**1. Introduction**

As more large scale photovoltaic power plants (LS-PVPPs) are being installed, the electrical system can face some challenges related to four key areas: (i) active power control, (ii) reactive power control, (iii) voltage support and (iv) frequency support [1]. Thus, many countries have updated their grid codes to permit a smooth interaction between these power plants with the transmission system. For instance, Puerto Rico requires that these PVPPs behaves similar to conventional power plants despite the intermittent conditions [2]. Considering these changes on the grid codes there are two key aspects necessary to approach: active and reactive power control.

According to the grid codes presented by Puerto Rico, Romania, South Africa and Germany, the active power managemen<sup>t</sup> for LS-PVPPs should consider: power curtailment, ramp rate control and active power reserves (Figure 1) [2–5]. Power curtailment, also called as absolute control or limiting control, addresses the reduction of the possible active power that the power plant can generate during the day depending on the grid requirements [6]. This requirement prevents overloading at peak generation hours of PVPPs (around midday) or also when the demand is lower than the possible generated active power from the PVPP. However, due to the intermittency of the solar source, ramp rate control is also necessary to be addressed [7]. The aim of these ramp rates is to smooth the change from low to high solar irradiance and vicevesa, so the change does not affect the voltage or the frequency. As more LS-PVPPs are being introduced at the transmission system, the participation on frequency regulation is a new challenge. Thus, power reserve are already being considered in some of the grid codes. The power reserve is the reduction

of the output power during some hours of the day. This reduction can oscillate between 10 to 20% of the maximum possible that the PVPP can generate [1,8].

**Figure 1.** Active power variation applying the new control functions.

There are two main techniques used to manage the active power: (i) incorporate energy storage, and (ii) new control strategies of the PV generator [1]. In the literature, the main topology proposed is the use of the energy storage together with the PV inverter that are distributed along the PVPP [9–11]. For instance, the study developed by Muller et al, proposes the use of ultracapacitors together with a central inverter to manage the power transients due to the variability of solar irradiance. In this study, the output power fluctuations are reduced and it helps to comply the grid code limits [9]. Commonly, the managemen<sup>t</sup> of the active power relays on the charge of the battery during high solar irradiance and the discharge in high peak demand [12]. This type of control smooths the output power of the single PV generator during the day [11,13–19]. However, the incorporation of energy storage increases the cost of installation and operation of the LS-PVPP [20]. An alternative solution is the improvement of the control by considering the characteristics of the PV generator. Commonly, the active power is managed by the maximum power point tracker (MPPT) which is part of the overall control of the PV inverter. However, the MPPT cannot withstand the power curtailment, the power reserve or the ramp rates. Therefore, this tracker should not only consider the maximum power point but also the reference of active power given by the transmission system operator (TSO). Some studies propose this method for multistring topologies (two stage inverters) used in small applications [8,21,22]. However, thesestudieshavenotbeenappliedforcentralinvertersthatarecommonlyusedinLS-PVPPs[23].

In the case of reactive power, the new grid codes require that the LS-PVPP injects or absorbs reactive power according to a predefined relationship between the active and the reactive power (power factor (*p f*)) or an specific value of reactive power. The grid codes presented by China, Germany, South Africa, Romania, and Puerto Rico requires that the LS-PVPP works under an specific capability curve (Figure 2). From this curve, it can be seen that Puerto Rico has the strictest requirement (*Qmax* = ±0.623 p.u). Meanwhile, China, Germany, Romania and South Africa require a maximum reactive power close to ±0.33 p.u. To comply these grid codes, commonly STATCOMs or capacitors are added at the point of common coupling (PCC), as it is explained in [24]. However, limited research has been developed regarding the reactive power control of PV generators in LS-PVPPs without using extra equipment. For instance, Rakibuzzaman et al. [25] explain the control of reactive power and how the capability curve could influence in the response, but, the variation of ambient conditions is not considered on this approach. Additionally, R. Varma et al. and L. Luo are working on the control of LS-PVPPs as STATCOM to support the grid when power oscillation occurs [26,27]. However, it considers the remaining inverter capacity and depends on the solar irradiance behavior. From a general point of view without any specific source of energy, new types of reactive power control for grid tied inverters have been presented in [28,29]. These studies do not take into account the variation of solar irradiance during the day or the corresponding capability curves of the PV generator. It is worth to point out that the control of reactive power in a LS-PVPP has not commonly been developed considering the capability curves. There are four main parameters that characterize these curves: (i) modulation index, (ii) dc voltage, (iii) solar irradiance, and (iv) ambient temperature, as it is explained in [30,31]. From this research, it can be understood that the variation of the dc voltage and the modulation index can help to have the complete curve despite the variation of ambient conditions. Although, this can reduce the active power generated by the PV generator.

