A Study of UHF Partial Discharge Signal Propagation

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A Study of UHF Partial Discharge Signal Propagation

in Power Transformers using FDTD ModellingA. M. Ishak

University of Strathclyde

[email protected]

M. D. JuddUniversity of Strathclyde

[email protected]

W. H. SiewUniversity of Strathclyde

[email protected]

Abstract-The UHF method for locating partial discharge(PD) sources in power transformers has become increasingly

important in recent research. In order to strengthen ourunderstanding of what is required to locate PD by this method,it is important to study the propagation of electromagneticwaves from PD in transformers. This paper is concerned withultra-high frequency (UHF) PD signal propagation in powertransformers in the presence of conducting obstacles, which mayrepresent the core, winding or other internal structures. Theapproach is to use the FDTD (Finite-Difference Time-Domain)method which can model the propagation of electromagnetic

waves and their interactions with the structure of materials.FDTD is a technique widely used in microwave and radiofrequency applications. Being a time domain method, it isparticularly suitable for modelling the time-of-flight PD locationproblem. A 3D geometry has been created to represent a simpleoil-filled tank containing a PD source, UHF sensor and anobstacle to line-of-sight UHF signal propagation. The effect ofobstacles in delaying the arrival time and attenuating the leadingedge of the signal is assessed. Implications of the results for

accurate PD location by the UHF method are discussed andfurther improvements to the modelling study are proposed.

Index Terms-FDTD, Partial Discharge, transformer, UHFsignal.

I. INTRODUCTION

The insulation system of power transformers is an

important aspect for the reliable and safe operation of

electrical power networks. Partial discharge (PD) within

power transformers often indicates weaknesses in the

insulation [1]. It can be valuable to locate PD sources in

power transformers in order to determine what remedial

action is necessary. PD pulses usually involve rise times of

less than 1 ns, which will excite a signal in the UHF range

(300-3000 MHz) [2]. The PD location can be estimated by

timing the arrival of UHF signals at several sensors on a

transformer tank, as shown in Figure 1. The algorithm tolocate PD sources in power transformers by using three or

more sensors has been outlined in [3]. It can estimate the

shortest propagation path of UHF signals using a numerical

procedure to account for obstacles. If the signals are noisy,

they could first be denoised, as elaborated in [4] [5] [6].

This paper reports a study of the propagation of UHF PD

signals in power transformers using the FDTD method.

Example applications of FDTD can be found in the literature

[7] [8] [9]. This paper deals with the propagation of a

wavefront from a PD source to a sensor with an obstacle at

the centre of the tank. The effect on PD location is studied by

observing the differential time delay for a tank with and

without an obstacle.

Fig. 1. Example of UHF PD location in a small power transformer,

following the technique outlined in [1]. S1 - S4 are UHF sensors and thelines to them approximate the propagation path of signals from the PD source

to each sensor.

II. DESCRIPTION OF THE MODEL

Two kinds of obstacles (conducting cylinder and cuboid)

have been positioned in turn at the centre of a tank. The grid

system has been defined on an FDTD cell size of 0.01 m for

all axes.

The dimensions of the tank are 4 m 2 m 3 m, which

represents a small oil-filled power transformer. The thickness

of the tank wall is 0.02 m and it is defined as a perfect electric

conductor (PEC). One obstacle is a cylinder of size 3 m 1 m(height diameter), as shown in Figure 2. The second

obstacle tested was a cuboid of dimensions 1 m 1 m 3 m.

Table I summarises the material parameters used in the

simulation. The coordinates of the electric field point sensor

are {0.5 m, 1.0 m, 1.5 m} and a Gaussian PD current source

of 0.28 ns pulse width has been applied. This pulse width is

the default value selected by the software for this particular

mesh spacing, which ensures a broadband response and is

also in the appropriate range for a PD pulse. Three orthogonal

directions of PD current have been simulated in turn, which

UPEC2010

31st Aug - 3rd Sept 2010


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flows over a 1 cm path from the coordinates {3.50 m, 1.00 m,

1.50 m} to the points {3.51 m, 1.00 m, 1.50 m}, {3.50 m,

1.01 m, 1.50 m} and {3.50 m, 1.00 m, 1.51 m} respectively.

These directions represent the positivex-direction,y-direction

andz-direction of PD current sources.

y

(a)

z

(b)

Fig. 2. The tank and centrally-located cylindrical obstacle in the model

viewed (a) in thex-yplane, and (b) in thex-zplane.

TABLE ITHE MATERIALS OF THE MODEL TANK AND INTERNAL OBSTACLES

Parameter

Geometry

RelativePermeability

RelativePermittivity

Conductivity(S/m)

Mineral Oil 1 2.2 0

Conducting

Cylinder/Cuboid1 1 5.8 107

III. SIMULATION AND ANALYSIS

For each obstacle, six simulations were carried out using

the XFdtd 7.0 software, comprising each of the three

orthogonal PD current directions with and without the

obstacle. The simulation period of was 96.27 ns,

corresponding to 5000 time steps of 19.254 ps each. The

simulation time was typically 78 minutes on a powerful PC

workstation. Without the obstacle, the simulation took a fewseconds longer to complete because the volume within which

fields have to be computed is slightly larger.

