INNOVATION FOR SECURE AND
EFFICIENT TRANSMISSION GRIDS
CIGRÉ Belgium Conference
Crowne-Plaza – Le Palace
Brussels, Belgium | March 12 - 14, 2014
21, rue d’Artois, F-75008 PARIS
http : //www.cigre.org
VFT insulation coordination study of a 400 kV GIS
DLO, DANIEL LEO OLASON
HEF
Denmark
TEB, THOMAS EBDRUP
Energinet.dk
Denmark
FFS, FILIPE FARIA DA SILVA
Aalborg University
Denmark
CLB, CLAUS LETH BAK
Aalborg University
Denmark
daniel.olason@gmail.com
tebdrup2@gmail.com
SUMMARY
In November 2008 the Danish government decided that all overhead lines below 400 kV should be
replaced by underground cables. This is due to a demand of reducing the overall visibility of the
transmission system, sometimes referred to as the beautification of the transmission system. The
agreement furthermore included to reinforce some of the existing 400 kV transmission lines. This is
due to both increasing wind penetration and power flow between Scandinavia, Germany and possible
future connections. As a part of reinforcing the 400 kV transmission system in Jutland, Denmark, the
Danish TSO (Energinet.dk) is in the process of constructing a new gas insulated substation (GIS) in
Revsing. As a part of this process, new Eagle pylons will replace some of the existing Donau pylons.
The new Eagle type pylon is meant to reduce the visual impact of transmission lines.
The reliability of the substation in Revsing is of great importance as it is part of the 400 kV systems
backbone between Sweden, Norway, Germany and the offshore windfarms in Horns Rev. The design
of the insulation coordination for GIS must therefore be studied carefully.
During a disconnector operation in GIS, very fast transient (VFT) may generate overvoltages (VFTO)
inside the enclosure. Because the gas insulated system must be viewed as non-self-restoring, it is
important to ensure that the voltage inside the GIS does not exceed the insulation strength. This must
therefore be accounted for, when conducting an insulation coordination study of a GIS.
This article describes how the VFT phenomenon occurs inside the GIS and how it may generate
overvoltages. This includes an explanation of how it is generated, what causes it and why it is so fast.
A schematic consisting of the surge impedances from the manufacturer of the GIS is simulated and
compared to the same model with a number of capacitances added (representing the corresponding
component). These added capacitances were not modelled by the manufacturer, but were added in
order to further increase the level of detail. This is important as VFT may also be generated by circuit
breaker (BRK) operations, a ground switch or due to a fault [1]. A detailed model is more likely to
detect a VFT generated by e.g. a fault than a simplified model.
It is shown via simulations in a EMTP software, how the level of modelling detail affects the results.
As may be seen from the simulation results, there is a significant difference between the voltage
characteristics when simulating the GIS with and without the added components. The difference is
approximately 1.5 p.u., and is apparent 35 ns from when the disconnector operation is performed. It is
of interest to investigate what the primary cause of this difference is. A further analysis of this
difference, lead to a closer look at how the breaker is represented in the model from the manufacturer
and the model with the added components.
A comparison of the two models revealed the importance of detailed modelling, especially all of the
capacitances which are present in the GIS for e.g. circuit breaker, disconnector (DS) and spacers.
KEYWORDS
Very fast transients, GIS modelling, EMTP modelling, VFTO, VFT modelling.
