NAME :
Uwe Riechert
COUNTRY :
Switzerland
REGISTRATION NUMBER : 853
GROUP REF. : A3
PREF. SUBJECT : 1
QUESTION N° : Q.1-5
Mitigation of Very Fast Transient Overvoltages
Uwe Riechert
VFTO in GIS
During switching of disconnector switches (DS) in gas-insulated switchgear (GIS) a varying number of prestrikes and re-strikes occur. Due to the very short duration of the voltage collapse of a few nanoseconds at the
switching gap, travelling surges are generated in the GIS. The multiple refractions and reflections of these surges
at impedance discontinuities within the enclosures create complex waveforms (very fast transient overvoltages –
VFTO). VFTO in GIS are of greater concern at the highest rated voltages, for which the ratio of the lightning
impulse withstand voltage to the system voltage is lower. CIGRÉ WG A3.22 has collected VFTO simulation
results for GIS installations from 550 kV up to 1100 kV [1]. As the rated voltage increases, the difference
between the rated lightning impulse withstand voltage (LIWV) and the VFTO decreases, as shown in Figure 1.
Hence, VFTO can become the limiting dielectric stress which defines the dimensions of both the switchgear
equipment as well as the connected equipment in certain UHV cases.
Figure 1 Dependency of rated withstand voltages and calculated VFTO on rated voltage as per IEC
62271-203, calculated values from [1] (different symbols for different substation)
Summarizing the different experiences CIGRÉ SC D1 has proposed a procedure for the insulation co-ordination
in case of VFTO, following the general insulation co-ordination approach [2]. Basis for the insulation coordination is the calculation of the required VFTO withstand voltage for the different equipment by using:
• Co-ordination factor Kc (statistical distribution, inaccuracy of simulation, frequency of occurrence,
volume effect)
• Safety factor Ks (atmospheric correction if applicable, aging behaviour in service, quality of
installation)
• Test conversion factor Ktc (for a given equipment or insulation configuration, the factor to be applied to
the required withstand voltage, which describes the different withstand behaviour under VFTO stress
compared to the stress with standard LI voltages)
The CIGRÉ WG C4.306 has reviewed the insulation co-ordination practice in the UHV AC range and proposes
values for the different factors, giving a total factor of 1.15 [3]. The required VFTO withstand voltage is also
shown in Figure 1. If the maximum VFTO is below the LIWV, no measures need to be taken. Otherwise it is
necessary to design considering the VFTO level as dimensioning criteria or to suppress VFTO by damping
devices. A maximum mitigation effect of 20 % to 25 % is required to supress the VFTO peak below the LIWV
for UHV applications considering the worst cases, as shown in Figure 2.
Figure 2 Dependency of rated withstand voltages and calculated VFTO on rated voltage as per IEC
62271-203,(25 % mitigation)
Mitigation
For the different sources of VFTO and for the different equipment different mitigation methods are known. The
damping of VFTO by integration of a damping resistor is one proven technology. The way to overcome the
drawback of such unwieldy design is to use other internal damping measures.
The new proposal concerning insulation co-ordination requires detailed knowledge about the trapped charge
voltage (TCV) of the DS. The new DS model for the entire VFT process analysis allows for trapped charge
voltage calculation. The assumption of a more realistic TCV value leads to more realistic insulation coordination calculations. The application of a slow-acting DS provides significant reduction of the VFTO up to
25 %.
A new approach for damping is to implement compact electromagnetic high-frequency resonators with low
quality factor specially designed to cover a wider frequency range. The novelty of this idea is not only in
designing the resonators but also in dissipating the received VFTO energy. The VFTO damping effect of the
developed RF resonator tuned to the dominant harmonic component was confirmed by full-size experiments.
Rings of a nanocrystalline material placed around the GIS conductor leads also to a significant mitigation effect
(10 -20 %). Depending on number, material and size of the rings good results could be achieved. Even the
amplitude of the first peaks was significantly reduced by 20 %.
A summary of the different mitigation methods is given in Table 1. By combination of different methods an
increased mitigation effect could be achieved.
Table 1
Comparison of different mitigation measures
Method
Slow acting DS
(low trapped
charge voltage)
Service
experiences
>20 Years
Mitigation
effect
Example
15-25 %
VFTO for different contact speed and trapped charge
voltage behavior for 1100 kV DS
Full scale tests
Up to 25 %
voltage / kV
Nanocrystalline
material
500
400
300
200
100
0
-100
0
10-7
2·10-7
3·10-7
time / s
VFTOs without magnetic rings (continuous line) and with
20 magnetic rings (dashed line)
High frequency
RF resonator for
VFTO damping
Full scale tests
Up to 20 %
The measured VFTOs in frequency domain of a 550 kV
GIS with and without resonator
[1] CIGRÉ Brochure 456 Working Group A3.22: “Background of Technical Specifications for Substation
Equipment Exceeding 800 kV AC”, April 2011
[2] CIGRÉ Brochure SC D1: “Very Fast Transient Overvoltages (VFTO) in Gas-Insulated UHV Substations”,
to be published in 2012
[3] CIGRÉ Brochure Working Group C4.306: “Insulation coordination for UHV AC systems”, to be published
in 2012
[4] Riechert, U.; Bösch, M.; Smajic, J.; Shoory, A,; Szewczyk, M.; Piasecki, W.; Burow, S.; Tenbohlen, S.:
“Mitigation of Very Fast Transient Overvoltages in Gas Insulated UHV Substations”; CIGRÉ Report A3110, 44th CIGRÉ Session, August 26-August 31, 2012, Palais des Congrès de Paris, Paris, France CIGRÉ
Session 2012