Proposal for an aggregated solar PV power plant simulation model for grid code compliance

Electric Power Systems Research 213 (2022) 108676 Available online 4 August 2022 0378-7796/© 2022 Published by Elsevier B.V. Proposal for an aggregated solar PV power plant simulation model for grid code compliance Miguel Martínez-Lavín a , Raquel Villena-Ruiz b , d , Andr´ es Honrubia-Escribano b , Jesús C. Hern´ andez c , * , Emilio G´ omez-L´ azaro b a Certification Entity for Renewable Energies (CERE), Getafe (Madrid), 28906 Spain b Renewable Energy Research Institute and DIEEAC-ETSII-AB, Universidad de Castilla-La Mancha, Albacete, 02071 Spain c Department of Electrical Engineering, Universidad de J´ aen, J´ aen, 23071 Spain d Department of Automation, Electrical Engineering and Electronic Technology, Universidad Polit´ ecnica de Cartagena, Cartagena, 30202 Spain ARTICLE INFO Keywords: Grid code Solar PV power Simulation model Spanish NTS ABSTRACT Renewable energy and, in particular, solar PV power, will play a crucial role in achieving net-zero emissions scenarios over the coming decades. Increasing the share of renewables in countries’ energy mixes is therefore a major challenge that must be urgently addressed. Within this context, the present work proposes an innovative tool that may accelerate the commissioning process of new solar PV power plants, in the form of an aggregated simulation model to assess a power plant’s responses under the technical requirements established in the Spanish grid code. Our analyses, based on the comparison of the proposed model with a detailed model -both representing the same installation, show, from a quantitative and qualitative viewpoint, the accuracy and usefulness of the dynamic simulation model proposed. The aggregated model is shown to be ten times faster than the detailed equivalent, while the errors encountered in the simulation responses of both models are below 0.1% in all cases. It must also be noted that the present paper has allowed for the introduction, in the most recent version of the Spanish grid code and following approval of the Spanish Transmission System Operator (TSO), Red El´ ectrica de Espa˜ na (REE), of this new and advantageous PV power plant model. Finally, it is worth mentioning that this study could form the basis for encouraging other countries to also adopt this model in their grid codes as a new tool to help integrate PV power into power grids. 1. Introduction The increase in the installation of power plants based on Renewable Energy Sources (RES) in countries around the world [1,2] involves a change in the way power systems must be operated [3,4]. In order for the electricity supply to be provided under acceptable quality conditions [5,6], parameters such as frequency or voltage must be kept within certain limits [7]. For instance, frequency above or below the nominal value can result in the malfunctioning of rotating machines and industrial equipment, among other negative effects. Thus, due to the growing penetration of renewable energy in the existing electrical grids, new regulations [8], with new technical requirements [9], are being established. In this novel scenario with increased renewable power capacity in power systems [10], the control of certain variables is an arduous task. Electronic-based power plants [11,12], which have no rotational inertia [13], make controlling parameters such as frequency a challenge [14, 15]. This inertia reduction in power systems has already been addressed in some of these new regulations [16], known as grid codes. Indeed, these normative documents require new renewable power plants to be installed in such a way that they behave under certain conditions and contribute to frequency control [17]. Another of the aspects addressed by some national grid codes is the capability of these RES-based power plants to provide reactive power to manage voltage compensation [18]. Some of the tests defined in the grid codes concerning renewable facilities are thus aimed at determining whether the power plant under consideration is able to comply with the reactive power capability requirements demanded [19], analyzing these under different voltage and active power levels. How the reactive power control is conducted in a power plant [20] is also subject to analysis in grid codes. In this sense, different reactive power control modes are typically required to be available in newly commissioned power plants [21]. * Corresponding author. E-mail address: [email protected] (J.C. Hern´ andez). Contents lists available at ScienceDirect Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr https://doi.org/10.1016/j.epsr.2022.108676 Received 24 November 2021; Received in revised form 20 July 2022; Accepted 25 July 2022 Electric Power Systems Research 213 (2022) 108676 2 Voltage dips, which are common electrical disturbances causing significant voltage reductions usually caused by faults in the transmission and distribution networks [22], are also events regulated by the applicable specific legislation in each country [23,24]. In grid codes, the way power plants deal with this type of event is known as the fault-ride through (FRT) capability of the power plant [25]. More specifically, FRT capability is the capability of a certain power plant to remain connected to the grid during short periods of low network voltage [26], thus avoiding