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Session 5: Condition Monitoring of High Voltage Switchgear

High Voltage Conference 2015 IDC Technologies 1

Session 5:

Condition Monitoring of High Voltage Switchgear

Karl Haubner Applications Engineer, Doble Engineering

Part 1

CONDITION MONITORING OF METAL-CLAD SWITCHGEAR

Karl Haubner, Doble Engineering

Introduction

Metal Clad Switchgear is one of the key assets of the electrical distribution system. An in-service failure has significant consequences on the reliability of supply.

In the case of a failure large amounts of energy are released. Faults on the board can cascade to other compartments causing collateral damage within the switchgear and consequent loss of adjacent circuits and in the worst case scenario loss of the whole board. This is particularly true for switchboards with oil filled circuit breakers, resin bonded paper insulated bushings and compound insulated busbars.

Modern switchboards are designed to eliminate the potential to cause injuries by containing the arc by-products but older boards still in service are not designed to be arc resistant and any failure there is associated with a high risk of personal injury.

This paper describes some common and not so common condition monitoring techniques to assess the insulation condition of switchboards.

1. The need for Condition Monitoring Industry and most utilities are experiencing increased pressure to maximise the economic return on investments and are forced to operate aging switchgear.

The age of the switchboard population in industry and utility is increasing which means that an effective condition monitoring program is gaining importance. At the same time experience indicates that whilst newer arc resistant switchboards are much safer, the margin to withstand electrical stresses are reduced and many quality control issues have been detected early using effective commissioning testing

Age by itself is not a good indicator of estimated remaining life and performance. Insulation does deteriorate due to electrical, thermal and mechanical stress, chemical attacks and environmental contamination.

A condition-monitoring program to assess the condition of the switchboard is required to:

1. Assess the risk of failure. 2. Identify deteriorated components and initiate maintenance or refurbishment (if

possible). There is no point replacing oil circuit breakers with modern vacuum



breakers if the condition of the busbar or CT chamber insulation has deteriorated to an unacceptable degree.

3. Identify the priority for replacement or refurbishment to ensure that limited resources are distributed effectively.

An effective program also needs to take the type of insulation and arc interruption method employed into account. The test method best suited to detect insulation defect in a minimum or bulk oil circuit breaker will not be the same as for SF6 insulated GIS.

Apart from human error, problems with the mechanism (which will not be covered in detail in this paper) and operating outside of the rated capability, the main electrical insulation failure modes of metal clad switchgear are:

Failure of the insulation system due to elevated temperatures leading to thermal runaway conditions.

Surface tracking mainly associated with moisture, dust and contamination on insulating surfaces. For air insulated equipment this is the most common failure mode. In many cases tracking is triggered by other discharge phenomena such a corona or sparking type PD activity that as a by-product generates corrosive gases that deteriorate the insulation surfaces.

Partial Discharge activity in components such as epoxy insulators, VTs CTs, bushings etc.

Partial discharge activity from unscreened cables in switchgear or insufficient clearances such as voltage transformers to frame

Failure of cable terminations not part of but connected to the switchgear A range of techniques and tools are available to assess the condition of switchboards. The first distinction must be made between on and off-line testing. 2. Tests to determine the condition of the insulation Off-line testing is usually employed during the commissioning of new switchgear or during major outages. Once in service, the asset owner is somewhat reluctant to give access to equipment so the initial tests are important to provide background data and to verify the design, material and workmanship during the assembly of the board on site. In addition the switchboard can then be visually examined which is just as important as electrical testing.



3. Commission and Maintenance off-line testing of Switchgear

insulation 3.1. Insulation Resistance Measurement

The Insulation Resistance (IR) measurement is the oldest, most commonly used and at first glance simplest test. The Insulation Resistance tester generates a dc voltage typically between 0.5 to 10kV. The small current flowing through and over the insulation under tests is measured and the insulation resistance is directly indicated on the display. Temperature has a pronounced effect on the insulation resistance of insulation material. The value decreases rapidly with an increase in temperature. When individual values are to be compared with each other it is important that both measurements are taken under

similar conditions or readings are normalised to 20 C. Temperature correction factors for different switchgear types and makes are generally not available and generic correction tables are not applicable across all different insulation system. Here the application of the polarisation index (10 min reading/1 min reading) or the polarisation factor (60 sec reading/15 sec reading) test which is largely independent of temperature can improve the meaning of the measurement. However, many switchgear components do not return significant polarisation currents and the obtained PI results are meaningless. Insulation Resistance results are also time dependent and when comparing spot measurements it is important that the measurement times are the same. The significant contributing factor to Insulation Resistance results is humidity. The higher the moisture content in the air the lower the IR values, in particular if surface contamination across any component of the insulation system is present. No repeatable results can be obtained if the humidity exceed 70-80%. Temperature & humidity should be part of the test record. The test voltage (e.g. 5kV for 11 & 22kV Boards and 10kV for 33kV boards) is applied between one phase and the other phases connected to earth with all breakers closed and in the racked in position. Voltage application is typically for 60 seconds. The same procedure is then repeated for each phase. In addition the IR value across the open contacts of the Circuit Breaker is determined. IR values vary widely depending on the insulation system employed. Oil filled circuit breakers or compound insulated busbars can be as low as 5000 M without being defective but GIS and air insulated switchboards are expected to return IR values in the high G range if the humidity is low. One of the major disadvantages of the Insulation Resistance test is that defects in condenser type multi-layer insulation (which is common in older style paper insulated bushings used on oil circuit breaker trucks from 6.6 to 33kV and even some older 33kV busbars) cannot be detected using DC methods.

Figure 1 Commonly used IR tester



Figure 2: Failure of paper insulated condenser type circuit breaker bushing. Failure occurred when racking a spare breaker into the board. The breaker passed a simple IR test. 3.2 High Voltage Power Frequency Withstand Test As part of the commissioning process it is common to conduct a Power Frequency withstand test in accordance with the relevant standards. This test, also called High-Potential test, is made at voltages above the normal system voltage for a short duration, such as 1 minute. Test voltages in the field are typically reduced to 80% of the listed values: Rated voltage kV 3.6 7.2 12 17.5 24 36

Rated power frequency withstand voltage

kV 10 20 28 38 50 70

The Switchgear has passed the PFWT test if the test voltage can be maintained for 1 minute without puncture, flashover or disruptive discharge.

Observe and record:

Noticeable rise or decrease in test transformer primary & secondary currents Large fluctuations of the applied voltage Audible discharges Visible discharges Tripping of HV test set circuit breaker

In essence the PFWT is a go-no-go test and whilst it is essential that a newly assembled switchboard passes this test, little information about the quality of the insulation system is obtained. There are many examples of switchgear having successfully passed a HV withstand test having serious assembly issues.



Figure 3 Example of a switchboard having passed the HV withstand test successfully but subsequent PD testing identified an incorrect assembly of VT connections resulting in insufficient clearance between phases.

A High Voltage Withstand test can also indicate a loss of vacuum in VCBs. A healthy vacuum interrupter must withstand the applied test voltage above across the open contacts.

3.3 Dielectric Dissipation Factor (DDF) also called Dielectric Loss Angle

(DLA) tan or Power Factor Measurement

DDF is the ratio between the resistive power loss and the reactive power loss of the insulation material. This is equal to the tan of the angle and typical displayed as % Dissipation or Power Factor.

