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4th Workshop on Communication Networks and Power Systems (WCNPS 2019)
Investigation on Full-Converter-Based Wind PowerPlant Behavior During Short-Circuits
Jônatas S. CostaDept. of Electrical Eng.
University of Brasília (UnB)Brasília, Brazil
Rodrigo T. ToledoDept. of Electrical Eng.
University of Brasília (UnB)Brasília, Brazil
Letícia A. GamaDept. of Electrical Eng.
University of Brasília (UnB)Brasília, Brazil
Gabriel B. SantosDept. of Electrical Eng.
University of Brasília (UnB)Brasília, Brazil
Felipe V. LopesDept. of Electrical Eng.
University of Brasília (UnB)Brasília, Brazil
Paulo S. Pereira JrCONPROVE
Industry and CommerceUberlândia, Brazil
Gustavo S. SalgeCONPROVE
Industry and CommerceUberlândia, Brazil
Moisés J. B. B. DaviCONPROVE
Industry and CommerceUberlândia, Brazil
Abstract—This paper presents an investigation on the behaviorof a full-converter-based wind power plant (FCWPP) duringshort-circuits. To do so, an actual power network is taken intoaccount, in which a transmission line that connects a FCWPP toa traditional system fed by typical high inertia generation unitsis studied. The test system was modeled and tested by meansof Electromagnetic Transients Programs (EMTP), namely Alter-native Transients Program (ATP) and Power System Simulator(PS SIMUL). Single-phase fault scenarios at different positionson the studied line were tested and then, voltage and currentcharacteristics at both terminals were investigated. The obtainedresults show that the FCWPP control leads voltages and currentsto present different patterns from those verified when typical syn-chronous machines are considered, which can impair monitoringfunctions or even interfere disturbance analysis studies.
Index Terms—EMTP, fault analysis, full-converter, wind powerplant, short-circuit, transmission line, power systems.
I. INTRODUCTION
The gradual reduction of fossil fuels usage in power gener-ation systems has been a topic of global interest. Discussionsand solutions to problems such as environmental pollutionemitted by fossil fuel based power generation units havedictated the trend of exploring renewable energies [1].
Along with demographic growth, electricity demand hassubstantially increased over the years. In accordance with thesocial and environmental aspirations that call for clean energysources, Brazil has invested in alternative power generationsources, so that 86% of its generated electric power will derivefrom renewable sources by the end of 2027 [2]. Regardingthe wind power generation capacity, it is expected to increaseby up to 2000 MW per year mainly through the explorationof Brazil’s wind potential. As a result, challenging scenarioswill arise in the Brazilian power grid, since wind powerhas different technical and operational characteristics whencompared against commonly used power sources, such as nonelectronically coupled rotating machines.
Concerns arise about possible impacts that the growth ofwind sources connections may have on steady and transientstate. These complications emerge due to its intermittent op-eration and the use of electronic converters. The combinationof those factors can change the traditional power systemoperation features, also influencing in voltage and currentbehavior during short-circuits [3], [4].
Based on the aforementioned context, researches on possibleimpacts caused by wind power plants on power networksoperation have attracted the attention of utilities. Indeed, stud-ies need to include mechanical and system control modelingand also short-circuit analysis in order to cover differentpaths in power systems operation procedures that may beaffected by converter-interfaced generation units. Thus, thispaper investigates the voltage and current signals behaviorin a 500 kV/60 Hz transmission network that connects atypical power system with a FCWPP. In order to do so, apower system with wind power integration was reproducedin EMTPs using data taken from [4]. As a result, modelingaspects are firstly addressed and then short-circuit studies arecarried out by means of the comparison between voltage andcurrent signals measured over the test system. Obtained resultsdemonstrate that the FCWPP can result in atypical voltage andcurrent waveforms, which differ from those observed whentraditional power generation is considered.
II. FULL CONVERTER-BASED WIND GENERATION
According to the wind turbine types described in [5], there isa number of different possible configurations that may be usedto assemble the generation units. In this paper, the investigatedwind power unit is defined as Type 4, whose mathematicaldetails can be found in [4]. It is composed by a mechanicalturbine, a synchronous generator (SG), and a full-converterwith its associated controls, as depicted in Fig. 1.
