analysis of power system stability

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ANALYSIS OF POWER SYSTEM STABILITY

INTEGRATED M.TECH-Self Study Course

(Stage First Presentation)

Submitted By-

Bhupendra Kumar

Roll No.-094008


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BASIC CONCEPTS AND DEFINITIONS Power system stability may be broadly defined as that property of a

power system that enables it to remain in a state operating equilibiriumunder normal operating conditions and to regain an acceptable state ofequilibirium after being subjected to a disturbance.

Instability in a power system may be manifested in many different waysdepending on the system configuration and operating mode.Traditionally ,the stability problem has been one of maintainingSynchronus operation. Since power system rely on synchronus

machines for electrical power generation, a necessary condition forsatisfactory system is that all syn. machines should remain insynchronism.

This aspect of stability is influenced by the dynamics of generatorsrotor angles and power-angle relationships.


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Synchronus machinecharacteristics(cont)

SYNCHRONOUS MOTOR CHARACTERISTICS ( P versus )

The steady-state characteristics of a synchronous motorrepresented by phasor diagrams are shown as function of theload P with the excitation voltage E kept constant. The powerexpression is P = I V cos() = E V sin() / X. The basic phasorequation is V = E + jX I. The locus of E is a portion of a circle ofradius E centered at the origin of the complex plane.The locus ofI is also a circle of radius E/X centered on the imaginary axis at V/X. The power P versus the torque angle is also plotted.


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Synchronus machine

characteristics(cont)


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THE STABILITY PHENOMENA


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Transient Stability Analysis

For transient stability analysis we need to consider three systems

1. Prefault - before the fault occurs the system is assumed to be at anequilibrium point

2. Faulted - the fault changes the system equations, moving the systemaway from its equilibrium point

3. Post fault - after fault is cleared the system hopefully returns to a newoperating point


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VOLTAGE STABILITY & VOLTAGE COLLAPSE Voltage Stability-It refers to the ability of the system to maintain a

steady frequency, following a system drastic change resulting in asignificant imbalance between generated and demand power

Voltage stability margins-


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Factors affecting voltage stability Voltage stability is a problem in power systems which are heavily

loaded, faulted or have a shortage of reactive power. The natureof voltage stability can be analyzed by examining the production,transmission and consumption of reactive power.

The reactive characteristics of AC transmission lines,transformers and loads restrict the maximum of power systemtransfers.

The power system lacks the capability to transfer power over longdistances or through high reactance due to the requirement of alarge amount of reactive power at some critical value of power ordistance.

Transfer of reactive power is difficult due to extremely highreactive power losses, which is why the reactive power requiredfor voltage control is produced and consumed at the control area.


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Scenario of classic voltage collapse The large disturbance causes the network characteristics to

shrink dramatically. The characteristics of the network and loaddo not intersect at the instability point. A load increase beyondthe voltage collapse point results in loss of equilibrium, and thepower system can no longer operate. This will typically lead tocascading outages.

. The load voltage decreases, which in turn decreases the loaddemand and the loading of EHV transmission lines. The voltagecontrol of the system, however, quickly restores generatorterminal voltages by increasing excitation. The additionalreactive power flow at the transformers and transmission linescauses additional voltage drop at these components.


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PV-curve1. Power systems are operated in the upper part of the PV-curve.This part of the PV-curve is statically and dynamically stable.

2. The head of the curve is called the maximum loading point. Thecritical point where the solutions unite is the voltage collapse

point. The maximum loading point is more interesting from thepractical point of view than the true voltage collapse point,because the maximum of power system loading is achieved atthis point. The maximum loading point is the voltage collapsepoint when constant power loads are considered, but in generalthey are different.

The voltage dependence of loads affects the point of voltagecollapse. The power system becomes voltage unstable at the

voltage collapse point. Voltages decrease rapidly due to therequirement for an infinite amount of reactive power.


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PV-curve The lower part of the PV-curve (to the left of the voltage collapse point)

is statically stable, but dynamically unstable. The power system canonly operate in stable equilibrium so that the system dynamics act torestore the state to equilibrium when it is perturbed.

V1=400 kV and X=100 Ohm


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Classification of Voltage stabilitySmall-disturbance Voltage Stability- this category considers

small perturbations such as an incremental change insystem load. It is the load characteristics and voltagecontrol devices that determine the system capability tomaintain its steady-state bus voltages.

This problem is usually studied using power-flow-basedtools (steady state analysis). In that case the power systemcan be linearised around an operating point and theanalysis is typically based on eigenvalue and eigenvector

techniques


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Large-disturbance Voltage Stability Here, the concern is to maintain a steady bus voltages

following a large disturbance such as system faults,switching or loss of load, or loss of generation. This abilityis determined by the system and load characteristics, and

the interactions between the different voltage controldevices in the system.

Large disturbance voltage stability can be studied by usingnon-linear time domain simulations in the short-term timeframe and load-flow analysis in the long-term time frame

(steady-state dynamic analysis) The voltage stability is. however, a single problem on which

a combination of both linear and non-linear tools can beused.


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long-term voltage stability

The analysis of long-term voltage stability requires

detailed modeling of long-term dynamics

Two types of stability problems emerge in the long-termtime scale:

1. Frequency problems may appear after a majordisturbance resulting in power system islanding.

Frequency instability is related to the active powerimbalance between generators and loads. An islandmay be either under or over-generated when thesystem frequency either declines or rises.

2. Voltage problems


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Historical Review of Stability Problems


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other in the west.

stability program.


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And also


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Documents

analysis of power system stability