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PD Webinar series

Introduction to PD measurement and monitoring

October 7, 2021

PD measurement and localization of power transformers

October 21, 2021

PD measurement and localization of power cables

November 4, 2021

PD measurement and monitoring on rotating machines

November 7, 2021

© OMICRON

Page 4


Page 5

The content of this Fundamentals of PD course includes the necessary PD fundamentals, theory, practical

exercises and result assessment divided in single sections.

It is based on a collection of measurement experiences from the last few years, which have been gathered

through hundreds of PD measurements at various locations around the world.

At OMICRON, we believe that this comprehensive collection of knowledge will cover the basics of PD

diagnostics that today's test engineers require before addressing application-related PD measurements.

Please note the copyright clauses mentioned at the end of this presentation.

Introduction to PD measurement and monitoringFlorian Predl

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5

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01-Introduction-Fundamentals-of-PD

Agenda

Overview and motivation

PD theory (basics)

PD types and associated PRPD

PD sensor types

IEC 60270 requirements

Charge calibration and measurement

Noise mitigation techniques

Periodic PD measurement and permanent monitoring

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Page 6


Various assets have to suffer a lot of periodic or continuous forces, such as thermal stress, electrical stress,

aging and mechanical stress (shortened by the abbreviation TEAM).

Thermal stress: The temperature rises in the case of overload. Many temperature shifts lead to

premature aging of the insulation such as delamination or cracking of the

insulation

Electrical stress: Besides nominal voltage the isolation is also stressed by over-voltage due to

electrical faults

Ambient stress: Includes moisture, aggressive and reactive chemicals (gas, acids) and foreign

particles (metal parts, ash, carbon, lubricants)

Mechanical stress: Vibration, mechanical forces and different thermal expansion

Page 7

Ageing of insulation

Overstress of insulation

Damage to the insulation of various assets due to a combination of stresses

Page 7

Thermal stress

• Highest and lowest temperature

• Increase of current through overloadT

Electrical stress

• Rated Voltage

• Over VoltageE

Ambient stress

• Moisture

• Aggressive and reactive chemicals

• Abrasive particles

A

Mechanical stress

• Vibration

• Mechanical forces

• Different thermal expansion

M

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© OMICRON Academy


According to the IEC 60270 standard, partial discharges are “localized electrical discharges that only partially

bridge the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial

discharges are in general a consequence of local electrical stress concentrations in the insulation or on the

surface of the insulation." PD can occur in gaseous, liquid and solid insulating mediums used in assets that are

subjected to high electrical fields. It can be initiated by voids, cracks, or inclusions within a solid dielectric, at

interfaces within solid or liquid dielectrics, in bubbles within liquid dielectrics, or along the boundary of different

insulation materials.

PD can cause progressive and irreversible damage to liquid and solid insulation systems. With time, PD activity

becomes more intense and dangerous. The process of deterioration can propagate and develop until the

insulation is unable to withstand the electrical stress, leading to a flashover.

Page 8

What is partial discharge?

Partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress.

Definition from IEC 60270 specificationLocalized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor.

Page 8

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PD theory...

© OMICRON

Page 9


PD measurement is a reliable and non-intrusive method that can be used anytime to diagnose the insulation

condition of an electrical asset. Compared with other dielectric diagnostic methods, PD measurement provides

you with very sensitive information to help you effectively detect localized weak points in the insulation system.

In many cases, PD phenomena are the preliminary stage of a complete insulation breakdown, and as a result,

power transformers, generators, instrument transformers, cable systems, and switchgear have been checked

for PD for many years.

Because PD activity is often present well in advance of insulation failure, asset managers can assess it over

time and make informed strategic decisions regarding the timely repair or replacement of the equipment before

an unexpected outage occurs. PD detection is therefore essential to ensure the reliable, long-term operation of

your electrical equipment.

Partial discharge analysis – why?

Why PD analysis is necessary

Detection of critical defects and risk assessment

In many cases PD phenomena are the preliminary stage of a complete insulation breakdown

As a result transformers, generators, instrument transformers, cable systems, and switchgear have been checked for PD for many years

Page 10

Typical consequence of PD

(pressboard barrier of a power transformer)

© OMICRON



The integrity of the insulation in MV and HV equipment should be confirmed with PD measurement and

analysis during the development, manufacturing, commissioning and, depending on the asset type, the service

life of electrical equipment, so that it stays in good condition and is safe to operate.

The high amount of manual work at the manufacturing stage of an asset increases the likelihood of production

errors that can lead to its premature failure. There is a disproportionately high percentage of insulation failures

being observed within the first one to three years of service compared to the rest of an asset’s working life. PD

testing is therefore initially used for routine and factory acceptance testing after production to identify quality

issues.

After the asset leaves the manufacturer, improper handling during transport and installation can lead to internal

mechanical damage. An on-line PD measurement is then often used to commission new equipment on-site as

a final quality control check.

Partial discharge analysis – when?

development tests

following standards

controlled environment

type tests and factory acceptance tests

Common measurement for acceptance testing on MV and HV assets

based on asset and general standards e.g. IEC 62271-200

controlled environment

on-site measurement

often used during site acceptance testing or after failures

following the typical standards

often disturbed environment

Page 11

Close-up of an electrical tree as a result of

continues partial discharges

© OMICRON



In general, partial discharge can be broken down into two categories, one is internal partial discharge and the

other is external partial discharge.

Common examples for Internal PDs are Void (Cavity) discharges or Electrical treeing.

Common examples for External PDs are Corona discharges or Surface discharges.

The next slides explain the single PD sources in detail.

Page 12

Classification

Types of PDPage 12

Corona discharges Surface discharges

External PD

Void discharges

Internal PD in the insulation

Electrical treeing

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© OMICRON Academy


Surface discharge

Occurs at “triple points” – a point where a conductor, a good insulator (e.g. solid insulator) and a bad insulator

(e.g. air) meet.

Therefore it can be found at boundaries of different insulation materials (e.g. on bushings, end of cables)

Corona discharge

Occurs in gaseous dielectrics in the presence of inhomogeneous fields, usually not inside of objects

Can be typically heard as PD at overhead lines in a substation (transformer / distribution station)

Page 13

Types of partial discharge

External PD

Page 13

Surface dischargeCorona discharge

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© OMICRON Academy


The following examples shall give an overview of common cases of partial discharge in different locations of

our energy supply and their operating components. Corona is an electrical discharge mainly take place at sharp

edges, corners, points, small radii and as a result of high electric field strength at the transition of conductor

and insulator on overhead lines. Due to the ionization process in air, electrons break away from their original

orbit and recombine with the atoms. This recombination process releases energy in the form of light and glow

and often can observed at transmission lines in low light conditions. The same effect can be observed during

the day with sun-blind UV cameras, known as DayCor® cameras.

Corona discharge can affect the electrical power systems due to generation of corrosive products such as

ozone and nitrogen oxides which can create sulfuric acids under humid conditions. Those corrosive materials

can heavily harm polymeric insulations as well metallic conductors and other over headline components.

In addition, the occurrence of Corona discharge can influence the measurement of possible internal partial

discharges in the actual asset, since they usually predominate strongly with their high amplitude and repetition

rate.

Page 14

Evidence of PD: Overhead lines

Corona discharges at insulator

Page 14

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© OMICRON Academy


Stator bars: The change of color proves destructive PD activity, followed by a degradation of the surface

material. The black dust and ashes often lead to an increased surface conductivity, causing parasitic currents

and hot spots. These effects will further deteriorate the insulation – eventually leading to more partial

discharges.

Constructional problem such as imperfect distances (phase separator) or contamination of winding heads lead

to partial discharge at the end-winding area. Further consequences are further decomposition of the insulation

material at end-winding area which often lead to discharges.

Page 15

Evidence of PD: Rotating machines

Contamination & imperfect distance at end-winding area

Page 15

Contamination of end-winding areaImperfect distance between winding heads

and contamination

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© OMICRON Academy


Internal discharges in laminated material

Occurs when laminated materials delaminate, allowing the formation of gas bubbles or entire areas of gas

inside the insulator to form.

