Improvement Of Voltage SAG And Dynamic Stability Engineering Essay

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Introduction

Now a day's many wind power sources are connected to the existing utility grid. So, it is important to address that all the wind farms should be connected to the utility grid under any abnormal conditions. In the wind power industry, the majority of grid-connected wind turbines are equipped with doubly fed induction generator (DFIG) because of their advantages over other wind turbine generator (WTG) system. As the wind power penetration continually increases, power utilities concerns are shifting focus from the power quality issue to the stability problem caused by the wind power connection [4].

Increasing amount of wind turbine are connected to electrical power system in order to mitigate the negative environmental consequence of conventional electricity generation. In general voltage & frequency must be kept as stable as possible. This stability can be obtained by using FACTS devices. Induction generator are widely used as wind generator due to its simple rugged and maintenance free construction . An induction generator are connected with a wind turbine to generate electricity is the sink of reactive power[1].

Doubly fed induction generators (DFIGs) used in wind farms has many benefits such as reactive power control capability and power improvement. These generators have the ability to keep the voltage in the steady state when they are exposed to a small disturbance. Due to these characteristics and the control ability of capacity, utilizing of such generators in power system stability control is very useful. FACTS devices have been introduced in the network in order to solve problems such as voltage instability and reactive power limits. Static VAr compensator (SVC) and STATCOM can be considered as a pioneer of FACTS devices [3].

One of the major causes of voltage instability is the reactive power limit of the system. Improving the system's reactive power handling capacity via Flexible AC transmission System (FACTS) devices is a remedy for prevention of voltage instability and hence voltage collapse[8].

literature review

The paper presented by S.G Thakare, H.S. Dalvi and K.D Joshi states that the STATCOM, based on voltage source converter (VSC) PWM technique are used to stabilize grid connected squirrel cage wind generator system. They adopt a simple control strategy of STATCOM where only measurement of RMS age at the IG terminal is needed i.e. there is no need of reactive power measurement. Investigate Performance of conventional PI controller and fuzzy logic controller (FLC) as the control system of STATCOM. The voltage sag and swell improvement of WPGS is compared with pitch controller also. They presented here that induction generator steady state capacitor value can be reduced by certain percentage from the rated value where the rest of the reactive power drawn by induction generator at steady state will be supplied by the STATCOM. Hence they concluded that STATCOM with FLC gives better performance than STATCOM with conventional PI controller [1].

Li wang, senior member IEEE and Chia-Tien Hsiung has presented a strategy and control scheme based on a static synchronous compensator (STATCOM) for dynamic

stability improvement and voltage control of an integrated OWF(Offshore Wind Farm) and MCF (Marine Current Farm).

A damping controller of the STATCOM is designed by using modal control theory (pole placement) for dynamic stability improvement and a set of liberalized system equations in matrix form as

pX =AX + BU + VW

Y= CX + DU

Where X is the state vector,Y is the output vector, U is the external or compensated input vector, W is the disturbance input vector while A,B,C, and D are all constant matrices of appropriate dimensions. For STATCOM the disturbance input

Vector W and the external input vector U can be properly neglected by letting D = V= 0 and the state vector X can be partitioned into five sub state vectors as X = [XDFIG, XSCIG, XMECH, XELEC, XSTA]T

Hence concluded from the simulated results that the proposed

STATCOM joined with the designed damping controller is very effective to stabilize the studied system under disturbance conditions and also the voltage fluctuations of the AC bus subject to the active-power variations of the studied system can also be effectively controlled [2].

M.Mohammadi, M.Gitizadeh and A.Roosta in this paper presents the impact of static VAr compensator (SVC) on the dynamic stability of power system connected to DFIG. In order to improve the dynamic stability, control parameters of SVC need to be set optimally. The dynamic simulation results give us information such as the impact of SVC on the performance of DFIG and synchronous generators dynamic performance. MATLAB is used for executing the dynamic simulation. By choosing the appropriate objective function KSVC plays very important role in improving stability by SVC. Hence concluded that the presence of SVC has a salient effect

on improving the stability of wind power plant presence of SVC and has a great effect on the voltage stability[3]

Ranjan K.Behera and Wenzhong Goa in this paper present the impact of voltage sag in the internal dynamics of a grid connected Doubly Fed Induction Generator (DFIG) system. They propose a common dc-link of the rotor side converter (RSC) and grid side converter (GSC) supported static compensator (STATCOM) for the voltage ride through capability and the proposed system is connected to a small transmission network, where many types' loads are connected and hence observed that the voltage sag at the PCC of the grid affects the internal dynamics of DFIG such as flux demagnetization of the stator flux, high transients in the developed electromagnetic torque, and abnormal rotor current

spikes. which affects the performance of the DFIG during transient conditions and may leads to the malfunctioning of the power electronics converters. This topology will also support the reactive power requirement of the grid without affecting the performance of the dc-link voltage. Hence, this structure is useful in improving the transient stability, power oscillation damping, voltage stability and increase in power transfer limit of the connected power system. They modeled the proposed DFIG system along with its controller and simulated in MATLAB using Simulink and power system block-set toolboxes [4].