The DDF measurement is one the standard methods to characterise the condition of insulating materials. The value can vary from 0.1% up to 15% depending on the insulation system used. The method is most effective for older type switchgear in particular when condenser foil type stress control methods are employed. Impregnated paper insulation as found in older Metal-clad switchgear is prone to absorb moisture and the Dielectric Dissipation Factor is a significant indicator of moisture ingress. Many cases have been encountered where circuit breaker bushings returned acceptable Insulation Resistance readings but the DDF measurement revealed poor insulation values.



The following sketch -figure 4- indicates how the DDF measurement can detect partial degradation within a multi-layer insulation system.

Figure 4 = Detection of defects in multi-layer insulation.

Older type oil filled circuit breakers have a number of graded electrostatic screens through the insulation structure of the bushing. A DC test performed on the same insulation structure is not capable to detect a defect that is still surrounded by good insulation.

The DDF can be measured using a differential transformer ratio arm bridge, a schering bridge or instrumentation which measures total current and watt loss using a reference resistor.

Figure 5 Modern insulation analyser used to conduct DDF tests

As is the case for insulation resistance measurements, DDF values need to be corrected for temperature for bitumen and oil filled switchgear. At the same time the variation of the dissipation factor with temperature can also be a significant indicator of the insulation quality. The temperature dependence of the insulation is more pronounced with poor insulation quality and the possibility of insulation breakdown due to thermal runaway is higher. Thermal runaway occurs in unstable insulation at high temperatures when the high dielectric losses heat the insulation further which increases the temperature further, losses increase again, etc., until complete insulation failure occurs.

For modern oil epoxy or gas insulted metal clad Switchgear no correction is required.



The voltage dependence of the insulation tested is the other important criteria. Good insulation will show very little increase in DDF value up to 120 % of operating volts.

As the capacitance of the insulation under test increases, the ability of the DDF measurement to detect incipient localised faults is reduced. When testing, for example, the busbar insulation a large number of spout bushings are measured at the same time and the results indicate the average DDF. The true DDF of e.g. a single bushing with high DDF will be masked by the low DDF of the large number of good bushings. Thats why on a circuit breaker each bushing is tested separately.

The DDF measurement is sensitive to moisture ingress and as this is a typical defect found in older metal clad switchgear components, the testing technique is considered a significant indicator of deterioration and can detect defects in condenser type stress grading insulation systems.

The ability to detect localised insulation defects is reduced with increased capacitance of the test sample.

The DDF tip-up (voltage dependence) can be another indicator of insulation integrity.

A pass/fail criteria needs to take the type of insulation system employed into account. Older switchgear can have DDF values above 10% and is considered poor but still serviceable whereas modern air, gas or epoxy insulation systems have values below 0.1%.

For modern air or gas insulated boards with Epoxy Cast Resin insulated structures the absolute value of the DDF test is not as important as the DDF tip-up.

3.4 Partial Discharge Measurement (PD) Partial Discharge (PD) activity is produced by incipient faults in HV insulation and is regarded as one of the best indicator of insulation condition providing an early warning against insulation faults. By definition PD represent discharge events that are limited to only a small portion of the dielectric and only partially bridge the insulation between the electrodes. PD usually occurs due to local electric stress concentration at defects and hardly within a homogenous insulation structure were the electric field is a function of the applied voltage and the distance between the electrodes and is designed with some margin above the breakdown strength of the material. However in defective insulation systems there can be many locations where the electric field is not uniform. For example a void inside an otherwise homogenous dielectric will cause a localized field enhancement across it and if the dielectric strength of the gas or air inside the void is below the breakdown voltage determined by Paschens law, a partial discharge pulse is generated. Over time the fast moving electrons or ions can cause deterioration of the surrounding void wall insulation resulting in chemical decompositions of the material. This can lead to a complete breakdown of the insulation. PD testing, either as a factory test to ensure that the equipment is designed and build not to exceed permissible discharge levels or as a condition monitoring tool to trend the transition from acceptable to critical levels, is gaining importance. In principal the type of discharge can be categorized as external and internal discharges.



Figure 6 types of Discharge activity In most cases partial discharge activity has a detrimental effect on the insulation material. The spectrum of damages due to PD activity include heating, oxidation, chain scission of polymer molecules, stress cracking due to UV light, surface erosion, build-up of aggressive gases (e.g. Ozone) which in turn cause corrosion of metal surfaces, delamination, etc. Detection Circuit Although the following simplified a-b-c model depicting a single void surrounded by insulation was rejected by researchers because a cavity cannot be represented by a capacitance, it is often used to explain PD activity and how to detect it. The capacitance of the void and the series and parallel capacitances of the surrounding insulation is shows as a, b, c

Figure 7 simple a, b c model and current flow If a voltage is now applied across the test sample a proportional voltage will now appear across the void

Cc: Capacitance of void

Cb: Capacitance of solid in series w/void

Ca: Capacitance of the rest of the solid

Va: Applied voltage of solid

Vc: Voltage across void

V+/V

-: Inception volt. for PD in void

Gro

ups of discharges originate from a single void and give rise to current pulses (pos. and neg.)



Figure 8 Distribution across void surrounded by insulation. When the breakdown strength of the gas or air inside the void is exceeded the void capacitance is temporarily shorted so the voltage across the void drops to 0 V and a charge transfer from one side to the other side of the void occurs. Ca now needs to supply the lost charge to Cb to compensate. This is associated with some current pulses between Ca and Cb/Cc.

Figure 9 Typical current transients associated with a Discharge from a single void

Figure 10 Current flow in coupling capacitor If now a capacitor Ck with a series impedance is now connected across the circuit, Ck will supply some charge to Cb/a for Cc. This is detected as a current by the series impedance in series with the coupling capacitor. (Figure 10) This circuit is basically described in IEC60270 and represents the standard test circuit showing the test object, the coupling capacitor, the measurement impedance, the HV supply and the instrumentation. The coupling capacitor provides the compensating charge and functions as a High Pass Filter i.e. blocks power frequency but lets the HF PD signals through. These decoupled current pulses are converted by the measuring impedance into equivalent voltage pulses. The pulses are integrated to provide a measure of charge involved in the PD event at the point the sensor is connected too. As per IEC60270 the apparent partial discharge level is expresses in pC (pico Coulomb) Those pulses have very short time parameters in the ns range and are low in magnitude. For example for an apparent charge of 0.1 pC the corresponding voltage magnitude is about 100 V, if a time constant of 50 ns for the PD pulse & a 50 Ohm impedance is assumed. In many instances the background interference from non-PD sources is much higher than this.



Figure 11 Standard PD measurement circuit. Apart from replacing the coupling capacitor with for example a HF-CT there are several options as far as the PD instrumentation is concerned such as narrow band, wide band and ultra-wide band detectors going up to hundreds of MHz with the ability to resolve features of individual pulses. Each system has their own merits and disadvantages. The most common method employed nowadays is the wide band technique described in IEC 60270. With modern digital detectors the detection center frequencies and bandwidth are freely adjustable permitting investigation of the pattern a different frequencies and superior noise suppression. However it should be noted that the frequency range used is outside of the IEC guidelines and results expressed in pC may not be comparable with factory data or measurements taken at other detection frequencies. Figure 12 Block Diagram of modern Digital DP detector With these digital detectors every PD event is captured and recorded permitting advanced post recording waveform analysis. The test voltage applied is increased to up to 120 to 130% of line to ground voltage.

