Calculation of voltage to ground along a three-phase power transmissionline

GROUPS:
 Hi , all ! I hope you may help me. I am trying to solve a (3x3) system of differential equations of second order. It allows me to obtain the voltage to ground for each of the three phase system (with two grounding wires. Thus, original 5 x 5 system becomes in a 3x3 after a matrix reduction) along a power transmission lines.Well, I started by solving it step by step through Laplace transform in relation to x (length) and the result was not good, because both side conductors should have the same voltage (modulus) along line lenght, whereas voltage of the central conductor should be different. However, obtained results were side conductorswith different voltages (which is right), whereas the central one is another value (maybe its ok).Thus , I tried through Laplace with shorter calculations a new family of results(even more incoherent than before) came. At last, I tried to solve it in the time domain and a third family of results came (much more incoherent).I believe I may be failing but I don´t know where and how...I´ve just bought a personal version of Mathematica