Partial Discharge Detection in Power Transformers Using Accoustic Emmission Technique

ABSTRACT

Increased pressure on high voltage power distribution components has been

created in recent years by a demand to lower involved costs and extend equipment

lifetimes. This has led to a need for condition based maintenance, which requires a

continuous knowledge of equipment health.

Power transformers are a vital component in a power distribution network.

However, there are currently no established techniques to accurately monitor the

health and diagnose faults in real-time while the transformer is on-line. A major

factor in the degradation of power transformer insulation is partial discharging.

Left unattended, partial discharges will eventually cause complete insulation

failure. PDs generate a variety of signals, including electrical pulses that travel

through the windings of the transformer to the terminals. A difficulty with

detecting these pulses in an on-line environment is that they can be masked by

external electrical interference.

This project develops a method for detecting PD pulses and determining the

location of PD source while the transformer is on-line and subject to external

interference. The project begins with a review of current techniques and need for

power transformer monitoring and diagnosis. Then Acoustic analysis technique is

discussed with the involved circuitry. Then follows the method devised for

assessing the location and hence severity of the discharge.

Partial discharge detection by Acoustic emission technique.

ACKNOWLEDGEMENT

I have great pleasure in expressing my deep sense of gratitude to Mr. Mathur for

designing such a great training module that allowed us to get the feel of whole

NDPL in my one year of training. During my training in different departments, I

found many people who spare their valuable time to give me the confidence to start

this project.

I am thankful to Mr. Vijender Rathor (Grid In-charge - RG-6) and Mr. R K

Sharma (AM-APS NW) for their valuable suggestions and guidance. I am also

grateful to Mr.Ashu and my college Electronics Lab staff for providing me

necessary facilities in the department for the completion of this project.

I wish to acknowledge with profound gratitude and wish to thank all those

who have been helpful in completion of this project.

S Naved Masood

Partial discharge detection by Acoustic emission technique

INTRODUCTION

Quality of the insulation is essential for successful and reliable operation of any

power apparatus. Minor flaws and irregularities such as voids, surface

imperfections in the insulation are however inevitable and leads to partial

discharges which are characterized by an electrical breakdown at the localized

region of the electrical insulation. Towards assessing the quality of the insulation

of power apparatus both during the manufacture as well as in service, tests to

determine the soundness of the insulation/dielectric material are conducted which

include overstressing the insulation with high DC/ AC and surge voltages. The

disadvantages of this technique are that during the process of testing, the

equipment may get damaged if the insulation is faulty.

Although DC breakdown stress measurements indicate the instantaneous

robustness of solid dielectric materials, partial discharge (PD) behaviour leads to

aging and exposure-dependent reduction in breakdown stress.

Failure of high voltage insulation is the No.1 cause of HV system failures with

IEEE statistics indicating that electrical insulation deterioration causes up to 90%

of electrical failures of certain high voltage equipments. On-line PD testing of

Power transformer gives advance warning of pending insulation failures thus

allowing the Grid in-charge to take remedial action during planned outages. Unlike

off-line testing, on-line PD testing and monitoring gives an accurate picture of the

PTR's health and performance under service conditions.


PARTIAL DISCHARGE

DEFINITION

IEC: 60270 standards on partial discharge measurements define Partial discharge

as "A localized electrical discharge that only partially bridges the Insulation

between conductors and which mayor may not occur adjacent to a conductor".

Many experiences have shown that the insulation damage caused by internal PD

activity is a main detrimental factor influencing the continued reliable operation of

a power transformer. In particular, the erosion of the insulation is one of the main

deteriorative mechanisms leading to early failure. As it is suspected that PDs of

high magnitude develop shortly before a major failure, continuous monitoring of

large or critically located transformers for such PDs is very desirable. However, no

internationally recognized standards currently exist for the measurement of PDs in

on-line power transformers.


Partial Discharge

CAUSE OF PD

When high voltage is applied to a device that produces Partial Discharge it may be

observed that the effect starts at a certain voltage level, and once started, the

voltage must be reduced to a lower voltage before it ceases. These two voltages are

called the Inception and Extinction voltages. This effect is illustrated in the figure

given below.

The figure given below illustrates the repetitive nature of Partial Discharge. Once

Inception Voltage is reached, the frequency of partial the discharge will increase as

the AC voltage is approaches its peak value.

Partial Discharge

Partial Discharge Studies In Solid Sheet Insulation

Figure: Recurrence of discharges at ac voltages

EFFECTS OF PD

In typical AC voltage testing Partial Discharge cycles occurs many times during

the positive and negative peaks. In applications, if this happens with sufficient

magnitude over time, arcing in the voids will degrade the insulation.

