Download UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND

Paper Published on the16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009
UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND
ALTITUDE CORRECTIONS AS RECOMMENDED IN IEC STANDARDS
Dong Wu1*, Ming Li2 and Mats Kvarngren3
ABB HVDC, SE-771 80 Ludvika, Sweden, 2ABB Corporate Research, SE-721 78
Västerås, Sweden, 3STRI, SE-771 80 Ludvika, Sweden
*Email: [email protected]
1
Abstract: The dielectric strength of air is influenced by air density (temperature and pressure) and
humidity. Such effects need to be taken into account when external insulation is designed and
tested. Since the conditions of application and the conditions of the laboratory tests may be
different, it is often necessary to make corrections between different atmospheric conditions. For
engineers at manufacturers, utilities or high-voltage laboratories, they follow the relevant IEC
standards. However, atmospheric conditions influence the dielectric strength of air in a
complicated way. Simplified and generalized solutions may cause vacillations especially when
different recommendations are given in different standards without sufficient clarifications. It is
the intension of this paper to give an outline of such issues that may lead to uncertainties in the
application of various IEC standards regarding the atmospheric correction. Some proposals are
also given for discussion.
1.
INTRODUCTION
The dielectric strength of air is influenced by air
density (temperature and pressure) and humidity. Such
effects need to be taken into account when external
insulation is designed and tested. Since the conditions
at the application and the conditions at the laboratory
tests may be different, it is often necessary to make
corrections between different atmospheric conditions.
Creditable studies and reviews have been published on
the atmospheric corrections, e.g. [1, 2]. These studies
were the base for the recommendations in IEC
standards, e.g. [3-10]. For engineers at manufacturers,
utilities or high-voltage laboratories, they follow the
relevant IEC standards. However, atmospheric
conditions influence the dielectric strength of air in a
complicated way. Simplified and generalized solutions
may cause vacillations especially when different
recommendations are given in different standards
without sufficient clarifications.
Based on practical engineering experience, it is the
intension of this paper to give an outline of such issues
that may lead to uncertainties in the application of
various IEC standards regarding the atmospheric
correction. Some proposals are also given for
discussion.
2.
2.1.
CORRECTION FOR AIR INSULATION
Related parameters of air
The dielectric strength of air is influenced by the air
density (temperature and pressure) and humidity. The
influence of temperature and pressure can be taken into
account simultaneously, at least as a first
approximation, by the relative air density, δ, [2]:
δ=
p1 273 + t0
×
p0 273 + t1
(1)
Where: p0 and t0 (in degree C) are the pressure and
temperature at the standard reference conditions
respectively; p1 and t1 are that at other air conditions.
Thereafter the atmospheric conditions are converted
into mainly two parameters, the relative air density and
the absolute humidity. These two parameters have also
certain co-effects. It should be noted that for outdoor
conditions, it might be assumed that the effects of
ambient temperature and humidity tend to cancel each
other [4].
2.2.
Influence of air density
The breakdown of a non-uniform long air gap takes
often the processes as corona inception, streamer
propagation, leader formation and propagation, and
final jump. The streamer and leader processes are the
decisive processes. It has been concluded in literature
[1-2] that the influence of air density is most
significant on the streamer formation and propagation.
The air density has little influence on the leader
process. Therefore, as an approximation, one may
consider if the streamer dominates the breakdown
processes in a gap, the dielectric strength of this air gap
is proportional to relative air density. This is in
principle the case for shorter gaps, shorter than 2
meters. For longer gaps, the breakdown will be resulted
by both the streamer and the leader process. Therefore,
the dielectric strength of a longer air gap is, in many
cases, less than proportional to air density.
According to above approximations, the change of the
dielectric strength of air gaps with air density may be
evaluated by the value of δm. For breakdown caused
mainly by streamers, m is equal to one (m=1).
Otherwise, for most of the non-uniform long gaps, m is
smaller than one (m<1). Therefore, the main task for
air density correction is to evaluate m. However, the
value of m is not just a simple function of gap length, it
is also the function of gap structure, voltage type, as
well as the relative air density, δ, in a complicated way.
