Field-test of wind turbine by voltage source converter

One of the main challenges for wind energy development is making wind turbines efficient in terms of costs whilst maintaining safe and reliable operation. An important design criterion is to fulfil the grid codes given by the transmission system operators. Grid codes state how wind farms must perform when connected to the grid under normal and abnormal conditions. In this regard, it is well-known that not all technical requirements can be tested by using actual impedance-based test equipment. Therefore, test equipment comprising a fully rated voltage source converter in back-to-back configuration is proposed. Thanks to the full controllability of the applied voltage in terms of magnitude, phase and frequency, the use of voltage-source-converter-based test equipment provides more flexibility compared to actual test systems. As demonstrated in this paper, the investigated test device not only can recreate any type of fault, including its recovery ramp, but also can carry out steady-state tests, such as frequency variations and frequency scan, on the test object. Finally, test results from a 4.1 MW wind turbine and 8 MW test equipment located in Gothenburg, Sweden, are shown to validate the investigated grid code test methodology.


Questions from reviewer Henk Poulinder
This paper presents test results of new method to test if a wind turbine meets the grid codes using a voltage source converter. What I appreciate very much is that this paper does not report on more simulations (as many other publications do), but on a test setup that has been built to test wind turbines. Building a voltage source converter with a power level of 8 MW and controlling it with such dynamics that it can simulate grid faults is a huge engineering job. This setup makes it possible to do lots of other tests that the current test setups using voltage dividers cannot do. The authors presented this idea in earlier publications. This paper presents test results that show the equipment works. 2. Doing a frequency scan is something that is possible with a VSC. However, why is that useful? What can we do with the results? 3 p9, line 1, I think the ramp is 12.5 pu/s instead of 0.0125 pu/s. Is that correct? 4. I miss a reference to what seems to be a similar paper: C Saniter; J Janning, "Test Bench for Grid Code Simulations for Multi-MW Wind Turbines, Design and Control", IEEE Transactions on Power Electronics, 2008, Volume: 23, Pages: 1707 -1715. What does the paper under review add to this one? 5. There are too many language mistakes. A number of examples: p3 line 4: In every grid code it is specified => Every grid code specifies p3 line 6: In fig. 1 is shown => Fig. 1 shows p4 line 7: dependent of => dependent on p4 line 24: can be of used to obtain => can be used to obtain p7 line 10-13: unclear sentence p9 line 14: as later see =>?? p12 line 3: signal of the wind turbine converter are => signals of the wind turbine converter are p12 line 5: have being kept => have been kept p12 line 7: dip => deep Replay: Dear Henk, Thanks for the review, some comments follows, we have also updated the paper for most of your comments. Especially we have in a more clear way explained the benefits with frequency scan and improved the paper linguistically.
1. As explained in the text the LVRT control is not activated in the small dip test in fig 5 whereas in fig 6 the LVRT mode is activated and the control of active and reactive power starts a small oscillation in active and reactive power. And this can be a message for even larger transients 4. Thank you for the suggestion and for the interest in the topic. The abstract indicate that that paper describe the test setup and not the use of it in a real test with a large wind turbine A discussion will be added to our paper and the list of reference will be updated. In the new text can be read: "For example, the test setup presented in Saniter and Janning (2008) consists of a complex configuration of several VSCs and a three-winding transformer. The paper discusses a comparison between no-load tests and simulation results, while the effectiveness of actual tests remain" 5. Thank you for pointing this. The paper will be checked for proofread.
The editor also like us to explain the scientific value of the paper: This paper has demonstrated that the full characterisation of the wind turbine system can be carried out by using flexible VSC-based test equipment. The full controllability of the test device allows for testing of multiple grid scenarios, making it possible not only to determine the behaviour of the generating unit against common grid contingencies, but also to evaluate the performance of the generating unit in further improving overall grid reliability. This includes evaluating different operating modes of the wind turbine which can be of interest for the overall stability of the interconnected power system. The unique field tests presented in this report have provided an experimental validation of the proposed wind turbine testing methodology, particularly on the wind turbine impedance characterisation and on the evaluation of its steady-state and dynamic performance under different grid conditions.
The requirements for steady-state operation of the grid can be mainly categorized in ::::::::: categorised :::: into three groups: reactive power requirements for normal voltage operation range; reactive power requirements during nominal active power production; 15 and minimum operation time and active power curtailment during long-term frequency deviations.

