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Advanced PD Monitoring of Power Transformers Supporting Asset Management

www.ldic.de

Gregor Pudlo

Doble Lemke GermanySwitzerland

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Abstract

• Reasons for (PD) monitoring of transformers

• Partial Discharge Detection principles

• Electrical PD diagnostic techniques• PD measuring circuits according to IEC 60270

• Advanced tools for PD monitoring• Acoustic and electromagnetic PD measuring principle

• Cone-shaped drain valve UHF-sensors / Performance Check

• Case studies – UHF PD measurements

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Reasons: PD Monitoring of Power Transformers

• Power transformers are the most important and expensive

pieces of equipment in substations.• A new power transformer costs millions of dollars.• Unplanned outage and repair costs, penalties, etc. sum up• Transformers design life is 20-35 years.• With proper maintenance they can remain in service for 60

years.• As transformers age, their internal conditions degrade which

increases the risk of failures.• Time-based maintenance changes to condition-based

maintenance approaches

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Partial Discharge Detection Principles

Optical

Mechanical

Chemical

Acoustic

Opto-acoustic

HV

Electrical IEC 60270

HF/VHF/UHF

Optical Effects(Light)

Pressure Wave(Sound)

Discharge EffectsDielectric LossesHigh Frequency

Waves

Chemical Effects

Heat

Macroscopic-PhysicalEffects

Detection Methods

Draft IEC 62 478

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Electrical PD diagnostic techniquesPD measuring circuits according to IEC 60270

a) coupling device in series with the coupling capacitor b) measurement at a bushing tap

Components:AC voltage source U, optional blocking impedance Z, coupling capacitor CK, measuring impedance Zm, a measuring instrument M and generalized test object Ca.

[König, 1993]

frequently used circuit in test laboratories

often applied in on-site/on-line PD investigations

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Electrical PD diagnostic techniquesPD Measuring Impedance for Bushing Tap Installation

• Decoupling of PD signals from the bushing tap

• in a wide frequency range up to 30 MHz

• Lower cut-off frequency < 60 kHz

• Additional voltage signal for phase-resolved

measurements

• Superimposed PD and test voltage signal

• Very fast and safe over-voltage protection

• TNC-socket for signal output

• Dimensions: diameter 125 mm, length 120 mm;

Weight: 2 kg

• Special feature for continuous PD monitoring (IP 68)

500 kV Transformer Bushing

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IEC 62478 - Draft level

"Non-conventional PD-measurements“:

• Electromagnetic PD Detection in the (HF/VHF/UHF) - range

• Acoustic PD measurements

• Mixed acoustic and electromagnetic method

-> PD location in Power Transformers and Reactors

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Advanced tools for PD monitoring – Electromagn. PD TestsCone-shaped active drain valve UHF-sensors

• for the decoupling of electromagnetic PD signals from the inner of an oil-paper-insulated

transformer

• PD-signals in the UHF frequency range

(mainly 300 MHz – 1 GHz)

• sensors support „Performance/Sensitivity Check“ (high-frequency test impulses can be injected with

additionally integrated electrode)

• sensor head is grounded (for lower frequencies)

• sensor application at oil valves, which are available e.g. for oil filling or draining (liquid tightness is

tested for 120°C warm oil with 5 bar compression)

View from inside the transformer

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Performance Check

• Functional check of the whole measuring path including sensors and partial discharge (PD)

acquisition system through injection of high-frequency test impulses

• „single-port“ solutions are possible

• the Performance Check „controls“ e.g. the effect of a contact to grounded elements within

the oil valve.

Cone-shaped active UHF-sensor (DN 50/80 oil gate-valve)

UHF PD spectra for grounded and ungrounded sensor head

input path

output path

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Transformer

Drain valve

UHF PD probe DN80

Output

Input

Pre-amplifier

High-pass filter

Performance Check

Performance check at 750 MHz

Phase-resolved view in UHF PD device LDS-6/UHF

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Advanced tools for PD monitoring – Electromagn. PD TestsCase study B - on-line UHF PD measur. of a 200 MVA single-phase transformer

380/220kV, 200 MVA single-phase transformer and position of the gate-valve UHF-sensor

UHF pulse recorded on-line

corresponding UHF PD spectrum up to 1.5 GHz

• customer wanted to verify the hydrogen measurements and locate potential PD

• full in-service sensor application and measurement

• UHF PD signal could be used to enhance acoustic PD measurements

Application benefit:

0 50 100 150 200-30

-20

-10

0

10

20

30

40

ampl

itude

(V

)

time (ns)

