Aug 10 - Fitch Ratings has assigned a 'BBB-' rating to Williams Partners L.P.'s (WPZ) $750 million 3.35% senior notes due 2022 (senior notes). Senior note proceeds will be used to repay short-term debt and for general partnership purposes. The Rating Outlook is Positive. Key Rating Factors: WPZ's rating reflects the scale and scope of its operations, the predictability of cash flow generated by its pipelines and fee-based midstream operations, and its conservative financial practices. Also cons