International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

PLL Approach to Mitigate Symmetrical Faults in Weak AC Grid of DFIG Based Wind Turbine for Voltage Stability Analysis

Cholleti Sriram1, O. Rakesh2, M. Harish3, A. Sridhar4 1Assistant Professor, 2-4UG Scholars, EEE Department, Guru Nanak Institute of Technology, Hyderabad, India [email protected], [email protected]

Abstract: Study of Instability issues of the grid-connected of wind turbines (WTs) based on doubly fed induction doubly fed induction generator (DFIG) based wind turbines generators (DFIG) that intends to improve its low- (WTs) during low-voltage ride-through (LVRT) have got voltage ride through(LVRT) capability. The main little attention yet. In this paper, the small-signal behavior of objective of this work is to design an algorithm that DFIG WTs attached to weak AC grid with high impedances would enable the system to control the initial over during the period of LVRT is investigated, with special attention paid to the -side converter (RSC). Firstly, currents that appear in the generator during voltage based on the studied LVRT strategy, the influence of the sags, which can damage the RSC, without tripping it. high-impedance grid is summarized as the interaction As a difference with classical solutions, based on the between phase-looked loop (PLL) and rotor current installation of crowbar circuits, this operation mode controller (RCC). As modal analysis result indicates that the permits to keep the inverter connected to the generator, underdamped poles are dominated by PLL, complex torque something that would permit the injection of power to coefficient method (CTCM), which is conventionally applied the grid during the fault, as the new grid codes demand. in power system to study the interaction between mechanical A theoretical study of the dynamical behavior of the and electrical subsystems of synchronous generator (SG), is rotor voltage is also developed, in order to show that the generalized to analyze how the PLL-RCC interaction influence the phase motion of PLL. Then the concerned voltage at the rotor terminals required for the control small-signal stability of the PLL-synchronized DFIG system strategy implementation remains under controllable can be discerned by the developed complex phase limits. In order to validate the proposed control system coefficients. Impacts of PLL’s and RCC’s parameters are simulation, results have been collected using highlighted, as well as the system’s operating conditions MATLAB/SIMULINK and experimental tests have during LVRT. Finally, the analytical result is validated by been carried out in a scaled prototype [3]. experiment. For doubly fed induction generator (DFIG)-based wind Keywords: Doubly Fed Induction Generator, Phase Locked energy conversion systems (WECSs), large Loop, LVRT, Rotor Current Controlled. electromotive force will be induced in the rotor circuit I. INTRODUCTION during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, This paper aims to study how the small-signal behavior which may even destroy the rotor-side converter (RSC). of DFIG WTs is affected by the control parameters as To mitigate this problem, a new flux-linkage-tracking- well as operating point during severe grid faults, with based low-voltage ride-through (LVRT) control strategy the interaction between PLL and RCC highlighted. This is proposed to suppress the short-circuit rotor current. paper mainly deals with the instability issues of the grid Under the proposed control strategy, the rotor flux connected DFIG based wind turbine during low voltage linkage is controlled to track a reduced fraction of the ride through have taken special attention. During the changing flux linkage by switching the control fault conditions, there is more chance of losing the algorithm of RSC during grid faults. To validate the synchronization between the wind turbine and Grid. proposed control strategy, a case study of a typical 1.5- Due to this, there occurs voltage variations i.e.,sudden MW DFIG-based WECS is carried out by simulation increase or decrease of voltage levels so that the system using the full-order model in stability can be disturbed. In this, back to back SIMULINK/SimPowerSystems. In the case study, a converters are used for DGIG where RSC and GSC are comparison with a typical LVRT method based on RSC controlled by using the RCC and PLL so that the entire control is given, and the effect of the control parameter system can be stabilized. For the controlling of these on the control performance is also investigated. Finally, pulses, PI controllers are used and also helps in the validity of the proposed method is further verified improving transient stability. by means of laboratory experiments with a scaled-size The main aim of this paper is to maintain the Voltage DFIG system [4]. stability before the fault condition and also maintaining The outline of the paper is as follows: Section II the same voltage levels after the fault conditions also by discusses about description of paper. Section III compensating the faults using PI controllers. The output discusses about MATLAB simulation theory. Section pulses from PI controllers are given to GSC and RSC IV discusses about fault analysis in power system converter and active and reactive power delivered by Section V discusses DFIG based wind turbine the machine can be controlled.This paper presents a connected to grid Section VI discusses on Model new control strategy for the rotor-side converter (RSC) 128

