Google is harder to manipulate compared to other search engines because of various filters it incorporates in its algorithm to prevent spam. Minus 30 penalty is one such filter for handling spam in Google algorithm.
So, what is this minus 30 penalty? If your site was previously ranking high in Google search results, and suddenly its ranking drops by 30 positions, there is a chance that the minus 30 penalty is applied to your site. Normally, if you do a search on your domain name, you would expect to see your site at the first position; but if you see your site at 31st position, it is very likely that the minus 30 penalty is in force.
Most likely candidates to experience this penalty are affiliate sites which do not add much value for the site visitors. However, many non-affiliate sites also have reported this penalty. Though there is no proven theory which can identify sites which are likely to face this penalty, sites with excessive low quality inbound or outbound links and lots of non-unique content may have a minus 30 ranking penalty applied.
The only solution to avoid this penalty is to have unique content on your site, to get links from well trusted sites and to link to high quality sites. For detailed information, please refer to Google’s Webmaster Guidelines.