Voltage Stabilsation Technique

Voltage Stabilization Techniques in Power Systems

Name: Yao Zhou

ID: 3206261

Instructor:Dr.A.M.Sharaf

• Voltage Stabilization Techniques in Power Systems

• 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.

• What is the voltage stabilization in power system?

• 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.


• 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]


• 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]


• 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]


• 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.


• 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]

• The type of Dynamic / Nonlinear load can also cause the voltage instability.


• Classification of voltage stabilization:

voltage stability can be classified into the following subcategories:[2]

• Large-disturbance voltage stabilization:

• Large-disturbance voltage stabilization refers to the system’s ability to maintain steady voltages following large disturbances such as system faults, loss of generation, or circuit contingencies.


• 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.


• Small-disturbance voltage stabilization: [2]

• Small-disturbance voltage stability refers to the system’s 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.


• The time frame of interest for voltage stabilization problems may vary from a few seconds to tens of minutes.

• 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)

Therefore, voltage stabilization may be either a short-term or a long-term phenomenon. [2]


• 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)


• 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

• Voltage Stabilization System for Induction Generator in Stand Alone Mode

• 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]

• Abstract:


• 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.

• Introduction:

• 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]


• 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]


• Mathematical model for SEIG with terminal voltage stabilization system:

• Mathematical model of induction machine consideringsaturation as a set of non linear differential equations isbased 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.

• 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 offactors yield extreme value of the function and consequently giving best quality of voltage stabilization.


• 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.

• 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]


• Equations used in the mathematical model:

• Equations in the vector form for machine is shown as below:

:spatial vector of stator voltage;

and :currents' and flux linkages' vectors and resistances of stator and rotor respectively;

:angular velocity of rotor.

Where:

[3]


• Standard form of swing equation was used for describing mechanical motion of rotor:

Where: :inertia constant;

:mechanical torque;

:electrical torque.


• Equations for load and capacitors’ battery are written below:

Where: :load inductance;

:load resistance;

:total apparent capacitance both regulated and unregulated banks;

:total current through capacitors’ battery.

[3]


• The time domain discrete functions for discrete control block is shown below:

Where: u(k) :control action; e(k) :error of output value; :constant factors; k :number of sampling.

[3]


• Criterion function is used to obtain a vector that yields minimum value of during transient:

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

[3]


• 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;


• - 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.

• - 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;


• 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 system’s behavior in different working modes.[3]

• Numerical modelling of the system:

• Special software for modeling the system was prepared on base of methods and approach described above.

• Optimization calculations were provided for transientboth 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.


• 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.

• Calculations Of transients for various working modeswere 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]


• 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.

• This algorithm affords take into account non linearities of the machine characteristics and digital control block those are neglected by frequency domain analysis.

• 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.

• Reference:

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

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

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

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Voltage Stabilsation Technique

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