INSTRUMENT TRANSFORMERS INSULATION BEHAVIOUR TO...

ARWtr 2004INSTRUMENT TRANSFORMERS

INSULATION BEHAVIOUR TO HIGH VOLTAGE (HV) AND HIGH

FREQUENCY (HF) TRANSIENTS.Adolfo Ibero

HERMANOS ARTECHE S.A.Munga, Bilbao-Spain

www.arteche.com

Modern TransformersARWtr 2004 28 -30 October. Vigo Spain http://webs.uvigo.es/arwtr04

2Instrument Transformers insulation behaviour to H.V. and H.F transients

Advanced Research Workshop on Modern Transformers. October 2004ARWtr 2004http://webs.uvigo.es/arwtr04

IntroductionThe possible failure modes shall first be defined, following which, the equivalent circuit of the transformer insulation shall be described, with the limits on the validity of this model, the laboratory tests used, and the results and conclusions derived fromthese.



Possible Failure ModesThe initial effect of a high-voltage, high-frequencytransient on insulation is to produce a mechanicalshock wave in the insulation. This mechanical shockwave will produce a vibration that compresses andexpands the insulating material. This shock wavecould cause immediate fracture of the insulation at itsweakest point, which will normally coincide with themaximum electrical gradient, where the mechanicalgradient is also highest.



Possible Failure Modes

Fig.1




A second failure mode, which is a consequence of thefirst one, is partial discharge. These partial dischargesare caused by inadequate impregnation of theinsulation. The mechanical shock waves produce aninternal cavitation phenomenon in the oil, releasingany gases that have remained occluded in it. For thisreason, it is essential to impregnate the paper with oil that has been thoroughly de-gassed, and theinsulation must be very compact.




The third failure mode, involves partial dischargescaused by high resistivity of the Faraday screening ofthe insulation. The values of current (kiloamps) and frequency(megahertz) involved make the design of the Faradayscreening a complex task, since very flexible electrodes must be achieved, with a low transversal resistance to allow these high currents to flow, while atthe same time, they must not present difficulties whenit comes to their use in the manufacture of theequipment.



Equivalent circuit of the insulation of an inverted-type C.T.

C.T. Fig.2

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Advanced Research Workshop on Modern Transformers. October 2004ARWtr 2004


Fig.3

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The equivalent circuit shown in Fig. 3 can be used when thefollowing condition is satisfied: the applied voltage transientsmust have rise or fall times or equivalent oscillationfrequencies such that this circuit is valid in the space-time domain. In other words, the propagation times of theelectromagnetic phenomena through the length and breadthof the insulation must be much shorter than the rise or falltimes of the transient voltage waveforms applied to thetransformer.This is important in 420 kV Very High Voltage equipment, and becomes really critical at 525 kV and 765 kV.




When we mention high-voltage, high-frequency transients, we are really talking about over-voltage phenomena between the line and ground due to lightning and/or circuit breaker operations.In reality, the most damaging phenomenon for the insulation is a lightning-type over-voltage spike with an abrupt leading or trailing edge caused, for example, by external spark-gaps. This type of over-voltage provokes a "stepped wave" that propagates along the line until it reaches the instrument transformer, inducing high-frequency over-voltage and over-current surges in its insulation, which must be designed with adequate dimensions to withstand these.




In practice, and given thedimensions of the lines, theconnections between circuitbreakers and C.T., the physicaldimensions of the C.T. itselfand its pedestal, etc., thestepped wave is transformedinto a spike with a minimum falltime of 0.3 s.

Fig. 4

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The wave enters the dielectric through the H.V. head as conduction current / voltage. It passes through the H.V. Faraday screening and is converted into a displacementcurrent / electric field to cross the dielectric material andreach the low-voltage electrode.

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If we take into account that we are talking about thicknesses of 70 to 100 mm. of dielectric, and that this dielectric is oil-impregnated paper with a propagation constant of:

and therefore, a velocity of:

To cover 100 mm = 10 10-2 m, the time required is:

Therefore, tpropagation



Fig. 6

The other propagation path is via the length, breadth and thickness of the insulating bushing. The insulating bushing is really a bushing that allows the insulation to be reduced in size and compacted in order to make an element with the smallest possible physical dimensions.

Fig. 6




We have already seen that the thickness of the insulation is such that the propagation time through it is much shorter that the duration of the perturbation. However, an analysis is required of the length and width of the insulating bushing to ensure that it behaves as a short transmission line from the point of view of the high-voltage high-frequency transients that are applied.

If it behaves as a long transmission line, undesirable delay phenomena may occur in the voltage propagation and distribution that may cause failures in the insulation.



The cone of an insulating bushing have a length of up to about 3 metres at most.

The propagation velocity through oil-impregnated paper is: v = 1.76 108 m/sec.

The duration of the perturbation is 0.3 s.

The propagation time or delay along the length of the bushing will be:

Criterion for a short transmission line : 0,1tperturbacin tretardo

In the present case: 0.1 0.3 s = 0.03 sec 0.017 s

Therefore, this case can be regarded as a short transmission line.

seg 10 1,7 seg 10 1,7 seg

m 10 1,76m 3 t 2-8-8r

==

=



In addition to being physically short, for this to be valid, it isalso necessary that the electrical resistance of theconductors in the transmission line be practically zero.

Fig. 7




This implies that the screens used to make the bushingmust be metallic, otherwise the delay time will increaseto inadmissible values. This would no longer be a short transmission line, leading to problems with reflectedwaves along the line and consequently, undesirablevoltage distributions in the bushing.



If the above conditions are satisfied, the circuit shown in Fig. 3 can be taken as valid, with the discrete values of capacitance, resistance and self-induction shown in the figure representing the electrical behaviour of the insulation of the current transformer.

Fig. 3




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Impulse generator Spark gaps Metering CA-525 TransformerDivider

Equivalent circuit of the C.T. and the Chopped Lightning impulse instalation.




Recorded waveforms in a CA-525 kV C.T.V. peak value = 1270 ; = 5070 through the insulation.

VK

I

A

Frequency of the current through the insulation = 1600 kHz.

Test was repeated 600 times on the unit.




Results obtainedBeforeBefore After



Conclusions

This test allows definitive testing of the resistance ofthe insulation to this type of transient.

Problems may be expected when there are changes in the gas concentrations that cannot be explained by other phenomena, or the test pieces do not pass thepartial discharge tests after being subjected to thistest.



IntroductionPossible Failure ModesPossible Failure ModesPossible Failure ModesPossible Failure Modes

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