International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

working and its operation ,simulation results Section useful, for instance, for generators used in wind VII gives outlines of future scope and concludes the turbines. paper.

II. DFIG BASED WIND TURBINE Extensive research on LVRT of DFIG WTs has been focused on the significant stress issues (including overcurrent in rotor winding and over-voltage in DC bus), which is induced by the coupling between stator and rotor and the limited control voltage of rotor-side converter (RSC). However, possible stability issues during faults are not well described yet. DFIG is Fig. 1. DFIG Block Diagram directly coupled to grid and the back-to- back converter usually works with heavy dependence on information Doubly fed electrical generators are similar to AC about terminal voltage. As control loops usually play a electrical generators, but have additional features which crucial role in dynamic behavior, PLL-based vector allow them to run at speeds slightly above or below ,control, which is widely implemented in on-site LVRT their natural synchronous speed. This is useful for large strategy for DFIG WTs [5] and is the scenario variable speed wind turbines, because wind speed can considered in this paper, concerns us. The easily change suddenly. disturbed terminal voltage in high-impedance grid brings undesirable dynamics to control loops and C. High Impedance Grids: probably cause small-signal oscillatory instability to trip In general, the grid-connected inverter is tending toward DFIG WTs during faults. In addition, the oscillation instability when the grid impedance is higher. A weak out of control may trigger transient instability issues grid system[5] with high grid impedance is usually easy further at the time of grid fault clearance since transient to cause stability problems, including large harmonic stability is dependent upon initial states. Thus, secure distortion in the inverter current and the grid voltage, LVRT technologies necessitate attention to stability and the false tripping of the inverter. issues during faults when WTs are connected to high impedance grid. D. Small-Signal Stability: A. Wind power Generation: Small-signal stability analysis is about power system stability when subject to small disturbances. If power In wind power generation systems, wind drives turbines system oscillations caused by small disturbances can be that generate electricity. This process has the great suppressed, such that the deviations of system state advantage of being free from carbon dioxide (CO2) variables remain small for a long time, the power emissions. There are several types of wind turbines. system is stable. Horizontal-axis three-bladed turbines, which are the main stream for large-scale power generation, can E. Low-Voltage Ride-Through (LVRT): convert about 40% of wind energy into electricity. Low voltage ride through. In electrical power On the other hand, the principal drawback of wind engineering [fault ride through] (FRT), sometimes power generation is its volatility, that is, electricity under-voltage ride through (UVRT), is the capability of generation fluctuates and output is inconsistent owing to electric generators to stay connected in short periods of changes in wind speed and direction. Output can be lower electric network voltage (cf. voltage dip). leveled to a great extent by locating many wind turbines F. Phase-Locked Loop (PLL): in a large area[4]. If insufficient electricity is generated by wind power, other sources such as thermal or A phase-locked loop (PLL) is an electronic circuit with hydroelectric power generation can make up for the a voltage or voltage-driven oscillator that constantly shortfall. adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, B. Doubly Fed Induction Generator (DFIG): filter or recover a signal from a "noisy" Doubly-fed electric machines are electric motors or communications channel where data has been electric generators where both the field interrupted. windings and armature windings are separately III. MATLAB SIMULATION SOFTWARE connected to equipment outside the machine. By feeding adjustable frequency AC power to the field With simpowersystems, we build a model of a system windings, the can be made to rotate, just as we would assemble a physical system. The allowing variation in motor or generator speed. This is components in the model are connected by physical connections that represent ideal conduction paths. This approach describes the physical structure of the system 129

