In the modern power system the reactive power compensation is one of the main issues, the transmission of active power requires a difference in angular phase between voltages at the sending and receiving points (which is feasible within wide limits), whereas the transmission of reactive power requires a difference in magnitude of these same voltages (which is feasible only within very narrow limits). The reactive power is consumed not only by most of the network elements, but also by most of the consumer loads, so it must be supplied somewhere. If we can't transmit it very easily, then it ought to be generated where it is needed." (Reference Edited by T. J. E. Miller, Forward Page ix).Thus we need to work on the efficient methods by which VAR compensation can be applied easily and we can optimize the modern power system. VAR control technique can provides appropriate placement of compensation devices by which a desirable voltage profile can be achieved and at the same time minimizing the power losses in the system. This report discusses the transmission line requirements for reactive power compensation. In this report thyristor switched capacitor is explained which is a static VAR compensator used for reactive power management in electrical systems. Seminar Topic For Electrical and Electronics Engineering (EEE)

1. 1
Thyristor Switched Capacitor for reactive power management in
electrical systems
A seminar report submitted in partial fulfillment for the award of the degree of Bachelor of
Engineering in Electrical & Electronics Engineering of the Visvesvaraya Technological University,
Belagavi
Submitted by
Name: RAJEEV RANJAN
USN: 2GI12EE037
Staff Counselor Staff Counselor
Department of Electrical & Electronics Engineering
Karnataka Law Society’s
GOGTE INSTITUTE OF TECHNOLOGY
UDYAMBAG, BELAGAVI-59008
Visvesvaraya Technological University
2015-2016

2. 2
ABSTRACT
In the modern power system the reactive power compensation is one of the main issues, the
transmission of active power requires a difference in angular phase between voltages at the
sending and receiving points (which is feasible within wide limits), whereas the transmission of
reactive power requires a difference in magnitude of these same voltages (which is feasible only
within very narrow limits). The reactive power is consumed not only by most of the network
elements, but also by most of the consumer loads, so it must be supplied somewhere. If we can't
transmit it very easily, then it ought to be generated where it is needed." (Reference Edited by T.
J. E. Miller, Forward Page ix).Thus we need to work on the efficient methods by which VAR
compensation can be applied easily and we can optimize the modern power system. VAR control
technique can provides appropriate placement of compensation devices by which a desirable
voltage profile can be achieved and at the same time minimizing the power losses in the system.
This report discusses the transmission line requirements for reactive power compensation. In this
report thyristor switched capacitor is explained which is a static VAR compensator used for
reactive power management in electrical systems.

3. 3
CONTENTS
Page No.
1. Certificate 2
2. Abstract 3
3. Contentswithpage number 4
4. Introduction 5
 Reactive Power
 NeedForReactive PowerCompensation
5. Compensation Techniques 6-9
6. The thyristor-switched capacitor (TSC) 9-19
 Principlesof operation
 Switching transients
7. Voltage/Current Characteristics 19-20
8. Advantages and Disadvantages 20
9. Result 21
10. Conclusion 21
11. Future Developments And Requirements 21
12. References 22
13. Hard copy of publishedpaperrelatedto thistopic

