Journal of International Council on Electrical Engineering
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Stepwise inertial control of a wind turbine generator to minimize a second frequency dip Dejian Yang, Jinsik Lee & Yong Cheol Kang To cite this article: Dejian Yang, Jinsik Lee & Yong Cheol Kang (2016) Stepwise inertial control of a wind turbine generator to minimize a second frequency dip, Journal of International Council on Electrical Engineering, 6:1, 153-159, DOI: 10.1080/22348972.2016.1202396 To link to this article: http://dx.doi.org/10.1080/22348972.2016.1202396
© 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group Published online: 05 Jul 2016.
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Date: 19 August 2016, At: 03:26
Journal of International Council on Electrical Engineering, 2016 VOL. 6, NO. 1, 153–159 http://dx.doi.org/10.1080/22348972.2016.1202396
OPEN ACCESS
Stepwise inertial control of a wind turbine generator to minimize a second frequency dip Dejian Yanga, Jinsik Leea and Yong Cheol Kangb a
Department of Electrical Engineering, WeGAT Research Center, Chonbuk National University, Jeonju, Korea; bDepartment of Electrical Engineering, WeGAT Research Center, and Smart Grid Research Center, Chonbuk National University, Jeonju, Korea
ARTICLE HISTORY
ABSTRACT
Wind turbine generators (WTGs) in power systems with high wind penetration levels are encouraged or forced to participate in frequency control. A stepwise inertial control (SIC) scheme instantly increases WTG output to arrest the frequency drop for a preset period upon detecting a disturbance. After arresting the frequency drop, the output is rapidly reduced to recover the rotor speed. The reduction could cause a power deficit to the power system, which in turn results in a second frequency dip (SFD). This paper proposes an SIC scheme that can improve the frequency nadir (FN) and maximum rate of change of frequency (ROCOF) while minimizing an SFD. To achieve this, a reference function is separately defined prior to and after the FN. To improve the FN and maximum ROCOF, the output is instantly increased by adding a constant, which is proportional to the rotor speed, and maintaining it until the FN is reached. To minimize an SFD, the output is slowly reduced with the rotor speed. This reduction ensures a slow output reduction rate. The performance of the proposed scheme is investigated using an EMTP-RV simulator under different wind speeds and wind power penetration levels. Results clearly demonstrate that the proposed scheme can improve the FN and maximum ROCOF while ensuring a quick frequency recovery.
1. Introduction Wind power generation has rapidly developed over the last decade. The total worldwide installed wind generation capacity increased to 370 GW at the end of 2014 and is expected to reach 479 GW by 2016 and 666 GW by 2019.[1] System frequency should be maintained within the allowable range at all times to guarantee stable operations. To this end, if a large disturbance such as a generator trip occurs, synchronous generators (SGs) with spinning reserve provide frequency control, such as inertial response, primary control, and secondary control, to arrest the frequency decline and recover the frequency.[2] Variable-speed wind turbine generators (WTGs) such as doubly-fed induction generators (DFIGs) and fully-rated converter-based WTGs, perform maximum power point tracking (MPPT) operations to extract the maximum wind energy. Nevertheless, MPPT operations decouple the rotor speed of WTGs from the system frequency, thus rendering the WTGs unable to contribute to the supporting frequency during a disturbance. As a result, system inertia is reduced, and frequency instability
CONTACT Yong Cheol Kang
Received 4 February 2016 Accepted 14 June 2016 KEYWORDS
Doubly-fed induction generator; frequency nadir (FN); maximum rate of change of frequency (ROCOF); stepwise inertial control (SIC); second frequency dip (SFD); frequency recovery
may be triggered in the power system, particularly under high wind power penetration. To maintain the required level of system reliability, WTGs are forced to participate in frequency control.[3,4] A number of studies have explored the frequency control of WTGs, which can be classified into two groups: inertial control and power reserve control. The former temporarily releases the kinetic energy (KE) stored in the rotating masses in a WTG to arrest the frequency decline during the initial stage of a disturbance [5–9]; conversely, the latter releases the reserve power of a WTG to share deficient power in a power system.[10] Even though the latter provides better contribution than the former, the de-loaded operation of a WTG is always necessary in power reserve control; hence, such control causes a significant loss in annual wind energy. This paper focuses on the inertial control of a WTG and assumes that a WTG operates under MPPT prior to a disturbance. The inertial control of a WTG can be divided into two types: frequency-based inertial control [5,6] and stepwise inertial control (SIC).[7−9] This paper only focuses on
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© 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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SIC, which promptly increases WTG output and maintains it for the preset period to improve the frequency nadir (FN) and maximum rate of change of frequency (ROCOF). Because a large amount of KE is extracted in the preset period, the output power should be reduced to recover the rotor speed. To this end, the output decreases in a step-like manner [7,8] and in a ramp manner [9]; however, the power deficit to the power system could cause a second frequency dip (SFD), which negatively affects frequency recovery. An SIC scheme of a WTG proposed in [11] is extended in this work, and the results are presented accordingly. In the proposed scheme, the output is instantly increased by adding a constant and maintaining it until the FN is reached. Afterward, the output declines with the rotor speed, and the rotor speed eventually converges to a value within the stable operation range. The performance of the proposed scheme is investigated under various wind speeds and wind power penetration levels.
