Traditionally, differential protections are used in a substation to protect important plants such as busbars and transformers. They are unit protection schemes which provide good selectivity and fast operating times, but do not provide backup capability. Backup protections are usually provided by overcurrent, under impedance or distance zone 2/zone 3 schemes. They are less discriminative, and because of the need to coordinate, they will need to operate with a relatively long time delay. Digital substation provides a process bus which allows information to be shared between protection devices. This has extended the capability of these devices, such that it is possible to change the traditional backup protection philosophy and to devise new schemes without excessive cost. This paper uses a multi-agent system (MAS) approach to model the interactions between protection devices and to develop new techniques for backup protection. The MAS system is developed using JADE (Java Advanced Development Environment), whilst PSCAD provides simulation for the power system. The boundaries of the unit protection schemes in a substation are extended to provide backup capability, with improved performance as compared to the traditional schemes. Simulations on the 110kV substation are used to illustrate and to validate this approach. Test scenarios with different failure mechanisms are used to show the cooperation and the self-healing properties of the MAS.