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V. 1.1
Unbalanced load
Sags & Swells
Loop flow
Interruptions
- additional
benefits
Harmonics
- major benefits
(link to Web)
Reactive Power Factor
F a c t o r s & P h e n o m e n a
- major benefits
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V. 1.1
Asynchronous connection
The interconnected AC networks that tie the power
generation plants to the consumers are in most cases
large. The map below shows the European situation.
There is one grid in Western Europe, one in Eastern
Europe, one in the Nordic countries. Islands like Great
Britain, Ireland, Iceland, Sardinia, Corsica, Crete,
Gotland, etc. also have their own grid with no AC
connection to the continent. The other continents on
the globe have a similar situation.
Even if the networks in Europe have the same nominal
frequency, 50 cycles per second or Hertz (Hz), there is
always some variation, normally less than ± 0.1 Hz,
and in certain cases it may prove difficult or impossible
to connect them with AC because of stability concerns.
An AC tie between two asynchronous systems needs
to be very strong to not get overloaded. If a stable AC
tie would be too large for the economical power
exchange needs or if the networks wish to retain their
independence, than a HVDC link is the solution.
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European interconnected power grids.
And in other parts of the world (South America and
Japan) 50 and 60 Hz networks are bordering each
other and it would be impossible to exchange power
between them with an AC line or cable. HVDC is then
the only solution.
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V. 1.1
Bottlenecks
Constrained transmission paths or interfaces in an
interconnected electrical system
The term Bottlenecks is often interchangeable to
congested transmission paths or interfaces. A
transmission path or interface refers to a specific set of
transmission elements between two neighboring
control areas or utility systems in an interconnected
electrical system. A transmission path or interface
becomes congested when the allowed power transfer
capability is reached under normal operating
conditions or as a result of equipment failures and
system disturbance conditions. The key impacts of
Bottlenecks are reduction of system reliability,
inefficient utilization of transmission capacity and
generation resources, and restriction of healthy market
competition.The ability of the transmission systems to
deliver the energy is dependent on several main
factors that are constraining the system, including
thermal constraints, voltage constraints, and stability
constraints. These transmission limitations are usually
determined by performing detailed power flow and
stability studies for a range of anticipated system
operating conditions. Thermal limitations are the most
common constraints, as warming and consequently
sagging of the lines is caused by the current flowing in
the wires of the lines and other equipment. In some
situations, the effective transfer capability of
transmission path or interface may have to be reduced
from the calculated thermal limit to a level imposed by
voltage constraints or stability constraints.
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V. 1.1
Flicker
A fluctuation in system voltage that can lead to
noticeable changes in light output.
Voltage Flicker can either be a periodic or aperiodic
fluctuation in voltage magnitude i.e. the fluctuation may
occur continuously at regular intervals or only on
occasions. Voltage Flicker is normally a problem with
human perception of lamp ‘strobing’ effect but can also
affect power-processing equipment such as UPS
systems and power electronic devices.
Slowly
fluctuating periodic flickers, in the 0.5 – 30.0Hz range,
are considered to be noticeable by humans. A voltage
magnitude variation of as little as 1.0% may also be
noticeable.
The main sources of flicker are industrial loads
exhibiting continuous and rapid variations in the load
current magnitude. This type of loads includes electric
arc furnaces in the steel industry, welding machines,
large induction motors, and wind power generators.
High impedance in a power delivery system will
contribute further to the voltage drop created by the
line current variation.
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V. 1.1
Harmonics
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
non-sinusoidal and considered to be distorted.
Nonlinear loads draw currents that are non-sinusoidal
and thus create voltage drops in distribution conductors
that are non-sinusoidal. Typical nonlinear loads include
rectifiers, variable speed drives, and any other loads
based on solid-state conversion. Transformers and
reactors may also become nonlinear elements in a power
system during overvoltage conditions. Harmonics create
many concerns for utilities and customers alike. Typical
phenomena include neutral circuit overloading in three
phase circuits, motor and transformer overheating,
metering inaccuracies and control system malfunctions.