Cavity / Void discharges

Occur in voids or cavities within solid or liquid dielectrics (voids and cavities are usually filled with some kind of

gas).

Treeing

Continuous impact of discharges in solid dielectrics forming discharge channels (treeing) in organic materials

(e.g. cable insulation).

PD activity can occur on the surface or/and inside insulations. The cavity discharge and treeing are the most

dangerous processes for assets’ insulation systems being almost impossible to accurately localize them.

Page 16

Types of partial discharge

Internal PD

Page 16

Cavity / Void dischargesInternal discharges

in laminated material

Electrical treeing

© OMICRON

© OMICRON Academy


This is a video of a needle on the left side and a large electrode on the right. This needle is pushed into a

silicon mass in a glass jar. High voltage is applied and the treeing process can be witnessed.

The channels/tunnels formed by partial discharges, have the strong tendency to “follow“ the electrical field

lines. This takes into account that every channel/tunnel will change the electrical field on its own.

Since the discharges are within a glass housing, the conductivity on the right side is very limited. Therefore,

new channels/tunnels can be created even though others have already reached the right side. Without the

glass bottom of the jar on the right side, the first channel/tunnel reaching the right side would result in a total

breakdown – destroying most of the silicon around it.

Page 17

Source: IPH Berlin

Visualization of electrical breakdown

Video: process of electrical discharge (channel) in solid insulation

Page 17

Source: IPH Berlin

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Bushing insulation: Above pictures shows the dismantled bushing with the porcelain insulator removed in a

workshop. Water ingress – through a defective gasket – caused high partial discharge activity along the surface

of the paper insulation. Luckily the bushing could be taken out and replaced with a spare bushing relatively

quickly before a major breakdown happened.

Partial discharges are observed in power transformer windings if the insulation material between different

voltage potentials is aged, contaminated or faulty. It can be initiated by voids, cracks, or inclusions within a

solid dielectric, at interfaces within solid or liquid dielectrics, in bubbles within liquid dielectrics, or along the

boundary of different insulation materials. PD can progressively cause damages insulation materials in power

transformer bushings and windings, leading to their eventual failure and costly outages. Therefore it is

important to recognize the PD source, find it, and eliminate it if necessary

Page 18

Evidence of PD: Power transformers

Evidence of surface discharges on 123kV RBP bushing

Page 18

© OMICRON

© OMICRON Academy


Here the insulation of a cable was modified to clear it up. Therefore light can pass through and allows us to see

the results of a discharge. This discharge bridged the insulation and destroyed it.

If this would have happened in a real-case scenario, the total power (charge) inside the cable would have

discharged at this very spot. This discharge power (in a real-case scenario) would have destroyed a much

larger area around the discharge point.

Page 19

Evidence of PD: XLPE cablesPage 19

Source: IPH Berlin

Discharge channel in XLPE insulation

© OMICRON

© OMICRON Academy


There are several well-known approaches of partial discharge (PD) analysis. The most common ones are:

▪ The use of Phase-Resolved Partial Discharge (PRPD), with the PD charge over the phase angle

▪ The PD trend over time

▪ The PD charge dependency of the high voltage (Q(U))

▪ Statistical TDR (sTDR) for localization, where the travelling times in a cable are used to determine the PD

position

The reliable methods 3PARD and 3CFRD introduced by OMICRON for PD analysis are based on either time

synchronous measurements with three MPD measurement channels or multi-band measurements with one

MPD measurement channel for single-phase applications. Both methods help you to discriminate PD sources

from noise and simplify measurement analysis in environments with heavy interference.

Page 20

How to analyze Partial Discharge

Well known approaches:

Phase Resolved Partial Discharge Diagram (PRPD)

PD trend over time (Trend)

PD charge over voltage Q(U)

TDR and sTDR (PD localization)

OMICRON’s approaches:

3-Center Frequency Relation Diagram (3FREQ)

3-Phase Amplitude Relation Diagram (3PARD)

Page 20

The 3PARD diagram with selected PD cluster

PRPD of offline PD measurement at hydro generator

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© OMICRON Academy

07-Assessment-How-to-analyze-PD

PD events are often visualized in diagrams called phase-resolved PD (PRPD) patterns. A PRPD is an

important analysis tool that relies on the repetitive nature of PD. PRPDs can help with determining the cause

and severity of PD activity. They plot PD events over the AC high-voltage signal, where each PD event is

associated with a certain phase position of the AC signal (i.e. the position in the cycle of the AC signal where

the PD event occurs). That phase position is used as the horizontal position in the plot, while the charge of the

PD event is used as the vertical position in the plot. The color of the point given by the phase position and

charge of the PD events signifies the number of PD events with the same phase position and charge that have

occurred so far. PRPDs give rise to characteristic patterns that may be specific to certain types of PD in certain

types of assets.

PD events that are triggered by the AC voltage as it progresses through its cycle will always appear at the

same phase position. On the other hand, spurious noise and other transient signals that may look like PD but

that are not synchronous to the AC signal will “smear” across the PRPD.

Page 21

How to visualize Partial Discharge?

Phase Resolved Partial Discharge diagram (PRPD)

Page 21

...2 3 4 5 50

Time (ms)

Time (ms)

u(t)

Am

pli

tud

e

360°180°90° 270°

Trigger

1

Am

pli

tud

e

Trigger Trigger Trigger Trigger Trigger

© OMICRON

© OMICRON Academy

03a-Methods-Measuring-PD

The PD scope view shows the signal applied to the PD input of the primary-selected channel group, plotted as

voltage over time. The PD scope view works much like a regular oscilloscope.

The FFT view shows the frequency spectrum of trigger shots. As such, it is tied to the PD scope view, and will

only contain any data when the PD scope view has triggered. The horizontal axis represents the frequency of

the spectrum, while the vertical axis represents the spectral energy of the signal, given in dBm (relative to a

voltage of 1 mV).

Since a PRPD is a statistical diagram that aggregates the PD activity over many AC cycles, relevant

correlations will become visible over time. PD events that always occur at the same AC phase position will be

strongly located, forming colored points or clusters in the PRPD, whereas signals that have no relation to the

AC signal’s phase will be spread across the PRPD with no clear location. PRPDs thus make it relatively easy to

differentiate between AC-synchronous PD and disturbance pulses that are not synchronous to the AC.

Page 22

How to measure Partial Discharge?

Forms of analysis: Scope → FFT → PRPD

Page 22

3

PRPD

AC voltage

PD pattern

Frequency → FFT

2

Time → Scope

1

© OMICRON

© OMICRON Academy


A practical demo...

© OMICRON

Page 23


Due to the polarity effect, a tip on high-voltage potential always causes PD to occur at 270° first (negative half

cycle). As the voltage is increased, the pattern becomes wider.

In contrast, a tip on ground potential causes PD to occur at 90° (positive half cycle).

Page 24

Corona discharge – PD patternPage 24

t

V

PD inception voltage negative tip

PD inception voltage positive tip

t

V

PD inception voltage negative tip

PD inception voltage positive tip

HV

tip on high-voltage potential

tip on ground potential

HV

Corona

© OMICRON

© OMICRON Academy

14-Assessment-Interpretation-PRPD

Page 25

Corona discharge – polarity effectPage 25

• electrons move to positive tip

• lower field in front of tip

• higher field in space

• late discharge, early breakdown

• starting electron at tip

• pos. ions remain at tip

• higher field at tip

• early discharge, late breakdown

with space charge (lower local field)

+

d

E

Einc.

without space charge

+

+

+

with space charge

d

E

Einc.

+

without space charge

+

++

© OMICRON

Corona

© OMICRON Academy


[1] A few discharges of similar amplitude appear in the negative (270°) only.

[2] Raising the test voltage, the number of discharges is increasing, and the pattern becomes wider, while the

amplitude is more or less still the same.

