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110kV降压变电所电气一次系统设计,110,kV,降压,变电所,电气,一次,系统,设计

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European Journal of Scientific Research ISSN 1450-216X Vol.23 No.1 (2008), pp.141-148 EuroJournals Publishing, Inc. 2008 /ejsr.htm Substation System Simulation Models for Transformer Risk Assessment Analysis M. Z. A. Ab Kadir Department of Electrical and Electronics Engineering, Faculty of Engineering Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia E-mail: mzainal.my Tel: +60-389464362; Fax: +60-389466327 M. H. Mohamad Ariff Department of Electrical and Electronics Engineering, Faculty of Engineering Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia R. Mesron Engineering Department (Transmission and Substation), Tenaga Nasional Berhad MPE Building, Petaling Jaya, Selangor, Malaysia M. T. Salahuddin Engineering Department (Transmission and Substation), Tenaga Nasional Berhad MPE Building, Petaling Jaya, Selangor, Malaysia Abstract This paper comprises a study which is carried out to investigate and evaluate the effect of lightning stresses on the 132 kV substation in the way to improve its reliability in the event of active lightning activities. The paper also detailed the modelling parameters of substation for this transient analysis in order to evaluate the performance and to recommend such configuration to optimize its design to be not only to withstand the stresses but to be more cost effective. The modelling and simulation are carried out using one of the most powerful power system simulations tools that is PSCAD-EMTDC and the substation layout design is adapted from 132/11 kV Simpang Renggam Ayer Hitam substation, courtesy of TNB. The model is based on single phase line model as it was suggested by the IEEE to be adequate to represent the substation in transient analysis simulation. The outcome of this paper would be the results of lightning stresses in term of voltage level measured at particular points in substation. The results are then compared with the suggested BIL for assessment of transformer failure. Keywords: Insulation coordination, lightning, basic insulation lightning level, PSCAD/EMTDC 1. Introduction Typically, more than 2,000 thunderstorms are active throughout the world at a given moment, producing in the order of 100 lightning flashes per second (Hileman, 1999). Lightning activity in South Substation System Simulation Models for Transformer Risk Assessment Analysis 142 East Asia, especially in Malaysia, ranks as one of the highest in the world. Tenaga Nasional Berhad Research (TNBR) Malaysia has recorded as high as 320 kA lightning impulse current in Malaysia using their lightning detection network system (LDNS) (TNBR, 2008). Substations can be considered as the most crucial parts in power systems. This is because it consists of such an expensive equipment that is the power transformer which is the essential part to the system to operate as a feeder check point either as stepping up or stepping down the incoming line voltage. There are many studies describing how one could model the substation for transient analysis studies (IEEE Modelling and Analysis of System Transient WG, 1996, IEEE Power Engineering Society, 1999, IEEE Power Engineering Society, 1996, IEC, 1993, Hileman, 1999). The equivalent circuit of the substation is basically modelled by a group of important input parameters to represent the elements inside the substation. Usually for transient analysis studies, parts of transmission line properties must also be included in the overall simulation model such as the transmission line connected to the substation entrance with a few span, phase conductor and transmission line tower because the substation is normally regarded having a perfect shielding and the voltage stresses penetrating inside the substation only via the lines (Savic and Stojkovic, 1994). 2. Simulation modelling The case studies include four cases of investigation concerning the influence of arrester presence in the system. The lightning current was injected at the top of the last tower before entering the substation while voltage level at selected points are observed as the voltage propagate to the substation. Below are the lists of mentioned case studies: 1. With both arresters (SA1 and SA2) are installed 2. Only SA1 is installed 3. Only SA2 is installed 4. Without both arresters The voltage level at four crucial points had been monitored, that are E1 at the substation entrance, E2 at the first surge arrester to protect the capacitive voltage transformer, CVT, E3 at the second surge arrester and finally at the power transformer, E4. Two main locations or points will be observed which are the E2 and E4, where there are transformers located at these points. Figure 1 depicts the substation system modelled for case studies. Figure 1: Substation model for case studies Further of specific details relating to the model are described in Table 1. 