* UK gas system oversupplied by 4.5 mcm/day
* Wind power expected at 5 GW on Friday
LONDON, Aug 19 (Reuters) - British wholesale gas prices fell on Friday as domestic supplies of gas ramped up while maintenance ended, with higher forecasts of output from the country's wind farms curbing gas demand from power plants.
Gas for immediate delivery was down 1.5 pence at 28.25 pence per therm at 0904 GMT.
Gas for day-ahead delivery was down 0.65 pence at 28.45 p/therm.
With (Other OTC: WWTH - news) supply flows at 166 million cubic metres (mcm) and demand expected to be 161.5 mcm/day, Britain's gas system was 4.5 mcm oversupplied, National Grid (LSE: NG.L - news) data showed.
Gas supplies from the UK Continental Shelf (UKCS) were expected around 112 mcm/day on Friday up around 5 mcm on the previous day.
Annual maintenance at ConocoPhillips (NYSE: COP - news) ' Britannia gas field, which had cut production by around 8 mcm/d, ended on Aug. 18 while maintenance at Apache (NYSE: APA - news) 's SAGE terminal was due to end on Friday.
Wind power output in Britain was expected to peak at around 5 gigawatts (GW) on Friday and 5.6 GW on Saturday, National Grid data showed, up from levels below 2 GW on Thursday.
"(Gas) demand is currently 10 mcm down day on day due to lower CCGT (combined cycle gas turbine) demand following higher wind generation," said Marcel Boonaert, head of gas trading at Wingas UK.
In the Netherlands, the day-ahead price at the TTF hub was down 0.53 euro at 10.90 euros per megawatt-hour.
Flows through the Nord Stream pipeline, between Germany and Russia, begin ramping up on Thursday after annual maintenance ended, boosting gas supplies to Europe which was bearish for European gas prices, traders said.
In Europe's carbon market, the front-year EU allowance price was up 0.07 euro at 4.76 euros per tonne.
(Reporting by Susanna Twidale; editing by Susan Thomas)