1. Introduction
Of all non-dispatchable Renewable Energy Sources (RES), wind power is the most significant in terms of electricity generation in current power systems. In 2019, the total wind power capacity installed worldwide was 651 GW [
1], which is a 19% increase on the figure for 2018. China and the US top the list of countries in new wind power capacity installed during 2019, while Europe installed a total of 15.4 GW of wind capacity during the same year [
2]. In line with this, approximately 76 GW of new wind power capacity is expected to be installed around the world during 2020 [
1]. Thus, wind power is unquestionably one of the fastest-growing energy sources, and this is partly due to the maturity of this RES from a technological viewpoint.
Nevertheless, the importance of wind power does not only relate to its unstoppable growth, but also to its capacity to create employment and reduce emissions. Furthermore, wind power is a source of energy widely supported by society, represents an economic boost for countries, guarantees environmental sustainability and contributes to reducing energy prices. In Spain, approximately 24,000 people work in the wind power industry [
3]. The sector creates five times more jobs than other conventional power technologies, and operates in more than 200 manufacturing locations [
4]. Moreover, wind energy avoided the emission of 28 million tonnes of CO
, accounting for 0.31% of Spanish Gross Domestic Product (GDP) and reduced the price of the pool in Spain by 6.83 €/MWh in 2018 [
3].
Unlike conventional power plants, which are able to support and contribute to the stability of the transmission system, the stochastic nature of renewable resources may affect the integration of renewable power plants into power systems. This introduces uncertainty and affects system stability and reliability. Moreover, the planning of electricity supply is often affected by RES. This is also due to RES-based power plants being decoupled from the grid by converters, which makes this type of source insensitive to voltage and frequency control. Thus, the increasing integration of electronics-based power plants leads to forecasting errors and notable uncertainties.
Therefore, in order to maintain the reliability and stability of power systems, Wind Power Plants (WPP) should also be able to contribute to both frequency and voltage regulation [
5], and should be able to remain connected to the grid during faults [
6]. In this sense, most countries have developed their own grid interconnection agreements, usually issued by their corresponding Transmission System Operators (TSO). These grid codes typically define the requirements that WPPs must comply with under grid disturbances [
7], detailing the main steps that must be followed to certify their performance. For instance, these requirements are especially strict in countries with islanding power systems, such as Ireland and the UK [
8]. The different grid codes, which are increasingly more restrictive and demanding, are also a challenge for Wind Turbine (WT) manufacturers because they must be able to develop and adapt their technology and machines to the new requirements.
The majority of grid codes concerning WPPs collect information on the necessary requirements for Fault Ride-Through (FRT) capability [
9,
10], active power regulation and frequency control [
11], as well as reactive power–voltage regulation. Likewise, they establish the limits of grid voltage and frequency. Works such as [
12] provide an interesting overview of the technical requirements addressed by different national grid codes, discussing their common criteria in detail. There are other interesting works addressing national technical specifications, such as [
13], which compares China and US wind power integration grid codes, or [
14], which compares the grid code in Bangladesh with those defined in other countries. In [
15], the requirements for offshore generation of wind power are reviewed. A comparative analysis of different grid codes concerning offshore installations is conducted in [
16]. Sourkounis et al. [
17] describes the requirements for Low-Voltage-Ride-Through (LVRT) and active and reactive power control in several European countries, while the Turkish grid code is described in [
18]. Finally, [
19] performs a review of the grid codes implemented by different countries, with a particular focus on the adaptation of the Indonesian grid code towards the integration of renewables. In this line, one of the most comprehensive studies addressing the requirements established by a significant number of different grid codes is found in [
20]. This work reviews the requirements for wind power integration in 12 countries, providing updated information and covering subjects from reactive power and frequency issues to power forecasting.
In view of the above, there is clearly sufficient information about the technical requirements to be complied with by WPPs when connected to the grid in different countries, and comparative studies on this topic can also be found in the scientific literature. However, the behavioral validation of dynamic WT simulation models as part of the certification processes of wind power installations is an increasingly important aspect that has not yet been addressed in any scientific publication. This is because it is a new feature not included in all grid codes. WT simulation models are representations of actual WTs. Therefore, model validation is required to assess the quality and accuracy of the dynamic model, and this is done by comparing the simulation results with field measurements conducted on the actual WT. WT model validation is a four-step process: (i) WT model definition; (ii) field measurements; (iii) model simulation of measured grid events; and (iv) comparison of simulated and measured results. Finally, the dynamic WT simulation model is considered validated with regard to the actual WT if the deviation between both data series is kept within the limits defined in the validation guidelines considered.