A

D

P

Postprocessing

Computer

Software

User/ PanelA

D

PD-Signal

Voltage Signal

vPD

vU

Filtering Detection

Input Unit DAQ DSP PC

A

D

P

Postprocessing

Computer

Software

User/ PanelA

D

PD-Signal

Voltage Signal

vPD

vU

Filtering Detection

Input Unit DAQ DSP PC



Figure 13 PD Test Circuit

In addition to the magnitude and type of discharges the other important parameters such as PD inception voltage, PD extinction voltage and effect of time are recorded. All these parameters need to be considered before a valid judgment can be made. Displaying the PD data using the Phase Resolved Partial Discharge (PRPD) patterns also called phi-q-n diagram is recognized as the one of most effective methods to display the characteristics of PD signals and is used to classify different types of PD and interference signals. The method produces patterns that can be directly related to gas discharge events in the dielectric. The PD pattern reflects the sum of all individual pulses correct in time (phase position) on the X axis, magnitude on the Y axis and repetition rate which is color coded collected during a preselected acquisition time. The user can also select to display the pulse polarity. From this information the generation of 2 or 3 dimensional graphs is possible.

Figure 14 Phase resolved pattern from sharp point at HV potential producing corona.



Figure 15 Sparking type discharge from floating bushing screen in switchboard.

3.5 Location of partial discharges

After a discharge has been detected, its location is beneficial. If, for example, a single component can be identified for replacement without sectionalising the switchboard, significant time & cost savings are possible. Location of PD is possible with non-electrical and electrical means. Electrical measurements usually involve comparing arrival times between different sensors (they may be TEV sensors or Coupling capacitors) using a high speed oscilloscope. Non electrical methods uses several acoustic sensors.

Figure 16 Location of PD source using time of flight measurement



Figure 17 Non electric location methods use microphones to detect the panels that return the loudest indication of PD activity. If panels can be removed during off-line testing a corona camera can be helpful determining the origin of the PD activity.

Figure 18 Defect visualised using a corona camera during off-line testing

4. Non-intrusive on-line tests

An effective condition monitoring regime combines regular non-intrusive survey type measurements with less frequent off-line assessments. Often the initial survey test results trigger more detailed testings including monitoring over a longer period or initiating some off-line testing.



Figure 19 Condition Monitoring using a combination of Survey, focused monitoring and intrusive testing Whenever there is a PD event different Energy forms are released that can be detected using different techniques and instruments.

Figure 20 Energy Forms released by Partial Discharge Activity One of the most commonly used method is the detection of electromagnetic waves. A PD current pulse comprises moving charges which results in an emission of impulsive electromagnetic waves that travels in the surrounding media. This radiation can be detected using specialised spectrum analysers in the RFI range. 4.1 Partial Discharge Monitor using TEV Principles When a partial discharge inside the switchboard occurs, electromagnetic waves propagate away from the source and are capacitively coupled to the inner surface of the metalwork.

Figure 21 TEV Transient Earth Voltage Method



These pulses travel due to the skin effect on the surface of the metal and are able to propagate through an opening in the metal cladding (such as a gasket) onto the outside surface of the switchboard. The travelling steep current pulse can generate a transient earth voltage on the metal surface, which can be measured by attaching a capacitive probe to the metalwork.

This capacitive plate sensor probe is often integrated in handheld detectors permitting quick scanning of the switchboards.

Figure 22 Commercial handheld TEV detector to survey cable box of switchboard

The TEV detectors have a frequency range of around 2 80MHz and measurements are unavoidably influenced by other narrowband and background broadband noise signals in a substation environment that are in the same frequency range. These external interference signals may originate from air-conditioners, lights, radio and communication signal, power electronics such as variable speed drives or other HV plant such as overhead lines and substation equipment in close proximity to the board and are often of high magnitudes compared to the target signal leading to false positives. Its important to always compare to background readings taken from panels that cannot be associated with discharge activity such as LV boards instead of just reacting on absolute dB values.

Surface PD activity which is the major failure mode in air insulated switchgear has less energy and emits electromagnetic waves outside of the detectors frequency range and can in general not be detected using TEV principles. Detection sensitivity is naturally much lower than off-line PD testing and depend on the propagation path so that a direct correlation to IEC type measurements in pC are not possible.

Figure 23 Example of significant defect that was undetectable using TEV techniques

Despite these shortcomings the non-expert ease, speed of use and the ability to detect many types of high level PD activity non-intrusively makes the TEV method a popular choice.



TEV Sensors can also be used with more sophisticated instrumentation such as oscilloscopes, recorders or spectrum analysers.

Using an optimised spectrum analyser as shown in figure24 it is possible to display the captured signal in both the frequency domain and in a time resolved mode.

Figure 24 PD Instrumentation & TEV sensor on switchgear

RFI Frequency Spectrum from above instrumentation. (Black is Background), the PD causes an uplift compared to the background across a wide frequency range

Display of the TEV/RFI signal in the time domain at a spot frequency (6MHz BW) selected from above spectrum. This guarantees the best signal to noise ratio. The trace shows clear phase correlation an indication of true PD activity but also two types of PD. Similar to the phase resolved pattern from the off-line PD tests characterisation indicating the type of PD is possible

Figure 25 RFI spectrum between 50 & 1000MHz and time resolved trace with phase synchronisation @ 620 MHz



Some instruments have additional post processing features that characterise the type of PD activity. Algorithms automate the PD characterization process by analysis in the time domain extracting the features of each PD event such as Pulse rise and decay times, pulse width and other essential frequency components. These features are determined by the type of discharge

Figure 26 Example of automatic software based classification from on individual PD pulse implemented in a commercial detector To further investigate the influence of humidity and load conditions on Partial Discharge activity several manufacturers offer permanent monitoring solutions combining sometimes TEV and acoustic sensors.

Figure 27- Example of permanent monitoring system installed Another method is on-line monitoring via permanent installed PD coupling capacitors. This technique is not popular in Australia as retrofitting is intrusive and can compromise the BIL rating of the switchboard. As modern flash hazard rated boards are relatively sealed often having a double skin resulting in reduced or no effectiveness of TEV and Acoustic methods this policy may need to be re-evaluated. In theory possible but often subject to significant system interference is the monitoring via the existing build-in capacitive voltage indicators.



Figure 28- Example of PD detection via Voltage indicators 4.2 Ultrasonic Detection 4.2.1 Airborne type Ultrasonic Detectors.

Surface discharge activity, corona, and tracking are best detected with high sensitivity using an ultrasonic listening device. Ultrasound easily passes through air but is readily blocked by a solid surface. The detector is a directional microphone with amplification in the sonic and ultrasonic range. Typical detection frequencies are 30-500 kHz. The key to a successful ultrasonic survey with a high confidence level is to find opening in the switchboard were sound can escape.

There must be an uninterrupted air path between the discharge site and the instrument to allow the airborne ultrasound waves to be detected. Some of the older switchboard have ventilation vents or other small gaps between panels or covers suitable for surveying.



