Generic dynamic wind turbine models for power

Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx

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Generic dynamic wind turbine models for power system stability analysis: A comprehensive review ⁎

A. Honrubia-Escribanoa, , E. Gómez-Lázaroa, J. Fortmannb, P. Sørensenc, S. Martin-Martineza a b c

Renewable Energy Research Institute and DIEEAC-EDII-AB, Universidad de Castilla-La Mancha, 02071 Albacete, Spain HTW Berlin-University of Applied Sciences, Berlin, Germany Wind Energy Systems, Department of Wind Energy, Technical University of Denmark, Denmark

A R T I C L E I N F O

A BS T RAC T

Keywords: Generic model IEC 61400-27 Power system stability Standard model Wind turbine modeling Wind energy integration

In recent years, international working groups, mainly from the International Electrotechnical Commission (IEC) and the Western Electricity Coordinating Council (WECC), have made a major effort to develop generic — also known as simplified or standard— dynamic wind turbine models to be used for power system stability analysis. These models are required by power system operators to conduct the planning and operation activities of their networks since the use of detailed manufacturer models is not practical. This paper presents a comprehensive review of the work done in this field, based on the results obtained by IEC and WECC working groups in the course of their research, which have motivated the publication of the IEC 61400-27 in February 2015. The final published versions of the generic models developed according to the existing four wind turbine technology types are detailed, highlighting the subsequent changes made during the development phase. The main differences between IEC and WECC generic models are also analyzed. Not only is the final model structure presented but we also provide a complete description of the physical behavior of wind turbines facing power system stability problems. Results are thus of great interest to grid operators, software developers, wind farm owners and researchers focused on the integration of wind energy into power systems.

1. Introduction During the last few years, wind energy has positioned itself as the world's most promising renewable energy source. In 2014 and 2015 two consecutive records of new capacity installed in a single year were achieved, reaching 52 GW and 63 GW respectively, bringing the global total capacity close to 433 GW [1,2]. The main contribution to this figure was made in Asia, where China installed around 30 GW, which represents 48% of the world capacity installed in 2015. Europe grew by 6.3% compared to 2014, when Germany set a new record by installing more than 6 GW, 47% of all new EU installations [3]. This was the first time any country other than China or the US had installed more than 5 GW in a single year. In parallel to this capacity increase and even more important is the wind power contribution to the electricity demand coverage. Wind power met 11.4% of EU electricity demand

in 2015, up from 10.2% in 2014, 8% in 2013, 7% in 2012 and 6.3% in 2011 [4]. In the case of Denmark, wind power has covered more than 30% of demand since 2012 [5]. The Iberian Peninsula presents similar rates of wind power [6,7]. Specifically, during 2013 wind power in Spain was the leading contributor to demand coverage, covering approximately 21% of the electricity demand, which is similar to the shares achieved in 2014 and 2015 [8]. In order to properly integrate this considerable amount of wind energy into power systems, the effect that wind power generation has on the power system response must be analyzed in detail. This type of surveys are typically carried out by means of transient stability analysis [9–47], which are one of the power system stability categories [48]. In this sense, grid operators —considering both Transmission System Operators (TSOs) and Distribution System Operators (DSOs)—, wind turbine manufacturers, power system software developers and techni-

Abbreviations: CD, Committee Draft; CDV, Committee Draft for Voting; CIGRE, International Council on Large Electric Systems; CIRED, International Conference on Electricity Distribution; DFIG, Doubly Fed Induction Generator; DSO, Distribution System Operator; EMT, Electro-Magnetic Transient; EPRI, Electric Power Research Institute; EU, European Union; FDIS, Final Draft International Standard; FRT, Fault Ride Through; IEC, International Electrotechnical Commission; IEEE, Institute of Electrical and Electronics Engineers; IGBT, Insulated Gate Bipolar Transistor; IVGTF, Integration of Variable Generation Task Force; JWG, Joint Working Group; M & VWG, Modeling & Validation Work Group; NERC, North American Electric Reliability Corporation; PI, Proportional-Integral; R & D, Research and Development; REMTF, Renewable Energy Modeling Task Force; RMS, Root Mean Square; TSO, Transmission System Operator; UK, United Kingdom; US, United States; WECC, Western Electricity Coordinating Council; WT, Wind Turbine ⁎ Corresponding author. E-mail addresses: [email protected] (A. Honrubia-Escribano), [email protected] (E. Gómez-Lázaro), [email protected] (J. Fortmann), [email protected] (P. Sørensen), [email protected] (S. Martin-Martinez). http://dx.doi.org/10.1016/j.rser.2017.06.005 Received 23 March 2016; Received in revised form 5 January 2017; Accepted 1 June 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Honrubia-Escribano, A., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.06.005


A. Honrubia-Escribano et al.

the level of complexity of the generic models in accordance with the level of accuracy needed for stability studies. In addition, based on the adopted modular model structure [83], the models are designed to be easily implemented in any power system simulation software [95,14], ensure flexibility for future changes [96–98] and allow the sharing of the modules between different wind turbine types [67,72]. Under this framework, this paper aims to conduct a review of the generic wind turbine models developed by IEC and WECC working groups. In this sense, there are only a few studies in the scientific literature providing overall descriptions of generic models. There are also a number of other works comparing generic and detailed models or generic models and real measurement campaigns. However, a comprehensive description of the evolution of generic models, from the initial assumptions to the final models, is needed to understand the physical meaning of the behavior of wind turbines submitted to power system disturbances. In this line, due to not only the complexity of the wind turbine models but also the rapid development of the technological advances, a review is urgently required by the wind power industry. Hence, this paper conducts a review of previous works and fills the existing gaps associated with the motivations for the changes made to generic wind turbine models during the development of a new Standard IEC 61400-27. As will be noted, these key changes are mainly the result of contributions from wind turbine manufacturers. Furthermore, the differences between IEC and WECC generic models are highlighted, aiming at providing an improved understanding and usability of the models. As an additional contribution of this work and in order to cover a wider scope, references with public parameters related to the generic models developed are provided depending on the wind turbine topology. After this short introduction, the paper is organized as follows: Section 2 provides an overview of the development and the current status of generic dynamic wind turbine models. Section 3 presents the four generic wind turbine models and changes made during the research. Section 4 describes the most important modules included in each generic wind turbine model, i.e. the modules giving rise to discussions between the working groups. In short, Section 2 provides a view of the chronological developments whereas Sections 3 and 4 present the detailed technical results. Finally, Section 5 summarizes the conclusions of the paper.

