Cyber threats are serious concerns for power systems. For example, hackers may attack power control systems via interconnected enterprise networks. This paper proposes a risk assessment framework to enhance the resilience of power systems against cyber attacks. The duality element relative fuzzy evaluation method is employed to evaluate identified security vulnerabilities within cyber systems of power systems quantitatively. The attack graph is used to identify possible intrusion scenarios that exploit multiple vulnerabilities. An intrusion response system (IRS) is developed to monitor the impact of intrusion scenarios on power system dynamics in real time. IRS calculates the conditional Lyapunov exponents (CLEs) on line based on the phasor measurement unit data. Power system stability is predicted through the values of CLEs. Control actions based on CLEs will be suggested if power system instability is likely to happen. A generic wind farm control system is used for case study. The effectiveness of IRS is illustrated with the IEEE 39 bus system model.