a widespread loss of generation. In parallel to the introduction of new and more up-to-date technical requirements in the recently-released grid codes [27,28], some of these national normative documents have also introduced the possibility of employing dynamic power plant simulation models [29]. Using a dynamic computer simulation model during the certification and commissioning process of a new power plant has key advantages: i) avoiding having to conduct all the field tests, thus reducing costs; ii) reducing the time it takes for a new power plant to be commissioned; and iii) standardizing the response of a significant number of power installations, which may contribute to more accurate knowledge of how these will behave and may reduce forecasting errors [30]. Within this framework, a number of scientific publications have already addressed the response of renewable power plants under some of the technical requirements defined in grid codes. For instance, in the case of [31], a review of technical requirements of certain European countries’ grid codes and the Turkish grid code is conducted, focusing on the integration of large-scale offshore wind power plants, while the FRT capability requirements of solar Photovoltaic (PV) and wind power plants have been reviewed in papers such as [32,33,34]. The study in [32] comprehensively reviews the FRT capability requirement included in the grid codes of 38 countries, with a particular focus on both PV and wind power plants. In [33], the main existing methods of FRT capability control in grid-connected PV power plants are described, comparing these methods in terms of compliance with grid codes. Finally, [34] studies how PV power plants impact on the development of grid codes, and pays special attention to the requirements related to FRT capability of these RES-based power facilities. In contrast, few publications can be found on the implementation and assessment, in actual PV power plants, of technical requirements established in grid codes -other than FRT requirements. The study in [28] conducts a review of the integration requirements for grid-connected PV power plants defined in several countries, while [35] focuses on the active and reactive power behavior of a solar PV generator to comply with the requisites established in a number of grid codes. Lastly, [36] analyzes the behavior of a real PV power plant using a simulation model, thus evaluating its compliance with a number of technical requirements established in the new Spanish grid code. However, in view of the above, it can be seen that few works analyze the response of actual PV power plants. This, together with the fact that a large number of grid codes are updating their requirements to adapt to the new energy scenarios, makes it imperative to study the assessment of PV power plant compliance with grid codes. Furthermore, most of the studies in the literature focus on the evaluation and description of the technical requirements defined in grid codes, while few offer a description of the tools and resources used to assess the compliance of power plants with those requirements [37]. In addition, as stated, the use of computer simulation models as a novel option to certify and commission new power installations [30,38] is a milestone that requires in-depth discussion. In light of the above, the present paper aims to cover the lack of scientific publications addressing the response of actual renewable power plants when subject to compliance analysis with technical requirements defined in grid codes. Furthermore, and most importantly, this work has allowed for the introduction in the latest version of the Spanish grid code, following approval by the Spanish Transmission System Operator (TSO), Red El´ ectrica de Espa˜ na (REE), of a new mechanism that is intended to speed up the verification, certification and commissioning process of new PV power plants, namely, the use of aggregated power plant simulation models. Through the assessment of three technical requirements defined in the new Spanish grid code, this paper demonstrates the accuracy of the proposed aggregated PV power plant simulation model, comparing its responses with those of the detailed simulation model representing the same PV power plant. Finally, the paper shows the usefulness and practical applicability of the aggregated simulation model by listing a number of advantages arising from its implementation. The enormous number of renewable energy power plants being integrated into electrical networks, which are connected through power electronics, undeniably makes the study of the dynamic characteristics of power systems a crucial matter. The behavior of a specific power system under different scenarios -different generation and demand scenarios or power outages scenarios, as well as the evaluation of its weaknesses -related, for instance, to voltage regulation or powerfrequency control, among others, can be identified. However, as mentioned above, this transition towards a more sustainable energy paradigm requires the development of new, accurate and fast-response dynamic models, which represent not only the power system to be studied, but also the different power plants to be connected. These dynamic models will allow the performance of power systems to be determined in advance, and valuable information related to these models may also be