2. Topics Under
Reactive Power Voltage Control
Generation and Absorption of Reactive Power
Basics of Reactive Power Control
Excitation Systems - Modeling
Static and dynamic analysis
Stability Compensation
Method of Voltage Control
Tap-Changing Transformer
SVC (TCR+TSC)
STATCOM - Secondary voltage control
2
3. Excitation Systems
Exciter Ceiling Voltage: It is the maximum voltage that may be
attained by an exciter under specific conditions.
Excitation System Ceiling Voltage: It is the maximum
D.C. component system output voltage that is able to be attained,
by an excitation system under specified conditions.
Excitation System Requirements:
Meet specified response criteria.
Must be able to prevent damages.
Good operating flexibility.
Reliable, redundant and capable to isolate.
3
4. Excitation Systems
Types of Excitation System:
D.C Excitation system
A.C Excitation system
Static excitation system
DC Excitation System:
–This excitation system utilizes D.C generators as sources of
excitation power and provide current to the rotor of the
synchronous machine through slip rings.
–It may be either self excited or separately excited.
4
5. AC Excitation System:
–This excitation system utilizes alternators (A.C machines) as
sources of main generator excitation power. Usually, the exciter
is on the same shaft as the turbine generator.
–The A.C output or the exciter is rectified by either controlled or
un controlled rectifiers to provide the direct current for the
generator field.
Static Excitation System:
–All components in these are static or stationary. Static rectifiers,
controlled or uncontrolled, supply the excitation current directly
to the field of the main generator through slip rings.
–The supply of power to the rectifiers is from the main generator
through a transformer to step down the voltage to an
appropriate level or in some cases from auxiliary windings in the
generator.
5
8. Brushless AVR
Out of the three types of excitation systems, the modern excitation
system tend to be either “brushless (or) static design”.
Here the exciter consists of an 3ɸ alternator with
“rotating armature type and stationary field”. i.e.,
3ɸ armature on the rotor & field on the stator
The AC armature voltage is rectified by “diode bridge” mounted on
the rotating shaft, and then fed directly into the main generator field.
This design eliminates the need for slip rings and brushes.
Hence it is called as “Brushless Automatic
Voltage Regulator – Brushless AVR” .
8
9. Modelling of Typical Excitation System (or)
Modelling of Automatic Voltage Regulator
9
Assume that for some reasons generator terminal voltage has been
decreased.
This results in increased “error voltage” (e) which in turn, causes
increased values of VR, ie, Vf and if.
The “direct axis” generator flux increases as a result of increase in if,
thus, raising the magnitude of the terminal voltage to the required level.
(Thus Voltage is automatically regulated as our desired).
V
10. Modelling of Brushless AVR
10
Potential Transformer and Rectifier:
Using potential transformer (P.T), the terminal voltage of the
generator is stepped down to the value required for the control signal and then
rectified to get DC voltage proportional to the r.m.s value of terminal voltage.
Comparator:
The comparator compares the measured signal against the reference
DC signal .
The difference between these two signals produce an error voltage ‘Ve’
called “error signal”.
The error signal, ∆e = Δ - Δ . . . . . . . . . . . (1)
Taking Laplace transform of equation (1),
Δ (s) – Δ (s) = ∆e (s)
V
ref
V
ref
V V
ref
V V
11. 11
Block Diagram of Comparator
Model of Comparator
Amplifier:
The amplifier amplifies the input error signal depending on
the amplification factor.
There are various types of amplifiers used in excitation system.
They are tuned generator, amplidyne and electronic amplifier.
12. 12
In reality, the amplifier will have a time delay that can be
represented by a time constant TA shown in figure.
13. 13
The amplifier transfer function becomes;
Model of amplifier is shown below.
Model of Amplifier
Typical value of kA : 10 to 400
TA : 0.02 to 0.1 sec
14. 14
Exciter:
The purpose of exciter is to supply field current to the rotor field
of the synchronous generator.
Let Re – exciter field resistor
Le – exciter field inductance
From the equivalent circuit,
Input Voltage,
Circuit of an Exciter Equivalent circuit for field
winding of an exciter
15. 15
Output Voltage of an exciter or Field voltage of generator,
Taking Laplace transform of above equations,
Transfer function of the exciter,
17. 17
Synchronous Generator:
• Synchronous generator generates 3ɸ AC power at its terminals.