P.K. Dhal and C. Christober Asir Rajan in this paper paper presents a transient stability improvement using neural-fuzzy controller design for STATCOM with static synchronous time critical error and better damping system oscillations after a short circuit fault. A STATCOM can stabilize the power system by controlling the reactive power, increase the maximum active power flow and regulate the line voltages. They propose amplitude control by making the STATCOM voltage higher than the AC system voltage the current will lead the voltage by 90°. When AC system voltage and inverter-composed voltage are synchronized and have the same amplitude, the active and reactive power outputs are zero, they use multiple six-pulse inverters which are magnetically coupled through a complex zig-zag transformer in this topology. Hence concluded the effect of synchronous after the system disturbance can be compensated with designed NFC controller based on energy function Lyapunov and with proposed multilevel optimal modulation strategy

for STATCOM, the system is easily balanced [5].

Mr. Sunil. T.P and Mr. N. Loganathan in this paper use STATCOM instead of SVC to improve power quality and connected it at a point of common coupling (PCC). STATCOM will reduce the harmonics, voltage fluctuation, voltage dips and reactive power in the grid current by injecting superior reactive power in to the grid, because the reactive power drops of STATCOM are linear with voltage. They implement a bang-bang control scheme with the STATCOM to achieve fast dynamic response for the reduction of harmonics in grid current to improve the power quality at PCC [6].

Yan Zhang and Jovica V. Milanovic in this paper presents an approach to optimally select and allocate flexible ac transmission (FACTS) devices with a proposed method based on the optimization of a preselected objective function using simple and niching genetic algorithms (GA) in a distribution network in order to minimize the number of voltage sags at network buses. Their objective is to achieve the improvement in overall system sag performance. STATCOM and SVC can provide an efficient solution to voltage sag problems. Among SARFI, SIARFI, and SMARFI values the sag density table is

one of the most widely used techniques for describing system and individual bus voltage sag performance which gives voltage events with a voltage magnitude and duration within a

Certain range. The total number of sags in the network within each magnitude range is calculated as

Where λi,p,k is the fault frequency of fault type p (three phase, single phase, line-to-line, line-to-line-to-ground) at fault position i, Ui,p,k is the voltage sag magnitude at bus k (out of N buses) as a result of fault p(type of fault) at location i ,UL and UU are the lower and upper limits of the voltage range. They use fault positions method, it consists of simulating various faults at numerous points in the system, and calculating the resulting retained voltage at the selected number of buses[7].

Mehrdad Ahmadi Kamarposhti and Mostafa Alinezhad in this paper compare the merits and demerits of SVC and STATCOM in terms of Maximum Loading Point (MLP) in static voltage collapse study and their effects in Static Voltage Stability Margin Enhancement. Because Voltage instability is mainly associated with reactive power imbalance and at Maximum Loading Point (MLP) or voltage collapse point, both real and reactive power losses increase rapidly. A continuation power flow algorithm with smooth changes of loading level at various buses of the system is chosen for simulation purpose. STATCOM delivers constant reactive power at the limits compared to SVC, when the load of the system is increased; the effect of STATCOM in improving the voltage is more adequate than the SVC and when the maximum limit is reached, the SVC behaves exactly like a fixed shunt capacitor. STATCOM gives the maximum loading margin compared to other devices due to the reason STATCOM is installed at the weakest bus, hence concluded that SVC and STATCOM significantly affects the shape of the PV curve, which improves the critical point without masking the nose point by only shift out the PV curve, hence both SVC and STATCOM increase static voltage stability margin and power transfer capability [8].

iii. References

S.G Thakare, H.S. Dalvi and K.D Joshi "STATCOM Based Fuzzy Controller For Grid Connected Wind Generator," Second International Conference On Emerging Trends In Engineering And Technology,ICETET-09

Li wang, senior member IEEE and Chia-Tien Hsiung "Dynamic Stability Improvement Of An Integrated Grid Connected Offshore Wind Farm And Marine Current Farm Using A STATCOM" IEEE Transaction On Power Systems,Vol.26 No.2,MAY 2011

M.Mohammadi, M.Gitizadeh and A.Roosta "Dynamic Stability Improvement Of A Power System Incorporating DFIG Wind Power Plant Using Optimized Control Parameters Of A SVC," 2012 IEEE International Power Engineering And Optimization Conference (PEOCO2012),Meleka,Malaysia:6-7 June 2012

Ranjan K.Behera and Wenzhong Goa "Low Voltage Ride Throough And Performance Improvement Of A Grid Connected DFIG System" 2009 Third International Conference on power systems, Kharagpur, INDIA December 27-29

P.K.Dhal and C.Christober Asir Rajan "Transient Stability Improvement Using Neuro Fuzzy Controller Design For STATCOM" IEEE International Conference On Advances In Engineering And Management (ICAESM-2012) March 30,31,2012, 510

Mr. Sunil. T.P and Mr. N.Loganathan "Power Quality Improvement Of A Grid Connected Wind Energy Conversion System With Harmonics Reduction Using FACTS Device" IEEE International Conference On Advances In Engineering And Management (ICAESM-2012) March 30,31,2012, 415

Yan Zhang and Jovica V. Milanovic´, Fellow, IEEE "Global Voltage Sag Mitigation With Facts-Based Devices" IEEE Transactions On Power Delivery, Vol. 25, No. 4, October 2010

Mehrdad Ahmadi Kamarposhti and Mostafa Alinezhad "Comparison Of SVC And STATCOM In Static Voltage Stability Margin Enhancement" Word Academy Of Science, Engineering And Technology 50 2009

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