Causes of 10kV busbar undervoltage

Undervoltage on a 10kV busbar is typically caused by high load demand, inadequate transformer or feeder capacity, long feeder lengths, and insufficient reactive power support.

High Load Demand

When multiple large loads, such as motors or industrial equipment, are started simultaneously, the inrush current can cause a significant voltage dip on the busbar. For example, starting several Direct-On-Line (DOL) motors can lead to voltage drops exceeding allowable limits due to the heavy initial current draw, which may last several seconds .

Transformer and Feeder Limitations

Undervoltage can occur if the transformers feeding the busbar are undersized relative to the connected load. Similarly, feeders with insufficient current-carrying capacity increase the voltage drop along the line according to Ohm's Law (V = I × R), especially under high load conditions . Long feeder lengths or high-resistance conductors exacerbate this effect.

Reactive Power Deficiency

Reactive power is essential for maintaining voltage levels in AC systems. A lack of reactive power compensation, such as capacitor banks or synchronous condensers, can result in undervoltage, particularly under heavy inductive loads. Optimizing busbar voltage often involves adjusting reactive power resources to improve voltage qualification rates without overcompensating .

Load Configuration and Starting Methods

The method of starting large motors affects busbar voltage. DOL starting causes high inrush currents, whereas soft starters or star-delta starters can reduce the voltage dip. Improper coordination of multiple loads can amplify undervoltage events .

System Topology and Protection Considerations

Busbar configuration, including the number of segments and connected circuits, influences voltage stability. Complex bus arrangements may require dynamic adjustments and proper protection schemes to prevent voltage drops during switching or fault conditions . High-speed busbar protection ensures stability but does not directly prevent undervoltage; however, it can mitigate voltage fluctuations caused by faults.

Summary

In practice, 10kV busbar undervoltage is usually a combination of:

• High simultaneous load demand, especially motor starting currents
• Undersized transformers or feeders
• Long feeder runs or high-resistance lines
• Insufficient reactive power support
• Inadequate load management or starting methods Mitigation strategies include installing appropriately sized transformers, using reactive power compensation, employing soft starters for large motors, and optimizing busbar voltage through system studies and control schemes .