We have m=f(U/L, kgap, δ). Here kgap is the gap factor.
2.3.
G factor method
Based on creditable studies and test results obtained at
altitude over 3000 meters, the correction methods using
“G factor” were summarized and presented [1-2]. A
factor, G, is introduced to simplify the complicated
relation of m=f(U/L, kgap, δ) by a semi-empirical
approach m=f(G). The value of G is the ratio of the
mean electric field, E, at the breakdown voltage of a
given gap, and the average electric field of the positive
streamer, Es, at the same atmosphere.
G=
E
Es
(2)
With E=Ub/L, Es=Es0×δ×k, and Es0=500 kV/m.
In the above relations, Ub is the breakdown voltage of
the air gap and L is length of the discharge. k is the
correction for humidity; and Es0 is the average electric
field of the positive streamer at standard reference
atmosphere. It is intended to determine, by the value of
G, how much a breakdown will be contributed by
streamer at the given gap. G value is actually the
simplified presentation of the breakdown characterises
of the given gap. The functions of m=f(G) is given
Figure 1, with the value δ×k as index.
applied with an acceptable accuracy for cases where
0.9≤δ×k≤1.1 [2]. Such limitation may only be fulfilled
when correction is made between test results obtained
near sea level. For high altitude correction other curves
give better accuracy. This is an issue being overlooked
by this standard.
In this standard, it is recommended to use the 50%
breakdown voltage and the minimum discharge path to
evaluate G. When U50 is not available, Ub can be
assumed to be 1.1 times of the voltage level of the
withstand test. However, such assumption can only be
justified when the voltage level of the withstand test is
related to the value of U50 and the minimum discharge
path in such a way that it reflects the discharge
characteristics of the test object. Otherwise, errors will
be introduced to the value of E=Ub/L and thereafter to
G and m. For the purpose of insulation coordination
and the determination of type test voltage, the value of
U50 and the length of related minimum discharge path
are often not available. The test voltage specified has,
in many cases, a weak relation with an arcing distance
of the equipment. In such a case, E=Ub/L can not be
correctly estimated as recommended by this standard.
3.2.
IEC 60071-2 (1996-12)
In this standard, [4], the altitude correction is given in a
formula as:
ka = e
m
H
8150
(4)
Where: H is the altitude in meters. Actually, we can
relate ka in formula (4) with kt in formula (3) by:
m
ka=1/kt=1/δ .
m
G
Figure 1: The relations of m=f(G) given in [2].
3.
3.1.
RECOMMENDATIONS IN IEC
STANDARDS
IEC 60060-1 (1998-11)
In this standard [3], the correction factor, kt, is defined
as:
m
kt = δ ×kw= U/U0
(3)
Where U is the breakdown voltage at given site
conditions and U0 is the breakdown voltage at standard
reference conditions. G factor method is adopted in this
standard. Both m and w are the function of G.
In this standard, the curve with solid line in Figure 1,
for δ×k=1, is used, for m=f(G). This curve may only be
Figure 2: The relations of m=f(Ucw) given in [4]
Instead of using the relation m=f(G), as that in IEC
60060-1 [3], m is given here as the function of the coordination withstand voltage, i.e., m=f(Ucw). This is a
simplified and conservative approach, avoiding the
difficulties in obtain the relation of E=Ub/L. This is
very convenient for the purpose of insulation coordination and the determination of type test voltage
Different values for m are recommended for different
types of voltage and insulation. For lightning impulse
voltage (LI) and short duration AC voltage, m=1 is
recommended. For switching impulse voltage (SI), the
value of m is to be obtained through a group of curves
in Figure 2.
3.3.
In this standard [5], the normal service conditions
including altitudes not exceeding 1000 meters are
specified. The altitude correction recommended is said
to be applicable to 4000 meters with formula below:
m
H −1000
8150
(5)
This is the same formula as formula (4) but
avoided the correction for the altitude not
exceeding 1000 m. For SI, further simplifications
and approximations are made. Instead of the
curves in the form of m=f(Uw), as given in Figure
2, constant values of m for different type of
voltages are recommended. The correction is not
anymore the function of voltage level:
•
For AC, LI, and phase to phase SI: m=1
•
For Longitudinal SI: m=0.9
•
For phase to earth SI: m=0.75
3.4.