It produces tree-like patterns in the dielectric that lead to failure. This effect is

called Erosion breakdown.

There are two types of degradations:

• At normal operating stresses, internal and surface discharge cause

progressive degradation of insulation and eventual breakdown by electrical

Treeing but at higher stress they may cause breakdown by treeing soon after

voltage application.

• Surface discharge may also be caused by moisture and ionic contamination

on insulation and cause breakdown by Tracking.

Partial Discharge

Figure: Treeing and paper degradation

Figure: Treeing and paper degradation

PD induced degradation of the dielectric is roughly due to two processes; chemical

degradation and physical attack by bombardment of particles (nitrogen ions). The

interaction between PD in a cavity and surrounding dielectric is complex and many

effects have been identified and studied. Part of the complexity stems form the fact


Partial Discharge

t

Figure: Stages of PD Induced Damage at the Insulator Surface


that the dielectric is charged (aged) due to the PD activity but at the same time the

PD mechanism is affected by the aging dielectric. The aging process is now more

or less generally accepted to proceed along the following lines:

• The conductivity of the surface of the cavity increases due to the reaction

processes of humidity and the dissociation products of air as caused by the

PD.

• In the following stage the surface roughness is seen to increase due to the

charge carrier bombardment and deposition of PD by-products.

• Further PD activity leads to the formation of localized solid by-products, i.e.

crystals which have been positively identified as hydrated oxalic acids.

• The field enhancement at crystal tips leads to further intensification and

localization of the PD process and often pit formation is observed. As a

consequence tree growth is initiated.

• Eventually, the tree growth may lead to breakdown when the fillers are

present in the dielectric, the insulation between filler particles usually most

severely degraded.

Partial Discharge

OBJECTIVES OF DISCHARGE FINDINGS

The ultimate goal of discharge detection is to ascertain in a non-destructive test

whether an insulation construction has a sufficient life expectancy. At least four

steps are required for this judgment -

1. Detection

2. Measurement

3. Location

4. Evaluation

DETECTION

The observations determine with certainty whether discharges are present or not.

MEASUREMENT

If discharges are present, the magnitude of discharges must be ascertained. A

physical quantity must therefore be chosen which is both relevant to the

harmfulness of the discharges and can be measured with one of the discharge

detection methods.

LOCATION

After the discharges have been detected it is very important to locate their place. It

makes all the difference whether the discharge is located in a cable, or in its

terminal, or in a transformer, or in its bushings.

Location or determination of the actual site of the discharges in insulation has been

very important in insulation technology. If a discharge is located, the cause can in

most cases be determined and the fault can either be repaired or prevented in future


Partial Discharge

production. Location is also of importance in acceptance tests. If a discharge

occurs at such a position where it causes little or no damage to the insulation when

in service, the discharge can be tolerated. There are a number of general methods

of discharge location such as discharge pattern, X-ray or non-electrical methods.

There are also a few special methods such as scanning, traveling wave method,

ultrasonic methods and electromagnetic probe method.

EVALUATION

Estimation must be made of the type and the danger posed by the detected

discharges. An assessment of voltage life of the insulation must be made as a long

voltage life is required often as much as 30 to 40 years. The question arises that

what are the acceptance levels for the discharge magnitude? However, we can

show that it is not always possible to give an unambiguous answer. However, when

all the information that can be drawn from a well-performed discharge test is

collected, valuable information can be supplied on the quality of the insulation

under test.

PD DETECTION METHODS

Partial discharge measurement is an effective way to detect degradation of

insulation status or failures as a result of electrical stress. Partial discharge

measurement may divide into two method; electrical measurement and acoustic

signal detection.

Electrical measurement method features merits as a high sensitivity and precision

measurement but also has such demerits as vulnerability to noise. Further, in case

of ultra high voltage transformers, it has another critical shortcoming that the


Partial Discharge


coupling network cannot be installed during operation. Acoustic signal detection

method of partial discharge has lower sensitivity than electrical method but strong

protection from peripheral electromagnetic noise as insulated electrically while the

sensor can be easily installed during operation. In addition, we can find the

location where partial discharge arises by measuring the acoustic signals' time

difference of arrival (TOA) when multiple sensors are used.

ACOUSTIC EMISSION TECHNIQUE

Partial discharge events in the insulation produces pressure waves inside

transformer tank. These waves can be detected at tank walls using a high

sensitivity acoustic sensor attached at walls. The main components that are used

are:

Acoustic couplant

A material used at the structure-to-sensor interface to improve the transmission of

acoustic energy across the interface during acoustic emission monitoring.

