The integration of Information and Communication Technologies in the power grid has given rise to new applications such as Advanced Metering Infrastructure. However, these technologies have also exposed the grid to new forms of security attacks. It is challenging to analyse and find solutions for these attacks due to the complexity of the grid system. There is a need of simple mechanisms to model and study the security implications of new power applications. This paper introduces a simulation model to evaluate the impact of security attacks on the Advanced Metering Infrastructure of a microgrid. The model is evaluated by demonstrating three security attack scenarios which lead to theft of power theft, privacy loss, and power outage.