4. 4
4. INTRODUCTION
Reactive Power
Reactive power is the power that supplies the stored energy in reactive elements. Power, as we
know, consists oftwo components, active and reactive power. The total sum of active and reactive
power is called as apparent power.
In AC circuits, energy is stored temporarily in inductive and capacitive elements, which results in
the periodic reversal of the direction of flow of energy between the source and the load. The
average power after the completion of one whole cycle of the AC waveform is the real power,
and this is the usable energy of the system and is used to do work, whereas the portion of
power flow which is temporarily stored in the form of magnetic or electric fields and flows back
and forth in the transmission line due to inductive and capacitive network elements is known as
reactive power. This is the unused power which the system has to incur in order to transmitpower.
Inductors (reactors) are said to store or absorb reactive power, because they store energy in
the form of a magnetic field. Therefore, when a voltage is initially applied across a coil, a magnetic
field builds up, and the current reaches the full value after a certain period of time. This in turn
causes the current to lag the voltage in phase.
Need for Reactive power compensation.
The main reason for reactive power compensation in a system is: 1) the voltage
regulation;2) increased system stability; 3) better utilization of machines connected to the
system; 4) reducing losses associated with the system; and 5) to prevent voltage collapse as
well as voltage sag. The impedance of transmission lines and the need for lagging VAR by
most machines in a generating system results in the consumption of reactive power, thus
affecting the stability limits of the system as well as transmission lines. Unnecessary voltage
drops lead to increased losses which needs to be supplied by the source and in turn leading
to outages in the line due to increased stress on the system to carry this imaginary power. Thus
we can infer that the compensation of reactive power not only mitigates all these effects but also
helps in better transient response to faults and disturbances. In recent times there has been an
increased focus on the techniques used for the compensation and with better devices included
in the technology, the compensation is made more effective. It is very much required that the
lines be relieved of the obligation to carry the reactive power, which is better provided near the
generators or the loads. Shunt compensation can be installed near the load, in a distribution
substation or transmission substation.
5. COMPENSATION TECHNIQUES
The principles of both shunt and series reactive power compensation techniques are described
below:

5. 5
Shunt compensation
Fig 1.1
Fig 1.2

6. 6
The figure 1.1 comprises of a source V1, a power line and an inductive load. The figure
1.1 shows the system without any type of compensation. The phasor diagram of these is also
shown above. The active current Ip is in phase with the load voltage V2. Here, the load
is inductive and hence it requires reactive power for its proper operation and this has to be
supplied by the source, thus increasing the current from the generator and through the power
lines. Instead of the lines carrying this, if the reactive power can be supplied near the load, the
line current can be minimized, reducing the power losses and improving the voltage regulation
at the load terminals. This can be done in three ways: 1) A voltage source. 2) A current source.
3) A capacitor.
In this case, a current source device is used to compensate Iq, which is the reactive
component of the load current. In turn the voltage regulation of the system is improved and the
reactive current component from the source is reduced or almost eliminated. This is in case of
lagging compensation. For leading compensation, we require an inductor.
Therefore we can see that, a current source or a voltage source can be used for both leading
and lagging shunt compensation, the main advantages being the reactive power generated
is independent of the voltage at the point of connection.
Series compensation
Fig 1.3

7. 7
Fig 1.4
Series compensation can be implemented like shunt compensation, i.e. with a current or
a voltage source as shown in figure 1.4. We can see the results which are obtained by
series compensation through a voltage source and it is adjusted to have unity power factor
at V2. However series compensation techniques are different from shunt compensation
techniques, as capacitors are used mostly for series compensation techniques. In this
case, the voltage Vcomp has been added between the line and the load to change the
angle V2’. Now, this is the voltage at the load side. With proper adjustment of the
magnitude of Vcomp, unity power factor can be reached at V2
FACTS devices used
Flexible AC transmission system or FACTS devices used are:
1) VAR generators.
a) Fixed or mechanically switched capacitors.
b) Synchronous condensers.
c) Thyristorized VAR compensators.
1. Thyristors switched capacitors (TSCs).
2. Thyristor controlled reactor (TCRs).
3. Combined TSC and TCR.
4. Thyristor controlled series capacitor (TCSC).
2) Self Commutated VAR compensators.
a) Static synchronous compensators (STATCOMs).
b) Static synchronous series compensators (SSSCs).
c) Unified power flow controllers (UPFCs).
d) Dynamic voltage restorers (DVRs