International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

rather than deriving and implementing the equations for V. DFIG BASED WIND TURBINE CONNECTED the system. from the model, which closely resembles a TO GRID schematic, simpowersystems automatically constructs the differential algebraic equations (daes) that An aggregated wind farm model with DFIG based wind characterize the behavior of the system. These turbine for small-signal stability study is proposed to equations are integrated with the rest of the simulink deal with the situation that the wind turbines operate model. receiving different incoming wind speeds. New England test system is applied to assess the effect of the large We can use the sensor blocks in simpowersystems to scale wind farm on power system small-signal measure current and voltage in your power network, stability.The effect of wind power on the oscillations is and then pass these signals into standard simulink investigated by gradually replacing the power generated blocks. source blocks enable simulink signals to assign by the synchronous generators in the system by power values to the electrical variables current and voltage. from either constant or variable speed wind turbines, Sensor and source blocks connect a control while observing the movement of the eigenvalues algorithmdeveloped in simulink to a simpowersystems through the complex plane.The aim of this project is to network. study the small-signal stability of wind power system IV. FAULT ANAYLSIS based on DFIG. In an electric power system, a fault or fault current is A. Description of DFIG: any abnormal electric current. for example, a short Doubly fed induction generator (DFIG) is one of the circuit is a fault in which current bypasses the normal most popular wind turbines which includes an induction load. an open-circuit fault occurs if a circuit is generator with slip ring. Wind power is the most rapidly interrupted by some failure. in three-phase systems, a growing one since the 20th century due to its fault may involve one or more phases and ground, or reproducible, resourceful, and pollution-free may occur only between phases. in a "ground fault" or characteristics. Wind energy has a significant impact on "earth fault", current flows into the earth. dynamic behavior of power system during normal the prospective short-circuit current of a predictable operations and transient faults with larger penetration in fault can be calculated for most situations. in power the grid. This brings new challenges in the stability systems, protective devices can detect fault conditions issues and, therefore, the study of influence of wind and operate circuit breakers and other devices to limit power on power system transient stability has become a the loss of service due to a failure. very important issue nowadays. Some years ago, the A symmetric or balanced fault affects each of the three most common type of generators used in the wind phases equally. In transmission line faults, roughly 5% energy conversion system (WECS) was the squirrel are symmetric. This is in contrast to an asymmetrical cage induction generator (SCIG), a fixed speed wind fault, where the three phases are not affected turbine generator (WTG) system, which has a number equally.These are very severe faults and occur of drawbacks Most of the drawbacks can be avoided infrequently in the power systems. These are also called when variable speed WTGs are used. With the recent as balanced faults and are of two types namely line to progress in modern power electronics, wind turbine line to line to ground (L-L-L-G) and line to line to line with doubly fed induction generator (DFIG) has drawn (L-L-L).Only 2-5 percent of system faults are increasing attention. In the DFIG, the induction symmetrical faults. If these faults occur, system remains generator is grid-connected at the stator terminals as balanced but results in severe damage to the electrical well as at the rotor mains via a partially rated variable power system equipments. Analysis of these fault is frequency AC/DC/AC converter (VFC). The VFC easy and usually carried by per phase basis. Three phase consists of a rotor-side converter (RSC) and a grid-side fault analysis or information is required for selecting converter (GSC) connected back-to-back by a dc-link set-phase relays, rupturing capacity of the circuit capacitor. breakers and rating of the protective switchgear.