Figure 29- Examples of suitable opening to conduct ultrasonic survey

Where possible a small gap can be created at switchgear panels permitting effective access to the airspace to permitting more sensitive acoustic measurements. Another approach where access to the airspace for the ultrasonic inspection is limited is by installing acoustic windows as shown.

Figure 30- Examples of acoustic windows retro fitted to Metal Clad switchgear Instruments provide indications of the acoustic magnitude in dB or dBV but in practice this level can alter significantly just by changing the angle of the sensor by a small degree and classification purely on dB results are not conclusive. The operator can differentiate from the sound what type of PD activity such as corona, contact or tracking is active. In general acoustic measurements are not effected by electrical interference (but can be masked by excessive vibration in heavy current applications) and any acoustic emissions should be monitored and investigated regardless of the dB V level. Insulation breakdown in the solid section of the switchboard (e.g. insulated busbar, internal discharge in instrument transformers or bushing insulation) cannot be detected using ultrasound.

4.2.2 Contact Type Ultrasonic Detectors.

On well-sealed boards a contact type probe can be used. These have excellent sensitivity detecting PD on Gas Insulated Switchboards. On Air Insulated Switchboards contact type sensors have reduced sensitivity when compared to airborne Acoustic Emissions type and should only be used if there are openings.



Figure 31- Contact Type Ultrasonic sensors in use

Some instruments further visualise the acoustic activity by displaying the pattern as a phase resolved trace in the time resolved mode which permits further classification and recognition of the type of PD. Synchronisation to the power cycle can be either via an internal time clock or truly phase locked via a mains outlet.

Figure 32- Phase resolved Acoustic Signature

4.3 Other on-line Tests External thermo-graphic testing although sometimes used to detect OCB faults, is of limited use in metal clad switchgear. The problem needs to be in a very advanced stage to raise temperatures of the steel covers. Some asset owners install Infra-Red inspection windows (which can also be combined with acoustic windows) increasing the effectiveness of infra-red surveys on switchboards considerably.

5 Verification of on-line techniques compared to off-line tests

Appendix 1 shows results from tests conducted in the laboratory and provides a comparison in terms of sensitivity of the most common on-line techniques in use with reference to off-line PD measurements.

From the results obtained from this controlled study and based on the field experience of both authors the following conclusions can be made:



The Laboratory tests indicate that by far the most sensitive method and the only way the PD activity can be accurately characterised, the degree of severity quantified and compared to standards is an off-line PD measurement in accordance with IEC60270.

However surface PD activity can be detected with very good sensitivity using

Acoustic techniques. Here the airborne type acoustic sensors achieve a much better sensitivity than the contact type sensors. The dedicated ultrasonic detector used in the trial with the inbuilt variable frequency filter returned the best performance in terms of clarity and using the filter being able to focus on different types of activity and noise suppression.

The same surface PD activity can in general not be detected using methods that

rely on the transmission of electric magnetic radiation (TEV) unless PD activity is above 1000pC.

TEV/RFI methods can be strongly effected by external interference sometimes

totally preventing meaningful measurements. If the interference frequencies are narrow banded it can be beneficial to focus on frequencies outside of the interference band using filtering and to check at spot frequencies if activity is phase related. In case of broad banded interference such as seen in installations with variable speed drives, these methods are not effective. Ultra-wide band measurements can be even more subject to interference with algorithms in most cases not being able to differentiate between interference and true PD activity. Additional add-on filters can assist in these cases.

Strong Void PD activity, Sparking type PD and severe PD activity from cable

terminations can be detected using TEV techniques. Representing the signals in the time domain (phase resolved) can assist in separating interference from real PD.

Internal PD such as voids inside instrument transformers and insulators cannot

be detected using ultrasonics so a combination of both TEV/RFI and ultrasonics is essential to cover all defects.

Switchboards where no interference is present can be easily surveyed using

simple hand held detectors combining peak TEV signal indications or LED and ultrasonic tool. In case interference is present more sophisticated instrumentation reduces the number of false positives.

Considering that the main failure mode in air insulated epoxy resin based

switchboards is surfaces deterioration and subsequent tracking much more emphasis should be placed on effective acoustic measurements. This can be achieved by installing acoustic windows.

The corona camera is not useful for survey type assessments with Panel covers

fitted but is a good investigative pinpointing tool during off-line testing with covers removed.



6 Test Criteria It is impossible to establish a universal heath index or acceptance criteria that covers the whole range of switchboards and insulation system employed.

Very often guidelines are only applicable to a specific type, manufacturer and voltage range.

Modern Metal Clad Switchgear utilising gas or air insulation and epoxy resin structures have negligible losses and Dielectric Dissipation Factor Measurements (DDF) are not a significant indicator and not commonly applied but are still listed in the table.

Some simple guide lines have been established using the hand held detectors.

Figure 33- Some Guide Lines Handheld TEV detectors

Based on experience the following guide lines are proposed:

Acceptance Criteria Modern Metal Clad Switchgear

Test Method Acceptance Criteria

Insulation Resistance

> 20 000 M

(typical values found are > 100G)

Partial Discharge

Off-line IEC values

< 50 pC @ 120% Uo but take into account type of PD from Phase Resolved Signature

Ultrasonic No AE signal unless identified as non-relevant such as vibrations

TEV No signals exceeding 20dB, Pulse count



Figure 34 Acceptance Criteria Modern Air/Vacuum/Gas insulated with Epoxy Resin support

Example of Criteria for older 11kV bitumen insulated boards with Oil Circuit Breakers established earlier by Australian utilities.

Test Method If insulation indicators are worse than levels below classify into bad category

Insulation Resistance > 200 M

Partial Discharge

Off-line IEC values

< 100 pC @ 110% Uo but take into account type of PD from Phase Resolved Signature

DDF < 10% & no significant increase between 2 kV and system voltage

Ultrasonic No AE signal unless identified as non-relevant such as vibrations

TEV No signals exceeding 35dB, Pulse count



Figure 37- Condition Monitoring Maintenance Scheme



7. Case Studies

Case Study 1 11kV Switchboard with defective bus bar support

This case study presented shows how the described tests helped to determine defects on bushings / barrier boards on an 11kV busbar in modern air and epoxy insulated switchgear. These tests were conducted after some noise was heard with the new switchboard in service coupled with a strong ozone smell.

Initial testing using TEV and RFI principles did not return any indications of PD activity with typical values equal to background readings. The modern board is well sealed but both contact and airborne acoustic sensors placed at small gaps on the arc chute confirmed PD activity.

Figure 38- Acoustic tests showing PD activity

Whilst online PD methods relying on electromagnetic radiation failed to detect any PD an off-line test confirmed relative high levels of surface PD activity up to 2000pC on all phases.

Figure 39- Test Set-up

Figure 40- Phase Resolved PD Signature



Switchgear panels were then removed and the location of discharge activity was exactly pinpointed using ultrasonics and a corona camera.

Figure 41- Pinpointing techniques using both acoustic and corona camera with covers removed

The PD activity originated from the busbar support fibre glass rings that were not sealed and had absorbed moisture at some stage. There was also evidence of white power at the rings and the heat shrink / rubber interface typical for crystallisation associated with PD activity. The fibre glass material itself showed discoloration.

