SHANGHAI/SINGAPORE, May 15 (Reuters) - China's central bank left a key policy rate unchanged when rolling over maturing medium-term loans on Wednesday, in line with market expectations.

The People's Bank of China (PBOC) said it was keeping the rate on 125 billion yuan ($17.28 billion) in one-year medium-term lending facility (MLF) loans to some financial institutions unchanged at 2.50% from the previous operation.

In a Reuters poll of 32 market watchers, 27, or 84%, of all respondents expected the PBOC to leave the interest rate on MLF rate unchanged.

With 125 billion yuan in MLF loans set to expire this month, the operation resulted in zero fresh fund injection or withdrawal from the banking system.

The central bank also injected 2 billion yuan through seven-day reverse repos while keeping borrowing cost unchanged at 1.80%, it said in an online statement.

($1 = 7.2330 Chinese yuan)

(Reporting by Winni Zhou and Tom Westbrook; Editing by Tom Hogue)