PMPPT
Pref
Pstepwise
Rotor side Converter Controller
(a)
(b)
2. Conventional SIC schemes Figure 1 shows the operational features of two conventional SIC schemes suggested in [7] and [9]. For convenience, we refer to the former and the latter as Scheme #1 and Scheme #2, respectively. After detecting an event, the power reference Pref is switched from the reference for MPPT operation PMPPT to the reference for SIC Pstepwsie, as shown in Figure 1(a). In both schemes, a constant ΔPOP is added to P0, which denotes the power reference prior to an event. Hence, Pstepwsie instantly increases from Point A to Point B and is maintained from Point B to Point C to improve the FN and maximum ROCOF. To recover the rotor speed, Scheme #1 rapidly decreases the output at Point C, as shown in Figure 1(b). Such a large output reduction rate may cause an SFD, which negatively affects system frequency recovery. To decrease the output reduction rate, Scheme #2 operates in a ramp-like manner Pref instead of in a step-like manner (see Figure 1(c)). To ensure a slow reduction rate from Point C to Point D, a large Tdrop is required. However, setting Tdrop with a large value results in the extraction of a large amount of KE from a WTG. As a result, the rotor speed declines to the minimum operating speed limit. Conversely, setting Tdrop with a small value causes a rapid drop in the output, thereby causing an SFD. In [9], Tdrop was set to 10 s. In Scheme #1, ΔPOP and ΔPUP are set to 0.10 and 0.05 p.u. for high wind speeds and to 0.05 and 0.025 p.u. for low wind speeds, respectively, while TOP and TUP are set to 10.0 and 20.0 s, respectively. Conversely, in Scheme #2, ΔPOP is defined according to wind speed to improve the
(c) Figure 1. Operational characteristics of Scheme #1 and Scheme #2. (a) Conventional SIC schemes, (b) Reference function of scheme #1 [7], (c) Reference function of scheme #2 [9].
FN and maximum ROCOF relative to Scheme #1; ΔPrec is set to 0.05 p.u. Unlike that in Scheme #1, TUP in Scheme #2 is unfixed and lasts until Pstepwsie meets PMPPT.
3. Proposed SIC scheme The objectives of the proposed scheme are (1) to improve the FN and maximum ROCOF and (2) to ensure rapid frequency recovery by minimizing an SFD. To accomplish the first objective, the power reference is instantly increased from Point A to Point B and is maintained until the FN is reached from Point B to Point C (Figure 2). The power reference prior to the FN can be expressed by
Pstepwise = P0 + ΔP
(1)
where P0 and ΔP are the power references prior to a disturbance and a constant, respectively, which depend on the rotor speed prior to a disturbance. After reaching the FN, the second objective is accomplished by defining the power reference as
Pstepwsie = (P0 + ΔP)
𝜔r − 𝜔min 𝜔FN − 𝜔min
(2)
Journal of International Council on Electrical Engineering Asynchronous Motor 340 MW
M
26.4/ 345 kV
Static load 210 MW 80 MVAr
155
Aggregated DFIG-based WPP 154/33 kV
345/154 kV PCC
345/ 13.8 kV
(a)
SG1, SG2 100 MVA
SG3, SG4 150 MVA
SG5, SG6 200 MVA
Figure 3. Model system employed in this paper.
Blade
(b)
Gear box
Step-up Tr. DFIG
Back-to-back converter
Figure 4. DFIG configuration.
