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POWER SYSTEM CONTROL & STABILITY

(METHODS OF VOLTAGE CONTROL IN POWER SYSTEM)
TABLE OF CONTENTS
1
...
1 STEADY-STATE STABILITY ………………………………………………………
...
2

THE DYNAMIC STABILITY ………………………………………………………
...
3

TRANSIENT STABILITY…………………………………………………………
...
0 CLASSIFICATION OF POWER SYSTEM STABILITY…………………………
...
2 ROTOR ANGLE STABILITY………………………………………………………
...
3 FREQUENCY STABILITY……………………………………………………………10
2
...
10
2
...
10

2
...
7

LOAD TRIPPING AND MODULATION……………………………………………11

2
...
0 SYNCHRONIZING AND DAMPING
TORQUES……………………………………
...
1 SMALL-SIGNAL STABILITY OF MULTI-MACHINE SYSTEMS……………………15
3
...
3

ADVANTAGE OF TAP CHANGING TRANSFORMER………………………
...
4

VOLTAGE CONTROL BY SHUNT REACTORS………………………………16

3
...
6

VOLTAGE CONTROL BY SERIES CAPACITORS……………………………
...
7

VOLTAGE CONTROL BY STATIC SHUNT COMPENSATION………………18

3
...
0
VOLTAGE CONTROL BY FLEXIBLE AC TRANSMISSION (FACT)
DEVICES…………………………………………………………………19
4
...
2
EXCITATION CONTROL AND VOLTAGE REGULATION IN GENERATING
STATIONS…………………………………………………………………
...
3
VOLTAGE CONTROL OF TRANSMISSION SYSTEM USING STATIC VAR
COMPENSATOR……………………………………………………………24
4
...
24

4
...
0 SVC CONTROLLER…………………………………………………25
4
...
1 SVC V-I CHARACTERISTIC…………………………………………
...
6
...
27
4
...
28

5
...
29

5
...
30

5
...
1 AUTO-TRANSFORMER TAP-CHANGING……………………………31
5
...
2 BOOSTER TRANSFORMER…………………………………………
...
0

CONCLUSION……………………………………………………………
...
0

ANNEX……………………………………………………………………
...
0 INTRODUCTION:
Power system is defined as a network of one or more generating units, loads and power
transmission lines including the associated equipment connected to it
...
Power system stability problem
gets more pronounced in the case of interconnection of large power networks
...
Stability denotes the ability of an electric power system, for a given initial
operating condition, to regain a state of operating equilibrium after being subjected to a
physical disturbance, with most system variables bounded so that system integrity is
preserved
...
Dependability is a state of harmony
between restricting powers; shakiness results when an aggravation prompts a
supported awkwardness between the contradicting powers
...

The stability problem is concerned with the behavior of the synchronous machines after
a disturbance
...

(2) Transient stability
...
1 Steady-state stability: refers to the ability of the power system to regain
synchronism after small and slow disturbances, such as gradual power changes
...
2

The dynamic stability: is worried about little aggravations going on for quite a

while with the consideration of programmed control gadgets
...
3

Transient stability: considers manage the impacts of enormous, unexpected

aggravations like the event of a flaw, the abrupt blackout of a line or the abrupt
application or expulsion of burdens
...

Generally, the dependability issue has been one of keeping up simultaneous activity
...
'' This part of steadiness is affected by the
elements of generator rotor points and force point connections
...
For
instance, a framework consisting of a generator taking care of an enlistment engine can
get shaky because of breakdown of burden voltage
...
This kind of flimsiness can likewise happen on account of
burdens covering a broad region in an enormous framework
...
If not appropriately
organized, it is workable for the framework recurrence to get flimsy, and creating units
and burdens may at last be stumbled potentially prompting a framework power outage
...

Due to the high dimensionality and intricacy of security issues, it is vital to make working
on suppositions and to dissect explicit sorts of issues utilizing the correct level of detail
of framework portrayal
...


2
...
1 Need for Classification
Power system stability is a solitary issue; be that as it may, it is illogical to manage it all
things considered
...
Examination of solidness issues,
including recognizing fundamental factors that add to precariousness and concocting
strategies for improving stable activity is incredibly worked with by order of strength into
fitting classifications
...

The size of the disturbance considered indicates the most appropriate method of
calculation and

prediction of stability
...


FIGURE 1
...


Figure 1
...
The following are descriptions of the corresponding
forms of stability phenomena

6

2
...
It relies upon the capacity to
reestablish harmony between electromagnetic force and mechanical force of each
coordinated machine in the framework
...