[3] Again the test voltage is risen. The pattern widens further, and some single PD impulses become visible on

the positive half-cycle (90°) as well.

Page 26

Examples: Corona discharge

PD Simulator – sharp tip on HV potential

Page 26

1 2

3

© OMICRON

Corona

© OMICRON Academy


[1][2] Examples of Corona discharge on ground potential

[3] Corona discharge measured on a HV cable. The PD impulses are reflected in the cable and appear as PD

impulses with lower amplitude below the original pattern.

Page 27

Examples: Corona discharge

PD Simulator – sharp tip on ground potential

Page 27

original corona

Example: measurement on a HV cable results with multiple reflections

of PD impulse

1 2

3

1st reflection

2nd reflection

© OMICRON

Corona

© OMICRON Academy


PD generated on the surface of the insulation material and are located naturally at transition areas between HV

potential and insulation material.

Some type of surface discharge arise at the transition between potential e.g. HV electrode and insulation, if the

field control system is not adequate or effective, because of poorly designed contact points, contamination,

porosity, thermal effects. Other types of surface discharge result often from conductive contamination (carbon,

oily dust, abrasion etc.) or from damaged field grading materials.

Page 28

Surface discharges

Examples: Surface discharge

PD at high voltage potential

Page 28

HV electrode

Insulation material

Ground potential

Surface discharge channel

© OMICRON

© OMICRON Academy


Following example of surface discharge was observed during an offline PD measurement at 20kV switchgear

with all accessories connected. As a HV source the 70KV RTS resonant system was used.

Problems with the black insulation caps to the MV voltage VT were suspected. After exchanging the caps, the

surface discharge was gone.

Often dirty surface (dust or other particles,...) during the mounting of insulators can caused surface discharge

which can lead to degradation of the insulation material and further to total breakdown in near future.

Page 29

Examples: Surface dischargePage 29

Surface discharge 20kV switchgear accessories

© OMICRON

Surface discharges

© OMICRON Academy


The PD measurement was performed on a 36 kV MV switchgear. The test was done with a separated voltage

source. The PD sensor was coupling capacitor.

The partial discharge test was done in accordance with IEC 62271-200 and IEC 60270 standards. System

calibration was performed on each phase according to IEC60270. The voltage source was connected to each

phase successively, the other two phases and all the parts were grounded. The voltage was raised to 1.3Ur

(47kV) and then it was decreased to 1.1Ur (40kV).

The PRPD is recorded at 1.3Ur (47kV).

Page 30


36 kV MV switchgear, PRPD @ 47kVrms

Page 30

© OMICRON

Surface discharges

© OMICRON Academy


End terminations are used to manage the electrical field at the end of MV or HV cables, similar as bushings are

used at transformers. Different solutions exist due to the variety of insulation materials. The most common

techniques are geometric (HV/MV cable) and refractive/resistive field control (MV cable).

Semi conductive layers adjacent to the core conductor and the sheath are used to smoothen the field

distribution and to prevent areas with elevated electric fields.

A defect in the inner or outer semi conductive layers, like a tip, leads to an inhomogeneous electrical field. The

elevated field strength, once exceeding the dielectric strength of the insulation, will cause partial discharge and

the evolution of an electrical tree along the semiconductor surface.

Page 31

Surface discharges


PD due to inhomogeneous electric field distribution

Page 31

Figure: Cable end without field control –

Tangential concentration of the electric field at the surface of the insulation material.

© OMICRON

© OMICRON Academy


Surface discharges on the surface of a 10kV XLPE cable without termination. The negative half cycle has

higher discharges with a lower density (amount of PD per second).

The positive half cycle has lower discharges with higher density (amount of PD per second). Asymmetry is

caused by polarity effect (PD source on ground potential).

The test was performed in the lab, the PD sensor was a coupling capacitor.

Page 32


XLPE cable without termination

Page 32

© OMICRON

© OMICRON Academy


Let us assume that the dielectric of an ideal capacitor includes a gas void (upper left side figure), the equivalent

circuit diagram of this dielectric would look like shown in the lower left side figure. The capacitors CS and CF

form a capacitive divider. Thus the U1 drop voltage on CF is lower than the applied voltage Ut (right side

picture).

If the electric field strength in the insulation becomes higher than the dielectric strength of the gas inside the

void, a total breakdown will appear inside the void (represented as spark gap “S” in the equivalent circuit

diagram). In this moment, the spark gap “S” flashes over and U1 drops to zero and the discharge extinguishes.

The process is repeated when the electric field strength in the insulation becomes higher again than the

dielectric strength of the gas inside the void. This process appears at the zero crosses of Ut and depends on

the voltage gradient (around the peaks the voltage gradient tends to zero).

Page 33

Ut(t)

U1(t)

U t)UZ

UL

-UL

U1(t)

-UZ

t

t

I1(t)

Internal discharge (Void)Page 33

𝑞 = ∆𝑈1𝐶𝐹 𝑞 = න 𝑖1 𝑡 𝑑𝑡

CP

CF

CS

R1

S

U2

U1

Ut

Test object

CP/2 CP/2

2CS

2CS

CF

ɛr

ɛ0

I1(t)

IS(t)

Void discharges

Discharge mechanism

© OMICRON

© OMICRON Academy


Most probably many cracks in epoxy in cast-resin voltage transformer form a PD pattern of multiple clearly

visible “fingers” around the zero crossing.

Each bow-shaped patterns equals to the single voids in the epoxy insulation.

Page 34

Examples: Void dischargePage 34

Distribution voltage transformer

© OMICRON

© OMICRON Academy


The void discharges (bow shaped patterns) measured on this dry-type transformer indicate a defect in the cast

resin insulation on phase U which can develop into a breakdown and failure of the winding. A statement on the

remaining service life cannot be made because the progress of the defect is not known and a failure depends

on the operating parameters, loads and possible errors in the network.

The dry-type transformer was excited from the low-voltage winding with using the CPC 100 (control unit) +

MTR2 (exciter transformer). The PD measurement was performed phase-to-ground for each individual phase.

Page 35

Examples: Void discharge

Dry-type transformer: crack in epoxy insulation

Page 35

© OMICRON

© OMICRON Academy


The PD measurement was performed on a 1600 kVA, 20 kV/400V, oil type transformer (in the factory). The test

was done with a separated three-phase voltage source. The PD sensors were coupling capacitors. The PRPD

diagram is measured at 22 kV at phase V.

The pattern (symmetric) could be assigned to internal discharges (inside the transformer tank) due to metallic

particles on the surface of the insulation (based on the CIGRE Brochure 676, PD in transformers).

Page 36

Examples: Void discharge

1600 kVA, 20 kV, oil-type transformer

Page 36

© OMICRON

Void discharges

© OMICRON Academy


Following example of floating potential was observed during an offline PD measurement at 20kV switchgear

with all accessories connected. As a HV source the 70KV RTS resonant system was used.

Like contact PD connector or accessories which are not directly connected to HV potential or ground potential -

and are within with the surrounding electric field can cause floating potential PD. The discharge typically are

located close to the zero crossing of the applied AC voltage. Additionally, the patterns show strong symmetry in

positive and negative voltage half-waves.

Different to Contact PD the discharges of floating potential PD do not disappear with time but remain at a

certain amplitude.

Page 37

Examples: Floating potential

Floating potential at 20kV switchgear

Page 37

© OMICRON

© OMICRON Academy


Page 38

Overview PD coupling methods

Coupling capacitors

Measurement on bushings

High frequency current transformers

UHF sensors

Page 38

© OMICRON

© OMICRON Academy

06-Accessories-PD-coupling-sensors

A coupling capacitor (Ck) is a very common coupling method when performing a PD measurement as

described in the IEC 60270 standard. When a partial discharge event occurs, the coupling capacitor provides

the devices under test (DUT) with a displacement current, which is measurable at the coupling devices (CPL).

Such an approach provides additional information about the test voltage, which is needed for a phase-related

partial discharge (PRPD) measurement.