Therefore, the present paper addresses the validation processes of WT models detailed in three grid codes, namely the Spanish, German and South African grid codes. These countries were chosen since they use dynamic WT models as a novel option to verify the behavior of a WPP. We focus particularly in the case of Spain. Our paper thus provides information on when a WT simulation model can be considered validated and ready to be used as part of the WPP certification process in those countries, in addition to reviewing the most important international guidelines defining dynamic WT simulation models and their validation process. Moreover, aiming to demonstrate the practical applicability of these national validation guidelines, the present paper includes three application examples consisting of four voltage dip tests. On the one hand, a detailed dynamic WT model is validated according to the previous Spanish guidelines, and, on the other hand, the generic Doubly-Fed Induction Generator (DFIG) WT model defined by the International Electrotechnical Commission (IEC) is validated according to the new Spanish guidelines and the German ones. In this respect, it should be noted that, to the best of the authors’ knowledge, it is the first time in the scientific literature that the new Spanish guidelines have been followed to validate the performance of a WT simulation model.
The paper is structured as follows:
Section 2 presents the Spanish grid code, describing, with a particular focus on the validation criteria of dynamic WT simulation models, the previous guidelines and the new ones issued for such purpose.
Section 3 reviews the other two existing sets of guidelines in reference to the validation of dynamic WT models following national grid codes, issued by Germany and South Africa.
Section 4 presents two application examples of the previous Spanish validation guidelines. In addition, it describes two other application examples of compliance with both the new Spanish and German guidelines, which currently share the same validation criteria for WT models. Finally,
Section 5 summarizes the main conclusions obtained.
2. Spanish Grid Code: Use of Dynamic WT Simulation Models
Transient stability analyses are needed to determine whether a power system will respond adequately after a grid disturbance, being essential to ensuring the stability of the system after any type of event [
21]. Load changes, connection and disconnection of generators and faults are merely a few examples of grid disturbances. These transient analyses also help Distribution System Operators (DSO) and TSOs to plan network operation effectively, guaranteeing power supply and forecasting eventual power compensations from conventional power plants. In this way, they also significantly enhance network security and reliability.
With regard to wind energy, two international entities, the IEC and the Western Electricity Coordinating Council (WECC), through Standard IEC 61400-27-1 [
22] and the WECC Second Generation of Wind Turbine Models [
23], defined what are known as generic—or standard—WT simulation models for transient stability analyses. These generic WT simulation models cover the four main types of WT technologies available in the market [
24]. The variety of WT models developed by different manufacturers and the complexity and diversity of parameters of their corresponding simulation models prompted the development of alternative generic, publicly available and simplified WT simulation models. These were devised in order to obtain a generalized response, employing a reduced number of parameters and obtaining reliable responses [
25]. In addition to the development of dynamic WT simulation models, the IEC also developed its own set of validation guidelines to evaluate the simulations using field measurements of actual WTs. Works such as [
26] comprehensively describe the validation process of generic WT simulation models in the framework of Standard IEC 61400-27-1. In this sense, the importance of the IEC validation procedure and its relationship with the Spanish and German grid codes is discussed in
Section 2.2 and
Section 3.1.
In July 2019, Red Eléctrica de España (REE), the Spanish TSO, published the new technical supervision standard, ‘Norma Técnica de Supervisión’(NTS) [
27], for commissioning and grid integration of RES-based power plants. This standard makes it possible to assess the conformity of these renewable power plants in accordance with such new technical requirements. However, until that date, the certification of Spanish renewable power plants, and in particular wind power installations, had been carried out according to Operation Procedure 12.3 (PO 12.3) for FRT capability, which detailed how WPPs should behave under grid disturbances [
28]. In line with this, the so-called Procedure for Verification, Validation and Certification (PVVC) [
29] detailed the steps that should be followed to certify a Spanish WPP and make it comply with the technical requirements specified in PO 12.3. Moreover, the PVVC included the characteristics of the tests.
2.1. Po 12.3: Procedure for Verification, PVVC
Following the general verification procedure [
30,
31,
32] detailed in the PVVC document [
29], the dynamic WT model to be validated should be provided by the manufacturer and should represent the model of the actual WT that formed part of the WPP. Thus, after conducting the specified tests at the actual WT [
33], the field tests and the simulated responses of the WT model were compared and submitted to the validation criteria.
According to the PVVC, a WT dynamic model was validated if the following statement was complied with (see Equations (
1) and (
2)) [
29]: the absolute value of the difference between the field tests’ active and reactive power measured values (
and
) and the active and reactive power simulation values (
and
) did not exceed the nominal values (
and
) by 10% in at least 85% of the data series analyzed.