shared with entities entrusted with the operation of the power system. The present contribution is thus framed within this context, since it provides a simulation model that is replicable and scalable and that may be adopted by TSOs in other countries to help with the energy transition process ongoing across the world. The paper is structured as follows: Section 2 presents the new Spanish grid code, based on recent European Regulations, and the technical document released to monitor the compliance of new power plants with the national technical requirements established. Section 3 presents the methodology followed in the present work, describing the case study and the requirements analyzed. Section 4 details the results obtained when analyzing the performance of the aggregated PV power plant simulation model proposed. Finally, Section 5 summarizes the main conclusions obtained. 2. New Spanish grid code & Technical Supervision Standard Ministerial Order (MO) TED/749/2020 of July 16 [39] and Royal Decree (RD) 647/2020 of July 7 [40] are two of the documents in the new Spanish grid code. MO TED/749/2020 defines the technical requirements for the network connection of electricity generation and demand facilities, among other installations. MO TED/749/2020 is thus intended to ensure compliance with the obligations established in the European regulations on which Spain based its own grid code documents: Regulation EU 2016/631 [41], Regulation EU 2016/1388 [42] and Regulation EU 2016/1447 [43]. On the other hand, RD 647/2020 is intended to regulate different aspects required of the proper implementation of the requirements gathered in such European regulations. In particular, the aim of RD 647/2020 is to regulate certain aspects related to the operational notification procedures provided for by the European documents. Furthermore, with the objective of monitoring compliance with the technical requirements set out in Regulation EU 2016/631, the so-called Technical Supervision Standard, ‘Norma T´ ecnica de Supervisi´ on’ (NTS) [44], was released. The latest edition of the NTS, entitled “Technical standard for monitoring the conformity of electricity generation modules according to EU Regulation 2016/631 ”, was published in July 2021. MO TED/749/2020 and RD 647/2020, together with the NTS, replaced the previous Spanish grid code, which comprised different operation procedures, among which are Operation Procedure (PO) 12.2 “Installations connected to the transmission grid: minimum design, equipment, operation, safety and commissioning requirements” [45], and PO 12.3 “Voltage dip response requirements for wind power plants” [46]. PO 12.3 M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 3 sets out the required response of Spanish wind power plants under voltage dips [38,30], as its title states. The NTS, which prescribes the procedure for assessing the requirements for connection of generators to the grid, develops the aspects set out within “Title IV: Compliance monitoring” of Regulation EU 2016/631 [41]. This latter document establishes the responsibilities of the power-generating facility owner regarding the compliance of the installation with the requirements applicable under the aforementioned regulation. Specifically, the NTS regulates the aspects that require a greater level of detail to appropriately verify the compliance of the Electricity Generation Modules, ‘M´ odulos de Generaci´ on de Electricidad ’ (MGE), with the applicable technical requirements. According to the NTS, MGEs can be divided into Power Park Modules, ‘M´ odulos de Parque El´ ectrico’ (MPE), such as Photovoltaic (PV) power plants, and Synchronous Power Generation Modules, ‘M´ odulos de Generaci´ on de Electricidad Síncronos’ (MGES), such as conventional power plants. Moreover, depending on their maximum capacity and voltage level at their point of connection to the grid -or Point of Common Coupling (PCC), MGEs range from ‘Type A’ MGEs to ‘Type D’ MGEs, comprising power values from 0.8 kW to 50 MW, and voltage values at the PCC below and above 110 kV. In order to obtain a final certificate of the MGE under study, and according to Article IV of Regulation EU 2016/631, the conformity assessment of each requirement may be carried out through different modalities: •Compliance testing. MGE compliance tests for each requirement will be conducted by an accredited entity, which will draft a testing report and send the results to an authorized certification entity to be evaluated. The compliance of each requirement will be assessed independently, and a certificate of compliance through testing for each of the requirements will be issued. •Compliance simulations. MGE compliance simulations for each requirement will be conducted by an accredited entity, which will draft a simulations report and send the results to an authorized certification entity to be evaluated. The compliance of each requirement will be assessed independently, and a certificate of compliance through simulations for each of the requirements will be issued. The simulation models used in this process must be validated models according to the requirements established in the NTS [44]. •Equipment certificates. In addition to the MGEs, other components may be part of the power generation facilities and may be subject to assessment by the NTS. These include the Power Generation Units, ‘Unidades