vfto in gis

8/10/2019 VFTO in GIS

1/5

CAN FERRITE MATERIALS OR RESONANT ARRANGEMENTSREDUCE THE AMPLITUDES OF VFTO IN GIS?

S. Burow 1*, S. Tenbohlen 1, W. Khler 1, J. Smajic 2, A. Trger 3 and W. Holaus 31Institute of Power Transmission and High Voltage Technology, Pfaffenwaldring 47,

70569 Stuttgart, Universitt Stuttgart, Germany2 ABB Switzerland, Segelhofstrae 1K, 5405 Baden-Dttwil, Switzerland3 ABB Switzerland, Brown-Boveri-Strae 5, 8050 Zrich, Switzerland

*Email: [email protected]

Abstract: Very Fast Transient Overvoltages (VFTOs) in high voltage Gas InsulatedSwitchgears (GIS) are an undesired phenomenon which causes several problems ininsulation systems and connected high voltage equipment. Therefore a reliable and basicsolution of VFTO mitigation is of high interest. In order to investigate different dampingsolutions, a high voltage test setup which enables amplitudes of VFTOs up to 800 kV andmore is introduced. Also two possible damping solutions were investigated: First, VFTOsuppressing by means of ferrite materials was analysed under high voltage conditions.Different ferrite materials were tested. The second approach is a high-frequencyresonator which is placed inside the GIS. It gets stimulated by the VFTOs and due to aspecial resonator termination the VFTO energy is dissipated. The high voltage test setupand the investigation results of both possible damping solutions are presented in thispaper.

1 INTRODUCTION

Very Fast Transient Overvoltages (VFTOs) in highvoltage Gas Insulated Switchgears (GIS) areelectromagnetic waves propagating along the GIS.Due to an instantaneous change of voltage insidethe GIS VFTOs get initiated. The main reason forVFTOs in practice is a disconnector switchingoperation where steep voltage drops occur severaltimes within a closing or opening operation. As theVFTOs get multiply reflected at each GIStermination, VFTOs have a non-harmonic timedependence and cover a wide frequency rangefrom 100 kHz to 100 MHz. The amplitude of VFTOcan reach up to 3 times the phase-to-earth voltageand raise time is in a range of few ns [1],[2].

Therefore VFTOs can cause a high stress for theinsulation system of GIS and other directlyconnected high voltage equipment. The risk of aninsulation fault caused by VFTOs raises for 420 kVand higher system voltage levels. Presently,VFTOs are a limiting constraint for the dielectricdesign of ultra high voltage (UHV) GIS. Thereforea reliable and basic solution of VFTO mitigation isof great interest.

Disconnector switches in UHV GIS are alreadyequipped with a series resistor [3]. Unfortunately, ahigh effort on technical and electrical design ofresistor fitted disconnector switches is necessary.Therefore cost and maintenance of disconnectorswitches is much higher compared to conventionaldisconnector switches.

The aim of the presented work is to develop a veryreliable and basic solution of VFTO suppression.

Therefore a special test setup was designed and asuitable measurement system was installed. Thissetup enables investigations and measurementsunder high voltage conditions. In this paper thehigh voltage test setup and results of two differentVFTO mitigation solutions will be presented: (a)investigation of mitigation potential of ferrite ringcores in a GIS and (b) development of a specialdesigned electromagnetic radio-frequency (RF)resonator inside the GIS. As (a) was alreadydiscussed manifold [4], [5], (b) is a pretty newapproach.

2 EXPERIMENTAL SETUP

The experimental setup enables high voltage testswith different mitigation solutions. Specialcapacitive sensors ensure correct measurementresults.

2.1 High voltage test setup

The high voltage test setup consists mainly of a550 kV GIS from ABB type ELK-3. A schematicdiagram is shown in figure 1. The VFTOs aregenerated by a breakdown between the spark gapinside the GIS. For this purpose a standard highvoltage impulse generator feeds a surge voltagevia a bushing to the inner conductor of the GIS. Bymeans of a spark gap the inner conductor isseparated into two parts. The spark gap consists oftwo spherical electrodes and is situated inSF 6 atmosphere. As the surge voltage reaches thebreakdown voltage of the spark gap, a sparkover

occurs between the contacts and initiates theVFTOs. By means of the SF 6 pressure, it ispossible to control the breakdown voltage of thespark gap and thereby the amplitude of VFTOs as


2/5

well. The end of the inner conductor is connectedwith two cupper strips to the enclosure and ground.Thus it forms a shorted termination and thetransient voltage waves are reflected by a negativefactor close to -1. In a test vessel with a third orificeand flexible inner conductor different testingobjects (e.g. ferrite rings and resonator) couldeasily be installed. On both sides of the test vesselcapacitive voltage sensors are mounted in a flangeof the GIS enclosure and enable very accuratemeasurement. The testing arrangement ispresented in figure 2.

