So i found out solution myself, how to connect instance of (localdb) from Windows Service which runs as Local System:
I used this articles:
http://technet.microsoft.com/pl-pl/library/hh212961.aspx
http://dba.fyicenter.com/faq/sql_server_2/Verifying_a_Login_Name_with_SQLCMD_Tool.html
http://social.technet.microsoft.com/wiki/contents/articles/4609.troubleshoot-sql-server-2012-express-localdb.aspx
So i shared my localdb (as in 1st article) the problem was I couldn't connect with sqlcmd named pipe np://. I found anwer in 3rd article:
When using sqlcmd, ensure that you are using the SQL Server 2012
version (found in %Program Files%\Microsoft SQL
Server\110\Tools\Binn). If you have previous versions of sqlcmd
installed, calling sqlcmd alone from the command line will most likely
use the old version (which isn't localdb-aware) since the older path
appears first in your PATH environment variable. It may be a good
idea, in general, to manually adjust your PATH environment variable so
that the 110 versions are picked up first.
This small information in fact was crucial;)
so i created user: user with password: pass321!@.
In my windows service my Sql connectionString looks:
"Data Source=(localdb)\\.\\MyInstanceShared;Integrated Security=false;User Id=user;Password=pass321!@"
Integrated security set to false value is also important.
Maybe it will help somebody.