35kV busbar overvoltage start-up

Overvoltage on a 35 kV busbar during start-up is primarily caused by switching transients, especially when energizing shunt reactors or empty bus conditions, and can be mitigated through proper circuit breaker operation and reactive power management.

Causes of Overvoltage

1. Switching Transients from Shunt Reactors When a 35 kV busbar is energized or a shunt reactor is switched in, vacuum circuit breakers can generate high-frequency overvoltages due to current chopping and arc reignition phenomena. The strong arc-extinguishing ability of vacuum breakers can cause the current to be interrupted near zero, leading to electromagnetic oscillations between the inductive and capacitive elements of the system, producing overvoltage spikes .
2. Empty Bus Conditions Overvoltages are more pronounced when the busbar is initially unloaded. In such cases, the absence of load damping allows the transient voltage to reach higher amplitudes, increasing the risk of insulation stress and equipment damage .
3. System Stiffness and Resonance A stiff network with low impedance can exacerbate overvoltage during start-up. The interaction between bus capacitance and reactor inductance can create resonant oscillations, further amplifying voltage peaks .

Protection and Mitigation Strategies

1. Bus Protection Relays High-speed differential protection or percentage differential relays can detect abnormal currents and isolate the bus quickly, reducing the impact of overvoltage events . Overcurrent-based interlocking schemes are also used for distribution busbars, balancing speed and security.
2. Shunt Reactor Switching Control Gradual energization of shunt reactors, or using pre-insertion resistors, can limit the magnitude of switching overvoltages. Simulation studies using EMTP/ATP have shown that controlling the reignition behavior of vacuum circuit breakers significantly reduces peak voltages .
3. Reactive Power Compensation Ensuring adequate reactive power in the network before start-up can reduce voltage overshoot. Shunt reactors or capacitors should be switched in coordination with voltage monitoring to maintain system stability .
4. Equipment Design Considerations Proper insulation coordination, surge arresters, and busbar layout optimization help absorb transient energy and prevent insulation breakdown during start-up .

Practical Recommendations

• Conduct simulation studies of busbar energization to predict overvoltage levels.
• Use high-speed bus differential relays for critical 35 kV busbars to quickly isolate faults.
• Implement controlled switching for shunt reactors and consider pre-insertion resistors to reduce transient peaks.
• Maintain adequate reactive power and monitor voltage levels during start-up to prevent excessive overvoltage. By combining proper protection schemes, controlled switching, and reactive power management, the risk of damaging overvoltage during 35 kV busbar start-up can be effectively minimized.