6.1.1. The procedure for ESS Synchronization

ESS system voltage is followed by priority, and when the system voltage is transmitted to the power conversion device (PCS) of the ESS via the sensor, the voltage is controlled by following the system voltage.

When the synchronous signal is transmitted from the EMS to the PCS, the PCS follows the system frequency. The PCS adjusts the frequency so that the system can be stably maintained. In this case, the PCS controls the system using the current and perform charging and discharging actions in accordance with the pre-setting power outputs.

The ESS synchronization time would be approximately 0.5 seconds during which PCS turns on, and EMS generates a synchronization signal in the normal operation state in which charging and discharging are enabled.

#### 6.1.2. Procedure of Diesel Generator

When the synchronization signal is produced to the EMS in normal operation, the diesel generator adjusts the voltage based on the grid voltage and synchronizes with the system frequency. The synchronization controller of the diesel generator plays a role in maintaining the system stable by adjusting the frequency. At this time, the amount of electric power in the form of current control mode is limited, so that more current of electric power than the pre-set can be prevented from flowing to the system.

Once the EMS synchronization signal is generated in the normal operation state of the diesel generator, it takes about 3−4 seconds to synchronize to the system.

#### *6.2. Power Measurement Point and Selection Criteria When Synchronizing Hybrid Power Sources*

The effect of voltage and frequency on the system was investigated by measuring the synchronization time between the ship's combined power source in two times: Two seconds before synchronization and five seconds after synchronization. The measuring time was established based on Korean Classification Rule. Therefore, the built-in LabVIEW-based power analysis system for test bed experiment can store one power measurement data every 33.3 ms.

The ESS was configured to measure two seconds before synchronization and five seconds after synchronization in consideration of the short synchronization time; it could be confirmed through data that ESS took 7−10 seconds depending on the system condition when synchronizing.

The synchronization time of the diesel generator was designed to measure two seconds before synchronization and five seconds after synchronization in the same manner as ESS. However, in case of diesel generator, it took about 10−12 seconds because the synchronization time usually took 3−5 seconds, which was confirmed through the results.