Figure 42 showing defective support fibre glass board



Case Study 2 Multiple PD Sources 22kV Switchboard This case study portrays the findings on a 22kV switchboard which was removed from service due to audible discharge and a strong ozone smell emanating from the switchboard. Testing was requested by the Asset Manager in order to determine the levels of partial discharge present before performing fault finding to determine the source of the audible discharge.

Figure 43 Switchboard tested

Figure 44 Phase Resolved PD Signature showing several types of PD

The audible discharge was clearly identifiable to originate from one end of the switchboard therefore after initial testing which confirmed that the discharge activity was originating from the bus section and not the circuits; inspections were focused in this region. Inspections revealed extensive corrosion to copper components, severe surface deterioration of resin components and signs of localised heating on heat shrink. This inspection confirmed the offline measurement which indicated that multiple PD sources were present with different partial discharge characteristics.



Figure 45 Busbar connection at rear of spout with cover removed

Figure 46 Surface condition of spout Testing was repeated with the end busbar panel removed in order to utilise an ultrasonic detector to pinpoint exact sources of discharge but unfortunately this was unsuccessful due to limited access whilst maintaining safe clearances, although during these attempts visual discharge was observed. The lights were then turned off and visual discharge was very clear and easily distinguishable to be originated from between the spout and the surrounding frame. Closer inspection of these areas identified a build-up of discharge powder which had bridged the gap between the two as well as corrosion on the metal frame.

Figure 47 Tracking point between spout and frame of switchboard



The discharge in this case was clearly occurring over an extended period of time which went undetected until discharge levels were severe enough to be audibly heard during general substation inspections. This confirms the requirement for regular monitoring of electrical apparatus both online and offline which helps to minimise repair costs through early detection or in a worst case scenario save the apparatus from catastrophic failure. Case Study 3 Loose mounting bolts on spout flange This case study focuses on an older bitumen/pitch filled switchboard (oil circuit breaker). During a scheduled outage on a single CB, audible discharge was heard, upon racking down the CB. Testing staff were called to the site and through the use of an ultrasonic detector were able to pinpoint the discharge to the Red phase Front Bus spout. Based on the testers onsite assessment and recommendations, the Asset Manager decided to de-energise the Front Bus in order to perform offline testing and subsequent fault finding.

Figure 48 Switchboard tested showing Bitumen bus bar insulation & oil circuit breakers



Figure 49 Phase Resolved PD pattern During initial inspections it was noticed that the shutter was fouling and not functioning correctly and therefore had sustained damaged but at that stage this was not seen as significant. During offline testing, severe audible discharge could be heard when a voltage of only 1kV was applied to the busbar. The rear mounting bolts of the Red Front Bus spout were found to be loose which confirmed the floating potential pattern obtained with offline testing. After tightening the loose bolts, not only did the discharge activity disappear but the shutter no longer fouled and therefore operated smoothly.

Figure 50 PD Signature Loose rear mounting bolts on spout flange

Rear flange mounting bolts



Figure 51 Front Bus shutter in fouled position The switchboard was successfully returned to service, with follow up online testing and acoustic surveys confirming that the discharge activity has not returned.

Case Study 4 Secondary wiring of CT touching body of another CT

This case study details the findings as a result of a switchboard which was de-energised after general substation inspections identified audible discharge and the smell of ozone within the switch room. Offline testing of just the busbar was discharge free but severe audible discharge was present with all CB closed (thus including all circuits in the test). Simply closing one CB at a time allowed the discharge activity to be attributed to a single circuit on the switchboard.

Figure 52 Phase Resolved PD signature The circuit portion consisted of a spout, one stand-off insulator, one neon pickup insulator and two sets of CTs in series (mounted one below the other.)



Initial inspection of the circuit portion of the switchboard revealed extensive corrosion and surface deterioration of components. Parts deemed as unserviceable were replaced before performing follow-up offline testing which yielded still very high levels of discharge. Components of the circuit were segregated and tested individually (although still mounted in position) with no discharge present but when the two CTs were tested together the high level discharge was present as well as able to be heard audibly. Closer inspection between the two CTs identified severe surface deterioration of the CT body as well as damage to secondary wiring.

Figure 53 Surface deterioration found when Top CT was removed The CTs were replaced and secondary wiring damage repaired before retests were completed but large level discharge was still measured. Further investigation identified that the close proximity of the secondary wiring of the Top CT to the body of the Bottom CT. The discharge activity was successfully rectified by simply increasing the clearance of the secondary wiring from the CT body. It appears that only the combined field strength of both CTs being energised together was enough to create the discharge that was measured.

Figure 53 Secondary wiring in close proximity



Case Study 5 Defective CT busbar section

This case study presented shows how the described tests helped to determine defects on a CT busbar in an air and epoxy insulated 22kV switchgear.

Figure 54 Switchboard tested

Routine off-line testing of a section of the switchgear returned the following results:

DC Step voltage test (Ileakage @

30kV) A

Dielectric Dissipation Factor

% @10kV

Insulation Resistance @ 5kV

G

PD (pC)

@ 12.7

PD (pC)

@ 15.3

Suspect Section

L1 1000



Panels of the board were removed and PD activity was located (whilst the busbar was energised from the test transformer) using a PD probe. It was interesting to observe that no ultrasonic discharge activity could be detected as the defect was internal to the busbar insulation. A visual inspection of the L2 CT busbar also revealed pitting and burn marks of the screen conductor. It was concluded that the PD activity already caused further deterioration of the insulation evident from the reduction in insulation resistance and increase in DDF. The CT busbar was removed and was retested in the Laboratory. Similar high levels of partial discharge activity were found. The area of highest PD intensity was located to be at the end of the internal screen.

PD signature @ 16 kV

PD Pulse Rate - Phase

PD

Num

ber/

Sec

ond

Degree

0

1

2

3

4

5

0 45 90 135 180 225 270 315 360

Figure 56 Test of faulty Busbar section once removed from switchgear

Case Study 6 - Significant deterioration of a 30 year old 11 kV switchboard

The following test results indicate a significant deterioration of the rear busbar. Key indicators of the deterioration are high DC leakage, low IR (in comparison with other phases and other air-insulated switchboards), high DDF and DDF tip-up, and very high partial discharge activity. In particular partial discharge results did indicate a critical deterioration. The PD signature suggested an arcing in air type discharge. With the side cover removed the discharge source was located (using ultrasonics) to originate from the bus coupler panel. Visual examination revealed signs of severe discharge activity between the compound filled bus joint and the panel as per photo. Although the white phase also displayed signs of previous corona activity between the panel and the bus joint no partial discharge activity was detected during the tests probably due to the low humidity present during the tests. This further highlights the fact that often more than one type of test is required for a comprehensive analysis of the condition.

Location of highest PD

activity



Rear Busba

r

DC Step voltage

test (Ileakage @ 20 kV)

A

Dielectric Dissipation

Factor

@7.5kV %

Insulation Resistance @

5kV

G

PD (pC)

@ 6.3

PD (pC)

@ 7.6

PD

Inception & Extinction

L1 6.8 4.42% 93.1



By sectionalising circuits the activity was localised to one panel and a visual examination showed insufficient spring contact between the busbar and the screen of the bushing leaving it floating. After rearranging the spring acceptable discharge levels were obtained.