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V. 1.1
Interruptions
Occur when the supply voltage drops below 10% of the
nominal value
An Interruption occurs whenever a supply’s voltage drops
below 10% of the rated voltage for a period of time no
longer than one minute. It is differentiated from a voltage
sag in that the late is not a severe power quality problem.
The term sag covers voltage drops down to 10% of nominal
voltage whereas an interruption occurs at lower than 10%.
A Sustained Interruption occurs when this voltage decrease
remains for more than one minute.
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. Loss of stability
is usually characterized by out-of-tolerance voltage
magnitude conditions and frequency variations which
exceed electrical machine and transformer tolerances. This
phenomenon is often associated with faults and
deficiencies in a transmission system but can also be the
result of lack of generation resources. The concerns
created by interruptions are evident and include
inconvenience, loss of production time, loss of product, and
loss of service to critical facilities such as hospitals.
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V. 1.1
Long lines
Long lines need special consideration in the planning of a
power system.
This transmission carries more than 12,000 MW over 800
km. There is an HVDC system with two 600 kV bipoles of
3150 MW each is direct route to São Paulo while the three
800 kV shunt and series compensated AC lines has two
intermediate substations that allow connection to the local
grids.
For long AC lines one must consider i.e. the reactive power
compensation, the transient stability and switching
overvoltages and how many intermediate substations one
needs.
If the line length is longer than approx. 600 km one should
also consider if an HVDC alternative brings lower
investment costs and/or lower losses or if the inherent
controllability of an HVDC system brings with some other
benefits.
Another factor to consider is the land use
The figure at the right compares two 3,000 MW HVDC lines
for the 1,000 km Three Gorges - Shanghai transmission,
China, to five 500 kV AC lines that would have been used if
AC transmission had been selected.
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V. 1.1
Long cables
Cables have large capacitances and therefore, if fed with AC,
large reactive currents. Cables for DC are also less expensive
than for AC. One must distinguish between submarine cables
and land (underground) cables.
Submarine cables
Since no shunt reactor can be installed at intermediate points
(in the sea) and DC cables are less expensive, the majority of
cables > 50 km are for DC.
Underground cables
Long underground cables (> 50 km) have been generally
avoided since the cost for an overhead line was deemed to be
only 10 – 20 % of the cost for the cable. In many parts of the
world it is now almost impossible to get permission to build a
new overhead line. HVDC Light ® has changed the cost relation
and the cable solution is less expensive than before.
Laying of the 200 km Fenno-Skan HVDC cable (500 MW).
Laying of the 180 km Murraylink HVDC Light cable (220 MW).
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V. 1.1
Loop Flow
Unscheduled power flow on a given transmission path in an
interconnected electrical system
The terms Loop Flow and Parallel Path Flow are sometimes
used interchangeable to refer to the unscheduled power flows,
that is, the difference between the scheduled and actual power
flows, 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 neighboring
control areas or utility systems.
The reliability of an interconnected electrical system can be
characterized by its capability to move electric power from one
area to another through all transmission circuits or paths
between those areas under specified system conditions. The
transfer capability may be affected by the “contract path”
designated to wholesale power transactions, which assumes
that the transacted power would be confined to flow along an
artificially specified path through the involved transmission
systems. In reality, the actual path taken by a transaction may
be quite different from the designated routes, determined by
physical laws not by commercial agreements, thus involving the
use of transmission facilities outside the contracted systems.
These unexpected flow patterns may cause so-called Loop
Flow and Parallel Path Flow problems, which may limit the
amount of power these other systems can transfer for their own
purposes.
Transmission Loop Flows for 1000 KW scheduled Transfer from
Area A to Area C in an Interconnected System
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V. 1.1
Power Oscillations
Periodic variations in generator angle or line angle due to
transmission system disturbances
Oscillations of generator angle or line angle are generally
associated with transmission system disturbances and can
occur due to step changes in load, sudden change of generator
output, transmission line switching, and short circuits.
