Cable Testing Anomalies for Windpark Testing Applications

SEPTEMBER 4 - 6, 2019

Cable Testing Anomalies for Windpark Testing Applications: Cable Fault Location, Test and Diagnostics

Steven H SpikesCable Application Engineer

Megger

Sergio RazoArea Sales Manager

Megger



General Topics of Discussion:– Cable Fault Locating (CFL)

• TDR Utilization• Breakdown Voltage Test• Thumping / Surging• Pinpointing

– Cable Test and Diagnostics within Windparks

• Very Low Frequency (VLF) Testing• Tan Delta Testing• Partial Discharge (PD) Testing

Field Examples:• CFL Windfarm

Example• PD testing post

splice repair activity, w/ accompanying CFL Discussion

• PD testing before splice repair and investigation


Cable Testing Anomalies for Windpark Testing Applications: Cable Fault Location Overview

• Cable Fault Locating (CFL) – TDR Utilization– Breakdown Voltage Test– Thumping / Surging– Pinpointing– Sheath Fault Testing

• Surge / Thumper Unit Selection– Voltage– Unit Joule Sizing vs System

Capacitance– TDR Functions– Break Down Voltage / HiPot

Testing– Portability / Accessibility– Additional Special

Considerations for CFL activity in windfarms Example Downtown Network – 3-Mile

length. Up to 8-9miles (not uncommon)Example Windfarm String: 3.5-Mile OR 18,480ft length


Cable Testing Anomalies for Windpark Testing Applications: Cable Test and Diagnostics (Overview)


Test Type Very Low Frequency (VLF)

Tan Delta Partial Discharge (PD)

Criteria Pass/Fail 3 criteria: MeanTD, Stability, Differential

IEEE Analysis; PDIV, PDEV, Level, pattern

What it tests Overall cable quality, finds major defects in the cable

Quality of the insulation,corrosion on metallic shield. Good for water tree detection.

Checks terminations. Gaps or voids between layers or in the insulation; Not suitable for water tree detection

Cable Info Not much abouthealth

Trend data over time

Trend data over time

Recommended Standard(s)

IEEE 400.2 IEEE 400.2 IEEE 400.3, IEEE 400.4 (DAC)

Locates weak areas

No No Yes





Application Workmanship Test Preventive Withstand TestMonitored* Diagnostics Test

Test ObjectiveSubstantial workmanship issues

in Accessories (Splices, Terminations)

Local defects in cable insulation itself; Reveal defects in splices

and terminations

Test Voltage typically 1x U0≈ Operating voltage

1.7x U0 … 3x U0very old cable to new cable

Test Time approx. 5 min p. phase* 30-60 min p. phase*

Meaning and/orExpectation

of Test Result

Cable can be re-energized w/orisk of immediate failure

If passing a test 60 min / 1.7…3xU0: there are no substantial

defects, statistically 3-5 year fault free

Cable Testing Anomalies for Windpark Testing Applications: VLF Testing

* Monitored withstand test utilizes a trend indicator to monitor the progress of the test1. true leakage current, 2. partial discharge activity, 3. dissipation factor / tan delta


• Recommended VLF Testing to reduce and plan cable-fault related outages.– 30-minutes = 75% of Faults– 60-minutes = 97% of Faults

• 60-minute testing has a statistical improvement of 3 to 5 years of failure free operation with proper testing program.

• VLF Test unit selection and sizing for field test applications is important.

• VLF testing does not cause the initial damage within the cable system.

• VLF testing for detection of suspect workmanship and installation practices and rare factory defects.

Cable Testing Anomalies for Windpark Testing Applications: VLF Testing

“Dortmunder Energie und Wasser” Experience Report 1987-2007

• 35kV system used in windfarms requires the following test voltages:– Installation: 55kV Peak – Acceptance: 62kV Peak– Maintenance: 47kV Peak

Refer to IEEE400.2 Table 3: VLF withstand test voltages for sinusoidal and cosine-rectangular waveforms


Cable Testing Anomalies for Windpark Testing Applications: Tan Delta Testing

Conductive Carbonized path

Electrical Tree

Localized Critical Water Tree

Uniformly Distributed Water Tree Growth

• Detects global defects caused my aging within insulation material. Water-tree growth, UV, temperature aging, and among other items, exposure to chemicals.

• Water tree growth driven by AC-voltage, moisture and time.• Water-trees convert into electrical trees. This is a Cable

Fault.• Measure of conductive losses. New cable is ideal. • Tan Delta measurements that are evaluated are as follows:• Test is performed with three test levels, 0.5Uo, 1.0Uo, and

1.5Uo. Uo is assumed to be the operating voltage of the cable system. Ex: 13.2kV system would have a Uo of 7.62kV.

– Tan Delta – Tip-up Value; TanΔ @ 1.5Uo – TanΔ @ 0.5Uo• Mean Tan Delta• Delta Tan Delta• Standard Deviation





• Environmental factors play a large role in tan delta results.

• High humidity and contamination create issues when attempting to capture reliable results. Conditioned air may be required.

• Mitigation required to compensate– Covering sample under test in some fashion

to isolate from environment– Cleaning surface of sample under test to

reduce surface tracking– Repeating test to ensure reliable results

• Tan-delta measurements exhibiting unusual trending can be an indication of a failing cable system.

• Test set can be used for very short duration as modifications are made to the system.