cal consultants require dynamic models of wind power generation to simulate the behavior of wind turbines in power systems. Most of the wind generator models have not been widely standardized and validated, which is in contrast to traditional synchronous generators [32]. Based on requirements, two types of wind turbine models are commonly found in the literature: (a) EMT-type models —commonly known as detailed models—, which are associated with three-phase electromagnetic transient simulations, and (b) positive sequence (RMS-type) models —commonly known as generic, simplified or standard models—, which correspond to the electromechanical average over one line period of balanced systems. The first choice provides models able to determine the electrical and mechanical behavior of the particular wind turbines with maximum accuracy [27,38,44]. Actually, this type of models were initially used for power system planning [49], and have been developed by almost all wind turbine manufacturers [30]. Nevertheless, this level of precision is not suitable for large-scale grid studies for a wide range of reasons. On the one hand, detailed manufacturer models require a considerable amount of specific input parameters to characterize each wind turbine type, most of them out of the TSO concerning [17,50], being avoided the ease of use. The computational time cost is also directly influenced by the number of parameters [51,52]. Furthermore, detailed manufacturer models are commonly subjected to non-disclosure agreements [53–55], where all information is kept hidden from the user [21,35], and are intended for use in a specific simulation platform [14,56]. These concerns, together with the ability of the models to be updated by the rapid changes in wind turbine technology make the maintenance of detailed manufacturer models a complex and time-consuming task [57]. As a result, very detailed implementations of the control and to certain extent of the EMT-models are only required by grid operators under some specific conditions, such us very weak networks (relevant for example for Hydro-Québec TSO), proximity to DC connections (relevant for example for TenneT TSO) or risk of subsynchronous interactions (relevant for example for ERCOT operator [58]). On the other hand, for transient stability studies, grid operators commonly request RMS models from manufacturers, as this is mandatory in many countries such as Australia, Germany, Ireland, UK and some regions of America [59,60]. Considering the above findings, some international entities, mainly the IEC and the WECC, have been actively working during recent years to define generic wind turbine dynamic models [60–78]. In this sense, the term generic is related to a publicly standardized model able to represent the dynamic behavior of a wide variety of equipment and controls not proprietary to any wind turbine manufacturer [68,79]. These models are intended to compute transient stability simulations, where simulation time is between ten and thirty seconds [64] and wind speed is considered as a constant value [80–85,28]. The models are thus applicable to different dynamic events such as line switching, loss of generation or load units and power system faults [24,33–36]. Since RMS-based positive sequence models are used, the system stability analysis are focused on balanced grid events, such as three-phase short-circuits. Such short-circuit fault types are rare [86–89], but occur and reflect the worst-case dimensional scenario of main interest to grid operators [15,90]. However, in this regard, further work is currently being conducted since unbalanced faults may have a considerable effect on wind turbines implementing power electronic converters [65,91], such as Type III and Type IV wind turbines. The omission of unbalanced faults may also be considered a major impediment to wide acceptance and application of generic models in some countries like Germany and UK [77]. In this sense, generic models use integration time steps between one and ten milliseconds [92], which is in accordance with the typical time step used for RMS simulations [93,94]. In fact, some TSOs, such as The Irish or Swedish ones, for instance, state in their grid codes that wind turbine models must be computed with simulation time steps larger than 5 ms or 10 ms [53]. For this reason, the main objective of the IEC and WECC is to adjust

2. Current status of generic dynamic wind turbine models development Initially, the main focus of the development of dynamic stability models for wind turbines in network stability simulation platforms was on vendor-specific models [57,43]. Indeed, in 2009, more than 25 vendor-specific models were available using Siemens-PTI PSS®E [21], which have been in use worldwide to evaluate system stability. Most of these models are tested in [16] and the main concerns related to their implementation are given from a TSO perspective, such as the large differences in the model details provided by manufacturers, different simulation time steps and some initialization problems. In this line, in 2005, the WECC Modeling & Validation Work Group (M & VWG), under its Renewable Energy Modeling Task Force (REMTF), launched an effort towards the development and validation of generic dynamic wind turbine generator models, whose first publicly known reference is [14]. As a result, several prototype generic models were implemented in the Siemens-PTI PSS®E and General Electric PSLF simulation tools as standard-library models [23], the first generation of which was published in 2010 [62], where it is noted that a different number of modules is used to implement the same wind turbine model depending on the software tool. Since these models were primarily implemented in the US, responses to typical US grid code requirements were able to be emulated [71]. Due to the lack of real measurements, vendor detailed models were considered the only benchmark available for validating this first generation of generic models [40,70]. Therefore, 2



Fig. 1. World distribution of IEC and WECC memberships.