8. 8
6. THE THYRISTOR-SWITCHED CAPACITOR (TSC)
Thyristor switched capacitor is defined as 'a shunt- connected, thyristor-switched
capacitor whose effective reactance is varied in a stepwise manner by full- or zero-
conduction operation of the thyristor valve'.
Principles of operation
The principle of the TSC is shown in Figures 6.12 and 6.13. The susceptance is adjusted
by controlling the number of parallel capacitors in conduction. Each capacitor always
conducts for an integral number of half cycles. With k capacitors in
parallel, each controlled by a switch as in Figure 6.13, the total susceptance can be
equal to that of any combination of the k individual susceptances taken 0, 1, 2 . . . . or
k at a time. The total susceptance thus varies in a stepwise manner. In principle the
steps can be made as small and as numerous as desired, by having a sufficient
number of individually switched capacitors. For a given number k the maximum number
of steps will be obtained when no two combinations are equal, which requires at least
that all the individual susceptances be different. This degree of flexibility is not usually
sought in power system compensators because of the consequent complexity of the
controls, and because it is generally more economical to make most of the
susceptances equal. One compromise is the so-called binary system in which there
are (k - 1) equal susceptances B and one susceptance B/2. The half-susceptance
increases the number of combinations from k to 2k.
The relation between the compensator current and the number of capacitors
conducting is shown in Figure 6.14 (for constant terminal voltage). Ignoring switching
transients, the current is sinusoidal, that is, it contains no harmonics

9. 9
Fig. 6.12 Alternative arrangements of three-phase thyristor-switchet capacitor. (a)
delta-connected secondary, Delta-connected TSC; ant (b) wye-connected secondary,
wye-connected TSC (four-wire system).
Fig. 6.13 Principles of operation of TSC. Each phase of Figure 6.12 comprises of parallel
combinations of switched capacitors of this type
Fig. 6.14 Relationship between current and number of capacitors conducting in the
TSC.
Switching transients and the concept of transient-free
switching
When the c u r r e n t i n a n i n d i v i d u a l c a p a c i t o r r e a c h e s a natural
zero-crossing, the thyristors can be left unbated and no further current will flow. The
reactive power supplied to the power system ceases abruptly. The capacitor, however,
is left with a trapped charge (Figure 6.15(a)). Because of this charge, the voltage

10. 10
across the thyristors subsequently alternates between zero and twice the peak phase
voltage. The only instant when the thyristors can be gated again without transients
is when the voltage across them is zero (Figure 6.15(b)). This coincides with peak
phase voltage.
Fig. 6.15 Ideal transient-free switching waveforms. (a) Switching on; And (b) switching off.
Ideal transient-free switching
The simple case of a switched capacitor, with no other circuit elements than the voltage
supply, is used first to describe the im portant c o n c e p t of trans ientfree
s w i t c h i n g . Figure 6.16 shows the circuit.
With sinusoidal AC supply voltage v = v sin (ω0t + α), the thyristors can be gated into
conduction only at a peak value of voltage, that is, when
Fig. 6.16 Circuit for analysis of transient-free switching.

11. 11
(6.8)
Gating at any other instant would require the current i = Cdv/dt to have a discontinuous step
change at t = 0+. Such a step is impossible in practice because of inductance, which is
considered in the next section. To permit analysis of Figure 6.16, the gating must occur at a
voltage peak, and with this restriction the current is given by
(6.9)
Where α = ±π/2. Now ω0C = BC is the fundamental frequency susceptance of the capacitor,
and XC = 1/BC its reactance, so that with α = ±π/2
(6.10)
Where îAC is the peak value of the AC current, îAC=vBC= v/XC .
In the absence of other circuit elements, we must also specify that the capacitor be
precharged to the voltage VC0 = ±v, that is, it must hold the prior charge ±v/C. This is because
any prior DC voltage on the capacitor cannot be accounted for in the simple circuit of Figure
6.16. In practice this voltage would appear distributed across series inductance and resistance
with a portion across the thyristor switch.
With these restrictions, that is, dv/dt = 0 and VC0 = ±v at t = 0, we have the ideal case of
transient free switching, as illustrated in Figure 6.15. This concept is the basis for switching
control in the TSC. In principle, once each capacitor is charged to either the positive or the
negative system peak voltage, it is possible to switch any or all of the capacitors on or off for
any integral number of half cycles without transients
Switching transients in the general case
Under practical conditions, it is necessary to consider inductance and resistance. First
consider the addition of series inductance in Figure 6.16. In any practical TSC circuit,
there must always be at least enough series inductance to keep di/dt within the
capability of the thyristors. In some circuits there may be more than this minimum
inductance. In the following, resistance will be neglected because it is generally small
and its omission makes no significant difference to the calculation of the first few peaks
of voltage and current.
The presence of inductance and capacitance together makes the transients oscillatory.
The natural frequency of the transients will be shown to be a key factor in the magnitudes
of the voltages and currents after switching, yet it is not entirely under the designer's
control because the total series inductance includes the supply system inductance which,
if known at all, may be known only approximately. It also includes the inductance of the
step-down transformer (if used), which is subject to other constraints and cannot be
chosen freely.
It may not always be possible to connect the capacitor at a crest value of the supply
voltage. It is necessary to ask what other events in the supply voltage cycle can be

