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PV‐fed DVR for simultaneous real power injection and sag/swell mitigation in a wind farm

2018; Institution of Engineering and Technology; Volume: 11; Issue: 14 Linguagem: Inglês

10.1049/iet-pel.2018.5123

1755-4543

Subramaniyan Priyavarthini, Aravind Chellachi Kathiresan, C. Nagamani, G. Saravana Ilango,

Power System Optimization and Stability

IET Power ElectronicsVolume 11, Issue 14 p. 2385-2395 Research ArticleFree Access PV-fed DVR for simultaneous real power injection and sag/swell mitigation in a wind farm Subramaniyan Priyavarthini, Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this authorAravind Chellachi Kathiresan, orcid.org/0000-0002-3231-2637 Department of Electrical and Electronics Engineering, Mepco Schlenk Engineering College, Sivakasi, IndiaSearch for more papers by this authorChilakapati Nagamani, Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this authorSaravana Ilango Ganesan, Corresponding Author gsilango@nitt.edu Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this author Subramaniyan Priyavarthini, Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this authorAravind Chellachi Kathiresan, orcid.org/0000-0002-3231-2637 Department of Electrical and Electronics Engineering, Mepco Schlenk Engineering College, Sivakasi, IndiaSearch for more papers by this authorChilakapati Nagamani, Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this authorSaravana Ilango Ganesan, Corresponding Author gsilango@nitt.edu Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, IndiaSearch for more papers by this author First published: 25 October 2018 https://doi.org/10.1049/iet-pel.2018.5123Citations: 10AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract Generally, a dynamic voltage restorer (DVR) installed in a grid-connected wind farm, is utilised for mitigating sag/swell in grid voltage and hence stays idle under nominal grid voltage. To enhance the utility of such a DVR, this study proposes to install a solar photovoltaic (PV) plant in the DVR, so that in addition to addressing sag/swell in grid voltage, energy can be harvested and injected into the grid from the PV plant using the same DVR. Various modes of operations of the proposed PV-assisted DVR (PVDVR) are described, underlining the logically set priorities. Comprehensive mathematical and graphical analyses are developed to explore the boundaries of the operating region of each mode of PVDVR considering fluctuations in grid voltage and available PV power. Investigations are carried out in a 50 MW wind farm and a 0.75 kW laboratory induction machine (both grid connected) each with a suitable DVR. Results of simulations in power systems computer-aided design/electro-magnetic transient design and control and laboratory tests demonstrate improved utilisation of DVR when fed from PV plant. Nomenclature V, I voltage and line current wind farm impedance angle generator voltage angle and line current angle PVDVR voltage injection angle ρ grid PLL position P, Q, S real, reactive apparent power sl operating slip Z, X wind farm impedance, reactance R, L,C resistor, inductance and capacitor First subscripts g,c,t grid, compensator, terminal s, r, m, T stator, rotor, mutual and transformer Superscripts * reference 1 Introduction Owing to the increase in global environmental concerns and depletion of conventional energy resources, renewable energy sources are the most sought after for power generation. Wind energy conversion systems (WECSs) have witnessed a significant growth in recent years [1]. The solar photovoltaic (PV) system has made rapid strides in deployment due to the reduction in the cost of PV panels and advancements in power semiconductor technologies [2]. Owing to the complementary nature, a wind–PV hybrid power generation system offers higher reliability to obtain improved power output than a sole renewable power generation system [3-6]. Further through the wind–PV hybrid power generation system, the space available in wind farms can be effectively utilised for installing solar PV arrays. With increasing