NEW YORK--(BUSINESS WIRE)--Stronger cash operating earnings and lower interest costs are likely to sustain improvements in debt service coverage for public power and electric cooperative issuers, Fitch Ratings says. Stronger earnings are likely being driven by higher economic activity, timely rate increases and improved rate design. The sector's low exposure to variable rate debt (9% of total debt for Fitch-rated issuers) should further protect coverage metrics in the event of higher interest r