With this code you can get the central administration webapplication.
1: private static SPWebApplication GetCentralAdministrationWebApplication()
2: {
3: SPWebService cS = SPWebService.ContentService;
4: var service = cS.Farm.Services.GetValue<SPWebService>("WSS_Administration");
5: SPWebApplicationCollection webApplications = service.WebApplications;
6: foreach (SPWebApplication webApplication in webApplications)
7: {
8: return webApplication;
9: }
10: return null;
11: }
If you have a better way, let me know :-)
And here we go. A big Thank you goes to Axel Heer.
return SPAdministrationWebApplication.Local;