de Generaci´ on de Electricidad’ (UGE), which refer to the main electricity generation plants within the MGE, and the Additional MGE Components, ‘Componentes Adicionales del MGE’ (CAMGE), which are devices whose responses may affect the compliance of the MGE with the technical requirements defined. Power Plant Controllers (PPC) and Flexible Alternating Current Transmission Systems (FACTS) are examples of CAMGEs. In view of this, if the equipment certification path is followed to conduct the MGE conformity assessment of each requirement, an authorized certification entity will have to issue the UGEs and CAMGEs certificates, which in some cases will be enough to consider that the MGE complies with the technical requirements. However, in other cases, having the UGEs and CAMGEs certificates does not imply compliance of the MGE at the PCC with the technical requirements defined. In these cases, complementary simulations will be required at MGE level. Therefore, as mentioned, this paper addresses the use of a simplified or aggregated simulation model, instead of a detailed simulation model, of the MGE under study, to conduct the complementary simulations required when following the equipment certification path. Thus, we fill the gap in the literature on evaluating the response of actual renewable power plants in compliance with technical requirements defined in grid codes. Moreover, the analyses conducted allow a new tool to speed up the certification and commissioning process of new solar PV power plants to be introduced in the new Spanish grid code, with the authorization of REE. 3. Methodology This research enabled, in the second version of the new Spanish grid code, the use of an aggregated MGE simulation model of a PV power plant -instead of a detailed MGE simulation modelto conduct the complementary simulations required to obtain the final MGE certificate when following the equipment certification path described in Section 2. In order to do this, the simulation responses of both a detailed and an aggregated simulation model representing the MGE under study are compared, and error time series are calculated between both responses. That is, the possibility of employing an aggregated model is introduced for the PV MGEs -the type of MGE under study, which are of type MPE. Therefore, based on the outcomes of this research, the latest version of the NTS has introduced the possibility of using an aggregated PV power plant simulation model. It states that, in the cases where the MGE under study is of the PV MPE type formed by string inverters -inverters which, in this case, constitute the UGEs of the power plant and of which there are usually a significant number, it will be allowed to model the MPE through block aggregates in Low-Voltage (LV). In particular, the aggregation method proposed consists of grouping the inverters and lines of the LV side per each Medium-Voltage (MV) transformer. The LV lines are grouped, taking as length the median length of the cables, and taking the cross-section value as the median cross-section value of the cables that are grouped. Moreover, when aggregated, the lines and the inverters forming part of the MPE, the options “Number of parallel lines” and “Number of parallel units” are used, respectively, to indicate the number of elements aggregated in each case. In addition, the main parameters that must be configured -or knownin each of the elements that form part of the simulation model are as follows: •Inverter: i) rated apparent power; ii) technology (3PH, 3PH-E…); and iii) rated power factor. •Transformer: i) rated power; ii) transformer ratio; and iii) impedance. •Cables: i) length; ii) cross-section value; and iii) impedance. Finally, it should be noted that one of the main aspects to be considered during the modeling process of the aggregated simulation model is that it must be configured -and the accuracy of its responses must be checkedfor each specific inverter model that forms part of the PV power plant to be studied. This is because the analyses conducted in this case correspond to a certain inverter model. Once this is performed, it will be valid for all the PV power plants using the same device model. On the other hand, when it comes to the limitation of the aggregated model, it should be noted that it has only been used to assess power plant compliance with power-frequency requirements but not with FRT requirements. This means that, for the time being at least, the simulation model proposed cannot be used to check whether the PV power plant meets the FRT requirements or not. It is, however, necessary to evaluate this capability to guarantee optimum plant operation. 