The turbines play the role of converting the kinetic windpower into mechanical power, which in turn drives the syn-978-1-7281-2920-4/19/$31.00 © 2019 IEEE
4th Workshop on Communication Networks and Power Systems (WCNPS 2019)
==
=~
~=
Wind Turbine
Grid
Field excitation
SG
Rectifier Inverter
Fig. 1. Wind Turbine Evaluated.
chronous generator. This type of mechanical turbine dependson a power coefficient, which expresses the relation betweenthe available wind power and the one indeed extracted by thewind turbine itself [6].
The synchronous machine works as in other typical powergeneration applications. The rotor winding is fed by directcurrents, inducing a magnetic field which varies in time whenit passes through the stator coils. As a result, voltage signalsare induced in the generator terminals, supplying power toconnected loads. The amplitude and frequency of terminalvoltages depend on the machine mechanical speed, which mayfigure as an issue due to the variant wind speed. Therefore,power converter-based interface between the machine and thepower system is taken into account, making the machine ter-minal voltages to have well behaved frequency and amplitude,so that the generator can operate over a range of speeds [7].Despite the challenges that may arise when using converterinterfaced wind power plants, one should bear in mind theuse of power electronics is crucial for the proper operation ofthis kind of power generation [6].
Regarding the power conversion solutions, several optionshave been reported in the open literature and by manufacturers.In the evaluated FCWPP, the converter on the generator sideconsists of a passive rectifier bridge and serial boost DC/DCconverters. The rectifier unit is associated with another bucktype DC/DC converter, through which the field winding ofthe synchronous machine is fed. The grid side converter ischaracterized by a two-level voltage source inverter, whichis responsible for delivering all the power generated by themachine to the grid respecting the applicable standards. Thecontrol associated with the generator side converter is expectedto extract the maximum electrical power of the SG given theconditions of mechanical turbine supplied power, consisting ofthe so-called maximum power point tracking [8]. The machineexcitation control works by relating the turbine mechanicalspeed to its field current, producing a variable excitation whichdepends on the mechanical speed in the axis of engagementbetween turbine and generator. Finally, the control associatedwith the inverter unit aims to control the machine power factorand the power flow given the operating network conditions.By doing so, the FCWPP becomes capable of controlling itsstability under different network abnormalities.
III. STUDIED POWER SYSTEM
The power system presented in Fig. 2 was modeled byusing the ATP [9] and PS SIMUL [10] platforms, whichare EMTP software that allow the detailed modeling of theFCWPP control units. By doing so, differences between theEMTP simulations could be analyzed, attesting that the majorshort-circuit behavior of the FCWPP is properly representedby both simulation tools despite of slight deviations.
The test power system consists of a 500 kV/60 Hz transmis-sion line 239 km long that connects three wind power plants(local bus) and Thévenin equivalent circuits, which representthe power grid around the analyzed system (remote bus). Thetransmission line was modeled using the distributed parameterline model, constant in frequency, as fully transposed line. Thewind power units are composed of full-converter wind turbinegenerators, which were modeled in detail as reported in [4].The FCWPP behavior during short-circuits is evaluated bymonitoring the voltage and current signals at the local and re-mote terminals while applying faults at different points on theanalyzed transmission line. Aiming to evaluate cases that yieldadverse asymmetrical currents, phase-A-to-ground (AG) faultswere simulated, which in turn consist in the most common typein power networks. Assuming that typical protective relaysoperate in average times of about one power cycle and thatthe electromechanical opening of traditional circuit breakersusually occur in about two power cycles after the protectionoperation [11], three cycles before and after the fault inceptionwere analyzed in the presented results.
ATP and PS SIMUL simulations were carried out using1.0 µs time step, emulating real analog signals. A third orderlow-pass anti-aliasing Butterwoth filter with cutoff frequencyat 480 Hz was applied and then the filtered signals were re-sampled to 32 samples per cycle of 60 Hz in order to proceedwith the phasor-estimation process. In this paper, the modifiedcosine filter was utilized [12].
IV. ANALYSIS AND RESULTS
In order to evaluate occasional differences caused by dis-crepancies in the transients computation methodology in ATPand PS SIMUL, an specific comparison analysis was per-formed considering an AG fault at 10% of the monitoredline length. Voltage and current signals disregarding the anti-aliasing filter for each phase are displayed on Fig. 3.
As one can see, the compared waveforms conduct them-selves quite similarly. Regarding the voltages signals, it was
Fig. 2. Studied Power System.