12. 12
detected and used to initiate the gating of the thyristors, and what will be the resulting
transients.
Fig. 6.17 Circuit for analysis of practical capacitor switching.
The circuit is that of Figure 6.17. The voltage equation in terms of the Laplace transform
is
(6.11)
The supply voltage is given by v = v sin (ω0t + α). Time is measured from the first
instant when a thyristor is gated, corresponding to the angle α on the voltage
wave- form. By straightforward transform manipulation and inverse transformation we
get the instantaneous current expressed as
(6.12)
Where ωn is the natural frequency of the circuit
(6.13)
and
(6.14)
n is the per unit natural frequency.
The current has a fundamental frequency component iAC which leads the supply voltage
by π/2 radians. Its amplitude îAC is given by
(6.15)

13. 13
and is naturally proportional to the fundamental-frequency susceptance of the
capacitance and inductance in series, that is, Bcn2/ (n2-1). The term n2/ (n2-1) is a
magnification factor, which accounts for the partial series-tuning of the L-C circuit. If
there is appreciable inductance, n can be as low as 2.5, or even lower, and the
magnification factor can reach l .2 or higher. It is plotted in Figure 6.18. The last two
terms on the right-hand side of equation (6.12) represent the expected oscillatory
components of current having the frequency ωn. In practice, resistance causes these
terms to decay.
The next section considers the behavior of the oscillatory components under important
practical conditions.
Fig. 6.18 Voltage and current magnification factor n2/ (n2 - 1).
1. Necessary condition for transient free switching.
For transient-free switching, the oscillatory components of current in equation (6.12)
must be zero. This can happen only when the following two conditions are
simultaneously satisfied:
(6.16)
(6.17)
The first of these equations means that the thyristors must be gated at a positive or
negative crest of the supply voltage sinewave. The second one means that the
capacitors must also be precharged to the voltage vn2/ (n2 - 1) with the same polarity.
The presence of inductance means that for transient- free switching the capacitor must

14. 14
be 'overcharged' beyond v by the magnification factor n2/(n2 - 1). With low values of
n, this factor can be appreciable (Figure 6.18).
Of the two conditions necessary for transient-free switching, the precharging condition
expressed by equation (6.17) is strictly outside the control of the gating-control circuits
because VC0, n, and v can all vary during the period of non-conduction before the
thyristors are gated. The capacitor will be slowly discharging, reducing VC0; While the
supply system voltage and effective inductance may change in an unknown way,
changing n. In general, therefore, it will be impossible to guarantee perfect
transient-free reconnection.
In practice the control strategy should cause the thyristors to be gated in such a way as
to keep the oscillatory transients within acceptable limits. Of the two conditions given
by equations (6.16) and (6.17), the first one can in principle always be satisfied.
The second one can be approximately satisfied under normal conditions. For a range
of system voltages near 1 p.u., equation (6.17) will be nearly satisfied if the capacitor
does not discharge (during a non-conducting period) to a very low voltage: or if it is
kept precharged or 'topped up' to a voltage near ±vn2/(n+2 - 1).
2. Switching transients under non-ideal conditions.
There are some circumstances in which equations (6.16) and (6.17) are far from being
satisfied. One is when the capacitor is completely discharged, as for example when the
compensator has been switched off for a while. Then VC0 = 0. There is then no point on
the voltage wave when both conditions are simultaneously satisfied.
In the most general case Vco can have any value, depending on the condition under which
conduction last ceased and the time since it did so. The question then arises,
how does the amplitude of the oscillatory component depend on VC0? How can the gating
instants be chosen to minimize the oscillatory component?
Two practical choices of gating are: (a) at the instant when v =VC0, giving sin α =
VC0/v; and (b) when dv/dt = 0, giving cos α = 0. The first of these may never occur if
the capacitor is overcharged beyond v. The amplitude îosc of the oscillatory component
of current can be determined from equation (6.12) for the two alternative gating
angles. In Figures 6.19 and
6.20 The resulting value of îosc relative to îAC is shown as a function of VC0 and n, for
each of the two gating angles.
From these two figures it is apparent that if VC0 is exactly equal to v, the oscillatory
component of current is non-zero and has the same amplitude for both gating angles,
whatever the value of the natural frequency n. For any value of Vc0 less than v, gating
with v = VC0 always gives the smaller oscillatory component whatever the value of n.