penetration of WECS, rigorous grid codes are stipulated for the operation of wind farms to ensure grid stability. The grid codes also insist the wind farms be equipped to meet fault ride through (FRT) or low-voltage ride through (LVRT) to prevent generation loss and to ensure immediate fault clearance to avoid grid instability [7]. Among the power quality (PQ) problems, unsymmetrical voltage sag and swell harshly affect the WECS. With unbalanced grid voltage, negative sequence currents flow in the stator. The positive sequence currents along with the negative sequence currents cause pulsations in power and torque, thereby leading to mechanical stress in parts of the turbine-generator system. To overcome the PQ issues such as sag/swell [8, 9] and to ensure FRT, the incorporation of dynamic voltage restorer (DVR) in the WECS has been investigated in [10-13]. Although variable speed wind generator is becoming the preferred machine in the recently installed wind farms, several installations still employ fixed speed wind generators (FSWGs). Coping with harmful grid disturbances (such as sag/swell) along with the need for compliance as per grid codes advocate the use of series or shunt compensators for protection of these generators [14-16]. Among the possible remedies, the DVR for series compensation and static compensator for shunt compensation is considered reliable options for fixed and variable speed wind farms [17, 18]. A DVR facilitates control of reactive power during sag/swell enabling a quick recovery from the sudden change in grid voltage, thereby maintaining the stability of the wind farm. A detailed comparison of DVR topologies is given in [19]. The role of a DVR in a wind farm being primarily for protecting the generator against grid voltage disturbances, it stays idle during normal grid conditions. In such scenarios, where a DVR coexists with a WECS, it is beneficial to include a solar PV plant to feed the DVR, so that in addition to addressing sag/swell in grid voltage, energy can be harvested and injected into the grid from the PV plant using the same DVR during the daytime. The extent of real and reactive powers exchanged under various conditions of grid voltage and PV power with a series-connected inverter is analysed for a constant load [20] and a series-connected inverter powered by renewable energy is employed for microgrid application in [21]. Generally, a DVR is used to improve the PQ of a wind farm while shielding it from the grid-side disturbances. Since the grid-side disturbances are intermittent and occur for short durations, the utilisation of DVR is limited. This paper explores the concept of integrating a PV-assisted DVR (named as PVDVR) instead of a normal DVR in the grid-connected wind generation system. The operating regions of the PVDVR incorporating the variation of grid voltage and PV power are explored using a comprehensive mathematical and graphical analyses. The utilisation of the PVDVR is significantly improved by the integrated operation of PV power injection and mitigation of grid voltage fluctuations. A suitable control strategy facilitating various modes of operations considering the fluctuations in the grid voltage and PV power is described. The primary objective of the controller is to mitigate the voltage sag or swells. Yet when the grid is normal, or under marginal sag/swells, the focus is to extract the available energy in solar PV system using the same DVR. Since the proposal implies the installation of a PV plant to match the existing DVR, the extent of additional energy extraction depends on the ratings of the existing DVR. A comprehensive mathematical analysis of the boundaries of operation of PVDVR is carried out considering variation in PV power and the grid voltage in Section 2. Furthermore, a control strategy to facilitate various modes of operation is presented in Section 3. The efficacy of the proposed control strategy under various grid voltage and PV power conditions is validated using simulations and experiments in Section 4. 