Equipment for voltage control Effect of series capacitors  Effect of AVB/AVR Line drop compensation.

1. LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT–VI: Voltage Control
Department of Electrical and Electronics Engineering

2. SYLLABUS
Department of Electrical and Electronics Engineering
 Equipment for voltage control
 Effect of series capacitors
 Effect of AVB/AVR
 Line drop compensation.

3. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
1. BASIC DIFINITIONS
• Nominal voltage (Vb): The voltage that is assigned to the system or apparatus specifi ed, in given voltage
class or level.
• Rated voltage: The voltage at which the performance or output and the operating characteristics of an
apparatus or appliance is specified.
• Service voltage: The voltage measured between the two lines or supply points or ends of the service
entries or connection of the appliance, for example, at the terminals of the motor or at the output points
of the main switch, etc.
• Base voltage: The reference voltage of the system, for example, 230 V, single-phase, 400-V, 3-ph line
voltage, etc.
• Maximum voltage: The largest average voltage sustained for 1 minute or 5 minutes.
• Minimum voltage: The lowest average voltage sustained for 1 minute or 5 minutes.

4. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
• Voltage spread: The difference between maximum and minimum voltages, considering any transient dips
or over voltages due to a motor starting or fault clearing, etc.
• Voltage drop: The difference between sending end and receiving end voltage of a line or feeder. This may
be from transformer point to the service connection point of the consumer.
• Voltage regulation: The percentage voltage drop of a line or transformer.

5. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2. VOLTAGE CONTROL
The usual means of control in distribution systems are
(i) Incorporating voltage regulating devices such as induction regulators, buck-boost transformers, line
drop compensators, etc.
(ii) Applying of power factor correction and improving power factor and thus regulation of the line
(iii) Relocation of loads on the feeders such as (a) balancing the service connections on the 3 phases
equally, (b) increasing the size of feeder conductors, and (c) transferring loads to new feeders, etc.
(iv) Erecting new lines or substations for increased loads or increasing the voltage levels of the feeders.

6. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2.1 Feeder Voltage Regulators: Induction Regulators
• Induction regulators are similar to the three phase or single induction motors in construction, but
are static devices.
• The rotor is held stationary in position but can be adjusted either manually or automatically.
• The primary winding or output is connected to the line whose voltage is to be controlled.
• In a three phase regulator, the stator generates a rotating magnetic field of constant magnitude
and the rotor gets induced voltage in the secondary.
• The phase of the secondary voltage can be varied depending on the rotor position.
• The rotor voltage is added to the stator or line voltage and depending on the phase position, the
net voltage becomes more or less.

7. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
Fig. 1 Phasor diagram of induction
regulator

8. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2.2 Transformer Tap Changing
• This is the most common method used every where.
• Transformer tap changing can be either off load or
on load.
• On-load tap changing is used in 66 kV and above
substations and rarely in 22 or 33-kV substations.
• 11 kV and below rated transformers usually have
off load tap changing arrangement.
Fig. 2 Transformer tap changer

9. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
The additional equipment fitted for
on-load tap changer are
(i) Transformer winding with
required tapping points on the
high-voltage side
(ii) Center-tapping reactor
(iii) Two tapping switches
(iv) Two contactors or CBS
(v) Opening and closing mechanism
with timing schemes for opening
in a sequential manner
Fig. 3 Tap changing with reactor coil. Tap shifted from position 5 to 4

10. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2.3 Automatic Voltage Regulator
• The automatic voltage regulator is used to regulate the voltage.
• It takes the fluctuate voltage and changes them into a constant voltage.
• The fluctuation in the voltage mainly occurs due to the variation in load on the supply system.
• The variation in voltage damages the equipment of the power system.
• The variation in the voltage can be controlled by installing the voltage control equipment at
several places likes near the transformers, generator, feeders, etc.,
• The voltage regulator is provided in more than one point in the power system for controlling the
voltage variations.

11. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
Working Principle of Voltage Regulator
• It works on the principle of detection
of errors.
• The output voltage of an AC
generator obtained through a
potential transformer and then it is
rectified, filtered and compared with
a reference.
• The difference between the actual
voltage and the reference voltage is
known as the error voltage.

12. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
Application of the Automatic Voltage Regulator
The main functions of an AVR are as follows.
1.It controls the voltage of the system and has the operation of the machine nearer to the steady state
stability.
2.It divides the reactive load between the alternators operating in parallel.
3.The automatic voltage regulators reduce the overvoltages which occur because of the sudden loss of
load on the system.
4.It increases the excitation of the system under fault conditions so that the maximum synchronising
power exists at the time of clearance of the fault.

13. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
Step Voltage Regulator
• A step voltage regulator is an
auto transformer with a load
tap changer.
• The voltage rise or lowering is
obtained by changing the
tappings of the series winding
connected with the
autotransformer.
Fig. 5 Voltage step regulator and its regulator unit

14. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2.4 Line Drop Compensator
When load current flows through feeder
lines voltage drop occurs in the line
resistance and reactance.
As the current varies, the voltage drop also
varies. In order to keep load end voltage
constant, voltage proportional to (IR) and
(IX) are injected into the line with the
circuit energized by current from the
secondary of CT.
Fig.6 Line drop compensator

15. Voltage Control: System Planning and Automation
Department of Electrical and Electronics Engineering
2.5 Voltage Boosters
A booster is device used to increase the voltage of a feeder line
proportional to the load current. It is mainly used in such feeders as
‘TRACTION’ lines at a convenient point.
The advantage are
(a) it is separate from the source transformer and hence can be put
at any point on the feeder
(b) it can be isolated from the supply without interrupting the main
feeder for maintenance
(c) it is simple, reliable and can be worked with any feeder, but
regulating transformer and booster are more expensive than a
transformer with tap-changing unit.
Fig. 7 Line voltage booster