Relationship between the Angels and the Holy Spirit
In the New Testament the angels are deeply involved in some manner or another in the economy of grace. Since grace is a divine effect which is related in a certain way to the divine missions, it happens that the angels are associated with the missions of the Son and of the Holy Spirit. This intimacy with the Holy Spirit is so profound that the angels, as appropriated instruments of the Holy Spirit, can be introduced in His lieu into formula of the NT which were otherwise  Trinitarian. For instance:
I charge you before God and Christ Jesus and the elect angels, that you observe these things impartially. (1 Tim 5,21)
For whoever is ashamed of Me and My words, of him will the Son of Man be ashamed when He comes in His glory and that of the Father and of the holy angels (Lk 9,26)207
The association of the angels with the SPIRIT is not only fostered by the fact that the former are also called spirits, –”Are they not all ministering spirits, sent for service, for the sake of those who shall inherit salvation?” (Heb 1,14) –, but that also in their mode of apparition and operation they again coincide. The Pentecostal promise fulfills itself with a rush of wind and tongues of fire (Acts 2,2-4). Recall, that concerning the angels it is written: “He makes his angels winds (spirits) “ and his servants flames of fire” (Heb 1,7; cf. Ps 103,4). By the Spirit of God, Christ casts out demons (Mt 12,28), and Michael, together with his angels, casts the dragon and all the rebellious angels out of heaven (Apoc 12,7-9). The Spirit is the principle of prophecy (Lk 1,41.67; 2,25f; Acts 7,55), while the communication of prophecy pertains to the principal mission of the angels.208 St. Stephen is full of the Spirit (Acts 6,3.11), and so it is that his face appears as the face of an angel (Acts 6,15).
Going further, it is an angel of the Lord who sends Philip out onto the Gaza road leading out desert wards (Acts 8,26) where he beholds an Ethiopian minister riding along. Now it is no longer the angel but the Spirit Who directs him to approach and instruct the man (Acts 8,29). It is not the result of a confusion which leads Luke to refer first to the angel and then to the Spirit. The relation between the Spirit and the angel is that between master and minister, between agent and instrument. Thus Luke is free to refer to either depending upon the point of reference. Theologically, however, it is important to refer to the absolute authority of the Spirit, for the full initiation of a eunuch is contrary to all Hebraic custom. Similar is the motive in the case of Cornelius the centurion. Whereas he receives a vision from an angel (Acts 10,3ff), Peter receives his instruction from the Spirit (Acts 10,19). In the one instance it is a question of the agent, in the other, a question of the divine authority, a point which Peter is at pains to make while justifying himself before the community of Jerusalem for having taken pagans into  the Church (Acts 11,12.15-18).
(Fr. Wagner- The Mission of the Holy Angels in the Economy of Salvation)