4
3) Exhaust in a defined volume. In a number of tests, the expelled gas was not exhausted to the environ- 5
ment, but to a collector tank of 1.2 m3. Pressure was measured in this tank as well. In Figure 5-9, pressure 6
measurements of SF6 and air are combined for the two-tank situation. It can be concluded that:
7
a. The pressure rise in the arcing volume in case of air is much faster and reaches higher values than in 8
SF6. This is in accordance with all earlier investigations [Daalder1989, Dullni1994, Pettinga1989];
9
b. The pressure relief disc operates at a (somewhat) higher pressure in air than in SF6. This is (probably)
10
because the inertia of the disc bursting process causes the disc to operate at a somewhat higher 11
pressure when the rate of pressure rise is higher. This phenomenon is frequently observed in tests. 12
c. The pressure in the exhaust volume reaches a higher value in the SF6 case than in the case with air.
13
From these tests it follows that the arc energy of arcs in air in a worst-case situation could be regarded 14
as being lower than in SF6.
15
d. Pressure inside the arc compartment. As shown by several authors the maximum pressure in a closed 16
arc compartment is higher if it is filled with air instead of SF6. This can be true even when the arc
17
energy in SF6 is higher than in air. The reason is the larger heat capacity of SF6, which compensates
18
for the higher arc energy. The smaller heat capacity of air leads to a faster pressure rise and an earlier 19
bursting of the rupture disc. When the pressure relief disk opens it limits the pressure rise in the arcing 20
compartment. There is a tendency for the opening to occur at a higher pressure in air due to inertia 21
effects. From this it follows that the arc compartment will be stressed in a comparable or even more 22
severe way by an arc in air. 23
e. Exhaust of gases via an intermediate compartment. If the overpressure of the arc compartment is 24
directly discharged into the environment (room), the hot gas stream will affect the indicators 25
immediately. However, in general, metal-enclosed switchgear consists of several compartments with 26
only the "arcing" compartment filled with SF6. In this case, hot SF6 first of all will exhaust to a neigh-
27
bouring air-filled compartment (intermediate room, e.g. cable compartment, pad mount enclosure) 28
within the switchgear before leaving it e.g. through fissures. 29
4) Ignition of indicators. Ignition of any material means exothermic reactions start to run. This happens 30
when a certain activation energy level is exceeded. Cotton consists of carbon-hydrogen-oxygen 1
compounds. In air, ignition is an oxidation process, mainly the reaction of carbon with oxygen to produce 2
CO and CO2. A similar process (without oxidation) happens with SF6. To reach the activation energy, heat
3
must be transferred from the gas stream to the cotton. Given the complexity of the energy transfer 4
(turbulence, chemical reactions, strongly inhomogeneous time-dependent flow and temperature patterns), 5
modelling of this process is beyond the reach of the Working Group A3.24. The influential parameters are 6
gas temperature, heat transfer duration, thermal conductivity and gas velocity. They act as follows: 7
a. Gas temperature: The higher the temperature difference between gas stream and indicator (as with 8
air), the higher is the energy flux applied to the indicator. 9
b. Heat transfer duration: The longer the duration of the gas flow (as in SF6), the more energy will be
10
transferred to the indicators. 11
c. Thermal conductivity: In the temperature range from 1500 to 4000 K the thermal conductivity of SF6
12
is larger than that of air, enlarging the energy flux applied to the indicator. 13
d. Gas velocity: The higher the gas velocity (as with air), the higher is the turbulence in enlarging the 14
temperature at the boundary of the gas stream (increasing the temperature gradient to the indicator). 15
Moreover, with higher gas velocity, the boundary layer between hot gas and the surface of the indicator 16
is thinner. Higher turbulence and a thinner boundary layer improve heat conduction to the indicator and 17
can lead to easier ignition of indicators. 18
Given that these various contributing factors have both ignition-enhancing and -impeding effects in air and 19
SF6 further research is necessary to determine the overall effect.
20
5.3 Summary
21 22
Based on the results, the authors conclude that relevant differences exist in the behaviour of fault arcs in SF6 and
23
in air, and in their effects on switchgear and the environment. 24
Replacing SF6 by air (all other parameters being equal) in internal arc testing leads to comparable or higher
25
pressure rise in the arcing compartment. Pressure rise in an adjacent compartment or a switchgear room into which 26
the exhaust gas is released may be lower in tests with an air-filled arc compartment than in tests with an SF6-filled
27
arcing compartment. No conclusions exist on other criteria required to pass an internal arc test such as the ignition 28
of the cotton indicators and enclosure burn-through. This needs further detailed investigation. 29
The following conclusions are drawn: 30
a. Arc compartment: The mechanical stress of the arcing compartment filled with air is higher than 31
with SF6, i.e. if criterion 1, 2 of IEC 62271-200 are fulfilled with an air test, this will be true for SF6
32
as well. 33
b. Intermediate compartment: When exhaust gas from the arcing compartment is released into 34
adjacent compartment(s) the mechanical stress of it is larger in tests with SF6 than with air.
c. Indicators: With the main focus directed to the ignition of the indicators, the following conclusions 1
are drawn if air is used instead of SF6 during internal arcing tests (criterion 4 of IEC 62271-200):
2
For worst-case situations (e.g. long arc duration) the arc energy in SF6 can be higher than
3
in air. 4
With air, the higher gas temperature as well as the thinner boundary layer in front of the 5
indicators will increase the heat flux applied to the indicators. 6
Thermal conductivity of the gas determines the heat flux as well, hot SF6 has a higher
7
conductivity than air in the relevant temperature range. 8
The gas stream duration is longer with SF6 and it cools down slower than with air.
