2.4.2. Storage System

The application of the ESS in renewable energy should consider both the own operational state and the generated signals from the grid operator. In general, there is a standardized form of the ESS in an industrial power system. (1) Usually, the ESS connection point provides the point of common coupling (PCC) for renewable energy. (2) To maintain the DC level of the battery, an ESS should apply a distinct DC/DC conversion device. (3) An ESS performs charge/discharge based on imposed limitation (voltage) or a direct order. In case of voltage focused solutions, previous research utilized the specified voltage level as the limit, which was utilized to decide whether the ESS was attached or detached to the grid. Figure 3 describes the concept with a flywheel solution, which employs a super-conducting rotating device [31]. This application relies on a bi-directional controller to control electrical flow and determine whether it charges or discharges by detecting the voltage limitation. In this topology, the controller can strictly manage the voltage levels within the specified range. However, the supplied power from the ESS cannot be defined as a constant value which is able to utilize power balancing. The proposed plan proposes storing and releasing real power with a strict control signal instead of voltage limitation. A bi-directional PCS focuses on designated order to handle entire output power from the DC network.

**Figure 3.** Voltage limitation-based ESS control scheme.

In case of order for ESS, both charging and discharging signals can be represented with the connected PV as follows: 

$$P\_{ESS}(t) = P\_{dc} - \sum P\_{PV}(t) \tag{6}$$

In order to correctly extract the power signal, the proposed ESS controller adopts additional DC analysis blocks to enhance the accuracy of charging/discharging signals.