Requirements for steady-state condition of the grid
A TSO can require reactive power injection from the wind farms to support overall system voltage control during normal operation of the grid ::: grid :::::::: operation. Usually, reactive power requirements are delimited inside :::::: within a minimum power factor range that goes from 0.95 lagging to 0.925 leadingand for , : an active power set-point :: of between 0.05 pu and 1 pu ; and within 25 a nominal voltage that varies :::::: varying between 0.9 pu and 1.15 ::: 1.1 pu.
3 Description of the testing ::: test facility

VSC-based testing system description
The test equipment is rated in : at : 8 MVA at 10.5 kV. The wind turbine is coupled to the testing ::: test device at the primary of the coupling transformer, as shown in Fig. 2. The secondary of the transformer is rated at 9.35 kV and its impedance 0.08 pu.
An LCL :::: LCL filter bank is placed in order to remove the harmonic content produced by the turbine-side VSC :::: VSC :::: itself. This 5 converter controls the AC voltage imposed to :: on the wind turbine system, while the grid-side converter is controlling ::::::: controls the DC-link voltage by exchanging active power with the interconnecting grid. The testing ::: test : equipment is interfaced with the grid means of LCL :::: 10.5 ::: kV ::: grid ::::: using ::::: again :: an :::: LCL : filter bank and coupling transformer, which grid-side is again 10.5 kV.
Note that the AC grid is decoupled from the tested object when performing the test; meaning that the strength of the grid is not a major limitation. Finally, between . ::::::: Finally, the wind turbine and the testing equipment are interconnected ::: test ::::::::: equipment ::: are 10 :::::::: connected : by a 300 m cable.
In this installation, only the three-phase voltages and three-phase currents at the PCC are sampled by a computer located in the control room of the HVDC station ::::: HVDC :::::: station :::::: control ::::: room. The instantaneous active and reactive power are calculated off-line.

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a dedicated open-loop voltage control is implemented. The output of the controller is the reference value for the output voltage of the VSC :::: VSC :::::::: terminal :::::: voltage. Finally, a PWM modulator is used to compute the switching signals of the converter (Espinoza, 2016

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The wind turbine under test is rated in :: at 4.1 MVA at 10.5 kV.  One of the first tests carried out on the testing facility corresponds ::: test :::::: facility ::: on ::::::: January ::::: 13th, ::::: 2015, :::::::::::: corresponded : to a voltage dip at full power production. The following tests were conducted on January 13th, 2015. The first attempt was to select a relatively small voltage variation, with a smooth transition between normal operation level and retained level. In order to 10 safely conduct the experiment ::::: safely while learning the dynamics of the system, the voltage is ::: was : reduced from 1 pu to 0.9 pu for 100 ms. The results are shown in Fig. 5. In the following and in all plots, a base voltage of 10.5 kV and base power of 4.1 MVA is utilized ::: used.
power production during the voltage reduction, the generating unit increases the magnitude of the current, while maintaining a constant power production, as also depicted in Fig. 5. During the voltage variation, the wind turbine maintains its active power set-point, injecting a steady 0.9 pu of active power into the imposed grid. From the figure, it is possible to observe ::: can ::: be ::::::: observed : that the wind turbine does not engage its LVRT control. Thus, the reactive power is reduced only :::: only :::::: reduced : due to the reduction of the voltage across the AC-link between the wind turbine and the testing ::: test equipment.
The wind turbine active power set-point is maintained at 1 pu, while the reactive power is boosted :: 0.2 ::: pu : at :::::::: pre-fault, : reaching a mean value of 0.4 pu during the voltage dip. Finally, it is possible to observe that the pre-fault reactive power exchange at the measurement point is 0.2 pu. The wind turbine injects an additional 0.2 pu of reactive power when detecting the voltage dip at its terminals. Therefore, a total of 0.4 pu is maintained until the voltage is increased back to 1 pu.
The voltage starts to recover ::: The :::::: voltage ::::: starts ::: its ::::::: recovery : at 0.18 s with a :: as ::: per ::: the ramp function. Observe :::: Note : that the current is also reduced at the same time that the reactive power is brought back to 0.2 pu. The active power oscillates at 0.23 s while :::: when : the voltage has reached a steady-state level of 1 pu. Observe :::::: Finally, :::::: observe : that the current is above 1 pu during 5 the voltage dip, meaning that the wind turbine have over-current capabilities and it is capable to momentarily increase the output current beyond nominal values during the voltage dip :::::::::: contingency.

