Power transmission system of hybrid electric vehicle

A power transmission system of a hybrid electric vehicle, comprising:

• an input shaft into which power of an engine is input;
• an output shaft outputting transmitted rotational power through an output gear;
• a first planetary gear set having three rotatable elements having a first sun gear, a first planetary carrier, and a first ring gear;
• a second planetary gear set having three rotatable elements having a second sun gear, a second planetary carrier, and a second ring gear;
• a third planetary gear set having three rotatable elements having a third sun gear, a third planetary carrier, and a third ring gear;
• a first shaft directly connected with a first motor/generator while directly connecting one rotatable element among the rotatable elements of the first planetary gear set and one rotatable element among the rotatable elements of the second planetary gear set;
• a second shaft directly connected with the output shaft while directly connecting another rotatable element of the first planetary gear set and one rotatable element among the rotatable elements of the third planetary gear set;
• a third shaft directly connecting a remaining rotatable element of the first planetary gear set and the input shaft;
• a fourth shaft directly connecting another rotatable element of the second planetary gear set with a transmission housing;
• a fifth shaft directly connected to a remaining rotatable element of the second planetary gear set;
• a sixth shaft directly connecting another rotatable element of the third planetary gear set and a second motor/generator;
• and
• a seventh shaft selectively connectable with the transmission housing while selectively connecting the one rotatable element of the third planetary gear set shaft.