TDR Cable Fault Pre-Locator Case Study: 150kV Substation Diagnostics at PLN Cawang Jakarta

Project Background

In March 2026, the engineering team at XZH TEST was contracted by PT PLN (Persero), Indonesia's state-owned electricity utility, to conduct a comprehensive cable fault diagnostic campaign at the 150kV Cawang GIS Substation in East Jakarta. The substation serves as a critical node in the Jakarta-Banten transmission ring, supplying power to over 400,000 residential and industrial customers across the city's eastern corridor. The facility houses six 150kV gas-insulated switchgear (GIS) bays, four 150/20kV power transformers rated at 60MVA each, and approximately 28 kilometers of XLPE-insulated underground power cables connecting the transformers to the 20kV distribution switchgear.

The scope of work involved diagnostic testing on 14 medium-voltage (20kV) and high-voltage (150kV) cable circuits that had been in service for 11 to 17 years without comprehensive fault location testing. PLN's asset management division required the following deliverables: precise fault distance measurement on two known-fault circuits, baseline TDR signature acquisition for all 14 cables, propagation velocity (Vp) calibration for each cable type, and integration of test results into PLN's APK-AMS (Asset Performance Knowledge — Asset Management System) database.

The testing was scheduled during a planned 72-hour maintenance window to minimize load shedding impact. All tests were conducted in accordance with IEC 60229, IEEE 400.2, and PLN's internal technical guideline ED-02-031 on underground cable field testing procedures.

Existing Problems

During the pre-test site survey and historical data review, our team identified the following operational issues that had been escalating over the preceding 18 months:

1. Cable Fault Cannot Be Located. Feeder CB-07 (20kV, serving the Cawang-Kampung Melayu corridor) had tripped on earth-fault protection four times in six months. Two previous fault-locating attempts by a local contractor using a basic TDR cable fault locator with 10MHz sampling had failed to identify the fault position, resulting in the circuit being left de-energized and customers supplied via an overloaded backup feeder.
2. Frequent Transformer Tripping. Transformer T2 (150/20kV, 60MVA) had recorded three Buchholz relay alarms and one differential protection trip in the preceding quarter. Dissolved gas analysis (DGA) indicated thermal fault indicators in the 300-700°C range, but the root cause — whether cable-related partial discharge or internal winding degradation — remained unconfirmed.
3. CT Ratio Abnormal. The current transformer on feeder CB-03 exhibited a ratio error of -2.8% during the last scheduled secondary injection test, exceeding the IEC 61869-2 Class 0.5 accuracy limit. The substation SCADA historian showed progressive ratio drift over 14 months, raising concerns about incorrect protection relay operation.
4. Circuit Breaker Slow Opening. The 150kV SF6 circuit breaker associated with incomer bay B-02 showed an opening time of 58ms during the last timing test, 16% above the manufacturer's rated 50ms specification and approaching the IEEE C37.09 maximum permissible deviation of 20%.
5. Maintenance Time Too Long. PLN's quarterly cable maintenance cycle for the Cawang substation required an average of 4.8 days per circuit, primarily because the existing fault pre-location process using a 10MHz single-pulse TDR instrument required multiple attempts with iterative Vp adjustments and manual waveform interpretation by a senior engineer stationed 90km away in Bandung.

Engineer Analysis

After reviewing the five problem areas, we conducted a structured root-cause analysis addressing each issue through the lens of relevant international standards.

Cable Fault Location Failure. The previous contractor's inability to locate the CB-07 earth fault was attributable to three technical shortcomings. First, the 10MHz sampling rate of their TDR cable fault locator yielded a theoretical minimum resolution of approximately 10 meters at a Vp of 0.67 (typical for XLPE), which is insufficient for detecting high-resistance faults exhibiting weak reflection coefficients below 0.15. Per IEEE 400.2-2013 Section 7.3, arc reflection and surge pulse methods with sampling rates exceeding 100MHz are recommended when fault resistance exceeds 500Ω. Second, the contractor used a default Vp of 0.67 for all cable types without performing on-site velocity calibration on a known-length healthy phase, violating the procedure outlined in IEC 60229 Annex B. Third, they employed only low-voltage TDR mode, which cannot break down the high-resistance oxide layer at the fault point — this requires high-voltage flashover (DECAY) or ARC multi-shot methodology to ionize the fault gap and generate a detectable reflection.