• The terminal voltage of the generator is maintained constant
during its varying load conditions, with the help of excitation
system.
The terminal voltage (V) of the generator equals to the difference
between induced emf (E) and drop across the armature (Vdrop).
∆V = ∆E - Vdrop
The relationship between Vf and V depends on the
generator loading.
At no load the drop can be neglected,
Hence, V = E (neglect drop)
18. 18
Taking Laplace transform,
Hence, ∆V(s) = ∆E(s)
Circuit diagram of a
Synchronous Generator
Equivalent circuit for
field winding of a
Synchronous Generator
21. 21
The Synchronous generator model is shown below,
Model of Generator
Typical values of kf : 0.7 to 1
T’do : 1.0 to 2.0 sec
Combining all the individual blocks, we get the closed loop
model of AVR as shown below:
Closed Loop Model of AVR
23. Static Analysis of Automatic Voltage
Regulator Loop (or) Steady State Response
23
1. The automatic voltage regulator must regulate the terminal
voltage within the required static accuracy limit.
2. It must have sufficient speed response
3. It must be stable
The block diagram of AVR is shown below:
Closed Loop Model of AVR
Initial error, . . . . . . . . (1)
V
24. 24
AVR block after reduction
From the above figure,
Open loop T.F.,
At initial condition, . . . . . . . (2)
∆𝑒𝑜 must be less than some specified percentage P of the reference
voltage ∆𝑉𝑟𝑒𝑓𝑜.
The static accuracy specification is :
. . . . . . . . (3)
25. 25
For a constant input, the transfer function is obtained by setting s = 0.
Substituting equation (2) in (1), we get,
. . . (4)
If K increases, ∆𝒆𝒐 decreases. So static error decreases with an
increased loop gain.
26. 26
To find the value of K:
Consider the equation (3),
4 ]
. . . (5)
If ∆𝒆𝒐is less than 1%, Kmustexceed 99%.
Steady State response for a closed Loop Transfer Function:
28. Dynamic Analysis of
Automatic Voltage Regulator (AVR) Loop
28
Block Diagram of AVR
From the above diagram,
Open loop T.F.,
29. 29
Taking inverse Laplace transform,
The response depends upon the eigen values (or) closed loop poles,
which are obtained from the characteristic equation 1 + G(s) = 0.
Findroots of characteristic equation [Eigen values] 𝐬𝟏, 𝐬𝟐, 𝐬𝟑
Case I: Roots are real and distinct
The open loop transfer function G(s) is of 𝟑𝒓𝒅 order.
There are three eigen values 𝐬𝟏, 𝐬𝟐 𝐚𝐧𝐝 𝐬𝟑.
Transient response =
30. 30
Case II: Two roots (Eigen values) are complex conjugate (σ + jω)
The transient response is
For AVR loop to be stable, the transient components must vanish
with time.
All the eigen values are located in the left half of s-plane. Then the
loop possesses good tracking ability (i.e) the system is stable.
For high speed response, the loop possesses eigen values located
far away to the left from origin s-plane.
The closer the eigen value is located to the jω axis, the
more dominant it becomes.
31. Stability Compensation
31
• Stability compensation improves the dynamic response
characteristics without affecting the static loop gain.
• Even for a small amplifier gain of kA, AVR step response is
not satisfactory.
• Thus, we must increase the relative stability by introducing a
controller, which would add a zero to the AVR open loop transfer
function.
The block diagram of AVR as shown below:
Block Diagram of an AVR
32. 32
• High loop gain is needed for static accuracy, but this causes
undesirable dynamic response (i.e) possibly instability.
• This conflict situation can be avoided by adding series and/or
feedback stability compensationto the AVR loop.
Consider the addition of a series phase lead compensator as
shown below:
Series compensator with unity feedback
Transfer function of series compensator is:
𝑮𝒔 = 1 + s 𝑻𝒄
where, 𝑻𝒄 is the compensator time constant
Open loop T.F = (1 + s 𝑻𝒄 )
33. 33
Series compensator network will not affect the static loop gain (K)
i.e.,
and thus maintains the static accuracy.
But the dynamic characteristics will change.
If we tune, 𝑻𝒄 = 𝑻𝒆
∴ Open loop T.F becomes
Root Loci:
Number of zeros, z = 0
Number of poles, p = 2
35. 35
Root loci for zero compensated loop
Low loop gain (a) still results in negative eigen values, the
dominant poles 𝐬𝟐 yields sluggish (lethargic (or) inactive)
response.