Gelled glycerin and silicone grease are particularly efficient and are recommended.

Sensors

In general, two types of sensors can be used for detecting acoustic waves in solids:

accelerometers and acoustic emission sensors. The output of both types of sensors

is proportional to the acceleration of material to which the sensor is attached.

Accelerometers are designed to achieve flat frequency response, and can be used

up to 50 kHz. Acoustic emission sensors work with a large variety of frequency

ranges from 30 kHz to 1 MHz. It is well known that the acoustic frequency range

of PDs is around 40 kHz to 200 kHz, therefore only acoustic emission sensors can

be used for the PD measurement in transformers. Acoustic emission sensors are

resonant sensors. A single sensor can be used only within a narrow frequency

band. Damped acoustic emission piezo-electric sensors are generally used to

transform the particle motion produced by an elastic wave into an electrical signal.

These are mounted externally on the transformer tank outer surface or mounted on

a wave-guide that is submerged in the oil inside the transformer tank. These

sensors have sensitive range from 20 kHz to 500 kHz.

These sensors have output is usually inversely proportional to bandwidth, often a

Partial Discharge detection by acoustic emission technique

Acoustic Emission technique

AE sensors are connected to filtering de-couplers to separate signal from the power


sensor with a narrower bandwidth centered at either 60 kHz or 150 kHz is used in

PD detection.

Benefits of the externally mounted sensors:

• Position of PD source can be obtained by reconfiguring the sensors.

• Flexibility to move the system to another transformer.

Benefits of the internally mounted sensors:

• Clear and louder measurement with a better signal to noise ratio.

• Once installed, the sensor cannot move around to achieve a clear line of the

PD source.


source as the sensor do not provide separate cables for power and signal lines.

But in my experiment, I have used a condenser microphone that does not need a

de-coupler. No doubt this microphone only works in low freq range, but there are

microphones available in market from National semiconductors that have a quite

nice sensing power in freq range of around 150 KHz.

The microphone used by me was not even having a respectable sensing to high

audible range thus I used a pull up resistor that further enhances its sensing power.

Vcc

High resistance (Pull up resistor)

+ To amplifier

Microphone



But if it had been a AE sensor like R151-AST, PAC with operating frequency

range of 50 - 200 KHz and resonant frequency of 150 K Hz then a de-coupler

circuit should have been there.

After the signal is collected, it is not strong enough to be detected by CRO or other

methods. Thus this signal is then passed through a wideband amplifier that

includes functions to cover the frequency characteristics of the sensor to measure

acoustic signal with high sensitivity though they are equipped with an embedded

preamplifier.

As shown in figure below, a low-noise amplifier was designed and fabricated to

have wideband characteristics to acquire 40 db gains using the low-noise, wide

band operational amplifier whose gain-bandwidth is 70 MHz.

Figure: De-Coupler circuit



- V e r

9 ci

A C2 --Vcc

Figure: Low-noise amplifier

E J ' L ' i ] m ' m ' L \HV.\




Acoustic emissions from a fault

• PD/Arcing

• Overheating

• Mechanical problem

Extraneous noise to be removed:

• Particle impacts.

• Rain drops hitting the tank

• Pump operating noise (Areas/pockets of turbulent pump flow)

• OLTC Operations

• Noise due to vibration of components

An electrical signal (trigger) can also be used along with acoustic signals for more

reliable measurement of the acoustic activity. Its various advantages are:

• Provides the confirmation that the acoustic sensors are locating a PD event

as opposed to another acoustic noise source.

• The electrical signal is a convenient trigger that can be used to start the data

acquisition at the acoustic sensors.

• When PD pattern is detected in both AE and HFCT, it is confirmed that the

signal is generating from the inside of transformer.

• When PD pattern is detected only in HFCT, it is confirmed that the signal is

generating from the outside of transformer.

• When there is any mechanical fault in the transformer it is picked up by

Acoustic sensor and not by the Electrical sensor.

Hence, the combined acoustic- electrical PD locator system is more suitable for use

in the field.

EXPERIMENTS AND SETUP

An amplifier circuit is first designed which gives gain to the input signal.

The input can be seen on the CRO, right now the input is in the form of sinusoidal

pulse of frequency 1 KHz that can be seen on CRO.

In the figure below, the corresponding output of the amplifier can be seen.

Partial Discharge detection by acoustic emission

Experiments and Setup

i, , — ~ ~

To make a sample, three circular discs of diameter 12 cm each are made from the

Perspex sheet with the help of a lathe machine and then one hole of required sized

along the diameter with spacing of 3 mm is drilled at the centre of one of the discs.