Transformer Tripping. The correlation between Buchholz alarms and DGA thermal fault indicators pointed toward either partial discharge activity in the cable termination box or internal winding hot-spot formation. IEEE C57.104-2019 guidelines for DGA interpretation classify the ethylene-to-acetylene ratio of 3.2:1 observed in T2 as indicative of thermal fault exceeding 500°C in oil-impregnated paper. However, without a baseline TDR signature of the transformer-to-switchgear cable segment, it was impossible to determine whether transient overvoltages from cable PD were contributing to insulation stress at the transformer bushing.

CT Ratio Anomaly. The progressive nature of the ratio error in CB-03's CT suggested either secondary circuit burden drift due to contact resistance increase in terminal blocks, or partial shorted turns in the CT secondary winding accelerated by thermal cycling. IEC 61869-2 mandates annual ratio verification with burden measurement, yet PLN's records showed the last burden test was 22 months prior.

Breaker Timing Degradation. The 16% opening time increase in B-02 was consistent with SF6 gas density reduction (measured at 0.62MPa versus nominal 0.70MPa) combined with increased mechanical friction in the operating mechanism linkage. ANSI/IEEE C37.09-1999 Section 6.3.2 specifies that opening time shall not exceed 20% of rated value, placing B-02 within the warning band but below the trip threshold — a condition that demands corrective maintenance during the next planned outage window.

Extended Maintenance Duration. The 4.8-day average per circuit was directly linked to the absence of a high-performance cable fault pre-locator with automated waveform capture and multi-method testing capability. Each iterative Vp adjustment cycle consumed 3-4 hours, and the manual nature of waveform interpretation introduced operator-dependent variability that necessitated senior engineer verification before dispatching excavation crews.

Equipment Used

For this diagnostic campaign, we deployed the XZH TEST XHGG502 TDR Cable Fault Pre-Locator, a professional-grade Time Domain Reflectometer engineered for power cable diagnostics across transmission, distribution, and industrial networks. The instrument was selected based on its alignment with the technical requirements identified during the root-cause analysis phase.

Testing Procedure

The following Step 1 through Step 12 testing sequence was executed for each of the 14 cable circuits, with the known-fault circuit CB-07 receiving additional high-voltage flashover testing in Step 8.

Step 1 — Safety Preparation and Permit Verification. All team members completed the PLN Level 2 electrical safety briefing. A Permit-to-Work (PTW) was obtained from the substation control room. The circuit under test was confirmed isolated, locked-out, and tagged-out (LOTO) at both ends per PLN SOP-02-P2. A portable earth was applied and verified at the test location. The exclusion zone was demarcated with safety cones and barrier tape at a 3-meter radius for LV pulse testing and 8-meter radius for HV flashover testing.

Step 2 — Cable Identification and Documentation. Cable ID tags were cross-referenced against PLN's single-line diagram (SLD Rev. 12, dated 2025-09-14). Cable type (XLPE 1×400mm² Cu, 12/20kV), route length from as-built drawings (2,840m for CB-07), and known splice locations at chainage 760m and 1,930m were recorded in the test log. Digital photographs of cable terminations at both ends were taken for the final report appendix.

Step 3 — Visual Inspection and Termination Cleaning. Both cable ends were visually inspected for signs of tracking, carbon deposits, swelling, or insulation cracking. Termination surfaces were cleaned with anhydrous isopropyl alcohol and lint-free wipes to remove semi-conductive residue that could affect pulse injection. The screen-to-earth connection integrity was verified with a low-resistance ohmmeter (readings ≤0.1Ω at both ends).

Step 4 — Insulation Resistance Pre-Check. A 5kV DC insulation resistance test was performed between each phase conductor and earth using a calibrated 5kV Megger MIT525. Readings were recorded at 15s, 60s, and 600s intervals to compute the polarization index (PI) and dielectric absorption ratio (DAR). CB-07 Phase-B returned IR(60s) = 18MΩ and PI = 1.1, confirming the presence of moisture ingress or insulation degradation consistent with the reported earth fault.

Step 5 — XHGG502 Setup and Grounding. The cable fault pre-locator was positioned on a stable, dry surface within the test zone. The instrument's protective earth terminal was connected to the substation earth bar using a 10mm² green/yellow braided copper lead (length 3m, resistance verified ≤10mΩ). AC mains power was supplied via an isolation transformer (1:1, 2kVA) to eliminate common-mode noise from the substation auxiliary supply. The XHGG502 was powered on and allowed a 2-minute warm-up period for the touch screen controller and sampling FPGA to reach thermal equilibrium.