of 2012, being implemented in several commercial software vendors during 2013 —the same two as in the first generation plus PowerWorld and Powertech simulation tools— [97], and finally published in 2014 [74] together with two application guides [75,92]. Moreover, IEC models are already implemented in the latest versions of DIgSILENT PowerFactory and further efforts are being made towards the implementation with other software vendors, such as Eurostag, Neplan and Siemens-PTI PSS®E [77]. As these models were developed jointly, they are very similar, although some differences are found based on the difference in the way model validation is approached in North America versus other regions in the world. Specifically, EU grid codes pay more attention to the behavior during the fault whereas North America focuses on the post-fault response —a comprehensive review of recent grid code regulations is found in [102]—. In addition to the extensive testing of these WECC 2nd generation models [47], which facilitate higher model fidelity during fault conditions, one of the new features of this second generation was a move towards a more modular approach. In order to facilitate model verification, in 2014 the Electric Power Research Institute (EPRI) launched the Renewable Energy Model Validation tool, which allows for parameter fitting and validation of some generic wind turbine models defined by IEC and WECC [103]. One of the original approaches of the IEC group focused on an easy model implementation and maintenance is the definition of three model parameter categories [41]. In parallel to these developments and in order to cover a similar objective, the International Council on Large Electric Systems (CIGRE), has just established a joint working group focused on the modeling of converter-based generation for power system analysis — JWG C4/C6.35/CIRED—, whose first meeting was held in 2014. This group consists of 44 members, more of whom are from industry than from university/R & D. Although this CIGRE group studies inverterbased sources and coordinates with IEC and WECC working groups to avoid duplicating efforts, this approach is mainly focused on photovoltaics [79]. As a summary, Table 1 presents the main contributions focused on generic dynamic wind turbine model development found through a scientific review carried out according to the publication year. This classification shows the main stakeholder involved in each work published. It is worth noting the considerable interest shown by wind turbine manufacturers and grid operators. Specifically, the Spanish TSO developed a simplified model to be used for planning studies [19], which was used afterwards by one manufacturer as the basis for the

model verification against real measurements has been one of the main objectives of the WECC working group since 2010 [57]. Considering the inputs from the first generation of WECC models as well as other stakeholders, and focused on the same aim, the IEC set up a working group in October 2009 to deal with the generic models for wind power generation, whose first publicly known contribution was published in 2011 [27]. Some of the members of the IEC committee had collaborated previously with the WECC, the NERC IVGTF and related IEEE working groups, which evidences the close collaboration among these entities in order to ensure maximum benefit to the industry globally [99]. In this sense, Fig. 1 shows the world distribution of IEC and WECC memberships, where the main areas affected by each working group are clearly observed. The IEC group approached this work first by creating two different Parts of the Standard: Part 1 focused on wind turbine models and validation procedures whereas Part 2 focused on wind power plant models and validation procedures. The meetings held during 2010 discussed the standard model validation procedure, which has been the basis of several discussions due to the different countries’ validation requirements. In 2011, the work focused on the development of generic models [36], the first Committee Draft (CD) of Part 1 being submitted at the end of 2011 [33]. Two years later was circulated the Committee Draft for Voting (CDV) and the subsequent IEC 61400-27-1 Final Draft International Standard (FDIS) was issued in mid-2014 [72], finally being published in February 2015 [100]. Once IEC 61400-27-1 was published, work on Part 2 had hardly started when two major concerns were detected: (a) Part 2 would present a considerable amount of duplications from Part 1 and (b) the interdependencies between wind turbine models and wind power plant models would pose serious challenges to the updates of both Parts. To overcome these problems, it was decided to modify the initial IEC 61400-27 structure, including the models (both wind turbines and power plants) in 27-1, and in 27-2 the validation procedures —based on the measurements and tests specified in the IEC 61400-21 series [101]—. As a consequence, a second edition of the IEC Standard, which considers this new structure, is being prepared. Both Parts of this second edition, which will replace the currently published first edition of 27-1, are expected to be published in 2017 [77]. In this sense, the current status at IEC level is that the development of the wind turbine models has been essentially completed whereas wind power plant models development has just commenced. As a result of the collaboration between WECC and IEC working groups [96], the WECC 2nd generation models was approved at the end

3

Year

2003 2003, 2003, 2006 2007 2007 2007

2009 2009 2009 2009, 2010 2010 2010 2010 2011 2011 2011 2012 2012 2012 2012 2013 2013 2013, 2014 2014

Refs.

[10] [80,106,12] [14] [15] [107]

[51] [21] [105,104] [59,61,108] [83] [84] [95] [57,31] [25] [26] [60] [34] [35] [36] [68,42] [39] [40,44,70] [71]

R & D Entity and grid operator WT manufacturer (Siemens Wind Power) R & D Entity and grid operator WT manufacturer (REpower Systems AG) Grid operator WT manufacturer (GE Energy) WT manufacturer (ABB) R & D Entity, WT manufacturer WT manufacturer (Alstom Wind) WT manufacturer (ABB) WT manufacturer, GAMESA R & D Entity, WT manufacturer R & D Entity R & D Entity R & D Entity, WT manufacturer R & D Entity R & D Entity, WT manufacturer WT manufacturer (REpower Systems AG)

R & D Entity WT manufacturer (GE Energy) R & D Entities, WT manufacturer and grid operator WT manufacturer (Siemens Wind Power) R & D Entity

Stakeholder

Table 1 Summary of the main contributions to the development of generic wind turbine models.

Simplified pitch angle control model Generic wind turbine model structure Generic wind turbine model development Generic wind turbine model development Generic wind turbine model development 1.5, 1.6, 2.5 and 3.6 MW simplified wind turbine models 2.5 MW generic wind turbine model Generic wind turbine model development 3 MW simplified wind turbine model 2 MW generic wind turbine model Generic electrical generator model Generic wind turbine model development Generic wind turbine model development Generic wind turbine model development Generic wind turbine model development Generic wind turbine model development Generic wind turbine model development Generic electrical generator model

Generic wind turbine model development 1.5 and 3.6 MW simplified wind turbine models Simplified aerodynamic and drive train models 3.6 MW simplified wind turbine model 180 kW simplified wind turbine model

Main contribution

DIgSILENT PowerFactory GE PSLF MATLAB® DIgSILENT PowerFactory, Siemens PSS®E DIgSILENT PowerFactory, Siemens PSS®E Siemens PSS®E Siemens PSS®E Siemens PSS®E – – GE PSLF – – MATLAB® – – DIgSILENT PowerFactory RTDS – – MATLAB®, DIgSILENT PowerFactory Simulink®, DIgSILENT PowerFactory –