The PD measurement setup according to IEC 60270:2015 includes:

Blocking impedance – an impedance or a filter can be introduced at high voltage to reduce background noise

from the power supply.

Coupling capacitor (here MCC 117 or MCC 210) for decoupling high-frequent PD signals and AC voltage.

Coupling device (here CPL1/2) is usually a four-terminal network (quadripole), converting input currents to

output voltage signals.

PD measuring unit (here MPD 800) for processing all measurement data.

In the most commonly used method, the coupling capacitor Ck and the coupling device CD are connected in

series. The main advantage of this scheme is that the measuring device is not affected in case of a total

Page 39

Standard PD measurement setup

Setup accordance with IEC 60270:2015

DUT... device under test

Ck ... coupling capacitor

CD... coupling device

Z ... blocking impedance

Page 39

DUT

Cstray

not

measured

Z

CD

Ck

Fiber-optic

cable

USB

MPD 800

MCU2

CPL1/2

(optional)

𝑈𝑡𝑡

iPD ... high frequent current

© OMICRON

© OMICRON Academy


breakdown of the test object Ca. In this case the stray-capacitances given by the surrounding

of the setup are not considering for the re-charging process and not part of the measured

charge.


What are we measuring?

© OMICRON

Page 40

B

A

CP/2 CP/2e0

2CS

CF

2CS er


The above example shows a 3-channel offline PD measurement on a dry-type transformer that has been

excited on all 3-phases from the secondary side.

The high diameter tube from the HV side of transformers to the coupling capacitors must be noticed. The goal

of using a high-diameter tube is to reduce the field strength on the cable surface between the test object and

coupling capacitors. Thus, a PD free test setup is obtained and high-accuracy measurements may be

performed.

If the diameter of the connection wires between the test object and the coupling capacitors is too small, Corona

discharges have to be expected.

Here, special connection wires have been used. Apart from the increased diameter, the connection wires show

toroid's (anti-corona-rings) on both sides.

The base of the coupling capacitors is isolated against the ground. The connection to ground is done by using

the coupling devices / quadripole / CPL . Therefore, all high-frequency signals pass through the coupling device

and can be measured by the MPD 600.

Page 41

Application: Dry type transformer (manufacturer)Page 41

3x coupling capacitors @ terminals

© OMICRON

Conventional measurement

Capacitive coupling

possible to state the actual measured charge

3x coupling capacitors @ terminals

© OMICRON Academy


Left picture shows an offline PD measurement on a MV switchgear panel with an external voltage source and

coupling capacitor (MCC 210) being used.

Right picture shows an offline PD measurement on a MV voltage cable with VLF (0.1Hz) source of B2

electronics. The HV cable of the VLF source is connected to an external HV filter, an external TD (tan delta)

measurement device and then to the coupling capacitor (MCC 210).

Page 42

Application: MV GIS and MV cables Page 42

PD measurement with 50Hz source PD measurement with VLF source (0.1Hz)

Offline PD measurement with coupling capacitors

© OMICRON

© OMICRON Academy


Page 43


Coupling capacitors



UHF sensors

Page 43

© OMICRON

© OMICRON Academy


Bushing adapters connected to the measurement tap allow partial discharge measurements to be performed

using the capacitive graded bushings of a power transformer instead of using large coupling capacitors.

Bushings with rated voltage above 110kV are equipped with a bushing measurement tap where a metallic

conductor is connected at the last capacitive layer. Through this capacitive voltage divider high voltages can be

measured, and which is used for measurement and diagnostics purposes. The usual purpose of the bushing

measurement tap is measure capacitance C1 and power/dissipation factor (tan δ). The measurement tap can

also be used for permanent voltage measurements or in this case partial discharge measurements.

For preforming PD measurements on power transformers there are several advantages when tapped bushings

are available:

▪ NO external coupling capacitor is required

▪ Less background noise in the measuring system

▪ The coupling device is directly connected to the measuring tap

▪ On-line measurements are possible in case of permanent installed measuring units

▪ Nevertheless, the installation of the measuring set up must be done when the transformer is disconnected.

Page 44

Transformer: HV bushing taps

Measurement setups

Advantages using bushing measuring tap NO external coupling capacitor is requested Less background noise in the measuring system The coupling device (quadripole) is directly connected to the measuring tap On-line measurements are possible in case of permanent installed measuring units

Page 44

Bushing without measuring tap Bushing with measuring tap

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Offline PD measurement on power transformers in substation environment are often performed by means of a

(external diesel) generator and step-up transformer connected to low voltage winding.

The actual PD measurement is performed on the HV bushings via the measurement taps. In this case bushing

tap adapters are used for the connection to the MPD 800 units.

Using long fiber-optic cables the workplace with the MCU2 and Notebook is set-up in the work area with safe

distance to the power transformer.

Page 45

Decoupling over bushing measuring tapSecondary voltage injection with step-up-transformer

Work area in safe distance

Application: Power transformers (substation)Page 45

Installing the bushing tap adapter

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Page 46


Coupling capacitors



UHF sensors

Page 46

© OMICRON

© OMICRON Academy


Partial discharges in solid insulations also induce high-frequent PD signals into the earth system. The high-

frequent PD impulses travel along the earth conductor, through the installed high frequency current transformer

where the induced voltage signal is measured and processed by the PD measurement system. High frequency

current transformers (HFCT) consist of ferromagnetic split-core and induction coil designed for the

measurement of transient signal (PD and noise).

HFCTs are a common alternative to coupling capacitors and are originally used for online PD measurement via

the ground conductor in an earthing network. The main benefit of using HFCTs is the possibility to measure PD

pulses not at high voltage potential but at grounding connections without opening them.

Page 47

High frequency current transformers (HFCT)

Principle of HFCT sensors

HFCTs consist of ferromagnetic split-core and induction coil

Measurement of transient signals (PD and noise)

Originally used for online PD measurement via the earthing system

Page 47

PD

Noise

Ground conductor

HFCT

u(t)

MPD 800© OMICRON

© OMICRON Academy


Typically the HFCTs and PD measurement units are installed at the near-end or far-end termination and at if

accessible at joints locations, via the cross-bonding boxes. Only two MPD 800, using three of the four available

PD inputs, are required to perform a three-phase simultaneous at the near-end terminal.

In case of short cable tracks a manually laid parallel fiber optic cable allows additional simultaneous

measurement in joints locations and far-end terminal.

In case of long cable tracks pre-installed parallel fiber optic cables are required to cover the longer distances

between terminations and joints.

Page 48

Cable PD measurements with HFCT

Example setup with MPD 800

Page 48

Termination

Joint

Fiber optic cable

MCU2

NotebookUSB

MPD 800

RBP1

Coaxial cable

MCT 120

© OMICRON

© OMICRON Academy


For on-line PD measurements on cables, an HFCT is normally installed around the cable sheath of the joints

and on the grounding point on the end terminations.

Page 49

High frequency current transformers (HFCT)

GIS termination: 220kV XLPE cable, 3x single-core

Page 49

© OMICRON

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Page 50


Coupling capacitors



UHF sensors

Page 50

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© OMICRON Academy


A brief description of the two methods (IEC and UHF) is provided here.

Until now there is no standard procedure to calibrate UHF measurements.

Page 51

UHF PD measurement

E / H

IEC compliant PD measurement

I / Qiec

PD measurement using IEC and UHF setupPage 51

IEC measurement UHF measurement

Dispersion Compensation current Electromagnetic field

Coupling Discrete capacitor Antenna

Frequency kHz – some MHz 100-2000 MHz

Calibration Small setups, low

frequencies

Magnitude and damping

depends on position of defect

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© OMICRON Academy


Page 52

UHF measurement equipment

Measurement setup @ oil-drain valve of power transformer

Page 52

© OMICRON

© OMICRON Academy


Example of connecting a MPD600, the UHF converter (UHF 608 or UHF620) and a battery to a preinstalled

UHF sensor at a GIS.

