1. Introduction

Submarine optical cable systems form the backbone of global telecommunications, carrying over 95% of intercontinental data traffic. These systems span thousands of kilometres across ocean floors and require continuous electrical power to operate the optical amplifiers (repeaters) that regenerate signals along the route.

A fundamental concept in submarine cable powering is the virtual ground — a dynamic zero-voltage reference point that enables fault-tolerant operation and ensures system continuity even when cable damage occurs. Unlike a physical ground connection, the virtual ground is determined by the balance of voltages applied from both terminal stations and can be relocated automatically to accommodate faults.

This article explains the principles of virtual ground, its role in double-ended power feeding configurations, and how submarine systems automatically adapt to shunt faults by relocating the virtual ground point.

2. Background: Submarine Cable Power Architecture

2.1 Power Feeding Fundamentals

Submarine cable systems use a series power feeding architecture where Power Feed Equipment (PFE) installed at terminal stations supplies a constant direct current (DC) to energize submerged repeaters connected in series along the cable. The electrical circuit comprises:

• Power conductor: The central metallic conductor within the submarine cable carries the DC current.
• Return path: Seawater serves as the return current path, with earth electrodes at each terminal station completing the circuit.
• Repeaters: Optical amplifiers connected in series, each creating a voltage drop as current flows through them.

Modern PFEs can generate voltages exceeding 15 kV, enabling transpacific cables spanning over 10,000 km to be powered effectively. The feeding current for optical amplifier systems is typically 1.0 A ± 0.3 A, with higher currents used for dense wavelength-division multiplexing (DWDM) systems supporting more transmission wavelengths.

2.2 Double-Ended Power Feeding

The standard powering topology for point-to-point submarine cables is double-ended power feeding. In this configuration, PFEs at both terminal stations apply voltages of equal magnitude but opposite polarity to the cable:

• Station A applies +7 kV (positive polarity)
• Station B applies −7 kV (negative polarity)
• Total voltage across the system: 14 kV

This balanced configuration provides several advantages: redundancy (if one PFE fails, the other can power the entire system), reduced maximum voltage stress on individual components, and the ability to maintain operation during certain fault conditions.

3. The Virtual Ground Concept

3.1 Definition and Location

Virtual ground is the point along the submarine cable where the voltage potential equals zero relative to earth (sea ground). Unlike a physical ground connection, this is a dynamic reference point determined by the balance of voltages applied from both terminal stations.

In a double-ended feeding configuration with balanced voltages, the virtual ground naturally occurs at the electrical midpoint of the cable. The voltage profile along the cable forms a linear gradient:

• Maximum positive voltage at Station A
• Zero voltage (virtual ground) at the midpoint
• Maximum negative voltage at Station B

3.2 Voltage Profile Along the Cable

The voltage at any point along the cable can be approximated by:

The position of the virtual ground depends on the ratio of voltages applied by each PFE:

3.3 Benefits of Virtual Ground

The virtual ground concept provides several operational benefits:

1. Reduced dielectric stress: No single point experiences the full system voltage, reducing the risk of insulation breakdown.
2. Fault tolerance: The system can adjust to cable faults by relocating the virtual ground.
3. Operational flexibility: Voltage levels can be adjusted to accommodate changes in system configuration.
4. Equipment redundancy: Either PFE can assume full system powering if the other fails.

4. Shunt Fault Operation and Virtual Ground Relocation

4.1 What is a Shunt Fault?

A shunt fault occurs when physical damage to the submarine cable exposes the power conductor to seawater, creating an unintended electrical path to ground. Common causes include anchor strikes, fishing activity, earthquakes, and abrasion against rocky seabeds.

When a shunt fault occurs, the cable is effectively grounded at the fault location, which creates a new reference point in the power feeding circuit.

4.2 Automatic Voltage Adjustment

The key capability that enables continued operation during a shunt fault is the ability of both PFEs to automatically adjust their output voltages. The system operates in constant current mode, meaning the PFEs will adjust their voltages to maintain the specified line current (typically 1.0 A) regardless of changes in the electrical circuit.

When a shunt fault occurs:

1. The fault creates a new ground point at the damage location.
2. Both PFEs detect the change in electrical conditions.
3. The PFEs automatically adjust their output voltages through mutual coordination.
4. The virtual ground relocates to coincide with the fault location.
5. The system continues operating with all repeaters between the stations and the fault remaining powered.

4.3 Voltage Redistribution Example

Consider a 6,000 km cable system with a shunt fault occurring at 4,800 km from Station A (80% of the cable length):

5. Constant Current Operation

5.1 Why Constant Current?

Submarine cable systems operate in constant current mode rather than constant voltage mode for several important reasons:

• Repeater stability: Optical amplifiers require a stable current to maintain consistent amplification characteristics and transmission performance.
• Automatic fault balancing: The system naturally rebalances around shunt faults without requiring external intervention.
• Voltage distribution: Current creates predictable voltage drops along the cable, reducing the maximum voltage stress at any single point.

5.2 Master-Master Current Control

In a double-ended configuration, both PFEs operate in a master-master current control mode. Each PFE continuously monitors the line current and adjusts its output voltage to maintain the specified current value. This coordination occurs automatically, even across thousands of kilometres, ensuring:

• Line current remains at the desired value (typically 1.0 A)
• Virtual ground position adjusts dynamically to circuit conditions
• System operation continues even if one PFE experiences a fault

6. Practical Considerations

6.1 Surge Protection

When a shunt fault occurs, a surge current flows as the electrical charge stored in the cable capacitance discharges to earth. This surge can exceed 100 A, with peak currents subject to the sea ground resistance. Repeaters and branching units incorporate surge protection circuits capable of handling currents exceeding 200 A for long pulses and 450 A/15 kV for short pulses, ensuring the submersible plant is not damaged.

6.2 Single-End Feeding Capability

Modern PFEs with voltages exceeding 15 kV enable single-end feeding capability, where one PFE can power the entire cable system regardless of fault location. This provides significant operational benefits:

• Continued operation during PFE maintenance
• Resilience against PFE failure
• Flexibility in fault accommodation

6.3 Fault Localisation

Once the system has stabilised following a shunt fault, the fault location can be precisely determined. In a double-ended configuration, the fault point is estimated by matching the feeding current at both terminal stations to zero current flowing to the shunt fault point. The distance is calculated using a voltage/current master table that accounts for:

• Cable resistance at sea temperature
• Voltage drops across repeaters and branching units
• Earth ground resistance
• Earth potential difference

7. Virtual Ground in Branched Networks

Modern submarine cable networks often incorporate multiple landing points using power-switched branching units (PSBUs). These networks introduce additional complexity in virtual ground management but also provide greater flexibility for fault accommodation.

PSBUs contain arrays of high-voltage switches that can reconfigure the cable powering architecture in-service. When a shunt fault occurs in a branched network, the PSBU states and PFE voltages can be adjusted so that the virtual ground of each powered segment occurs at the appropriate location, potentially allowing the entire network to remain in service.

The transition between powering states requires careful coordination: PFEs must be adjusted so that voltage levels at all BU ports are within an acceptable switching range before any reconfiguration occurs.

8. Conclusion

Virtual ground is a fundamental concept in submarine cable power systems that enables fault-tolerant operation and system resilience. By understanding how double-ended power feeding creates a dynamic zero-voltage reference point, operators can appreciate the sophisticated engineering that keeps global communications flowing even when cable damage occurs.

As submarine cable systems continue to evolve with higher capacities and more complex network topologies, the virtual ground principle remains central to ensuring reliable, continuous operation of the world's undersea telecommunications infrastructure.