Power system software

III All III III, IV I, II III, IV IV All III III III IV I III III, IV III I III

III III III, IV IV I

WT Type

– WECC WECC IEC IEC WECC WECC WECC – – IEC IEC IEC IEC WECC IEC IEC IEC

– WECC WECC – –

Work group



4



development of a simplified model for its real Type III wind turbine [25]. In addition, Table 1 also specifies the wind turbine technology analyzed in the contribution according to the Types defined in Section 3, as well as the international working group the contribution was intended for. One of the main observations resulting from this table is the considerable effort made in the development of Type III and Type IV wind turbine models. Actually, these modern wind turbine types are the most commonly sold and installed topologies in the current market [85], representing the main issue of the WECC TSO group based on the interconnection requests received [14]. Furthermore, the simulation software used to model the system is included in Table 1. Grid operators normally assess short term power system stability using a network model implemented in a specific power system simulation tool, such as —on the evidence of Table 1—, PSS®E from Siemens Power Technologies International, PowerFactory from DIgSILENT and PSLF from GE Energy. Indeed, Hydro-Qubec, Svenska Kraftnat and REE —Canadian, Swedish and Spanish TSOs— commonly use Siemens-PTI PSS®E [104,105], whereas other system operators (specially European TSOs) such as Energinet. DK, National Grid and Transpower —Danish, British and New Zealand TSOs— use PowerFactory [32,44]. North American TSOs use either PSS®E or PSLF to conduct these power system studies [11,49], which has motivated the implementation of the WECC generic models mainly in these software platforms. This diversity of software implementations means that no software developer has found a single solution (i.e. only one model) to each wind turbine specific model. In addition, IEC generic modeling is a recent development and hence there is a lack of specific information. Therefore, the detailed knowledge of the physical behavior of each wind turbine model is needed in order to avoid misunderstandings in the model creation phase. To deal with these concerns, the present paper aims to fill the gaps in the scientific literature, by combining all the different extant versions of generic modeling.

purposes, the blade pitch angle may be changed depending on the voltage dip depth in order to avoid rapid increase of rotor speed [111]. Therefore, for power system stability analysis, two submodels of Type I have finally been accepted at IEC level, as summarized in Table 2. However, the inclusion of Type IB has been a matter of some discussion by IEC members because this FRT capability is not generally implemented in Type I and Type II wind turbines equipped with pitch control. However, it can be found in some existing wind turbines in the US, such as the Mitsubishi MWT1000A (Type IB) or Vestas V80 (Type II) [112]. Hence, it was finally agreed to make IEC models compatible with WECC models [72]. Considering the previous ideas, Fig. 3 shows the block diagram of these Type IA-IB & Type II wind turbine topologies. A complete set of the acronyms used to identify the parameters included in this figure can be found in Appendix A. Black arrows are used to indicate the structure common to the wind turbine types. Grey is used to point out the modifications added by the FRT control. As previously noted, the blade pitch angle will be modified depending on the voltage dip, uwtt, and this change will affect the aerodynamic power, paero. Therefore, the only difference between Type IA and IB is how the aerodynamic power is computed. Dashed grey arrows are used to represent the additional control of the rotor resistance given by Type II. The main modules of the models, such as the aerodynamic or mechanical blocks, are described in Section 4. However, there is no description of the generator system used in these Type I and Type II, which are modeled by asynchronous generators models, because such asynchronous generator models have already been implemented in every power system simulation tool for stability analysis [83]. 3.2. Type III & Type IV: variable speed operation The main feature of these pitch-controlled wind turbine topologies is the use of an AC/DC/AC bi-directional power converter, as shown in Fig. 2c and d, and summarized in Table 2. Depending on the FRT solution implemented for the Type III wind turbine, two different topologies are defined at IEC level, which is explained in Section 4.3. Furthermore, according to the full scale power converter configuration, two Type IV models have been defined at IEC level [73]: Type IVA, where choppers are connected to the DC link and hence the aerodynamic and mechanical components of the wind turbine are not modeled, while in the Type IVB choppers are not used and hence postfault power oscillations are injected and a two-mass mechanical model is required for modeling [67], as is described in Section 4.2. Under these assumptions, Fig. 4 shows the block diagram of these IEC Type III & Type IV wind turbine topologies. Black arrows and modules are used to indicate structure common to these wind turbine types. Grey is used to include the additional modules needed by Type IVB and Type III wind turbines. Finally, one dashed grey arrow identifies how the pitch angle, θ, is the input to the aerodynamic module of a Type III wind turbine. A detailed review about this module as well as the mechanical and electrical models is provided in Section 4. These IEC models are in line with the 2nd generation of the models published by WECC working group, although considerable modifications were carried out in comparison with the first generation of WECC models. Specifically, the first generation of WECC Type III was only truly indicative of one vendors design —GE Energy— [31].

3. Generic wind turbine models definition From the beginning of mass production of wind turbines, wind generation technology has evolved greatly to better accommodate a variable rotor speed operation [21]. Based on the different types of machines available in the market, four generic wind turbine topologies, which have the largest market share across the world [23], have been defined [14], Fig. 2: (a) directly connected induction machines, Fig. 2a; (b) variable rotor resistance induction machines, Fig. 2b; (c) doubly-fed induction machines (DFIG), Fig. 2c and (d) machines connected to the grid via a full size frequency converter, Fig. 2d. The main details of each model Type are provided in the following sections. Table 2 provides a summary. 3.1. Type I & Type II: fixed-speed & variable slip These wind turbine topologies comprise induction machines and represent the oldest type of wind turbines [109,36]. As observed in Table 2, the major difference between them is the rotor configuration and corresponding speed operation. Since these induction generator topologies are directly connected to the grid (see Fig. 2a and b), the power factor during steady-state operation is around 0.9 leading, i.e. absorbing reactive power. Therefore, capacitors, which are assumed to be constant for short-term simulations [35,44], are connected to the generator terminals to improve the power factor [105]. With regard to the control systems, these wind turbines use very simple controls. The smaller and older configurations of the Type I concept ( < 1 MW [92]) used a passive stall mechanism (i.e. no pitch control) to limit the aerodynamic torque imposed by the wind, while the newer and larger Type I designs commonly employ either an active stall or pitch control mechanism [82,110], the latter control also being commonly used in Type II. In addition, for Fault-Ride-Through (FRT)

