Themain objective of this paper is to establish a predictive power controlstrategy which is used to improve the stability of power systems. This has beendone in the presence of wind farms based on Doubly Fed Induction GeneratorDFIG using Static Synchronous Series Compensator SSSC and Super CapacitorEnergy Storage System SCESS.
In our proposed system, SCESS is used to controlthe active power in the Grid Side Converter GSC and SSSC is used to reducethe low frequency oscillations. SCESS is composed of a capacitor bank and adual switch DC/DC converter which is connected to DFIG through the DC link. Inthis method, it is used to control the active and reactive power of the RotorSide Converter RSC as well as damping controller design for SCESS and SSSC.For improving the power system stability, an SSSC is used, which is an modern power quality FACTS device that employs a voltagesource converter connected in series to a transmission line through atransformer. Thus the system has been simulated and tested on MATLAB/SIMULINK.1. Introduction1.
1MotivationEnergyis a key to human development, and renewable energy can ensure energy needs aremet while protecting local environments and populations. Even other sources ofenergy are finite and will someday be depleted. Now a day’s renewable energy isthe major source for the power generation. Among renewable resources, windenergy and solar energy is emerging technology. In that wind energy is thecommon energy to produce electrical signals all around the world. Due tooscillating behaviour of wind power, there will be some oscillations inelectrical grids while transmitting the electrical signals.
In a power systems,these fluctuations from stable system and considering the power quality, it isan serious issue in power system 1, 2. For this we are using an Super CapacitorEnergy Storage System (SCESS) and Flexible AC Transmission System(FACTS)devices, which is used as a balancer to diminish oscillations and enhancedamping in power system 3,4. To eradicate this fluctuations andto improve damping, we can done more number of studies in the field of powersystem. In those studies there will be some pros and cons in it. The mainobjective of this paper is to increase the stability of the power system withthe help of predictive control strategy. Because of the cost effectiveness anddirect power control, Doubly Fed Induction Generator DFIG is taken as acommon type of variable speed wind turbine 5. Here, rotor of DFIG is coupledto the grid along with the back-to-back bi-directional converter and stator iscoupled to grid.
More number of studies have been done for the DFIG basedcontrol systems to improve stability such as Sensitivity Analysis and Hybrid Wavelet-PSO-ANFIS6, eigen value sensitivity analysis 7, state feedback 8. We have analayzedthe PI controllers, neural networks and fuzzy logic types but it also havingsome complexity and cons in it 9,10. Now a days, Super Capacitor EnergyStorage System plays an vital role in reducing the fluctuations of DFIG powersystem 11,12. For compensating the reactive power, Fly-Wheel Energy StorageSystem (FESS) 13, 14 and Super conducting Magnetic Energy Storage(SMES)15-17. For reducing the low frequency oscillations in power system, we canuse the FACTS devices. SSSC is an advanced type of FACTS devices in which it isconnected in series along transmission lines 18,19. There are various methodshave been developed for designing an controller of SSSC 20-25. Advancedcontrol method known as model predictive control (MPC) is mostly used inindustries and in research 26.
2. Power System Design2.1Doubly Fed Induction Generator TypeInDFIG based wind turbine system, the grid side converter and rotor sideconverter are connected as back-to-back converter through DC link. To maintainthe balance between grid side converter and rotor side converter, DC link isused. 2.2Design of Rotor Side ConverterRotorSide Converter is used to extract the maximum power from wind, to control theactive and reactive power of DFIG and it gives an reactive power needed by theinduction generator. In this controller, active power and voltage are regulatedby Pqr and Pdr respectively.
Using the reactive powercontrol, the voltage control is made by measuring Xsw and bymeasuring the turbine speed Trw, the reactive power control is made.These control signals are compared along with the reference signals by PIcontroller for all the parameters, the reference signals (Kqr_ref, Kdr_ref)are measured. For producing the error signal, current reference signal iscompared to the reference signals which are in d-q axis. These referencesignals are amplified by current signals to produce Pqr and Pdrsignal and it is send to Pulse Width Modulation.
Signals from PWM is send toinverter to act as switching mode 7. 2.3Design of Grid Side ConverterGridSide Converter is used to keep the constant DC voltage link and for controllingthe reactive power of the system. In this paper, reference signal of DC link (Pdc_ref)is compared with the Pdc by PI controller. The current referencesignals (Kqg_ref, Kdq_ref) is compared along with measuredvalue. After combining the current signals, Pdg, Pqg areproduced and these signals are send to Pulse Width Modulation.
The pulses fromPWM is send to inverter to act as switching mode 7. 2.3Design of SCESS converter SuperCapacitor Energy Storage System is used to control the DC link voltage. In thispaper, damping controller is designed, which is used to control the reactivepower of the system.
SCESS converter consists of capacitor bank and dual switchDC/DC converter. This is connected to DFIG through DC link. The switches S1and S2 is used as boost and buck modes in the system. The converterwill works as boost mode, when S1 opens. The converter will works asbuck mode, when S2 opens. 2.4Design of Static Synchronous Series CompensatorSSSCis a type of FACTS devices which is connected in series with transmission line.Depending upon voltage source inverter, SSSC is used as a reactive powercompensator.
SSSC can operate in capacitive and inductive modes. SSSC consistsof voltage source inverter, series transformer, capacitor and control block. 3. Controller SectionFordetermining the future response of the system, a new powerful technique knownas model predictive control was used in the system 27, 28. For each problem,optimal point can be determined by using this method. For state estimation ofthe process, we can predict the input and output of the system.
By using thiscontrol strategy, for each sampling interval, a optimization problem is solved init. The changes can be made in the system as an error signal. In this method, predictivehorizon is measured in the system for future response.