The rotor point security issue includes the investigation of the electromechanical
motion’s characteristics in power frameworks
...
The
component by which interconnected simultaneous machines keep up synchronism with
each other is through reestablishing powers, which act at whatever point there are
powers having a tendency to speed up or decelerate at least one machine regarding
different machines
...
On the off chance that the framework is bothered, this harmony is
disturbed, bringing about speed increase or deceleration of the rotors of the machines
as indicated by the laws of movement of a pivoting body
...
The subsequent rakish contrast moves part of
the heap from the sluggish machine to the quick machine, contingent upon the force
point relationship
...
The force point relationship, as talked about above, is profoundly
nonlinear
...
For some random circumstance, the dependability of the framework relies
upon whether the deviations in rakish places of the rotors result in inadequate
reestablishing forces
...

Damping force part, in stage with the speed deviation System solidness relies upon the
presence of the two segments of force for every one of the simultaneous machines
...

For accommodation in investigation and for acquiring valuable understanding into the
idea of solidness issues, it is helpful to describe rotor point security regarding the
accompanying two classes:
a
...
The disturbances are
considered to be sufficiently small that linearization of system equations is permissible
for purposes of analysis
...
Small signal stability depends on the
initial operating state of the system
...

In today’s practical power systems, small signal stability is largely a problem of
insufficient damping of oscillations
...
The stability of the following
types of oscillations is of concern:
Local modes or machine-system modes, associated with the swinging of units at a
generating station with respect to the rest of the power system
...

Inter area modes, associated with the swinging of many machines in one part of the
system against machines in other parts
...

Control modes, associated with generating units and other controls
...
Torsional modes, associated with the
turbine-generator shaft system rotational components
...

b
...
The subsequent framework reaction
includes enormous trips of generator rotor points and is impacted by the nonlinear force
point relationship
...
Generally,
the unsettling influence modifies the framework with the end goal that the
post-aggravation consistent state activity will be unique in relation to that preceding the
unsettling influence
...
In enormous force
frameworks, transient flimsiness may not generally happen as first swing unsteadiness
related with a solitary mode; it very well may be because of expanded pinnacle
deviation brought about by superposition of a few methods of wavering causing huge
trips of rotor point past the primary swing
...
It might
reach out to 10 sec for exceptionally enormous frameworks with prevailing between
region swings
...
It is
unrealistic and uneconomical to plan the frameworks to be steady for each conceivable
possibility
...


2
...
Recurrence precariousness
prompts stumbling of creating units and additionally stacks
...

Over the course of a frequency instability, the trademark seasons of the cycles and dev

10

frosts that are initiated by the huge changes in recurrence and other framework factors
will go from only seconds, comparing to the reactions of gadgets, for example,
generator controls and securities, to a few minutes, relating to the reactions of gadgets,
for example, main player energy supply frameworks and burden voltage controllers
...


2
...
It is depending on transient
disturbance

2
...
Linearization of system equations is permissible for
analysis
...
6 Dynamic Braking: Shunt dynamic brakes utilizing mechanical exchanging have
been utilized inconsistently
...
One appealing technique not needing exchanging is
impartial-to-ground resistors in generator venture up transformers; slowing down
consequently results for ground issues which are generally normal
...

Thyristor exchanging of dynamic brakes has been proposed
...


2
...
Interruptible mechanical burden is
ordinarily utilized
...
Maybe than stumbling upon
enormous squares of mechanical burden, it could be feasible to trip low need business
and private burden, for example, space and water warmers, or forced air systems
...
The attainability
of this control relies upon execution of direct burden control as a feature of interest side
12

administration and on the establishment of high velocity correspondence connected to
customers with fast actuators at load gadgets
...

Load tripping is additionally utilized for voltage security
...
It is additionally conceivable to tweak
loads like radiators to sodden motions
...
8

Reactive Power Compensation Switching or Modulation

Controlled series or shunt compensation improves stability, with series compensation
generally being the most powerful
...
For continuous modulation, thyristor phase
control of a reactor (TCR) is used
...
The operating times of circuit breakers are usually adequate, especially for inter
area oscillations
...
In addition to previously mentioned advantages, power
electronic control has advantages in sub synchronous resonance performance
...
The main
application is for full or partial outages of the parallel Pacific HVDC Intertie (event-driven
control using transfer trip over microwave radio)
...
Response-based control using an impedance relay was also used for some
years, and new response-based controls are being investigated
...
Thyristor-controlled series compensation
(TCSC) allows significant time–current dependent increase in series reactance over
nominal reactance
...