Depending on the characteristics of the power system, the
oscillations may last for 3 -20 seconds after a severe fault.
Drawn out oscillations that last for a few seconds or more are
usually the result of very light damping in the system and are
pronounced at power transfers that approach the line’s stability
limit. During such angular oscillation period significant cycle
variations in voltages, currents, transmission line flows will take
place. It is important to damp these oscillations as quickly as
possible because they cause mechanical wear in power plants
and many power quality problems. The system is also more
vulnerable if further disturbances occur.
The active power oscillations on a transmission line tend to limit
the amount of power that may be transferred, thus may result in
stability concerns or utilization restrictions on the corridors
between control areas or utility systems. This is due to the fact
that higher power transfers can lead to less damping and thus
more severe and possibly unstable oscillations.
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V. 1.1
Reactive Power Factor
Effects of reactive power on the efficiency of transmission and
distribution
Reactive power is defined as the product of the rms voltage,
current, and the sine of the difference in phase angle between
the two. It is used to describe the effects of a generator, a load,
or other network equipment, which on the average neither
supplies nor consumes power. Synchronous generators,
overhead lines, underground cables, transformers, loads and
compensating devices are the main sources and sinks of
reactive power, which either produce or absorb reactive power
in the systems. To maintain efficient transmission and
distribution, it is necessary to improve the reactive power
balance in a system by controlling the production, absorption,
and flow of reactive power at all levels in the system. By
contrast, inefficient reactive power management can result in
high network losses, equipment overloading, unacceptable
voltage levels, even voltage instability and outages resulting
from voltage collapse. Local reactive power devices for voltage
regulation and power factor correction are also important
especially for balancing the reactive power demand of large and
fluctuating industrial loads.
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V. 1.1
Sags and Swells
Short duration decrease/increase (sag/swell) in supply voltage
A Voltage Sag or Voltage Dig is a decrease in supply voltage of
10% to 90% that lasts in duration from half a cycle to one
minute. A Voltage Swell is an increase in supply voltage of 10%
to 80% for the same duration.
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
distribution supply. Sags can be created by sudden but brief
changes in load such as transformer and motor inrush and short
circuit-type faults. A sag may also be created by a step change
in load followed by a slow response of a voltage regulator. A
voltage swell may occur by the reverse of the above events.
Electronic equipment is usually the main victim of sags, as they
do not contain sufficient internal energy to ‘ride through’ the
disturbance. Electric motors tend to suffer less from voltage
sags, as motor and load inertias will ‘ride through’ the sag if it is
short enough in duration.
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V. 1.1
Unbalanced Load
A load which does not draw balanced current from a balanced
three-phases supply
An unbalanced load is a load which does not draw balanced
current from a balanced three-phase supply.
Typical
unbalanced loads are loads which are connected phase-toneutral and also loads which are connected phase-to-phase.
Such loads are not capable of drawing balanced three-phase
currents. They are usually termed single-phase loads.
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. This unequal voltage
drop leads to unbalanced voltages at delivery points in the
system. Blown fuses on balanced loads such as three-phase
motors or capacitor banks will also create unbalanced voltage in
the same fashion as the single-phase and phase-phase
connected loads. Unbalanced voltage may also arise from
impedance imbalances in the circuits that deliver electricity such
as untransposed overhead transmission lines.
Such
imbalances give the appearance of an unbalanced load to
generation units.
An unbalanced supply may have a disturbing or even
damaging effect on motors, generators, poly-phase converters,
and other equipment. The foremost concern with unbalanced
voltage is overheating in three-phase induction motors. The
percent current imbalance drawn by a motor may be 6 to 10
times the voltage imbalance, creating an increase in losses and
in turn an increase in motor temperature. This condition may
lead to motor failure.
