在平面直角坐标系\(xOy\)中，过点\(P(-5,a)\)作圆\(x^{2}+y^{2}-2ax+2y-1=0\)的两条切线，切点分别为\(M(x_{1},y_{1})\)，\(N(x_{2},y_{2})\)，且\(\dfrac{y_{2}\mathrm{{-}}y_{1}}{x_{2}\mathrm{{-}}x_{1}}+\dfrac{x_{1}{+}x_{2}\mathrm{{-}}2}{y_{1}{+}y_{2}}=0\)，则实数\(a\)的值为____\(.\)