Fig. 3. Construction of Wind Turbine Fig. 2. LLLG and LLL Faults

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International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

DFIG for Double Fed Induction Generator, a generating the generated active power and the active power principle widely used in wind turbines. It is based on delivered to the grid. Low voltage ride through an induction generator with a multiphase wound rotor requirement. As per LVRT requirement, during dip and a multiphase slip ring assembly with brushes for occurrence, the wind power generation plantdemands access to the rotor windings. It is possible to avoid the management of this mismatch, which is a challenge for multiphase slip ring assembly (see brushless doubly-fed the WECS. must remain connected to the grid and in electric machines), but there are problems with addition, it has to deliver reactive power into the grid to efficiency, cost and size. A better alternative is aid the utility to hold the grid voltage. a brushless wound-rotor doubly-fed electric machine. LVRT fulfillment is required by the wind generators The principle of the DFIG is that rotor windings are when the voltage in the grid is temporarily reduced due connected to the grid via slip rings and back-to- to a fault or large load change in the grid. The required back voltage source converter that controls both the LVRT behavior is defined in grid codes issued by the rotor and the grid currents. Thus rotor frequency can grid operators in order to maintain system stability, freely differ from the grid frequency (50 or 60 Hz). By thereby reducing the risk of voltage collapse. The grid using the converter to control the rotor currents, it is codes were originally developed considering the possible to adjust the active and reactive power fed to synchronous generator generally used in conventional the grid from the stator independently of the generator's power plants, whereas Wind Turbine Generators turning speed. The control principle used is either the (WTG) have different characteristics as compared to two-axis current vector control or direct torque control conventional power plants. As per Indian Wind Grid (DTC). DTC has turned out to have better stability than Code (IWGC), wind farms connected to 66kV and current vector control especially when high reactive above shall have the operating region as shown in figure currents are required from the generator. during system faults.

Fig. 4. Schematic Representation of DFIG Based Wind Turbine VII. SIMULATION RESULTS The doubly-fed generator rotors are typically wound with 2 to 3 times the number of turns of the stator. This means that the rotor voltages will be higher and currents respectively lower. Thus in the typical ± 30% operational speed range around the synchronous speed, the rated current of the converter is accordingly lower which leads to a lower cost of the converter. The drawback is that controlled operation outside the operational speed range is impossible because of the higher than rated rotor voltage. Further, the voltage transients due to the grid disturbances (three- and two- phase voltage dips, especially) will also be magnified. In order to prevent high rotor voltages - and high currents resulting from these voltages - from destroying the IGBTs and diodes of the converter, a protection circuit (called crowbar) is used. VI. LVRT PROBLEMS Under the compensated condition, two blocks(GSC and RSC) are added additionally to reduce the effect of fault Low Voltage Ride-Through (LVRT) is one of the most on the system. In GSC converter controller, PI dominant grid connection requirements to be met by controller is used to reduce the transient effect caused Wind Energy Conversion Systems (WECS). In presence due to the fault. Taking the grid references values, it of grid voltage dips, a mismatch is produced between tries to maintain the constant voltage level between the 131

International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

grid and wind turbine. This is done by controlling the and RCC for this studied LVRT strategy, and modal pulse levels of the converter circuit. analysis result indicates that the unstable poles of this PLL-synchronized DFIG system are dominated by PLL. Thus, in light of stability analysis of mechanical motion of SG, CTCM is generalized to study how the phase motion of PLL is affected by the PLL-RCC interaction. Then the concerned small-signal behavior, including the impacts of control parameters and operating points, can be discerned by the developed complex phase coefficients. As a result, the following conclusions can be given as: PLL-based DFIG wind turbines connected to high-impedance grid may be at risk of small-signal Fig. 5. Output Voltage without Compensation oscillatory instability during deep voltage sag due to insufficient damping, which can be qualified and quantified by the generalized complex phase Deep voltage sag, along with induced great power angle, sharply increases the coefficients. negative damping contributed by rotor current control and probably brings The wide range of control parameters endangers small- signal stability. Either insufficient damping to DFIG system during LVRT. higher bandwidth of PLL or lower bandwidth of RCC tends to aggravate the interaction and decrease the net damping of DFIG system. For modeling of DFIG WTs under weak grid Fig. 6. Output Current without Compensation condition, it is not recommended to omit the dynamics of RCC Unlike stress issue (transient over voltage and over current), the concerned if the bandwidth of RCC is not high enough. Small-signal stability of sub- synchronous frequency range during LVRT become worse under low rotor speed condition. Reducing the active rotor current is helpful to improve the damping during deep voltage sag. In future, we can modify the proposed control system with the efficient controller. For example instead of PI controller we can use Artificial Neural Network (ANN) which is more efficient than the PI controller to get