Figure 60 Insufficient spring contact of screen to HV conductor

Case Study 8 - Commissioning Issues on 6.6kV AIS Switchboard

During Commissioning of the new Switchboard the L3 (C) Phase showed elevated PD levels. Although the PD levels were relatively low the phase resolved PD pattern suggested an internal void discharge with good symmetry between positive and negative half cycles

Figure 61 Phase resolved PD pattern & off-line test results

Acoustic location was not possible but by switching off sections of the switchboard and removing links and components the PD was pinpointed to originate from a faulty voltage indicator. Separate tests at component level confirmed the defective part.

Figure 62 Defective Voltage Indicator



Case Study 9 - Commissioning issues on 33kV GIS Switchboard

During commissioning of a new 33kV GIS Switchboard one Phase showed high PD levels only slight above system voltage. The PD patterns suggested a sparking type discharge from floating components.

Figure 63 Test Set-up & Phase resolved PD pattern

A subsequent examination of components showed several parts of the switchgear were not grounded. (Installation error)

Figure 64 Showing some of the components that were not connected to ground during the assembly.



Part 2

CONDITION MONITORING OF GAS INSULATED SWITCHGEAR GIS

1. Introduction

GIS can be defined as switchgear where the conductors and contacts are insulated are by pressurised Sulphur Hexafluoride gas (SF6). The SF6 gas is used both as switching and insulation medium. Due to their compactness, immunity against environmental conditions such as pollution, their very high reliability, low maintenance and long service life they are now a popular choice for both distribution and transmission voltage levels. There are now several GIS installations in Western Australia and the monitoring of these installations in service is covered in this second part on Switchgear monitoring.

Figure 65 Outdoor GIS substation in Perth WA



Gas insulated Switchgear is considered as overall very reliable however failures still occur.

Figure 66 Source: Cigre TB 513 Final Report of the 2004 - 2007 International Enquiry on Reliability of High Voltage Equipment Part 5 GIS 2012

Figure 67 Source: Cigre TB 525 Risk Assessment on Defects in GIS based on PD Diagnostics 2013 The GIS Equipment is often installed at strategically important locations in the network and since the time to repair Gas Insulated Switchgear can be considerable longer than with conventional switchgear any failure can have serious consequences.

2. Failure modes in GIS

Despite stringent QA procedures during manufacturing and assembly failures still occur.



Figure 68 Failures in GIS Typical defects in GIS are: Protrusions Any sharp points on earthed and live parts in the GIS in the vicinity of on electric field causes a field enhancement. This reduces the ability of the GIS to withstand Switching and Lightning Surges. Any sharp protrusions exceeding a size of 1-2mm are harmful. Protrusions may originate from production, transportation and from assembly. Particles This is a common problem and exists in almost every GIS Particle with several mm length can reduce the AC withstand level considerably. If they settle on epoxy spacers and insulators surface treeing my result leading to flashovers. Particles can be introduced from contamination during manufacturing or assembly on site or from metal abrasion due to vibrations or from moving parts from e.g. switching operations Floating Components Floating Electrical and mechanical loose shields or other metal parts not bonded to either HV or ground potential can generate large discharges that degrades the SF6 gas. Internal PD Internal PD from Voids and defects in spacers and epoxy components such as insulators are due to manufacturing issues and factory routine tests should detect these before shipment.



Protrusion

Electrically floating

shield

Particle on s pacer

Free particleProtrusion on

earth potential

Void in

s pacer

Figure 69 Type of failures

3. Monitoring Methods

The majority of users conduct a High Voltage withstand test combined with a Partial Discharge test during commission of GIS. These measurement sensitivity during these tests should be in the order of 5pC or better when using a conventional IEC60270 complied PD measurement system. This is hard to achieve using a conventional test system with open air connections and its preferable to use a bolt on totally enclosed transformer to maintain a screened measurement circuit.

Figure 70 Frequency tuned resonant bolt on test sets for on-site testing of GIS (Source: Siemens) Once energised the following monitoring options are available:

3.1 Chemical Analysis of SF6 gases

Permanent installed SF6 gas gauges usually monitor the Gas Density which provides an indication that a sufficient quantity of gas is present in the chamber. Monitored. These are typically equipped with relay contact that are connected to the SCADA system to signal the state of the Gas pressure/density.

Figure 71 SF6 density gauge installed on GIS CB chamber



In addition it is common to conduct annual SF6 quality check using portable gas analysers.

Figure 72 Example of SF6 analyser commonly used in Australia Parameters measured using include Humidity/Moisture and by-products Moisture and SF6 are normally non-reactive with each other but in the presence of a high temperature from arcing the hydrogen and oxygen of the water vapor may react with the sulfur and fluorine of the SF6 to create hydrofluoric acid (HF), sulfuric acid (H2SO4, and sulfur dioxide (SO2) These by-products reduce the dielectric breakdown strength inside the GIS across insulating surfaces and causes corrosion. The amount of acid is related to the amount of moisture. SF6 must have a certain level of purity to ensure safe operation of the GIS. Purity Safe limits for moisture and purity are included in guidelines and standards but vary between companies.



Figure 73 Example of limits sets by one utility considering various sources.

3.2 Partial Discharge Measurements (electrical)

Onsite Partial Discharge Measurements fall into two categories. Conventional PD measurement in the low frequency (



Figure 75 UHF Test Circuit To maintain good measurement sensitivity a high bandwidth across the whole measurement path is desirable. The typical frequencies of interest for GIS is between 250KkHz to 1.5GHz. Due to the near perfect shielding and a detection frequency of > 200MHz, away from most interference signals, this method permits the most sensitive PD measurement. A calibration of the UHF method in terms of pC is not possible although parallel measurements have confirmed sensitive levels of



Figure 77 External Window and spacer sensor. Measurements with the external spacer or spacer sensors are not as interference immune as the fully shielded measurement with internal integrated UHF sensors UHF Measurement results Measurements are taken initially in the spectrum analyser mode and if possible compared to background or historical readings.

Figure 78 UHF signal detected by Spectrum Analyser Circuit From the spectrum view a frequency of interest is selected and a time resolved or Point of wave measurement is taken to identify the type of fault. The pattern is synchronised to the mains frequency via a wireless adapter.

Figure 79 UHF Signal in Point of Wave/ Phase synchronised view Various knowledge rules are available to categorise the PD patterns based on shape, statistical parameters and frequency content 3.2.2 Acoustic PD measurements A valid alternative or a complementary technique to assess the GIS for any defects is the acoustic measurement already covered in part 1 of the paper.

Background without test

voltage, no fault

With test voltage applied,

large uplift across wide

spectrum from

background response



Acoustic measurements are performed by using an external piezoelectric sensor and a portable instrument as shown. The sensor picks up the acoustic waves/sound that propagate in the enclosure due to emitted acoustic signals from defects inside the GIS. Each section of the GIS enclosure separated by flanges should be tested with at least one measuring point. The acoustic signals from the described defects

are generally wide banded - partial discharges in the range of 10-100 kHz and particles up to several MHz The acoustic signals from the defects may vary widely from continuous signals from corona to pulse shaped signals from for example moving particles. The shape of the acoustic signal will depend on the type of source, the propagation path of the signal and the sensor characteristics. The measured parameters of the acoustic signal can be displayed and evaluated in three different measuring modes the continuous mode, the particle mode and the phase resolved mode.