SurgeGenerator

SparkGap

TestVessel

Bushing GroundStrips

Sensor1

Sensor2

Figure 1: Schematic diagram of high voltage testsetup: GIS with spark gap, capacitive voltagesensors and a flexible test vessel for different testinstallations

Figure 2: The high voltage test setup in thelaboratory of University of Stuttgart: ABB 550 kVGIS type ELK-3 with bushing

By means of this setup it is possible to generateVFTOs with an amplitude up to 800 kV and more.This depends on the SF 6 pressure and thedistance between the spherical contacts of thespark gap. The dominant harmonic component ofthe VFTO frequency spectrum is approximately7.5 MHz and is related to the total length of the

GIS.

2.2 Voltage measurement system

For a precise measurement of these VFTOs twocapacitive voltage sensors were built up andmounted inside the GIS. The sensors consistmainly of a double layer board and are presentedin figure 3. The layer facing to the grounded GISenclosure is connected to the enclosure by theflange and a copper RF-seal directly. The secondlayer forms a capacitance as well to the groundedlayer as to the inner conductor of the GIS. So ahigh frequent capacitive voltage divider is arrangedwith a divider ratio of some 10 000. The top layer isconnected to a BNC-connector outside the GISflange where the divided VFTO voltage can bemeasured by an oscilloscope.

1

2

3

Figure 3: The capacitive sensor which was madeat University of Stuttgart: It consists mainly of a

double layer board (1) which forms a capacitivevoltage divider. The board and a copper RF-seal(2) are mounted on a flange cap (3).

The signals of the Sensors are recorded by anOscilloscope (LeCroy waveRunner 104 MXi, usedBandwidth: 200 MHz). Before analyzing theresults, always 10 signals were superimposed toavoid impacts caused by small variation of thebreakdown voltage. The frequency spectrum of theresults is computed by a Fast FourierTransformation with Matlab.

3 FERRITE MATERIAL FOR VFTO DAMPING

Ferrite rings around the inner conductor for VFTOdamping were already investigated and results arepresented e.g. in [4]. In the majority of cases ferriterings were tested in low voltage setups orsimulations of their behaviour under influence ofVFTOs were carried out. The results of theseinvestigations show at least a slight VFTOmitigation. In this chapter the result of tests withdifferent ferrite rings and ferrite arrangements in ahigh voltage setup will be discussed. Therefore therings were located inside the test vessel andVFTOs were generated as described in theprevious chapter.


3/5

3.1 Test with ferrite rings

Rings of different ferrite materials wereinvestigated and the number of rings inside theGIS was variegated from 1 to 6 rings. The ringshad either an inner diameter of about 10.4 cm,were 1.7 cm thick and had a length of 2.6 cm orhad an inner diameter of about 1.5 cm, were2.4 cm thick and had a length of 2.5 cm. A smallerring with an inner radius of 4.5 cm, thickness of1.5 cm and length of 2.6 cm was also used. Theused rings are presented in figure 4. For the testsVFTOs with an amplitude up to 800 kV were used.

1

2

Figure 4: Different kind of ferrite rings (1) and asheared ferrite ring arrangement with gaps ofepoxy resin (2)

All measurements show no mitigation of VFTOs.For example the result of a measurement with6 rings and VFTOs with an amplitude of 700 kV is

presented in figure 5. It is compared to ameasurement without ferrite rings.

0 10,5 1,5 2time/s

0

500

-500

v o l t a g e / k V

2

1

Figure 5: Comparison of VFTOs with a maximumamplitude of about 700 kV: The darker linerepresent VFTOs without rings (1). VFTOs with 6ferrite rings are drawn in the brighter colour (2). Nodamping is visible.

In comparison to most previous studies theresulting current through the rings is much higherand equals real currents of VFTOs in a GIS. Theimpedance of the GIS setup is approximately 70 .Therefore VFTOs with an amplitude of 700 kV leadto a current of about 10 kA. Thus, the intensity ofthe magnetic field around the conductor is also

very high. If a current of this amplitude flowsthrough a ferrite ring, the high magnetic fieldsaturates the ferrite material completely. Theresults of these measurements demonstrate clearlythat a saturated ferrite ring cannot suppressVFTOs.

3.2 Test with ferrite arrangements A possibility of reducing the effective magnetic fieldintensity in a ferrite ring is an insertion of gaps withlower permeability in the ring. Through the socalled shearing of ferrite rings, the magnetic fieldstrength which saturates the material could beincreased. But shearing the rings with a material oflower permeability reduces the effectivepermeability too. Therefore, the damping efficiencyof the whole ring arrangement decreases. Figure 4shows a sheared arrangement of ferrite segmentswith plates of epoxy resin as known from conductorboards. Test with this ring arrangement howevershow no VFTO suppressing effect.

