In an ungrounded system, what is the return path for the capacitive coupling between the high voltage line and the earth to maintain the flow?

In an ungrounded (or isolated grounding) power system, there indeed exists capacitive coupling between high-voltage lines and the earth, forming a capacitive current path. This phenomenon occurs because the electric field between the high-voltage conductors and their surrounding environment, including the ground, causes uneven charge distribution, resulting in what is known as "earth capacitance."

Return Path Through Capacitive Coupling

1. Earth Capacitance: Each high-voltage line has a certain amount of earth capacitance relative to the ground. These capacitances exist between the conductor and the ground as well as between different conductors. Even without direct electrical connections, capacitive currents arise due to the presence of the electric field.

2. Capacitive Current Flow: In an ungrounded system, if a single-phase-to-ground fault occurs, the current does not flow directly back to the source through the earth as it would in a grounded system. Instead, it returns to the source via the mutual inductance and capacitance between lines, transformer winding capacitance, and other distributed parameter elements, forming a complete circuit. This process mainly relies on the capacitances between the three-phase conductors and between the conductors and the ground to provide a complete current loop.

3. System Behavior: In an ungrounded system, when a single-phase-to-ground fault occurs, the fault current is typically very small due to the lack of an effective low-impedance grounding path. The current is primarily a displacement current caused by the aforementioned capacitances. This makes such faults less noticeable initially but could lead to further insulation degradation over time and potentially more severe faults if left unresolved.

4. Protection Measures: To detect such faults, specialized monitoring equipment, such as ground fault indicators or highly sensitive relay protection devices, are often installed to identify and locate the fault. Additionally, some designs may use a neutral point grounded through an arc suppression coil to limit the fault current during a single-phase-to-ground fault.

In summary, in an ungrounded system, the return path that allows capacitive coupling between high-voltage lines and the earth to maintain current flow is primarily achieved through the capacitances between the conductors and between the conductors and the ground. While this design can reduce certain types of short-circuit currents, it also requires careful monitoring for potential ground faults to ensure timely intervention.