A key parameter of interest in the output data is the

estimated differential time delay of PD signal that is

introduced by the addition of the obstacle.

The absolute distances between the sensor and PD source

were calculated based on simple geometry. The geometrical

minimum distances which the PD signals would have to

propagate with the two conducting obstacles are calculated as

illustrated in Figure 3. The differences in geometrical

minimum distances with and without obstacles will be used

as reference values to assess the differential time delaysresulting from the FDTD simulations. The theoretical values

for the differential time delay were calculated using the speed

of light in oil transformer, which equals 2 108

m/s [10].

Table II lists the expected differential time delays.

(a)

(b)

Fig. 3. The calculation of geometrical minimum distances for (a)cylindrical obstacle, and (b) cuboid obstacle

x

x


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TABLE IIPROPAGATION TIME DELAY BASED ON GEOMETRIC CALCULATION

The electric field sensor included in the modelling software

delivers the absolute magnitude of electric field as a function

of time. The total electric field results from the vector sum of

the Cartesian components, which can also be exported

separately. By way of example, Figure 4 shows the variation

of total electric field magnitudeEwith time and that of thex,

yand zcomponents of E(with and without obstacles) for an

x-directed PD current source. Note that the dominant electric

field component of the radiated PD signal is in the direction

of PD current flow.As is evident from Figure 4, only the initial part of the

UHF signal response to PD is available from the simulation.

Much more time would be needed to simulate the whole UHF

resonance. The signals will still be reflecting and refracting

inside the oil-filled tank long after 100 ns of propagation

time, but in this study we are only interested in the initial

response.

Enlarged views of the PD signals from Figure 4 are shown

in Figure 5. Their amplitude is very small at the leading edges

but it is noticeable from Figure 5 that the signals without the

cylindrical obstacle arrive about 0.8 ns before the signals with

the cylinder present, which is in line with expectation.Examination of the orthogonal components of the electric

field indicates that the x-component is the dominant

contributor to the overall electric field magnitude at the

sensor. In this regard, it is interesting to observe that the

initial peak of the electric field is actually larger in amplitude

when the cylinder is present (despite the delayed arrival).

This touches on some of the issues of the complexity of

propagation around conducting obstacles, which provide

much scope for future detailed investigation. For example, if

we consider the radiation pattern of the x-directed PD source

to be similar to that of a small dipole then the PD sensor

would lie on a projection of the line over which the current

flows. The radiation pattern suggests that the field component

parallel to the current flow will be large but that on the axis of

current flow will be negligible. Since the boundary conditions

on a conducting cylinder require that the electric field is

perpendicular to its surface, it is likely that the presence of the

cylinder assists with steering a surface wave around its

surface. By considering the shortest path shown in Figure

3(a), it is apparent that the angle of the radiated field leaving

the PD source along this path will one for which the x-

component ought to be larger.

Signal arrival times were calculated based on the total

electric field magnitude according to the following procedure:

The electric field data is squared and then a threshold is

defined as a certain percentage of the peak value of this

squared data. The first time at which the squaredE-field data

crosses the threshold is taken as the absolute value of arrival

time. The absolute arrival time is of little value in itself, but

subtracting pairs of arrival times between data sets allows thenecessary differential arrival times to be obtained. Figure 6

shows the differential time delays for the obstacles, including

the effects of the three different PD current source directions

and the different threshold levels used to calculate signal

arrival times. The differential time delay for the simulated

electric field has been compared with the expected

differential time delay from Table II, revealing the potential

for several ns of timing error.

Fig. 4. Absolute total and three absolute components of PD signals with

and without cylinder (Positivex-directions of PD current source)

Practical experience has led us to conclude that, even for

relatively noise-free signals, a threshold of about 1% is aboutthe minimum that can give repeatable results with UHF

signals from power transformers. In these simulations, we

have been able to apply thresholds several orders of

magnitude smaller, since the output data is noise-free. Even

so, the levels of differential timing error evident in Figure 6

are still greater than might have been expected for such

impractically low thresholds. Further studies are needed to

see if this may be a consequence of the very simple model

perhaps the structure is too simple and symmetrical to allow

for the conversion between orthogonal field components that

ObstacleObstacle

Configuration

Geometric

Distance

(m)

Absolute

PropagationTime

(ns)

Expected

DifferentialTime Delay

(ns)

CylinderWithout 3.000 15.00

0.81With 3.162 15.81

Cuboid Without 3.000 15.00 1.18With 3.236 16.18


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may be possible with many more conductors are present at

varying angles to the radiated electric field.

Note also that the effect of the response of UHF sensors (as

reported in [11]) has not been taken into account in this work,

and will be a further contributory factor to the timing

accuracy.