4. Main generic modules definition Because of the technological complexity of some wind turbine components, IEC and WECC working groups have maintained interesting discussions and remarkable results have been obtained related to some specific modules. Particularly, the aerodynamic and mechanical drive train systems, as well as the generator and power converter representations, described in vendor specific models are quite different and complex. For this reason, this section details the development of the main generic modules of the wind turbines previously defined in Section 3. 5



Fig. 2. Wind turbine types.

unfamiliar with wind turbine aerodynamics. Under these assumptions, the development of generic linearized aerodynamic models involves a reduced number of parameters, facilitating the simulation of large power systems. Due to the complex behavior of wind turbine rotor dynamics, three types of generic aerodynamic models have been developed during recent years by IEC and WECC working groups depending on the wind turbine Type. These are summarized in Fig. 5 (see Appendix A for parameter definition).

4.1. Wind turbine rotor dynamics module: aerodynamic model The aerodynamic model output, paero in Figs. 3 and 4, which represents the mechanical power delivered to the rotor shaft, is a complex non-linear function of pitch angle, shaft speed and wind speed [84]. In addition, blade profile characteristics are manufacturerspecific, may to some extent be confidential [14] and a large number of parameters are needed to define their behavior [85]. Furthermore, this modeling process is extremely difficult for a grid operator

Table 2 Summary of main characteristics of IEC wind turbine model types. Parameter

Type I IA

Type II

Type III

IB

IIIA

Type IV IIIB

IVA

Generator type

Induction, squirrel cage

Induction, wound rotor

Induction, doubly-fed

Either induction or synchronous

Speed operation

Fixed (1–2% slip)

Fixed (up to 10% slip)

Variable (−30% to +40% slip)

Fully variable

FRT capability

No

Yes (pitch control)

Yes No crowbar

6

Crowbar

Yes Chopper

IVB

No chopper



constant for power system simulations involving grid disturbances, as presented in Fig. 5c. The WECC working group developed the first linearized model, called the one-dimensional model, which is detailed in [12]. This considers that paero depends only on the pitch angle variation, dp / dθ . One year later, the evaluation of the influence of the rotor speed variation, dp / dω , on the paero is analyzed in [14], but it was concluded that this term should be removed because of the large inertia of the wind turbine rotor. Therefore, the WECC first generation of models accepted the one-dimensional model [62], which is indicated in grey in Fig. 5c. However, in [61] it is stated that the applicability of this one-dimensional model is limited to power system faults cleared in the range between 150 ms and 200 ms. In fact, considering the typical duration of faults measured in renewable energy power plants [87,89], the effect of the rotor speed change on paero is compared with real measurements in [61], highlighting that the omission of this term may a have a serious consequence on model accuracy. For this reason, WECC 2nd generation models evaluated the addition of this term in their aerodynamic models [31], though it was finally decided not to include it to avoid increasing the number of parameters used by the generic models [68,92]. Nevertheless, looking for a wider compatibility and usability of the generic models, the IEC working group has accepted the use of both linearized aerodynamic models [72]. The black blocks in Fig. 5c show the contribution of this dp / dω term to the complete two-dimensional aerodynamic model. The knowledge behind these linearized aerodynamic models is found in [36,85]. Once the block diagram of a model is defined, the next step is the addition of suitable parameters. For this reason, the present paper provides references with public model parameters found in the scientific review conducted. With regard to the aerodynamic model, parameters for the linearized representation are provided by the authors in [12,61,36,85] based on field campaigns conducted in different Type III wind turbines. Parameters for both the one-dimensional and the pseudo aerodynamic model are found in [62,112,75].

Fig. 3. Block diagram of generic wind turbine Type IA, Type IB & Type II and associated control systems.

Fig. 4. Block diagram of generic wind turbine Type III & Type IV.

4.1.1. Constant aerodynamic torque model The simplest approach to represent the effects of wind is by assuming a constant power or constant torque [61]. This may be the case of Type IV wind turbines where power output is completely differentiated from the wind source [34]. In addition, due to the fixed pitch angle in stall-controlled wind turbines —Type IA—, the mechanical turbine torque may be considered as a constant value equal to the steady-state initial value, Tinit, [27]. It is important to note that the turbine torque is the parameter kept constant, but not the power, paero, because of its dependence on the rotor speed, ωrotor, during the turbine speed excursions, Fig. 5a.

4.2. Drive train module: mechanical model The drive train is the only element of the wind turbine mechanical components to be considered for modeling due to its significant influence on power fluctuations [81,73]. It is composed of several rotating masses together with the corresponding shafts and a gear box is included in some designs [36]. Based on the comparison of different types of drive train models for transient stability analysis conducted in [13], it is concluded that the reduced order two-mass shaft model is more appropriate for this purpose, which is in line with most of the contributions found in the literature, such as [113,20,114,29,69]. Fig. 6 shows this two-mass representation of the mechanical system, which is basically composed of the wind turbine rotor and generator inertias, Hrotor and Hgen, respectively, connected through a relatively flexible shaft with torsional stiffness, ktg, and damping, Dtg. As may be deduced from this figure, the aerodynamic torque, Taero —calculated from paero deduced previously in Section 4.1—, is applied to the rotor inertia to accelerate it. The electrical torque, Tgen, is also applied to the generator inertia to balance the speeding-up of the shaft. The corresponding discrepancy between ωrotor and ωgen is responsible for the mainshaft wind-up in proportion to the stiffness coefficient. In parallel, some natural damping also acts to resist this speed difference. The torque contribution of ktg and Dtg is commonly referred to as “shaft torque” and is associated with both the stored potential energy and the friction losses in the twisted shaft [18]. Therefore, this two-mass model represents the mechanical response of the system when it is submitted to a torque imbalance, for example due to short-circuits, or wind gust or modification of the wind turbine active power setpoint [73]. The influence of Dtg coefficient on the response of the wind turbine model against a voltage dip is examined in [14,107,35], concluding that this mechanical damping has a significant effect. It should be noted that Dtg is a fitted parameter in order to emulate the net damping

