Mitigating the Rising Threat of Short-Circuit Currents in Power Systems: Advanced Limiting and Interruption Techniques​

1. Introduction
The rapid expansion of power systems, characterized by increasing generator and transformer capacities, has led to significantly higher system fault levels. Elevated short-circuit currents pose dual threats: intensified electrodynamic stresses on critical equipment like main transformers, and the requirement for circuit breakers with exponentially higher interrupting capacities, driving up costs and demanding extreme reliability. Particularly in industries like metallurgy and petrochemicals—exemplified by offshore oil platforms with their compact, high-load networks—failure to swiftly restrict or isolate fault currents can cause catastrophic equipment damage (deformation from dynamic forces, insulation breakdown from thermal stress) and widespread production disruptions. Prompt mitigation is crucial to prevent extensive operational and financial losses.

2. The Hazards of High Short-Circuit Currents
Short-circuit currents, vastly exceeding normal operating currents, inflict severe stress throughout the power system:

• Equipment Stress & Sizing: Components (transformers, CTs, busbars, cables) endure massive dynamic forces, necessitating oversized equipment. Switchgear (isolators, circuit breakers) must withstand both the thermal (I²t) and dynamic (peak withstand) stresses.

• Thermal Degradation: Excessive heat accelerates insulation aging, reduces equipment lifespan, and can cause terminal overheating or conductor melting/fusing.

• Dynamic Stability Failure: The peak electromagnetic force during fault transients can exceed equipment withstand limits, causing mechanical failure.

• CB Interruption Failure: Excessive fault levels may surpass a breaker's interrupting rating, preventing fault clearance and escalating incidents.

• EMI & Safety Hazards: Earth faults induce electromagnetic interference in nearby comms/signaling systems and create hazardous step/touch potentials near the fault point, endangering personnel and assets.

3. Case Study: Offshore Platform (LIBRA Project)
The LIBRA offshore oil platform (referenced single-line diagram) requires multiple synchronized diesel generators. Calculated fault currents regularly exceed 50kA. While switchgear can be reinforced structurally to handle high peak forces (~150kA), sourcing and integrating 63kA breakers is prohibitively expensive and space-intensive compared to 50kA units. The client opted for 50kA rated equipment.

• Normal Operation: All bus-tie breakers closed; generators feeding combined busbars. Max fault current (34.5kA) is within the 50kA switchgear rating.

• Maintenance Scenario: All bus-ties closed; generators connected. During generator transfer for maintenance, a potential fault current of 51.7kA exceeds the switchgear's 50kA short-time withstand and breaker interrupting ratings. Failure to isolate such a fault risks total platform blackout (eHouse system).

4. Solutions for Short-Circuit Current Mitigation
4.1 Limiting Fault Current Magnitude

• Series Current Limiting Reactors (CLRs): The primary solution in HV/MV networks. Installed at transformer LV/MV sides or on lines, CLRs effectively reduce both RMS and peak fault currents to match switchgear ratings (e.g., reducing 63kA/168kA peak to 32.74kA/83kA peak as shown in representative diagrams). However, conventional CLRs introduce continuous losses, voltage drop, and electromagnetic interference.

• Zero-Loss Deep Current Limiting (DLCL) Technology: Overcomes CLR drawbacks. Comprises:

• Fast Commutator Switch (closes during normal operation, near-zero loss).

• Deep Current Limiter (reactor).

• Ultra-rapid Fault Detector & Sensors.

• Upon fault detection (<0.4ms) and prediction (~2-3ms), the commutator switch opens within 7-8ms at the first current zero, inserting the DLCL reactor. Post-fault clearance or successful auto-reclose, the commutator recloses, bypassing the reactor. Features include self-healing on false trips, remote monitoring, and modular application up to EHV levels via series-connected units. DLCL enables deep limitation without inherent losses or voltage issues.

Variants of Zero-Loss Limiters:

• a. Ultra-Fast Switch (UFS) Type: Uses an electromagnetic repulsion-driven vacuum interrupter (<5ms opening, <8ms closing) paralleling the reactor. Normal current flows through the closed UFS; fault current forces the UFS open, inserting the reactor. High speed enables true "current-zero switching". Advantages: Very fast, reliable, long mechanical life (100k ops).

• b. Self-Restoring Pyrotechnic Current Limiter Type: Employs a pyrotechnic "exploding bridge" device paralleling the reactor. Normal current flows through the solid bridge (zero loss). On fault, the bridge ruptures within 1-3ms, inserting the reactor faster than a UFS. Key innovation: A backup limiter circuit allows system restoration (via a breaker closing) without immediate maintenance. The main limiter is replaced only during scheduled outages (estimated every 3-5 years), drastically reducing operational disruption and cost compared to traditional sacrificial devices. While CLRs/DLCL represent an investment, their system-wide benefits justify the cost.

4.2 Fault Current Interruption

• High Voltage Current Limiters (HVCLs - Fuse-Based): Ideal where brief supply interruption is acceptable or for critical protection. Pioneered by EPRI/G&W over 40 years, HVCLs offer ultra-fast (<2-5ms) interruption (63-200kA @ 12kV), continuous currents up to 6300A, indoor/outdoor use, and high cost-effectiveness. They work by using a solid copper path for normal current. A sensor triggers diversion of fault current into a parallel, current-limiting fuse which clears the fault before its first peak. Benefits:

• Prevents thermal/dynamic damage to downstream equipment.

• Mitigates arc flash hazards (safety critical).

• Enables continued use of lower-rated (cheaper/smaller) switchgear (e.g., 50kA vs. 63kA).

• Avoids continuous losses associated with CLRs (significant operational savings).

• Facilitates system upgrades without full switchgear replacement.

• Application to LIBRA Platform: Installing HVCLs on the bus-tie between Generator sections (G2-G3 buses - see schematic) effectively limits potential fault currents below the critical 50kA threshold. During generator maintenance scenarios, HVCLs activate to isolate faults within the switchgear's capability. HVCLs can be fixed-mounted (high current) or withdrawable (low current). Paired with a parallel backup HVCL and a breaker, continuous supply can be maintained post-fault.

4.3 Time-Delayed Fault Interruption
In systems with high X/R ratios (indicating slow DC component decay in asymmetric fault currents), the first current zero crossing can be delayed. This forces breakers to withstand the arc for longer, imposing severe thermal and mechanical stress. For circuits fed by small generators or with low X/R ratios, the DC component decays rapidly (within ~100ms or 3-5 cycles). Implementing intentional time delays (~100ms) allows breakers to interrupt a mostly symmetrical current. This leverages the switchgear's inherent peak withstand capability and requires only a robust breaker mechanism, offering a reliable and economical solution for specific applications like generator outlets.

5. Conclusion
Mitigating the destructive thermal and dynamic effects of high short-circuit currents is paramount for power system security and asset protection. While series reactors remain fundamental, their inherent losses and space requirements are drawbacks. Advanced solutions like Zero-Loss DLCL devices (using Ultra-Fast Switches or Self-Restoring Pyro-Limiters) offer efficient limitation while maintaining supply continuity. HV Current Limiters provide ultra-rapid, definitive fault interruption and isolation without reactors, ideal for protection in multi-source networks (offshore platforms, renewables) and bus-tie applications, offering significant space savings, enhanced safety (arc flash reduction), and enabling the use of cost-effective switchgear. Time-delayed interruption presents a highly economical and reliable option specifically where fault current asymmetry decays rapidly. The optimal strategy depends on system configuration, criticality of supply continuity, safety requirements, and cost-benefit analysis.