better compensation of Voltage sag during fault Fig. 7. Output Voltage with Compensation condition. This thesis routed many ways for the future researchers to work further on the grid. The steady state and dynamical behaviour analysis of shunt active filter with gridconnected PMSG and DFIG based WECS. When the rotor attains the super synchronous speed for DFIG based WECS, the rotor The THD for experimental analysis is slightly higher than the simulation result, which will lead to further research in future connection. The hardware result of reactive power is not exactly zero when using matrix gets more oscillation. Propose a new design for reduce the rotor oscillations. Experimental setup for the proposed Fig. 8. Output Current with Compensation system is carried out only by using laboratory converter. This will lead to develop a new design in future. wind VIII. CONCLUSION emulator and future researchers may use real wind In this paper, small-signal behavior during deep voltage turbine sag within the frequency range of tens rad/s is investigated for DFIG based WTs connected to a high- IX. REFERENCES impedance grid. The high grid impedance induces [1] Network Code on Requirements for Grid significant interaction between control loops, i.e., PLL Connection Applicable to All Generators, European 132

International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com

Network of Transmission System Operators for Electricity, Apr. 2016. [2] Requirements for the Interconnection of Distributed Generation to the Hydro-Québec Medium-voltage Distribution System, Hydro-Québec, C anada, Feb. 2009. [3] F. K. A. Lima, A. Luna, P. Rodriguez, E. Watanabe, and F. Blaabjerg, “Rotor voltage dynamics in the doubly fed induction generator during grid faults,” IEEE Trans. Power Electron., vol. 25, no. 1, pp. 118–130, Jan. 2010. [4] S. Xiao, G. Yang, H. L. Zhou and H. Geng, “An LVRT control strategy based on flux linkage tracking for DFIG-based WECS,” IEEE Trans. Ind. Electron., vol. 60, no.7, pp. 2820-2832, Jul. 2013. [5] R. Pena, R. Cardenas, G. Asher, “Overview of control systems for the operation of DFIGs in wind energy applications,” IEEE Trans. Ind. Electron., vol. 60, no. 7, pp. 2776-2798, Jul. 2013. [6] I. Erlich, F. Shewarega, S. Engelhardt, J. Kretschmann, J. Fortmann and F. Koch, “Effect of wind turbine output current during faults on grid voltage and the transient stability of wind parks,” presented at the 2009 IEEE Power & Energy Society General Meeting, Calgary, AB, Canada, Jun. 26-30, 2009. [7] O. Goksu, “Control of wind turbines during symmetrical and asymmetrical grid faults,” Ph.D. dissertation, Aalborg Univ., Aalborg, Denmark, 2010. [8] D. Dong, B. Wen, D. Boroyevich, P. Mattavelli, and Y. Xue, “Analysis of phase-locked loop low- frequency stability in three-phase grid connected power converters considering impedance interactions,” IEEE Trans. Ind. Electron., vol. 62, no. 1, pp. 310–321, Jan. 2015. [9] O. Goksu, R. Teodorescu, C. L.Bak, F. Iov, “Instability of wind turbine converters during current injection to low voltage grid faults and PLL frequency based stability solution,” IEEE Trans. Power Syst., vol. 29, no. 4, pp. 1683–1691, Jul. 2014.

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