143 M. Z. A. Ab Kadir, M. H. Mohamad Ariff, R. Mesron and M. T. Salahuddin Table 1: Key parameters used for modelling the system. Model Sub-Component Details / Reference Lightning Strike Double exponential current source with a varying front time according to the peak current (CIGRE, 1991) and negative polarity. Overhead Line Phase Conductor Single phase conductor, 300 surge impedance, lowest phase conductor at 20 m. Modelled with frequency dependent travelling wave model. Distance of 50 m between tower and substation section. Main Structure Surge impedance of 155 and travelling wave velocity of speed of light modelled with a Bergeron model Tower Tower Footing DC resistance of 10 in a soil resistivity of 100 m. Soil ionisation modelled (Woodford, 1998). Substation Equipments The overall substation model derived from substation layout drawings. The buswork and conductors between the discontinuity points inside the substation, and connections between the substation equipments are represented by line sections. The substation equipments, such as circuit breakers, substation transformers, and isolators, are represented by their stray capacitances to ground as in Table 2. Table 2: Comparison of capacitors value between TNB calculation and IEEE recommendations. Capacitor TNB (calc) IEEE recommendations CCVT 7592.98 pF 8000 pF CISOa 116.12 pF 100 pF CT 385.16 pF 250 pF CB 92.25 pF 100 pF CISOc 116.12 pF 100 pF CBb 82.78 pF 80 pF CB 92.25 pF 100 pF CTx 1485.13 pF 2027 pF Table 2 describes the comparison of capacitor value between TNB calculation approach and IEEE recommendation base on 115 kV US substation system model. For this work, TNB calculation approach of capacitor values was adapted to model the system as it more or less agreed with the value recommended by IEEE WG 3.4.11 (1992) and for the actual analysis. The distance between each substation equipments are as below: Tsub1 =3.0 m Tsub5 = 4.0 m Tsub9 = 14.5 m Tsub2 =3.5 m Tsub6 = 4.5 m Tsub10 = 3.0 m Tsub3 =3.5 m Tsub7 = 3.0 m Tsub11 = 3.0 m Tsub4 =3.0 m Tsub8 = 3.0 m Tsub12 = 5.0 m 3. Surge arrester modelling Several models of arrester had been described elsewhere in literature (IEC, 1993, Martinez, 2004 and IEEE WG 3.4.11, 1992). Most of the arrester model must include two nonlinear resistances A0 and A1 as shown in Figure 2. However for different approach, it is basically using different type of lumped parameter arrangement. The frequency-dependent surge arrester model which was recommended by IEEE WG 3.4.11 (1992) is used in this work. This is because the model was reported as the most accurate representation based on single phase line model (Goudarzi and Mohseni, 2004). Adjustment procedure of parameters is described in by IEEE WG 3.4.11(1992). Substation System Simulation Models for Transformer Risk Assessment Analysis 144 Figure 2: IEEE Frequency-dependent model 4. Results and discussions 4.1 Assessment of transformer failure The transformer BIL according to data given by TNB is 550 kV with 20% of safety margin. Results in Figures 3, 4, 5 and 6 show that the worst case appeared when there was no arrester installed. The probability of failure was as high as 82% compared to its opposite case which the latter appeared to give 0% of probability failure. For the case when there is only one surge arrester is installed to protect the transformer, the best option was to install the protective device near the crucial point. Otherwise, if only SA1 was installed, the probability of transformer failure was reported to have about 10% of failure risk. Figure 3: With both arresters (Case 1) 145 M. Z. A. Ab Kadir, M. H. Mohamad Ariff, R. Mesron and M. T. Salahuddin Figure 4: Only SA1 (substation entrance) is installed (Case 2) Figure 5: Only SA2 (near transformer) is installed (Case 3) Figure 6: Without both arresters (Case 4) Table 3 describes the summary of our case study which the worst case was when there was no arrester installed, since 35 kA of lightning current level had already caused the transformer at point E4 to breakdown. For assessing the best arrester placement, it really depends on the asset to be protected in the