What does the 35kV bus voltage depend on

Voltage variation on a 35 kV bus is influenced by load changes, system impedance, and bus design, typically maintained within ±5% of nominal voltage for reliable operation.

Factors Affecting Voltage Variation

Load Fluctuations: Sudden changes in load can cause voltage drops or rises on the bus. For a 35 kV bus, the system must be designed to handle peak currents without exceeding voltage limits, considering the rated continuous current of bus and feeders, which typically ranges from 1250 A to 3150 A. System Impedance: The impedance of transformers, cables, and busbars affects voltage drop.

For example, a 75 MVA transformer at 34. 5 kV produces a rated current of 1. 255 kA, and the distribution among parallel cables influences voltage stability.

Bus Configuration: Metal-enclosed or cable bus systems must be designed to minimize unbalance and voltage drop. Proper spacing, parallel conductor arrangements, and cooling considerations help maintain voltage within acceptable limits. Protection and Switching Devices: Vacuum breakers, relays, and disconnect switches help isolate faults and maintain voltage stability.

Accurate CT/VT metering and protective relays (50/51, 27/59, 46, 49) ensure that voltage deviations are detected and corrected promptly.

Design Considerations

Insulation and BIL Ratings: 35 kV busgear typically has a BIL of 170 kV, ensuring insulation withstands transient overvoltages.

Short-

Circuit Withstand: Busbars must handle short-time currents of 25–40 kA without excessive voltage drop or damage. Cable Bus and Busway: Proper sizing of cables or busbars, including parallel runs and spacing, reduces voltage drop and unbalance. For example, using multiple parallel cables per phase improves ampacity and voltage stability.

Environmental Factors: Altitude, temperature, and pollution degree affect insulation and voltage variation, requiring appropriate creepage and clearance design.

Operational Guidelines

Maintain voltage within ±5% of nominal 35 kV to ensure equipment protection and system reliability. Monitor bus voltage using accurate VT/CT measurements integrated with SCADA systems for real-time control. Ensure proper coordination of surge arresters and grounding to mitigate transient overvoltages.

Regular maintenance of bus connections, vacuum interrupters, and cable terminations reduces voltage variation caused by contact resistance or aging. By considering these factors, a 35 kV bus system can maintain stable voltage under varying load conditions, ensuring safe and reliable operation of connected equipment.