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NETA PowerTest 2018 Spotlight PresentationBy Bret Hammonds

NETA PowerTest 2018 Spotlight Presentation

By Bret Hammonds

As the electrical power industry has grown in its understanding of the potential benefits

associated with partial discharge testing, it has been recognized for its value as a

diagnostic tool. The identification of partial discharge has now become a significant

component of system reliability programs and the associated planning for critical

electrical systems.

During this 15-minute presentation we will discuss considerations and challenges relating

to partial discharge testing and its ability to impact electrical insulating systems and their

associated dielectric qualities.

• Insulation Resistance

• Power Factor (Various)

• Voltage Withstand (High Potential Testing)

• Tan Delta (Cables, Rotating Apparatus…)

• Partial Discharge

Online

Offline

Field Testing of Electrical Insulation

Insulation Resistance

• Spot Tests

• Dielectric Absorption Ratio

• Polarization Index

• Step Voltage

1 kV, 5 kV, 10 kV, 10 kV, 15 kV

Field Testing of Electrical Insulation

Insulation Power Factor

Field Testing of Electrical Insulation

Voltage Withstand (High Potential Testing)

• AC Power System Frequency

• VLF (0.1 Hz)

• Sinewave

• Cosine Rectangular Waveform

Tan Delta (Cables, Rotating Apparatus…)

• TDM – A mean (or avg.) Tan ẟ at Power System Frequency

• TDTS – Time Stability of Tan ẟ Values (Std. Dev.)• Tip-Up – Voltage Dependency

Partial Discharge• Online

• Offline

• Introduction of Damped AC (DAC)

Field Testing of Electrical Insulation

Voltage Withstand – High Potential

HistoricalMuch of this type of testing was performed using DC

Problem: Potential for leaving space charges

Solution: Use AC

Field Testing of Electrical Insulation

Voltage Withstand – High Potential

Problem: Size and Weight of 60 Hz power supply didn’t make it practical for

field testing – Right back where we started.

Solution: Test at Lower Frequency 0.1 Hz. (VLF)

Power System

Frequency

Very Low

Frequency

60 Hz /100pF @ 16 KVV2 x 2πƒC = 9,650 VA

0.1 Hz / 100pF @ 16 KVV2 x 2πƒC = 16 VA

The required 60 Hz power supply would have to be

~ 600 Times

Larger than the required VLF power supply

100 pF is used for the

purposes of this example, but

will vary in relation to the

capacitance of the item being

tested.

Field Testing of Electrical Insulation

Voltage Withstand – High Potential

Problem: VLF Sinewave Doesn’t Precisely Correlate to Insulation

Breakdown Values Using 60 Hz. AC

Solution: Cosine Rectangular Waveform. The transition between positive

and negative more closely resembles that of a 60 Hz sinewave.

0.1 Hz SinewaveẟV/ẟt = Approx. 5 seconds.(1/2 Cycle at 0.1 Hz)

60 Hz SinewaveẟV/ẟt = Approx. 8 msec(1/2 cycle at 60 Hz)

0.1 Hz Cosine RectangularẟV/ẟt = Approx. 2 to 6 msec(per IEEE 400.2 30 to 250 Hz.)

Field Testing of Electrical Insulation

Diagnostic Tests

Life Cycle

I always knew that I would get

old…I just never knew how

long it would take to get there.

Field Testing of Electrical Insulation

Reliability

Diagnostic Tests

Tangent Delta – an measure of electrical losses that can be used in trending the current

condition of electrical insulation and the speed at which it may be deteriorating.

Problem: What works best for Withstand Testing may not be appropriate for Tan ẟTests

Solution: Must use Sinusoidal Waveform to look at quadrature components of current

Evaluation Criteria

• TDTS (Std. Dev.) – Time Stability of Tan Delta (at least 6 measurements at equivalent intervals)

• DTD (TipUp) - higher voltage dependency as service age increases.(0.5 Uo to 1.5 Uo)

• TDM – Mean value of Tangent Delta at Rated Voltage

While Tan ẟ testing might be very helpful in making judgements regarding the age and condition of insulation, it may not be helpful in localizing the source of identified problems unless combined with other technologies.

Field Testing of Electrical Insulation

Offline PD

Partial Discharge - An electric discharge which only partially bridges the insulation between conductors.

Notes: 1. Partial discharges occur when the local electric-field intensity exceeds the dielectric strength of the dielectric involved, resulting in local ionization and breakdown. (IEEE 100 Complete Dictionary)

Field Testing of Electrical Insulation

PDMWT

WithstandPD

PD

VLF Sinewave VLF Cosine Rectangular

WithstandTan ẟ

Damped AC (DAC)

Waveform Applications

0.1 Hz

30 – 250 Hz

Online PD

PD Sensor Technologies

Acoustic

• Airborne (~40 kHz)

• Contact (20 kHz – 300 kHz)

Electromagnetic

• TEV – Transient Earth Voltage (3 MHz –

100 MHz)

• UHF – Ultra High Frequency (300 MHz

– 1.5 GHz)

• High Frequency Current Sensor (500

kHz – 50 MHz)

Field Testing of Electrical Insulation

Online PD

Field Testing of Electrical Insulation

Online

Advantages Disadvantages

• Does not require interruption of power• Non-Invasive…Testing is performed with test

items in services• Less Time Consuming • Costs Less to perform

• Limited to identifying PD at rated voltages,which could already be a problem

• Unable to control supply voltage and are therefore unable to determine PDIV/PDEV

Offline

Advantages Disadvantages

• Predictive and helpful Current Status and Planning of Maint./Replacment

• Able to determine PDIV/PDEV

• Requires De-Energizing Items to be tested• More costly to perform - Requires Additional

test Equipment• More time consuming

Advantages & Disadvantages of

PD Testing Types

Combination of test methods already discussed:

Monitored Withstand – A test in which a voltage of a predetermined

magnitude is applied for a predetermined time. During the test other

properties of the test object are monitored and these are used together

with the withstand results to determine it’s condition. IEEE Std. 400.2-2013

• VLF Withstand

• Tan ẟ (TD) – (TD, DTD, TDST) – (Sinusoidal Waveform)

• Partial Discharge (PD)

Monitored Withstand Test – (MWT)

Equipment Rental Options

EA Technology

UltraTEV Plus+ Partial

Discharge Detector

Partial Discharge VLF Hipot Tan Delta

PMDT PDetector

Handheld Partial

Discharge Detector

Omicron OMS 605

Portable Partial

Discharge (PD)

Monitoring System

High Voltage VLF-

65E 65kV VLF Tester

HV Diagnostics

HVA34 34kV VLF /

DC High Voltage

Test Instrument

HV Diagnostics

TD90-MC 90kV

Portable Tan Delta

Measurement System

HV Diagnostics

TD30 30kV Tan

Delta Bridge

For more information on these products see our extensive catalog

at protecequip.com/products

The Electrical Test and Measurement

Equipment Rental Experts