Reactive power control during voltage dip
A similar test has been ::: was carried out on the 17th of May of 2016. Unfortunately, the wind turbine was operating at low wind speed. However, the lack of produced active power ::::: active ::::: power :::::::: produced : made the variations in the reactive power more 10 prominent, as later shown ::::: shown :::: later in this section. This experiment consists in ::::::: consisted :: of : a voltage variation from 1 pu to 0.75 for about 200 ms. In order to :: To : avoid an oscillatory response similar to the ones ::::: those experienced in the previous experiment, the down-slope ramp of the controlled VSC voltage is set at 0.02 ::: was ::: set :: at :::: 0.04 : pu/ms and the recovery ramp is set at 0.006 ::::::: recovery ::::: ramp :: at :::::: 0.0067 : pu/ms. From the voltage waveform given in Fig. 7, it is possible to observe that the voltage is controlled by the testing ::: test : equipment in an effective and smooth way. Moreover, due to reduced ramp-rate :::: slope 15 at dip inception , as compared to 10 times faster in Fig. 6, the oscillatory response is ::::: shown :: in :::::: Fig. 6 ::: was not triggered.
The active power was set to 0.15 pu during this test. Moreover, the ::: The reactive power seen in Fig. 7 : at ::::::: pre-fault : is mainly due to the capacitor bank of the local LCL filter placed at the terminals of the turbine-side VSC of the testing :: test : equipment, while the variation observed during the voltage dip is due to the control action of the wind turbine.

Testing for unbalance voltage dip
In this section :::: This :::::: section ::::::: studies the response of the wind turbine under unbalanced voltage dipis studied. This test was carried out again on the :::::::: conducted ::: on : 17th of May of 2016, at :::: under : low wind conditions. The :::: Here, ::: the : turbine-side VSC 5 of the actual testing ::: test equipment is controlled in :: an : open-loop and the voltage in phase a and b :::::: voltages ::: in ::::: phases :: a :::: and : b are dropped from 1 pu to 0.7 pu for 200 ms. The resulting PCC voltage is given in Fig. 8. During the duration of the test, the voltage in phase c is maintained at 1 pu from the VSC output. However, due the negative sequence across the AC circuit between test-VSC ::: test ::::: device : and wind turbine, phase c appears to be slight :::::: slightly lower than 1 pu at the measurement point.
. For a better understanding of the dynamics inside the wind turbine converter, the physical magnitudes of speed, voltage and If the voltage reduction is not dip enough ::: does ::: not ::: dip :::::::::: sufficiently, the wind turbine might have some extra room in its rating in order to maintain normal power production at a reduced voltage (see Fig. 5). This can be done by increasing the current immediately after detecting a voltage dip. In Fig. 9 it is possible to observe ::: this :::::: regard, :::::: Fig. 9 ::::: shows : the effect of a voltage 15 dip in the wind turbine converter. The voltage imposed is measured at 1 pu and is dropped to 0.74 pu for ::::::::::: approximately : 150 13 0 0  msapproximately. The : . :::: The :::: VSC : line current is also plotted in Fig. 9 and it increases fast ::::::: increases :::::: rapidly : when the voltage dip is detected at 0.08 s. The :: Its : pre-fault value is 0.85 pu and rises to 1.3 pu during the dip.
The DC voltage (also shown in Fig. 9 : ) : is dropped during the test. This can be attributed to the fast increasing in the :::: rapid ::::::: increase :: in wind turbine current, which can be faster than the time constant of the DC link capacitor, allowing :::::::: capacitors :: at :: the ::::: wind :::::: turbine :::::::: converter. :::: This :::::: allows : a normal power flow during the dip while affecting slightly or even decreasing :::::: slightly Finally, it is interesting to see the decoupling that exist ::::: exists between the grid-side VSC and the generator-side VSC of the wind turbine. Figure 10 shows the generator torque and generator speed when the grid voltage experiences a dip. If the conditions are met so that :: for : the wind turbine VSC response is :: to :: be : smooth, the generator is not affected. Here ::::::::: unaffected. ::::: Here, the torque is maintained constant at : at : a :::::::: constant 1 pu and the speed is ::: also : controlled at 1 puas well, meaning that . :::: This :::::: means the wind turbine system is operated at full power. Note that the wind turbine converter have ::: has over-current capabilities in order 5 to maintain a constant power flow during transients.