3
...
For very severe disturbances and operating conditions,
loss of synchronism (instability) occurs on the first forward swing within about 1 s
...


3
...
Coordinated machines, and the related
excitation frameworks and central players
...


14

Static and dynamic (engine) loads
...
For framework soundness, it is proper to disregard the transmission
organization and machine stator drifters
...
The outcome is that the total framework model comprises
countless customary differential and logarithmic conditions
...
Conversely dependability of the nonlinear framework
relies upon the kind and extent of information, and the underlying state
...
A
few techniques have been utilized viably for a long time, both at producing plants and in
transmission organizations
...
We
give an expansive study of accessible steadiness control procedures with accentuation
on executed controls, and on new and arising innovation
...
The assignment of voltage control is

15

firmly connected with fluctuating burden conditions and relating prerequisites of
receptive force remuneration
...


3
...
The OLTC voltage regulation is naturally operated by changing
the number of turns in one winding of the transformer to physically alter the ratios of the
transformer
...
The OLTC voltage regulation is naturally operated by
changing the number of turns in one winding of the transformer to physically alter the
ratios of the transformer
...
This is done
by changing the turn’s ratio of the transformer the voltage ratio and the secondary
voltage is changed and voltage control is obtained
...
The voltage control of the range
+ 15 to -15 % can be achieved by tap changing transformers While Off load tap
changing voltage control is achieved by adjustment of voltage ratio when the circuit is
off position by tap changing
...


3
...

During light load conditions, it is usually required to lower network voltages, avoid
under excited operation of generators

3
...
Because of the inductive nature of the Shunt
Reactor, it is utilized at whatever point there is need for remuneration of capacitive
reactance
...
During framework light burden
conditions, regular voltages increment past the ordinary working levels and such a
condition requests extra inductive burdens to keep up framework voltage levels inside
the typical reach
...
The shunt capacitance of this lines should

17

be killed by exchanging in the shunt reactor
...
Shunt reactors might be associated with the low voltage
tertiary twisting of a transformer by means of a reasonable electrical switch, EHV shunt
reactors might be associated with the transmission line with no electrical switch
...
5 VOLTAGE CONTROL BY SHUNT CAPACITOR: This strategy is utilized to improve
the force factor
...
To redress, a
shunt capacitor is associated which draws current driving the source voltage
...

Shunt capacitors are generally exchanged during high loads
...
3 to 0
...
The shunt capacitors give
driving VARs there by the all-out KVA stacking of the substation transformer and the
current is decreased
...
Shunt capacitors are exchanged when KVA request on the
circulation line goes up and voltage on the transport descends
...
6 VOLTAGE CONTROL BY SERIES CAPACITORS
Series compensation is the method of improving the system voltage by connecting a
capacitor in series with the transmission line
...
It improves the power transfer capability of the line
...
One of the major
drawbacks of series capacitors is that high over-voltages are produced across the
capacitor terminals under short circuit conditions
...
7 VOLTAGE CONTROL BY STATIC SHUNT COMPENSATION:
Static Var Compensators (SVCs) are devices that can quickly and reliably control line
voltages
...
g
...
In addition, an SVC can also increase transfer capability, reduce
losses, mitigate active power oscillations and prevent overvoltages at loss of load
...
An SVC typically includes a combination of at least two of
the given items below (e
...
, TCR/FC or TCR/TSC/FC):
19

(a)Thyristor controlled reactor (TCR),
(b)Thyristor switched capacitor (TSC),
(c)Harmonic filter (FC),
(d)Mechanically switched capacitor bank (MSC) or reactor bank (MSR)
...
During low loads, the thyristors in reactor circuits are made to conduct for
longer duration in each cycle
...


3
...
It is a property
of such an engine that it takes slacking kVA, when the field current is under a specific
worth, and a main kVA when the field current is over this worth
...
The genuine force it takes will be little
...
The primary benefit of simultaneous stage modifiers is the capacity to
convey the receptive force that can be changed, not normal for static shunt capacitors
...


20

To decide the Var to be provided by a coordinated condenser for keeping zero guideline
for a given transmission line for changing burdens

4
...
Recent advances in the power system handling capabilities of
static switches have made the use of the voltage source inverter (VSI) feasible at both
transmission and distribution levels
...


4
...

The static coordinated compensator (STATCOM) depends on the rule that a voltage
source inverter creates a controllable AC voltage source behind a transformer spillage
reactance so the voltage contrast across the reactance produces dynamic and receptive
force trade between the STATCOM and the transmission organization
...

