Electricity distribution networks are large complex systems that are continuously evolving. Agent-based models are a useful way of exploring possible future scenarios for these networks. This paper introduces MODAM, a software framework developed to support building large-scale agent-based models for electricity distribution network planning. This paper describes how models can be assembled in an automated manner at runtime, even though an agent may be composed of aspects that come together from separate components. The Module Manager, which weaves the components together in an automated manner, is described in this paper using formal specifications written in Z, and the specification is validated using the ZLive animation tool.