The helical distortion of a compact tokamak plasma with currents in modular Furth-Hartman type coils is investigated with respect to the impact on the magnetic field structure and to the ideal magnetohydrodynamic stability. A sequence of configurations, in which the current in the helical coils is increased up to the level of that of the main toroidal coils, is generated. The magnetic field structure acquires a ripple along the field lines when the currents in both sets of coils becomes comparable. The magnitude of this ripple depends on the modulation of the Furth-Hartman coils on their winding surface. The local ideal ballooning stability properties deteriorate with the increasing helical deformation of the plasma shape because the normal magnetic field line curvature becomes larger in regions where the helical contribution enhances the toroidicity component while the local magnetic shear vanishes. The global external kink stability properties, in contrast, improve because the normal curvature on average does not change while the local magnetic shear acquires a noticeable helical modulation.