The most progressive answer for remunerate responsive force is the utilization of a
Voltage Source Converter (VSC) consolidated as a variable wellspring of receptive
force
...
The STATCOM is proper for voltage control since it might quickly
infuse or assimilate receptive ability to settle voltage journeys, and has been generally
demonstrated in modern applications
...
Be that as it may, a STATCOM/ESS blend can give a preferred
powerful exhibition over an independent STATCOM
...


Fig 2
...
Static synchronous compensator

4
...
The terminal voltage V of synchronous generators are given by V = E –
IX The generators have excitation and automatic voltage regulation systems (AVR)
...
The reactive power shared by a generator
depends upon its excitation level the terminal voltage of the synchronous
generator is held within the permissible limits by automatic voltage regulators
(AVR) systems
4
...
One
reason for introducing the Static Var Compensator is to improve the voltage profile and
build the framework load capacity
...
At the point when the framework voltage is high it assimilates the
responsive force
...


23

The circumstance of increment transient, oscillatory and voltage unsteadiness, which
are currently the issue can be amended by utilizing Static Var Compensator
...
The best way to save the
arrangement of voltage breakdown is through control receptive force
...

At the point when it will ingest the receptive force TCR are associated in the
transmission line
...


4
...
0 The Thyristor switched capacitor (TSC)
It comprises a capacitor, a bidirectional thyristor valve, and a moderately little flood
current restricting reactor
...
At the point when a
capacitor is exchanged into a sinusoidal voltage source, an enormous current floods
through the capacitor if the underlying voltage across the capacitor is not quite the same
as the stock voltage at the moment of exchanging
...
In any case, the
pace of progress of current will be exceptionally high, past the rating of the thyristor
gadgets (acting as switches)
...


24

4
...
1 The Thyristor controlled reactor (TCR)
It consists of a bidirectional thyristor valve, and a current limiting reactor is connected in
series
...
Under high voltage conditions,
the SVC will be at its inductive limit and the TCR current may exceed its rated value
...
To avoid interaction with the voltage regulator function, a time delay is
introduced (of the order of 100 ms)
...
Series Capacitors have been used in long
distance EHV transmission lines for increasing power transfer
...

However, the problem of SSR has deterred system planners from going in a big way for
series compensation
...
Even when a shunt capacitor is located at the
Midpoint of a long line, it requires a much larger rating to achieve the same level of
increase in power transfer as a series
...


4
...
It
is a variable impedance gadget where the current through a reactor is controlled
utilizing consecutive associated thyristor valves
...
The significant distinction

25

is that thyristor valves utilized in SVC are appraised for lower voltages as the SVC is
associated with an EHV line through a stage down transformer or associated with the
tertiary twisting of a force transformer
...
Here the
goal is to give dynamic force factor improvement and furthermore balance the flows on
the Compensators started in the last part of the seventies
...

i
...


Improve stability with fast acting voltage regulation

iii
...


Damp sub synchronous frequency oscillations due to torsional modes

v
...
0 Static Var Compensator

4
...
0 SVC CONTROLLER
SVC Controller fusing voltage controller
...
The AC channel is essentially a score channel to take out the sign
26

segment of recurrence relating to the equal reverberation in the framework seen from
the SVC transport
...
The SVC voltage controller
tends to destabilize this full method of swaying and the score channel is pointed toward
beating this issue
...

Assuming arrangement capacitors are utilized alongside SVC, they can cause equal
reverberation with an adjoining shunt reactor
...


4
...
1 SVC V-I CHARACTERISTIC
The SVC can be worked in two unique modes: In voltage guideline mode and in var
control mode (the SVC Susceptance is kept steady) when the SVC is worked in voltage
guideline mode, it executes the accompanying V-I Characteristic
...
Be that as it may, a voltage hang
is ordinarily utilized (normally somewhere in the range of 1% and 4% at greatest
responsive force yield), and the V-I trademark has the slope shown

27

Figure 4 SVC V-I Characteristic

The voltage at which SVC neither absorbs nor generates reactive power is the
reference voltage (Vref)
...
The
slope of the characteristics reflects a Change in voltage with compensator current and,
therefore can be considered as slope reactance, resulting the SVC Response to the
voltage
variation
...
6
...
In
this range, the SVC is represented as a PV-node at an auxiliary bus with P = 0 and V =

28

Vref
...
3
...


