Control of Power Flow

Power flow control uses several key components: generator prime mover and excitation control, shunt capacitor banks and reactors, static var systems, and regulating transformers. Prime mover control adjusts real power generation, while excitation control manages reactive power. Shunt devices compensate for reactive power demand, and transformers regulate voltages and power angles to maintain system stability and efficiency.

A generator under steady-state conditions is represented by its Thévenin equivalent circuit, which includes terminal voltage, excitation voltage, power angle, and synchronous reactance. This model allows calculation of the generator's current, real power, and reactive power. The power angle directly influences real power output, while excitation voltage primarily controls reactive power generation in the system.

Increasing the power angle boosts real power output from the generator. However, this adjustment simultaneously decreases reactive power generation. This trade-off is fundamental to generator control: operators must balance real and reactive power requirements by carefully adjusting the power angle to meet system demands while maintaining voltage stability.

Shunt capacitor banks increase bus voltage magnitude by compensating for reactive power demand, while shunt reactors decrease it. Adding a capacitor absorbs negative reactive power, reducing overall reactive power demand. These devices provide localized voltage support and help maintain desired voltage profiles across the power system, improving stability and efficiency.

Tap-changing and voltage-regulating transformers control bus voltages and reactive power flows by adjusting the turns ratio. Phase-angle regulating transformers control bus angles and real power flows instead. Both types alter the transformer's turns ratio, which the power flow program evaluates to determine system impacts and ensure the maximum power flow and line loadability remain within safe operating limits.

Raising excitation voltage increases reactive power generation and slightly decreases the power angle. This adjustment allows operators to boost reactive power support to the system while maintaining real power output relatively stable. Excitation control is essential for voltage regulation and reactive power compensation in steady-state power system operation.

Power flow studies use trial and error to adjust generation levels and control settings, evaluating effects of switching lines, transformers, loads, and generators. These simulations ensure the system meets desired equipment loadings and voltage profiles while accommodating load growth, new transmission lines, transformers, and generation additions, maintaining stability and reliability.