In the NATO warship, the shipboard electric power system is usually designed to meet STANAG 1008 standard requirements. As in all the standards, STANAG 1008 specifies individual and global harmonic distortion limitations for the grid voltage. But STANAG 1008 standard also imposes the control of an unusual criterion called voltage deviation factor. This paper discusses on the origin, the meaning and the practical interest of this deviation factor. Its definition is mathematically formalized and a procedure is described in order to estimate deviation factor for any time signal. Causes and consequences of high deviation factor are examined regarding high power converter side.