4th Workshop on Communication Networks and Power Systems (WCNPS 2019)
Fig. 3. Waveforms obtained from EMTP simulations using ATP and PS SIMUL.
Fig. 4. Comparison Scatter Plot.
observed a sag on phase A, as expected, at both line terminals,while the sound phases experience a smooth swelling. It shouldbe noted that vL,A oscillates with high frequencies after faultinception, as expected due to the proximity of the fault inrelation to the local bus (wind power plant terminal).
The local current signals show in all phases a slightincreasing at the wind power plant side, which are muchsmaller than those observed at the remote terminal, i.e., at thetraditional power network side. This is an interesting aspect
to be investigated especially because the fault occurs close tothe local line end and this abnormal characteristic could leadprotective functions to misoperate.
To provide a more thorough comparison between ATP andPS SIMUL simulations, Fig. 4 depicts a scatter plot that showsthe correlation between the signals obtained by means of bothprograms. In other words, the scatter plot creates a Cartesianpoint with samples taken from ATP and PS SIMUL, therebyplotted points are expected to concentrate themselves on thecoincidence central line highlighted in the figures.
From the obtained scatter plots, despite slight differences, itis noted that most samples take place around the coincidenceline, demonstrating that the major FCWPP behavior undershort-circuit conditions is properly represented by both ATPand PS SIMUL platforms. It proves that the use of differentEMTP softwares should not result in significant differences inthe FCWPP operation during faults, specially when only fun-damental component analysis is considered. Thus, from nowon, short-circuit contributions will be evaluated consideringonly PS SIMUL data.
Aiming to demonstrate the different behavior of voltagesand currents at local and remote buses at which FCWPP andtraditional power network are connected, respectively, Fig. 5illustrates scatter plots obtained by considering the absolutevalue of voltage and current fundamental components at bothline ends. Results for faults at 10%, 30%, 50%, 70% and 90%of the line length (taking the local line terminal as reference)are tested.
4th Workshop on Communication Networks and Power Systems (WCNPS 2019)
(a)
Fault Period
Steady State
(b)
Fig. 5. Scatter plot of: (a) Currents; (b) Voltages.
From Figs. 5, it is concluded that significant voltage andcurrent variation at the traditional power network terminalare verified, which vary their values depending on the faultlocation. On the other hand, at the wind power plant side, itis noted that voltages and currents converge for very similarvalues, irrespective of the fault distance, which differs inrelation to the expected behavior for traditional generationsources. This behavior is due to the associated control schemesof power electronic converters on wind power plants. Indeed,one can see that the areas with greater density of scatterplot samples are vertically aligned, proving that the FCWPPelectrical quantities do not significantly vary when comparedto the variations observed at the traditional power grid.
From Fig. 5(a), it can be observed that all points take placeabove the coincidence line, demonstrating that very low faultcurrent contributions are expected from the modeled FCWPP.A similar behavior is verified in Fig. 5(b), attesting that voltagesags at the FCWPP terminal were greater than those observedat the traditional power network line side.
It is worth to mention that the obtained results bring up
questions on the atypical behavior of converter-interfacedwind power generators under short-circuit conditions. Indeed,significant voltage sags and specially the low fault currentcontributions may compromise the system protection opera-tion, so that appropriated schemes should be developed. Infuture works, it is intended to analyze the impact of themodeled FCWPP on protective relays under different faulttypes, detailing the relation of obtained voltage and currentwaveforms with turbine control aspects. In addition, it isexpected to evaluate other wind power generator topologies,indicating potential solutions for challenging scenarios thattraditional protective devices may face.
V. CONCLUSIONS
In this paper, an actual power system was modeled andtested with the aim to compare the behavior of voltage andcurrent signals considering short-circuits on a 500 kV/60 Hztransmission line that connects full-converter wind powerplants to a power network with traditional generation units.Firstly, different EMTP programs were used to evaluate dif-ferences between transient simulations, demonstrating thatthe major system behavior is properly represented despiteslight differences. Then, scatter plots were plotted in orderto compare voltages and currents measured at the converter-interfaced wind power plant line side and those taken fromthe power network terminal. The obtained results show thatvoltage sags are greater at the wind power plant side, wherevery low current contributions are verified. These operationalfeatures consist in challenging scenarios for protective deviceswhich must be considered in actual systems.
REFERENCES
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