## 4 USPVPs Control for Applying EL Technique  
### Power Inverter Controller Structure  
As discussed in the previous section, industrial high-power grid-tie converters usually use a single-stage inverter topology in PV applications [48; 49]. A three-level NPC type-I three-phase topology has been selected for the purposes of this study; however, nevertheless, the same techniques could be applied in the same way to other topologies.  
Figure 3 shows a classical DG converter control block scheme. The controller is divided into four layers [50; 51]:  
* **High level controller:** The highest level control is responsible for providing the right active and reactive power set-points to the lower level controllers. Its objective is to maximise the active power generation and to help the utility grid with the reactive power injection.  
Figure 2: Representative power inverter topologies for utility-scale photovoltaic plants (USPVPs) applications: (**a**) two-level three-phase inverter, (**b**) three-level three-phase NPC I-type inverter, (**c**) three-level three-phase neutral point clamped (NPC) T-type inverter, (**d**) three-level three-phase flying capacitor inverter and (**e**) three-level three-phase NPC I-type modified inverter.  
* **Middle level controller:** The middle level controller is tasked to follow the higher level control references. Besides, it is in charge of checking the power inverter limitations due to restrictions (such as temperature and over voltages), and setting up priorities whenever necessary.
* **Low level controller:** The low level controller includes both the instantaneous current controller based on a proportional-integrative-derivative (PID) controller, a direct-quadrature-zero (DQ0) transformation and the modulation control technique, which provides the switching pulse width modulation (PWM) signal to the power converter semiconductors.
* **Hardware level:** Finally, the hardware level is in charge of translating the control signals to the physical pulses of the converter in order to produce the expected voltage signal behaviour by the power converter.  
In particular, the EL control action is performed inside the high level controller layer. Figure 3 highlights in blue the blocks concerning EL functionality. The PV power inverter has a maximum power point tracking (MPPT) algorithm that looks for the optimum working point of the inverter in order to get the maximum output power from the PV panels [52; 53; 54; 55].  
The proposed algorithm introduces a switch that allows to change between the MPPT controller and a specific DC current controller for the EL operation. Therefore, the high-level goal, instead of being the maximum output power generation, is to establish a predefined negative DC current reference, in order to force the PV panels to work as light emitters. Consequently, the controller finds the right DC voltage reference in which the current set-point is acquired by means of, for example, a PID controller. Theoretically, the transition between the EL and the MPPT working modes could be done on the fly; however, it is necessary to stop the device in order to change the combiner box breakers state conveniently [56; 57].