Your question asks about present or past tense, but your last example appears to be talking more about active or passive voice.
In relation to tense, your writing should make clear whether the things you refer to in your paper are in the past, present or future. The tenses you select must correspond with the facts (the style guides linked below also expand on this).
As for voice, some universities still favour the passive voice - e.g.
use passive verbs to avoid stating the ‘doer’
- Birmingham City University, UK
(Note, though, that the instruction to use passive voice is itself written using the active voice.)
Style guides at other universities now prefer the active voice, reversing this tradition.
Here are some examples:
Writing in the active voice almost always improves the clarity of writing.
- Duke University, USA
Use personal pronouns – I, you, us, we – as though you’re talking one-on-one.
- Monash University, Australia
You could try using: ... This essay discusses the importance of ...
- De Montfort University, UK
If in doubt, consult your institution's and publication's style guides.