Step 6 — Vp Calibration on Healthy Phase. Using the healthy Phase-A of CB-07 as reference, the TDR was connected via the low-voltage pulse output BNC to the phase conductor. A known cable length of 2,840m (from as-built records) was entered. The instrument's Auto-Vp function transmitted a 2μS-wide, 500V pulse and captured the open-circuit reflection from the far end. The measured round-trip time of 28.38μS yielded a calibrated Vp of 0.668 (XLPE). This value was saved to the internal cable library and applied to all subsequent measurements on the CB-07 circuit.

Step 7 — Low-Voltage TDR Survey. With Vp = 0.668 confirmed, the XHGG502 was switched to 400MHz sampling with 0.05μS pulse width for maximum resolution. A complete TDR trace was acquired on Phase-A (healthy), Phase-B (faulted), and Phase-C (healthy). The Phase-B trace displayed a pronounced negative-polarity reflection at a cursor-measured distance of 1,830m from the test end, indicating a low-resistance shunt (short-to-earth) at that position. The reflection coefficient of -0.72 confirmed a near-solid earth fault with fault resistance estimated at 8-15Ω. The Phase-A and Phase-C traces served as differential comparison baselines, clearly highlighting the anomaly on Phase-B.

Step 8 — High-Voltage Flashover (DECAY) Verification. To confirm the fault location under dynamic breakdown conditions, the pulse coupler (40kV DC rated) was connected between the XHGG502 and the Phase-B conductor. A DC high-voltage source was ramped to 18kV at 1kV/s. At 14.2kV, an acoustic discharge was audible from the cable — the fault gap had broken down. The XHGG502, operating in automatic continuous sampling mode, captured the transient flashover waveform. Cursor measurement on the decaying oscillation trace confirmed the fault distance at 1,831m, within 0.1% of the LV pulse measurement, providing dual-method confirmation suitable for excavation authorization.

Step 9 — ARC Multi-Shot Capture. With the fault now ionized, the ARC multi-shot mode was activated. The instrument automatically triggered the high-voltage source and captured eight successive arc reflection pulses within a 2-second window. All eight traces overlaid with fault distance readings between 1,829m and 1,832m (mean 1,830.5m, standard deviation 1.1m). This data provided statistical confidence for the excavation crew and was exported as a multi-trace PNG overlay for the final report.

Step 10 — Healthy Circuit Baseline Acquisition. For the 12 non-faulted circuits, a complete LV pulse TDR signature was acquired at 100MHz sampling (adequate resolution for baseline trending). Each trace was saved with metadata including cable ID, date, time, Vp setting, operator name, and ambient temperature (28.6°C at time of testing). These baseline signatures were stored for future differential comparison — any subsequent fault on these circuits can be rapidly identified by subtracting the healthy baseline from the faulted trace.

Step 11 — Data Export and Report Generation. All 14 test records were exported from the XHGG502 via USB 3.0 as individual CSV waveform files and a consolidated PDF report generated directly on the instrument. The report included: waveform screenshot with cursor measurements, test parameters (sampling rate, pulse width, Vp, gain settings), cable metadata, ambient conditions, and operator digital signature. The CSV files were formatted with column headers compatible with PLN's APK-AMS import template.

Step 12 — Site Restoration and Handover. All test connections were removed from the cable terminations. The portable earth was removed last, per safety protocol. The exclusion zone barriers were dismantled. The PTW was closed out at the substation control room with the shift supervisor's signature. A preliminary verbal briefing was delivered to PLN's asset manager, and the digital test report package was emailed to the PLN engineering team via the XHGG502's built-in 4G connection before leaving site.

Testing Results

The following tables summarize the key diagnostic data collected during the Cawang Substation campaign.

The dual-method fault distance confirmation on CB-07 — with only 1-meter deviation between TDR and DECAY measurements over a 2,840-meter cable — provided the confidence level required for PLN to authorize a precision excavation at chainage 1,830m. The excavation revealed a mechanically damaged cable joint where a construction pile had grazed the outer sheath during adjacent civil works three years prior, allowing gradual moisture ingress that eventually formed the low-resistance earth path detected in our measurements.