4.1.2. Pseudo aerodynamic model Under a voltage dip situation, Type IB and Type II wind turbines use the pitch control to cut down the mechanical turbine power to avoid over-speeding, which is commonly called “pseudo governor model” or “pitch control power model” —depending on WECC [112] or IEC terminology [83,40], respectively—. The WECC working group published a first generation of the pseudo governor model [62,57], but due to some problems detected in certain circumstances, such as frequency excursions [31], a second generation was developed in line with the IEC working group, which is shown in Fig. 5b. As shown in this figure, under normal operation, this controller regulates the aerodynamic power, paero, following the initial power, pinit. Fault operation is activated if two conditions are met: (1) the wind turbine terminal voltage, uwtt, is lower than the voltage FRT threshold, uFRT, and (2) pinit is larger than the minimum wind turbine mechanical power value that has to be considered to have a remarkable effect on the power generated, pset. As a consequence of the activation of this FRT mode, the controller adjusts the blade angle to constraint paero to a minimum power setting, pmin. Finally, paero is also limited according to the ramping rates for increasing or decreasing aerodynamic power, dpmax or dpmin. 4.1.3. Linearized aerodynamic model Type III wind turbines present a different aerodynamic response to the previous designs. Therefore, a linearization of the aerodynamic model has been performed considering that wind speed is kept 7



Fig. 5. Block diagram of aerodynamic model.* IEC 61400-27-1 ed.1.0 “Copyright © 2015 IEC Geneva, Switzerland. 〈www.iec.ch〉”.

[84,32,28]. Other manufacturers also recommend it for Type III wind turbines [80,106]. This is in contrast to some other manufacturers who recommend the use of the two-mass approach not only for fixed speed but also for variable speed wind turbines [22]. Considering previous contributions, the WECC guidelines allow for either a one-mass or a two-mass representation of the mechanical system depending on the wind turbine Type: Type I & Type II may use either the one or twomass mechanical model, whereas Type III & Type IV use the two-mass representation [64,75,74]. In contrast, the IEC Standard has defined the two-mass approach shown in Fig. 6 as the mechanical model to be used for every wind turbine Type. One common relevant issue for both working groups is the location of the generator inertia depending on the simulation software used to implement the mechanical model [44].

perceived by the wind turbine post-fault behavior [92]. In real equipment implementations, active damping controllers are used to damp drive train oscillations, which are significantly different from the generic two-mass mechanical model shown in Fig. 6. In addition, both stiffness and damping are subjected to aging effects due to the wind turbine operation [85]. For these reasons, some efforts have been made to consider the addition of active damping controls in the generic models [115,98]. However, for large-scale power system simulations, the added complexity of trying to model active drive train damping has not been justified by either the IEC or the WECC working group. Some manufacturers recommend the use of the simple one-mass shaft model in cases where the power converter decouples the generator from the network, such as in a Type IV wind turbine 8



in the rotor together with large power injected into the DC circuit [117–119]. Therefore, the rotor and rotor-side converter need to be protected against over-current and over-voltage. In this regard, a DCchopper and/or an AC-crowbar are the solutions used by manufacturers to evacuate the surplus energy generated during the power system fault, which are shown in grey in Fig. 2c and are reviewed in [120]. Specifically, ABB and GAMESA designs use a crowbar on their commercial Type III wind turbines [115]. The crowbar was used in older DFIG designs, whereas the chopper and/or a higher rating of the rotor-side converter has been implemented in recent Type III wind turbines [108,71]. The crowbar is basically a resistance installed in the rotor circuit for a short period to disconnect the rotor-side converter [81,82]. Therefore, during crowbar operation, a Type III wind turbine is operated as a Type I, which involves two performance dynamic characteristics: delay on the delivery of voltage control functionality, and active and reactive power transients after fault inception and fault clearance [60,119]. The chopper is an IGBT-switch resistance parallel connected to the dc-link [29]. As a result, it is possible to control the whole converter during chopper activation. Since the switching of these protection devices depends on the dclink voltage and/or the rotor current values, detailed EMT models are needed [38]. For this reason, different approaches have been proposed for an appropriate emulation of the crowbar operation for power system stability studies. A specific model allowing a detailed representation computed under a RMS simulation was presented in [121], although reduced integration time steps were used. The approach presented in [26] is based on encoding the crowbar activation time depending on both the network voltage dip magnitude and the generator speed. Based on [26], the dependency of generator speed is substituted by the initial power in [60], presenting a good response in comparison with real measurements from a 2 MW GAMESA wind turbine. In this sense, the measured response of several Type III wind turbines submitted to voltage dips is compared with the Type III WECC model response in [115,98], where erroneous behavior of wind turbines equipped with crowbar is observed. As a consequence, WECC members evaluated including the simplified crowbar model proposed in [60], although this was finally rejected because these additions added more complication and did not necessarily yield greater fidelity for the intended purpose and main aim of these generic models [38]. In contrast, based on the motivations given by different manufacturers —basically ABB and GAMESA— [56,60], the IEC working group decided to define two Type III versions aiming at an improved model accuracy during grid faults, as summarized in Table 2. Considering previous findings, modeling the generic Type III generator system is based on the DFIG voltage and flux equations, which are well known from the literature, although stator flux transients are not considered because it is a common assumption for stability simulations, in line with [122,104,59,105,108]. The simplifications made to obtain a generic Type III generator system are presented in [105], which is based on GE Energy design [80,106,84], and was adopted by the WECC first generation of generic models [54]. In the case of REpower vendor —currently known as Senvion—, a DFIG generator model for stability analysis was first developed in [121] and later improved in [108,71,85], with regard to the model response accuracy, especially during grid faults and voltage recovery. In summary, Fig. 7 presents this Type III generator system model, which is the final solution adopted by both IEC and WECC entities for wind turbines without a crowbar, parameters of which are defined in Appendix A. The output of the generator model is the current injected into the grid, iwtt, whereas the inputs are the active and reactive power commands, i pcmd and iqcmd , which are given by the active and reactive power control modules. These are out of the scope of the present paper due to the wide diversity of control concepts implemented by manufacturers. If a Type III wind turbine equipped with crowbar requires modeling, it is only necessary to act on these i pcmd and iqcmd . Further details about generic crowbar modeling are presented in [60]—. The air-gap power,

Fig. 6. Block diagram of two-mass mechanical model.

