Driver not specified for this database connection
You've already provided the target DB name with the second param to addConnectionInfo(), so you just need to remove the 'default' element from the array, and shift the connection params up:

$old_database = array (
  'database' =&gt; 'grantexpert_registration',
  'username' =&gt; 'grantexpert_usr',
  'password' =&gt; 'xxxxxx',
  'host' =&gt; '62.168.127.59',
  'port' =&gt; '5432',
  'driver' =&gt; 'pgsql',
  'prefix' =&gt; '',
);

Database::addConnectionInfo('olddatabase', 'default', $old_database);
db_set_active('olddatabase');