Customer Benefits

The Cawang Substation diagnostic campaign delivered the following operational outcomes for PLN:

• Targeted Excavation Instead of Trial Digging. By pinpointing the CB-07 fault to within ±1m, PLN avoided the traditional approach of excavating multiple trial holes along a suspected 500-meter fault zone. A single 3m × 2m excavation at chainage 1,830m directly exposed the damaged joint, reducing the civil works scope from 12 man-days to 1.5 man-days and eliminating traffic disruption on Jalan Raya Bogor, a major Jakarta arterial road under which the cable is buried.
• Avoided Unnecessary Cable Replacement. The healthy-phase TDR signatures confirmed that Phases A and C of CB-07, plus all phases of the remaining 13 circuits, exhibited no impedance anomalies requiring intervention. This evidence-based finding prevented a scheduled replacement of CB-07's entire 2,840m cable run — a capital expenditure estimated at IDR 4.3 billion (approximately USD 265,000) — which had been proposed based on the assumption of widespread insulation degradation following the Phase-B fault.
• Reduced Troubleshooting Time from Days to Hours. The 14-circuit baseline acquisition and dual-method fault location were completed within 18 hours of the 72-hour maintenance window, compared to the 67 hours historically required for similar scope. The automated waveform capture and on-board reporting capability of the XHGG502 eliminated the multi-hour iterative Vp adjustment cycles and the need for off-site senior engineer waveform interpretation that had previously dominated the testing timeline.
• Verified Equipment Condition for Asset Planning. The CT ratio, burden, and excitation tests on CB-03 provided quantitative justification for CT replacement — the 125% burden loading and -2.8% ratio error clearly exceeded the IEC 61869-2 Class 0.5 envelope. Similarly, the B-02 breaker timing and SF6 density data supported a scheduled overhaul at the next 6-month maintenance window rather than an emergency shutdown. PLN's asset management team integrated all 14 baseline TDR signatures into APK-AMS, creating a permanent reference for future differential fault location that will further reduce diagnostic time on subsequent faults.
• Improved Safety Through Reduced Site Exposure. The 18-hour testing duration, compared with the estimated 67 hours for conventional methods, reduced field crew exposure to high-voltage test areas by 73%. No safety incidents were recorded during the campaign. The LOTO and exclusion zone protocols, combined with the XHGG502's remote waveform streaming capability that allowed the senior engineer to participate from Bandung without traveling to site, contributed to this unblemished safety record.

Engineer's Notes

Common Mistakes to Avoid. The single most frequent error we observe in TDR-based underground cable fault detection is the use of a default Vp value without on-site calibration. In this project, the calibrated Vp of 0.668 differed from the cable manufacturer's datasheet value of 0.67 by only 0.3%, yet this 0.002 difference translated to a 6-meter error over 3km — enough to miss a buried joint by two excavation lengths. Always calibrate Vp on a known-length healthy phase; never trust the datasheet alone. A second common mistake is attempting HV flashover testing without first verifying that the cable's insulation resistance can safely withstand the applied voltage. Our 5kV IR pre-check on CB-07 Phase-B identified the 18MΩ reading, which was adequate for controlled flashover at 14.2kV but would have been dangerous on a cable with IR below 1MΩ.

Environmental Considerations. Jakarta's tropical climate presents specific challenges for power cable testing. Ambient temperature during our test window was 28.6°C with 82% relative humidity. At these humidity levels, condensation on BNC connector surfaces can introduce reflection artifacts that mimic low-amplitude cable faults. We mitigated this by applying dielectric grease to all BNC connections and using connectors with IP65-rated boots. The afternoon thunderstorm that occurred during Day 2 of testing forced a 90-minute suspension while we moved equipment under the substation canopy — the XHGG502's IP54 rating provided adequate protection against wind-driven rain during the brief exposure, but we do not recommend continuous operation in precipitation without additional shelter.

Safety Requirements Beyond Standard Protocol. While PLN's SOP-02-P2 covers standard LOTO and earthing procedures, we implemented two additional safety measures based on our experience with cable fault pre-locator field work in Southeast Asian substations. First, we verified the absence of induced voltage on the disconnected cable using a non-contact voltage detector before and after portable earth application — the 150kV GIS busbar's electromagnetic field can induce 50-200V on parallel de-energized 20kV cables over the 2.8km parallel run in the cable trench. Second, during HV flashover testing, we stationed a safety observer with a rescue hook at the test area perimeter, equipped with a two-way radio on a channel separate from the test team's channel to avoid communication interference during discharge events.

