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
[email protected]
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.
I NTRODUCTION
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-
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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 I NSULATION LEVELS) Standard Rated Maximum Standard Rated Short Duration System Lightning Impulse Standard Power Frequency Voltage Withstand Voltage Withstand Voltage (kVrms) or BIL (kV peak ) (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 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 (U ps) 410 kV Lightning impulse protective level (U pl) 500 kV Number of lines connected to th e bus 2 Number of conductors/phase 2 BFR (Back flash rate), MTBF 2 FO/100 km/yr, 100 years 1,000 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 (U p2/Ue2) 1.53, 1.5 Earth fault factor, load rejection factor 1.5, 1.4 Overvoltages originating from substation 1 (Ue2, U p2) 1.9, 2.9 pu Overvoltages originating from substation 2 (Ue2, U p2) 3.0, 4.5 pu α 0.50 Safety factor (K sf ) 1.05, 1.15 σf /CFO 0.07 0.035 σfp/CFO0 Gap factor 0.3 0.5, 0.67 Α, K L
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 Volt age peak from v oltage calculations – equation s in standard 1 or simulation 2
m k n i e d u t i t l A
Calculating relat ive air density (δ)
Comp ute t he BIL for non-self restoring insulatio ns ( i.e.; t ransformer in ternal insulations) Comp ute t he BIL for self restoring insulations (i.e. ; t ransformer external insulations and oth ers)
Comp are t he calculated BIL with t he sta ndard required BIL
Selected BIL
BIL calcula tions
Comp ute t he phase to ground clearance (S pg ) and ph ase to p hase clearance (S pp =1.1xS pg )
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 K s Comp ute t he time for surge tr avel, T distance to flashover, d m and surge steepn ess, S
s c I - i t s V i r r e e t t c s a e r r r a h A c
Comp ute ar rester vo ltage, arrester current and arrester resistance K 1 and K 2 Comp ute t he voltage magnitude fo r equipment s in substation (i.e. ; t ransf ormer, ar rester bus connection)
A and B
Input data
1
Comp ute the voltage for equipments in substat ion (i.e. ; breaker, swit ch and bus) Voltage calculations-equations in standard
(a)
Input data Input data
MT BF and BFR K s Input data
s c I i - t s V i r r e e t t s c a e r r r a h A c
Comput e the time for surge travel, T distance t o flashover, d m and surge steepn ess, S
CFO VPF
Calculating alt itude cor rection factor (δm), ph ase to ground clearance (S pg ) and ph ase to phase clearances (S pp )
(b) Solve fo r con stant G0 and stan dard crit ical flashover v oltage (CFOs)
2
Surge arrest er model (ZNO - exp onential current dependent resistor, type 92)
Inco ming surge model RAMP - ramp between zero and a const ant, type 12
tr ansformer model (capacitance : 2 , 4 nF)
Expo nent, m=0.5 Input data Altitude in km
Calculating relat ive air density (δ)
Comp ute t he basic switc hing impulse insulation level phase to ground (BSL pg ) and ph ase to ph ase (BSL pp )
Bus, break er 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.
(b) Input data o i t a r O F C /
o i t a r
σ
σ
f
O F C /
) 0
From Fig. 3(a), at the sea level, the phase to ground clearance can be calculated by applying equation given in (1).
p f
Compute the cr itical flashover volt ages (CFO, CFO0 , CFO p )
(a)
T erm inate process, solution reached for insulatio n lev els 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.
3
V ( e e s g a t a h l p o o v t d e n s a a t h s P h t i w
Compute the basic switch ing impulse insulation levels phase to ground (BSL pg ) an d phase t o phase (BSL pp )
Adjust m and δm
yes
Input data
) 3 V d ( n e u g a o t r l g o o v t d e s n a a t h s h P t i w
no
Check if m and δm is sufficiently small
α
and K L
S pg =
Input data ) g k ( r o t c a f p a G
Compute t he phase to ground clearance (S pg ) and phase to phase clearance (S pp )
8
(1)
3400 × k g −1 CFO
Where
V 3 ×V CFO = base σ f 1− 3 CFO And the phase to ground BSL can also be calculated by applying equation given in (2).
CFO
BSL pg = CFO 1 − 1.28
σ f
(2)
The phase to phase clearance can be calculated by applying equation given in (3).
S pg =
8
(3)
Earth fault factor
3400 × k g − 1 CFO p
Load reject ion factor
Input data
Representative voltages and overvoltages (Urp)
Where
CFO0 and V 30 CFO p = ×V CFO0 = base 1 − α (1 − K L ) σ fp 1− 3 CFO And the phase to ground BSL can also be calculated by applying equation given in (4). BSL pp
=
CFO
CFO p 1 − 1.28
σ fp
Temporary overvoltages
Coordination factor (K c)
where A = altitude above the sea level, km.
S pp
=
8
(6)
3400 × k g × δ m − 1 CFO
3400 × k g × δ CFO p
m
Where CFO spg
=
CFO spg
Simplified statistical approach
Switching withstand voltages
Lightning withstand voltages
Altit ude correction factor (K a) K sf
m H
Input data
Int ernal insulation : Urw=Ucw·K sf Ext ernal insulation : Urw=Ucw·K sf ·K a
Required with stan d voltages (Urw) Po wer f requency with stan d voltages, Urw (s)
Switching withstand voltages
Lightn ing with stand volt ages, Urw (s)
(7) Power frequency withstand volt ages, Urw (c)
− 1
The phase to ground BSL can be calculated by applying equation given in (8). BSL pg =
U pl
Test conversion factor (K tc) for ran ge I
8
Coordination factor (K cd )
Power frequency withstand voltages
Input data
The phase to ground clearance ( S pg ) can be calculated by applying equation given in (6) and phase to phase clearance (S pp) can also be calculated by applying equation given in (7). S pg =
U ps
Fast fro nt overvoltages
Coordinat ion with stand vo ltages (Ucw)
(5)
U p2 /Ue2
(4)
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
Slow front overvoltages (case p eak m ethod or phase peak method)
Compare Urw (s) and Urw (c)
Lightning withstand volt ages, Urw (c)
Compare Urw (s) and Urw (c)
(8)
1.0471
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 )
Stan dard Rated Sho rt Durat ion Power Frequency Withstand Voltage
Sta ndard Rated Lightning Impulse Withstand Voltage
Rated or st andard 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, K tc 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, U rp 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 (U pl and U ps) can reduced the overvoltages in some degree. After applying the coordination factor, K c the coordination withstand voltages, U cw can be found. Taking into account the altitude correction factor, K a for external insulation and safety factor, K sf for both external and internal insulations, the required withstand voltages, U rw(s) can be calculated. Converting the required switching withstand voltages to power frequency and lightning withstand voltages, U rw(c) by multiplying test conversion factor, K tc. Comparison the required withstand voltages from calculations and conversions and the rated or standard insulation level, U w for short duration power frequency and lightning impulse withstand voltages as shown in Table I can be achieved. III.
R ESULTS
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 I NSULATION 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 I NSULATION LEVELS AND CLEARANCES APPLIED IEC STANDARD Calculated Insulation Selected 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. R EFERENCES [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 Coordination Definitions, Principles, and Rules. [3] IEEE Std. 1313.1-1996, IEEE Standard for Insulation Coordination Definitions, 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.
