*3.5. PCSs*

The current project BESS contains two bidirectional PCSs to perform DC/AC and AC/DC conversions. In addition, it controls voltage and frequency, ensuring that the electricity output meets desired connection requirements.

The BESS uses two bidirectional 500 kW PCSs, connected on the DC side to the power bank battery. The PCSs in question contain anti-islanding protection, in which the inverter detects problems in the electrical grid, such as a power outage, and switches off to interrupt the supply. This protection is needed because, after electrical grid problem occurrences, it is assumed that workers will be dispatched to deal with the issue; therefore, it is necessary that the electric power lines are entirely safe and electric current free.

Another functionality of PCSs is stability for under and over voltage/frequency ranges, in which the inverter does not trip if the anomaly duration exceeds a specific period. This function is an essential feature to improve grid stability.

The PCSs' operating modes are:


The PCSs' operating modes are in four quadrants, as illustrated in Figure 4, both in on-grid and off-grid modes, which means that active power and reactive power can be in four characteristics:


**Figure 4.** Four-quadrant operation of PCSs.

Control System

The PCS installed on the BESS operates in grid-connected and islanded mode. Active and reactive power control (P-Q Control) is used in grid-connected mode, while constant voltage and frequency control (V-F Control) is employed in islanded mode. These two control strategies are based on [22] and detailed below.

(a) P-Q Control

When the PCS is connected to the electrical grid, it operates in P-Q Control mode. Active and reactive power based on instantaneous active and reactive power theory are shown in Equation (1) [22].

$$\begin{cases} p = 1.5(\nu\_d i\_d + \nu\_q i\_q) \\ q = 1.5(\nu\_q i\_d - \nu\_d i\_q) \end{cases} \tag{1}$$

When the *q* component of the voltage is zero and assuming that the voltage vector is in the *d*-axis direction, Equation (1) can be represented by:

$$\begin{cases} p = 1.5\nu\_d i\_d\\ q = -1.5\nu\_d i\_q \end{cases} \tag{2}$$

The reference current can be calculated by:

$$\begin{cases} i\_{drcf} = \frac{P\_{ref}}{1.5\nu\_d} \\ i\_{qrcf} = -\frac{Q\_{ref}}{1.5\nu\_d} \end{cases} \tag{3}$$

where *Pref* represents active power and *Qref* represents reactive power, the expected output values. Figure 5 illustrates the simplified P-Q Control block diagram.

**Figure 5.** Block diagram of the P-Q Control structure [22]. Reprinted from [22] with permission (License 978-1-4799-7720-8/14) from U.S. Department of Energy Office of Scientific and Technical Information [22].