15. 15
Fig. 6.19 Amplitude of oscillatory current component. Thyristors gated when v = Vco
Fig. 6.20 Amplitude of oscillatory current component. Thyristors gated when dv/dt = 0.

16. 16
The conditions for transient-free switching appear in Figure 6.20 in terms of the
precharge voltage required for two particular natural frequencies corresponding to n =
2.3 and n = 3.6.
Switching a discharged capacitor
In this case VC0 = 0. The two gating angles discussed were: (a) when v = VC0 = 0; and
(b) when dv/dt = 0 (c os α = 0). In the former case only equation (6.17) is satisfied.
From equation (6.12) it can be seen that in the second case (gating when dv/dt = 0)
the oscillatory component of current is greater than in the first case (gating when v =
VC0 = 0). An example is shown in Figure 6.21 and Figure 6.22.
Fig. 6.21 switching a discharge capacitor; Circuit diagram.
The reactances are chosen such at îAC = 1 p.u. and the natural frequency is given by
n = XC / (Xt + Xs) = 3.6 p.u. In case (a), the amplitude of the oscillatory component of
current is exactly equal to îac .In case (b), the oscillatory component has the amplitude
nîAC and much higher current peaks a r e experienced. The capacitor experiences
higher voltage peaks and the supply voltage distortion is greater

17. 1
Fig. 6.22 switching transients with discharge capacitor. (a) Gating when V = VC0 = 0;
(b) Gating when dv/dt = 0.
7. VOLTAGE / CURRENT CHARACTERISTICS
In a TSC for transmission system application the cost of the thyristor switches and other
complications make it desirable to minimizethe number of parallel capacitor units. A figure
of 3 or 4 is typical of existing or presently planned installations. With so few capacitors a
smooth voltage/current characteristic is unobtainable, and a stepped characteristic is
obtained (Figure 35). The capacitor characteristics 1, 2, and 3 intersect the system
voltage/current characteristic at discretepoints, and operation can be at any of these points
depending on the number of capacitors conducting. With two capacitors conducting,
operation would be at point A.
In order to obtain a smoother voltage/current characteristic it is usual to have a parallel-
connected TCR which "interpolates" between the capacitor characteristics. If the TCR
characteristic has a small positive slop, the resultant characteristic is shown by the heavy-
line segments in figure 35. This construction shows that the TCR current rating must be
little larger than that of one capacitor bank at rated voltage, otherwise deadbands arise as
shown by the shading in Figure 35. The increase in rating enables the heavy segments to
be extended to the left through the deadband.