Figure 5 SVC Operating Within Control Range

A static var compensator (SVC) is used to regulate voltage on a 33 kV, 3×50 MVA
system
...
When system voltage is high it absorbs reactive power (SVC inductive)
...
The Static Var Compensator
block is a Phasor model representing the SVC static and dynamic characteristics at the
system fundamental frequency
...
0 pu
...
03 p
...
97 p
...
015 p
...
By simulating the SVC V-I characteristic curve is obtained
...
0667 pu/50 MVA
Xs = 0
...
0345s
The voltage/current characteristic of the compensator at the 33 kV transport is shown in
Fig
...
The slant of the control range is ostensibly 3%
...
For a direct
voltage-current trademark, a voltage change of +3% produces an inductive receptive
power of 50 MVar
...
7 IMPORTANCE OF VOLTAGE CONTROL
At the point when the heap on the stockpile framework changes, the voltage at the
customer's terminals likewise changes
...
In instance of lighting load, the light attributes are extremely touchy to changes of
voltage
...
Then
again, if the inventory voltage is 6% over the evaluated esteem, the existence of the
light might be diminished by half because of fast weakening of the fiber
...
In instance of force load consisting of acceptance engines, the voltage varieties may
cause whimsical activity
...
Then again, if the voltage is excessively low, it
will diminish the beginning force of the engine impressively
...
Too wide varieties of voltage cause extreme warming of dispersion transformers
...
With the pattern
towards bigger and bigger interconnected frameworks, it has gotten important to utilize
proper techniques for voltage control
...
0 LOCATION OF VOLTAGE CONTROL EQUIPMENT
In a modern power system, there are several elements between the generating station
and the consumers
...
Firstly, the power network is very extensive and there is a
considerable voltage drop in transmission and distribution systems
...
For these
reasons, it is necessary to provide individual means of voltage control for each circuit or
group of circuits
...


Generating stations

ii
...


The feeders if the drop exceeds the permissible limits

31

5
...
In every technique, the framework
voltage is changed as per the heap to acquire a genuinely consistent voltage at the
purchaser's finish of the framework
...
c
...


By excitation control

ii
...


Auto-transformer tap changing

iv
...


Induction regulators

vi
...
However, method (vi) is reserved
for the voltage control of a transmission line
...


5
...
1 AUTO-TRANSFORMER TAP-CHANGING
Autotransformer has a solitary twisting with two end terminals, at least one terminal at
transitional tap focuses, its essential voltage is applied across two of the terminals, the
optional voltage taken from two terminals, and continually having one terminal is regular

32

with the essential and auxiliary the current streams straightforwardly from the
contribution to the yield, and just more modest part inductively autotransformer
...

Fig6
...
Here, a mid-tapped
auto-transformer or reactor is utilized
...
One end, say an of this transformer is associated with a progression of
switches across the odd recordings and the opposite end B is associated with switches
across even recordings
...
In the typical
activity, there are no inductive voltage drops across the auto-transformer
...
beneath, obviously with switch 5 shut, least optional turns are in the circuit and
subsequently the yield voltage will be the most reduced
...

Assume now it is wanted to modify the tapping point from position 5 to situate 4 to raise
the yield voltage
...
Thus, tapping can be
changed without intruding on the stockpile
...
At the
point when the short-circuiting switch is opened, the heap current moves through
one-portion of the reactor curl so that there is a voltage drop across the reactor
...
A circling current moves through this neighborhood circuit
however it is restricted to a low value because of high reactance of the reactor
...
0 Auto-Transformer Tap-Changing

5
...
2 BOOSTER TRANSFORMER
Sometimes it is desired to control the voltage of a transmission line at a point far away
from the main transformer
...
7
...
The primary of this transformer
is supplied from a regulating transformer fitted with on-load tap-changing gear
...


34

Fig
...
Notwithstanding, there might be significant voltage drop among AA and BB
because of the genuinely long feeder and tapping of burdens
...
By
changing the tap-ping on the controlling transformer, the greatness of the voltage
infused into the line can be different
...
This strategy for voltage
control has three detriments
...
Besides, it is less effective attributable to misfortunes in the supporter and
thirdly more floor space is required
...
8 shows a three-phase booster transformer
...
8
...
0 CONCLUSION
The quick expansion in load request powers systems to work close as far as possible
because of monetary and ecological limitations
...
The goal in power
frameworks activity and control is to serve energy with pronounced upsides of voltage
and frequency with least power loss
...


7
...
0 Classification of power system stability………………………………
...
0
...
21
Fig 3
...
0 SVC V-I Characteristic…………………………………………………
...
0 SVC Operating Within Control Range…………………………………27
Fig 6
...
7 Booster Transformer……………………………………………………………
...
8
...
34

37



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