**Figure 2.** Reactive power requirements.

Thus, the objective of this paper is to propose a control of active and reactive power for a PV generator applied in LS-PVPPs for grid code compliance. In this paper, the PV generator has a three phase central inverter (one stage of inversion). To control the active power, two main targets are accomplished: (i) Power curtailment, and (ii) Power reserves, by using an adaptation of the Maximum Power Point Tracker (MPPT). For the reactive power control, two considerations are addressed: (i) preference of active over reactive power and (ii) preference of reactive over active power. For this control, the instant capability curves are considered by the adjustment of the dc voltage and the modulation index depending on the solar irradiance and temperature that affects to the production of active power. To validate this study, a LS-PVPP is modeled and simulated in DIgSILENT PowerFactory under different ambient conditions. The paper is structured as follows: Section 2 explains the configuration and the control structure of a PVPP. The active power control is explained in Section 3, meanwhile the reactive power control is detailed in Section 4. Then, the simulations and the results are presented in Section 5. Finally, the discussion and the conclusions are in Sections 6 and 7 respectively.

## **2. Configuration and Control Structure**

In a LS-PVPP, tens to hundreds of PV generators are interconnected through a collection grid in order to increase the power. ABB, SMA, Danfoss, and First Solar have described some topologies for this distribution as radial, ring and star [23]. The main difference among them is the reliability and the cost [32]. In the current paper, radial configuration is considered as it is the most used topology. In the case of the PV generator, the configuration can be central, string or multistring. The most used configuration is the central one where the PV array is interconnected to a single stage inverter [33,34]. Then, the inverter is connected with a three winding transformer (Figure 3).

**Figure 3.** PV generator in central configuration.

As many PV generators are interconnected in a LS-PVPP, a central controller is necessary. Additionally, each PV generator has to perform its local control. Thus, a hierarchical control architecture is considered, as it is illustrated in (Figure 4), where the first stage is the transmission system operator (TSO) who sends the requirements, then the second stage is the power plant control (PPC) and the third stage is the PV generator's local control.

The control of the LS-PVPP is focused in two main tasks: (i) apply grid support actions, for example in case of disturbances, and (ii) coordinate the control of active and reactive power according to TSO's requirements [11,24]. For the second task, the PPC uses a Proportional-Integral (PI) controller to reduce the error between the reference given by the TSO and the power available in the grid. Then, the total active or reactive power calculated by the controller is divided by the total number of PV generators in the LS-PVPP and this is the reference value under which the PV generators should respond (Figure 5) [24,32]. After these references are calculated, the PV generator develops its corresponding control according to grid code requirements and the behavior of the internal grid to keep ac voltage and frequency constant.

**Figure 4.** Proposed control architecture for a large scale photovoltaic power plant (LS-PVPP).

A general control structure of a PV generator is illustrated in Figure 6. The PV generator has three main tasks: (i) MPPT, (ii) the inverter control, and (iii) the control of active and reactive power. For the first task, the MPPT, the aim is to look for the *vmpp* at each solar irradiance and temperature according to the P-V curves characteristics. To address this, algorithms as perturb and observe, hill climbing, incremental conductance as the more known and others as fuzzy control and swarm optimization have been developed [35,36].

**Figure 5.** Power plant control (**a**) Active power and (**b**) Reactive power.

**Figure 6.** General control of a PV generator.

The second task, it is the one in charge of the general inverter control to interconnect the PV generator with the internal grid of the PVPP. This control performs the grid synchronization, the voltage modulation, the dc voltage regulation and the current loop. The third task, it is in charge of the delivering of the power demanded by the PPC. This control should consider the PQ capability curves of the PV generator analysed in [30] and the variation of ambient conditions as solar irradiance and ambient temperature.

## **3. Active Power Control**

To address the active power control of a PV generator is nesessary to understand the limitations that it has as a system (PV array and the PV inverter). The active power production capability of a PV generator can be presented by a P-G curve (Figure 7). From the figure, four main regions can be

identified. Region I is the starting phase, Region II is the controlling phase, Region III is the clipping phase and Region IV is the shut off phase. To go from one region to other three main points are considered: cut-in, rated and cut out solar irradiance. The solar irradiance at which the PV generator first starts to generate power is named as "cut-in solar irradiance". As the solar irradiance increases, the PV generator starts to work with the MPPT control. When the rated power is reached, the point of solar irradiance is the one named as "rated solar irradiance". Eventhough, the PV array can generate more power due to higher solar irradiance, the inverter limits the generation of active power and loses the ability to track the MPP. Thus, in this region the PV generator waste available PV power. As the solar irradiance increases, the cell temperature also does it. The active power is reduced and the system cannot track any longer the active power. When this point is reached (cut out solar irradiance), the PV generator has to shut off.

**Figure 7.** Control areas of a PV generator in a Power vs Solar irradiance curve (P-G curve).

Taking into consideration this curve, the present section proposes an active power control of a PV generator for power curtailment and active power reserves.