18. 2
Fig: Effect of paralleling a TCS and TCR before control systems are properly
coordinated.
With fixed TCR controls (i.e., fixed knee voltage and slope) the voltage/current
characteristic of this hybrid TSC/TCRis still stepped, giving rise to the possibility of bistable
operation. It is therefore necessary adjust the TCR knee voltage and slope by a small
amount every time a switch is in or out. This can in principle be done either by open-loop
or closed-loop (current feedback) modification of the control system giving a continuous
V/I characteristic as in Figure 14. A further sophistication in the control system is to
incorporate a hysteresis effect so the capacitors are switched in at a lower voltage than
that at which are switched out. This helps to prevent a "hunting" instability which can arise
if the system characteristic intersects the compensator characteristic near the junction of
two segments.
8. ADVANTAGES AND DISADVANTAGES
In the chapter 2 of the book Edited by T. J. E. Miller, the following are listed as
advantages of using reactive compensation:
1. Limit Rapid Voltage Increase or Decline.
2. Limit Slow Voltage Increase or Decline.
3. Reactive Power Support at DC Converter Terminals.
4. Increase Short Circuit Levels.
5. Decrease Short Circuit Levels.
6. Improving Steady-state Stability.
7. Improve Dynamic Stability
8. Improve Transient Stability
9. Limit Fast Wave-front Overvoltages due to lighting, switching etc.
A TSC is usually a three phase assembly, connected either in a delta or a star
arrangement. Unlike the TCR, a TSC generates no harmonics and so requires no filtering.
For this reason, some SVCs have been built with only TSCs .This can lead to a relatively
cost-effective solution where the SVC only requires capacitive reactive power, although a
disadvantage is that the reactive power output can only be varied in steps. Continuously
variable reactive power output is only possible where the SVC contains a TCR or another
variable element such as a STATCOM.

19. 9. RESULT
 Improvedvoltage regulation
 Reduced power losses
 Increased utilization of equipment
 Improved power factor
10. CONCLUSION
The results shows that, when we use the compensation device, it can balance the voltage and
current to normal levels, as we know that there is reactive power due to capacitive and
inductive elements in the grid which can make the current and voltage phase difference,
due to which the real power in the system reduces from the ideal level, but after we introduce
the compensation device, the current and voltage waveforms have same phase, and due
to this compensation effect, power in the system achieves to the desired value with stable
voltage.
11. FUTURE DEVELOPMENTS AND REQUIREMENTS
Although the demand for electric power in world is increasing only slowly, the trends in worldwide
generation and transmission suggest that reactive power compensation will become increasingly
important. This follows from a wide commitment to ac transmission, together with economic
factors which necessitate the maximum utilization of generation and transmission facilities. In
addition, there is no evidence that large irregular loads like arc furnaces will becomeless common.
AS far as bulk transmission is concerned, the continued development of remote hydro resources
worldwide should continue to provide a demand for EHV compensators. Among the competitive
issues in this field are power losses and harmonics, and innovations that improve performance in
these areas are greatly to be desired. Overvoltage performance is another area where
improvements are desirable. Most static compensators, including the polyphase saturated
reactor, are limited in their ability to hold down the system voltage under emergency.
Developments in HVDC transmission also generate reactive power control requirements which
compensators can help to fulfill.

20. 1
12. REFERENCES
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Duluth, MN, USA, Date of Conference: 2002 Page(s): 507 - 508 vol.1 Print ISBN:0-7803-
7322-7, INSPEC Accession Number: 7386352, DOI:10.1109/PESW.2002.985054 ,
Publisher: IEEE
2. T.J.E. Miller, “Reactive power control in electric systems” (New York: John Wiley & Sons,
1982).
3. Akwukwaegbu I. O, Okwe Gerald Ibe “Concepts of Reactive Power Control and Voltage
Stability Methods in Power System Network” IOSR Journal of Computer Engineering
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2013), PP 15-25
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a
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7. P. Sakis Meliopoulos, George Christoforidis, “Effects of DC Ground Elect rode on
Converter Transformers” IEEE Transactions on Power Delivery, 1989, pp. 995 – 10021.
8. Juan dixon, senior member, IEEE, “Reactive Power Compensation Technologies: State-
of-the-Art Review” Proceedings of the IEEE, VOL.93, NO.12, december2005
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