(c) Figure 2. Operational characteristics of the proposed scheme. (a) Control scheme, (b) Proposed reference function, (c) Power-speed locus.
where ωr, ωmin, and ωFN are the rotor speed, minimum rotor speed, and rotor speed at the instant of the FN, respectively. When the rotor speed decreases, the power reference also decreases. Consequently, the reduction rate from Point C to Point D decreases, thereby minimizing an SFD. Furthermore, the proposed scheme can prevent the rotor speed from reaching the minimum value during inertial control because Pstepwsie meets the mechanical input curve at Point D, which is always located in the stable operating region. Therefore, the proposed scheme is able to generate a significant amount of power to improve the FN and maximum ROCOF; in addition, it decreases the reduction rate to minimize the SFD, thereby ensuring rapid frequency recovery.
Figure 5. Power curve of the WPP used in this paper.
wind power plant (WPP), a static load of 210 MW and 80 MVAr, and a 340-MW asynchronous motor. 4.1. Synchronous generators The model system includes two 200-MVA SGs, two 150MVA SGs, and two 100-MVA SGs, all of which are steam turbine generators. Their steam turbine governor model is the IEEEG1 steam model, and the droop gains are set to 5%, which is a typical value for the governor setting of SGs in the Korean power system. 4.2. DFIG-based WPP
4. Model system To investigate the performance of the proposed scheme, a simple model system (Figure 3) is selected. The model system consists of six SGs, an aggregated DFIG-based
Figure 4 shows the configuration of the DFIG used in this paper. The stable operation range of the rotor speed is 0.7–1.25 p.u. The cut-in, rated, and cut-out wind speeds are 4, 11, and 25 m/s, respectively (Figure 5).
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5. Case studies The performance of the proposed scheme is investigated and compared with that of the ‘MPPT operation,’ Scheme #1, and Scheme #2 under varying wind speeds of 10.5 and 7.5 m/s, and wind power penetration levels of 18.2 and 27.3%. The installed capacity of a WPP for the two wind power penetration levels is set to 100 and 150 MW, respectively. In addition, SG6, which generates 100 MW, is tripped out at 40 s as a disturbance. In Scheme #1, TOP and TUP are set to 10.0 and 20.0 s, respectively; ΔPOP and ΔPUP are respectively set to 0.10 and 0.05 p.u. for Case 1 and to 0.05 and 0.025 p.u. for Cases 2 and 3, as suggested in [7]. In Scheme #2, TOP is set to 9.0 s for Case 1 and to 11.0 s for Cases 2 and 3 while Tdrop is set to 10.0 s, as used in [9]; ΔPOP is set to different values under different wind speeds (Table 1) while 0.05 p.u. is used for ΔPrec for all cases. The estimated ΔP for the proposed scheme is shown in Table 1.
Table 1. Comparison of ΔP (p.u.) for all cases. Scheme #1 Scheme #2 Proposed scheme
Case 1 0.10 0.165 0.22
Case 2 0.05 0.08 0.16
Case 3 0.05 0.08 0.16
5.1. Effects of wind speed Wind speed affects the performance of the proposed inertial control scheme and results in different levels of KE in the WTG. In this subsection, we investigate the performance of the proposed inertial control scheme for two cases with wind speeds of 10.5 and 7.5 m/s. 5.1.1. Case 1: wind speed of 10.5 m/s and wind power penetration level of 18.2% Figure 6 presents the results for Case 1. In this case, ΔP is set to 0.22 p.u. in the proposed scheme and to 0.10 and 0.165 p.u. in Schemes #1 and #2, respectively. The FNs for the ‘MPPT operation,’ Scheme #1, Scheme #2, and the proposed scheme are 58.98, 59.09, 59.15, and 59.22 Hz, respectively, as shown in Figure 6(a). The FN for the proposed scheme is greater than the FNs for Schemes #1 and #2 by 0.12 and 0.07 Hz, respectively. Moreover, the maximum ROCOFs for the ‘MPPT operation,’ Scheme #1, Scheme #2, and the proposed scheme are −0.757, −0.721, −0.715, and −0.624 Hz/s, respectively. The maximum ROCOF in the proposed scheme is the lowest because the proposed scheme has greater power injection than the conventional schemes before the FN is reached (43.33 s), as shown in Figure 6(b). Severe SFDs start at 50.1 s in Scheme #1 and at 59.3 s in Scheme #2. The size of the SFD in Scheme #1 is 0.13 Hz, and that in Scheme #2 is 0.15 Hz. Size is defined by the difference between the frequency before an SFD and the second FN. By contrast, no SFD occurs in the proposed scheme because of the slow output reduction rate; therefore, the proposed scheme recovers frequency more rapidly than Schemes #1 and #2 (Figure 6(a)).
