3.2.2. Energy Storage System (ESS)

ESS refers to a small/medium-sized electrical storage facility that is to store electrical energy and use it when necessary in aids of a distributed power source in a micro-grid. As shown in Figure 6, ESS comprises the battery and power conditioning system (PCS).

**Figure 6.** Configuration diagram energy storage system (ESS).

A lead-acid type battery was applied to the test bed. According to Table 2, it was modelled based on the two-way grid connected type.


**Table 2.** Specification of ESS.

The PCS is designed to perform bidirectional power control for DC power and AC power between the grid and the rechargeable battery, to improve the reliability of the power system, and to supply the stored energy quickly at peak power demand.

Therefore, The PCS is to balance active powers from hybrid power systems when the load fluctuation, accidental power supply, or load drop occurs by means of charging or discharging the battery and to contribute to enhancing system stabilization by adjusting the frequency. In addition, it has a functionality to monitor the state of charge (SOC) of the battery in real time and to control the temperature, current and voltage so that it can make the system to be operated with high reliability. It also provides surge protection, automatic overcharge / overload protection as well as overvoltage protection [29,30].

#### 3.2.3. Diesel Generator System

A 50 kW diesel generator used in the test bed is a revolving-field type using a permanent magnet. This system is soundproofed to 75 db or less. As shown in Figure 7, and Tables 3 and 4, it has the synchronous speed of 1800 rpm with four poles.

**Figure 7.** Appearance of diesel generator.


**Table 3.** Major specification of diesel generator engine.

**Table 4.** Major specification of diesel generator.


3.2.4. Load Bank

The load bank is a device designed to provide an electrical load for testing power sources, such as generators or uninterruptible power sources. In the case of a load bank used in a test bed, a power line is constructed for load testing, and the current load factor is calculated in the EMS and adjusts the load output from the load bank. The consumption of the voltage and current during this operation was monitored by an analog signal.

As shown in Table 5, the 300 kW load bank adopted the second type of iron chrome (FCHW-2) with high resistivity and low resistance against temperature increase and used a forced air-cooled load bank which was connected in parallel so that the load capacity could be adjusted.



#### 3.2.5. Energy Management System

EMS is a control system configured to monitor the voltage, current, output amount, and system status of each device in real time to operate the system stable. It is to balance the load of the load bank in real time according to the load variations so that each device can be synchronized properly [31–35].

The EMS and each power source—namely MCFC, diesel generator and ESS—are configured to send and receive status and operation commands of the device via the interface as presented in Figure 8.

**Figure 8.** Integrated power control system configuration.

The power source (fuel cell, battery, diesel generator) constituting the hybrid system was controlled by EMS, according to the output range. In the output range of 0−100 kW, the fuel cell produces an output of 100 kW as the base load. When the required load is less than 100 kW, the battery is set to be charged, or the energy is sent to load leveler.

In addition, in the 100−130 kW output period, the battery power is additionally supplied to the load with fuel cell power, and the diesel generator is controlled to operate in the output period of 130 kW or more. When the proposed hybrid system was operated on the basis of the above conditions, the power quality was analyzed by measuring the voltage variation rate and frequency variation rate for each scenario.