3.1. Case study The solar PV power plant analyzed has a nominal power of 42 MW, and is a Type D MPE, since the voltage at the point where the power plant is connected is above 110 kV, specifically 132 kV. It is formed by a High-Voltage (HV)/MV transformer, i.e., a power plant transformer, and ten MV/LV transformers. The total number of inverters is 465. Each inverter is of 105 kW rated power. This configuration represents an actual solar PV power plant located in Spain. M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 4 The power system simulation software tool used to conduct the simulations of both the detailed and the aggregated PV power plant model is DIgSILENT PowerFactory, version 2019 SP3. Based on the original structure of the PV power plant described above and the aggregation method explained (see Section 3), Fig. 1 shows the proposed aggregated structure of the generation facility, implemented in DIgSILENT PowerFactory. The number of inverters grouped in each case can be seen, resulting in ten equivalent UGEs, one per each MV transformer. The inverter model used is provided by a well-known manufacturer (which cannot be named due to a confidentiality agreement) in the DIgSILENT simulation environment. This means that the response of the PV power plant dynamic model is therefore controlled by a complex set of equations defining the behavior of the inverters, modeled by the manufacturer. 3.2. Technical requirements analyzed at MPE level through complementary simulations In order to check whether an aggregated PV power plant simulation model could be used as part of the certification and commissioning process of an actual new PV power plant in Spain, the complementary simulations defined in Sections 5.1, 5.2, 5.3, 5.7 and 5.8 of the NTS were conducted. The technical requirements analyzed are those addressing the power-frequency regulation modes, the reactive power capability and the reactive power control modes of the PV power plant. They are listed as follows: •Section 5.1: Power-frequency limited over-frequency regulation mode. •Section 5.2: Power-frequency limited sub-frequency regulation mode. •Section 5.3: Power-frequency regulation mode. •Section 5.7: Reactive power capability. •Section 5.8: Reactive power control. The complementary simulations corresponding to these technical requirements are conducted using both a detailed and an aggregated PV power plant model, and once their responses are obtained, they are compared with each other, and error time series are estimated. In this sense, Section 4 further details the advantages of using the aggregated simulation model. 3.2.1. Power-frequency regulation modes The three above-mentioned power-frequency regulation modes defined in the NTS are studied at the PCC of the PV facility, which has a rated voltage of 132 kV and is located upstream the power plant transformer. To set the frequency at the PCC to the values required to conduct the tests, an ideal voltage source was implemented along with the simulation models. Moreover, with the objective of obtaining the increments defined by the Spanish grid code with respect to the rated Fig. 1. Aggregated model proposed for the solar PV power plant under consideration. M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 5 power of the PV power plant, the statism or droop of each inverter was configured accordingly. Therefore, to assess whether an aggregated PV power plant simulation model, rather than a detailed one (a possibility already included), can be used as part of the certification process of an actual new PV facility in Spain, and when evaluating the power-frequency regulation modes, a number of variables were compared after conducting the simulations using both models. These variables are defined below according to [39] and [41], with some shown in Fig. 2. In particular, the speed of response of the PV facility when operating under the RMPFL-S, RMPFL-O or RMPFL is characterized by the time parameters shown in Fig. 2. This means that the response of the MPE to activate the previously listed regulation modes cannot last longer than the time values set as limits, as shown in this figure. Some of these are specified in Table 1. •Δf: deviation of frequency f from 50 Hz (Δf =f - 50). •Initial active power (P ini ): pre-disturbance active power of the MGE. This coincides with the test power. •Maximum active power (P max ): maximum capacity of the MGE. In practical terms, the maximum capacity of the MGE is equal to its rated power. •Registered active power (P reg ): measured active power of the MGE during the tests. •Expected active power (P exp ): expected active power response of the MGE. •Statism (s): technical characteristic of a generating unit, which determines the percentage variation of power for each unit of percentage variation of frequency. •Power deviation (ΔP): difference of active power with respect to the initial active power P ini . I.e., ΔP is equal to the final active power (P) minus the P ini (ΔP =P - P ini ). •Initial delay time (t a ): activation time of the power-frequency regulation modes. This value coincides with the time it takes a variation of 1% of the expected power response (ΔP) against a deviation of frequency (Δf) to occur. •Response time (t r ): time it takes 90% of the measured power (ΔP) against a deviation of frequency (Δf) to be reached, without including the initial delay time (t a ). •Setting time (t e ). Time it takes the response to stabilize within a band of ±5% of the power deviation, (ΔP), against a frequency deviation Δf, without including the initial delay time (t a ). Regarding the Power-Frequency Limited Over-Frequency Regulation Mode (RMPFL-O), the aim of the tests conducted is to verify that the MPE is able to activate the supply of power-frequency regulation reserves. This is done according to Article 13.2 of Regulation EU 2016/631 [41] with regard to the limited frequency-over frequency sensitive model, and Article 1.3 of MO TED/749/2020 of July 16 [39]. The tests conducted under the Power-Frequency Limited SubFrequency Regulation Mode (RMPFLS) are also used to verify that the MPE is able to activate the supply of power-frequency regulation reserves. However, this is done according to Article 15.2.c of Regulation EU 2016/631 [41] with regard to the limited frequency-under frequency sensitive mode, and Article 1.7 of MO TED/749/2020 of July 16 [39]. Finally, under the Power-Frequency Regulation Mode (RMPFL), the tests are conducted to verify that the MPE is also able to activate the supply of power-frequency regulation reserves. In this case, however, it is done according to Article 15.2.d of Regulation EU 2016/631 [41] with regard to the situations where the frequency sensitive mode is operating, and Article 1.8 of MO TED/749/2020 of July 16 [39]. The allowable limit values of the main parameters defined previously and shown in Fig. 2, i.e., the acceptance criteria, are specified in Table 1 Fig. 2. Example of power response showing the main parameters considered in the evaluation of the models’ behavior. Table 1 Main initial conditions and acceptance criteria when conducting the complementary simulations of the RMPFL-O and RMPFL-S regulation modes. Initial conditions Acceptance criteria P ini (%) Δf (Hz) s (%) t a (s) t e (s) t r (s) P reg −P exp (%) RMPFL-O P ini =P max 0.20 5 ≤30** ≤20 • ≤30 •• ≤2 • ≤10 •• ≤2 • ±5% ⋅ P max RMPFL-S P ini =60%*⋅ P max -0.20 5 ≤30** ≤20 • ≤30 •• ≤2 • ≤10 •• ±5% ⋅ P max * The initial active power value of the MPE will be equal to 60% of Pmax, so there will be a saturation of the RMPFL-S mode response after a 40% increase of Pmax when reaching the maximum power of the power plant. ** In cases where ta ≥2 s, the MGE owner must provide evidence to the Transmission System Operator (TSO) that justifies this value. • For reductions in active power. •• For increases in active power. M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 6 for the power-frequency regulation modes RMPFL-O and RMPFL-S. Moreover, Table 1 also shows the initial conditions that must be set to conduct the complementary simulations that will allow the MPE certificate to be obtained with regard to the RMPFL-O and RMPFL-S regulation modes. Finally, regarding the RMPFL regulation mode, P ini must be set to 80% of P max and statism to 5%, as in the previous cases. However, in this case, it is necessary to set the insensitivity of the response of the RMPFL regulation mode to a frequency variation of 10 mHz. 3.2.2. Reactive power capability Regarding the reactive power capability technical requirement, the aim of the tests is to verify that the MPE is able to supply the reactive power required at, and below, its maximum capacity. This is done according to Article 21.3 of Regulation EU 2016/631 [41] with regard to reactive power capability and voltage stability, and Article 2.3.2 of MO TED/749/2020 of July 16 [39]. In order to conduct the complementary simulations that will allow the MPE final certificate to be obtained with regard to the reactive power capability technical requirement, different voltage levels are set at the PCC using an infinite electrical network. Moreover, under these voltage levels, a number of load flows, setting different active power levels, are conducted to analyze the reactive power capability of the PV power plant. This is done according to Section 5.7 of the NTS [47]. 3.2.3. Reactive power control at MPE According to Article 21.3d of Regulation EU 2016/631 [41] and Article 2.3.3 of MO TED/749/2020 of July 16 [39], and as defined in Fig. 3. Simulation response of the PV power plant models -aggregated and detailedunder the power-frequency limited over-frequency regulation mode. Table 2 Comparison between the aggregated and the detailed PV power plant simulation models under the RMPFL-O regulation mode. P vs. over freq.; 50.2 Hz; s =5%; P ini =P max Simulation Point f (Hz) Registered Active Power Error (%) t r (s) Error t a Error (s) t e Error (s) 90% Registered Active Power (%) 50 0.08 – – – 50.2 0.07 – – – 50.4 0.07 0.007 0.001 0.007 50.6 0.07 0.008 0.001 0.008 50.8 0.06 0.009 0.001 0.009 51 0.06 0.009 0.000 0.009 51.2 0.05 0.009 0.001 0.009 51.4 0.05 0.008 0.001 0.008 M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 7 Section 5.8. of the NTS [47], there are three reactive power control modes: (i) voltage control mode; (ii) reactive power control mode; and (iii) power factor control mode. To obtain the final MPE certificate with regard to the voltage control mode via conducting the complementary simulations, the MPE will have to inject a value of active power equal to at least 80% of P max , and control slopes will be set to 2% and 7% -referring to the relation reactive power/voltage. Moreover, the complementary simulations under this voltage control mode are conducted by providing reactive power setpoints to the inverters based on the voltage at the PCC, while the voltage level at the PCC is set employing a voltage source. This is done according to Section 5.8.2.2. of the NTS [47]. Under the reactive power control mode, to obtain the final MPE certificate for this technical requirement, as in the previous case, the MPE will have to inject a value of active power equal to at least 80% of P max . The reactive power setpoint will be null when the simulation is started, and voltage at the MPE terminals will coincide with the nominal voltage. Different reactive power setpoints will then be established during the simulations. Section 5.8.2.3. of the NTS [47] further details the settings needed to conduct the complementary simulations in this case. Regarding the last reactive power control mode, i.e., the power factor control mode, the MPE will have to inject an active power value equal to at least 80% of P max , and the voltage at the MPE terminals will coincide with the nominal voltage. Different power factor setpoints will then be set during the simulations. All these settings are described in Section 5.8.2.1. of the NTS [47]. 