1) Voltage control in power systems involves maintaining voltage levels within prescribed limits despite varying loads through reactive power balance. 2) Methods of voltage control include tap changing transformers, shunt reactors, synchronous phase modifiers, shunt capacitors, series capacitors, and static VAR systems. 3) Static VAR systems provide faster switching than mechanical methods and allow transient-free operation through thyristor control of reactors and capacitors.

1. Voltage Control

2. Introduction
• The reactive power is proportional to the magnitude of the voltage
drop inline.
• The voltage at the consumer terminals must be maintained constant
within prescribed limits irrespective of the type and magnitude of the
load.
• For maintaining the voltages at their prescribed values at all times, it
is necessary to maintain the balance of reactive power in the system.
• In practice it is difficult to maintain the balance of reactive powers
because of varied unpredictable demands of the consumers.
• Therefore an unbalance always exists between the supply and
demand conditions of reactive power.

3. Introduction
• The active power is only supplied by generators but the reactive power may
be supplied from several sources.
• The following are the generators of reactive power
• Synchronous generators and synchronous motors
• Static capacitors
• Distributed capacitance of overhead lines and cables
• The following are the consumers of reactive power
• Synchronous machines
• Inductive static loads
• Induction motors
• Distributed inductance of overhead lines and cables
• Transformer inductance

4. Methods of Voltage Control
• The voltages at different buses of the power system vary with the
changes in load.
• The voltage is normally high at light load conditions and low at heavy
load conditions.
• To following methods are used for voltage control in a power system:-
1. Tap changing transformers
2. Shunt reactors
3. Synchronous phase modifiers
4. Shunt capacitors
5. Series capacitors
6. Static VAR systems (SVS)

5. Shunt Compensation
• Adjustment of the system voltage by means of shunt reactive
elements is known as shunt compensation. The shunt compensation
consists of
a.) static shunt compensation
b.) synchronous compensation
The static shunt compensation uses shunt reactors, shunt capacitors
and static var system . In synchronous compensation, synchrononus
phase modifiers are used.

6. Tap Changing Transformers
• The change of voltage is affected by changing the number of turns of
the transformer provided with taps.
• For sufficiently close control of voltage, taps are usually provided on
the high voltage winding of the transformer.
• There are two types of tap changing transformers:
i) Off load tap changing transformer
ii) On load tap changing transformer

7. Off-load tap-changing transformer
• In this method, the transformer
is disconnected from the main
supply when the tap setting is to
be changed. The tap setting is
usually done manually. The off
load tap changing transformer is
shown in the figure below

8. On-load tap-changing transformer
• In order that the supply may not be interrupted, on-load tap changing
transformer are used. Such a transformer is known as a tap-changing under
load transformer.
• While tapping, two essential conditions are to be fulfilled.
• The load circuit should not be broken to avoid arcing and prevent the damage of
contacts.
• No parts of the windings should be short–circuited while adjusting the tap.
• The tap changing employing a center tapped reactor R show in the figure
above. Here S is the diverter switch, and 1, 2, 3 are selector switch. The
transformer is in operation with switches 1 and S closed.
• To change to tap 2, switch S is opened, and 2 is closed. Switch 1 is then
opened, and S closed to complete the tap change. It is to be noted that the
diverter switch operates on load, and no current flows in the selector
switches during tap changing.
• It is to be noted that the diverter switch operates on load, and no current
flows in the selector switches during tap changing. During the tap change,
only half of the reactance which limits the current is connected in the circuit.

9. Image of Tap Changing Transformer

10. Shunt Reactors
• A shunt reactor is and inductive element connected between line and
neutral.
• It compensates for capacitive current from transmission lines or
underground cables.
• They are used in long distance EHV and UHV transmission lines.
• They are used when the line is to be charged or when the line is lightly
loaded.
• They are switched off under normal loaded conditions of the line.
• They are installed in sending end substations, receiving end substations
and intermediate substations of long EHV and UHV AC lines.
• For very long lines the shunt reactors are installed at an interval of about
300 km in intermediate substations to limit the voltage at the
intermediate points during low loads.
• Their construction is identical to power transformer except that an air gap
is provided within the reactor core to prevent magnetic saturation.