Page 53

UHF measurement equipmentPage 53

© OMICRON

© OMICRON Academy


Since charge (q) is calculated by integral current over time 𝑖 𝑡 , the partial discharge (PD) data acquisition unit

detects the voltage drop across the known effective resistor of the coupling devices (CD) in the test circuit. This

resistor will be R, 𝑡1 and 𝑡2 , which are defined by the user of the measurement system.

Seen from a geometrical point of view, both mathematical expressions on the top of the slide are describing an

area below a current curve.

Since current cannot be measured easily, the voltage drop over a known resistor (shunt) will be used.

Charge measurement by frequency domain integration will be covered later in detail.

Page 54

How to measure Partial Discharge?Page 54

q = 𝑡1𝑡2𝑖 𝑡 𝑑𝑡 =

1

𝑅𝑡1𝑡2𝑢 𝑡 𝑑𝑡

q

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© OMICRON Academy


The PD Measurement according IEC 60270 is the basis for many applications, different assets and different

voltage levels. This is reflected in a variety of IEC, CIGRE and IEEE guides and standards which refer to the

IEC 60270 standard. Therefore, the IEC 60270 standard is very important for acceptance measurements in the

test fields of manufacturers as part of their type and routine tests on high-voltage equipment.

The IEC 60270 recommends two different filter settings. These include wide band measurement and narrow

band measurement. For a wide band measurement, the recommendation is:

o Lower frequency limit and above or equal to 30 kHz and below or equal to 100 kHz

o Higher frequency limit below or equal to 1 MHz

o Bandwidth of 100 kHz to 900 kHz

o Polarity detection can be possible

The on-site partial discharge measurement is often conducted with a filter setting out of the recommended

range by IEC 60270 to avoid a high noise condition. The MPD data acquisition unit allows users to adjust the

filter setting to find out the optimized SNR (Signal-to-Noise-Ratio) to ensure high sensitivity for the PD

measurement and a high robustness against noise for further analysis.

Page 55

Frequency limits per IEC 60270:2015

For wide band measurement the recommendation is:

Left frequency limit below 100kHz and above 30kHz

Right frequency limit below 1MHz

Bandwidth of 100kHz to 900kHz

Polarity detection can be possible

Page 55

Fre

qu

ency r

espon

se

Frequency

measurement filter

Frequency10kHz 100kHz 1MHz 10MHz

30kHz 500kHz

© OMICRON

© OMICRON Academy


PD signals in general are very broad-band signals. The unavoidable low-pass characteristic of the test object

(here cable) influences the signal. Higher frequencies will get damped while the signal propagates from the

fault to the sensor. The amount of damping as well as the frequencies that are damped depend mainly on the

distance between fault and sensor.

"Visibility" of PD

The low-pass characteristic of the cable damps higher signal frequencies. Higher measurement frequency

reduces the "visibility". It is not possible to measure PD far away from the measuring point at high frequencies.

Therefore, the center frequency should be selected as low as possible to see far inside the cable. On the other

hand, using a high center frequency allows for a very sensitive measurement that focuses on the point of

measurement, which is often the termination.

Disturbances Noise

Disturbances and noise are not equally distributed over the frequency range. There might be certain

frequencies which are noisier than others. Very often, noise is dominant in the low frequency range. It might be

possible to avoid disturbances and to get a better signal-to-noise ratio by carefully choosing the measuring

Page 56

High frequency signal propagation

PD is a HF signal – insulation is a low pass, filtering HF signals

Page 56

fA good filter settings

t

i

Area = q

= 10pC

Area = q

= 10pC

Area = q

= 10pC

Area = q

= 10pC

wrong filter

settings

(too high)

Measurement

should be done

with low

frequencies! This

isn’t possible with

Cc<<

© OMICRON

© OMICRON Academy


frequency and bandwidth.


Using a higher frequency range often allows a less disturbed measurement, while it limits the range from a

sensor in which faults are detectable. Vice versa, lower frequencies are often more disturbed but allow for a

'deeper' look inside the cable. In this example the damping effect of a calibrator impulse along an HV cable is

shown in dependence of the measuring frequency.

Page 57

High frequency signal propagation

Damping effect vs. distance vs. measurement frequency

Page 57

Damping of a calibration impulse along a HV cable in

dependence of the measuring frequency - each joint

represents 500m

Synchronous multichannel PD measurement –

damping along a HV XLPE cable

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© OMICRON Academy


The partial discharge (PD) activity is quantified by the secondary measuring unit called electric charge. The

real electric charge involved by the PD activity inside the insulation’s voids is higher (even much higher, in

some cases) than the apparent electric charge, which can be measured at the accessible points of the test

objects. The ratio between the real and apparent charge depends on the void dimensions, shape and the

insulation thickness, as well.

Page 58

Why charge calibration?Page 58

Winding

Core

Winding Core

qm

q’ < qmq’ < qm

q << qm q << qm

real charge qm >> apparent charge q

Insulation

© OMICRON

© OMICRON Academy

03b-Methods-Charge-Calibration

The calibration process is carried out before the actual PD measurement and is done by injecting short duration

current pulses of known charge magnitude, across the terminals of the test object. A calibration factor (also

called k factor) is therefore calculated by the software of the PD instrument. This calibration factor is simply the

ratio of the known injected charge magnitude over the measured charge magnitude. It is only valid for a specific

test object for a specific frequency range, a specific test setup and PD instrument. If any of those parameters

are modified, the calibration process must be repeated.

Note: don’t connect the calibrator directly at the coupling capacitor which will results in wrong readings (lower

PD levels).

Page 59

How to perform charge calibration

The set up has to be complete, ready for measurement

No voltage shall be applied

Connect the calibrator as close as possible to the test object

Perform the charge calibration

Page 59

Fiber-optic

cable

USB

MPD 800

MCU2

ut(

t)

CD -> PD & V

Ck

CaCstray

not

measured

Z

CPL1/2

(optional)

© OMICRON

© OMICRON Academy


Page 60

Important to know

Changes in measurement environment

Change in measurement setup Change of HV connection cable

Change of grounding cable

Change of any PD measurement

components

Change from one phase to next phase*

Change of PD measurement

settings

Change of center frequency 𝑓𝑚Change of bandwidth Δ𝑓

Page 60

Calibration has to be repeated if:

Calibration at generator terminal side

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Noise in PD measurements

© OMICRON

Page 61


1. Sources of noise coupled into PD measurement circuit

2. Background noise coupled through mains supply voltage of HV source:

3. Background noise coupled through grounding system of mains supply of HV source

4. Background noise and internal PD coupled through HV source → use appropriate blocking impedance

(manufacturer specific)

5. External PD due to inappropriate grounding system (other “noisy” devices on the same ground potential

e.g. machines with excitation, construction site, factory environment,...)

6. Outer noise (interference) due to neighbored and live HV systems (assets, over headlines,) or external

sources of interference (radio frequency communication,...)

