The power system of the target vessel was constructed as a DC microgrid-based distribution system with a DC 1000V level voltage for the performance testing of a heterogeneous EFPS in a marine environment. This topology complies with international maritime regulations and DC distribution standards [15], minimizes the energy conversion stages compared to existing AC systems to increase efficiency, and facilitates the interconnected operation of heterogeneous eco-friendly power sources with different dynamic characteristics [16].

The main bus is composed of two independent main busbars. Main Bus No. 1 is configured with a combination of Main Generator No. 1 (AC) and Fuel Cell (DC), whereas Main Bus No. 2 is configured with a combination of Main Generator No. 2 (AC), Battery (DC), and Test Generator (AC or DC). The dual-fuel generator, battery, and fuel cell are all EFPSs subject to the sea trial and are applied to the main bus in a standalone or hybrid form, depending on the test requirements.

Each power source is connected to the main DC bus through a grid converter. AC/DC converters are used for the main generator (AC output), whereas DC/DC converters are used for the battery and fuel cell (DC output). These converters exhibit bidirectional functionality depending on the nature of the test object. Figure 1 shows a single-line diagram (SLD) of the DC microgrid configuration of the target vessel.

Figure 1: 
Single-line diagram (SLD) of the target-vessel DC microgrid

This system applies an operating strategy that leverages high-load devices already installed on a vessel as substitutes for the test load. The IEMS plays a pivotal role in efficiently managing these dynamic loads and creating the conditions necessary for testing [17].

To verify the macroscopic operational management logic of the IEMS and ensure simulation efficiency, an average-value model is applied instead of a detailed switching model for each grid converter [18]. This model is suitable for verifying the effectiveness of high-level control commands, because it effectively reflects the instantaneous load distribution characteristics of the converter within the second-scale control timeframe of the IEMS.

The IEMS comprises a combination of a computer-based supervisory control and data acquisition (SCADA) system and a programmable logic controller (PLC)-based local control unit, considering the ease of system configuration and maintenance. In particular, core logic such as power source commissioning/isolation and mode transition control, which require real-time data processing and high reliability, is configured on a PLC-based control system. This PLC-based control system is suitable for functional testing and simulator development of the power management system of a ship and can satisfy the high reliability requirements of marine operating environments [19].

The IEMS classifies the operating state into Base Mode (M1), ST Mode (M2), and EFPS Mode (M3) according to the connection status of the switchboard and the operational objective. Table 1 lists the details of the operating state classification.

DC Voltage Control and Test Power Source Commissioning Conditions: The main DC bus voltage is established by the output of the main generator-side grid converter, and the EFPS, which is the test object, operates in a grid-following manner by tracking the main bus voltage. The IEMS explicitly incorporates safety control logic that immediately outputs a stop command for the EFPS based on a grid-converter fault signal, such as a voltage tracking failure, during EFPS operation. Figure 2 shows the overall operating-mode state transition diagram.

Figure 2: 
State transition diagram for the IEMS operating modes

The primary role of the IEMS is to secure the operational stability of the switchboard system of the vessel and establish a safe test environment through the main-generator output limiting management and EFPS load-sharing control. The load profile designed to verify this is not merely a simple load pattern, but also serves as an input signal to trigger the IEMS mode transition (start phase) and safety return (end phase) logic. In this study, the core operating logic was verified by configuring the start and end segments of the profile as follows:

• 1) Initial Mode Transition and Interlock Release Verification (Start Phase): The initial segment of the load profile was designed to verify the transition process from normal operation (M1) through test preparation (M2) to entry into the test mode (M3). Figure 3 illustrates this initial stage, showing a pattern in which the main generator load gradually decreases, reaches, and stabilizes at a minimum fixed load level. The IEMS detects this physical load change, confirms that the essential condition for the M3 transition is met, and executes the logic to release the interlock for EFPS commissioning.
• 2) Verification of Test Termination and Emergency Return Sequence (End Phase): The ability to safely return during a normal test termination or in the event of an emergency is the most critical safety function of an IEMS. Figure 4 illustrates the termination section of the load profile, which simulates abrupt load fluctuation scenarios that occur when the test load is removed or when the EFPS is isolated. We verified that the IEMS immediately isolates the test object (EFPS) and performs a forced mode change to Base Mode (M1), in which the main generator stably takes over the remaining load to restore the system.
• 3) Verification of Physical Implementation Accuracy of the Load Sharing Logic:
• ㆍ The load profile is applied to induce a sudden load fluctuation during EFPS Mode (M3) operation, thereby verifying whether the IEMS outputs the load-limiting command to the converter in a timely manner. Successful execution of this control command is confirmed by observing the load applied to the main bus, where the main generator output is maintained without exceeding the set minimum fixed load.
• ㆍ Ultimately, by confirming that all incremental loads applied through the load profile are accurately shared solely by the EFPS, it is proven that the implementation of the IEMS operating logic precisely achieves the power distribution objective of the switchboard system.

