua Musang line. The results are then compared with the actual tripping records and the existing simulation results obtained using TFLASH-EPRI software. 2. Brief Descriptions of Kuala Krai - Gua Musang Transmission Line Details of the 132kV Kuala Krai – Gua Musang line and tower configuration (Figure 1) are shown below, respectively [6]. Table 1: Line details

ITEM DETAILS No of towers 295 Line length (km) 112.81 Conductor Data 1 x 300mm2 Batang Insulator Data 14 disc x 146mm Line sag (max) 8.89m Average Ground Flash Density 4 flashes/km2/year No of Tripping (Jan2004 – July2007) 13 Backflashover Rate 4.19 flashes/100km/year Ground Resistance (Ohm) 2 - 558 Soil Resistivity (Ohm-m) 200 - 3000 Number of shield wires 2

535 M. Z. A. Ab Kadir, J. Sardi, W. F. Wan Ahmad, H. Hizam and J. Jasni

Figure 1: Tower configurations and dimensions

4.32m

3.66m

3.66m

18.9

5.7m

8.84

3. Modelling for Backflashover Analysis A. Overhead Transmission Lines

Overhead transmission line is modelled using Frequency Dependent (Phase) Model which uses curve fitting to duplicate the frequency response of a line or cable. It is the most advanced time domain model available as it represents the full frequency dependence of all line parameters (including the effect of a frequency dependent transform). It is useful for studies wherever the transient or harmonic behaviour of the line or cable is important [7]. Each span is represented by a multiphase untransposed line model and the phase conductor and shield wire are explicitly modelled between towers [2]. B. Transmission Tower

Tower is modelled using several segments of single conductor distributed parameter model or Bergeron model [2]. The surge impedance of the transmission line tower and the tower travel time of wave propagation down the tower are required. Surge impedance for each tower in the Kuala Krai – Gua Musang line is given by TNB and its value range 100 – 200 ohm. For each case of tower structures, the travel time from tower top to ground can be estimated as

ch

=τ (1)

Where h is height of the tower in and c is the speed of light, 3 x 108 ms-1 C.Tower Footing Resistance

Tower footing is determine using current dependence of tower footing resistance given by [4]

g

ot

II

RR+

=1

(2)

being Ro the footing resistance at low current and low frequency, Ig the limiting current to initiate sufficient soil ionization, I the stroke current through the resistance. The limiting current is given by


221

o

og R

EI ρπ×= (3)

where ρ is the soil resistivity (ohm-m) and Eo is the soil ionization gradient (≈ 300kV/m). D. Insulator Coordination gap Flash Model

The comprehensive analyses of the discharge development have confirmed that, discharge development always consists of three different phases: corona inception, streamer and leader propagation. Time to breakdown tc can be expressed as a sum of three components:

tc = ti + ts +tl (4) where ti describes the corona inception time, ts the time the streamers need to cross the gap or to meet the streamers from the opposite electrode, and tl the leader propagation time. As corona inception voltage is far below the breakdown voltage and taking into account the high rate of rise of the applied voltage, corona inception time can be neglected without introducing large errors [3]. For streamer propagation time:

95.025.11

50

−⎟⎟⎠

⎞⎜⎜⎝

⎛=

EE

ts

(5)

where E is the maximum gradient in gap before breakdown (kV/m) and E50 is the average gradient at CFO voltage.

For leader propagation time

( ) ( )⎥⎦

⎤⎢⎣

⎡−

−= 0E

LgtVtKV

dtdL (6)

being V(t) the voltage across gap (kV), L the leader length (m), g the gap length and constants K and E0 for air gaps, post insulators, negative polarity lightning are equals to 1.0 m2/kV2sec and 670 kV/m. E. Lightning Stroke

Lightning stroke is represented by a current source of negative polarity. The peak current is statistically related to the steepness or time to crest of the current waveform. The steepness increases as the peak current increases, however, the front time increases with peak current [4]. F.Simulation Details

Figure 2 shows the last 10 towers model with its span length between towers for Kuala Krai – Gua Musang line. The last tower connected to a capacitor which represents Gua Musang substation. Tower 10 is terminated with a matching impedance to avoid reflections. Ground resistance and soil resistivity for tower 1 and tower 6 from Gua Musang substation are shown as per Figure 2. Simulation is done by injected a set of lightning current (0-200kA) to tower 1 and 6 at different cases. Maximum voltage level at substation is then recorded for each value of a stroke.