2
Introduction
The insulation material used in GIS is Sulphur hexauoride gas (SF6) which greatly increases the
insulation strength from 27 kV/cm∙bar for air, to 89 kV/ cm∙bar [2, p.348]. The GIS is pressurized and
the operating pressure of the GIS systems is 4.5 to 5.3 bar. The high insulating strength of SF6 has
however some disadvantages as well. The type of gas used (SF6) is one of the factors responsible for
the generation of very fast transients (VFT) inside the GIS. The overvoltage generated by VFT may
range from 1.7 - 2 p.u. according to IEC 60071-4. There are however other sources which indicate
higher overvoltages e.g. as high as 2.5 p.u. [3, p.1]. Even though the overvoltage is only reported to
reach levels below 3.0 p.u. it must be investigated, especially for higher voltage systems. This is due to
the fact that as the voltage level of the system rises, the ratio between the switching withstand levels
and the system voltage decreases as shown in table 1. The switching impulse withstand level (SWIL)
is used as a reference of the overvoltage [5], as VFT often originate from a switching event. According
to [4, p.612], a 20% safety margin should be applied in insulation coordination studies of gas insulated
substations.
Table 1: Switching impulse withstand level (SWIL) in comparison to the system voltage IEC60071-1.
Highest voltage for
equipment Um [kVRMS]
24
52
123
145
245
420
765
Switching impulse withstand
level (SIWL) [kVPEAK]
145
250
550
650
950
1050
1550
Ratio [p.u.]
Ratio [p.u.] with 20 %
safety margin.
5.92
4.71
4.38
4.39
3.80
2.45
1.98
7.40
5.89
5.48
5.49
4.75
3.06
2.48
Even if the overvoltage does not reach the SIWL, the overvoltage may speed up the aging and
degradation process of the GIS [5, p.1]. It is however questionable if it should be compared to the
lightning impulse withstand level (LIWL) instead of SIWL. The reason for this is that the fast rise of
the VFT should perhaps rather be placed in a protective category with lightings
Origin of very fast transient overvoltage (VFTO)
A VFT is a result of an instantaneous change in the voltage inside the GIS. In most cases this change
in voltage is due to the opening or closing operation of a disconnector (DS). VFT may however also
be generated by circuit breaker (BRK) operations, a ground switch or due to a fault [3, p.1]. An
example of a DS operation is shown in figure 1, where a part of the GIS in Revsing is illustrated.
DS
Supply
side
DS
BRK
DS
DS
BRK
BRK
CT
DS
Load
side
BRK
CT
DS
CT
BRK
CT
DS
VT
SF6/air
Bushing
Bay 2
KAS2
DS
BRK
BRK
CT
DS
VT
Propagating
wave
BRK
CT
DS
DS
DS
DS
DS
VT
SF6/air
Bushing
Bay 3
EDR1
SF6/air
Bushing
Bay 4
TJE
Figure 1: During a DS operation a travelling wave is generated which may cause overvoltages in the
GIS. The dashed line indicates the current path in this specific switching scenario.
VFT has two main characteristics
1. Are in the highest frequency range in power systems: 1 to 50 MHz [3, p.1].
3
The reason for the high frequency is the overall compactness and construction of the GIS. This means
that from a modelling perspective it may be considered as
several short sections of transmission lines in series, each with
its own surge impedance. An example is shown in figure 2,
where a closed disconnector is modelled according to [6]. This
results in a vast number of discontinuities. In Revsing there
are 8 bays with 4 DS for each bay, resulting in a total number
of 32 DS, again resulting in a total number of 224 surge
impedance to be modelled, only with respect to the DS. This
results in many reflections and refractions of the travelling
wave occurring at the points of discontinuity, which may
superimpose each other. As a result, high frequency
Figure 2: Modelling of a closed
overvoltages will appear in the GIS [7].
disconnector in GIS [6].
2. Have a rise time of 4 to 100 ns.
The reason for the fast rise time of the VFT is due to several factors, which are further explained later
in this paper.
Trapped charge and its influence on VFTO
During a DS operation e.g. as shown in figure 1, numerous discharges (pre- or re-strikes) occur due to
the relative slow speed of the moving contacts [12]. Figure 3 shows possible voltage restrikes during
an opening sequence of a DS from an ideal capacitive floating section of the system. The floating
section is the section denoted as load side DS in figure 6. The disconnection of capacitive loads results
in trapped charges, which influence the amplitude of the VFTO, when closing the DS. The exact
amount of trapped charges depends on the disconnection and it may be explained by the following
sequence for figure 3:
1. A disconnection occurs and the voltage in the load side remains constant while the voltage in the
source continues oscillating at power frequency.
