Canada crude-Oil sands upkeep lifts synthetic

* Sept synthetic quoted at $9.50/bbl over WTI

* Maintenance planned at Suncor, Nexen oil sands plants

* WCS quoted at $15/bbl under WTI

CALGARY, Alberta, Aug 15 (Reuters) - Canadian synthetic crude prices extended recent sharp gains on Wednesday as players prepared for tighter supplies due to scheduled maintenance at two oil sands plants.

Light synthetic for September delivery last sold for $9.50 a barrel above benchmark West Texas Intermediate, up $2.50 a barrel from Tuesday to its highest premium since Oct. 28, according to Shorcan Energy Brokers.

Differentials for the crude, derived from the Alberta tar sands, have strengthened steadily since July 30 as some refineries that run it completed turnarounds and on anticipation of outages at Suncor Energy Inc's and Nexen Inc's production facilities, market sources have said.

Suncor plans to take its U2 upgrader down for planned maintenance in September. The company declined to say how long the unit will be down or what the impact on production will be. Suncor's oil sands mining operation in Alberta has a capacity of about 350,000 bpd.

Nexen on Tuesday took the upgrader at its Long Lake oil sands facility down for six weeks of planned work. The operation produced 33,700 barrels a day in the second quarter, less than half its design capacity.

Heavy crude strengthened as well on Wednesday. Western Canada Select heavy blend was quoted at $15 a barrel under WTI, compared with $16.25 under a day earlier.