in three phase circuit calculation of line-line voltage can be from
Vry = 2 * Vphase voltage COS(30);
Vry = √3 * Vphase voltage;
sometimes phase voltage of Vr and Vy can be different level, when how do we calculate Line-Line voltage?
line-to-line voltage is nothing just a vector subtraction of two phase voltage vector. it depends on the angle between vectors and their magnitudes.
the indicated formulas are for balanced system. you can obtain any other case just by a simple vector subtraction.
Different phase voltage level is caused by asymmetrical load. In this case the phase angle between phases can differ from nominal value of 120 degree too. It's not possible then to calculate the phase-to-phase voltage without knowing the actual phase angle.