Figure: The Three Perspex Sheet Constituting A Sample



8

(4 '////////// '//A//////, < \ \ \ \ \ \ \ \ 1 | 2 l k \ \ \ \ \ V O < ' / / / / / / / / / /

5

EARTH

Figure: Cell With Electrodes And Samples

Note: 1: The Sample

2: Cavity In The Sample

3: Oil Resistive Adhesive Tape

4: HV Electrode Extending Outside By Means Of A Rod

5: Earthed Electrode

6: Transformer Oil

7: Cubical Tank with Top and Sides Made Of Perspex

8: Corona Shield


7 6

3


Figure: Experimental setup in lab

PD POSITIONING

The diagnostic technique for transformers using acoustic signal can estimate the

insulation status and find the defection spot where PD occurs.

The spot of defection can be found by two ways;

• Electric-acoustic method.

• Acoustic-acoustic method.



The electric-acoustic method measures PD signal using a HFCT (High Frequency

Current Transformer) and an AE sensor, and calculates the spot from the arrival

time difference between electric and acoustic signal.

However, it is difficult to detect PD signal with HFCT because the PD signal is

much too small and many different high frequency noises exist on the ground wire.

On the contrary, the acoustic-acoustic method has advantages of the possibility of

insulation from electrical circuit and no influence from electrical noise.

In this project, I have used the acoustic-acoustic method to find the spot on the two

plane dimension by the arrival time difference of acoustic signals.

Let us assume the propagation velocity of the acoustic signal is v

Then the distance 1 l , 2 l and 3 l from the sensors AE1, AE2, and AE3 in the

figure below can be calculated as following equations ;

Figure: PD positioning method using AE sensors



The experimental apparatus consists of metallic enclosure, discharge electrode and

four AE sensors.

I have marked plane-coordinates on the enclosure to calculate the spot, and

installed AE sensors as shown in Figure below

Figure: Configuration of the apparatus

A sample calculation based on the experimental analysis is shown below. This can

prove to be really helpful in calculating the positioning of PD activity with a

proven 3% error.



pv /div, 100 ^ d i v ]

Figure: Sample results on CRO to show the involved calculations.

From the Equation (1) ~ (3) and the plane-coordinates of the enclosure, we can

derive the following equations:



*w = -• C# - SB? +• >-a - VO-10D0) 2 + ( j ; - lS0) 2 ) ( 4 ) v

f3_j =1 - 75>)2 + (,y - 740): -*j{x-1000) 3 + (v-ISO}") ( 5 ) v

Using equations (4), (5) and (6). We can calculate the position of the source of the signal thus the location of the PD activity.


BIBLIOGRAPHY 1. Solid Dielectric Material Influence On Fast Impulse Partial Discharge

behaviour, A.T. Wilder, W.K. Eickelberg and J.W. Wilder, IEEE 2005.

2. Degradation Of Solid Dielectrics Due To Internal Partial Discharge:

Some Thoughts On Progress Made And Where To Go Now,Peter H.F.

Morshuis, IEEE transaction on Dielectric and electrical insulation, vol

12,No 5; October 2005.

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1048.

4. E. Howells, E. T. Norton, Detection of Partial discharge in Transformers

Using Acoustic Emission Techniques, IEEE Transaction on Power

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5. L. E. Lungaard, Acoustic Partial Discharge Detection, Fundamental

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pp. 25-31.

6. J. P. Steiner, W. L. Weeks, E. S. Furgason, Acoustic Emission from

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Symposium, 1984, pp. 938-943.

7. H. Kawada, M. Honda, T. inoue, T. Amemiya, Partial discharges

automatic monitor for oil-filled Power transformer, IEEE Transaction on

Power Apparatus and System, Vol.PAS-103, No.6, 1986, pp. 1045-1048.

8. Mistras Holding Group,"Acoustic PD Measurement Manual", November

2005.

Partial discharge detection by acoustic emission technique

BIBLIOGRAPHY

9. L.E Lundgaard, "Partial Discharge XIV, Acoustic Partial Discharge

Detection-Practical Application", IEEE Electrical Insulation Magazine,

Vol. 8, No. 5, September/October 1992, pp. 34-43.

lO.Ekram Husain and R.S. Nema, "Analysis of Paschen curves for Air, N2

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on Electrical Insulation Vol. EI-17 No.4, August 1982.

11.M. U. Zuberi, A. Masood, E. Husain & A. Anwar "Estimation of

Discharge Inception Voltages at Different Pressures in the Ambient

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Partial discharge detection by acoustic emission technique

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Partial Discharge Detection in Power Transformers Using Accoustic Emmission Technique