To address the frequency-security risks resulting from source-load fluctuations and fault impacts

this paper proposes methods for assessing the power-imbalance situation

optimizing and controlling the operation mode

and hierarchical defense against security risks in the new-type power system. First

the main manifestations of power-imbalance risks in the new-type power system are analyzed. The safe-operation boundaries are defined based on six safety-constraint indicators related to power-grid frequency fluctuations. Next

hierarchical assessment indicators for power-grid frequency security risks are proposed

and a power-grid security-situation assessment model is established accordingly. Then

considering load characteristics

fault effects

and energy storage resources

an optimization adjustment model for the day-ahead operation mode is established with the objective of minimizing the power output deviation. Finally

considering the under frequency load shedding control

a hierarchical control model for power-grid security defense is established for scenarios where source-load fluctuations exceed expectations. The calculation results of the improved IEEE 39-bus system case demonstrate that the proposed methods can effectively evaluate the power- imbalance risk situation of the power grid

rectify the safe-operation domain of the power grid

and achieve proactive defense against power-grid security risks.