Comparison of IEEE and IEC Standards for Calculations of Insulation Levels and Electrical Clearances for 230 kV Air Insulated Substation T. Thanasaksiri Department of Electrical Engineering Faculty of Engineering Chiang Mai University Thailand 50200 tthanacmu@gmail.com Abstract-This paper compares the calculations of insulation levels and electrical clearances for 230 kV air insulated substation based on IEEE and IEC standards. The IEEE Std. 1427 can be applied for phase to ground and phase to phase insulations and electrical clearances calculations. Besides calculations, the simulation tool, EMTP as presents in IEEE Std. 1313.2 can be helpful for estimation the crest voltages at any location in substation. According to IEEE Std. 1427 which taking into account the basic switching impulse insulation levels (BSL), the iterative method is also required. At this voltage level, the procedure for calculations refer to IEC 60071-2 in range I can be applied. To compile with IEC 60071-2 for calculations the insulation levels and electrical clearances, the iteration process accounting for the standard rated switching impulse withstand voltage or BSL is not required but the test conversion factors have to be considered. The relation between insulation levels and electrical clearances applying IEEE and IEC standards are approximately linear. The insulation levels and electrical clearances when applied both standards are not significantly difference. I. INTRODUCTION IEEE standard 1427-2006 [1], for equipment in Class I (1.2-242 kV), the standard insulation withstand level include low frequency, short duration withstand voltage (phase to ground) and standard rated lightning impulse withstand voltage or BIL (phase to ground). For equipment in Class II (362-800 kV), the standard insulation withstand level include BIL (phase to ground) and BSL (phase to ground). IEEE Std. C62.82.1-2010 [2] (revision of IEEE Std. 1313.1-1996 [3]), for equipment in Class I (15 kV to 242 kV), the standard insulation withstand level include low frequency, short duration withstand voltage and BIL. For equipment in Class II (362-1200 kV), the standard insulation withstand level include BIL and BSL. IEC standard 60071-1/2006 [4], for equipment in Range I (3.6 kV to 245 kV), the standard insulation withstand level include standard rated short duration power frequency withstand voltage and standard rated lightning impulse withstand voltage. For equipment in Range II (300- 978-1-4673-49749-0/16/$31.00 © 2016 IEEE 800 kV), the standard insulation withstand level include BIL and BSL. This paper compares the insulation levels and electrical clearances of 230 kV air insulated substation applying IEEE Std. 1427 with IEC 60071-2. The standard insulation levels (phase to ground) for equipment in Class I from IEEE or Range I from IEC of the system voltage (phase to phase) being considered are shown in Table I [1], [2], [3], [4]. For the voltage level being considered, the low frequency, short duration withstand voltage and BIL are mainly factors which can be leading to the final insulation levels and electrical clearances of all equipment in substation but the effect of switching surge, BSL to insulation levels can also dominated the insulation levels and clearances which received from BIL and short duration withstand voltage [1], [5]. TABLE I COMPARISON OF STANDARD WITHSTAND VOLTAGE (STANDARDS INSULATION LEVELS) Standard Rated Standard Rated Maximum Short Duration Lightning Impulse System Standard Power Frequency Withstand Voltage Voltage Withstand Voltage or BIL (kVpeak) (kVrms) (kVrms) 275 650 IEEE Std. 325 750 C62.82.1 360 825 242 395 900 480 975 IEEE Std. 1050 1427 IEC 60071-1 II. 245 (275) (325) 360 395 460 (650) (750) 850 950 1050 CALCULATIONS AND COMPARISONS The purpose of this study is to compare the insulation levels and electrical clearances for 230 kV air insulated substation applying the IEEE Std. 1427 and simulation tool, EMTP [6] as presents in IEEE Std. 1313.2 [7] with IEC 60071-2 [8]. To calculate the phase to ground clearance based on the lightning surge, the BIL