The Gemara here in Chulin says that there are 45 Tzadikim in whose merit the world continues to exist, divided into two groups of 30 and 15 (the Gemara is in doubt whether the 30 are in Eretz Yisrael or in Chutz la'Aretz). When the Midrash (Bereishis Rabah 45) says that there are no less than 30 Tzadikim, it is referring to the group of 30 Tzadikim (either in Eretz Yisrael or in Chutz la'Aretz) mentioned by the Gemara here. The Margolios ha'Yam (Sanhedrin 97b) mentions that the Zohar (Vayera 105b) also says that there are 30 Tzadikim, referring to the 30 in Eretz Yisrael (or in Chutz la'Aretz).

The Margolios ha'Yam also cites the Sefer ha'Bahir (Ma'amar 101) that states that there are 36 Tzadikim in whose merit the world exists, and the Tikunei Zohar (51) says that there are 36 in Chutz la'Aretz and 36 in Eretz Yisrael. It appears that the Gemara and Midrashim are discussing different types of Tzadikim. There are 45 "like Avraham Avinu" and another 72 who are on a lower level, but nevertheless are great Tzadikim. These numbers may correspond to Hash-m's Name. Just as there are different Names of Hash-m, such as one with 45 letters and one with 72 letters, so, too, there are different levels of Tzadikim.

M. Kornfeld