Challenges

Model integration at scale

• Consolidate over 20 ETAP models from EPCC contractors into one coherent model of ~5000 buses.
• Validate data consistency and ensure all models use the same network topology and settings.

Multi-vendor, multi-voltage system

• Voltage levels from 400 V up to 275 kV, with mixed transmission and industrial practices.
• Need one common model for short-circuit, transient stability, harmonic and reliability studies.

Solidly grounded 275 kV system at both ends

• 275 kV transformers at sending and receiving ends are solidly grounded to optimise insulation cost.
• This raises the question of circulating earth current and sympathetic relay operation during single-line-to-ground (SLG) faults.

Operating modes: radial vs ring

• 275 kV system normally operates radially, but can form a ring (N+1) via interconnections between key substations.
• In ring mode, earth-fault currents can be bidirectional, complicating 67N settings.

Grid constraints

• Short-circuit contribution at the grid interconnection must be limited to 50%, requiring reactors at the tie point and careful protection coordination.

Need to go beyond traditional TCC curves

• Conventional time-current characteristic (TCC) checks from EPCC contractors looked acceptable.
• PETRONAS needed a more advanced technique to reveal hidden mis-operations.

Selected applications

To address these challenges, PETRONAS used ETAP as the common platform:

ETAP digital-twin modeling

• Integration of all EPCC models into a single, validated ETAP project representing the entire RAPID power system.
• Single source of truth for short-circuit, load flow, transient stability, harmonics and reliability analyses.

PD Sequence-of-Operation (Star Sequencer)

• Simulation of single-line-to-ground faults on 275 kV transformer feeders.
• Step-by-step replay of relay operations to detect sympathetic tripping and coordination issues that are not visible on TCC curves.

Sequence network analysis

• Construction of positive, negative and zero-sequence networks for SLG faults to understand zero-sequence paths and circulating currents.

Earthing & protection coordination studies

• Evaluation of earth-fault current distribution in radial and ring configurations.
• Assessment of existing EPCC protection settings (51N, 67N, differential, distance and overcurrent backup).

Key findings

1. Sympathetic earth-fault relay operation

Using PD Sequence-of-Operation, PETRONAS simulated a line-to-ground fault on one 275 kV transformer feeder:

• Healthy 275 kV feeders experienced circulating earth-fault current greater than 1 p.u., reaching about 364 A against a line rating of 250 A.
• Earth current flowed in both directions on some feeders in ring configuration.
• As a result, 51N and 67N relays on healthy feeders picked up unnecessarily.

If the main differential protection failed to clear the fault, these sympathetic operations could cause a wider outage of the complex.

2. Cause: double-ended solid grounding

Sequence network analysis showed that:

• Solidly grounded transformers at both ends provide a zero-sequence path through healthy feeders.
• This allows circulating earth current to flow during SLG faults, even on feeders without faults.
• At 33 kV, the same effect exists but is mitigated by neutral earthing resistors, so currents are smaller.

3. Limitations of directional 67N

Some consultants assumed that directional earth-fault protection (67N) would solve the issue. Simulations showed:

• Because the system can operate radially or in a ring, earth-fault current direction is not fixed.
• 67N alone cannot reliably discriminate sympathetic operation in all configurations.

Main customer benefits

Reliable integrated model

• PETRONAS obtained a single, high-fidelity ETAP model of the entire RAPID complex, validated across all EPCC designs.
• Multiple engineering teams and even university collaborators can now run different types of studies on the same dataset.

Uncovering of hidden protection issues

• PD Sequence-of-Operation revealed sympathetic relay behaviour that appeared acceptable in conventional TCC reviews.
• PETRONAS could proactively address these issues before energising the system.

Clear, transmission-grade recommendations

• From the study, PETRONAS issued corporate guidelines for 275 kV protection on RAPID and future projects (see below).

Improved system reliability & selectivity

• By aligning practices with 132/275 kV transmission norms, the risk of widespread outages from a single SLG fault is significantly reduced.

PETRONAS Recommendations for 275 kV protection

Based on the ETAP study and industry best practice, PETRONAS recommended that RAPID and future projects adopt the following:

Install double main differential protection

• 275 kV transformer feeders should have two independent differential protections, as is common in transmission systems, to ensure fast and reliable fault clearing.

Use 67N only as backup

• Directional earth-fault (67N) may be enabled only as backup to single-main protection, with pickup settings above the simulated circulating earth current and looking toward the transformer.

No stand-alone earth-fault O/C on 275 kV lines

• In line with typical practice at 132 kV and 275 kV, there is no requirement for separate earth-fault overcurrent protection on these solidly grounded lines.
  Rely instead on differential, distance and backup phase overcurrent schemes.

Customer perspectives