The invention relates to a modular oil trench (15) for a power generation unit (1) of a power plant, comprising at least one trench module (16) having an essentially U-shaped cross-section defined by two side walls (26, 27) and a bottom wall (28) adapted for forming at least a part of the a trench (18, 19) and providing a front face (29) and/or a rear face (30) for connecting the at least one trench module (16) to another trench module (16) in an oil-proof and fire-proof manner. Furthermore, the invention relates to a Power generation assembly (0, 0') for a power plant, comprising at least one trench supporting element (17) adapted to support at least one trench module (16). Finally, the present invention relates to method of assembling a power generation assembly (0) a power plant with a table (2) for supporting a power generation unit (1) of the power generation assembly (0), wherein a trench system (14) for the power generation unit is formed of at least one pre-fabricated trench module (16) mounted to the table (2).