Figure 2: Simulation for backflashover analysis

Z T1T2T3T4T5T6

T7T8T9 T10

50m

296m400m 190m745m180m 720m440m 260m 680m

109km

Case 1 Case 2

C

R=558P=3000

R=47.83P=3000

V

Since the BIL of the transformer at substation (132 kV) is 550 kV in each case, the probability of the transformer damage can easily be known by comparing this value of backflashover rate (BFR) and the voltage level recorded. The BFR for each case is then calculated using equations below [5]:-

(7)

(8)

⎟⎟⎠

⎞⎜⎜⎝

⎛=

1028 6.0hNN gS (9)

where BFR = backflasover rate, flashes/100km-year Ng = ground flash density, flashes/km2/year h = tower height, m Ic = crictical current, kA

4. Results and Discussions As shown in Table 2, BFRs for both cases seem in good agreement with the actual BFR that was recorded by TNB. This is particularly true when compared to another existing simulation results computed by TFLASH-EPRI program for the same line. It is expected that the footing resistance of the tower will help in increasing the withstand capability and possibility of breakdown failure and thus increase the critical current recorded at the tower. However, because of shorter span length between tower 1 and substation, the effect of proper substation grounding has strongly influenced and affected the tower 1 top voltage and its critical current. This is more than the effect of the footing resistance at tower 1 itself. For this reason, tower 1 top voltage is decreased which then also increased the critical current and decreased the BFR. These results are in good agreement obtained by other workers [8-9].


Table 2: Comparison Of BFR and critical current between simulation (PSCAD and TFLASH) and actual data

a. Actual Values of Footing Resistances and Soil Resistivities IC(kA) BFR Vmax at substation (kV) Probability of transformer damages, %

Tower1 80 4.1 601.81 63 Tower6 69 5.8 1168.78 67 Actual Data - 4.19 - - TFLASH - 6.5 - -

b. Average Values of Footing Resistances and Soil Resistivities IC(kA) BFR Vmax at substation (kV) Probability of transformer damages, % Tower1 126 1.33 868 42.5 Tower6 79 4.22 1282 60.7 Actual Data - 4.19 - - TFLASH - 6.5 - -

Probability of transformer damages compute for both cases are high as tower 1 and tower 6

have high ground resistances coupled with high soil resistivity. Although tower 6 is far from substation, it gives higher voltage at substation entrance and probability of transformer damages than tower 1. This also because of strong effect from proper substation grounding which reduces tower 1 top voltage and probability of transformer damages. 5. Conclusion The tower footing resistance is an extremely important parameter in determination of lightning flashover rates. The peak overvoltage occurring at the top of the tower is partly determined by the apparent tower footing resistance. This is because the voltage reflection from the tower base will arrive sooner at the tower top than reflections from the adjacent towers after return of the first reflection. The lower the tower footing resistance, the more negative reflections produced from the tower base towards the tower top and these hence help to lower the peak voltage at the tower top.

The influence of the tower footing resistance on the tower top voltage is determined by its value which is also surge current dependent. The larger the surge current, the smaller the tower footing resistance and the more negative reflection produced. 6. Acknowledgement The author wants to express her sincere gratitude to the electrical engineers of the Tenaga Nasional Berhad (Transmission and Substation) for their kind supply of various technical data and cooperation.


References [1] A. R. Hileman, Insulation Coordination for Power System (Marcel Dekker, Inc., 1999) ISBN:

0824799577. [2] IEEE Modelling and Analysis of System Transients working Group, “Modelling guidelines for

fast front transients,” IEEE Transaction on Power Delivery, vol. 11, pp. 493-506, 1996 [3] CIGRE, “Guide to procedures for estimating the lightning performance of transmission lines,”

CIGRE Brochure 63, 1991 [4] T.Irwin and H.M Ryan, “Transmission and Distribution: Part 2,” in High Voltage Engineering

and Testing: IEE Power Series 17, Edited by H.M Ryan, Second Edition ed, IEE,1998 [5] IEEE Power Engineering Society, “IEEE guide for improving the lightning performance of

transmission lines,” IEEE Std. 1243-1997, 1997 [6] N. Abdullah, Lightning Performance Analysis of 132kV Kuala Krai-Gua Musang and 275kV

Kg. Awah-Paka Transmission Lines’, Draft Report, TNBR, August 2007 [7] Woodford, D “Introduction to PSCAD/EMTDC V3,” Manitoba HVDC Research Centre Inc,

2001. [8] J.A Martinez, F. Castro-Aranda, “Modelling overhead transmission lines for line arrester

studies”, IEEE Power Engineering Society General Meeting, 2004 [9] M.Z.A Ab Kadir, “Improved coordination gap model for insulation coordination studies”, PhD

Thesis, University of Manchester, UK, 2006.

Evaluation of a 132 KV Transmission Line Performance

Documents

Transcript of Evaluation of a 132 KV Transmission Line Performance