11. Shunt Capacitors
• A shunt capacitor is a capacitor connected in parallel with the lines.
• They are installed near the load terminals in receiving end
substations, distribution substations and in switching substations.
• They inject leading reactive voltamperes and are arranged in three
phase banks.
• A serious disadvantage with shunt capacitors is Ferranti effect which
is overcome by providing the capacitor bank with fixed and variable
elements.
• They are switched in during heavy and low power factor loads.
• In transmission lines shunt capacitors are connected either to the
tertiary winding of power transformer or to the busbars.

12. Series Compensation
• Adjustment of the system voltage by connecting capacitors in series
with the line is called static series compensation.
• The following are the advantages of series compensation
• Increase in power transfer capability
• Improvement in system stablilty
• Load division among parallel lines
• control of voltage
• series capacitors are very commonly used with EHV and UHV lines.

13. Location of Series Capacitors
• Many technical and economic considerations decide the location of
series capacitors.
• They maybe located at the sending end, receiving end or at the centre
of the line.
• They are sometimes distributed at two or more points along the line.
• The degree of compensation and the characteristic of the line decide
the location of the capacitors.
• The rating of the series capacitor is given by
Qc = 3I2 XC x 10-6 MVAr
where I is the line current

14. Synchronous Phase Modifiers
• A synchronous phase modifier is a synchronous motor running
without a mechanical load.
• it is connected in parallel with the load at the receiving end.
• it can both generate or absorb reactive voltamperes (VAr) by varying
the excitation of its field winding.
• It is also known as synchronous condensor, synchronous capacitor ans
synchronous compensator.
• It is connected at the receiving end of the line to the tertiary of the
power transformers.
• Its installation, operation and maintenance is not easy
• It is difficult to increase its capacity in order to cope with the
increasing demand.

15. Static VAR Systems (SVS)
• In EHV transmission when the voltage at a bus falls below the reference value,
capacitive VARs have to be injected.
• When the voltage becomes higher than the reference value, inductive VARs
have to be supplied to lower the bus voltage.
• In conventional methods of shunt compensation, shunt reactors are
connected during low loads and shunt capacitors are connected during heavy
loads or low lagging power factor loads.
• Such switching operations are very slow because of greater time (3-4 cycles)
required for the operation of the circuit breaker.
• The C.B. are not suitable for frequent switching during voltage variations.
• The above limitations have been overcome by static var systems (SVS)
• In static var system, thyristors are used as switching devices instead of circuit
breakers
• This results in faster switching than mechanical switching and it is also
possible to have transient free operation by controlling the switching instant.

16. SVS Schemes
Some of the most commonly used Static VAr Schemes are as follows:
1. Thyristor controlled reactor (TCR)
2. Thyristor switched capacitor (TSC)
3. Fixed capacitor, thyristor controlled reactor (FC-TCR) scheme
4. Thyistor switched capacitor, thyristor controlled reactor (TSC-TCR)
scheme

17. Thyristor Controlled Reactor (TCR)
• A single phase thyristor
controlled reactor is shown in fig.
• The current through the reactor
can be varied by controlling the
firing angles of back to back pair
of thyristors connected is series
with the reactor.
• The TCR scheme is used in EHV
lines for providing lagging VAR
during low loads or load
rejections.

18. Thyristor Controlled Capacitor (TSC)
• A single phase thyristor switched
capacitor is shown in fig.
• The current through the capacitor
can be varied by controlling the
firing angles of back to back
thyristors connected is series with
the capacitor.
• The TSC scheme is used in EHV lines
for providing leading VARs during
high loads.

19. FC-TCR Type VAR Compensator
• The fixed capacitor, thyistor
controlled reactor is shown in fig.
• This scheme provides discrete
leading VARs from the capacitors
and continously lagging VARs from
the thryistor switched reactors.
• Leading VARs are supplied by two or
more capacitor banks.
• Harmonics are generated due to
switching operation.
• The TCR and the secondary winding
of the coupling transformer in delta
are connected in delta for
eliminating third harmonics.