2. As time passes, the electrical potential between the two increases.
3. If the voltage breakdown level is exceeded, sparking occurs.
4. The current flowing through the spark will charge the capacitance on the load side to a voltage
equal to the supply voltage.
5. During the charging process the insulation strength between the two contacts increases as the
contact distance increases and the spark will eventually extinguish.
6. The process may repeat itself, resulting in a staircase type waveform.
Voltage
AC
Supply
voltage
Load
voltage
Figure 3: Voltage on each side of a DS during the opening sequence, which can lead to a trapped
charge on the load side
The amplitude of the trapped charge will have consequences on the magnitude of the travelling wave
which is transmitted during sparking, when the DS is closed again. The worst case scenario would be
if the voltage potential on the floating section would be 1 p.u. and the voltage on the other side of the
DS would be -1 p.u. or vice-versa. This would result in a 2 p.u. between the DS contacts. There are
however limits to the amplitude of the trapped charge. That is to say that the voltage amplitude will
according to various sources never reach 1 p.u. e.g. IEC 60071-4 states that the maximum trapped
charge will reach 0.5 p.u., but according to [5] and generally throughout this study a worst case
scenario of 1 p.u. should and will be investigated.
4
Origin of VFT, why is it very fast?
The breakdown field strength (E/p)0 of an insulating gas is dependent on the difference between the
ionization coefficient α and the attachment coefficient η. These coefficients are defined as follows:
Ionization coefficient α: Considering a swarm of
electrons moving in a gas under a constant field, the
growth of ionization rate is defined in terms of the
number of ionizing collisions per electron per cm
travel in the gas parallel to the applied field [9, p.39].
Attachment coefficient η: The removal of electrons
from the swarm is determined by an attachment
coefficient [9, p.39].
In other words α describes how fast free electrons are
created, whereas η is describes the gas ability to
absorb free electrons. Shown in figure 4 is the Figure 4: Effective ionisation coefficients in
relationship between the effective ionization and the air and SF6 [9, p.40].
breakdown field strength. Figure 4 indicates that SF6
is a “brittle” gas, as a slight increase in electrical field will increase the rate of ionisation much faster
than e.g. for air. That means that the breakdown process for SF6 is faster than the breakdown process
for e.g. air. The rise time of the VFT may be determined by the following equation [6, p.1].
𝑘𝑇
𝑡𝑧 = 13.3
[ns]
𝐸
( 𝑝 ) ∙ 𝑝 ∙ 𝜂ℎ
0
Where:
kT = Toepler spark constant = 0.5∙10-2 [V∙sec/m] , for: air, N2 and SF6.
(E/p)0 = Breakdown field strength [V/m∙bar].
ηh = Field efficiency factor (1 for a uniform field and 0 for radius of curvature approaching zero) [2,
p.203].
p = Gas pressure [bar].
The breakdown field strength (E/p)0 will according to [8, p.4], increase in proportion to the pressure of
the insulation gas. Meaning that at e.g 5 bar the breakdown field strength becomes 89 kV/cm∙bar∙5 =
445 kV/cm. Given a GIS pressure of 5 bar and a uniform field, the rise time is equal to 14.9 ns. This
correlates well with the definition of VFT which defines the rise time as between 4-100 ns.
Modelling components
All components are modelled in general according to IEC 60071-4. Detailed information regarding the
GIS components from the manufacturer was not available. Due to this reason and the fact that IEC
60071-4 does not specify values for each component, the authors searched for other sources to obtain
values for a similar system. Each component value and modelling method is shown in figure 6.