IEC 60076-1 (2000-04) and IEC 60076-3
(2000-03)
In these two standards for power transformers [6-7],
the normal service conditions including altitudes not
exceeding 1000 meters are specified [6]. The altitude
correction is made directly on the gap distance, i.e. “if
the transformer is specified for operation at an altitude
higher than 1000 m, the clearance requirements shall
be increased by 1% for every 100 m by which the
altitude exceeds 1000 m” [7].
3.5.
IEC 60137 (2008-07)
In this standard [8], it is said that “bushings
corresponding to this standard are declared suitable for
operation at any altitude not exceeding 1000 m”. The
altitude correction recommended is applicable to 4000
meters with formula (5). Constant values for m are
also recommended as:
3.6.
•
m =1 for power frequency and LI;
•
m =0.75 for SI.
IEC 60044-8 (2002-07)
This standard [9] has exactly the same approach for the
altitude correction as that in IEC 62271-1 [5].
IEC 60168 (2001-04)
In this standard [10], no recommendation is given for
altitude correction. If the atmospheric conditions at the
time of test differ from standard reference atmosphere,
then corrections shall be made according to IEC
60060-1[3].
4.
4.1.
IEC 62271-1 (2007-10)
ka = e
3.7.
DISCUSSIONS AND PROPOSALS
Application of IEC 60060-1
The atmospheric correction recommended by this
standard [3] is most accurate when breakdown tests
have been performed on air gap or dry insulators. In
such cases, the dielectric characteristics of the test
objects can be obtained. The value of E=Ub/L can be
evaluated accurately.
This correction becomes difficult to apply when
correction is to be made on withstand test voltage of
equipments. This is because in some cases, the relation
between the withstand voltage and U50 is not 1.1 times.
In some cases the discharge path may not be
determined by the voltage level of this test but by other
constrains, such as creepage or installation
requirement. In some cases, the shortest discharge path
is not the critical insulation under this test voltage. The
value of E=Ub/L in such cases may not be correctly
evaluated. Even if one may introduce certain type of
iterative calculation procedure between the test voltage
and the correction factor, the uncertainty in evaluate
E=Ub/L can not be solved. However, as long as the
corrections are made for relative small difference in air
density from standard reference conditions, i.e.,
0.9≤δ×k≤1.1, the magnitude of the error introduced
may still be tolerated.
The extended application of the relation m=f(G) in this
standard to high altitude correction will lead to a
increased error. The correction as recommended today
is not suitable for altitude correction. One solution
would be to adopt the complete relation of m=f(G) as
that given in Figure 1 into this standard. The other
solution is to use the recommendations in IEC 60071-2
for altitude correction, limiting the application of this
standard in the range of 0.9≤δ×k≤1.1.
4.2.
Application of IEC 60071-2
The atmospheric correction recommended by this
standard is most suitable for the purpose of insulation
co-ordination and the determination of type test
voltage. Since the correction factor is related directly to
the co-ordination withstand voltage.
One importance issue in this correction is whether or
not to make correction for the altitude not exceeding
1000 meters. On this point this standards differs from
many other IEC standards [5-9]. In this discussion,
there are actually four different atmospheric conditions
in the context.
1.
Standard
reference
conditions
with
temperature of 20oC, air pressure of 101.3
kPa, and absolute humidity of 11g/m3
2.
Normal service conditions (conditions that
specified for various HV equipment in
relevant standards) with maximum ambient
temperature of, e.g., 40oC, altitude not
exceeding 1000 meters, and …
3.
Specific
site
conditions
(application
conditions) with altitude of, e.g., 1600 meters,
and…
4.
Laboratory test conditions (at the day of
testing) with ambient temperature of, e.g.,
25oC, air pressure of, e.g., 100.0 kPa and
relative humidity of, e.g., 40%.
In several IEC standards, the values of m for different
voltage types are recommended.
