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  • International Electrical Engineering Journal (IEEJ) Vol. 4 (2013) No. 1, pp. 907-913 ISSN 2078-2365

    907 Power System Transient Stability Analysis with High Wind Power Penetration

    Abstract Some countries did not have adequate fuel

    and water power resources, which led them to look for

    alternative ways of generating electricity such as wind power,

    solar power, geothermal power and biomass power, called

    renewable energy. Wind energy is one of the most available and

    exploitable forms of renewable energy due to their advantages.

    However the high penetration of wind power systems in the

    electrical network has introduced new issues in the stability

    and transient operation of power system. The majority of wind

    farms installed are using fixed speed wind turbines equipped

    with squirrel cage induction generator (SCIG). Therefore, the

    analysis of power system dynamics with the SCIG wind

    turbines has become a very important research issue, especially

    during transient faults. This paper provides an assessment of

    wind penetration effects on the power system transient

    stability. The wind generators considered are the squirrel cage

    induction generator (SCIG), which is a fixed speed.

    Index Terms Power system, Squirrel Cage Induction

    Generator (SCIG), Wind Penetration, Transient stability,

    Critical Clearing Time (CCT)

    I. INTRODUCTION

    Wind generators are primarily classified as fixed speed

    or variable speed. Due to its low maintenance cost and simple

    construction, squirrel cage induction generator (SCIG) is

    mostly used for wind power generation [1]. SCIGs directly

    connect to the grid and they dont have convertor like DFIGs.

    Because of lack of convertor and robust control procedure,

    SCIGs are more sensitive to wind speed variations rather

    than DFIGs and mechanical parameters like wind turbine

    inertia constant and shaft stiffness coefficient have

    remarkable impact on operation of this kind of wind

    generators. Moreover they are more sensitive to fluctuations

    and faults in power system rather than DFIGs [2].

    One of important issues engineers have to face is the

    impact of SCIGs wind turbines penetration on the transient

    stability of power system. Transient stability entails the

    evaluation of a power systems ability to withstand large

    Prof. Tarek Bouktir acknowledges support from MESRS (Algeria), grant

    number J0203020080004

    disturbances and to survive the transition to another

    operating condition. These disturbances may be faults such

    as a short circuit on a transmission line, loss of a generator,

    loss of a load, gain of load or loss of a portion of

    transmission network [3]. A number of studies have been

    conducted on power system transient stability with high

    penetration of SCIG based wind farms, but they have

    considered simple network

    structures [4], [5], [6]. In the present work, the impact of

    SCIG wind farms installation and penetration on transient

    stability is demonstrated using the IEEE 30-bus system.

    Using this network, simulation has been carried out for two

    different cases and different penetration levels during three

    phases to ground fault:

    Case 1: single SCIG wind farm has been connected to grid.

    Case 2: the network has been modified by connecting two SCIG wind farms.

    Simulation results show that wind farm consist of constant

    speed wind turbine in high penetration condition is

    remarkably influential in transient stability.

    The paper is organized as follows. Section II briefly

    introduces the mathematical models of power system and

    wind generator. The Optimal Power Flow (OPF) formulation

    is presented in Section III. In section IV, the detail case

    studies focusing on the impacts of fixed speed grid-connected

    wind farms on IEEE-30 bus test system are carried out.

    Finally the conclusions are summarized in Section V.

    II. POWER SYSTEM MODELLING

    A. Power System Modelling

    The power system model consists of synchronous

    generator, transmission network and static load models,

    which are presented below.

    The machine classical electromechanical model is

    represented by the following differential equations [7]:

    Power System Transient Stability Analysis with

    High Wind Power Penetration

    Amroune Mohammed 1, Bouktir Tarek

    2

    Department of Electrical Engineering, University of Setif 1, Algeria 1 amrounemohammed@yahoo.fr

    2 tbouktir@gmail.com

  • International Electrical Engineering Journal (IEEJ) Vol. 4 (2013) No. 1, pp. 907-913 ISSN 2078-2365

    908 Power System Transient Stability Analysis with High Wind Power Penetration

    2

    2

    i

    i s

    i

    mi ei Di

    i

    d

    dt

    d fP P P

    Hdt

    (1)

    Where:

    D

    dP D

    dt

    D is the generator damping coefficient, H is the inertia

    constant of machine expected on the common MVA base, Pm

    is the mechanical input power and Pe is the electrical output.