Figure 3: 
Load profile for initial load fluctuation and mode transition sequence

Figure 4: 
Load profile for test termination and emergency return sequence

The load profile designed according to these verification goals was used as the core input data to verify the IEMS control logic in the simulation-based operation verification strategy outlined in Section 4.

The load profile for verification was designed to reflect various load fluctuation scenarios required under the test conditions and served as the core input data for IEMS operational logic decisions. Figure 5 shows a typical load profile for the EFPS Load Test across the entire sequence, including the EFPS-Mode entry process. This profile encompasses the entire sequence, starting from the M1 state, where the main generator load decreases (M2 sequence) to reach the minimum fixed load, followed by EFPS commissioning and a subsequent load increase.

Figure 5: 
Load profile for the entire sequence of the EFPS-Mode load test

This simulation verification focuses on proving whether the IEMS high-level control logic accurately distributes and manages the output between the main power source and the EFPS under load fluctuation conditions, excluding the analysis of voltage fluctuation characteristics. The main verification goals were as follows:

• 1) Mode Transition Condition Detection and Interlock Release Verification (M1 -> M2 -> M3):
• ㆍ Verification Goal: To confirm the precise timing of the operating logic when the IEMS accurately detects that the main generator load has reached the set minimum fixed load level in ST Mode (M2) and subsequently releases the interlock condition for EFPS commissioning. This confirms the capability of the IEMS for load-state monitoring and logical decision-making in the mode transition sequence.
• 2) Verification of EFPS Mode Output Limitation and Load-Sharing Accuracy:
• ㆍ Verification Goal: To confirm whether the main generator output is maintained without fluctuation at the minimum fixed load level through the IEMS load-limiting command during EFPS Mode (M3) operation.
• ㆍ Verification Content: To validate the effectiveness of the power distribution logic, all applied incremental loads were entirely allocated to the EFPS rather than to the main generator.
• 3) Verification of Emergency Situation Sequence and Safety Return Logic:
• ㆍ Verification Goal: To confirm whether the IEMS recognizes a fault signal during testing, the mode specified by the operator is overridden, and an immediate isolation command for the test power source is executed.
• ㆍ Verification of the speed and stability of the protection interlock; the switchboard operating status is forcibly returned to the Base Mode (M1) logic state, thereby blocking the restart of the test power source while the EFPS output swiftly returns to zero.

The simulation was configured primarily around the core of the IEMS control logic with the generator capacity information and initial limit settings specified as parameters. Operator commands for Start, Stop, and Emergency Stop were directly input to the control logic. The bus power requirements and output waveforms of the parallel-connected converters were measured and analyzed. In the waveform plots, the main generator output limit setpoint designated by the IEMS is shown as a red horizontal line, the main generator (Gen.1/2) output as a blue line, and the test power source (EFPS) output as a green line.

The IEMS HMI was utilized to demonstrate the sequential execution of the internal logic by displaying the operating status, mode flags, interlock status, and other related information.

1) Load Stabilization Verification During Mode Transition

• • Waveform Analysis: The main generator output (blue line) precisely reached and stabilized at a minimum fixed load level of 200 kW after decreasing. This signifies that the core condition for M2 mode was successfully achieved, allowing the test power source (green line) to subsequently start and enable entry into M3 mode.

2) Output Limitation and Load-Sharing Accuracy During EFPS Mode Operation

• • Waveform Analysis: Despite the dynamic fluctuation in the load of the test power source (green line) during EFPS Mode operation, the main generator output (blue line) remained constant, closely adhering to the limit value (red line) specified by the IEMS.
• • Discussion/Consideration: This confirms that the IEMS successfully achieved the main generator protection objective, and the accuracy of the load-sharing logic—through which the EFPS takes on all incremental loads—was demonstrated by simulation.

3) Verification of Safe Return Sequence During Normal Shutdown and Emergency Stop

• • Normal Shutdown Analysis: At the moment of termination (Figure 11), it was confirmed that the EFPS output (green line) decreased, and the limit setting value (red line) was released. Simultaneously, the main generator output (blue line) took over the remaining load, resulting in a sequential and safe return to Base Mode.
• • Emergency Stop Analysis: Upon receiving an emergency stop command, the output of the test power source (green line) was immediately isolated to zero, and the limit setting value (red line) was released (Figure 12). This validates the safety protection interlock logic, in which the IEMS forcibly terminates the mode and swiftly returns to Base Mode through the isolation of the test power source operated with high reliability.