2 System description and analysis The schematic diagram of the proposed system is shown in Fig. 1a. The input of the DC–DC converter is connected from the solar PV array and the output is connected at the DC link. The PVDVR is connected through three-phase diode bridge rectifier to the DC-link capacitor and is connected to the ac grid through three single-phase inverters (fed from a common DC link) with inductor–capacitor filter and series injection transformers. The equivalent circuit and the phasor diagram are shown in Figs. 1b and c, respectively. The wind farm generators are lumped together and modelled as a single induction generator for the analysis. Source convention is followed for the grid and PVDVR while load convention is followed for the lumped wind farm circuit. Fig. 1Open in figure viewerPowerPoint Schematic diagram of the proposed system (a) Proposed PVDVR in wind farm, (b) Equivalent circuit, (c) Phasor diagram Considering the condition of grid voltage and the available PV power, there are four possible operating modes for the PVDVR as follows: Mode-1: Injecting PV power into the grid under nominal grid voltage (when PV power is available). Mode-2: Simultaneous sag/swell mitigation and PV power injection (marginal sag/swell in grid voltage and PV power available). Mode-3: Sag/swell mitigation only (deep sag/swell in grid voltage beyond mode-2) though PV power is available. Mode-4: Sag/swell mitigation only (PV power not available). When the grid voltage is nominal, the PVDVR is operated in Mode-1, solely to extract energy from the PV plant and inject real power into the grid. In Mode-2, the available PV power is injected along with sag/swell mitigation for marginal sag/swell in grid voltage. In Mode-3, the depth of sag/swell is greater than that in Mode-2 and sag/swell mitigation alone is possible, even if PV power is available. The Mode-4 is selected when PV power is not available and only mitigation of sag/swell is required. Thus, based on the level of grid voltage and available PV power, the mode of operation of PVDVR is chosen. 2.1 Steady-state analysis of operating regions of PVDVR 2.1.1 Mathematical analysis This analysis considers that the generator terminal voltage is always maintained at 1.0 pu with PVDVR. From the equivalent circuit (Fig. 1b), the loop voltage equation can be written as follows: (1)By solving (1), the magnitude and angle of injection of voltage can be derived as follows: (2)and (3)The real power and reactive power injected by the PVDVR can be expressed as follows: (4)and (5)where is the equivalent impedance of the wind farm and is constant for an FSWG. From (4) and (5), the following equation is obtained: (6)Thus, (6) is the equation of a circle in the P–Q-plane with centre at and radius is given by denoting the real and reactive powers exchanged by the wind farm. Furthermore, for a given available PV power, (4) can be rewritten to determine the phase angle δ to be maintained between grid and generator voltages (7)To get a real value of the angle δ, the following constraint should be met: (8)For a certain available PV power Pc, the minimum grid voltage at which the PVDVR can be operated in Mode-2 (sag mitigation and PV power extraction) can be determined from constraint (8). The range of grid voltages where the Mode-2 operation is feasible is explained in the next section. Under an unsymmetrical grid voltage sag/swell, the angle δ will be different in each phase and this may cause a dissymmetry among three phases in the DVR output. To prevent this, the PVDVR should be operated in Mode-3 where no real power is injected to the grid and hence there is no constraint on the angle δ. Simple in-phase compensation is adopted in such a scenario. 