Frequently Asked Questions

Q1: What is a TDR cable fault locator and how does it work?
A Time Domain Reflectometer (TDR) transmits a low-voltage electrical pulse into a cable and measures the time required for any reflection to return from an impedance discontinuity — such as an open circuit, short circuit, or partial damage point. By knowing the pulse's propagation velocity through the cable insulation, the instrument calculates the exact distance to the fault. Modern instruments like the XHGG502 achieve 0.3-meter resolution by sampling at 400MHz, capturing reflections that slower instruments miss.

Q2: What cable types can the XHGG502 cable fault pre-locator test?
The XHGG502 is compatible with XLPE, PILC (paper-insulated lead-covered), EPR, and PVC-insulated power cables rated up to 35kV, as well as control cables, communication cables, and street lighting circuits. The selectable output impedance (25-120Ω) and adjustable pulse width (0.05μS-2μS) allow optimal matching to a wide range of cable constructions and cross-sectional areas.

Q3: How does ARC multi-shot differ from standard TDR measurement?
Standard TDR uses a single low-voltage pulse and may not generate a detectable reflection from high-resistance faults (>500Ω) because the pulse energy is insufficient to break down the oxide or carbonized layer at the fault point. ARC multi-shot technology applies a high-voltage surge to ionize the fault gap, then fires the TDR pulse during the arc's conductive window. The instrument automatically captures multiple successive arc events (up to eight shots) and overlays the traces, dramatically improving fault identification reliability on intermittent and high-impedance faults.

Q4: What is the maximum testing distance for underground cable fault detection?
The XHGG502 supports test distances up to 80km, although the practical limit depends on cable type, condition, and the magnitude of the fault reflection. On XLPE-insulated cables with low attenuation characteristics (typically <1.5dB/km at the test frequency), distances beyond 50km are routinely achievable. On older PILC cables with higher dielectric losses, the effective range may be reduced to 20-30km.

Q5: Is the XHGG502 suitable for live-line testing?
No. The XHGG502 is designed for testing on de-energized, isolated, and earthed cables only. Attempting to connect the pulse output to an energized cable will damage the instrument's input protection circuitry and create a severe arc-flash hazard. Always verify isolation using a qualified voltage detector before connecting any cable fault locator, regardless of manufacturer claims.

Q6: How long does a typical cable fault location test take?
For a single cable circuit with known parameters (cable type, length, and a healthy phase available for Vp calibration), a complete LV pulse TDR survey can be completed in 15-20 minutes. Adding HV flashover and ARC multi-shot verification extends the testing time to approximately 45-60 minutes per faulted phase. The Cawang Substation campaign — covering 14 circuits including one faulted circuit with dual-method verification — was completed in 18 hours by a two-person team.

Q7: What training is required to operate the XHGG502?
Operators should possess a fundamental understanding of Time Domain Reflectometry principles, cable construction types, and electrical safety protocols for substation environments. Engineers with a bachelor's degree in electrical engineering and one year of field testing experience can achieve proficiency within two days of hands-on training. XZH TEST provides a comprehensive operator training program covering instrument setup, Vp calibration, multi-method testing, waveform interpretation, and report generation.

Q8: Can the XHGG502 test submarine or subsea cables?
Yes, the instrument supports fault location on submarine power cables within its 80km range capability. The key consideration for subsea cable diagnostics is the cable's attenuation characteristics, which vary significantly with insulation type (XLPE, EPR, or mass-impregnated paper) and whether the cable incorporates an integrated fiber optic element. For cables exceeding 50km in length, we recommend a preliminary attenuation assessment before committing to a fault location campaign.

Q9: How are test results documented and shared with stakeholders?
The XHGG502 generates PDF test reports directly on the instrument, including waveform screenshots with cursor measurements, test parameter summaries, cable metadata, ambient conditions, and operator digital signatures. Waveform data can also be exported as CSV files for integration with third-party analysis software or asset management databases such as APK-AMS, Maximo, or SAP PM. Built-in WiFi and 4G connectivity enable immediate email distribution of reports to remote stakeholders from the test site.

Q10: What warranty and after-sales support does XZH TEST provide?
Each XHGG502 includes a 12-month manufacturer warranty covering parts and labor, with extended warranty packages up to 36 months available. XZH TEST maintains spare parts inventory (pulse couplers, battery packs, printer modules) at our Xi'an, China headquarters with 48-hour dispatch. Technical support is available via email, phone, and video conference during China business hours (UTC+8), with emergency after-hours support for critical fault-finding campaigns.