Analyzing the application of crowbar and stat com to overcome voltage sags with actual - PDF document

International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal , Volume 4 , Issue 3 , March 2014 ) 468 Analyzing t he Application o f Crowbar And Statcom To Overcome Voltage Sags With Actual Field Data A. D. Thirumoorthy 1 , C. Deepthi 2 1 B.64, Sreevatsa gardens, Thudiyalur, Coimbatore, Tamil Nadu, India . 2 Department of EEE, Karunya University, Coimbatore, Tamil Nadu, India Abstract — Wind power generation is increasing rapidly in India and wind integration with the grid is considered as a promising source of energy, Double Fed Induction Generator (DFIG) being the most commonly used wind turbine generator, when integrated with the grid show challenges during grid faults. This paper focuses on the challenges faced by DFIG during the occurrence of voltage sags and the Low Voltage Ride Through (LVRT) methods to overcome it. The crowbar protection method is used to ride through voltage sags and FACTS device STATCOM is used to quickly sense the voltage sag and overcome it. Furthermore, simulation has done using MATLAB/ SIMULINK; analyses the performance of combined crowbar protection and STATCOM on DFIG during voltage sags. The results shows t hat crowbar protection method is a reliable method to ride through the fault and STATCOM helps to sense and overcome voltage sags quickly. Moreover, the grid code requirements are met . Keywords — Crowbar protection, Double Fed Induction Generator (DFIG) , Low Voltage Ride Through (LVRT), STATCOM . I. I NTRODUCTION The utilization of renewable energy resources like the wind, solar, biomass etc. are increasing to meet the energy demand and to have sustainable growth. In sustainable energy system, energy conservation and the use of renewable energy resource are the key paradigm. The need to integrate the renewable energy like wind energy into the power system is to reduce the environment impact produced due to the conventional plant and to meet the required energy demands. However, the integration of wind energy into the existing powe r system presents technical challenges like voltage regulation, stability, power quality problems. The issue of power quality is of great importance to wind turbine (WT) [1] [2]. Thus, power system operators have created grid codes which determine how WT‘s should operate during grid disturbances [2]. The grid codes covers rules considering the fault ride through behavior as well as the steady state active power and reactive power production. During the occurrence of grid faults grid codes prescribe that WT ‘s must stay connected to the grid and should support the grid by generating reactive power to support the and restore the grid voltage quickly after the fault [3]. Doubly Fed Induction Generator (DFIG) is dominant among the various wind turbine concepts due to its variable speed operation, its separately controllable active and reactive power and its partial rated power converter. The stator of the DFIG is directly connected with the grid and the rotor is connected to the grid through the power electronic converters. The back – to – back converters consists of two voltage source converters (ac - dc - ac) having a dc link capacitor connecting them. The generator side converter takes the variable frequency voltage and converts it into dc voltage. The grid side c onverter has the ac voltage from the dc link as input and voltage at grid parameters as output. The gearbox has the role of matching the speed between the blades and the rotor. The transformer couples the generator to the grid adjusts the voltage of the ma chine to that of the grid. DFIG‘s are sensitive to grid voltage disturbances; symmetrical and unsymmetrical voltage dips and hence; requires additional protection for the rotor side power electronic converter [3], [4]. During voltage sag high currents are induced in the rotor circuit which can damage the Rotor Side Converter (RSC) and cause serious fatigue to the generator if no protection is applied [4] [5]. In order to protect the generator, disconnects itself from the grid. This sudden disconnection cau ses huge outages in the grid. Hence, a proper protection is needed for DFIG to prevent it from over voltages and provide excellent voltage ride through (VRT) capability. Conventionally, a resistive network called crowbar is connected to the rotor side conv erter. In case of rotor over currents, the rotor side converter is disabled and crowbar is activated during voltage sags to provide safe path for the over currents. In order to sense the voltage dip and to satisfy the dip quickly FACTS device STATCOM is u sed in addition with the crowbar. