The successful implementation of Smart Grid depends on the success of the information exchange between devices in the respective systems in a Smart Grid domain. Digital Substation is one of the key systems in Smart Grid domain where information regarding power system network are obtained to execute various functions in a Smart Grid domain. Interoperability, which is the ability of two or more devices to exchange information with each other, is a vital feature for digital substation. To ensure interoperability in a multi-vendor environment for Digital Substation, it is desirable for devices to able to communicate using an open communication standard that is implemented internationally. For power utilities, such as Tenaga Nasional Berhad (TNB), it is imperative to ensure that interoperability between multi-vendor devices is thoroughly tested before the devices are deployed in actual digital substations. This paper will highlight a case study by TNB to verify and validate interoperability between multi-vendor digital substation devices using an open communication standard which is IEC 61850.