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Voltage Stabilization Techniques in Power Systems

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Voltage Stabilization Techniques in Power Systems Instructor:Dr.A.M.Sharaf Name: Yao Zhou ID: 3206261 * Voltage Stabilization Techniques in Power Systems Power system ... – PowerPoint PPT presentation

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Title: Voltage Stabilization Techniques in Power Systems


1
Voltage Stabilization Techniques in Power Systems
InstructorDr.A.M.Sharaf
Name Yao Zhou
ID 3206261
2
  • Voltage Stabilization Techniques in Power
    Systems
  • What is the voltage stabilization in power
    system?
  • Power system stability may be broadly defined
    according to different operating conditions, an
    important problem which is frequently considered
    is the problem of voltage stabilization.
  • This important issue of power system control is
    to maintain steady acceptable voltage under
    normal operating and disturbed conditions, which
    is referred as the problem of voltage
    stabilization.

3
  • Voltage Stabilization Techniques in Power
    Systems
  • Voltage stabilization refers to the ability of a
    power system to maintain steady voltages at all
    buses in the system after being subjected to a
    disturbance from a given initial operating
    condition. It depends on the ability to
    maintain/restore equilibrium between load demand
    and load supply from the power system. 1

4
  • Voltage Stabilization Techniques in Power
    Systems
  • Why we need voltage stabilization?
  • Instability that may result occurs in the form
    of a progressive fall or rise of voltages of some
    buses.
  • A possible outcome of voltage instability is
    loss of load in an area, or tripping of
    transmission lines and other elements by their
    protective systems leading to cascading outages.
    Loss of synchronism of some generators may result
    from these outages or from operating conditions
    that violate field current limit. 1

5
  • Voltage Stabilization Techniques in Power
    Systems
  • What cause voltage instability?
  • The driving force for voltage instability is
    usually the loads. In response to a disturbance,
    power consumed by the loads should be restored.
  • A situation causing voltage instability occurs
    when load dynamics attempt to restore power
    consumption beyond the capability of the
    transmission network and the connected
    generation.1

6
  • Voltage Stabilization Techniques in Power
    Systems
  • A major factor contributing to voltage
    instability is the voltage drop that occurs when
    active and reactive power flow through of the
    transmission network this
  • limits the capability of the transmission network
    for power transfer and voltage support.
  • Voltage stability is threatened when a
    disturbance increases the reactive power demand
    beyond the sustainable capacity of the available
    reactive power resources.

7
  • Voltage Stabilization Techniques in Power
    Systems
  • The type of Dynamic / Nonlinear load can also
    cause the voltage instability.
  • While the most common form of voltage
    instability is the power frequency variation, the
    progressive drop of bus voltages, and the
    overvoltage instability.
  • Overvoltage can be caused by a capacitive
    behavior of the network as well as by under
    excitation limiters preventing generators and
    synchronous compensators from absorbing the
    excess reactive power. 1

8
  • Voltage Stabilization Techniques in Power
    Systems
  • Classification of voltage stabilization

voltage stability can be classified into the
following subcategories2
  • Large-disturbance voltage stabilization
  • Large-disturbance voltage stabilization refers
    to the systems ability to maintain steady
    voltages following large disturbances such as
    system faults, loss of generation, or circuit
    contingencies.

9
  • Voltage Stabilization Techniques in Power
    Systems
  • This ability is determined by the system and
    load characteristics, and the interactions of
    both continuous and discrete controls and
    protections.
  • Determination of large-disturbance voltage
    stabilization requires the examination of the
    nonlinear response of the power system over a
    period of time sufficient to capture the
    performance and interactions of such devices as
    motors, underload transformer tap changers, and
    generator field-current limiters.

10
  • Voltage Stabilization Techniques in Power
    Systems
  • Small-disturbance voltage stabilization 2
  • Small-disturbance voltage stability refers to
    the systems ability to maintain steady voltages
    when subjected to small perturbations such as
    incremental changes in system load.
  • This form of stability is influenced by the
    characteristics of loads, continuous controls,
    and discrete controls at a given instant of time.
  • This concept is useful in determining, at any
    instant, how the system voltages will respond to
    small system changes.

