**3. Hybrid Propulsion System Characterization**

The hybrid propulsion system under study consists of low speed 2-stroke diesel engine, driving directly the propulsion shaft. A permanent magne<sup>t</sup> synchronous machine is coupled to the diesel engine using a single-stage gearbox. This geared mechanical transmission system enables the combination of the mechanical and electrical prime movers in the same kinematic drivetrain, as shown in Figure 2

**Figure 2.** Hybrid propulsion system configuration.

The synchronous machine is connected to the main ship's grid using a back-to-back power converter. This configuration enables bidirectional power flow, between the electric drive and the ship's grid, thus enabling the electric drive to operate in power take-off (PTO) or power take-in (PTI) modes, depending on the direction of the power flow, as shown in Figure 3. Moreover, the use of the back-to-back converter configuration decouples the electric drive and grid control schemes. This enables the grid side control scheme to be synchronized referred to the ship's main busbar frequency, independently of the electric drive operational frequency, and therefore the diesel engine operational speed [16].

**Figure 3.** Power flow in (**a**) power take-off operation mode; (**b**) power take-in operation mode.

**Power take-off mode:** in this mode, the main diesel engine supplies the power needed for the propulsion *PS* as well as for the ship's consumers *PG* by forcing the electric drive to operate within the generator region. Depending on the ship's load and the required propulsion power, all or some of the generator sets (GS) are turned off, as shown in Figure 3a. The engine developed power *PB* = *PS* + *PG*.

**Power take in mode:** in this mode the electric drive is forced to operate within the motor region, an auxiliary motor, allowing to reduce the main engine's load, as shown in Figure 3b. Depending on the required shaft power, the system operates in booster mode, when both electric and diesel provide deliver power to shaft, or diesel-electric mode, when only the electric drive delivers power. Therefore, the engine developed power *PB* = *PS* − *PG*.
