Operating limits of underexcited synchronous generator

Standard power chart of synchronous generator has its limit defined by practical stability limit and by field current minimum in the underexcitation quadrant. However, during the time, some generators have shown certain sensitivity during the permanent underexcited operation even in the stabile operating area due to local heating of magnetic conductive parts in the end region of the machine. Underexcited operation is usually required in case of raised network voltage while the active power demands are lowered. Precise knowledge of limits of underexcited operation ability enables precise planning of system reactive power trading. Some world experiences and axial magnetic field measurements in the end region of synchronous hydro and turbine generator are presented. Based on mentioned measurements, it is possible to define real power chart which almost always deviates in capacitive operating area compared to manufacturer's diagram.