From Fig. 6, it is assumed that the built-in induction generator model in the simulation software will not include its inertia equation since it is included in the mechanical model. With regard to the parameters associated with the mechanical model, more information has been published than for the aerodynamic model. Current wind turbines, where gear ratios increase to 100, stiffness values in the range of 0.15–0.40 pu are observed [16], compared to values in the range of 20–80 pu for a conventional synchronous generator. In this sense, the matching between inertia constants and inertia moments as well as between pu units and physical units are fully detailed in [36,116]. Specific mechanical model parameters are provided depending on the wind turbine Type, as follows:

• • • •

Type IA wind turbine data are provided in [107,35,40], for a 180 kW and a 2.3 MW rated power wind turbine, respectively. Type III with 1.5 MW and Type IV with 2 MW rated power data are found in [105,43,73], respectively. GE Energy provides the mechanical parameters of three specific Type III and one Type IV design, which correspond to commercial wind turbines in the MW range, in [84]. In addition, WECC reports, such as in [62,115,98,75], as well as in CIGRE reports in [18], present mechanical model data.

4.3. Generator and converter module: Type III & Type IV generator systems In contrast to the generator systems defined for Type I & Type II wind turbines, where IEC and WECC working groups recommend the use of the standard asynchronous generator included in the specific software application library, the development of the generator systems for Type III & Type IV wind turbines has been the subject of several discussions held during recent years. Furthermore, based on the comparison of the response provided by modern Type III & Type IV wind turbines during grid faults with regard to active and reactive currents, slight differences between both technologies have been found. For this reason, only one modular approach that allows the model to be implemented either for Type III or Type IV designs by changing the model parameters has been considered. The most important issue involved in the development of the generic Type III generator system is the modeling of the crowbar system, which is needed to meet FRT requirements of recent grid codes [105]. Riding through grid faults with a DFIG will imply large currents 9



Fig. 8. Block diagram of pitch angle control model.

either electric or hydraulic, is modeled with its corresponding servo time constant, Tθ , as the pitch angle is not able to change instantaneously. As indicated in Fig. 8, the pitch angle limitation, θmax and θmin, and the pitch angle rate of change, dθmax and dθmin, which mainly depends on the wind turbine size [55], must be considered. Specifically, the rate-of-change limitation parameter plays a key role during power system faults because it avoids overspeeding during faults by controlling how fast the aerodynamic power is reduced [81,45]. In addition, the speed and power integrators represented in Fig. 8 are anti-windup —also known as non-windup—. This means that when the pitch angle is at its lower limit, θmin, the integrator is blocked if the input error is negative and, as a consequence, moving further down while θ remains at its minimum value is avoided. Similarly, the integrator is blocked if the input error is positive and θ reaches its upper limit. Under these assumptions, the WECC 2nd generation models and the IEC working group adopted the pitch angle control model shown in Fig. 8. However, the first generation of the WECC pitch angle control model did not include the cross coupling gain, Kpx, which was added after discussions held with the IEC group [68,47,92]. In fact, this is the only difference between the two generations of the WECC models. This Kpx plays an important role due to the relation between rotor speed and power in Type III wind turbines. Specifically, in a DFIG design, the turbine is already operating at rated rotor speed before reaching the nominal wind speed. Therefore, under certain conditions, such as a voltage dip, the pitch could be activated (θ > 0) although the active power generated is lower than 1 pu. To avoid this situation, Kpx gain is used to modify the signal introduced into the rotor speed error in proportion to power deviation. With regard to the parameters associated with the pitch angle control module, in comparison with the generic models analyzed in the previous sections, a wide variety of studies have been published [80,124,81,18,123,111,105,84,114,28,36,45,55]. This is because the generic pitch angle control model is quite similar to most of the specific pitch angle control systems found in the literature.

Fig. 7. Block diagram of Type III & Type IV generator system model, [100].a IEC 61400-27-1 ed.1.0 “Copyright © 2015 IEC Geneva, Switzerland. 〈www.iec.ch〉”.

pag, is an additional output of the model, which is an input for the mechanical model depicted in Fig. 6. The most relevant difference between a Type III and a Type IV wind turbine generator system is depicted in grey in Fig. 7, which is the term 1/ xs . Since the stator of a Type III wind turbine is directly coupled to the network, a sudden voltage variation will cause a change in the reactive power flow. Therefore, the term 1/ xs is added in a Type III wind turbine to simulate these electromagnetic transients [19]. However, with regard to the WECC generic models, this term was added in the WECC 2nd generation models based on the discussions held with IEC members [115,98], which was the most relevant important modification to the WECC first generation models. With regard to the parameters associated with the generator system model, GE Energy collects them for their 1.5, 1.6 and 3.6 MW Type III and 2.5 MW Type IV wind turbines in [84], which have been used by WECC members to analyze generic DFIG model response in [54]. 4.4. Pitch angle control module When wind speed exceeds its nominal value, pitch angle control is one of the most common methods used to limit the wind turbine aerodynamic torque [36,110]. Generator power, rotor speed and/or wind speed are some of the pitch control variables typically used [123]. However, wind speed is not a suitable variable because wind speed measurements are not accurate enough [81,121]. Therefore, two PI controllers —one associated with rotor speed and the other with generator power— have been widely used in the literature to model pitch control. Based on a detailed pitch angle control implementation, two different types of simplified models for pitch control are simulated and compared in [51], concluding that the generator power compensation loop may be ignored because it is used in some designs only, in line with [10,18,28]. Nevertheless, most of the authors, including some manufacturers, prefer the implementation of the two PI controllers [80,121,84,54], the block diagram of which is shown in Fig. 8 and the parameters defined in Appendix A. The upper region of Fig. 8 shows the rotor speed control involved in the adjustment of the rotational speed of the rotor, ωrotor —which is derived from the mechanical model presented in Section 4.2—, to its reference value, ωref, while the power compensation control is below. In fact, external power curtailment demands, from either the grid operator or the wind power plant control, require the implementation of this additional power control [85]. Furthermore, on the right side, the pitch actuator, which may be