Figure 6. Results for Case 1. (a) system frequency, (b) Active power of the WPP, (c) Rotor speed.
In Scheme #1, the rotor speed decreases from 40.0 to 50.0 s and recovers from 50.0 to 70.0 s. In Scheme #2, the rotor speed decreases from 40.0 to 59.3 s and recovers from 59.3 s. In the proposed scheme, the rotor speed decreases and eventually converges to 1.03 p.u., which indicates that the proposed scheme can prevent the rotor speed from declining to the minimum value. Note that the proposed scheme releases less KE than Scheme #2, although it performs better in terms of FN, maximum ROCOF, and frequency recovery. 5.1.2. Case 2: wind speed of 7.5 m/s and wind power penetration level of 18.2% Figure 7 presents the results for Case 2, which is identical to Case 1 except for a lower wind speed. In this
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Figure 7. Results for Case 2. (a) system frequency, (b) Active power of the WPP, (c) Rotor speed.
Figure 8. Results for Case 3. (a) system frequency, (b) Active power of the WPP, (c) Rotor speed.
case, ΔP is set to 0.16 p.u. in the proposed scheme and to 0.05 and 0.08 p.u. in Schemes #1 and #2, respectively. The ΔP values for Case 2 are lower than those for the previous case because of the lower wind speeds. The FN for the proposed scheme is 59.19 Hz, which is greater than those of ‘MPPT operation’ by 0.21 Hz, higher than those of Scheme #1 and Scheme #2 by 0.17 and 0.12 Hz, respectively. The maximum ROCOFs for the ‘MPPT operation,’ Scheme #1, Scheme #2, and the proposed scheme are −0.757, −0.722, −0.716, and −0.635 Hz/s, respectively. In the proposed scheme, the FN is the highest, and the maximum ROCOF is the lowest because of the large power injection at the early stage of the disturbance. An SFD in Scheme #1 starts at 50.1 s when the output starts decreasing, and the size of the SFD is 0.04 Hz. An SFD in Scheme #2 starts at 51.8 s, at which point the output starts decreasing with a size of 0.12 Hz. The sizes of the SFDs in Case 2 are smaller than those in Case 1 because of the lower incremental power. As in Case 1, no SFD is caused in the proposed scheme in Case 2.
As shown in Figure 7(c), the rotor speed converges to 0.76 p.u., which is greater than the minimum speed limit. Note that as in Case 1, the proposed scheme in Case 2 releases less KE than Scheme #2 but performs better in terms of FN, maximum ROCOF, and frequency recovery. The results for the above two cases indicate that the proposed scheme can improve the FN and maximum ROCOF with rapid frequency recovery. 5.2. Effects of wind power penetration levels The performance of the proposed inertial control scheme is also affected by wind power penetration levels. Thus, we validate the performance of the proposed inertial control scheme for a wind speed of 7.5 m/s with a high wind power penetration level of 27.3%. 5.2.1. Case 3: wind speed of 7.5 m/s and wind power penetration level of 27.3% Figure 8 shows the results for Case 3, which is identical to Case 2 except for a higher wind power penetration level.
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6. Conclusions This paper proposes an SIC scheme to improve the FN and the maximum ROCOF while ensuring a rapid frequency recovery by minimizing an SFD. To improve the FN and maximum ROCOF, the proposed scheme instantly increases the power reference by adding a constant and maintaining it until the FN is reached. This constant power is proportional to the rotor speed. After the FN is reached, the power reference is reduced with the rotor speed to guarantee a slow output reduction rate. The results clearly indicate that the proposed scheme can improve the FN and maximum ROCOF while ensuring a rapid frequency recovery by minimizing an SFD even under low wind conditions and high wind power penetration levels.
Disclosure statement Figure 9. Inertial response results for all cases. (a) Frequency nadir, (b) Maximum ROCOF.