4. Results In order to analyze the outcomes obtained, the present work is based on the premise that the simulation responses of both PV power plant models -detailed and aggregatedfirst analyzed separately, are within the permissible limits in all cases and for all the technical requirements analyzed and described in the previous sections. Therefore, the present section focuses on showing the results obtained when comparing the Fig. 4. Simulation response of the PV power plant models -aggregated and detailedunder the power-frequency limited sub-frequency regulation mode. M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 8 simulation responses of both models. In this way, we demonstrate the validity and usefulness of the aggregated PV power plant simulation model, which can be used to certify and commission a new PV power plant following approval of the Spanish TSO as a result of this work. Sections 4.1, 4.2 and 4.3 show the comparative analysis results achieved when examining the power-frequency, the reactive power capability and the reactive power control regulation modes, respectively. In all cases, the error values are calculated as the difference between the measured values of the detailed PV power plant model response and the values of the aggregated PV power plant model response. 4.1. Power-frequency regulation modes: Comparative analysis Imbalances between generation and demand constantly occur in power systems. This causes frequency rises and falls that may lead to significant problems if this parameter reaches extreme values falling outside the limits established. As explained in Section 1, electronicbased power plants have no rotational inertia, and this has an impact on the frequency dynamics of power systems. In particular, frequency dynamics are faster when there are low levels of rotational inertia in a region because of the high share of renewables. This implies that the time to respond to those frequency changes decreases. Therefore, the operational risks associated with these faster frequency dynamics must be urgently addressed, and that is why stricter power-frequency requirements are being demanded from renewable power plants. Section 4.1.1 shows the results obtained when comparing the responses of both simulation PV power plant models, once the complementary -or additionalsimulations are conducted to obtain the final MPE certificate under the requirement “power-frequency limited overfrequency regulation mode”. Section 4.1.2 and Section 4.1.3 do the same to obtain the final MPE certificates under the requirements “power-frequency limited sub-frequency regulation mode” and “powerfrequency regulation mode”, respectively. In all the figures shown in the present section, the frequency variations and the active power responses corresponding to the detailed model are represented by dashed lines, while continuous lines are used to represent the frequency variations and power responses of the aggregated PV power plant model. 4.1.1. Power-frequency limited over-frequency regulation mode As mentioned, the new grid codes demand PV power plants contribute to frequency regulation in power systems. In the case of the RMPFL-O regulation mode, the power plant must guarantee reductions in active power when frequency rises occur. Fig. 3 shows the simulation responses of the detailed and the aggregated PV power plant models when conducting the complementary simulations required to obtain the final MPE certificate for the RMPFL-O regulation mode technical requirement. In particular, Fig. 3a shows the frequency variations set to assess the active power response of the MPE, while Fig. 3b shows that power response. Finally, Fig. 3c shows two enlarged views -or zoomed areasthat allow the minor differences between the responses of both models to be better observed. As can be seen in all these figures, slight differences -of thousandthsmay be observed between the active power responses of the detailed and the aggregated MPE model. Table 2 shows, as an example, the specific error values obtained when calculating the difference between the P reg , t r , t a and t e parameters of both models (see Section 3.2.1, which explains the parameters and the limits established for each of them when analyzing the power-frequency technical requirements). The error in P reg is estimated in percentage -the maximum value obtained is of eight hundredth, while the error in parameters t r , t a and t e is estimated in seconds. For these temporary parameters, the maximum error value obtained is of a nine hundredth. All this highlights the great similarity between the responses of the detailed model and the aggregated model proposed. 