7. Background noise coupled through inappropriate grounding of coupling capacitor

Page 62

Origin of background noise

Sources of noise coupled into PD measurement circuit

Page 62

Source: A.Küchler – Hochspannungstechnik,

Grundlagen-Technologie-Anwendungen

Blocking

impedance Z

HV source

Mains

supply

𝑪𝒂

CD

DUT

PD

M

Outer noise

1

2

3

4

5

6

7

Coupling

capacitor 𝑪𝒌

© OMICRON

© OMICRON Academy

10-Noise-suppression-Basic

Quantitative measurements of partial discharge magnitudes are often obscured by interference caused by

disturbances which fall into two categories:

o Disturbances which occur even if the test circuit is not energized

o Switching operations in other circuits

o Commutating machines

o HV tests in the vicinity

o Radio transmissions

Page 63

Types of disturbances acc. IEC 60270

Disturbance when test circuit is NOT energized

Switching operations in neighbored circuits

Commutating machines

HV tests in the neighbored environment

Radio transmissions

Page 63

Non-phase stable interference (example PD simulator) Phase stable interference from (neighbored) excitation circuit

© OMICRON

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11-Noise-suppression-Gating

Disturbance when test circuit is energized (not related to test object)

o Usually increase with increasing voltage.

o Internal PD in HV source

o On the HV connectors (surface discharges, corona discharges)

o Sparking of imperfectly earthed objects or imperfect connections in the area of the HV (contact noise, metal

parts at floating potential)

o In the mains supply due to solid-state switching devices (thyristors)

Page 64

Types of disturbances acc. IEC 60270

Disturbance when test circuit is energized (not related to test object)

Inappropriate HV source (internal PD)

Solid-state switching devices (thyristors) in mains-supply of HV source

Unsuitable HV connectors (causing external PD)

Non-PD free connections in HV area

External PD due to ungrounded HV source, DUT or other components

Page 64

Phase-stable interference (excitation 6-pulse bridge) Internal PD of HV source without blocking impedance

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The measurements are affected by disturbances which should be low enough to permit a sufficiently sensitive and

accurate measurement of the PD quantity to be monitored. As

disturbances may coincide with PD pulses and as they are often superimposed on the measured quantities, the

background noise level should preferably be less than 50 % of a

specified permissible partial discharge magnitude, if not otherwise specified by a relevant technical committee.

For acceptance tests and type tests on high-voltage equipment, the background noise level should be recorded.

Signal gating by time window, polarity discrimination, or similar methods can result in the loss of true partial discharge

signals if those signals occur concurrently with the disturbance or the gated-out (inhibited) part of the cycle. For this

reason, the signal should not be blocked by the gate for more than 2 % of each test voltage period in alternating voltage

systems, nor by more than 2 % of the cumulative test time in direct voltage systems.

If, however, several mains-synchronized interference sources per period are present, the blocking interval limit may be

increased to 10 % of the test voltage period. Hence, this gating

shall be set before the full test voltage is applied and these settings shall not be altered during the test.

Page 65

Phase and amplitude (window) gating

IEC 60270: Chapter 10: DISTURBANCES (G3.3.1)

Page 65

According to IEC 60270:2015o Neglect high readings!!!

o DC: not more than 2% of measuring time

o AC: NOT MORE THAN 2% of period

o Heavy disturbances: ≤ 10% of period

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The above example was recorded before charge calibration before for an offline PD measurement on hydro

generator.

Page 66

Examples: disturbance / noise

Example 1: non-stable noise pattern – Part 1

Page 66

Ground noise level

Non-phase stable noise

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The above example was recorded before charge calibration before for an offline PD measurement on hydro

generator.

Page 67


Example 1: non-stable noise pattern – Part 2

Page 67

Ground noise level


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Page 68


Example 2: non-stable noise pattern

Page 68

Ground noise level

Switching noise

HV source


from additional safety

equipment

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© OMICRON Academy


The above example was recorded during an online PD measurement on hydro generator.

This six pulses are obviously caused by the generator excitation system due to its stable phase position and

high repetition rate. In this case also multiple reflections, with damped amplitude are visible.

In such cases selection of different measurement frequency / bandwidth, use of unit-gating or 3PARD can be

sufficient tools for noise suppression.

Page 69


Example 3: Online PD measurement – generator excitation noise

Page 69

Micro void PD +

Ground noise level


from additional safety

equipment

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© OMICRON Academy


The above example was recorded during an online PD measurement on a 4.7 MW, 6.6 kV hydro generator.

Right picture shows the measurement setup.

Pattern Nr. 1 shows typical slot discharge PD due to mechanical abrasion of the Outer Corona Protection

(OCP) of the stator bars.

Pattern Nr.2 shows external interference from the connected power grid, most probably from sure arrestors

nearby over headlines.

Page 70


Example 4: Online PD measurement at generator

Page 70

Slot

discharge

PD

phase

stable

noise

coupled

from

power grid

1

2

1

2

1

2

© OMICRON

© OMICRON Academy


In these pictures three different PD measurements setups in HV labs are shown. In such cases the blocking

impedance is customized to the HV source or ordered as a standard component of the HV source

manufacturer.

In addition, the use of corona rings is essential to ensure a homogeneous electrical field distribution and to

avoid external partial discharges (corona discharge), which can otherwise occur due to sharp edges at the

connection points.

Page 71

Suppression of internal PD (HV source)Page 71

Blocking impedance solutions in different HV labs

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Here is an example of PD measurement on a medium-voltage switchgear using the 70kV RTS resonance

system. For a PD-free measurement setup, HV connection cables with screwable shield electrodes. are used

here. These standard accessories is very flexible and suitable for various PD applications up to 70kV.

Page 72

HV connection techniques

Connection techniques (field examples)

Avoid external PD at sharp connection points

Connection cables with shield electrodes > approx. 20kVrms

Corona rings to harmonize electric field distribution > approx. 30kVrms

Page 72

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Same procedure shall be applied for onsite measurements. The use of tubes and corona rings prevents the

occurrence of unnecessary partial discharges caused by your own measurement setup. When using tubes,

make sure that the connection to the contact points is improved using conductive adhesive tape (copper

adhesive tape). Otherwise, additional contact PD can occur, which can have a negative effect on the

implementation of the actual PD measurement.

Attention: still use a standard laboratory cable for the HV connection from coupling capacitor to the device

under test. The additional tubes are just put over to increase the diameter of the HV potential!

Page 73

HV connection techniques

Connection techniques (field examples)

Avoid external PD at sharp connection points

Corona rings to harmonize electric field distribution > approx. 30kVrms

Tubes to increase diameter of conductor > approx. 70kV

Page 73

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Another major influence on reducing the basic noise level is to use short and direct ground connections from

the coupling capacitor to the device under test. In simple words: reducing the antenna effect of our HV and

ground connections. Shorter HV and ground loops are significantly reducing the coupling of external

interference into our PD measurement setup.

If all measures for interference suppression are applied, the basic noise level can be reduced significantly

through hardware measures.

Additional noise suppression methods within MPD Suite software and optional Gating Units are explained in

the following slides.

Page 74

Grounding techniques

Ground loop (antenna effect)

Page 74

Short HV and ground connections with the MPD 800 setup

Conventional PD measurement

PD signal obscured

in the noise band

PD signal with reduced

noise level

MPD 800 and RBP1

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© OMICRON Academy


The following example describes the problem faced during an online PD measurement on a hydropower

generator. The measurement was carried out by means of coupling capacitors installed on all three phase

terminals.

After the charge calibration had been performed at standstill condition, the generator was synchronized with the

electrical grid afterwards. Measurement with the standard setting: 𝑓𝑚 = 250kHz and Δf = 300KHz, phase-

unstable interference caused by the excitation device of the machine were more dominant than the measured

PD discharge of the winding insulation. Luckily charge calibration was performed with different center

frequencies in advance and the settings were saved as configuration files.

Page 75

Noise suppression (software)

Selection of measurement frequency

Variation measurement filter (𝑓𝑚 and Δf)

Selection of frequency range less exposed to noise interference

Page 75

Fre

qu

ency r

espon

se

Frequency

measurement filter

𝑓𝑚 = 250kHz and Δf = 300KHz 𝑓𝑚 = 1 MHz and Δf = 300KHz

© OMICRON

© OMICRON Academy


Window gating of phase and amplitude (PRPD gating)

Phase/amplitude gates allow the MPD 800 to eliminate frequency-stable signals with a certain amplitude and

fixed phase position, for example converter pulses, drives, irrelevant PD. You can easily define the gating

areas by marking them with the mouse. These areas will be excluded during the subsequent PD measurement.

This is very useful to, for example, suppress inverter noise, which will usually happen at fixed phase positions,

but can have very large charge levels.

Page 76

Phase and amplitude (window) gating

When to use?

Eliminates frequency-stable signals with a certain amplitude and fixed phase position

How to use?

Users can easily define gating areas by marking them with a mouse

These areas will be excluded during subsequent PD measurement

Page 76

Measurement example using phase/amplitude

window gating in the PRPD diagram.

