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.