In this case, ΔP for the proposed scheme, Scheme #1, and Scheme #2 are the same as those in Case 2 because the same wind speed is used. The FN for the proposed scheme is 59.25 Hz, which is greater than the FN values for Scheme #1 and Scheme #2 by 0.19 and 0.14 Hz, respectively. Furthermore, the FNs increase because of the high wind power penetration levels. The FN for ‘MPPT operation’ is reduced from 58.98 to 58.97 Hz because of the reduced system inertia. The maximum ROCOFs for the ‘MPPT operation,’ Scheme #1, Scheme #2, and the proposed scheme are −0.758, −0.703, −0.694, and −0.610 Hz/s, respectively. The sizes of the SFDs are 0.07 Hz in Scheme #1 and 0.116 Hz in Scheme #2. Compared with those in Case 2, the SFD sizes in Case 3 increase with increasing wind power penetration levels. As expected, and similar to the previous cases, the proposed scheme in Case 3 does not cause an SFD. In addition, the proposed scheme can ensure faster frequency recovery in comparison with conventional schemes. The results clearly indicate that the proposed scheme improves the FN and maximum ROCOF while ensuring a rapid frequency recovery with minimal SFD even under high wind penetration levels. Figure 9 shows a comparison of the results for all cases in terms of FN and maximum ROCOF. Because of the larger greater power injection in the proposed scheme is higher than that in the conventional schemes, the FN is also significantly greater, and the maximum ROCOF is significantly lower. Furthermore, system frequency recovery in the proposed scheme is faster than those in the conventional schemes.
No potential conflict of interest was reported by the authors.
Funding This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) [grant number 2010-0028509].
Notes on contributors Dejian Yang received his BSc degree from Mudanjiang Normal College, China, in 2013. He is currently pursuing an MSc degree from the Department of Electrical Engineering, Chonbuk National University. He is an assistant researcher at the Wind energy Grid-Adaptive Technology (WeGAT) Research Center supported by the Ministry of Science, ICT, and Future Planning (MSIP), Korea. His research interests include the frequency support of WPPs. Jinsik Lee received his BS and MS degrees from the Department of Electrical Engineering at Chonbuk National University, Korea, in 2011 and 2013, respectively. He is currently pursuing a PhD degree at Chonbuk National University while serving as an assistant researcher at the WeGAT Research Center, which is supported by the MSIP, Korea. His research interests include plant-level control schemes for WPPs. Yong Cheol Kang received his BSc, MSc, and PhD degrees in electrical engineering from Seoul National University, Korea, in 1991, 1993, and 1997, respectively. He has been with Chonbuk National University, Korea, since 1999. He is currently a professor at Chonbuk National University and the director of the WeGAT Research Center, which is supported by the MSIP, Korea. At present, he is a member
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of the International Electrotechnical Commission working group TC88/WG27. His research interests include the development of control and protection techniques for WPPs.
References [1] Global Wind Energy Council. Global wind report: annual market update 2014; 2015. Available from: http://www.gwec.net/wp-content/uploads/2015/03/ GWEC_Global_Wind_2014_Report_LR.pdf [2] Ackermann T. Wind power in power systems. 2nd ed. England: John Wiley & Sons; 2012. [3] Hydro Québec. Transmission provider technical requirements for the connection of power plants to the Hydro Québec transmission system. Montréal: Hydro Québec; 2009. [4] EirGrid. EirGrid grid code [online]. version 5.0. Dublin. 2010. Available from: http://www.eirgrid.com/media/ GridCodeVersion5.pdf [5] Morren J, de Haan S, Kling WL, et al. Wind turbines emulating inertia and supporting primary frequency control. IEEE Trans. Power Syst. 2006;21:433–434.
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[6] Lee J, Kim J, Kim Y-H, et al. Rotor speed-based droop of a wind generator in a wind power plant for the virtual inertial control. J. Elect. Eng. Technol. 2013;8:742–749. [7] Ullah NR, Thiringer T, Karlsson D. Temporary primary frequency control support by variable speed wind turbines—potential and applications. IEEE Trans. Power Syst. 2008;23:601–612. [8] Hansen AD, Altin M, Margaris ID, et al. Analysis of the short-term overproduction capability of variable speed wind turbines. Renewable Energy. 2014;68:326–336. [9] Itani SE, Annakkage U, Joos G. Short-term frequency support utilizing inertial response of DFIG wind turbines. Proc. IEEE Power & Energy Society General Meeting; 2011 Jul, Detroit, USA. [10] de Almeida RG, Lopes JAP. Participation of doubly fed induction wind generators in system frequency regulation. IEEE Trans. Power Syst. 2007;22:944– 950. [11] Yang D, Lee J, Kang YC. Stepwise inertial control of a wind turbine generator to minimize a second frequency dip. 21st ICEE; 2015 Jul, Hong Kong, China.