Generating the spark
This process may be modelled according to [5] and [10] with an exponentially decaying resistance in
series with a small resistance. This is based on a worst case assumption were a spark of maximum
amplitude is considered. The spark resistance is shown in the following equation [10].
𝑅 = 𝑅𝑎𝑟𝑐 + 𝑅𝑜𝑝𝑒𝑛 ∙ 𝑒 −𝑡/𝜏
[Ω]
Where:
Rarc is the arcing resistance = 0.5 Ω.
Ropen is the resistance of the gap = 1012 Ω.
τ is a time constant = 0.6∙10-9 s.
5
The implementation of the spark resistance equation in EMTP and simulation results from EMTP is
shown in figure 5, where the variable resistance is a function of time. As can be seen from figure 5 the
Figure 5:Simulations results from PSCAD/EMTP and the implemetation of the spark generator.
value of the variable resistance decays from 1 TΩ to zero in app. 20 ns. This correlates well with the
limits between 4 - 100 ns for VFT, more specifically the 14.9 ns as previously mentioned.
Overall GIS model
As detailed geometrical information regarding the GIS was not available, the possibility to construct a
detailed model of the GIS was limited. That is to say that including every component such as every
section, elbow, spacer and so forth was not possible. What was available, was a schematic of the GIS
model constructed by the manufacturer. The problem was that this model is a somewhat simplified
model, compared to the recommended level of detail, making it difficult for the authors to determine if
it is sufficient in detail for the study of VFT. A custom model is therefore constructed. The main
reason why the model from the manufacturer cannot be used to construct a detailed model, is that the
only geometrical data available was the length of the ducts and for each length the surge impedance
was given. This is in fact the only data available from the manufacturer regarding modelling of the
GIS. It is however of interest to investigate among other, if the model should be constructed in greater
detail. There are however some components which may be added to possible improve the level of
detail. It is therefore of interest to implement these components, and to compare the simulation results.
The following components are implemented as they were not accounted (or the value was unknown)
for in the manufacturer model:
Capacitance for DS, circuit breaker (BRK), surge arresters, Transformer and Reactor.
These components are documented by IEC60071 as standard components to model a GIS. Other
components require more detailed geometrical information.
Shown in figure 6 is the overall custom model to be implemented in EMTP. The components colored
in grey and the voltage transformers were the only available data from the manufacturer.
Simulation results
The following case study will be simulated for each model:
A trapped charge of 1 p.u. will be simulated in order to account for the worst case scenario.
The following locations will be measured for each model:
Load side DS, OHL terminal, Transformer terminal, Reactor terminal.
6
SF6/air
Bushing
58 Ω / 3.7 m
58 Ω / 4.1 m
58 Ω / 3.5 m
15 pF
58 Ω / 39.9 m
120 pF
51 Ω / 0.9 m
4 nF
Surge arrester
242 Ω / 3 m
Transformer
80 pF
15 pF
58 Ω / 1 m
15 pF
58 Ω / 1 m
58 Ω / 2.8 m
15 pF
58 Ω / 5 m
Voltage
transformer
DS
58 Ω / 5 m
SF6/air
Bushing
58 Ω / 3.7 m
58 Ω / 4.1 m
58 Ω / 3.5 m
15 pF
58 Ω / 39.9 m
120 pF
51 Ω / 0.9 m
4 nF
Surge arrester
242 Ω / 3 m
Reactor
80 pF
Voltage
transformer
350 pF
58 Ω / 1 m
10 pF
58 Ω / 2.7 m
Circuit breaker
15 pF
15 pF
DS
Load side DS
15 pF
350 pF
58 Ω / 1 m
58 Ω / 2.8 m
58 Ω / 5 m
58 Ω / 5 m
15 pF
58 Ω / 1 m
15 pF
DS
OHL
58 Ω / 5 m
58 Ω / 5 m
SF6/air
Bushing
58 Ω / 3.7 m
58 Ω / 4.1 m
58 Ω / 3.5 m
58 Ω / 77.5 m
51 Ω / 0.9 m
242 Ω / 3 m
15 pF
58 Ω / 5 m
80 pF
Voltage
transformer
DS
58 Ω / 2.8 m
58 Ω / 2.1 m
15 pF
80 pF
Voltage
transformer
Figure 6: The overall custom model to be implemented in an EMTP software.