2.1.2 PV rating By neglecting the losses in the converter, the desired PV power rating can be calculated from (4) as follows: (9)Therefore, the PV rating can be determined from (9) by taking nominal values for Vg and Vt as 1.0 pu and by substituting Z∠ for the nearly constant effective impedance of the FSWG. 2.1.3 Volt–ampere (VA) rating of PVDVR The VA rating of PVDVR can be determined by knowing the PV rating, which is chosen based on the wind farm rating. The maximum VA rating can be obtained by determining the maximum real power (Pc–max) and the corresponding reactive power (Qc−max) injected by PVDVR at nominal grid voltage (Vg = 1.0 pu) and this can be observed from operating point ‘C’ in graphical representation from Fig. 2b (graphical analysis is explained in detail in the next section). Thus, the VA rating of PVDVR is given by (10) Fig. 2Open in figure viewerPowerPoint Operating regions of PVDVR in a wind farm (a) Grid-connected 50 MW wind farm with PVDVR, (b) Magnified view of Fig. 2a showing the operating boundaries of PVDVR, (c) Grid-connected 0.75 kW wind generator with PVDVR, (d) Magnified view of Fig. 2c showing the operating boundaries of PVDVR The VA rating constraint in (10) is important to prevent the overloading of the inverter during PVDVR control. Generally, the VA rating of DVR should be equal to that of the wind farm to meet the fault conditions and thus the capacity of DVR is sufficient to install the PV power plant in addition. 2.1.4 Graphic interpretation Figs. 2a and b present the operating region of the 50 MW wind farm system and Figs. 2c and d show the case of 0.75 kW induction machine setup. The concentric circles in Figs. 2a and c are drawn using (6) for different grid voltages Vg. A. Operating region of the 50 MW wind farm (Test system 1) The concentric circles with the centre ‘u’ (PIG = –1 pu and QIG = 0.5 pu) in Fig. 2a represent the real power injected and reactive power absorbed by the wind farm under various grid voltages, whereas the generator voltage is maintained constant at 1.0 pu by the PVDVR. The two circles with centre at ‘k’ represent the VA ratings of PVDVR. The circle in the magnified view (Fig. 2b) represents the operating area of the PVDVR with a VA rating of 0.5 pu. In Fig. 2b, Gs (or Gs2–Gs8) denote the sag event, while Gsw (or Gsw2–Gsw8) denotes the swell event and the nominal grid condition is denoted as Gs0 (Vg = 1.0 pu). Fig. 2b is sub-divided into three possible operating regions as follows: Region 1 (R1) – In this region, only sag mitigation is possible since there is no overlap of PV power band with region R2. This implies that at R1, the PVDVR will be operated either in Mode-3 or Mode-4 based on PV power availability and grid voltage variation. Region 2: Region R2 has an overlap of PV power band with a marginal range of sag and swell in grid voltage. Thus in this region, the PVDVR can be operated in Mode-2 for simultaneous PV power injection and sag/swell mitigation or can be operated in Mode-1 for PV power injection alone under nominal grid voltage. The further expanded view of R2 is shown adjacent to Fig. 2b to illustrate possible operating points under different grid voltages and PV power conditions. With a nominal grid voltage (arc Gs0 marked by points A, B and C in Fig. 2b), the range of injectable PV power is zero to Pc2 (or 0.12 pu for the 50 MW system). Pc2, the x-coordinate of point ‘C’ on the arc Gs0 denotes the maximum injectable PV power under nominal grid voltage. Hence, the limit Pc2 is considered as the PV rating necessary to facilitate real power injection and maintain the generator voltage at 1.0 pu. Under nominal grid voltage, the operating point lies at ‘A’ for zero PV power and shifts to operating point ‘B’ for PV power of Pc1 and to ‘C’ for maximum PV power of Pc2 (Mode-1 operation). The points ‘B1’ and ‘B2’ represent sag and swell conditions, respectively, when the PV power is Pc1 (Mode-2 operation). The points ‘A1’ and ‘A2’ lie on the y-axis, representing PV power is zero and hence only sag and swell mitigations can take place. Region 3: R3 – only swell mitigation is possible in this region. Hence, Mode-3 or Mode-4 operations can take place, based on PV power availability and grid voltage condition. From Figs. 2b and d, the range of grid voltages corresponding to different levels of available PV power for Mode-2 operation are listed in Table 1 for both the test systems. Table 1. Range of grid voltage for various PV powers Test system Available PV power, pu Range of Vg, pu 50 MW 0.12 (maximum PV rating) 1.35≥Vg≥1.0 0.06 1.3≥Vg≥0.95 0.015 1.25≥Vg≥0.9 0.75 kW 0.59 (maximum PV rating) 1.6≥Vg≥1.0 0.3 1.58≥Vg≥0.8 0.1 1.55≥Vg≥0.7 B. Operating region of 0.75 kW machine (Test system 2) The concentric circles with centre ‘m’ (PIG = –1 pu and QIG = 1.25 pu) in Fig. 2c represent the real power injected and reactive power absorbed by the wind generator under various grid voltages, whereas the generator voltage is maintained constant by the PVDVR. In Fig. 2c, two concentric circles with centre at ‘n’ represent the operating region of PVDVR with VA rating of 1.0. and 0.273 pu. With a DVR rating of 1.0 pu, the maximum injectable PV power is 0.59 pu, whereas with a DVR rating of 0.273 pu (as in the laboratory test setup), the maximum injectable PV power is 0.1 pu. Considering the injectable PV power of 0.1 pu, (Fig. 2d), the operation is at ‘D’ under nominal grid voltage, whereas ‘E’ and ‘F’ represent the operation points under 10% sag and 10% swell, respectively. Following a reduction of 25% PV power, the operating point shifts from ‘D’ to ‘D1’ under nominal grid voltage and shifts from ‘D1’ to ‘E1’ under 10% sag in grid voltage as shown in Fig. 2d. Thus, for a given DVR rating, and an anticipated range of grid voltages, the power rating of PV plant required for Mode-2 operation is given in Table 1. Furthermore, the boundaries of operating regions R1, R2 and R3 for test system 2 are shown in Fig. 2d. Thus, it is inferred from graphical interpretation, the mode of operation of PVDVR is chosen based on the grid voltage condition under certain PV power availability. 2.1.5 Utilisation factor The enhanced utility of a PVDVR can be explained quantitatively by defining the utilisation factor (UF) of a DVR. The UF is ‘the ratio of the total time that the DVR is in use to the maximum possible time that it could be in use’. The duration and the percentage of grid voltage sag/swell vary based on the grid condition. Let ‘x’ be a number of hours with sag/swell occurrence in a certain block of the time period (say, a month or a year) and ‘T’ is the total time in hours in that block period that the DVR could be in use. Hence, for a conventional DVR, the UF is given as follows: (11)Furthermore, considering ‘y’ hours of PV power injection for a certain block of the time period, the UF for a PVDVR is given by (12)For illustration, an average of 6 h/day is considered as the generation period of the PV system. The details of grid voltage sag/swell events in the wind farm in a selected location [22] are considered for UF calculation. The number of recorded events of sag is 26 and swell is 3 in a period of 15 days as per the report [22] and time duration of sag event recorded varies from 6 ms to 10 s and swell event recorded is 20 ms. The total time durations of the sag/swell events in a period of 15 days and the total time ‘T’ in 15 days (360 h) are used to calculate the UF for illustration. From Table 2, it can be observed that the UF of the proposed PVDVR is far greater than the conventional DVR [10, 12, 13]. However, the percentage of UF calculated is only illustrative and may vary based on the time duration of events occurring. Thus, by using the PVDVR for extracting PV power during the daytime, the utilisation is significantly improved compared with conventional DVR. Table 2. Comparison of conventional DVR and the proposed PVDVR DVR Status of DVR at a nominal grid voltage UF conventional idle (zero voltage injection) 0.5–1% PVDVR active (during PV power generation) 25–30% 3 Control strategy The configuration of the PVDVR with a grid-connected wind generation system is shown in Fig. 3a. The control strategy of the proposed system is shown in Fig. 3b. Fig. 3Open in figure viewerPowerPoint Configuration of the PVDVR with a grid-connected wind generation system (a) Schematic diagram of the experimental setup, (b) Control strategy of the PVDVR in wind farm 3.1 DC-link voltage control The DC-link voltage has to be regulated to support power exchange between the DC link and the grid. In other words, the DC-link voltage is controlled to achieve active power balance through control of the phase angle of generator voltage with respect to grid voltage [23, 24]. In a similar approach, the command or reference value of the phase angle of generator voltage corresponding to the real power to be injected (from the PV plant) can be computed from the DC-link voltage control. The phase angle δ is obtained from DC-link voltage control for Mode-1 and Mode-2 operations of DVR (R2 region). The phase angle is maintained at δ = 0° for Mode-3 and Mode-4 operations of PVDVR (R1 and R3 regions). 