Continuous Change in Sprint backlog during sprint

Are the team happy to accept these ad-hoc requests, and are the previously agreed Sprint Goals still being met?

Team is getting frustrated on this continous change,

But unfortunetly from last 7 to 8 sprints things are not falling well due to this continous adoc request.

I am seeking on how can I convence PO on this.

There are two ways to look at this.

The first is from the "rules of the game" perspective - the Scrum Guide. The Sprint Backlog is owned and maintained by the Development Team. Although the Product Owner may ask for changes based on a changing environment, the Development Team is the one who needs to accept those changes into the Sprint Backlog. Since the work is likely to not be understood and refined by the members of the Development Team, there is some risk in taking on the work. In the event that the Sprint Goal becomes obsolete, the Product Owner does have the ability to cancel the Sprint, and there is guidance for how to handle done and in progress work when a Sprint is cancelled.

The second perspective is to understand why this is happening. Why are so many requests coming in that simply cannot wait until they are refined and scheduled for an upcoming Sprint? There are many reasons that could account for this, but understanding where these requests are coming from, why they are so important, and why they cannot wait for a future Sprint would be the first steps to take to solving the root problems.

To add on to @Thomas Owens' post there is a 3rd way to look at it.  

If the Product Owner is constantly asking for additional work to be added to the Sprint Backlog, the Development Team should meet his request with items that will have to be removed from the Sprint in order to accomodate the request. As you say, the past sprints have not gone well which I am going to assume means that the Development Team is not completing the work that was originally included in the Sprint Backlog.  This evidence supports that the requests for additional unrefined, not well understood work addition is impacting the Development's Team to make and maintain a forecasted body of work that will lead to a potentially releasable increment.  I would be encouraging the Development Team to make this clear by negotiating with the Product Owner.  Often this technique will lead to @Thomas Owens' second perspective.