0.00 0.25 0.50 0.75 1.00 1.25 1.500.0

0.1

0.2

0.3

0.4

0.5

0.6

frequency (GHz)

ampl

itude

(Vs)

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Advanced tools for PD monitoring – Electromagn. PD TestsCase study C - on-line UHF PD measur. of a 3-phase power transformer 600 MVA

gate-valve UHF-sensor used at the 3-phase power transformer 600 MVA, 21/380kV

UHF pulse recorded on-line

corresponding UHF PD spectrum up to 1.0 GHz

• gas-in-oil hydrogen indication „useless“ due to active hot spot

• customer wanted to test for PD and check a potential load dependency of PD

• full in-service sensor application and measurement

• Inception and extinction voltage could be identified, load dependency excluded

Application benefit:

-500 -250 0 250 500 750 1000-150

-100

-50

0

50

100

150

ampl

itude

(m

V)

time (ns)

0.00 0.25 0.50 0.75 1.000

2

4

6

8

10

12

ampl

itude

(V

s)

frequency (GHz)

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UHF PD Measurements and delivered UHF PD Probe DN50/80

TaiwanTaipeh

ChinaShangai

BelgiumLinkebeek

GermanyBerlin HeidenauRiesa

Ireland & UKArdnacrusha CashlaDublin Louth

Philippines Manila

Saudi Arabia JeddahJizan

UAEDubai

EgyptKairo

ColombiaBogota

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UHF PD measurements in Ireland

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UHF PD measurements in Ireland

Application benefit:

• Tested measurements have been performed using non-conventional decoupling methods taking advantage of electromagnetic field coupling

• PD measurements on the power transformers have been functioned on the drain valves of the 3- phase 110/225kV power transformers with the help of the UHF probe DN 50

• Three transformers found with minor PD activity, one transformer with clear PD

Locations:

• Louth

• Dublin

• Ardnacrusha

• Cashla

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Drain Valve

Applied UHF Drain Valve Sensor

Mounting of the UHF Probe DN50 (with Adapter DN50/DN80)

Measuring System LDS-6/UHF

Measuring Instrument:

• PD Measuring System LDS-6/UHF• UHF Probe DN50 (with Adapter DN50/DN80)• UHF Calibrator LDC-6/UHF• Pre Amplifier LDA-5/GIS

UHF PD measurements in Ireland

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V ≈ 550 mV at measuring frequency 560 MHz

V ≈ 238 mV at measuring frequency 560 MHz

Left drain valve Right drain valve

V ≈ 139 mV at measuring frequency 560 MHz

V ≈ 128 mV at measuring frequency 520 MHz

UHF PD measurement in Louth, Dublin, Ardnacrusha, Cashla (Ireland)

Louth

T2xxxLeft drain valve Right drain valve

The measurements on both valves showed PD activity from inside the transformer.

Not any critical PD levels above the detection sensitivity have been measured in Dublin, Ardnacrusha and Cashla.

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UHF PD measurements in Jizan, Saudi Arabia

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UHF PD measurements in Jizan, Saudi Arabia

Drain Valve Power Transformer

on 33/13,8 KV Power Transformers “A” and “B”

Measuring Instrument:

• UHF Probe DN50

• Calibrator LDC-6/UHF

• PD measurement instrument LDS-6/UHF

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UHF PD measurements in Jizan, Saudi Arabia

PRPD Pattern at the measuring frequency of 400 MHz V = 177 mV

“A”

PRPD Pattern at the measuring frequency of 400 MHz V = 288 mV

“B”

Not any critical PD levels above the detection sensitivity have been measured.

Application benefit:

• For examination and diagnostics of the insulation quality of high voltage apparatus the measurement of PD is accomplished by sensitive UHF method on-site

• UHF probe applied in full operation of transformer, no outage needed

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Doble Lemke AGRheinfelden, SwitzerlandTel.: +41 61 836 8000www.ldic.ch

Doble Lemke GmbHKesselsdorf/Dresden,GermanyTel.: +49 35204 3900 0www.ldic.de

Conclusion• sensitive UHF PD measurement with UHF probes applicable in

service provide advantageous robust and easy PD decoupling • PD location - especially in transformers of great importance - is

possible in mixed and all-acoustic way• significant diagnostic information are derived from advanced

tools for PD monitoring • valuable application benefits e.g. on-line quick start of PD

monitoring, enabling time-based maintenance, “diagnostic double

checks”, etc. are at hand