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Under excited Synchronous phase modifier works as
Shunt capacitor
Shunt Reactor
Series capacitor
Series Reactor
Shunt Reactor
A synchronous phase modifier (or synchronous condenser) is a synchronous motor running at no load. When it is under-excited, it draws lagging reactive power from the grid, effectively acting as an inductive load or a shunt reactor to stabilize voltage.
A synchronous phase modifier (or synchronous condenser) is a synchronous motor running at no load. When it is under-excited, it draws lagging reactive power from the grid, effectively acting as an inductive load or a shunt reactor to stabilize voltage.
Q=XsтАЛVtтАЛ(EbтАЛcos╬┤тИТVtтАЛ)тАЛ тАФ Reactive power output equation
EbтАЛ=k╧Х╧Й тАФ Induced back EMF
The excitation of a synchronous machine dictates its reactive power profile. When over-excited, the machine acts as a source of reactive power (like a capacitor). Conversely, when under-excited, the back EMF (EbтАЛ) becomes less than the terminal voltage (VtтАЛ). This condition causes the machine to draw lagging reactive power from the system, behaving as an inductor to suppress over-voltages in EHV transmission lines.
An under-excited synchronous condenser absorbs lagging reactive power.
Used in EHV lines to counteract the Ferranti effect under light load conditions.
Synchronous modifiers allow continuous adjustment of reactive power by varying the field current (IfтАЛ).
Under-excitation means the field current is reduced below the value required for unity power factor.
Continuous control of reactive power.
Capable of absorbing and supplying reactive power as needed.
Improves system transient stability.
High capital cost compared to static capacitors/reactors.
Higher maintenance requirements due to rotating parts.
Adds noise and mechanical losses to the system.
Voltage regulation in long EHV transmission lines.
Grid stability support in large industrial power systems.
The Ferranti effect in long transmission lines causes voltage rise at the receiving end during light loads, which is mitigated by shunt reactors or under-excited synchronous condensers.
Option A (Shunt capacitor) is the behavior during over-excitation.
B is correct тАФ An under-excited synchronous phase modifier acts as a shunt reactor by absorbing lagging reactive power from the power system.
Remember: Over-excited = Capacitor (leading kVAR), Under-excited = Inductor/Reactor (lagging kVAR).