2.2. Spanish New Technical Supervision Standard: NTS
Edition 1 of the new guidelines issued to make new power generation units comply with the Spanish grid code, the NTS, was published on 18 July 2019. A year later, in July 2020, after several meetings, analyses and comments, the working group responsible for the monitoring of compliance of the generation sources with the Spanish grid code, coordinated by REE, released the draft document of the NTS Second Edition for its supervision. During this process, the Spanish Wind Energy Association, ‘Asociación Empresarial Eólica’ (AEE), played a decisive role, defending the advantages of wind power.
In order to facilitate understanding of the way to proceed, the NTS defines three main types of power units: (i) power generation modules, ‘Módulos de Generación de Electricidad’ (MGE); (ii) power generation units, ‘Unidades de Generación de Electricidad’ (UGE); and (iii) additional MGE components, ‘Componentes Adicionales del MGE’ (CAMGE), such as Flexible Alternating Current Transmission Systems (FACTS). MGEs are composed of UGEs and CAMGEs. Moreover, the NTS defines a list of technical requirements to be complied with by MGEs, such as FRT capability, active power recovery after faults and power-frequency control, among others.
Prior to the commercial operation of the power plant and the issuance of a final operation notification, there are three different ways to proceed in order to obtain the final certificate of the MGE:
Conformity assessment procedure through equipment certificates. This consists of first obtaining the equipment certificates, i.e., obtaining certificates for the UGE and CAMGE units. These certificates shall then be provided to the power plants’ owners to obtain the final certificate of the MGE.
Conformity assessment procedure through testing. This procedure may be followed with two objectives: (1) to obtain conformity with the technical requirement considered through directly testing the MGE, or (2) to certify the UGEs and CAMGEs for this requirement through testing.
Conformity assessment procedure through simulation. This procedure may be followed with two objectives: (1) to obtain conformity with the technical requirement considered through directly simulating the MGE, or (2) to certify the UGEs and CAMGEs for this requirement through simulation.
Further information regarding the procedures described above to obtain the final MGE certificate is explained in greater detail in [
27]. However, since this paper focuses on the simulation and validation of dynamic WT models as part of the WPP certification processes, the conformity assessment procedure through simulationis discussed in detail:
Having a dynamic simulation model of MGE, UGE and/or CAMGE validated by an authorized certification entity, according to the validation guidelines described below.
Conducting the simulation of the validated dynamic models according to the technical requirements, which is conducted by an accredited entity.
Assessment of the simulation results by an authorized certification entity, and issuance of an equipment certificate for the UGE, CAMGE and/or MGE when the evaluation is positive.
The main objective of dynamic simulation models is to represent the electrical behavior of actual devices in a precise manner, using a simulation software tool. Let us assume that the dynamic simulation model considered in this case, i.e., the dynamic simulation model of the UGE under consideration, is a WT. The use of both RMS and Electromagnetic Transient (EMT) models is considered acceptable for the purposes described [
34,
35]. However, the WT manufacturer, together with the accredited and the authorized certification entities, determine which model—RMS or EMT—is more appropriate to use [
27]. This decision depends on the technical requirement to be assessed and the typical frequency of the electrical phenomenon under consideration.
Figure 1 shows the general scheme to validate a dynamic WT model to be employed in the conformity assessment procedure through simulation, adapted to the specific technical requirements of FRT capability.
A dynamic WT simulation model is considered validated and suitable to be used as part of the Spanish WPP certification process after following the steps depicted in
Figure 1. In other words, the dynamic model, once validated according to the guidelines which the paper are focused on (and once obtained the certificate of ‘validated model’, as shown in
Figure 1), is demonstrated to behave in a sufficiently accurate manner and therefore is ready to be used for simulating the whole set of technical requirements that the generation units and/or the installation must comply with. Finally, if every requirement established in the grid code is fulfilled, the corresponding entity will give the final certificate to commission the wind power plant.
Thus, according to
Figure 1a competent accredited entity proceeds with the simulations. These must be consistent with the voltage dip tests conducted by an accredited entity equipped to carry out the required tests in laboratories (LAB), or by an Authorized Certifier (AC). The dynamic WT model is provided by the manufacturer (MAN). The testing report and the simulation results are then provided to an AC, who proceeds to issue a validated model certificate if the errors between the simulated responses of the model and the measurements are within the limits established. It is especially important to underline that the WT simulation model must be validated against measurements corresponding to all the technical requirements defined in the NTS [
27]. The acceptance criteria of the simulation model, together with the validation procedure, are described below.
According to Equation (
3), in the particular case of voltage dips, the error time series (
) are calculated as the difference between the simulated time series (
) and the measured time series (
) for the time window defined in [
36].
Three validation errors or validation performance indicators are now calculated for each of the variables considered during each of the time windows defined within the whole voltage dip window, and based on the error time series (
) (see [
26,
36] for more information about the so-called quasi-steady state sub-windows): (i) pre-fault window, which starts 1 s before the start of the voltage dip; (ii) fault window, which starts when the fault occurs and ends when the fault is cleared; and (iii) post-fault window, which is extended 5 s from voltage dip clearance. The validation errors are:
Mean Error (ME). ME is defined as the mean value of the error over the corresponding time window. It is related to the steady-state performance of the model (see Equation (
4)).
