Here is how to create a Scheduled Refresh to an Database that does not accept domain credentials in PowerPivot:Create 2 SSS IDs:
a. Create a SSS application using Group leaving application Page URL to none.
b. Utilize the Windows Username/Password template.
c. Setup members that need to access this SSS application ID.
d. Then set credentials with domain windows credentials.
b. Utilize Username/Password for example: (“Oracle Username” “Oracle Password”) template.
d. Then set credentials with Oracle credentials.
On the PowerPivot "Configure Service Application Settings" page, set the "PowerPivot Unattended Data Refresh Account" to PowerPivotDataRefresh.
Browse to the PowerPivot Gallery > Locate the workbook you want to Schedule a Data Refresh for > Click “Manage Data Refresh”:
Under “Data Refresh” tick “Enable”
Under “Schedule Details” tick “Also refresh as soon as possible”
Under Credentials > Select "Use the data refresh account configured by the administrator"
Unselect "All Data Sources"
(In this case) Select "Custom"
Under "Data Source Schedule:" > Select "Use Default Schedule"
Under "Data Source Credentials:" > Select "Connect using the credentials saved in the Secure Store Service (SSS) to log on to the data source. Enter the ID used to look up the credentials in the SSS ID box.”