3.2 DC–DC converter control The DC–DC converter is used to control the power flow from the PV array to the DC link, thereby implementing the MPPT algorithm. The comparison of various MPPT tracking techniques is presented in [25] from which the performance of the classical perturbation and observation technique is found favourable to be adopted in the proposed system. 3.3 PVDVR (inverter) control A double loop control is implemented for the three single-phase inverters individually for effective dynamic control of the PVDVR. The inner loop controls the output current of the inverter and the outer loop controls the injected voltage. Thus, the reference to the sinusoidal pulse-width modulation (SPWM) block is obtained corresponding to the available PV power and grid voltage. 3.4 DC-chopper control A DC-chopper circuit is incorporated across the DC-link capacitor to maintain the DC-link voltage below the threshold value. The DC-chopper circuit is operated when Vdc>Vdc−th or else kept idle (Fig. 3b). The DC chopper is operated under Mode-3 and Mode-4 operations of PVDVR. The DC-link control, boost converter control and the inverter control are combined together as shown in Fig. 3b to generate the reference for the output voltage of PVDVR in order to inject the PV power and also to maintain the generator voltage at 1.0 pu. The reference signal for generator voltage Vt is given as follows: (13)where ρ(t) is obtained from phase-locked loop (PLL). The synchronous reference frame (SRF)-PLL is implemented for grid synchronisation as shown in Fig. 3b. The generator voltage reference from (13) is compared with the grid voltage and the error is processed in the double loop control to deduce the reference signal for switching the devices in PVDVR. The voltage reference for PVDVR obtained from the double loop control is further fed to an SPWM block to generate pulses to the three single-phase inverters. 4 Results and discussion The effectiveness of the proposed system is verified under various operating conditions of grid voltage and PV power through power systems computer-aided design/electro-magnetic transient design and control (PSCAD/EMTDC) simulations and experiments. The system parameters are given in Table 3 and ‘Nomenclature’ section. Table 3. System parameters Parameters Simulation Experiment base apparent power 57.5 MVA 1.2 kVA rated active power 50 MW 0.75 kW rated voltage (line to line), V 690 380 , pu 0.0108 0.0311 , pu 0.107 0.081 , pu 0.01214 0.0566 , pu 0.1407 0.081 , pu 4.4 0.95 PV farm power rating 6 MW (0.12 pu) 75 W (0.1 pu) PVDVR inverter VA rating, pu 1.0 0.273 series transformer voltage 1.5/11 kV 60/60 V filter inductance, mH 4 3.8 filter capacitance, μF 180 120 DC-link voltage 1.5 kV 300 V 4.1 Simulation results The test system 1 consists of 25 fixed speed induction generators each of 2 MW rating connected to the grid through a transformer, and 6 MW (0.12 pu) rated PV plant. This system is simulated in PSCAD/EMTDC. The 6 MW PV power plant is realised with an array of 775 strings with 31 panels each. The rating of each PV panel is 250 W with an open-circuit voltage of 37.8 V and a short-circuit current of 8.7 A at 1000 W/m2 irradiation and 25°C temperature. The PV array is connected to a single-stage boost converter with an input and output DC voltage of 976 and 1500 V, respectively. However, in a practical implementation, the high-voltage requirements may be met by choosing a multistage DC–DC converter based on the available switch ratings. A VA rating of 1.0 pu is chosen for PVDVR. The DC-link voltage of the inverter