Using DBCA to add 3rd instance on standby database failed

Problem description:
Try to use DBCA to add 3rd instance on a standby database, but it faile


Cause of problem:
While adding 3rd instance on primary database, it created undo,redo, thread information for 3rd instance and the change is propagated to standby.

select inst_id, thread#, status, enabled, instance from gv$thread;
select group#, thread#, sequence#,status from gv$log;

Solution:

1. copy init*** and orapw*** to 3rd node

2. Modify the spfile:
alter system set thread=3 scope=spfile sid='xxxx'; (sid is for third instance)
alter system set instance_number=3 scope=spfile sid='xxxx';
alter system set undo_tablespace=xxxx scope=spfile sid='xxxx';
alter system set cluster_database_instances=3 scope=spfile

3. Bounce the standy database:

4. Use Sevctl to add 3rd instance and start it.