is required. The data need for BIL calculations can be found in Table II. TABLE III DATA FOR BSL CALCULATIONS APPLYING IEEE AND IEC STANDARDS Data description Values Maximum system voltage (Us) 245 kV Transmission line phase to ground withstand voltage, V3 2.50 pu Transmission line phase to phase withstand voltage, V30 2.80 pu Switching surge flashover rate (SSFOR) 1/100 Ratio 2% of energization and re-energization (Up2/Ue2) 1.53, 1.5 Earth fault factor, load rejection factor 1.5, 1.4 Overvoltages originating from substation 1 (Ue2, Up2) 1.9, 2.9 pu Overvoltages originating from substation 2 (Ue2, Up2) 3.0, 4.5 pu α 0.50 Safety factor (Ksf) 1.05, 1.15 σf/CFO 0.07 σfp/CFO0 0.035 Gap factor 0.3 Α, KL 0.5, 0.67 The sequence of determining the insulation levels and electrical clearances based on the lightning surge at and above the sea levels follow IEEE Std. 1427 are shown in Fig. 1 and 2. The voltage calculations can be performed by applying the equations appears in the standards as shown in Fig. 2(a) or simulation via EMTP as shown in Fig. 2(b). The calculations and system modeling using digital simulation include incoming surge model, surge arrester model, transformer model and line model [9], [10]. More detailed for calculations and computer simulation using EMTP can be found in [5], [7], [9] and [10] and the results of insulation levels and clearances can be found in [11]. The reason for considering the insulation levels and clearances based on the switching surge is for the system voltage not greater than 242 kV (IEEE) or 245 kV (IEC), the clearances are mainly based on lightning surge but switching surge is involved and would affect insulation level as well [1]. To calculate the phase to ground clearance based on the switching surge, the BSL is required. The data need for BSL calculations can be found in Table III. The sequence of determining the insulation levels and electrical clearances based on the switching surge at and above the sea levels follow IEEE Std. 1427 is also shown in Fig. 3. The calculations of phase to ground and phase to phase clearances and BSL at the sea level can be directly calculated as shown in Fig. 390(a) but the phase to ground and phase to phase clearances and BSL above the sea levels, the solutions require an iterative process [1], [12] as shown in Fig. 3(b). Input data Altitude in km Voltage peak from voltage calculations – equations in standard 1 or simulation 2 Calculating relative air density (δ) Compute the BIL for non-self restoring insulations (i.e.; transformer internal insulations) Compute the BIL for self restoring insulations (i.e.; transformer external insulations and others) Compare the calculated BIL with the standard required BIL Selected BIL BIL calculations Compute the phase to ground clearance (Spg ) and phase to phase clearance (Spp =1.1xSpg ) Fig. 1. Sequence of determining the insulation levels and electrical clearances based on lightning surge at and above the sea levels follow IEEE Std. 1427. Input data Input data MT BF and BFR Ks Compute the time for surge travel, T distance to flashover, d m and surge steepness, S Arrester V-I characteristics TABLE II DATA FOR BIL CALCULATIONS APPLYING IEEE AND IEC STANDARDS Data description Values Maximum system voltage (Us) 245 kV Line surge impedance, span length 488 Ω, 250 m CFO 1,300 kV Switching impulse protective level (Ups) 410 kV Lightning impulse protective level (Upl) 500 kV Number of lines connected to the bus 2 Number of conductors/phase 2 BFR (Back flash rate), MTBF 2 FO/100 km/yr, 100 years 1,000 Compute arrester voltage, arrester current and arrester resistance K1 and K2 Compute the voltage magnitude for equipments in substation (i.e. ; transformer, arrester bus connection) A and B Input data 1 Compute the voltage for equipments in substation (i.e. ; breaker, switch and bus) Voltage calculations-equations in standard (a) Input data Calculating relative air density (δ) Input data Input data Compute the time for surge travel, T distance to flashover, d m and surge steepness, S CFO VPF (b) Solve for constant G0 and standard critical