4.1.2. Power-frequency limited sub-frequency regulation mode Regarding the RMPFL-S regulation mode, the PV power plant must guarantee increases in active power when frequency drops occur, so that these installations can also contribute to regulating network frequency. Fig. 4 shows the simulation responses of the detailed and aggregated PV power plant models when conducting the complementary simulations required to obtain the final MPE certificate for the RMPFL-S regulation mode technical requirement. In particular, Fig. 4a shows the frequency variations set, while Fig. 4b shows the power response of the power plant. Finally, Fig. 4c shows an enlarged view of a limited zone, so that the responses of both models can be better observed. As in the case of the RMPFL-O regulation mode, only slight differences may be seen between the responses of the detailed and the aggregated MPE simulation models. In this sense, Fig. 5 shows a box plot where the error values obtained between the detailed and the aggregated PV power plant models for the parameters analyzed under the RMPFL-O and RMPFL-S technical requirements are represented. This type of graphic is based on a standardized method where the center line within each box represents the median -the value of the central position variable once the dataset is ordered. The upper and lower edges that form each box indicate the 75th and 25th percentiles of the distribution, while the outliers are represented by a small black cross and the whiskers extend to the most extreme values -those not considered as outliersthat form the dataset. In the boxplot shown in Fig. 5, the error values in P reg are represented within the shaded area, with their corresponding Y axes placed on the left side of the figure. The error values in t r , t a and t e are represented within the unshaded area, with their corresponding Y axes placed on the right side of the figure. Finally, blue and red represent the error between the models for the RMPFL-O and the RMPFL-S technical requirements, respectively. In the case of P reg , the errors between the models for the RMPFL-O requirement are higher than the errors calculated for the RMPFL-S requirement. The same occurs with the errors in t r and t e , higher for the RMPFL-O technical requirement. In the case of parameter t a , more similar error values are obtained, although the same trend is observed. It can be stated, therefore, that the medians of the datasets for all the analyzed parameters of the RMPFL-S requirement are lower, in all cases, than the corresponding medians of the datasets for the RMPFL-O requirement. Indeed, the values of errors obtained for the RMPFL-S requirement are all lower than the errors for the RMPFL-O requirement, considering even the outliers. Thus, it can be concluded that, although the aggregated model performs with absolute precision in all cases, it performs even better when operating under the RMPFL-S regulation mode. Despite slight differences between the error values depending on the Fig. 5. Error values obtained between the detailed and the aggregated PV power plant model for the main parameters analyzed under the RMPFL-O and RMPFL-S technical requirements. M. Martínez-Lavín et al. Electric Power Systems Research 213 (2022) 108676 9 requirement analyzed, the main interest of the graphic depicted in Fig. 5 lies in reflecting how low these errors are in all cases. Thus, all these analyses made it possible to justify, to the Spanish TSO, the feasibility of introducing the aggregated PV power plant simulation model as a new tool to commission PV power plants. 4.1.3. Power-frequency regulation mode The last power-frequency regulation mode analyzed, the RMPFL technical requirement, is intended to prove that the PV power plant is capable of varying its active power response depending on frequency behavior, that is, whether it increases or decreases. Fig. 6 shows the simulation responses of the detailed and the aggregated MPE models when conducting the complementary simulations required to obtain the final MPE certificate for the RMPFL regulation mode technical requirement. In particular, Fig. 6a shows the frequency variations set, while Fig. 6b shows the power response of the PV power plant. Finally, Fig. 6c shows an enlarged view of a certain zone, which allows the responses of both models to be analyzed more precisely. As in the cases of the RMPFL-O and RMPFL-S regulation modes, the differences between the responses of both simulation models are kept to a minimum. 4.2. Reactive power capability: Comparative analysis Voltage regulation of power systems is a technical requirement hitherto provided almost exclusively by conventional power plants, i.e., by synchronous generators. This was because RES-based power plants previously only accounted for a small percentage of the total power capacity installed and had no voltage regulation capability. However, the growing penetration of these renewable power plants has led to the need for these generation facilities to contribute to voltage and reactive power regulation of power systems. Thus, newly released grid codes establish these power electronic-based facilities to comply with demanding reactive power capability requirements. Indeed, in some cases, if needed to comply with these interconnection technical requirements, reactive power support equipment, such as static var compensators, may be added at the power plant level. Fig. 6. Simulation response of the PV power plant models -aggregated and detailedunder the power-frequency regulation mode. M. Martínez-Lavín et al.