These different conditions represent different severities
for the external insulation design and test. In Figure 3,
the relations between the different atmospheric
conditions are sketched out.
The recommendation of m=1 for LI is justified. The
mean field of a rod-plane gap under positive LI is
higher than 500 kV/m, i.e., E=Ub/L≥500 kV/m.
Severity
Specific site conditions
kt
Theoretically kt
Normal service conditions
Standard reference conditions
kt
Lab. test conditions
Figure 3: Atmospheric correction between different
ambient conditions
Theoretically, for the design and test of external
insulation, atmospheric corrections should be applied
between the specific site conditions, laboratory test
conditions and the standard reference conditions.
However, for most equipment, there are normal service
conditions specified in the relevant IEC standards. For
economical reasons and industrial practice, equipments
have to be designed to withstand the required withstand
voltage within the range of normal service conditions.
The differences between the standard reference
conditions and the normal service conditions have been
included in the design. This means that for equipment
used at a location with altitude not exceeding 1000
meters; no altitude correction will be necessary. This
industrial practice has been supported by vast
operational experience and adopted by many IEC
standards. This is especially true for the cases when
deterministic method is used for insulation
coordination, e.g. for HVDC systems (IEC60071-5,
under revision). The effect of the air density in this
range, where H 1000 meters, has already been
included in the margins commonly adopted for
insulation design.
For equipment that will be used at the specific site
conditions more severe than the normal service
conditions, e.g., at an altitude higher than 1000 meters,
atmospheric corrections between these two conditions
are necessary for both the design and test of the
external insulation. For equipment which will be tested
at a laboratory where conditions at the day of testing
differ from standard reference conditions, atmospheric
correction is necessary for the test voltage between
these two conditions.
Taking this discussion into account, it is justified to use
formula like that in IEC 62271-1, i.e., (5) instead of
formula in IEC 60071-2, (4).
4.3.
Values of m for different types of voltage
The recommendation of m=1 for AC is a conservative
approach. The non-linear characteristic of the AC
breakdown voltage against gap length indicate the
same breakdown process as that appears under SI [2].
For long gaps, there will be m<1. On the other hand, a
rod-plane gap of 2 meters will have a U50 for short
duration AC of 620 kV. This voltage corresponds to a
phase to phase voltage of 1074 kV. Therefore, if AC is
the dimensioning voltage, with the system voltage
available today, relative short gaps will be required.
Taking this into consideration, the conservative
approach of using m=1 may be justified. Otherwise for
longer gaps, the value of m should be similar to what is
used for SI. Therefore, the recommendations would be:
•
For gaps of lengths shorter than 2 meters, m=1
•
For gaps of lengths longer than 2 meters, use
the same m value as that for SI obtained in
Figure 2
Note that peak AC voltage should be used as SI in
Figure 2.
No correction for DC was included in most of these
standards. In Cigré report a linear relation between the
short duration DC breakdown voltage and the length of
rod-plane gap has been reported up to 1000 kV with
mean breakdown field in the level of 500 kV/m [2].
Since the gap length is short, m=1 would be used.
However, other literature has reported slightly nonlinearity from 2 to 4 meters of gap lengths [11]. The
mean breakdown field is in the level of 400 kV/m. This
will result in: G=0.8 and m=0.6. More laboratory
studies are needed for longer gaps. However, with the
limited results today, the recommendations would be:
•
For gaps of lengths shorter 2 meters, m=1
•
For gaps of lengths longer than 2 meters,
m=0.6.
The recommendation of use m=0.75 for all SI level is a
conservative approach. This value may be justified for
EHV systems. For UHV system, relations in Figure 2
should be followed.
4.4.
2.
The correction method recommendation in
IEC 60071-2 is convenient to use for purpose
of insulation co-ordination and the
determination of withstand test voltages. It
can be applied for altitude correction for
altitudes up to 4000 m, as being recommended
by several other IEC standards, before any
research results prove otherwise. It is a
conservative approximation of G factor
method.
3.