5. Conclusions Wind energy must be integrated in the power systems in a proper manner. To evaluate the effects of wind power generation on network performance, transient stability analysis, which need dynamic models of wind turbines, are conducted by system operators. For this purpose, in recent years, generic dynamic models of wind turbines have been defined by several organizations, mainly IEC and WECC. As a result, the international standard IEC 61400-27 was published in February 2015. Nevertheless, because of the wind technology complexity as well as its rapid development, a review was required to understand the reasons motivating the finally accepted generic model designs. In this line, since the generic models are simplifications of very complex 10



discussion. In this sense, due to the reasons given by some manufacturers, two versions of the Type III generator system have been defined by IEC, while the WECC working group has accepted only one model. With regard to the control systems of wind turbines, the pitch angle control module has been detailed It has been justified that both IEC and WECC working groups have accepted a model that contains two controllers, one related to the rotor speed control and the other related to the power compensation, which are connected through a cross coupling gain. The review conducted in this paper is thus of particular interest for system operators, wind turbine manufacturers, software developers, technical consultants and other stakeholders interested in the integration of wind power into electric energy systems. Furthermore, not only the model block diagrams but also references with public model parameters have been included aiming to improve the usability of the generic dynamic wind turbine models for power system stability analysis.

systems, a further understanding of the changes made to the models was needed. Therefore, models that have been subjected to major discussions and changes during their development have been described in this paper. Furthermore, this paper specifies the main differences between IEC and WECC models, which are mainly based on the different validation requirements imposed in North America and Europe. With regard to wind turbine topologies, both working groups decided to focus on the common four commercial wind turbine designs, from Type I to Type IV. Although the recently published IEC models and WECC 2nd generation models share common characteristics, some noticeable differences have been identified. In contrast to the WECC generic models, the IEC working group decided to split Type I and Type IV wind turbines into two different topologies depending on the FRT behavior. Furthermore, WECC 2nd generation models for Type III and Type IV wind turbines were subjected to considerable modifications in comparison with WECC first generation due to the lack of real measurements. Specifically, the first generation of WECC Type III was only truly indicative of one vendor's design. On the other hand, the IEC and WECC working groups have held interesting discussions and major results have been obtained related to some specific generic wind turbine modules. Firstly, to represent the aerodynamic response of the wind turbine, these working groups have developed three types of generic aerodynamic models depending on the wind turbine Type: constant aerodynamic torque model for some Type I and Type IV wind turbines, pseudo aerodynamic model for some Type I and Type II, while a linearized aerodynamic model has been developed to represent Type III wind turbines. Furthermore, both working groups agree that the mechanical representation of wind turbines is depicted by a two-mass shaft model, although the WECC working group also allows the use of the single-mass equivalent for Type I and Type II wind turbines. In addition, the generator and converter module of Type III and Type IV has been the matter of much

Acknowledgment This work was supported by the “Ministerio de Economía y Competitividad” and the European Union —FEDER Funds, ENE2016-78214-C2-1-R—, the research stay funded by “Universidad de Castilla-La Mancha” and the grant provided by Fundación IBERDROLA ESPAÑA. The authors thanks the International Electrotechnical Commission (IEC) for permission to reproduce Information from its International Standards. All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved. Further information on the IEC is available from http://www.iec.ch. IEC has no responsibility for the placement and context in which the extracts and contents are reproduced by the authors, nor is IEC in any way responsible for the other content or accuracy therein.

Appendix A. Glossary of model parameters

Parameter

Definition

θ θ0 θmax θmin ωgen ωref ωrotor ωs Caero dθmax dθmin di pmax

Blade pitch angle Initial blade pitch angle Maximum blade pitch angle Minimum blade pitch angle Generator rotational speed Reference wind turbine rotor rotational speed Wind turbine rotor rotational speed Synchronous rotational speed Offset for term relating the change of aerodynamic power with pitch angle Maximum blade pitch positive ramp rate Maximum blade pitch negative ramp rate Maximum active current ramp rate

diqmax Dtg Hgen Hrotor igen i pcmd i pmax iqcmd

Maximum reactive current ramp rate

iqmax

Maximum reactive current

iwtt Kaero Kpx

Wind turbine terminal current Slope for term relating the change of aerodynamic power with pitch angle Pitch cross coupling gain

Drive train damping Generator inertia Wind turbine rotor inertia Generator current Active current command Maximum active current Reactive current command

11



ktg paero pag pinit pmin pord pref pset pwtt pwttref Rrot Tθ Taero Tgen Tinit uFRT ugen uwtt Vwind xs

Drive train torsional stiffness Aerodynamic power Generator air-gap power Initial power (parameter set by the load flow) Minimum power setting Power order Reference power Minimum wind turbine power value considered to have a remarkable effect on the power generated Wind turbine terminal active power Wind turbine terminal active power reference Rotor resistance Pitch servo time constant Aerodynamic torque Generator torque Initial aerodynamic torque voltage fault-ride-through threshold Generator voltage Wind turbine terminal voltage Wind speed Electromagnetic transient reactance

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