Power System voltage control has a hierarchy structure with three
levels: the primary, secondary, and the tertiary voltage control.
Over the past 20 yrs, one of the most successful measures
proposed to improve Power System voltage regulation has been
the application of secondary voltage control, initiated by the
French Electricity Company.
Important Of Features
Sags & Swells
fluctuation in system voltage that can
lead to noticeable changes in light
Flicker can either be a periodic
or non-periodic fluctuation in voltage
magnitude i.e. the fluctuation may
occur continuously at regular intervals
or only on occasions.
How To Reduce Flickers
Installation of a MiniCap to reduce flicker during large motor
Minicap is nothing but the Installation of
the feeder strengthens the network .
a series capacitor in
•Reduced voltage fluctuations (flicker)
•Improved voltage profile along the line
•Easier starting of large motors
Harmonics are associated with steady-state
waveform distortion of currents and voltages.
Harmonics are components that make up a
waveform where each component has a
frequency that is an integral multiple of the
fundamental frequency. The term Harmonic is
normally applied to waveform components that
have frequencies other than the fundamental
frequency. For a 50 Hz or 60Hz system the
fundamental frequency is 50HZ or 60Hz. A
waveform that contains any components other
than the fundamental frequency is nonsinusoidal and considered to be distorted.
How To Reduce Harmonics
filters near the harmonic
sources can effectively reduce
Improved Power Factor.
Reduced Transmission Losses.
Increased Transmission Capability.
Improved Voltage Control.
Improved Power Quality.
Occur when the supply voltage drops below 10% of the nominal
An interruption is usually caused by downstream faults that are
cleared by breakers or fuses. A sustained interruption is caused
by upstream breaker or fuse operation. Upstream breakers may
operate due to short-circuits, overloads, and loss of stability on
the bulk power system.
Long lines need special consideration in the
planning of a power system.
For long AC lines one must consider i.e. the
reactive power compensation, the transient
stability and switching over voltages and how
many intermediate substations one needs.
Area of application:-Expressway
A HVDC transmission line costs less than
an AC line for the same transmission
Lower investment cost
Lower right-of-way requirement for DC lines than for AC lines
HVDC does not contribute to the short circuit current
Basically Long cables are of two types
Under Ground Cables
Submarine Cables are used for long distance water crossing.
Unscheduled power flow on a given transmission path in an
interconnected electrical system.
Unscheduled power flows on transmission lines or facilities may
result in a violation of reliability criteria and decrease available
transfer capability between neighbouring control areas or utility
Area of application:
Interconnected power systems
HVDC can be controlled to transmit contracted
amounts of power and alleviate unwanted loop
An HVDC link can alternatively be controlled to
minimize total network losses
An HVDC link can never be overloaded
Reactive Power Factor
Reactive power is defined as the product of the r.m.s. voltage,
current, and the sine of the difference in phase angle between the
To maintain efficient transmission and distribution, it is necessary
to improve the reactive power factor.
How to Improve power factor
The use of the MiniCap on a distribution
feeder provides self-regulated reactive power
for improved power factor at the utility
Increased power factor at the utility source.
Easier starting of large motors .
Improved voltage regulation and reactive power
Self-regulation Improved power factor, Reduced
transmission losses, Increased transmission
Improved voltage control, Improved power quality,
Eliminates harmonics S.
Sags & Swells
Short duration decrease/increase (sag/swell) in
Voltage sags are one of the most commonly
occurring power quality problems. They are
usually generated inside a facility but may also
be a result of a momentary voltage drop in the
Electronic equipment is usually the main victim
of sags, as they do not contain sufficient internal
energy to ‘ride through’ the disturbance.
Post-disturbance excursions of voltages at some buses in the
power system out of the steady operation region.
Voltage instability is basically caused by an unavailability of
reactive power support in an area of the network, where the
voltage drops uncontrollably.
A load which does not draw balanced current from a balanced
A single-phase load, since it does not draw a balanced threephase current, will create unequal voltage drops across the
series impedances of the delivery system.
Area of application:-Railway
Feeder connected to the Public
Modern electric rail system is a major source
of unbalanced loads.
STATCOM can elegantly be used to restore
voltage and current balance in the grid, and
to mitigate voltage fluctuations generated by
the traction loads.
Power Factor Correction
Power Quality Improvement, Flicker Mitigation
Grid Voltage Support
The new electricity supply industry presents increasing
challenges for stable and secure operation of power systems.
State-of-the-art methods and tools have advanced our
capabilities significantly facing the challenges
comprehensive stability analysis tools
coordinated design of robust stability controls
on-line dynamic security assessment
Industry yet to take full advantage of these developments!
Need to review and improve
The reliability criteria.