9
Both gas streams with, i.e. with SF6 or air, are longer than the distances from the panel to the cotton indicators
10
even considering the reflections at walls and ceilings. Although more test results would be needed to confirm this, 11
the indications are that, in this respect, the effects are comparable. 12
Also, the authors noted that IEC / IEEE standards currently do not request a pressure measurement in the arc 13
compartment during testing. A lot of useful information which could be used in the design of the switchgear can be 14
obtained from the measurement of the overpressure curve. 15
16
REFERENCES: 17
[Bjørtuft2005]: T. Bjørtuft, O. Granhaug, S.T. Hagen, J.H. Kuhlefelt, G. Salge, P.K. Skryten, S. Stangeherlin, 18
"Internal Arc Fault Testing of Gas Insulated Metal Enclosed MV Switchgear", 18th Int. Conf. on El. Distr. (CIRED), 19
2005. 20
[CIGRE WG 23.03]: CIGRE WG 23.03, “Pressure Rise in Metal-Enclosed Switchgear of Single Phase Enclosure 21
Type due to Internal Arc. Evaluation of various International Test Results and Study of Calculation Procedure”, 22
Electra 93, pp. 25-52, 1984. 23
[Daalder1989]: J.E. Daalder, O. Lillevik, A. Rein, W. Rondeel, "Arcing in SF6-MV-Switchgear. Pressure Rise in
24
Equipment Room", Int. Conf. on El. Distr (CIRED), 1989. 25
[Dullni1994]: E. Dullni, M. Schumacher, G. Pietsch, "Pressure Rise in a Switchroom Due to an Internal Arc in a 26
Switchboard", Proc. 6th Int. Symp. on Short-Circuit Currents in Power Systems, Sept. 6-8, 1994. 27
[Pettinga1989]: J.A.J. Pettinga, "Pressure-rise tests due to a High-Current Internal Arc in a MV Cubicle Model", Int. 28
Conf. on El. Distr. (CIRED), 1989 and KEMA internal report 00880-DZO 88-2046 (88-10), 1988. 29
[Trinh1992]: N. Giao Trinh, “Risk of Burn-Through – a Quantitative Assessment of the Capability of Gas Insulated 30
Equipment to Withstand Internal Arcs”, IEEE Trans. On Pow. Del., vol. 7 no. 1, 1992. 31
32 33
6 EFFECT OF INTERNAL ARC ON STRUCTURES
1
6.1 Introduction
2
The internal arc is initiated at a particular place in the switchgear during a type-test (see IEC62271-203 for high 3
voltage GIS switchgears for example). There are three major effects which affect the switchgear and adjacent 4
personnel. 5
1. Mechanical stress on the switchgear due to the overpressure 6
2. Mechanical stress on the building walls due to the overpressure 7
3. Burn-through 8
Sections 6.2, 6.3 and 6.4 describe and evaluate these effects in detail. 9
•
The first effect (see Section 6.2) is the overpressure generated by the arc that creates mechanical stress 10on the enclosure structure causing deformation of walls, blown out doors, etc. Generic constructions are 11
shown in Figure 6-1. 12
a. In Figure 6-1 (a), a typical construction of air insulated switchgear (AIS) with grounded enclosure 13
used in medium voltage is shown. The walls of the enclosure are made of a sheet metal with a 14
certain thickness connected by bolts spaced by a certain distance. For simplicity, the doors are not 15
shown in the figure. The pressure relief devices are flaps which are structurally weaker than the 16
main enclosure, so that in case of high pressure inside the panel they will be first to rupture and 17
release the excess pressure. Usually the flaps start to open at an internal pressure around 20 to 30 18
% higher than the external pressure. 19
b. Figure 6-1 (b) shows a typical construction of a gas insulated enclosure used in medium voltage 20
switchgear (GIS). The walls of the enclosure are made of a sheet metal with a certain thickness 21
and the walls of the enclosure could be welded or bolted together. The pressure relief device is 22
usually a rupture disk which opens at a specific overpressure, for example 200-300 kPa depending 23
on the application and the protection philosophy. 24
c. Figure 6-1 (c) shows a typical construction of a high voltage GIS busbar arrangement. The external 25
envelope is composed of a metallic cylinder with a certain thickness which contains a concentric 26
conductor. Insulating spacers support the conductors and divide the GIS into separate gas 27
compartments. The pressure relief device is usually a rupture disk which opens at a specific 28
overpressure, for example 1000-1500 kPa depending on the application and the protection 29
philosophy. 30
• The second effect (see Section 6.3) has an impact on the switchgear room and building. Hot gases will be 31
ejected through the pressure relief device of the switchgear enclosure and cause pressure buildup in the 32
rooms and buildings. The pressure relief devices of the installation room start to open when the 33
overpressure reaches a certain value. 34
• The third effect (see Section 6.4) is the “burn-through” effect. This effect is caused by the arc which can 1
burn on a surface of the metallic enclosure (like a switchgear wall or panel, or GIS bus duct). This melts 2
and then punctures the wall of the enclosure. 3