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal , Volume 4 , Issue 3 , March 2014 ) 469 The study concentrates on utilizing crowbar to overcome voltage sags and STATCOM to overcome faults quickly. The paper is organized as follows. The section II provides the understanding of DFIG. Section III focuses on the designing of crowbar and STATCOM using MATLAB/SIMULINK. Section IV describes how an LVRT method help in improving the power quality during grid faults. Section V describes the result analysis of DFIG during its normal operation, on the occurrence of fault and the power quality improvement using crowbar and STATCOM. II. U NDERSTANDING O F DFIG DFIG is basically an induction generator, with multiphase stator winding that is directly connected to the electrical grid and a three - phase rotor winding which is also con nected to the grid through slip rings, but partially rated power electronics converters (PEC) F ig. 1 DFIG With LVRT Methods Recently, designs without brushes have also been introduced. Alternatively, the rotor power could be magnetically transferred using the brushless doubly - fed induction generator (BDIG), however, presently, BDIG‘s are larger and more expensive than slip - ring option and hence not common. They are called ‗doubly - fed‘ since stator and rotor windings are participating in energy conversion proces s. In the super - synchronous speed operation mode of the DFIG the power is delivered to the grid directly by the stator. At the same time power flows from DFIG rotor as well through the PEC to the grid or simply, the rotor supplies power to the grid (i.e.) ‗positive resistance is inserted‘. But then as the wind speeds up still further, the additional power is spilled away by pitching the rotor blades out of the wind. Simultaneously, the PEC in the rotor circuit compensates the difference between the mechani cal and electrical frequency of the wind power plant (WPP) by injecting a current in the rotor circuit with variable frequency. At low wind speeds, when DFIG has to operate in sub - synchronous speeds (positive slip) the PEC continues to enable the DFIG to generate power because the PEC has a bidirectional power flow. In such a case, a ‗negative resistance is inserted into the rotor circuit to make up the energy deficit (i.e.) PEC may ‗borrow‘ power from the line for the under - speed rotor, which is passes on to the stator, still allowing the stator to feed the network with 50Hz power supply. The stator appears to be supplying 130% of the power to the grid. Note that the generator rotor has ‗borrowed‘ 30% leaving the line with 100% for theoretical losses of DF IG. When power flow is in the reverse condition, the grid side PEC will act as a converter and the rotor side PEC will act as an inverter. The DFIG rotor is connected to the grid through three phase variable frequency bi - directional back - to - back four quad rant PEC. This consists of two controllable IGBT - based pulse width modulated (PWM) voltage source inverters (VSI) linked by a DC - link bus which generates a voltage with amplitude and phase being continuously and rapidly controlled to effectively perform reactive power control. The dc - link capacitor acts like an energy storage to keep voltage variations (or ripple) in the dc - link voltage small. The gear box helps the wind turbine to match the speed between the winds and rotor. The transformer couples the generator to the grid. III. D ESIGNING O F C ROWBAR A ND S TATCOM A. Crowbar To protect the RSC from tripping due to over currents active crowbar is used in the rotor circuit. The crowbar limits the currents and provides a safe path for the currents by short circuiting the rotor by a set of resistors. When the crowbar is activated the RSC pulses are completely disabled and the machine behaves like a squirrel cage induction machine directly coupled to the grid. The magnetisation of the machine that was provided by the RSC during the normal operation is lost and the machine large of a mount of reactive power from the grid, which actually grid codes. Though, crowbar circuit triggering produces high stress to the mechanical components of the system crowbar protection is reliable because of its simple construction and low cost. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal , Volume 4 , Issue 3 , March 2014 ) 470 Crowbar pro tection can be designed with symmetric three phase y - connected resistance. It is connected to the rotor through a controllable breaker. Though, this is not the real case (in reality, the crowbar may be made up of one resistance fed through a switched recti fier bridge), but it may be sufficient to assess the overall impact of a crowbar protection on the LVRT. The breaker is normally open, but it is closed short - circuiting the rotor through the resistance if either the rotor current or the DC - link capacitor v oltage becomes too high. At the same time the switching of the RSC is stopped. The value of the crowbar resistance is chosen as 20 times the rotor resistance. The choice of the crowbar resistance is important because, as it determines how much reactive pow er the DFIG will draw while the crowbar is inserted. The crowbar is disconnected and the RSC is reinserted when the rotor current and DC - link voltage return to their normal operating range [7]. B. STATCOM A STATCOM, also known as an advanced static VAR compe nsator, is a shunt connected FACTS device. It generates a set of balanced three - phase sinusoidal voltages at fundamental frequency, with rapidly controllable amplitude and phase angle. In this paper, STATCOM is modelled in MATLAB/SIMULINK using an IGBT PWM converter with a dc - link capacitor. The objective of STATCOM is to ride through voltage dip quickly. IV. LVRT M ETHODS T O O VERCOME G RID F AULTS On the occurrence of single phase to ground fault, the wind turbine trips itself from the grid when crowbar is not available. This sudden disconnection should be avoided, as it produces huge outages. The inclusion of crowbar with the DFIG system is that the wind turbine does not trip even when severe fault occurs in the grid or in the generator system. The crowbar prot ection system is connected at the first stage of the dip (i.e.) before 10 ms of the occurrence of the fault. This early detection of the fault is done with the help of STATCOM. Though, the speed of the Induction generator (IG) decreases, DFIG continues its operation with a short - circuited rotor circuit as SCIG. At this time, the rotor current will be flowing through the crowbar the crowbar resistor. After the fault clearance, the crowbar resistance is still connected for 10 ms and then RSC is reconnected. H ence, the generator is protected from damages and moreover, there is continuous production of electricity. F ig. 2 MATLAB/SIMULINK block diagram V. R ESULT A NALYSIS To verify the effect of crowbar and STATCOM on DFIG during grid faults, a simulation is carried o ut using MATLB/SIMULINK software. In this simulation Pethappampatty wind farm in Coimbatore, Tamil Nadu is taken for examination. The Substation consists of four wind farm feeder namely Pukkulam, SV Patty, Eluppanagaram and Ponneri connected to the 33kV fe eder. The power transformers, 33/110 kV and 110/230kV are used to step up the voltage from 11kV to 230 kV for connecting the wind electric system to the 230kV grid. During the normal operation the wind farm produces 9 MW of output power. A. Behavior of DFIG w ithout crowbar and STATCOM: In this paper, only three feeders are considered and from the analysis it is found that, there were two short duration Instantaneous sag for 0.5 to 20 cycles produced heavy increase in current. F ig. 3 Occurrences of sag in the field recorded by power analyzer – Fluke 434. On the occurrence of voltge sags, the voltage in the circuit drops drastically. As a result, current increases sharply. This causes severe damage to the generator. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal , Volume 4 , Issue 3 , March 2014 ) 471 Fig. 4 Current increase Fig. 5 Voltage variation At this moment, the active power drops to zero and DFIG absorbs a lot of reactive power from the grid. According to grid codes, absorption of large amount of reactive power is not permitted. Fig. 6 Meter reading Fig . 7 Power variation F ig. 8 Frequency variation Hence, from the simulation results it is concluded that without crowbar and STATCOM voltage dips will cause heavy destruction on converter system of the generator. Moreover, the generator separates itself qu ickly from the grid which violates grid codes. B. Behavior of DFIG with crowbar and STATCOM: Crowbar and STATCOM LVRT methods are used to prevent the DFIF from being disconnected from the grid during severe voltage dips. The STATCOM senses the voltage dip qu ickly and the crowbar will get connected to the grid within 10 ms of sag occurrence. The crowbar will stay connected to the grid even after 10 ms of fault clearance. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com ( ISSN 2250 - 2459, ISO 9001:2008 Certified Journal , Volume 4 , Issue 3 , March 2014 ) 472 F ig. 9 Output analysis with crowbar and STATCOM It is found that, even during fault condit ions the wind farm generates 3.5 to 4 MW of power and the absorption of reactive power absorption is also reduced. Moreover, the generator stays connected to the grid as mentioned by the grid codes. VI. C ONCLUSION With the reference field data taken at Pethappampatty wind farm, Coimbatore, Tamil Nadu, India; this paper has analyzed the effect of voltage sags with crowbar and STATCOM on DFIG. The simulation results proved that crowbar and STATCOM methods are reliable LVRT methods to overcome voltage sags. 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