For equipment with a specified normal service
conditions including altitudes not exceeding
1000 m, altitude correction should only be
applied to correct the altitude exceeding 1000
m as that given in formula (5):
Altitude correction for creepage
Along hydrophilic surfaces, when pollution has been
wetted, dry-band related discharge activities may take
place. This is a short gap with streamer breakdown.
The dry-band can become wetted again after the
current flow though the arc in air. Such activities can
occur several times and a full flashover of the insulator
may take place. In this process, the change of air
density will change the dielectric strength of the air. If
it was pure air breakdown in the form of streamer, then
the correction would be kt=δm with m=1. However, the
conditions of both surfaces in series and parallel to the
dry-bend activity contribute to the effects. For real
insulator, the change of pollution level and the change
of creepage distance in parallel with the air gap will
change the interaction between the air gap and surface
activities. The insulator shed profile play also an
important role. This is again a complicated
phenomenon [12].
ka = e
4.
Although wide differences exist in literature, it is
recommended that a constant value of m should be
used with m=0.5 for AC and 0.35 for DC [12]. The
correction is to be applied for voltage, the same as in
equation (3):
kt = U/U0=δm
It is the same as for air insulation correction; no
correction for the creepage distance is needed for
altitude up to 1000 meters. In pollution test, for the
insulators of the same shed profile and at a given
pollution level, the relationship between the U50 and the
creepage distance is in most cases linear [12].
Therefore, one can replace the voltage in equation (3)
with creepage distance, L:
kc = L/ L0 =δa
-m
(6)
Here L is the creepage distance for a high altitude
and L0 is the creepage distance for altitudes up to
1000 meters.
5.
CONCLUSIONS
For the recommendations in IEC standards on the
atmospheric and altitude correction the following may
be considered.
1.
The correction method recommendation in
IEC 60060-1 is most accurate for breakdown
test performed at the condition where
0.9≤δ×k≤1.1. It is inaccurate when used for
high altitude correction. It is inconvenient and
inaccurate when used for insulation coordination and the determination of withstand
test voltages.
5.
6.
m
H −1000
8150
the recommended value for m is proposed to
be related to gap length as:
•
For LI: m=1.
•
For SI: using curves in Figure 2.
•
For AC: when gap length is shorter than 2
meters, m=1. When gap length is longer
than 2 meters, use the same value of m for
SI with the same voltage level to AC peak
voltage.
•
For DC: when gap length is shorter than 2
meters, m=1. When gap length is longer
than 2 meters, m=0.6.
Altitude correction for creepage should be
introduced into IEC standard as that
recommended by Cigré Review [12].
REFERENCES
[1] K. Feser, A. Pigini, ”Influence of atmospheric
conditions on the dielectric strength of external
insulation” Paper prepared at the request of the
Chairman of SC 33, Electra No. 112.
[2] WG 33-07, “Guidelines for the evaluation of the
dielectric strength of external insulation” Cigré
Brochre 72.
[3] IEC 60060-1, High-voltage test techniques – Part
1: General definitions and test requirements.
Edition 2.0, 1989-11
[4] IEC 60071-2, Insulation co-ordination – Part 2:
Application guide. Third edition, 1996-12
[5] IEC 62271-1, High-voltage switchgear and
controlgear – Part 1: Common specifications.
Edition 1.0, 2007-10
[6] IEC 60076-1, Power transformers – Part 1:
General. Edition 2.1, 2000-04
[7] IEC 60076-3, Power transformers – Part 3:
Insulation levels, dielectric tests and external
clearances in air. Second edition, 2000-03
[8] IEC 60137, Insulated bushings for alternating
voltage above 1000 kV. Edition 6.0, 2008-07
[9] IEC 60044-8, Instrument transformers – Part 8:
Electronic current transformers. First edition,
2002-07
[10] IEC 60168, Test on indoor and outdoor post
insulators of ceramic material or glass for systems
with nominal voltage greater than 1000 V. Edition
4.2, 2001-04
[11] S.J. Huang, Z.H. He, Z.B. Wen, M.G. Wang,
“Flashover tests on large air gaps with DC
voltage”, ICPST’94 Beijing, China
[12] Taskforce 33-04-01: “Polluted insulators: a review
of current knowledge” Cigré Brochre 158