* Eskom says some power plants operating below capacity
* Unions have threatened 'total shutdown'
* Company produces more than 90 pct of South Africa's power
* President oversaw changes at Eskom to clean up governance (Adds Gordhan quote)
JOHANNESBURG, June 14 (Reuters) - About 1,000 union members picketed Eskom's headquarters on Thursday in a wage dispute that threatens the cash-strapped South African utility's ability to deliver power.
There have been no reports of outages in Africa's most industrialised economy but Eskom said some plants had cut output, with shortages more likely now during peak winter demand.
Labour unions have threatened a total shutdown of Eskom's operations unless it meets their demands for a 15 percent increase in salaries. Eskom wants to keep salaries flat.
The pickets come a day after protests at about 10 Eskom power plants forced the utility to switch off some generating units because trucks carrying coal and buses ferrying staff were blocked.
Eskom, which produces more than 90 percent of South Africa's power, narrowly avoided a liquidity crunch early this year and was embroiled in corruption scandals.
The picketing workers demanded Eskom Chief Executive Phakamani Hadebe address them, and about 12 police cars were stationed outside the company's Megawatt Park headquarters to protect Eskom staff and equipment.
The National Union of Mineworkers (NUM) and National Union of Metalworkers of South Africa (NUMSA), who say they represent more than 20,000 of Eskom's 47,000 employees, said that thousands of their members would picket at Megawatt Park.
Thabiso Masha, an Eskom employee who works in the research department in Germiston, said: "Zero percent is nonsense, we won't stand for it. Petrol is going up, VAT is going up, so our pay is decreasing".
Another employee, who works in Eskom's distribution operation in Johannesburg but asked not to be named, said: "It's not the workers' fault that the company is suffering because of corruption. We are not the root cause ... They are preparing for job cuts."
Two workers held up a placard emblazoned with "0% equals 0 Megawatt". Another read: "To hell austerity measures. Workers cannot pay for sins of management."
Eskom spokesman Khulu Phasiwe said that some power stations were not generating at full capacity because of "acts of sabotage and intimidation".
"There have been several incidents of road blockades, attacks on staff and wilful damage of electricity infrastructure. As a result, all road coal deliveries have been stopped for security reasons," Eskom said in a statement.
Public Enterprises Minister Pravin Gordhan, who oversees Eskom, said the blockages could lead to power shortages.
"I didnt anticipate the blockages, we might end up in a situation where through lack of coal supply and not because of poor operations, there might be a deficit in terms of electricity supply," he said at a conference in Johannesburg.
Stabilising Eskom's finances is a priority for President Cyril Ramaphosa as he looks to rekindle growth after nine years of stagnation under his predecessor, Zuma.
Ramaphosa oversaw the appointment of a new board and chief executive at Eskom in an attempt to clean up governance and set the company on a firmer financial footing.
Separately, South Africa's energy regulator NERSA gave Eskom the go-ahead to recover 32.69 billion rand ($2.5 billion) of costs incurred over the past three years through higher tariffs, about half what the utility had requested.
Eskom said it would study the decision before commenting. ($1 = 13.1788 rand) (Additional reporting by Olivia Kumwenda-Mtambo Editing by James Macharia and Jon Boyle)