Mean Absolute Error (MAE). MAE is the mean value of the absolute error. It is also concerned with the steady-state performance of the model, albeit based on the mean deviation (see Equation (
5)).
Maximum Absolute Error (MXE). MXE is the maximum value of the absolute error. It focuses on giving information about the transient performance of the model (see Equation (
6)).
In Equations (
3)–(
6),
x represents the variable to be compared (for instance, active power),
n is the indices of the vectors and
N is the total number of samples used. In this sense, it should be noted that the integration time step defined during the simulation will determine the number of samples used. IEC generic models use integration time steps in the order of 1 ms to 10 ms. Thus, for instance, during the validation process of a generic WT model executed with a simulation time step of 5 ms and a time window for comparison of 6.6 s, the total number of samples considered will be 1320.
After these calculations, the simulation results are provided to the AC, and the model is subsequently validated.
Table 1 summarizes the variables and results required, corresponding to positive sequence components. The table also includes the acceptable limits.
As was discussed in
Section 2, Standard IEC 61400-27-2 is closely related to the Spanish NTS. This is because the NTS based its validation guidelines for WT dynamic models on the validation guidelines issued by the IEC, developed to validate the behavior of generic WT simulation models [
36].
5. Conclusions
A number of countries include the use of dynamic WT simulation models as part of the WPPs certification process. These WT models are used either to determine in advance the behavior of the new wind power installations seeking connection to the grid, or are directly used as a key element in the process of the WPP obtaining the certificate of compliance with the specified technical requirements. In this sense, only three countries are known to include these WT simulation models in their grid codes as part of the commissioning of WPPs: Spain, Germany and South Africa. This work conducts a review of the requirements established in these grid codes to validate the behavior of dynamic WT models. In particular, the study evaluates the accuracy of these models’ required responses when subjected to voltage dips, which allows the FRT capability of WPPs to be assessed.
In the case of the Spanish grid code, both the previous and the new validation guidelines are reviewed. According to the previous ones, the PVVC, dynamic WT simulation models can be used during the so-called general verification procedure of WPPs. During the second stage of this process, ‘wind turbine model simulation and validation’, the WT model provided by the manufacturer and representing the actual WT model that forms part of the installation must be subjected to the same voltage dip as that measured at the actual WT. The field tests and the simulated responses of the WT model can then be compared. The model can thus be considered validated if the absolute value of the difference between the values of active and reactive power obtained in the field tests and the active and reactive power simulation values do not exceed the nominal values by 10% in at least 85% of the data series analyzed.
However, the validation criteria considered by the Spanish grid code were changed. In 2019, a new grid code was issued in Spain: the NTS. A new working structure was defined, and new validation criteria for dynamic WT models were adopted. Following the so-called ’conformity assessment procedure through simulation’, dynamic simulation models of power generation modules and/or units can be used to obtain a positive equipment certificate as long as these models are validated according to the new validation guidelines. These new guidelines involve estimating an error times series, obtained as the difference between the simulated and the measured data. Three validation errors or validation performance indicators must also be estimated. Therefore, the dynamic model is considered validated if such validation errors are below the thresholds established for this purpose. At this point, two aspects must be highlighted: (i) the German and Spanish grid codes follow the same validation procedure and criteria; (ii) both grid codes are based on the guidelines issued by the IEC. This IEC validation procedure was developed to test the accuracy of the responses of the so-called generic or standard WT simulation models, also defined by this international entity.
The South African grid code, the latest version of which was issued in 2019, establishes that the national SO and TNSPs can require accurate dynamic WT simulation models to assess in advance the impact of the integration of WPPs on the stability, security and dynamic performance of the national network. During this assessment process, the WPP owner shall provide information on the dynamic modeling data. Moreover, after commissioning the wind power installation, WPP or WT electrical simulation models validated with field measurements shall be provided by the owner to these entities. In this sense, the South African grid code, approved by the NERSA, defines several types of errors. In addition, it also defines acceptable limits of voltage and current deviation for assessment of the simulation models’ accuracy, so that these can be validated with field measurements.
Finally, our application examples consisted of the validation of a detailed WT simulation model according to the previous Spanish PVVC, and the validation of the generic IEC DFIG or Type 3 WT model according to the criteria shared by the Spanish NTS and the German TG 4. These examples successfully demonstrated the practical applicability of the guidelines. Indeed, the simulation models comply with the criteria established in all cases, which means they are suitable for use as part of the certification and commissioning process of WPPs.