flashover voltage (CFOs) 2 Surge arrester model (ZNO - exponential current dependent resistor, type 92) Incoming surge model RAMP - ramp between zero and a constant, type 12 transformer model (capacitance : 2, 4 nF) Calculating altitude correction factor (δm), phase to ground clearance (Spg ) and phase to phase clearances (Spp ) Arrester V-I characteristics MT BF and BFR Ks Exponent, m=0.5 Input data Altitude in km Compute the basic switching impulse insulation level phase to ground (BSLpg ) and phase to phase (BSLpp ) Bus, breaker and line models (distributed parameter model-Clarke) Voltage calculations-EMTP simulation Fig. 2. Sequence of voltage calculations follow IEEE Std. 1427 a) equations given in standards b) EMTP simulation. Check if m and δm is sufficiently small Phase to phase withstand voltage (V30) σfp /CFO ratio σf/CFO ratio Phase to ground withstand voltage (V3 ) (b) From Fig. 3(a), at the sea level, the phase to ground clearance can be calculated by applying equation given in (1). α and KL S pg = Input data Gap factor (kg) (a) T erminate process, solution reached for insulation levels and air clearances Fig. 3. Sequence of determining the insulation levels and electrical clearances based on the switching surge follow IEEE Std. 1427 (a) at the sea level (b) above the sea levels. Compute the critical flashover voltages (CFO, CFO0 , CFOp ) Compute the basic switching impulse insulation levels phase to ground (BSLpg ) and phase to phase (BSLpp ) Adjust m and δm yes Input data Input data no Compute the phase to ground clearance (Spg ) and phase to phase clearance (Spp ) 8 3400 × k g −1 CFO (1) Where V3 ×V CFO = base σf 1− 3 CFO And the phase to ground BSL can also be calculated by applying equation given in (2). σ BSL pg = CFO 1 − 1.28 f CFO (2) The phase to phase clearance can be calculated by applying equation given in (3). S pg = 8 3400 × k g CFO p (3) Earth fault factor − 1 Load rejection factor Representative voltages and overvoltages (Urp) Where CFO p = CFO0 1 − α (1 − K L ) and V30 ×V CFO0 = base σ fp 1− 3 CFO T emporary overvoltages And the phase to ground BSL can also be calculated by applying equation given in (4). σ BSL pp = CFO p 1 − 1.28 fp CFO 8 3400 × k g × δ m − 1 CFO p (7) Ksf Lightning withstand voltages m H Input data Internal insulation : Urw=Ucw·Ksf External insulation : Urw=Ucw·Ksf·Ka Required withstand voltages (Urw) Power frequency withstand voltages, Urw (s) Switching withstand voltages Power frequency withstand voltages, Urw (c) Lightning withstand voltages, Urw (s) Compare Urw (s) and Urw (c) Lightning withstand voltages, Urw (c) Compare Urw (s) and Urw (c) (8) Where CFOspg = G0 × 500 × S pg and by solving the quadratic equation given in (9), the constant G0 can be found. m = 1.25G0 ( G0 − 0.2 ) Switching withstand voltages T est conversion factor (Ktc) for range I The phase to ground BSL can be calculated by applying equation given in (8). 1.0471 Upl Simplified statistical approach Altitude correction factor (Ka) Input data (6) CFOspg Coordination factor (Kcd ) Power frequency withstand voltages (5) 8 = 3400 × k g × δ m − 1 CFO BSL pg = Ups Fast front overvoltages Coordination withstand voltages (Ucw) The phase to ground clearance (Spg) can be calculated by applying equation given in (6) and phase to phase clearance (Spp) can also be calculated by applying equation given in (7). S pp = Up2 /Ue2 (4) where A = altitude above the sea level, km. S pg Slow front overvoltages (case peak method or phase peak method) Coordination factor (Kc) From Fig. 3(b), the insulation levels and clearances above the sea levels can be calculated as follow, starting with the altitude adjustments by applying equation given in (5), the relative air density, δ can be calculated. δ = 0.997 − 0.106 × A Input data Standard Rated Short Duration Power Frequency Withstand Voltage Standard Rated Lightning Impulse Withstand Voltage Rated or standard insulation level (Uw) (9) The phase to phase BSL can also be calculated by applying equation similar to equation (8). Recalculating m and δ m until the solutions are within the