Increasing loop gain (b) results in oscillatory response.
The damping of the oscillatory term will however, not decrease
with increasing gain as was the case in uncompensated system.
So, the system is stable.
36. Feedback Stability Compensation
36
• Consider the addition of feedback stability compensation
(stabilizer).
• Even for a small amplifier gain of kA,AVR step response is
not satisfactory.
• Thus, we must increase the relative stability by introducing a
controller, which would add a zero to the AVR open loop transfer
function.
The block diagram of AVR with feedback stability compensation
is shown below:
Block Diagram of an AVR with feedback stability compensation
• By proper adjustment of 𝑲𝒔 and 𝑻𝒔 , a satisfactory response can
37. Methods of Voltage Control
37
• Voltage level control is accomplished by controlling the
generation, absorption and reactive power flow at all levels in
the system.
The following are the methods of voltage control.
1. By excitation control
2. By static shunt capacitors
3. By static series capacitors
4. By static shunt reactors
5. By synchronous condensers
Other methods for voltage control:
1. Tap-changing transformer
2. Booster transformer
3. Regulating transformer
4. Static VAR compensators (SVC: (TCR + TSC))
5. STATCOM
38. Methods of Voltage Control
38
TAP-CHANGING TRANSFORMER :
• All power transformers on transmission lines are provided with
taps for control of secondary voltage.
• The tap changing transformers do not control voltage by
regulating the flow of reactive VARs but by changing
transformation ratio.
• There are two types of tap changing transformers.
(a) Off-load tap changing transformer
(b) On-load (under-load) tap changing transformer
OFF-LOAD TAP CHANGING TRANSFORMER :
Off-load tap changing transformer
39. 39
• The off-load tap changing transformer requires the disconnection
of the transformer when the tap setting is to be changed.
• Off-load tap changers are used when it is to be operated in
frequently due to load growth (or) some seasonal change.
ON-LOAD TAP CHANGING TRANSFORMER (OLTC):
• The On-load tap changing transformer is used when changes in
transformer ratio to be needed frequently, and no need to switch off
the transformer to change the tap of transformer.
• It is used on power transformers, auto transformers and
bulk distribution transformers and at other points of load service.
• The modern practice is to use on-load tap changing transformer
which is shown in the following figure.
40. 40
• In the position shown, the voltage is maximum and since the
currents divide equally and flow in opposition through the coil
between 𝑸𝟏 and 𝑸𝟐, the resultant flux is zero and hence
On-load tap changing transformer
41. 41
Voltage Reduction in OLTC:
To reduce the voltage, the following operations are required in
sequence.
1. Open 𝑸𝟏
2. Move selector switch 𝑺𝟏 to the next contact
3. Close 𝑸𝟏
4. Open 𝑸𝟐
5. Move selector switch 𝑺𝟐 to the next contact
6. Close 𝑸𝟐
Thus, six operations are required for one change in tap position.
The voltage change between taps is often 1.25 percent of the
nominal voltage.
42. 42
Applications of Tap-changing Transformers:
• Transformer with tap-changing facility constitute an important
means of controlling voltage throughout the system at all
voltage levels.
• Theses are usually present throughout the network
interconnecting transmission systems of different levels.
• During lightly loaded condition, it is usually required to lower
the network voltage, to reduce line charging and avoid
under excited operation of generators.
• Transformers with off-load tap –changing facilities can also help
to maintain satisfactory voltage profiles, while transformers
with OLTC can be used to take care of daily, hourly and
minute-by-minute variation in system conditions.
43. Methods of Voltage Control
43
STATIC SHUNT CAPACITORS:
• Shunt capacitor banks are used to supply reactive power at both
transmission and distribution levels; along lines or substations and
loads.
• Capacitors are either directly connected to a busbar (or) the
tertiary winding of a main transformer. They may be switched
ON and OFF depending on the changes in load demand.
• When they are in parallel with a load having a lagging
power factor, the capacitors supply reactive power.
• As the voltage reduces, so does the reactive power output, when it
is required the most. This is called the “destabilizing effect” of
power capacitors.
44. 44
Rise in Voltage due to Shunt Capacitance:
The equivalent circuit of a short transmission line with
static shunt capacitor is shown below.
Equivalent Circuit
Voltage drop without the shunt capacitor is,
Voltage drop with shunt capacitor is,
45. 45
Capacitor raises the voltage,
Voltage profile of a radial feeder having a capacitor is shown below.
