The student union at the Faculty of Engineering is called TLTH. TLTH has a PhD student section (in Swedish called Dokt or Doktorandsektionen), that is working with improving quality of PhD education at LTH by coordination of PhD student representatives and with mentorship programmes for PhD students (PLUME and MentLife). Furthermore, the PhD student section sometimes arranges social activities for PhD students.
You can find more information about the PhD student section and TLTH on their webpages: http://dokt.tlth.se and http://tlth.se. As a PhD student at the Faculty of Engineering your union is TLTH and therefore not LDK (Lund's Doctoral Student Union), however the two unions cooperate on PhD education issues.