Shown in figure 7 is the simulation results for voltage simulated at the load side DS, for both the
manufacturer and custom model. The maximum overvoltage at the DS is well below 2 p.u for both of
the models. There is an apparent difference in the simulation results between the two types of models.
Figure 7 : Simulation results from PSCAD/EMTP, comparing the model constructed by the
manufacturer and the custom model, measured at the DS terminal.
Shown in figure 8 is the simulation results for the voltage simulated the OHL terminal, for both the
manufacturer and custom model.
7
Figure 8: Simulation results from PSCAD/EMTP, comparing the model constructed by the
manufacturer and the custom model, measured at the DS terminal.
The maximum overvoltage at the OHL is well below 2 p.u for both of the models. There is an apparent
difference in the simulation results between the two types of models. Shown in table 2 are all of the
peak voltage p.u. measured for each model. Marked in red/bold is the maximum simulated
overvoltage. Not only are the waveforms very different, but it is apparent that the overvoltage for the
custom model is higher than for the manufacturer model. They are however both well below 2 p.u. and
thereby well below the SIWL limit.
Table 2: P.u. values for the different measuring points for the manufacturer and custom.
Model
Manufacturer
Custom
DS
1.447
1.699
OHL
1.323
1.758
Transformer
1.032
1.045
Reactor
1.032
1.050
A closer look at the simulation previously shown in figure 7 reveals a special area of interest where the
main difference is shown. Further analysis of the cause of this difference leads to a closer look at the
termination of the breaker in the two types of models.
Shown in figure 9 is the breaker termination, used as the breaker model in the manufacturer model.
The model is terminated by an open end, representing an open circuit breaker. The graph shown in
figure 9 is a simulation of the manufacturer model and the custom model, this time using the open
circuit breaker model shown in figure 9, for the custom model as well (replacing the one shown in
figure 6). This was performed in order to see if the main difference between the manufacturer model
and the custom model shown in figure 7 would disappear.
58 Ω / 5 m
58 Ω / 5 m
58 Ω / 1 m
58 Ω / 2.7 m
Figure 9 : Simulation results from PSCAD/EMTP, comparing the model constructed by the
manufacturer and the custom model, measured at the DS terminal using the open circuit breaker model
(shown on the right side).
As can be seen from the simulation in figure 9 the main difference disappeared. This shows the
importance of simulating, using the correct values for the capacitors, when representing a breaker in
8
GIS. For this study the voltage at the DS will result in a higher value with capacitors representing the
breaker, instead of simulating with an ideal open end.
Conclusion
As may be seen from table 2 that the maximum voltage will never reach the switching limits of 2.45
p.u. As was mentioned earlier, it is however questionable if it should be compared to the switching
voltage limit, were VFT should perhaps rather be placed in a protective category with lightings. This
would increase the allowable p.u. overvoltage to be as high as 3.32 p.u. placing it even further away
from the limit.
Modelling the GIS should be done in great detail.
Especially modelling the capacitance of components
is very important as components capacitances greatly
influence the simulation result. Modelling each small
detail may however be troublesome and difficult to
simulate. If the geometrical data is available a
simplification, as shown in figure 10, would however
be possible.
Figure 10: The figure on the left side is the
original illustration of the GIS and the
figure on the right is the possible equivalent
representation of the GIS [11].
There is a need to model all of the capacitances in detail and as is shown in this paper, the capacitance
of the breaker is important as it greatly influences the results. This is evident as modelling with the
correct components (capacitances) resulted in a VFTO of 1.758 p.u. compared to 1.323 p.u. without
the corresponding components.
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