Thanks, your suggestions works but there is still the problem that small numbers are represented in scientific notation (like 1e-6) in the output. However, I found today by myself another way which gives me exactly what I want:

gtabU0beta = 
 SetAccuracy[
  Table[{NumberForm[nu[j, 1.0], 
     ScientificNotationThreshold -> {-20, 20}], 
    NumberForm[0.0, ScientificNotationThreshold -> {-100, 100}], 
    NumberForm[-1.0/nu[j, 1.0], 
     ScientificNotationThreshold -> {-100, 100}]}, {j, 0, 29999}], 16]

So I use just the number form for my output and set a threshold for scientific notation. This gives me in the export file exactly what I need when exporting the data set as "Table".