Testing for frequency deviation
In this case study, the voltage applied to the wind turbine is controlled at 1 pu and only the :::::::::: fundamental : frequency is varied.
First ::::: Firstly, this section shows the frequency scan test carried out on the 20th of June, 2016, when the wind turbine was 15 operating at 0.6 pu power of production. Afterwards, the result from : of : the frequency scan test , carried out on the 17th of May, 2016, at low power production, is :::: then given. The current and voltage are :::: again measured at the HVDC station , where ::::::::: controlling the voltage applied to the wind turbineis controlled. The frequency scan is performed :::::::: conducted by adding a voltage
The magnitude of the admittance is plotted in :: as white dots. The resulting measured points in Fig. 12 suggest that the wind turbine presents a positive real part for frequencies below 22 Hz, with a maximum measured value of 2.8 pu at 10 Hz. On the other hand, a relatively high non-passivity behaviour is exhibit :::: seen : for frequencies above 30 Hz, meaning that the wind 5 turbine could resonate if these frequencies are encountered in the network. The minimum value for Re(Y w (jω)) is -1.5 pu and can be observed at the last scanned frequency of 34 Hz.
The scanned imaginary part of the admittance Im(Y w (jω)) is also shown in Fig. 12 in : as : green dots. The turbine seems to present a ::::: exhibit : capacitive behaviour for most of the studied frequency range. The :::: Note ::: that ::: the : reactive power set-point at the measurement point is dependent on both the configuration of the filtering stage at the terminals of the VSC-HVDC :::::::: terminals 10 and on the wind turbine ::::: ÔÇÖs : reactive power controller. The minimum value of Im(Y w (jω)) is found to be -2.4 pu at 18 Hz, exhibiting its maximum capacitance in the scanned frequency range. For frequencies above 20 Hz, Im(Y w (jω)) increases up to a maximum of -0.4 pu measured at 34 Hz, corresponding to its minimum capacitance in the synchronous :::::::::::::: sub-synchronous range. Observe, however, that ::: the reactive power set-point shown Fig. 12 is kept relatively constant at 0.2 pu (0.9 MVAr) at 50 Hz during all ::::::::: throughout the test.
The tests carried out on the actual wind turbine system include ::::::: included ::: not :::: only : balance and unbalanced voltage dips, defined by different retained voltage and different ramp-rates, as well as :: but :::: also : frequency variation tests and frequency scan :::: scans.
The results shows ::::: show that a LVRT control strategy is implemented on :: in the tested system injecting ::: that :::::: injects reactive power when a voltage dip is detected. Moreover, it has been shown that the generating unit maintains a smooth control of the 15 reactive power output :::: even during unbalanced voltage dips, at least for low power set-points. These results demonstrate that a VSC-based testing ::: test : device can be used to evaluate how well a wind turbine system can withstand the technical requirements given in the Grid Codes ::: grid ::::: codes.
The multi-megawatt FPC wind turbine system has also been characterized :::::::::: characterised : in the sub-synchronous range by means of ::::: using the interconnected VSC-based testing ::: test : equipment. The frequency scanning technique has been demonstrated by 20 field test and the input admittance of the generating unit has been evaluated for ::::: under two operating conditions. The frequency trend of the scanned turbine exhibits a non-passive behaviour at higher frequencies within the sub-synchronous range : , while also exhibiting capacitive behaviour throughout the whole ::::::: complete : scanned range.
The unique field tests presented in this report have provided an experimental validation of the proposed wind turbine testing methodology, particularly on the wind turbine impedance characterization :::::::::::: characterisation : and on the evaluation of its steady-state 25 and dynamic performance against different conditions of the grid :::: under ::::::: different :::: grid ::::::::: conditions.