tolerance. Fig. 4. Sequence of determining the insulation levels (BIL and BSL) and electrical clearances (phase to ground and phase to phase clearances) at and above the sea levels follow IEC 60071-2. For calculating the insulation levels and electrical clearances in 230 kV air insulated substation applying IEC 60071-2 for range I, the sequence of insulation level and electrical clearances calculations is shown in Fig. 4. The process can be directly calculated, which means no iteration required but the test conversion factor, Ktc has to be considered in order to convert the required switching impulse withstand voltages to short duration power frequency and lightning impulse withstand voltages. To calculate the insulation levels and electrical clearances applying IEC 60071-2, the data need for calculations can also be found in Table II and III. From Fig. 4 the process is starting from determining the representative overvoltages, Urp accounting for temporary, slow front and fast front overvoltages. Two factors play significantly roles which affected to representative overvoltages are earth fault and load rejection factors but the lightning and switching protective levels of protective devices (Upl and Ups) can reduced the overvoltages in some degree. After applying the coordination factor, Kc the coordination withstand voltages, Ucw can be found. Taking into account the altitude correction factor, Ka for external insulation and safety factor, Ksf for both external and internal insulations, the required withstand voltages, Urw(s) can be calculated. Converting the required switching withstand voltages to power frequency and lightning withstand voltages, Urw(c) by multiplying test conversion factor, Ktc. Comparison the required withstand voltages from calculations and conversions and the rated or standard insulation level, Uw for short duration power frequency and lightning impulse withstand voltages as shown in Table I can be achieved. III. RESULTS Depending on the methods from IEEE Std. 1427 or IEEE Std. 1313.2, the selected BIL should be approximately 825-850 kV and the required electrical clearances should be within 1.2-1.6 m phase to ground and 1.2-1.75 phase to phase [11]. Refer to both IEEE standards, the recommended insulation levels and electrical clearances at the sea level based on BIL are shown in Fig. 5. For example, at the selected insulation level of 650 kV, the minimum electrical clearances phase to ground and phase to phase should be 1.235 and 1.360 m respectively. At the selected insulation level of 825 kV, the minimum electrical clearances phase to ground and phase to phase should be 1.570 and 1.725 m respectively. The phase to phase clearance is greater than the phase to ground clearance approximately by 10%. The insulation strength decreases as a linear function of the relative air density [1] which means at the altitude of 2 km above the sea level, the BIL and clearances must be divided by the relative air density (0.79). From Fig. 5, the insulation level of 650 kV can be applied at the sea level with the clearances of 1.235 m phase to ground and 1.360 m phase to phase but at the altitude of 2 km above the sea level the insulation should be 650/0.79=823 kV with the clearances of 1.235/0.79=1.563 m and 1.36/0.79=1.722 m phase to phase. The BIL and clearances are well within the values as recommended. TABLE III INSULATION LEVELS AND CLEARANCES BASED ON BSL Phase to ground Phase to phase Calculations Sea level 2 km Sea level 2 km Required BSL (kV) 569 730 707 871 Clearances (m) 1.32 1.67 1.54 1.97 As shown in Table III [11], at the sea level, the required BSL are 569 kV phase to ground and 707 kV phase to phase. The minimum clearances are 1.32 m phase to ground and 1.54 m phase to phase. At the altitude of 2 km above the sea level, the required BSL are 730 kV phase to ground and 871 kV phase to phase. The minimum electrical clearance at 2 km elevation from sea level should be 1.67 m phase to ground and 1.97 m phase to phase. From the system being studied, to follow IEEE Std. 1427 and 1313.2, taking into account both BIL and BSL, the insulation level at the sea level should be 825 kV and the minimum clearances are 1.60 m phase to ground and 1.75 m phase to phase. At the altitude of 2 km, the insulation should be 900 kV and the minimum electrical clearances are 1.71 m phase to ground and 1.97 m phase to phase (not include the safety clearances). 