A reactor at a nuclear power station in Ukraine has been disconnected from the grid following an electrical failure but radiation levels were not elevated, authorities said on Tuesday.

The second reactor of the Yuzhno-Ukrainskaya Nuclear Power Station in the south of Ukraine was put on minimal capacity following the failure of its main transformer and the subsequent breakage of the high voltage power line late Monday, the emergencies ministry said.

"Reactor No 2 has been switched to the minimum capacity and unplugged from Ukraine's energy grid," the emergencies ministry said in a statement on its website.

"Radiation and fire safety levels are normal," it said, adding that the nuclear power station's employees were taking steps to bring the situation under control.

Repair work at the station was continuing Tuesday afternoon.

"Specialists are looking at what exactly happened. Equipment is being changed," plant spokeswoman Vlada Tishkova told AFP by phone.

According to the current plan, the reactor will go back online Thursday.

"Everything is okay. The level of radiation is what we always have," Tishkova said.

The nuclear plant operator said in a separate statement that scheduled repair work was also under way at the third reactor. Its first reactor was functioning in a normal mode.

Ukraine is home to the now defunct Chernobyl nuclear plant whose fourth reactor exploded in April 1986 with fallout hitting the three Soviet republics along with a large part of Europe.

The Chernobyl nuclear plant is located about 100 kilometres (60 miles) north of Kiev and close to the borders with Russia and Belarus. The explosion remains the world's worst nuclear disaster.