Voltage profile
46. 46
Advantages:
• These are less costly and Flexibility of installation and operation
• Efficiency of transmission and distribution of power is high
Disadvantages:
• They cannot be overloaded
• The reactive power supplied by static capacitors tends to decrease in
case of voltage dip on the bus because KVAR α𝑉2
Problems Associated with Shunt Capacitors :
• Switching inrush currents at higher frequencies and switching
overvoltages
• Harmonic resonance problems
• Limited overvoltage withstand capability
47. 47
Applications of using Shunt Capacitor to Distribution and
Transmission System:
• Shunt capacitors are used in distribution system to:
Improve power factor
Improve feeder voltage control
• Power factor correction
• Feeder voltage control
• Voltage regulation
• Reducing power loss
48. Methods of Voltage Control
48
STATIC SERIES CAPACITORS:
• It is connected in series to compensate the inductive reactance of
line. This reduces the transfer reactance between the buses to
which the line is connected.
• It increases maximum power that can be transmitted and
reduces the reactive power loss.
• Under fault conditions, the voltage across the capacitor rises, and
unlike a shunt capacitor, a series capacitor experiences many times
its rated voltage due to fault currents.
• A zinc oxide varistor in parallel with the capacitor may be
adequate to limit this voltage.
49. 49
The schematic diagram of a series capacitor installation is shown in
below fig.
Schematic diagram of a series capacitor installation
50. 50
Phasor diagram when series capacitor is connected on a line
Drawbacks of Series Capacitor:
• High over-voltage is produced across the capacitor terminals
under short circuit conditions. Therefore, very high protective
equipment is used. (Ex: Spark gap)
51. 51
Comparison between Shunt and Series Capacitors
Shunt Capacitors Series Capacitors
It is connected in parallel with the
line
It is connected in series with the
line
It is mainly used for power factor
correction at the load terminal of
low voltage
It is mainly used to compensate
the effect of series reactance
If the load VAR requirement is
small, shunt capacitors are of
high use
If the load VAR requirement is
small, series capacitors are of
small use
If the total line reactance is high,
shunt capacitors are not effective
If the total line reactance is high,
series capacitors are very
effective and stability is
improved
52. 52
Problems Associated with Series Capacitors :
• Locking of synchronous motor during starting
• Hunting of synchronous motor at high load due to high R/X ratio
• Ferro resonance occurs between transformers and
series capacitors which produces harmonic over voltages
Advantages:
Series capacitors are used:
• To improve voltage regulation of distribution and industrial feeders
• To reduce light flicker problems
• To improve system stability
Applications:
The applications of Series capacitors are:
• Voltage rise due to reactive current
• By passing the capacitor during faults and reinsertion after
fault clearing
53. Methods of Voltage Control
53
STATIC VAR COMPENSATORS (SVC):
• SVCs are located in receiving end substations and distribution
systems for smooth and stepless variation of compensation of
reactive power injected into the line, by shunt capacitors and
shunt reactors.
The below figure shows the schematic diagram of
Static VAR Compensator.
Static VAR Compensator
54. Methods of Voltage Control
54
STATIC VAR COMPENSATORS (SVC):
• The reactor control is done by an antiparallel thyristor assembly.
• The firing angle of thyristors governs the voltage across the
inductor, so the reactor current and reactive power absorption by
the inductor can be controlled.
• Let 𝑸𝒄- reactive power charging by the capacitor
𝑸𝒍- reactive power absorbed by the inductor
Net reactive power injected into the bus 𝑸 = 𝑸𝒄- 𝑸𝒍
By varying 𝑄𝑙, 𝑄𝑐 can be controlled
For light load condition : 𝑸𝒍>𝑸𝒄
For heavy load condition : 𝑸𝒄> 𝑸𝒍
55. 55
Advantages of SVC:
• Bus voltage can be controlled
• Improves system stability, voltage stability
• Reduces power oscillations
• Minimize transmission loss
Types of SVC:
Saturated Reactors [SR]
Thyristor Controlled Reactors [TCR]
Thyristor Switched Capacitors [TSC]
Thyristor Switched Reactors [TSR]
Thyristor Controlled Transformers [TCT]
Fixed Capacitor and Thyristor Controlled Reactors [FC – TCR]
Thyristor Switched Capacitor [TSC] and
Thyristor Controlled Reactor [TCR]
Self (or) Line Commutated Converter [SCC / LCC]
56. 56
STATIC VAR COMPENSATORS (TSC - TCR):
• To control the current through a reactor, with new elements
TCR and TSC to meet the reactive power generation and
absorption demands.
