Even though the Quality of Service (QoS) concept is widely known, in most cases the network traffic continues being treated in a classic way, with the Best Effort paradigm. One reason for this point is that QoS implementation is a complex task. However, critical infrastructure networks such as power substation communication networks, require a careful management of the traffic due to their reliability requirements. In this context, Software Defined Networking (SDN) can provide capabilities to guarantee QoS thanks to its ability for programming the network, making easier the QoS deployment. This paper presents an architecture to implement QoS inside a Substation Automation System, governed by the IEC 61850 standard, using SDN. Our testbed integrates an Opendaylight controller, which uses OVSDB and Open Flow southbound protocols to manage OpenvSwitches inside a Mininet framework. Experimental validation shows that our proposal contributes to guarantee demanding requirements of GOOSE traffic in this type of networks.