Fig. 5. Expanded views of the total E-field and its components with and

without cylinder for PD current source in the positive x-directions. Labels

show the magnitude of the first peak in the signal received at the sensor.

IV. DISCUSSION AND CONCLUSIONS

Accurate onset time determination for UHF signals is

important for locating PD sources in power transformers. A

study of the propagation of UHF signals excited by PD in a

simple power transformer model has been carried out using

FDTD simulation software.

Since there is no noise present in the output of this

simulation, the threshold method for computing the

propagation time can be set to very small values (e.g.,

0.00001%). When comparing differences in signal arrival

times for obstacles that block line-of-sight propagation of

electromagnetic waves, it was found that several nano-

seconds of timing error may occur in the differential time

delay compared with expected changes that were only about 1

ns. This raises issues concerning the accuracy of PD location

by the method of assuming the minimal delay signal path,

which require further investigation though the evaluation of

results from more detailed and realistic models.

(a)

(b)

Fig. 6. Differential time delay for simulated PD signals with (a)

cylindrical obstacle, theoretical delay = 0.81 ns, and (b) cuboid obstacle,theoretical delay = 1.18 ns.

The direction of flow of PD current plays a significant role

that can affect the observed differential time delay by 1 or 2

ns when all other parameters are kept constant. The total

electric field at the sensor seems to be predominantly

composed of the component corresponding to the direction of

PD current flow.

In a real transformer, with a much more complicated

structure and arrangement of conductors, the situation may be

different in that the electric fields radiated by the PD source

may not be able to retain such ideal polarisation. This may in

fact lead to more accurate observation of the expecteddifferential arrival times, and is a topic that will be

investigated in future work.

ACKNOWLEDGEMENTS

A. M. Ishak would like to acknowledge the support of

colleagues in the High Voltage Technologies Group. He also

wishes to thank Ministry of Higher Education, Malaysia and

National Defence University of Malaysia for their funding.

Differential Time Delay (ns)

Differential Time Delay (ns)

Threshold

Level

(%)

Threshold

Level

(%)


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REFERENCES

[1] M. D. Judd, L. Yang and I. B. B. Hunter, Partial Discharge Monitoringfor Power Transformers Using UHF Sensors Part 1: Sensors and SignalInterpretation, IEEE Elect. Insul. Mag., vol. 21, no. 2, pp. 5-14,March/April 2005.

[2] M. D. Judd, O. Farish and B. F. Hampton, The Excitation of UHFSignals by Partial Discharges in GIS, IEEE Transactions onDielectrics and Electrical Insulation,vol. 3, no. 2, pp. 213-228, April1996.

[3] L. Yang and M. D. Judd, Propagation characteristics of UHF signals intransformers for locating partial discharge sources, Proc. 13thInternational Symposium of High Voltage Engineering, Netherlands,August 2003.

[4] Shim I, Soraghan JJ and SIEW WH, Digital Signal Processing appliedto the Detection of Partial Discharge: An overview, IEEE ElectricalInsulation Magazine, vol. 16, no. 3, pp. 6-12, May/June 2000.

[5] Shim I, Soraghan JJ and SIEW WH, Application of Digital SignalProcessing to the Detection of Partial Discharge Part 2: Optimized A/DConversion,IEEE Electrical Insulation Magazine, vol. 16, no. 4, pp.11-15, July/Aug 2000.

[6] Shim I, Soraghan JJ and SIEW WH, Detection of PD Utilising DigitalSignal Processing Methods Part 3: Open-Loop Noise Reduction,IEEEElectrical Insulation Magazine, vol. 17, no. 1, pp. 6 13, Jan/Feb 2001.

[7] D. Pommerenke, R. Jobava and R. Heinrich, Numerical simulation ofpartial discharge propagation in cable joints using the finite differencetime domain method, IEEE Electrical Insulation Magazine, vol. 18,no. 6, November/December 2002.

[8] L. Yang, M. D. Judd and G. Costa, Simulating Propagation of UHFSignals for PD Monitoring in Transformers Using the Finite DifferenceTime Domain Technique, Annual Report Conf. on ElectricalInsulation and Dielectric Phenomena, Millennium Harvest HouseHotel, Boulder, Colorado, USA, pp. 410-413, 17-20 October 2004.

[9] M. D. Judd, L. Yang and I. J. Craddock, Locating Partial Dischargesusing UHF Measurements: A Study of Signal Propagation using theFinite-Difference Time-Domain Method, 14th InternationalSymposium on High Voltage Engineering, Tsinghua University,Beijing, China, 25-29 August 2005.

[10] A. Convery and M. D. Judd, Measurement of propagationcharacteristics for UHF signals in transformer insulation materials,Proc. 13thInt. Symp. onHigh VoltageEngineering, Delft, August2003.

[11] P. J. G. Orr, A. J. Reid and M. D. Judd, Sensor Response

Characteristics for UHF Location of PD Sources, InternationalConference on Condition Monitoring and Diagnosis, Beijing, China,21-24 April 2008.

A Study of UHF Partial Discharge Signal Propagation

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