LIGHTNING OVERVOLTAGES AND THE QUALITY OF SUPPLY: A CASE STUDY OF A SUBSTATION

Transcription

1 LIGHTNING OVERVOLTAGES AND THE QUALITY OF SUPPLY: A CASE STUDY OF A SUBSTATION Andreas SUMPER sumper@citcea.upc.es Antoni SUDRIÀ sudria@citcea.upc.es Samuel GALCERAN galceran@citcea.upc.es Joan RULL rull@citcea.upc.es Daniel BERNADÓ Dirección de Explotación y Calidad de Suministro Calidad de la Explotación FECSA-ENDESA-Spain dbernado@fecsa.es Beatriz RAFOLS Dirección de Explotación y Calidad de Suministro Calidad de la Explotación FECSA-ENDESA-Spain brafols@fecsa.es 1 INTRODUCTION The main source of transient overvoltages on utility systems is lightning flashes, which cause important transient overvoltages. The consequences of these transient overvoltages, as well known, are insulation stress, short interruptions, voltage dips due to tripping, and finally, long interruptions due to permanent faults. This results in power quality problems for the customer and damage or lifetime reduction of the substation equipment. In order to protect the substation equipment from the overvoltages it is necessary to apply surge arresters. For the right application of surge arresters it is also necessary to study the interaction between the arresters and the other station equipment and the power line. In the following paper we will discuss a realised case study of a 220kV / 25 kv substation in semi-rural area in the north-east of Spain. At fist, the statistical data of power interruptions are analyzed to discover potential for improvements and, at last, the results of one of the realised simulation are shown in the second part of this article. 2 GENERAL 2.1 Typical network configuration The typical configuration of a substation in the northeastern region of Spain is represented in figure 1. The studied substation is connected to the transportation network by two 220 kv lines. There are two levels of distribution voltages, namely 66 kv and 25 kv rated voltage. The 66 kv distribution network serves to supply a local chemical industry by cable, and in this case, it will not be part of the study. The 25 kv distribution network is used to supply the semirural area nearby the substation by cable and is followed by an overhead line (figure 2). In this figure we can observe that the substation is connected to 8 distribution lines by an underground cable, which connects the substation with a disconnector (cable section 2). The length of this section is between 20 and 45 meters. The length of the cable connection between the disconnector and the overhead line (cable section 1) reaches between 35 and 263 meters. The poles supporting the overhead line are usually made of steel with a medium ground impedance of 12 Ω. The 25 kv level is used in this region in general for the distribution, which is not a standard rated voltage established by the IEC. This means that the equipment used in 25 kv substations and lines has to be the highest voltage for the equipment U m of 36 kv (1 kv BIL), which means an overinsulation of the equipment in relationship with the rated voltage. Furthermore, the switching events of two capacitor banks with each 5,4 MVA should be considered. In this case, the transient overvoltages due to capacitor switching were determinated by numerical simulation and are not considered as critical. 220 kv 0/60/40 MVA 66 kv 25 kv 25 kv Figure 1: Configuration of the studied substation Figure 2: Configuration of the studied 25 kv line 2.2 Lightning as origin of transient overvoltage As mentioned, lightning is the main source of transient overvoltages. Due to the lack of shielding, the overvoltages can be divided into overvoltages due Lines 40 MVA 5,4 MVA 5,4 MVA kv Cable Sect. 1 Cable Sect. 2 Substation 3 CEA_Sumper_A1 Session 2 Paper No 2-1 -

