Reasons for High Voltage on 35kV Busbar

High voltage on a 35kV busbar is often caused by single-phase-to-ground faults, neutral displacement, ferroresonance, or voltage transformer anomalies, which can lead to overvoltage on the healthy phases.Common Causes1. Single-Phase-to-Ground Faults When a single-phase-to-ground fault occurs on a 35kV busbar, the voltage of the faulted phase drops, while the other two phase-to-ground voltages rise significantly, often by a factor of √3, even though line-to-line voltages remain unchanged. This phenomenon is due to the neutral point displacement caused by the fault, which shifts the voltage distribution across the phases. Voltage transformers (VTs) may indicate dimming on the faulted phase and brightening on the healthy phases, signaling overvoltage conditions on the unaffected phases . 2. Ferroresonance Ferroresonance can occur in lightly loaded or unloaded bus sections, especially when voltage transformers or long cable runs are involved. This nonlinear resonance can cause sudden voltage spikes on the busbar, potentially exceeding normal operating levels. It is often triggered by switching operations or the presence of series capacitance in the system . 3. Phase Loss or VT Fuse Blowouts If a voltage transformer experiences a blown fuse or phase loss, the remaining phases can experience elevated voltages. The imbalance caused by the missing phase leads to overvoltage on the healthy phases, which can be detected by VT indicators and may trigger alarms in SCADA systems . 4. Neutral Point Displacement In systems with grounded or compensated neutrals, any fault or imbalance can shift the neutral potential, causing overvoltage on the non-faulted phases. This is particularly relevant in 35kV auxiliary busbars where the neutral is connected through arc suppression coils or grounding resistors . 5. Protection System and Relay Effects High-impedance bus differential relays and overcurrent protection schemes are designed to detect faults quickly. However, during external faults or CT saturation, the busbar may temporarily experience voltage fluctuations. While relays may not trip immediately, alarms are triggered, and the busbar voltage can rise until the fault is isolated . 6. Human Error and Operational Factors A significant proportion of busbar overvoltage events are caused by operational mistakes, such as incorrect switching, improper isolation, or maintenance errors. These can lead to temporary overvoltage conditions or phase-to-phase imbalances .Mitigation MeasuresBusbar Protection: Implementing unit busbar protection with sensitive ground fault detection ensures rapid isolation of faults, minimizing overvoltage duration .Arc Suppression Coils: Properly rated arc suppression coils help compensate for ground faults and limit voltage rise on healthy phases .Voltage Transformer Maintenance: Regular inspection of VTs and fuses prevents overvoltage due to transformer anomalies .Ferroresonance Prevention: Avoid lightly loaded configurations and use damping resistors or proper switching sequences to reduce ferroresonance risk . Understanding these causes and implementing appropriate protection and operational practices is essential to maintain safe voltage levels on a 35kV busbar and prevent equipment damage or service disruption.