11
  • Voltage Stabilization Techniques in Power
    Systems
  • The time frame of interest for voltage
    stabilization problems may vary from a few
    seconds to tens of minutes.

Therefore, voltage stabilization may be either a
short-term or a long-term phenomenon. 2
  • Short-term voltage stabilization involves
    dynamics of fast acting load components such as
    induction motors, electronically controlled
    loads. (several seconds)
  • Long-term voltage stabilization involves slower
    acting equipment such as tap-changing
    transformers, generator current limiters.
    (several mins)

12
  • Voltage Stabilization Techniques in Power
    Systems
  • Some methods used for voltage stabilization
  • Using reactive power compensation technology,
    the capacity of the transmission and distribution
    system can be significantly enhanced.
  • Fixed capacitor bank compensation
  • Back-to-back phase control of a
    thyristor-controlled reactor (TCR) or thyristor
    switched capacitor (TSC)
  • FACTS (flexible alternation current transmission
    system) devices such as modulated power filter
    compensator (MPFC), Static Synchronous
    Compensator (STACOM), Static Synchronous Series
    Compensator (SSCC), and Unified Power Flow
    Controller (UPFC)

13
  • Voltage Stabilization Techniques in Power
    Systems
  • System planning optimization and power flow
    forecasting are also very important to the system
    voltage stabilization condition.
  • Optimization algorithm
  • Artificial intelligence based on nonlinear
    programming
  • Dynamic programming

14
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Abstract
  • I will introduce a method of terminal voltage
    stabilization for self excited induction
    generator (SEIG) in stand alone mode.
  • The method is based on looking for minimum of
    criterion function that is weighted sum of some
    system parameters.
  • Overall method is based on well established
    theory for time domain modeling of induction
    machines affords to take into account non
    linearities of induction machine and digital
    control block and seems to be reliable and
    accurate.3

15
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Introduction
  • SEIG have been increasingly used in isolated
    power supplying systems. Main drawback of
    induction generators is well-known property
    sufficiently decrease terminal voltage under
    increasing load.
  • Linearised models of generator and equivalent
    continuous (analog) transfer functions of
    discrete control block are frequently used for
    solving the problem of voltage stabilization.
  • This way demands extended analysis in frequency
    domain in order to obtain actual tuning that
    provides required performance and stability for
    determined structure of control algorithm.3

16
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Nevertheless non linear behavior of induction
    machine working in generator mode in many cases
    can not be neglected.
  • In non linear models, time domain analysis along
    with optimization technique also can be used for
    determining some unknown parameters of a system
    such as factors of control scheme. This approach
    was successfully applied for tuning power
    stabilizer system and it seems to be promising
    use the same way for obtaining tuning of discrete
    control algorithm.3

17
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Mathematical model for SEIG with terminal
    voltage stabilization system
  • Mathematical model of induction machine
    considering
  • saturation as a set of non linear differential
    equations is
  • based on well-established theory and provides
    accurate representation both steady state and
    transient modes.
  • Weighted sum of some parameters of transient
    could be defined as a criterion function.

18
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Numerical resolving machine equations along with
    equations describing excitation system and
    control algorithm yields necessary parameters for
    calculating criterion function as qualitative
    characteristic of a system stability and voltage
    stabilization during transients.
  • On the base modeling of transient optimization,
  • calculations should be provided in order to get
    values of
  • factors yield extreme value of the function and
    consequently giving best quality of voltage
    stabilization.

19
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • In isolated power supplying system with SEIG,
    Stabilization unit includes power part that is
    thyristor switches and capacitor bank. The bank
    is divided into two parts regulated and
    unregulated.
  • Unregulated part is used for excitation and
    presents three single phase delta connected
    capacitors. Regulated part is a set of three
    phase capacitors, those capacitances are binary
    weighted.
  • Those capacitors could be turned on and off by
    thyristor switches of power part.

20
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Microprocessor based digital control block is
    used for measurement of input values,
    implementation algorithm and generating fire
    pulses for thyristors.
  • General scheme of SEIG with stabilization unit is
    presented on Fig. 1.