• Adherence to IEEE 400.2 recommendations is paramount to ensuring reliable results interpretation.


Cable Test and Diagnostics (CTD) : Partial Discharge (PD) Testing• Partial Discharge (PD) testing is exceptionally sensitive to

contamination, air-gaps, imperfect test connections.• Due to this sensitivity, it can detect minor defects in

extruded XLPE, missing strands, voids / regions where the insulation is breaking down, as well as cable accessories (terminations, splices, elbows, t-bodies) that are trending toward failure.

• Connections to test cable must be as close to the a properly sealed and voltage graded environment as is expected with any cable accessory connections.

• The selection of the correct cable components, proper workmanship practices during construction and installation, and the conditions under which testing is occurring will influence results.

• Reduction of background noise, and proper calibration is important.

• Determination of when the PD activity begins and ends is paramount to determining criticality of measured PD as well as the magnitude. (Ex: below 1000pC versus 30,000pC)


BGN measurement at 0 kV (de-energized test set)

Increase of test voltage in steps (N = 3 … 5)Service-aged cables: 1.7 U0

New cable: 2.0 U0

Defining of PDIV Defining of PDEV (if PDIV > U0)

Cable Test and Diagnostics (CTD) : Partial Discharge (PD) Testing• PD testing requires measurements to be taken at

multiple test levels.• Goal is to obtain the following information:

– PDIV – Partial Discharge Inception Voltage (PDIV)– PDEV – Partial Discharge Extinction Voltage (PDEV)– Magnitude of the PD events– PD event count


Partial Discharge Testing


Cable Fault Location (CFL) Test Example No.1: Windfarm Example

Open Switch

T1 T2 T3 T4 Tx Ty T12 T13 T14 Tz

Example Circuit: Approximately 6.1 miles OR 32,331ft


Cable Fault Location (CFL) Test Example No.1: Windfarm Example

Example Circuit: Approximately 6.1 miles OR 32,331ft

• System Attributes / Issues:– Substation getaway to first

structure at 28,000ft+ length– Limited sectionalizing capabilities– Exceptionally high system

capacitances– Limited vehicle access.

• Solutions / Mitigation:– Ideally add junction-boxes at set

interval to allow for sectionalizing.– Appropriately sized thumper to

match cable capacitances.– TDR unit for range-finding and

pin-pointer required to locating cable-fault.


Open Switch

T1 T2 T3 T4 Tx Ty T12 T13 T14 Tz

Partial Discharge Field Test Example No.2: Windfarm in Texas PD measurements after Splice Repair

• System Attributes / Issues:– Substation getaway to first structure

at 28,000ft+ length– Limited sectionalizing capabilities– Exceptionally high system

capacitances limit the cable test equipment effectiveness [CFL / CTD]

28,000ft / 8,534mJunction

BoxJunction

Box

Collector Substation

6,256ft / 1,906m

• Mitigation and Benefits:– Add junction-boxes within the

28,000ft run of cable.– CFL and CTD gains.– Restoration time-improvements– Reduction in loss-of-generation


• CFL / CTD Strategies– System Topography– TDR usage and

effectiveness– Thumper/Surge Unit– VLF, Tan Delta, and PD

Units– High Humidity and

Contamination

• Results Interpretation and Findings:– PD activity localized at Connection terminal at first

junction-box, a splice in the middle of the string toward the next junction-box, and activity at the open-terminal end at the next junction-box.

– Measured PD activity at repaired splice is between 400pC and 900pC. This value is below 1000pC and can be viewed as acceptable for existing facilities.

Partial Discharge Field Test Example No.2: Windfarm in Texas PD measurements Post-Splice Repair


Partial Discharge Field Test Example No.3: Windfarm in Mexico, Air-Void in Splice Detected by PD Unit, with Post Mitigation

• Wind Park in Mexico; 35kV XLPE Cable; Length of 6.5km (21,326ft)• PD testing used to check system, activity detected within splice.

PD Activity at Splice Location


Partial Discharge Field Test Example No.3: Windfarm in Mexico, Air-Void in Splice Detected by PD Unit, with Post Mitigation

High Noise at Input PD Activity at Splice Location


Cable Fault Locating (CFL)• TDR usage is crucial. Surging /

Thumping systems for location requires a distance to a cable fault location to expedite restoration of service.

• Surge/Thumper sizing based upon capacitance and Voltage of system to be tested and fault located.

• Surge/Thump Modes used can improve chances of fault location.

• Pinpointing Methods rely upon sufficient energy is present at fault location to be detected.

• System designs can be improved by adding additional sectionalizing locations to allow for a divide and conquer strategy to quickly isolate the faulted cable segment and obtain an approximate footage distance to the fault.

Cable Testing Anomalies for Windpark Testing Applications: Summary / Conclusion

Cable Test and Diagnostic (CTD)• VLF, Tan Delta, and PD units must be

sized and selected appropriately for the cable to be tested.

• Long cable runs pose challenges due to high capacitance values and signal loss.

• Environmental conditions plays a big role in the results derived from VLF, Tan Delta, and PD testing. Field testing practices and strategies can mitigate for these influences OR eliminate them entirely.

• Proper testing practices can reduce outages by locating areas of concern long before a cable-failure occurs.

• Most cable damage occurs from the day of installation. Preventative measures during installation forestall future failures.


Questions?


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Cable Testing Anomalies for Windpark Testing Applications