Uninvited involvement in postgraduate supervision What is your opinion about another member of staff, uninvited, requiring postgraduate students to discuss their research with him, not even informing the actual supervisor and even requiring / insisting on significant changes? This uninvited intruder has considerably less supervision experience, research experience and in fact lower indices and international accreditation than the supervisor but is the head of department. There is no problem in terms of relationships between the various students and supervisor (in fact those relationships are good - quite possibly excellent). When the topic is raised by the supervisor with the HoD he says it is normal academic practice. He has never indicated that he thinks there is any problem with the current supervision.
My impression from your question is that you are very upset by this. My first advise is not to take any of this personally, and assume that everybody involved means well (even if the execution of those intentions may leave room for improvement). 

The process you describe does not sound uncommon to me, though the execution seems (based on the limited information in your question) awkward. 

In academia advisors have a lot of power over the people they advise. The vast majority of advisors handle that power correctly. But if you hand many people such power, then there will always be some who abuse it. Abuse can take a lot of forms, it can range from not adequately advising the advisee, up to abusing their labor or worse. So it is sound policy to regularly (say once a year) "check up" on all advisees, no matter if there are complaints or not. That way the university can keep an eye on whether the advisees are "on track" and give the advisees a more or less secure possibility to voice other concerns. 

As an well intentioned (or so I say) advisor, I see such a process as a potentially helpful "second set of eyes". However, a lot depends on how such a process is implemented. At the very least, the purpose of this process was not clearly communicated to you, nor the status of his "advise".