2 to direct lightning and overvoltage due to indirect lightning. According to [1] the number of direct strokes per year can be evaluated by the following formula: N d = K N ( b +,5 H where: g ) 1 (1) N g is the ground flash density H is the average height of the line b is the horizontal distance between the outside conductors K o is the orographic coefficient The expected number of direct strokes for the studied lines (N g = 1,8; H = 5m; b = 2m and K o = 1,8) is 12,02 per year and 0 km. The average lightning current in year 1999 was 16,6 A. The expected number of induced lightning overvoltages higher than a given value U (kv) is shown in the following formula [1]: ( 1 c) Ni = 0,19 3,5 + 2,5 Log N g H (2) U where c is the coupling factor (for unshielded lines c = 0) and the parameters N g and H are the same as in formula (1). For our studied line the number of induced lightning overvoltages which exceed the lightning insulation level (BIL) of 1 kv, are,36 strikes per year and 0 km. The total flashover rate is 22,38 flashovers for the studied line, if shielding by nearby objects is not considered. 3 OVERVOLTAGES AND POWER QUALITY Overvoltages have a great influence on the quality of supply. On one hand, the direct influence of transient overvoltages stresses insulation and can lead to equipment damage. On the other hand, permanent faults, caused by transient overvoltages, are the main origin of interruptions. Short interruptions are mainly caused by breaker tripping (breaker opens for minimum 20 cycles and recloses) to clear earth faults. Sensitive equipment will almost surely trip during this type of interruption. Long interruptions are mainly caused by the earth faults, which are not clearable by breaker tripping, or insulation faults and damage to MV equipment. 3,75 Figure 3 shows the power interruptions per line and year which are caused by lightning. It is clearly seen that lines like Line A, are more affected by a higher number of interruptions than lines like Line M, which can be explained by the constructive differences (the lines M and N are the only line with shielding cables). On the other hand, due to the statistical likelihood of lightning flashes, lines like line A have a high fluctuation between the four studied years Line A Line B Line C Line D Line E Line F Line G Line H Line I Line J Line K Line L Line M Line N Line O Figure 3: Power quality events due to lightning in the studied MV network in north eastern Spain. In figure 4 the percentage of interruptions, with reference to the duration of interruption, in the period from 1999 to 2002 is shown. It can be seen that the number of interruption decreases with the duration of the interruption. The same conclusion is valid for in figure 5, which presents the interrupted transformer in relation with the duration of interruption. Depending on the connected power and number of consumers the interrupted power can vary strongly, which is evidenced by the second bars in figure 4 and 5. % of total interruptions,00% 60,00%,00% 40,00%,00% 20,00%,00% 0,00% Interruptions due to lightning Duration in min. Figure 4: Number of events due to lightning Statisical data In the next section, statistical data about power interruption MV network which is connected to the substation is analysed. The data was collected by the system SGI (Sistema de Gestión de Incidencias) and refers exclusively to the MV network caused by lightning events from the year 1999 to CEA_Sumper_A1 Session 2 Paper No 2-2 -

3 Transformer Power Power interruptions due to lightning At last we can analyze the effects of transient overvoltages on long interruptions and the type of failure which caused the long term interruption. (figure 8). The main types of failure are caused by the intermediate protection, and in the case of a permanent short circuit fault, the main feeder breaker disconnects the line [4]. Nearby 26% of the failures are caused by insulation failures of apparatus Duration in min Figure 5: Transformer power of interruptions due to lightning Interruptions with durations of shorter than three minutes, after EN 160 [3], are defined as short interruption and interruptions longer than three minutes are considered as long interruptions. After this division, it is seen that 44,6 % of all interruptions in the studied network are short interruptions and 56.4% are long interruptions (see figure 6). Long interruptions effects 11,3% Insulation Failure 26,4% 47,2% 15,1% Main Breaker Interruption Intermediate Protections Without Failure or no Defect Localized Figure 8: Relationship between short term interruptions and long term interruptions in the investigated network. 44,6% Percentage of the type of interruption due to lightning ,4% Long interruptions Short interruptions Figure 6: Relationship between short interruptions and long interruptions in the investigated network. Figure 7 presents the long interruptions in the studied network. The number of interruptions, like in figure 4, decreases to the duration as the interruption increases. % of total interruptions 25,00% 20,00% 15,00%,00% 5,00% 0,00% Long interruptions due to lightning Duration in min. Figure 7: Relationship between short term interruptions and long term interruptions in the investigated network. 3.2 Conclusion from the statistics An overvoltage disturbance leads to an intervention by the power system protection, which in nonredundant networks, like radial MV networks, causes an interruption for a number of customers. To reduce the duration of interruption, fuse saving is applied, which means that the main breaker is working with fast tripping. Permanent faults lead to main beaker or intermediate protection interruptions. Moreover, with 26,4 % long interruptions due to insulation faults, it is recommendable to revise methods to protect equipment (e.g. surge arrester) and the overvoltages proceeding from lightning events. 4 MIXED NETWORK AND SUBSTATION OVERVOLTAGE PROTECTION In our case it is necessary to improve the substation protection from overvoltages. On one hand, we have the possibility to protect the overhead line itself against lightning strikes. In some literature [2] overinsulation of a line, addition of shielding, or application of surge arrester on every pole are presented as methods of protection. All these methods entail high investments. On the other hand it is possible to protect only the susceptible apparatus with surge arresters, which is also the case in our substation. All substations, which are connected via a cable to an overhead line, are in danger with regard to possible overvoltages caused by lightning flashes. The studied substation is also connected to a mixed network with alternating parts, overhead lines, and cables. The overhead lines are unshielded and therefore at risk for lightning overvoltages. As is well known, the CEA_Sumper_A1 Session 2 Paper No 2-3 -