Figure 80 Knowledge rules to identify defects based on the acoustic signature The measurement is immune against most types of external interference, has good sensitivity for detection of the most common types of defects. Once a defect is detected the origin can be localised to at least between two flanges and a risk assessment based on source characterisation is possible. The following UHF and corresponding signals were detected using a portable device that combines both UHF and acoustic modes.



UHF Mode- Electrical

Acoustic Mode

UHF Spectrum up to 1GHz shows

PD activity indicated by the large

diversion from Background and

high Peak to Average ratio

In Time /Phase resolved mode at

711.2MHz 6 MHz BW high

signals but poor relation to power

cycle

Both Bar and Oscilloscope

mode show high signal

levels with poor power

cycle correlation



Figure 81 UHF and Acoustic Signatures for same defect

The particle plot (signal amplitude vs. flight time) shows whether the signal is due to a particle. The flight times provide information on whether the particle moves into the high-field region. Signal amplitudes provide information on the particle length. Based on the parameters of the signal some estimation about size and risks can be made.

Phase synchronized mode

shows some phase

correlation

Interval/particle mode

clearly shows a bouncing

particle inside the GIS



Part 3

Time, Motion and Travel Testing of switchgear

Whilst so far the discussions have been limited to the assessment of the insulation quality of Switchgear it should be noted that the circuit breaker and its operating mechanism itself is basically a mechanical device. Life expectancy of HV circuit breakers is at least 30 years but under normal operating conditions the breaker will operate less than 10 minutes and under fault conditions it will operate less than 1 minute throughout its service life. During the Time and Motion test various timing and motion measurements are be made. After the test is complete the results are compared to the manufacturers specifications. During the test we may identify problems in the:

Mechanism, Linkages & Shock Absorbers the most common problems discovered are lubrication issues, mechanical binding, or mechanical interference. These types of problems may be identified by abnormal timing, travel, or velocity results.

Main & Resistor Arcing Contact Systems these types of problems are most frequently identified by excessive contact bounce on the contact timing plots.

Control Circuit these types of problems can be found directly or indirectly during the motion and timing test. Most frequently they are first identified indirectly, where the problem in the control circuit causes the CB to fail to operate or to fall outside the manufacturers timing specifications. During the subsequent investigation problems in the control circuit can also be identified directly using the instruments Analog or Auxiliary channels.



Figure 82 Typical setup for timing without motion transducers The above circuit just measures the contact state of the break per phase but in addition the trip and close currents are measured to obtain the timing. To further analyse the breaker other parameters are recorded via Auxiliary channels that monitor the state of contacts in the circuit breakers control circuit whilst analogue channels can measure any voltage or current of interest such as DC supply voltage or current and voltage during dynamic resistance tests. These are common timing parameters recorded for such a test.

Opening Time (Trip Test O) Closing Time (Close Test C) Reclosing Time (Reclose Test O-C) Reclose Open-Close Time (Reclose Test O-C) Trip-free Dwell Time (also known as Close-Open Time) (Trip-free Test CO)

Most instrument provide the initial data from each test in graphical format but in addition tables are available indicating numerical data

Figure 83 Plot & data from a TDR test showing parameters of all connected channels The numerical data for each test can be shown and if manufactures specification are available in the test plan automated pass/fail indications are given. For detailed analysis of circuit breakers Motion Channels are used to measure the motion of the circuit breaker using transducers. Motion transducers that are temporarily connected to predetermined points on the mechanism convert the measured mechanical motion into an electrical signal that is communicated to the Instrument.

Figure 84 Motion transducers temporarily connected



Including motion measurement in the measurement protocol provides Travel Plot and Velocity Plots. The Travel Plot shows the movement of circuit breaker over time. The Velocity plot shows the velocity over time. From these Travel and Velocity Plots, travel and velocity parameters are calculated. Average Velocity Total Travel Overtravel Rebound Contact Wipe Figure 85 Travel and Velocity Plots and table Contact Resistance The contact resistance tests using a 4 wire DC method injecting typically at least 100A to 200A are essential to identify any contact deterioration. A popular requirement now is the so called Dynamic Resistance measurement that monitors the voltage drop across the contact continuously. Either a DC source is integrated in the instrument or a separate DC source such as a battery or the output of the Resistance Meter can be used whilst utilizing analogue channels to measure voltage and current. The actual value of the calculated resistance is of no importance but the plot can identify additional features. This is a complimentary measurement to the above test but is required if graphite arcing contacts are used. Voltage drop measurement results depends on breaker contact system design and analysis is best done by comparison with previous tests.

Figure 86 Dynamic Resistance (Voltage drop) measurement. Minimum voltage required to trip the breaker and oil tests are common but exceed the scope of this paper. Whilst it is essential for Transmission type Circuit Breakers to monitor travel & velocity by attaching motion transducer to the mechanism these test are usually not conducted on distribution type breakers although timing measurements should be conducted at least during commissioning and major maintenance activities.



References:

1. Balcombe, H. The assessment and management of older oil-filled switchgear, Power Engineering Journal December 1997

2. Bradwell, A. and Bates, G. Analysis of dielectric measurements on switchgear

bushings in British Rail 25 kV electrification switching stations, IEE Proceedings, Vol.132, Pt.B, No.1, January 1985

3. Brown, P. Non-intrusive partial discharge measurements on high voltage

switchgear 4. Caviagelli, G., Kopaczynski, D., Lachman, M. and Levi, R. AC Power Factor

Versus DC Insulation Resistance Measurements 5. Douglas, R. and Booth, N. Testing Ageing Switchgear, IE Australia Electric

Energy Conference, 1992 6. EA Technology PDM3, Ultratev+ User Manual 7. HVPD PDsurveyor Air & Longshot User Manual 8. Ultra probe 2000 User Manual

9. Hilder, D. Partial-discharge measurements for insulation quality, Power

Engineering Journal, March 1992 10. James, R., Phung B. and Blackburn, T. Partial Discharge Phenomena

Characteristics, Interaction With Materials, Interpretation, ESAA Short Course, Brisbane, April 1999

11. Koenig, D. and Y. Narayana Rao Partial Discharges in Electrical Power Apparatus VDE Verlag

12. Lachman, M., Doble Engineering , 1998 Seminar Course Notes 13. Lemke Diagnostics GmbH Differential PD-probe LDP-5 Users Manual 14. Lundgaard, L. Partial Discharge Part XIV: Acoustic Partial Discharge

Detection Practical Application, IEEE Electrical Insulation Magazine, September/October 1992-Vol.8, No.5

15. Mettam, J. Insulation Aging In 12kV Switchgear A Users Perspective,

Proceedings of the 4th International Conference on Properties and Applications of Dielectric Materials, July 3-8, 1994, Brisbane Australia

16. Neil Davies, Simon Goldthorpe Testing distribution switchgear for partial discharge in the Laboratory and the field, Paper 804, CIRED Prague, 2009.

