CONDITION ASSESSMENT OF MV CABLES AT SOUTH TEXAS PROJECT

NUCLEAR PLANT

S. Banerjee1, H. Sedding1 & D. Dayton2

1Kinectrics Inc., CANADA2South Texas Project NOC, USA

Project Background

• Medium Voltage (4.16kV – 13.8kVL-L), tape shielded pink EPR cables at risk of submergence identified as requiring a formal ageing management program at STPNOC

• Agreed test methodology based upon established Ontario Hydro practice for testing MV nuclear plant cable

1. AC Hi-Pot testing (can be power or very low frequency, 60Hz PF applied at STP)

2. Partial discharge (PD) in conjunction with an overvoltage test - point defects

3. Low frequency dielectric spectroscopy or VLF tanδ – moisture ingress/water trees

4. STP Test Population: Initial 26 cables (72 conductors) Spring 2013, additional 12 cables (36 conductors) for fall outage November 2013

Project Scope

AC Hi-Pot Testing• Testing at power frequency from 20 Hz to 300 Hz using resonant

power supplies (variable inductance and variable frequency) • Simulates electrical stress under service conditions• Recommended high potential test level ranges from ~ 1.7U0 to 2.5U0

• Highly effective at detecting weak spots in insulation

Figure 1: Withstand voltage as function of test frequency

PD Testing - Theory

LV ELEC

HV ELEC

Eins

Evoid iPD

BPD

iPD.HV.ind

iPD.LV.ind

VHV.AC

Conditions• Void• Free Electron• Evoid > Ebreakdown

Result Current induced on LV and HV electrodes Magnitude depends on void size and

location Rise-time depends on void size and shape

Partial Discharge Extinction Voltage(PDEV) and Inception Voltage (IV) PDEV – void is already discharging, hence

represents a natural process PDIV is obtained from artificially over-

stressing a void without an initial supply of free electrons

PD RELATES TO INSULATION INTEGRITY

PD Testing – Electrical Detection• A partial discharge internal to the cable insulation will induce a

current on both the cable core and the cable screen (semiconductor tape layer).

• The pulse shape of both currents will initially be similar but the magnitude will differ depending on the radial proximity of the PD

• Both currents will propagate towards either end of the cable

PD Signal Along cable core

PD Signal Along Cable Screen

vPD(t)

RPD

CPD vPD(t)

Off-Line PD Monitoring

On-Line PD Monitoring

AC Hipot & PD Testing Criteria

• 60Hz withstand test (duration 5 minutes) followed by PD test• Voltage levels, PD criteria developed 30 years ago at Ontario

Hydro• Basis is PDEV being sufficiently above operational voltage

Voltage Class (kV) Overvoltage (kV) PDEV (kV)5 9kV, 5 min. 4.58 13kV, 5 min 7.515 19kV, 5 min 13

Condition For 5kV Cable For 15kV Cable

Good PDEV>4.5kV PDEV>13kVEvaluate 2.4kV<PDEV≤4.5kV 8kV<PDEV≤13kV

Not Acceptable PDEV≤2.4kV PDEV≤8kV

PD Test Methodology @ STP

1. Sensitivity check, injection of known pulse magnitude (pC) at both ends if accessible

2. Complete 5 min. withstand test, monitoring Phase Resolved PD Pattern (PRPD) at maximum test voltage

3. Lower test voltage to measure electrical Partial Discharge Extinction Voltage (PDEV)

4. Raise voltage back to measure electrical Partial Discharge Inception Voltage (PDIV)

PD Test Methodology @ STP (Cont’d)

5. Assess acoustic PD (using ultrasonic probe) at MV cable terminations, where accessible

6. Analysis – based on PRPD patterns, PDEV levels, PD magnitudes, acoustic results/severity

AC Hipot & PD Testing Equipment

1. Capacitive Voltage Divider A

2. PD Injection Capacitor

3. Capacitive Voltage Divider B

4. 30kV Hipotronics AC Resonant Test Transformer

6. 60Hz Isolation Transformer

7. Omicron MPD 600 PD Analyzer

5. Biddle 37000 PD Detection System

8. Test Transformer Controller

PD Testing Case Study

• Reactor Coolant Pump and Circulating Water Pump Motors- 13.8 KV MV cables (2 per phase)

• 15kV rated L-L, 13.8kV operational L-L voltage, 250-750MCM conductors, 133% insulation Okonite Pink EPR, tinned copper tape shield

• Terminated using 15kV nuclear qualified heat shrink kits (Tyco/Raychem)

• Circuits lengths ranging from 600 – 2200 ft.• Cables ~ 26 years in service underground

PD Testing Case Study (Cont’d)

Figure: Example of Circulating Water Pump Cable Test Circuit (Outdoor motor end –Junction Box removed, tested from Switchgear side)

Results – PD Sensitivity Check

500pC Injected PD Pulse on HV Conductor at TEST END, fcenter = 250kHz, ∆f = 300kHz

Circulating Water Pump Circuit – 2144’ LengthNote: PD testing of these cables included one splice in a manhole

approx 300 feet from the motor end

2500pC Injected PD Pulse on HV Cond. at REMOTE END, fcenter = 250kHz, ∆f = 300kHz, 4.1x lower sensitivity than Test End

500pC Injected PD Pulse on HV Cond. at TEST END, fcenter = 250kHz, ∆f = 300kHz

Typical PD Test Results• Defect in cable termination

PD @ 15kVL-G PD @ 13kVL-G (PDEV acceptance level)

• Electrical PDEV = 8.5kV, PDIV = 9.0kV, • Audible PD using Ultrasonic Probe at both Test and Remote Ends

Typical PD Test Results• Cable termination replaced

PD @ 15kVL-G

• Electrical PDEV > 15kV, PDIV > 15kV

• No audible PD using Ultrasonic Probe at either Test or Remote Ends

Typical Termination Defects

• All tested cables successfully passed 60Hz withstand and DS/VLF Tan δ requirements, however PD testing revealed numerous, localized defects in 13.8kV terminations – PD eliminated following repair in 100% of cases

• Testing clearly illustrates that PD can be successfully detected up to remote ends of lengthy, pink EPR cable circuits with tape shields

Summary

1. Know up front the size and weight of your test equipment when contracting the service. This caused STP delays in the testing when accessing or changing room locations.

2. PD showed deficiencies in the terminations that were not anticipated (construction legacy)

3. Scheduling issues –Operations stopped cable testing when PD was found and would not allow restart until deficiencies were corrected.

4. Ensure you have adequate MV Termination Kits available during testing and their age (problem with tape provided in aged termination kits)

STP Lessons Learned

• Need to review number of Grounding Buggies available to support cable testing which was in addition to the station’s needs.

STP Lessons Learned

QUESTIONS

?