4 lightning overvoltages propagate in the network and can stress cable terminals and other apparatus connected to the cable and line. It is necessary take measures to protect the cable terminals and apparatus with surge arresters. 4.1 Configuration of the simulated network The main feeders of our substation with initial cable sections is shown in figure 9. The overhead line is connected at point 1 to a cable network, which consists of two cable sections. At point 2 the cable section 1 ends and is connected to an air disconnector (see figure ). The cable endings at point 1 and 2 are protected with a surge arrester. The second cable section stretches from point 2 to point 3 to the main breaker in the indoor substation. The cable sections at the studied substation vary from 35 m to 263 m. Overvoltages can cause damage to station apparatus at point 3 due to the reflected waves at the conjunctions. Therefore it is necessary to verify if the overvoltage protection of the two applied surge arresters is sufficient [5]. different lengths of cables. The EMTP is probably the most widely-used tool for the analysis of such transient and travelling waves. For the computation, the system was divided into suitable parts which were substituted by equivalent circuits. Analysis of the equivalent lumped-constants network is carried out by means of EMTP. Each branch (transformer, cable, transmission line) together with the lightning waveform constitute the input to EMTP of which the output is the transient waveform. The simulation was carried out with the following parameters: Overhead line: LINE CONSTANTS 3 phase, steel pole, surge impedance 0Ω, v= 3. 5 km/s; total length 5,5 km. Cable: XPLE cable; Saenger (General Cable); Size = 6 mm 2 ; R = Ω/km; C = 0,334 µf/km Surge arrester: Type ASEA 42XBD, used MODEL: Type 99 Lightning source: Type Heidler, 15 ka, distance to substation: 0,5 km The length of the cable sections are shown in table Line Line Cable Cable Figure 9: Circuit used to investigate overvoltages Figure : Air disconnector at point Simulation and simulation parameters To study the transient on high voltage lines and in underground cables and in substations, numerical computations with the well known Electromagnetic Transient Program (EMTP/ATP) were carried out for Line Cable section 1 Cable section 2 Line E-F 253 m m Line G-H 35 m 45 m Line I-J 263 m 38,5 m Line A-B 112 m 22 m Table 1: Cable section length used for the simulation 4.3 Simulation results The results of the simulations are represented in figures 11 to 14. The figures show the overvoltages in the points 1 to 3 during the application of a lightning surge with a lightning current of 15 ka at 0,5 km distance to the substation. Figure 12 shows the most inconvenient cable length proportion with 112m and 22 m in spite of the damping function of the first section of the cable. Mainly, the oscillating overvoltage at the end of the cable is caused by the steepness of the overvoltage in point 2. In figure 11, the proportion between the two cable sections is 253 to and it is clearly shown that the rising overvoltage in point 2 is less steep, which leads to lower magnitude of the overvoltage in point 3. In the four cases it was shown that the substation overvoltage protection is sufficient with the application of two surge arresters at point 1 and point 2. A third surge arrester at point 3 is not necessary. In all simulated cases, the overvoltage at every point at the cable network is under the limits of the basic insulation level (BIL) of 1 kv. CEA_Sumper_A1 Session 2 Paper No 2-4 -

5 Line E-F; Sect.1= 253m; Sect.2= m Lines A-B, Sect.1= 112 m; Sect.2= 22m [ms] [ms] 0. Figure 11: Simulated overvoltages on lines E-F Line G-H; Sect.1= 35 m; Sect.2= 45 m; [ms] 0. Figure 12: Simulated overvoltages on lines G-H Line I-J; Sect.1= 263m; Sect.2= 38,5m [ms] 0. Figure 13: Simulated overvoltages on lines I-J Figure 14: Simulated overvoltages on lines A-B 5 CONCLUSIONS At first, statistical data from power interruptions due to lightning events of the studied network was analyzed. Due to a high activity of lightning and the resulting overvoltages, an increased rate of insulation failures was detected. The medium voltage substation, which is connected via two cable sections to the overhead line, is protected against lightning surges by two surge arrester, installed at the begin and end of the first cable section. Based on what was found in the simulation, an overvoltage protection in the substation is not necessary. 6 REFERENCES [1] Porrino A. (Joint Cired/Cigre Working Group 05); Protection of MV and LV Network against Lightning. Part I: Basic Information; 14 th International Conference and Exhibition on Electricity Distribution, Part 1, Subject area 2: Disturbances and overvoltages; IEE Press; 1997; [2] Porrino A. (Joint Cired/Cigre Working Group 05); Protetion of MV and LV Network against Lightning. Part II: Application to MV Networks, 14 th International Conference and Exibition on Electricity Distribution, Part 1, Subject area 2: Disturbances and Overvoltages; IEE Press; 1997; [3] Bollen, M. H. J.; Understanding Power Quality, Voltage Sags and Interrumptuions; IEEE Press; 2000; IEEE New York; US, [4] Dugan, R. C.; McGranaghan F. M.; Beaty H. W.; Electrical Power System Quality; McGraw-Hill; 1996; New York; US, [5] Balzer G.; Overvoltage Protection for MV substations.; 14 th International Conference and Exibition on Electricity Distribution, Part 1, Subject area 2: Disturbances and overvoltages; IEE Press; 1997; CEA_Sumper_A1 Session 2 Paper No 2-5 -