is 1.5 kV and the series transformer rating is 1.5/11 kV based on the PVDVR rating. The switches in the inverter are fired at a switching frequency of 5 kHz. The voltage, current and phase angle of PVDVR, grid and wind farm are given in Table 4 which gives the details of real and reactive power balances at various grid voltage and PV power variations. The real and reactive powers are calculated and are converted to per unit values with the base values as given in Table 4. The following cases are considered for investigation. Table 4. Real and reactive powers exchanged in the system (simulation 50 MW system) VBase = 690/√3 V = 1.0 pu, IL−Base = 46.95 kA = 1.0 pu, PBase and QBase = 50 × 106 = 1 pu and φ = 153.43° Figure Mode of operation Grid PVDVR Wind farm (Vt=1.0 pu) Vg, pu φg, deg Pg, pu Qg, pu Vc, pu φc, deg Pc, pu Qc, pu Pw, pu Qw, pu Case 1 Fig. 4a mode-1 1.0 161.73 –1.06 +0.35 0.144 68.2 + 0.06 +0.15 –1.0 +0.5 mode-2 0.95 175.68 –1.06 +0.08 0.38 81.9 +0.06 +0.42 –1.0 +0.5 mode-2 1.20 141.3 –1.06 +0.85 0.316 99.73 +0.06 –0.35 –1.0 +0.5 Case 2 Fig. 4b idle 1.0 153.43 –1.0 +0.5 0 0 0 0 –1.0 +0.5 mode-4 0.15 153.43 –0.15 +0.089 0.847 153.43 −0.85 +0.41 –1.0 +0.5 mode-4 1.20 153.43 –1.2 +0.602 0.21 –27 +0.2 –0.102 –1.0 +0.5 Case 3 Fig. 5a mode-1 1.0 177.44 –1.12 +0.05 0.416 75.07 +0.12 +0.45 –1.0 +0.5 mode-3 1.0 in R 153.43 –0.333 0.167 0 0 0 0 –1.0 +0.5 0.5 in Y 153.43 –0.167 0.083 0.5 153.43 –0.167 0.083 0.5 in B 153.43 –0.167 0.083 0.5 153.43 –0.167 0.083 Case 4 Fig. 5b mode-1 1.0 177.44 –1.12 +0.05 0.416 75.07 +0.12 +0.45 –1.0 +0.5 mode-1 161.73 –1.06 +0.35 0.35 68.2 + 0.06 +0.15 –1.0 +0.5 idle 153.43 –1.0 +0.5 0 0 0 0 –1.0 +0.5 Source convention (grid and DVR). P or Q is + ve = supplying. P or Q is −ve = absorbing. Load convention (wind farm). P or Q is + ve = absorbing. P or Q is −ve = supplying. Fig. 4Open in figure viewerPowerPoint System response under (a) Case 1: symmetrical sag (5%) and swell (20%) in grid voltage with simultaneous PV power injection, (b) Case 2: symmetrical sag (85%) and swell (20%) in grid voltage with zero PV power Fig. 5Open in figure viewerPowerPoint System response under (a) Case 3: unsymmetrical sag in grid voltage with available PV power, (b) Case 4: change in available PV power 4.1.1 Case 1: symmetrical sag/swell in grid voltage with PV power injection The available PV power is considered to be 0.06 pu, which corresponds to operating point ‘B’ in Fig. 2b. Fig. 4a shows that during nominal grid voltage, the PVDVR operates in Mode-1 and injects 0.06 and 0.13 pu of real and reactive powers, respectively (corresponds to ‘B’ in Fig. 2b). For a 5% sag (Fig. 4a), the operating point shifts from ‘B’ to ‘B1’ (Mode-1–Mode-2), whereby real power of 0.06 pu and reactive power of 0.45 pu is injected to compensate the sag and to maintain the reactive power requirement of wind farm since the reactive power supplied by grid reduces during sag. For 20% of symmetrical swell in grid voltage (Fig. 4a), the operating point shifts from ‘B’ to ‘B5’ (Mode-1–Mode-2), whereby PVDVR supplies a real power of 0.06 pu and absorbs the excess reactive power from the grid to maintain the generator voltage at 1.0 pu. 4.1.2 Case 2: symmetrical sag/swell in grid voltage with zero PV power (night time) The LVRT capability of the proposed system is tested as per grid code [14] and 85% of symmetrical sag is introduced in grid voltage at t = 0.45 s (Fig. 4b), wherein the PVDVR injects 0.85 pu voltage (in-phase) to the grid voltage and absorbs a part of real power since the real power absorbed by the grid reduces under sag. The real power absorbed by PVDVR is dissipated through the braking resistors using DC chopper to maintain the DC-link voltage. Furthermore, the PVDVR injects reactive power to mitigate the sag and maintains the generator voltage at 1.0 pu. Furthermore, 20% of symmetrical swell is introduced at t = 0.7 s (Fig. 4b), wherein the PVDVR injects 0.2 pu of voltage to inject real power and absorb the reactive power from the grid. Under nominal grid voltage, the PVDVR does not exchange any real and reactive powers since it injects zero