The first one is good. The second one needs a conjunction like "so".

Hello and good morning Hisham. First to answer your question, no Having cannot replace "I had" in every situation in the Past Perfect tense. It cannot replace it at all because they are two totally different tenses. As far as the Past Perfect tense goes its form is as such: Subject + auxiliary 'Have' (in the past tense 'Had') + main verb. So the form is: Subject + Had + main verb The first sentence you wrote: "Having Received a letter, I travelled to London. is actually in the Present Perfect tense and not the Past Perfect tense. This is because of the usage of the present participle "Have" + '-ing'. The Present Perfect tense form is: Subject + auxiliary "Have" (in the present simple tense) + main verb (in the past tense) so the form is Subject + Have (-ing) + past tense main verb The second sentence you wrote: "I had received a letter, I travelled to London." is in the Past Perfect tense but not grammatically correct. It can be written in a number of ways like so: "I had received a letter, after which, I travelled to London." or "I travelled to London after I had received a letter." or "After I had received a letter, I travelled to London." The words 'After' and 'which' is used to denote the sequence of time to correlate the two events together. These are just some ideas on how to rewrite the sentence using the Past Perfect tense. I hope this helps clarify your question and how to use the Past Perfect and what the Past and Present Perfect tenses are.