TSC - TCR
57. 57
STATIC VAR COMPENSATORS (TSC - TCR):
• Each thyristor switch is built up from two thyristor stacks
connected in antiparallel.
• Each single phase thyristor switched capacitor consists of the
capacitor C, thyristor switch and reactor L to limit the current
through the thyristors and to prevent resonance with the
network.
• Switching of the capacitor is accomplished by separation of the
firing pulses to the antiparallel thyristors.
58. 58
STATIC VAR COMPENSATORS (TSC - TCR):
• The capacitor will then remain charged to the positive (or)
negative peak voltage and be prepared for the new transient free
switching on.
The V-I characteristics of an SVC (TSC-TCR) is shown below.
VI Characteristics of an SVC (TSC – TCR)
59. 59
STATIC VAR COMPENSATORS (TSC - TCR):
• A certain short-time overload capability is provided both in the
maximum inductive and capacitive regions.
• Voltage regulation with a given slope can be achieved in the
normal operating regions.
• The maximum capacitive current decreases linearly with the
system voltage, and the SVC becomes a fixed capacitor when the
maximum capacitive output is reached.
Advantages:
SVCs are suited to control the varying reactive power demand of
large fluctuating loads
Less maintenance
Possible to regulate the phases individually
60. Methods of Voltage Control
60
SECONDARY VOLTAGE CONTROL – STATCOM
(STATIC CONDENSERS):
• STATCOM is actually a shunt compensation device.
• It is a static synchronous generator operated as a shunt-connected
static VAR compensator (SVC) whose capacitive (or) inductive
output current can be controlled independently of the
A.C system voltage.
Major differences between SVC and STATCOM :
• Use of Gate Turn-Off Switch (GTO) in STATCOM compared to
use of conventional thyristors in SVC
• SVC is a voltage regulator and variable susceptance controller
whereas STATCOM is based on Voltage Source Converter (VSC)
61. 61
Major differences between SVC and STATCOM :
• Performance of STATCOM is superior to SVC because
Reactive power delivered in STATCOM = Voltage x Current
Reactive power delivered in SVC =
𝑽𝒐𝒍𝒕𝒂𝒈𝒆𝟐
𝑰𝒎𝒑𝒆𝒅𝒂𝒏𝒄𝒆
62. 62
The schematic arrangement of STATCOM based on
Voltage Source Converter (VSC) is shown below.
STATCOM based on Voltage Source Converter
63. 63
Working Principle:
• The operating principle is like a Synchronous Condenser.
• It is a 3ɸ inverter that is driven from the voltage across a
DC capacitor. VSC is coupled to circuit through a transformer,
which provides the safe operating voltage and small reactance.
• An inverter generates three phase voltages in phase with the
A.C system voltages.
• Reactive power exchange between the converter and the AC system
can be controlled by varying the amplitude of the three phase
output voltage of the converter (Eo).
64. 64
• If Eo > Einput : Current flows through the reactance from the
converter to the AC system and converter generates
capacitive reactance power.
• If Eo < Einput : Current flows from the AC system to the
converter and the converter absorbs inductive reactive power.
• If Eo = Einput : the reactive power exchange becomes zero and the
STATCOM is in floating state.
• The current lags if the inverter voltage is less than the
system voltage and leads if the inverter voltage is greater than the
system voltage.
65. 65
Features of STATCOM:
• The STATCOM is capable of supplying required reactive power
even at small values of bus voltages where reactive power supply
capability gets limited to its susceptance limit. The susceptance
decreases linearly with decrease in bus voltage.
• Due to susceptance limit, SVC cannot have a short time
overload capacity whereas STATCOM can have the same.
• STATCOM can serve as a real power exchanger if it has an
energy source at D.C bus, conversely supply D.C power.
• STATCOM can be designed as an active filter to
absorb system harmonics.
66. 66
The V-I characteristics of STATCOM is shown below.
VI Characteristics of a STATCOM
From the curve,
• The STATCOM can supply both capacitive and inductive
compensation.
• It Controls the output current (Ic max & IL max)
• It gives full output of capacitive generation independently of