© OMICRON

© OMICRON Academy


To reduce the effect of disturbances, such as inverter noise on the measurement results, you can use the

second MPD 800 input channel as a gating channel.

PD activity detected on a gating channel suppresses PD events that are simultaneously measured on

measurement (non-gating) channels.

The underlying method uses the (gating) signal of a sensor or other coupling close to the source of the

disturbance, which is dominated by the interfering signals. The signal of the measurement channel is not used

for the measurement result if an impulse of a certain size is measured on the gating channel.

Page 77

Channel (unit) gating

When to use?

Reduce disturbance (inverter noise) which affects the PRPD pattern

Phase stable or non-phase stable noise

Window gating is not sufficient

How to use?

The second MPD measurement channel can be used as a gating channel

Additional sensor (HFCT) as close as possible to source of noise

Page 77

Filtered measurement channel (upper PRPD) and the

gating channel (marked as GC ON) in real-time.

Unfiltered PRPD with 6-pulse disturbances.

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What is the principle of unit gating?

The principle of unit gating is very simple but requires synchronous measurement of a measurement channel

and gating channel.

While in the measurement channel PD pulses, asynchronous background noise and unwanted disturbance

signals are detected the a second gating channel is only used to detect the unwanted disturbance signal which

later shall be gated. For this reason a so called gating window is manually defined which only covers all

unwanted disturbance signals in the PRPD.

Page 78

Time axis

measurement

channel

PD pulses


Principle

Page 78

Time axis

Disturbance

Gating channel

active

Gate

rectangle

© OMICRON

© OMICRON Academy


Once the gating feature is active all pulses appearing inside the manually drawn gating window and triggered

by the gating unit will lead to suppression on all other measurement channels.

The disturbance signals disappear from the PRPD of the measurement channel.

Page 79

PD pulses


Principle

Page 79

Time axis

Gating channel

active

Disturbance

Time axis

Gate

rectangle

measurement

channel

© OMICRON

© OMICRON Academy


A practical demo...

© OMICRON

Page 80


Partial discharge (PD) events on one phase can also be detected on the other phases. Making a distinction

between different PD sources and superimposed noise pulses is a challenge due to this coupling. The

OMICRON MPD measurement and analysis system provides users with the following powerful tools for the

separation of different sources of interference and easy data visualization.

3-Phase Amplitude Relation Diagram (3PARD)

The 3-phase amplitude relation diagram (3PARD) simplifies the differentiation of various PD sources and PD

interferences. The three phases are measured synchronously. The combined results of three measurement

channels are displayed in a single 3PARD star diagram, which facilitates result comparison and separation of

impulse sources. To further increase the testing reliability, clusters are selected in the 3PARD and the resulting

PRPD diagrams show the filtered-out pulses in real time while graying out the residual pulses in the

background

Page 81

Separation of event sources

3-Phase-Correlation-Diagram (3PARD)

Eases the separation of multiple PD sources and disturbances

3 phases are measured fully synchronous with three MPD measurement channels

The results are displayed in the 3PARD-diagram and can be used for source separation

Page 81

The 3PARD diagram with selected PD cluster

Resulting PRPDs of the three measurement channels (due to the selected

cluster in 3PARD) and the gray unfiltered PRPD histogram displayed in the

background

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12-Noise-suppression-3PARD

A schematic diagram of how 3PARD works is shown here. Three synchronous channels - in this case the

phases L1, L2 and L3 - detect the same partial discharge pulse with different amplitudes within a user defined

time window.

If we assume that the original PD source occurs in Phase L1, with the highest amplitude measured of 900pC.

The other two channels also measure the PD pulse by cross-coupling from the neighbored phases.

In phase L2 which is closer to the source the PD is measured with an amplitude of 700pC.

In phase L3 which is further away to the source the PD is measured with an amplitude of 500pC.

Page 82

Multi-channel synchronous PD measurement (3PARD)

Coupling of PD source

Page 82

900 pC

700 pC

500 pC

PD source

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The amplitude of each phase is now transformed into a vector which, when graphically added, gives a point in

the 3PARD star diagram. If the PD source appears regularly, the various points form a cloud, also called a

"cluster".

Different PD sources form different clusters in the diagram.

Adding the vector obtained form the single phases results in a cluster formed between Phase L1 and Phase L2,

but closer to Phase L1.

Page 83


Source separation

Page 83

© OMICRON

PD

900 pC

700 pC

500 pCPhase 1

Phase 2

Phase 3

© OMICRON Academy


Typically, outside noise (or background noise) is equally coupled into all three phases through the

measurement setup. For this reason the amplitude of the interference is measured with the same magnitude in

the acquisition units.

Page 84


Coupling of outside noise (disturbance)

Page 84

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In case of symmetrical coupling of the disturbances, the addition of the vectors results in a cluster located

around the zero point of the vector diagram.

If the noise isn’t coupled symmetrically into measurement setup, the noise cluster will be shifted around zero

point into the direction of the phase with the highest noise amplitude.

Page 85


Source separation

Page 85

Phase 1

Phase 2

Phase 3

disturbance

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© OMICRON Academy


An example PD measurement on a cast resin transformer with induced voltage using 200Hz. (YD5

transformator)

In the three PRPDs several faults are visible, the QIEC value gives no indication where these faults are

situated.

The HV winding is connected in triangular.

Page 86

3PARD example: Cast resin transformer

Cast resin transformer Yd5 Induced voltage test up to approx. 43.5kV @ 200Hz Excitation from LV winding 3x Coupling capacitor + MPD units

Page 86

© OMICRON

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With back transformation of cluster “A” into the single PRPDs it becomes obvious that the PD source must be

located between phase L1 (measured charge: ~37pC) and phase L2 (measured charge: ~43pC).

Due to the delta connection of the primary winding, this leads to a PD fault approximately in the middle of the

winding between phase 1 and phase 2.

There are several pattern visible, most of them indicate an internal fault.

Page 87


Transformation of Cluster “A” to PRPD

PD source is most likely between phase L1 and L2 (approx. middle of winding)

Page 87

A

© OMICRON

© OMICRON Academy


With back transformation of the cluster “B” back into PRPDs it becomes obvious that the PD source must be

closer to the phase L1 (charge level: ~31pC).

Page 88


Transformation of Cluster “B” to PRPD


Page 88

B

© OMICRON

© OMICRON Academy


When transforming the cluster “C” back into PRPDs it becomes obvious that the PD are situated between

phase L1 (charge level: ~14pC) and phase L3 (charge level: ~21pC).

Due to the delta connection of the winding, the location of the PD source must be approximately in the middle

of the winding between phase L1 and phase L3.

There are several pattern visible, most of them indicate an internal discharge (void).

Page 89


Transformation of Cluster “C” to PRPD


Page 89

C

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The 3FREQ (3-center frequencies relation diagram) is a one-channel filtering tool using three digital filter

frequencies. It characterizes PD sources by their frequency signature.

Using a 3FREQ diagram, you can separate PD events such as surface discharge, corona and internal void

from disturbances. As with 3PARD, the PRPD diagram shows filtered out pulses while greying out the residual

pulses in the background to improve the testing reliability.

The 3FREQ diagram, also known as 3CFRD (3-center frequencies relation diagram), is a one-channel filtering

tool that uses three digital filter frequencies to characterize PD sources by their frequency signature.

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Separation of event sources

3-Center Frequency Relation Diagram (3CFRD/3FREQ)

Sources are separated based on their frequency behavior

One measurement channel with three simultaneous measurement frequencies

The results are displayed in the 3FREQ-diagram and can be used for source separation

Page 90

3FREQ diagram with

selected PD cluster

FFT diagram with selected

three measurement

frequencies

Resulting PRPDs of the three measurement frequencies (due to

the selected cluster in 3FREQ) and the gray unfiltered PRPD

histogram displayed in the background

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13-Noise-suppression-3FREQ

The 3FREQ filter uses three different center frequencies for PD analysis. Due to the one-channel measurement

approach you only need one MPD 800 device.

