You're right to question this. The setup you describe is common in Saudi Arabia and other regions, but the discrepancy you point out requires careful consideration. Here's a breakdown of why the FSCTs might be used on the OHL feeder but not for the transformer, and what factors influence that decision

Possible Reasons for Using FSCTs for OHL Feeders but Not the Transformer:

1. Focus on External Faults on the OHL



OHL Feeders are more exposed to external faults

Overhead lines are inherently more susceptible to faults caused by lightning strikes, tree contact, animal interference, and other external factors. The FSCTs provide fast and sensitive protection specifically for these faults on the OHL itself.

Distance to fault

Because OHLs are longer than the short 200m cable tail connection, the FSCTs need to detect faults happening along the entire OHL cable.2.

Transformer Protection Philosophy



Internal Fault Emphasis

Transformer protection schemes (differential, REF, etc.) are primarily designed to detect internal faults within the transformer itself. These schemes often rely on CTs located inside the transformer bushings or very close to the transformer terminals, providing highly localized fault detection.

Backup Protection for Cable Tails

While dedicated cable tail protection isn't always implemented with FSCTs for the transformer, the overall transformer protection scheme (differential, overcurrent, restricted earth fault) inherently provides some level of backup protection for the cable tail connection to the GIS. The sensitivity and speed might not be as high as with dedicated FSCTs, but it's still present.3.

Cost and Complexity



FSCTs add cost and complexity

Installing additional FSCTs just for a short cable tail protection for the transformer adds extra equipment, wiring, and commissioning effort. If the existing transformer protection is deemed adequate for the cable tail, this additional cost might be considered unnecessary.

Coordination Challenges

Adding more protection elements (like FSCTs) requires careful coordination to ensure proper tripping and avoid nuisance tripping.4.

Assumptions about Cable Tail Faults



Lower Probability of Cable Tail Faults

The assumption might be that the probability of a fault within the short cable tail connection to the transformer is relatively low, especially if high-quality cable and sealing ends are used and installation practices are rigorous. A cable fault, if it occurs, may likely be a phase to phase or ground fault that the transformer differential protection will detect.

Manufacturing Quality

Cable sealing ends are designed to withstand high voltages and currents. It's assumed that these sealing ends are manufactured to a high quality, thereby minimizing the risk of failure.5.

Reliance on Existing Protection Schemes



Differential Protection Dominance

Differential protection is typically the primary protection for power transformers. It detects faults by comparing current entering and leaving the transformer. While it primarily focuses on internal faults, it will detect faults in the cable tail connection if they are significant enough.

REF Protection

Restricted Earth Fault (REF) protection is highly sensitive to earth faults within the transformer winding. It might also offer some limited protection for earth faults in the cable tail close to the transformer.6.

Cable Sealing Ends

Yes, the use of cable sealing ends contributes to the decision. These sealing ends are designed to provide reliable insulation and protection at the cable termination points. They are typically tested to withstand high voltages and currents, minimizing the risk of flashover or breakdown in normal operating conditions. These sealing ends also provide a barrier against environmental factors such as moisture, dust, and pollution, which can degrade the insulation and lead to faults over time.

Why the Assumption Might Need Re-Evaluating

While the above reasons provide a rationale, the difference in approach should be critically assessed based on:

Cable Type and Quality

If the cable tails are of a type that's known to be less reliable or more prone to failure, dedicated protection should be considered.

Conduit Protection

If cable tails are not installed in protected conduits, but are subject to physical damage, a dedicated protection scheme is recommended.

Operating Experience

If the substation has experienced cable tail faults in the past, adding FSCTs for the transformer connection is justified.

Importance of the Transformer

For critical transformers, the added protection might be worth the cost and complexity, regardless of the perceived risk.*

Grid Codes and Standards

Local grid codes and standards might have specific requirements regarding cable tail protection. Ensure the installation complies with these regulations.

In conclusion:

The decision to use FSCTs on the OHL feeder but not the transformer cable tail likely stems from a combination of factors, including the greater exposure of OHLs to external faults, the primary focus on internal fault detection for transformers, and cost considerations. The robustness of the transformer protection scheme (differential, REF, overcurrent) also plays a role, along with assumptions about the reliability of the cable tail installation and sealing ends.However, it's essential to re-evaluate this approach based on the specific conditions, operating history, and criticality of the transformer. A risk assessment, considering the likelihood and consequences of cable tail faults, should be conducted to determine if additional protection is warranted. It is also important to consider local grid codes and standards to ensure the installation complies with these regulations. Flag for review