Fig.1 Structural scheme of SEIG with voltage
stabilization system and load
3
21
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Equations used in the mathematical model
  • Equations in the vector form for machine is
    shown as below

3
spatial vector of stator voltage
and currents'
and flux linkages' vectors and resistances of
stator and rotor respectively angular
velocity of rotor.
Where
22
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Standard form of swing equation was used for
    describing mechanical motion of rotor

Where inertia constant
mechanical torque
electrical torque.
23
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Equations for load and capacitors battery are
    written below

3
Where load inductance
load resistance total
apparent capacitance both regulated and
unregulated banks
total current through capacitors battery.
24
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • The time domain discrete functions for discrete
    control block is shown below

3
Where u(k) control action e(k)
error of output value
constant factors k number of
sampling.
25
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Criterion function is used to obtain a vector
    that yields minimum value of
    during transient

3
Where error of terminal voltage during
transient current
capacitance of turned on capacitors bank
capacitance of turned on
capacitors bank after the end of transient
r weight factor for control action
26
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • The optimization calculations were made with
    algorithm that uses co-ordinate search for
    criterion function minimum.3
  • In general, the calculation procedure could be
    described as follows
  • - determining of limiting (minimum and maximum)
    values for each factor

27
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • - calculating of transient for previously
    defined working point and values of factors with
    storing apparent values for voltage error and
    turned on capacitors on each step of numerical
    integration
  • - calculating of criterion function on the base
    of collected values of errors and control actions
  • - modification of factors values according to
    optimization algorithm, analysis of their values
    and finishing on calculations or calculation of
    another transient.

28
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Numerical modelling of the system
  • Special software for modeling the system was
    prepared on base of methods and approach
    described above.
  • Numerical modeling of the system for SEIG with
    terminal voltage stabilization was made in order
    to get values of factors for controlling
    difference equations and check out systems
    behavior in different working modes.3
  • Optimization calculations were provided for
    transient
  • both with resistive load and with
    resistive-inductive load in order to obtain
    values of factors. Values of full load current
    and reactive part of load current were taken as a
    disturbance.

29
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Calculations Of transients for various working
    modes
  • were made with obtained values of factors.
    Results of modeling show that algorithm according
    to difference equation and factors optimized for
    connection of resistive- inductive load with
    using full load current provides robust and fast
    voltage stabilization during transients. 3
  • Then we can use the equations and software tools
    referred above, to achieve the optimization for
    voltage stabilization by tuning of the digital
    control block according to the parameters of
    system.

30
  • Voltage Stabilization System for Induction
    Generator in Stand Alone Mode
  • Conclusion
  • The method for SEIG terminal voltage
    stabilization system tuning is based on direct
    modeling of transients in the system using
    well-known space vector theory.
  • The method is an alternative to methods
    employing frequency domain analysis for
    evaluation of system stability and tuning of
    control system those use linearised model of
    induction machine.
  • This algorithm affords take into account non
    linearities of the machine characteristics and
    digital control block those are neglected by
    frequency domain analysis.

31
  • Reference

1. Joon-Ho Choi and Jae-Chul Kim, Member, IEEE,
Advanced Voltage Regulation Method at the Power
Distribution Systems Interconnected with
Dispersed Storage and Generation Systems, IEEE
TRANSACTIONS ON POWER DELIVERY, VOL. 15, NO. 2,
APRIL 2000
2. Yi Guo, Member, IEEE, David J. Hill, Fellow,
IEEE, and Youyi Wang, Senior Member, IEEE,
Global Transient Stability and Voltage
Regulation for Power Systems, IEEE TRANSACTIONS
ON POWER SYSTEMS, VOL. 16, NO. 4, NOVEMBER 2001
3. Oleg Chtchetinine, Department of Electrical
Engineering, Nizhny Novgorod Technical
University, Nizhny Novgorod, Russia, Voltage
Stabilization System for Induction Generator in
Stand Alone Mode, IEEE Transactions on Energy
Conversion, Vol. 14, No. 3, September 1999
32
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