In summary the investigation of ferrite materialsresults in no mitigation of VFTOs. The reason ofthis is the high current of VFTOs which causes acompletely saturation of the ferrite material.Saturated ferrite material loses its damping abilityand can thus not suppress any VFTOs.

4 RF RESONATOR FOR VFTO DAMPING

A new approach for mitigation of VFTOs is acompact electromagnetic RF resonator withoptimised quality factor (Q). The resonator isinstalled inside the GIS and gets stimulated by theVFTOs. For example, a cavity of an electricshielding could serve as a resonator. The noveltyof this idea is not only the design of the low Qresonator. Also the dissipating of the receivedVFTO energy must be investigated.

4.1 RF resonator design

For basic investigations a resonator around theinner conductor was built as it is shown in figure 6.It is a tube which is connected to the inner

conductor on one side. The other side forms a thin,but long gap between resonator and innerconductor.

2

13

Figure 6: The resonator (1) around the innerconductor of the GIS (2) with a long gap betweenresonator and conductor (3)


4/5

The resonant frequency of the resonator is about57 MHz. This was determined by a specialarrangement, which enables a measurement of thevoltage in the gap between resonator and innerconductor (figure 7). Therefore the resonator wasmounted in a test vessel with cones on each side.On one side of the test arrangement a terminatingimpedance was placed so that reflection wasavoided. On the other side a waveform generatorwas connected via a BNC plug to the innerconductor of the test vessel. The output of thewaveform generator was a sinusoidal signal withan amplitude of 10 V. By steps of 1 MHz thefrequency of the signal was shifted from 1 MHz upto 100 MHz. A third orifice of the test vesselallowed direct access to the resonator. So thevoltage in the gap between resonator and innerconductor could be measured by a special kind offield probe made by University of Stuttgart.

Figure 7: The test vessel with cones in each sideand the mounted resonator on the inner conductor

Figure 8 shows the gap voltage measured by thefield probe. It changes as a function of the signalfrequency. As the amplitude of the input signal isconstant, the resonant frequency of this resonator

is achieved at 57 MHz when the gap voltagereaches its maximum.

frequency/MHz0

020 40 60 80 100

0,5

1

n o r m a

l i z e

d

g a p v o

l t a g e

Figure 8: Gap voltage measured by a field probeas a function of frequency: Resonator withoutexternal capacitor parallel to the gap (grey curve)and Resonator with 5 nF capacitor parallel to thegap (dashed curve)

The VFTO frequency spectrum of the GIS is afunction of the GIS length and termination. As thedominant harmonic component of the used testsetup is about 7.5 MHz, it is an inherent differencebetween the resonant frequency of the resonatorand the dominant harmonic of the test setup. For

an effective mitigation, both frequencies shouldequal each other as much as possible. Thereforethe resonator has to be adjusted.

It is possible to describe the resonator as a parallelresonant circuit. So the resonant frequency of theresonator is a function of its inductance andcapacitance:

(1)

While the gap between resonator and innerconductor forms the capacitance of the resonantcircuit, the cavity of the resonator represents theinductance. An increase of one or both parametersleads to a reduced resonant frequency of theresonator. To increase the values of theseparameters, several approaches were investigatedand are listed in table 1. All test with thesemodifications and combinations of them results in alower resonant frequency of the resonator.

Increase in Capacitance

Dimension of gapDielectric

properties ofgap

Extension ofgap length

Reduction ofgap width

Increased permittivity of

dielectricmedium

Increase in Inductance

Dimension of cavity Magnetic propertiesof cavity

Extension of cavityvolume

Increased permeabilityof cavity

Table 1: Different approaches for modifying theresonance frequency of the RF resonator

To reach an equality of the resonant frequency ofthe resonator and the dominant harmoniccomponent of the VFTO frequency spectrum, asimplification was done for the first laboratory tests:The capacitance of the gap between resonator andinner conductor was increased by an additional

external capacitor. It was connected parallel to thegap as shown in figure 9. Finally, an externalcapacitance of 5 nF leads to a maximum of gapvoltage at about 7.5 MHz. The gap voltage of theresonator with a 5 nF termination is shown as afunction of the signal frequency in figure 8.

In summary it can be concluded, that it is possibleto achieve a designated resonant frequency of theresonator with a certain design of the cavity andthe gap. For further tests the resonant frequencywas adjusted by the external capacitor.

4.2 Dissipation of EnergyFor an ideal stimulation of the resonator by theVFTOs, the resonant frequency of the resonator


5/5

vfto in gis

Documents