1.722 1.563 823 Fig. 5. The relation between minimum insulation levels, BIL and electrical clearances recommended by IEEE Std. 1427. When refer to IEC 60071-2 [4], the recommended insulation levels and electrical clearances based on BIL are shown in Fig. 6. The calculated and selected insulation levels at the altitude of 2 km above the sea level are given in Table IV. The selected BIL should be 850 kV and the required electrical clearances should be within 1.6-1.7 m phase to ground and 1.9-2.1 m phase to phase (not include the safety clearances). Fig. 6. The relation between minimum insulation levels, BIL and electrical clearances recommended by IEC 60071-2. TABLE IV INSULATION LEVELS AND CLEARANCES APPLIED IEC STANDARD Selected Insulation Calculated Insulation levels (kV) levels (kV) Type of insulations Phase to Phase to Phase to Phase to ground phase ground phase External 803 1046 850 1050 insulation Internal 705 798 750 850 insulation IV. CONCLUSIONS This paper compares the insulation levels and electrical clearances of 230 kV air insulated substation applying IEEE Std. 1427 with IEC 60071-2. For the voltage level being considered, the low frequency, short duration withstand voltage and BIL are mainly factors which can be leading to the final insulation levels and electrical clearances of all equipment in substation but the effect of switching impulse, BSL to insulation levels can also dominated the insulation levels and clearances which received from BIL and short duration withstand voltage. For the voltage level, both for IEEE (class I) and IEC (range I) standards, BIL calculation is much more complicated. Especially when taking into account the effect of switching impulse. According to IEEE Std. 1427 which taking into account the basic switching impulse insulation levels (BSL), the iterative method is also required. To compile with IEC 60071-2 for calculations the insulation levels and electrical clearances, the iteration process accounting for the standard rated switching impulse withstand voltage or BSL is not required but the test conversion factors have to be considered. The relation between insulation levels and electrical clearances applying both IEEE and IEC standards are approximately linear. The insulation levels and electrical clearances when applied both standards are not significantly difference. REFERENCES [1] IEEE Std. 1427-2006, IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air-Insulated Electrical Power Substations. [2] IEEE Std. C62.82.1-2010, IEEE Standard for Insulation CoordinationDefinitions, Principles, and Rules. [3] IEEE Std. 1313.1-1996, IEEE Standard for Insulation CoordinationDefinitions, Principles, and Rules. [4] IEC 60071-1, 2006, Insulation Coordination-Part 1 : Definitions, Principles, and Rules. [5] Andrew R. Hileman, Insulation Coordination for Power Systems, Marcel Dekker, 1999. [6] Hans Kristian HØidalen, ATPDraw version 5.9p3 for Windows 9x/NT/2000/XP/Vista/7, 2014. [7] IEEE Std. 1313.2-1999, IEEE Guide for Application of Insulation Coordination. [8] IEC 60071-2, 1996, Insulation Coordination-Part 2 : Application guide. [9] IEEE Modeling and Analysis of System Transients Working Group, “Modeling Guidelines for Fast Front Transients,” IEEE Transactions on Power Delivery, Vol. 11, No. 1, January, 1996, pp. 493-506. [10] Juan A. Martinez-Velasco, Power System Transients, CRC Press, 2010. [11] T. Thanasaksiri, “Insulation Level and Clearances for 230 kV Air Insulated Substation,” Proceedings of The ECTI International Conference, May 14-17, 2014, NakornRatchasima, THAILAND. [12] T. Thanasaksiri, “Iterative Method for Clearances and Insulation Levels Based on Switching Surge", Proceedings of The ECTI International Conference, June 24-27, 2015, Hua-Hin, THAILAND.