17. M. Boltze, S. Kornhuber, Various Methods of the partial discharge detection at switchgears, 2011 Doble Engineering Company -78th Annual International Doble Client Conference

18. Doble Lemke PD smart User Manual



19. Tettex Instruments, Information No. 21, Teilentladungs-Messtechnik 20. Tettex Instruments, Information No. 23, Teilentladungsmessung 21. TransiNor AIA User Manual 22. Doble PDS100 & DFA300 User Manual

23. A. Nesbitt, B.G. Stewart, S.G. McMeekin, Substation Surveillance using RFI and

complementary EMI detection techniques, 2011 Doble Engineering Company -78th Annual International Doble Client Conference

24. Sumereder C, Muhr, L Applied Risk Analysis for High Voltage Equipment,

Proceedings of the 9th International Conference on Properties and Applications of Dielectric Materials, July 2009, Harbin, China

25. U. Schichler, E. Kynast, High-voltage Tests and Measurements during the Life

Cycle of GIS, HIGHVOLT KOLLOQUIUM `07 26. R.G.A. Zoetmulder- S. Meijer- J.J. Smit, Delft University of Technology.

Conditional Based Maintenance with On-line Partial Discharge Measurements of HV and MV Switchgear Systems, C I R E D 17th International Conference on Electricity Distribution, 200

27. HV PD Course Notes 28. Transpower NZ, Metal Clad Switchgear Maintenance TPSS02.26 2002 29. Bernhard Fruth, PD Interpretation Scheme, PDTech 1999 30. E. Kuffel, W. S. Zaengl, and J. Kuffel, High Voltage Engineering: Fundamentals,

2nd ed Butterworth-Heinemann, 2000 31. F. Kreuger, Partial Discharge Detection in High Voltage Equipment, Butterworth

& Co., Kent, 1989 32. IEC Publication 60270, 2000, Partial Discharge Measurements 33. R. James, Q Su, Condition-Assessment-of-High-Voltage Insulation in Power

System Equipment, IET Power and Energy Series 53, The Institution of Engineering and Technology UK, 2008, ISBN 978-0-86341-737-5

34. E. Lemke, S. Belijn, E. Gulski, M. Muhr, E. Pultrum, T. Strehl, W. Hauschild, J.

Rickmann, and G. Rizzil, Guide for partial discharge measurements in compliance with IEC 602

35. Cigre Technical Brochure 513 - Final Report of the 2004 - 2007 International

Enquiry on Reliability of High Voltage Equipment, Part 6 Gas Insulated Switchgear (GIS) Practices (2012)



36. Cigre Technical Brochure 556- SF6 Analysis for AIS, GIS and MTS Condition Assessment (Feb 2014)

37. Cigre Technical Brochure 525- Risk Assessment on Defects in GIS based on PD

Diagnostics (2013)

38. Doble TDR 900-TDoble Manual.

39. Levi Jozef, Motion measurement and use of transducers for the Detection of circuit breaker characteristics 2011 Doble Engineering Company -78th Annual International Doble Client Conference

40. Levi Jozef, Radenko Ostojic Use of Micro Ohm Meter as a Power Source for

DRM testing of Dead Tank Circuit Breakers 2013 Doble Engineering Company -80th Annual International Doble Client Conference

Biography Karl Haubner joined Doble Engineering in 2004 and is employed as the High Voltage Test Application Engineer servicing the Asia-Pacific region. Prior to his appointment with Doble Engineering he worked for the Utility Western Power. As the Superintendent of the HV Test Laboratory he was responsible for all technical aspects of the test group such as development and introduction of new condition monitoring techniques on distribution, transmission and generating plant. In addition to his employment with Doble Karl also provides testing and consultancy services to the industry via his company High Voltage Solutions. He is the author of several technical papers on condition monitoring of HV assets and cable fault location and has delivered short courses on testing techniques at University and Industry level. Karl has a Diploma in Electrical Engineering and is member of Cigre D1, AS Committee EL-007, and of VDE.

High Voltage Conference 2015 IDC Technologies

55

Appendix 1 Verification of effectiveness and limitation of various on-line techniques used by conducting parallel on and off-line measurements using a range of commercial detectors. A ring main unit (RMU) that had been removed from service due to PD activity (signs of surface PD activity at the fuse holders evident) was energised from a HV test transformer. The RMU is gas insulated and all defects (existing or simulated) relate to the exterior connection points and not the busbar. The level of Discharge Activity present was measured via a 20nF coupling capacitor and an IEC 60270 type compliant PD detector (Doble-Lemke PDsmart) operating in the frequency range between 100-500 kHz as prescribed in the standard. The total test circuit was calibrated by injecting 10pC across the switchgear. The RMU was first energised as found and then measurements were taken at several voltage levels. Several defects were then introduced to simulate other faults than the ones present already on the switchboard but commonly found in switchgear Whilst the off-line PD measurements provide more statistical parameters then the pC value shown below for the purpose of this exercise only IEC pC values and the phase resolved PD patterns are listed. Phase synchronisation for the off-line measurement was via the coupling capacitor (true phase position) whereas synchronisation for the hand held RFI/EMI Spectrum Analyser and the Ultra-Wideband TEV analyser was from the Mains. Humidity and temperate conditions were recorded during the tests but are not shown as the aim was to compare measurement techniques. The transmission path from the PD source to sensor the metal cover was direct with the defect always in close proximity to the enclosure. This represents an ideal situation compared to example busbar defects in Metal Clad Switchgear were the transmission path may be longer and obstructed by other barrier material or additional covers.


56

Test

Off-line IEC 60270

TEV

Acoustics

Airborne

Acoustics

Contact

Acoustic

s

Airborne

RFI using unidirectional

Antenna

Acoustics (Phase

resolved)

Contact type Sensor

RFI/EMI

UHF/TEV sensor

Full spectrum & point

on wave (phase

resolved)

Corona Camera

Ultra-wide Band

TEV Sensor

As found at 3kV

30pC

1-2dB BG 1-4db

Audible 17dbm

Audible 15dm

Clearly audible

None detected with covers on

(Background~15dB)

Measurement ~15dB

As found 10kV

150-200pC

4dB Audible 19dbm Audible 20dbm

Clearly audible


~15dB

as found @ 14kV 500-600pC

13dB Pulse/ Cycle:

Audible 25dbm

Audible 20dbm

Clearly audible


~15dB


57

1.5 Severity 5

as found @ 14kV B Phase

800-1000pC

13dB Pulse/ Cycle: 0-3 Severity 1

Audible 46dbm

Audible 31dbm

Clearly audible

With covers removed

~15dB

Void Discharge insert insulator with voids

800-1200pC low pulse count

24dB Pulse/ Cycle: 0 Severity 0.16

-3dBV Not audible

-3dBV Not audible

Not audible

N/A

~ 15dB


58

Voltage indicator O/C

700-800pC

33dB Pulse/ Cycle: 56 Severity 2.08

-3dBV Not audible

-3dBV Not audible

Not audible

N/A

~ 26dB

Cable PD (Stress Control removed)

+2000pC

25dB Pulse/ Cycle: 5.5 Severity 84

Audible 25dbV

Audible fair to poor 19dbV

Clearly audible

~ 30dB

idc_hv conf_perth 2015_condition monitoring of high voltage switchgear_kh

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