3FREQ (3-center frequencies relation diagram) is a powerful tool used in order to separate multiple PD sources

which are overlapping in the PRPD (phase resolved partial discharge) diagram. It involves PD measurements

using only one acquisition unit (one-channel measurement) and three digital filters (with three different center

frequencies and the same bandwidth). In other words, this is a multi-spectral technique for multiple PD sources

analysis.

The test set up for one channel measurement consisting of a test object, coupling capacitor and coupling

device (measuring impedance) and measuring instrument (MPD unit) is shown above. To keep the circuit as

simple as possible, the voltage source and the blocking impedance (line filter) are not shown here. Also, the

coupling device (measuring impedance) is shown in simplified format.

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3-Center Frequencies Relation Diagram (3FREQ)Page 91

Source separation

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Source 2

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Source 1

Source 3

MPD 800

MPD 800

CD

Ck

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Different PD sources are characterized by different frequency responses (e.g. above, FFT pulse 3 includes

more high frequency components than FFT pulse 1, which includes more high frequency components than FFT

pulse 2). Thus, the difference in the time domain is also existing in the frequency domain.

The 3 center frequencies have to be placed on the position where the biggest differences between the spectra

of the pulses are visible.

Using the phasors (red, blue, green) corresponding to each filter and spectrum and plotting them in a 2D star

diagram (120° phase shift) different clusters corresponding to different PD sources are developed.

Nevertheless, if the PD sources are very close to each other inside the DUT, a clear separation using

3CFRD/3FREQ might not be possible.

Do not forget to place a filter according to IEC 60270 recommendation if standard compliant measurements are

required.

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3-Center Frequencies Relation Diagram (3FREQ)

Source separation

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fc3

fc2

fc1

FFT-Impuls 1

FFT-Impuls 2

FFT-Impuls 3

TE-Quelle 1

TE-Quelle 2

TE-Quelle 3

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All three back transformations in one view. For the different PRPDs now a separate analysis can be done, and

the results are shown above the individual diagrams.

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3-Center Frequencies Relation Diagram (3FREQ)Page 93

Surface discharge Internal voidCorona discharge

Phase-resolved pattern 3FREQ

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The integrity of the insulation in MV and HV equipment should be confirmed with PD measurement and

analysis during the development, manufacturing, and commissioning of electrical equipment.

Once the asset is in service, strategic decisions about maintenance must be made to ensure maximum

availability. Periodic PD measurements and continuous PD monitoring provide asset managers with the

required data to focus on at-risk assets and minimize unnecessary maintenance outages and costs.

Regular PD measurements performed during scheduled maintenance outages enable a trending of the asset’s

insulation condition, which is a powerful way to recognize a developing insulation fault in its early stage and to

plan maintenance accordingly to extend its service life.

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Extension of expected lifetime...

... through periodic testing and/or monitoring

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Development

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Periodic measurement

Periodic measurements can be offline or online measurements. For offline measurements the asset can be tested with an external

voltage source and the variation of the voltage level is possible. The measurement system can be set up for the best signal to noise ratio

to provide the lowest noise level. For online measurements only pre-installed sensors can be used. Otherwise, limited sensors are

available because the system is online (e.g. no installation of MCC possible). Both measurements provide data over a small period.

Temporary monitoring of PD activity

Temporary PD monitoring enables operators to observe changes in PD activity over short, continuous periods of time. Temporary PD

monitoring systems are designed for use with a variety of PD measurement sensors, including coupling capacitors for rotating machines,

bushing tap sensors and UHF sensors for power transformers, as well as high-frequency current transformers (HFCTs) for power cables.

These PD measurement sensors can be permanently installed and connected via a terminal box, which is also permanently installed at

the asset. This enables safe connections while the asset is on-line to avoid unnecessary downtime during setup. With the monitoring

software, asset managers can reliably assess the current insulation condition and identify which asset is most at risk of failure.

Permanent PD monitoring for high risk assets

A permanent on-line PD monitoring system is typically used on critical assets and assets showing signs of aging to assess insulation

condition for an indefinite period of time under normal operating conditions. This type of PD monitoring system consists of permanently-

installed PD sensors, a data acquisition device as well as monitoring and PD analysis software running on a central computer. Multiple

assets can be synchronously monitored at the same time and the data can be compared using the same software at the central

computer. A warning or alarm is triggered to alert the operator when PD activity exceeds defined limits.

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Periodic vs. Temporary vs. Continuous monitoring

Periodic measurementsSnapshot of the current parameters

Installation or pre-installation of setup components (e.g. coupling capacitor, HFCT, ...)

Non-permanent installations of measurement setup

Lowest amount of measurement data

Lowest noise level

Variation of voltage level possible

Temporary monitoring

Short time monitoring of parameters for minutes, hours, days or load cycles

Pre-installation of setup components (e.g. coupling capacitor, HFCT, ...)

Non-permanent installations of measurement setup

Reduced amount of measurement data

Continuous monitoring

Permanent monitoring with pre-installation of measurement setup

Permanent analyzation of parameters (dissipation factor, partial discharge,...)

Definition system configuration, setting thresholds levels for alarming

Huge amount of measurement data

Page 95

Online Monitoring: PD trend data

3PARD and PRPD diagrams

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One device for all assets and testing applications. You can use the MPD 800 for a wide range of testing

applications, beginning with the traditional power supply sector, at manufacturers or repair shops, in

laboratories or, for example, during diagnostic testing of motors in the industry sector. It supports you during

standards-compliant PD testing for routine and type testing, factory and site acceptance testing, as well as for

troubleshooting to localize or investigate PD sources in:

Power transformers

Power cables

Rotating machines

Gas-insulated switchgear (GIS) and medium-voltage switchgear

Industrial drives

Railway transportation

High-voltage components such as bushings, insulators, capacitors, cable terminations, busbars

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PD diagnostics: MPD 800

Standard-compliant partial discharge (PD) measurement and analysis for:

Routine and type testing

Factory and site acceptance testing

PD maintenance testing

Troubleshooting and localization of PD sources in the field

Applicable assets:

Power transformers

Power cables

Rotating machines

Gas-insulated switchgear (GIS) and Medium-voltage switchgear

Industrial drives

Railway transportation

High-voltage components, i.e. Bushings, Capacitors, Cable terminations, Busbars

Page 96

Beside its usage in the

traditional power supply sector,

you can also use MPD 800 for

diagnostic PD testing on

industrial drives.

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05a-Hardware-MPD-800

PD monitoring on power cables: MONCABLO

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2

Fiber optic communication3

4

5

Master control unit

Central computer & monitoring software

High frequency current

transformer (HFCT)

1

2 Data acquisition unit & enclosure

Monitoring cable terminations (example)

1 1 1

2

3

4

5


PD monitoring of power transformers: MONTRANO

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Bushing tap adapter

3

1

2

4

5

1

2

3

UHF Sensor

data acquisition unit

tan reference4

5

6

fiber optic communication

data processing and storage

1

1

6


PD monitoring on generators and motors: MONGEMO

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Capacitive

sensor

Data acquisition

unit in a

protected

enclosure

Central computer

and software

rotating

machine

11 1

2

3

1

2

3


PD monitoring on generators and motors: MONGEMO

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Cluster separation and pattern classification

According to the automatically generated report, the pattern of cluster 6 can be classified

as delamination of insulation tape layers (rmS2).


Temporary PD monitoring: MONTESTO 200

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Dry-type transformers Power transformers

OHL cable terminations MV cable terminations

GIS cable terminations Rotating machinesPower cables & accessories

MONTESTO 200


Temporary PD monitoring: MONTESTO 200

Periodic on-line PD measurements at assets equipped with pre-installed sensors:

Settings can be saved for each asset and retrieved whenever required

Results of multiple sessions on the same asset can be visualized and compared

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Asset

Sensors

Terminal box

MONTESTO 200


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The end

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