substation. If there is only power transformer available to be protected, then the best option is to have the SA2 installed as close as possible to the transformer to ensure the voltage level at the power Substation System Simulation Models for Transformer Risk Assessment Analysis 146 transformer within the safety margin in the presence of unpredicted very high transient current level injection. Table 3: Summary of the case studies for power transformer, TX at point E4. Case Study Breakdown I, kA Prob. of failure E4, kV Case i. 200 0% 259 kV Case ii. 200 10% 517 kV Case iii. 200 Less than 1% 319 kV Case iv. 35 kA 82% 2781 kV 4.2. Case comparison at selected current, I The purpose of this analysis is to observe the difference of voltages level at points E1, E2, E3 and E4 at selected lightning current. For current of 40 kA, the comparison of the voltages values can be seen in Table 4 while Tables 5 and 6 show the results for the lightning current at 80 kA and 120 kA respectively. Table 4: Voltage values at lightning current of 40 kA E1 E2 E3 E4 Case 1 759 216 204 208 Case 2 759 223 284 291 Case 3 759 306 220 225 Case 4 759 657 688 692 In the case of no surge arresters installed at substation (vandalism or failed to operate), this current has already caused the transformer damage due to the maximum voltage level that exceeded the suggested BIL of 550 kV. Table 5: Voltage values at lightning current of 80 kA E1 E2 E3 E4 Case 1 1321 228 220 231 Case 2 1321 239 397 407 Case 3 1321 408 240 245 Case 4 1321 1222 1278 1282 While in Table 5, case 1 and 2 are still in safe condition at lightning current of 80 kA. This shows that SA1 is very important for helping discharging the lightning voltage at the entrance of the substation and protect the equipments in the substation section. Failure in operation of SA1 will cause a significant effect in terms of voltage level up at point E2 (if the current goes high), as demonstrated in the next result. Table 6: Voltage values at lightning current of 120 kA E1 E2 E3 E4 Case 1 1794 235 226 241 Case 2 1794 256 452 471 Case 3 1794 498 261 274 Case 4 1794 1741 1818 1821 Result in Table 6 shows the voltages level at lightning current of 120 kA. Whilst the voltage level on case 1 and 2 are below the BIL value, the voltage at point E2 increases as the lightning current 147 M. Z. A. Ab Kadir, M. H. Mohamad Ariff, R. Mesron and M. T. Salahuddin increases to 120 kA, even though they are not exceeding the BIL value of the transformer. For case 4, the equipments especially the transformers were already damaged by overvoltage caused by the lightning impulse. 4.3 Result comparison for case 4 Figure 7 shows the I-V characteristic for case 4. The graph is clearly shown that without SA1 and SA2, low lightning current can already caused damage to the transformers and the value is around 25 to 35 kA for all points. Therefore, the utility should be concerned and aware of the surge arresters installation, which is used to protect the transformers in the substation section. More care and cautions are needed to avoid the damage and vandalisms which may affect the performance of transformers and other equipments in the substation section. Figure 7: I-V characteristics graph for case 4 I-V characteristic02040608010012014016018020022003006009001200150018002100240027003000Voltages, E max (kV)Lightning C urrent, I (kA ) E 1maxE 2maxE 3maxE 4max 5. Conclusion The modelling concept and parameter selection to model a substation were presented for the simulations of substation performance in order to investigate the failure probability of crucial component in the substation. The validation of the result was earlier compared with previous work done by Savic and Stojkovic to make sure the model was valid. The results showed that the default arrangement of TNB substation (Case 1) was well protected, but the cost for implementation of such arrangement is high. The placement of arrester is crucially needed in order to optimize the substation performance in term of its reliability and cost effective. The arrester is best suited for optimized performance if it is installed near to the crucial point compared to the substation entrance. In this case, it is very important to have this surge arrester as close as possible to the equipment to be protected. Substation System Simulation Models for Transformer Risk Assessment Analysis 148 References 1 Hileman, A. R., 1999. Insulation coordination for power system. 1st ed, Marcel Dekker Inc., New York. ISBN: 0824799577 2 TNBR, 2008. Lightning Detection Network