    The transmission network model is described by the

    steady-state matrix equation:

    bus bus busI Y V

    (2)

    Where Ibus is the injections current vector to the network, Vbus

    is the nodal voltages vector and Ybus is the nodal matrix

    admittance.

    The electrical power of the ith generator is given by [8]:

    2

    1

    cos

    ng

    ei i ii ij ij i j

    j

    P E G C

    (3)

    Where i = 1, 2, 3ng is the number of generators.

    Cij = |Ei||Ej||Yij| is the power transferred at bus ij, E is the

    magnitude of the internal voltage, Yij are the internal

    elements of matrix Ybus and Gii are the real values of the

    diagonal elements of Ybus.

    The static model of load is represented by load admittance YL

    defined by [8]:

    i i

    Li 2

    i

    P - jQY =

    V

    (4)

    B. Wind Generator Modelling

    The fixed-speed, squirrel cage induction generator

    (SCIG) is connected directly to the distribution grid through

    a transformer. There is a gear box which maces the

    generators speed to the frequency of the grid.

    During high wind speeds, the power extracted from the wind

    is limited by the stall effect of the generator. This prevents

    the mechanical power extracted from the wind from

    becoming too large. In most cases, a capacitor bank is

    connected to the fixed speed wind generator for reactive

    power compensation purposes. The capacitor bank

    minimizes the amount of reactive power that the generator

    draws from the grid [3].

    Fig.1. Representation of the fixed speed induction generator

    The Squirrel Cage Induction generator model is shown in

    Fig. 2. Where Rs represents the stator resistance, Xs

    represents the stator reactance; Xm is the magnetizing

    reactance, while Rr and Xr represent the rotor resistance and

    reactance, respectively.

    Fig.2. Equivalent circuit of the Squirrel Cage Induction generator

    [3]

    A standard detailed two-axis induction machine model is

    used to represent the induction generator. The relationship

    between the stator voltage, rotor voltage, the currents and the

    fluxes are given by the following equations [9]:

    ds s ds s qs ds

    qs s qs s ds qs

    dv = -R i - +

    dt

    dv = -R i + +

    dt

    (5)

    dr r dr s qr dr

    qr r qr s dr qr

    dv = 0 = R i - g +

    dt

    dv = 0 = R i + g +

    dt

    (6)

    Where Vs is the stator voltage while Vr represents the rotor

    voltage, s and r are the stator and rotor flux respectively,

    while s is the synchronous speed. The rotor voltage is zero

    because the rotor has been short-circuited in the Squirrel

    cage induction generator. The model is completed by the

    mechanical equation as given below [9]:

    r

    m e

    d 1= (T -T )

    dt 2H (7)

    H is the inertia constant; Tm is the mechanical torque; Te is

    the electrical torque and r is the generator speed.

    III. OPTIMAL POWER FLOW FORMULATION

    The OPF problem is considered as a general minimization

    problem with constraints and can be written in the following

    form [10], [11]:

    Minimize ( , )f x u

    (8)

    Subject to g( , ) 0x u

    (9)

    h( , ) 0x u

    (10)

    Where f(x,u) is the objective function; g(x,u) is the equality

    constraints and represent typical load flow equation; h(x,u) is

    Rs Xs

    Xr

    Rr Xm Vs

    Vr Vr= e-

    jwt Vr

    R

    v

    Grid

    Gear Box

  • International Electrical Engineering Journal (IEEJ) Vol. 4 (2013) No. 1, pp. 907-913 ISSN 2078-2365

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