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V. 1.1
Voltage Instability
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. Lack of reactive power may
essentially have two origins: firstly, a gradual increase of power
demand without the reactive part being met in some buses or
secondly, a sudden change in the network topology redirecting
the power flows in such a way that the required reactive power
cannot be delivered to some buses.
The relation between the active power consumed in the
considered area and the corresponding voltages is expressed in
a static way by the P-V curves (also called “nose” curves). The
increased values of loading are accompanied by a decrease in
voltage (except in case of a capacitive load). When the loading
is further increased, the maximum loadability point is reached,
beyond which no additional power can be transmitted to the
load under those conditions. In case of constant power loads
the voltage in the node becomes uncontrollable and decreases
rapidly. This may lead to the partial or complete collapse of a
power system.
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V. 1.1
Factors / Phenomena: Harmonics
Technology / System: Harmonic Filters
Example of application: Reducing harmonics in heavy industry
Harmonic Filters may be used to mitigate, and in some cases, eliminate
problems created by power system harmonics. Non-linear loads such as
rectifiers, converters, home electronic appliances, and electric arc
furnaces cause harmonics giving rise to extra losses in power equipment
such as transformers, motors and capacitors. They can also cause other,
probably more serious problems, when interfering with control systems
and electronic devices. Installing filters near the harmonic sources can
effectively reduce harmonics. For large, easily identifiable sources of
harmonics, conventional filters designed to meet the demands of the
actual application are the most cost efficient means of eliminating
harmonics. These filters consist of capacitor banks with suitable tuning
reactors and damping resistors. For small and medium size loads, active
filters, based on power electronic converters with high switching
frequency, may be a more attractive solution.
Benefits:
•Eliminates harmonics
•Improved Power Factor
•Reduced Transmission Losses
•Increased Transmission Capability
•Improved Voltage Control
•Improved Power Quality
Other applications:
more about Harmonic Filters and Harmonics
•Shunt Capacitors
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V. 1.1
Factors / Phenomena: Reactive Power Factor
Technology / System: Harmonic Filters
Example of application: Regulation of the power factor to increase the
transmission capability and reduce transmission losses as well as
reducing harmonics.
Harmonic Filters produced reactive power as well as mitigate, and in
some cases, eliminate problems created by power system harmonics.
Where the main need is power factor compensation the best solution can
still be a harmonic filter due to the amount of harmonics. Non-linear loads
such as rectifiers, converters, home electronic appliances, and electric arc
furnaces cause harmonics giving rise to extra losses in power equipment
such as transformers, motors and capacitors. They can also cause other,
probably more serious problems, when interfering with control systems
and electronic devices. Installing filters near the harmonic sources can
effectively reduce harmonics. For large, easily identifiable sources of
harmonics, conventional filters designed to meet the demands of the
actual application are the most cost efficient means of eliminating
harmonics as well as producing reactive power. These filters consist of
capacitor banks with suitable tuning reactors and damping resistors. For
small and medium size loads, active filters, based on power electronic
converters with high switching frequency, may be a more attractive
solution.
Benefits:
Improved power factor, Reduced transmission losses, Increased transmission capability
Improved voltage control, Improved power quality, Eliminates harmonics
Other applications:
more about Harmonic Filters and Reactive Power Factor
Shunt capacitors
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V. 1.1
Factors / Phenomena: Asynchronous connection
Technology / System: HVDC and HVDC Light®
Example of application: Interconnection of power systems
It is sometimes difficult or impossible to connect two AC networks due to
stability reasons. In such cases HVDC is the only way to make an
exchange of power between the two networks possible.
Several HVDC links interconnect AC system that are not running in
synchronism with each other. For example the Nordel power system in
Scandinavia is not synchronous with the UCTE grid in western continental
Europe even though the nominal frequencies are the same. And the
power system of eastern USA is not synchronous with that of western
USA. There are also HVDC links between networks with different nominal
frequencies (50 and 60 Hz) in Japan and South America.
Direct current transmissions in the form of classical HVDC or HVDC
Light® are the only efficient means of controlling power flow in a network.
HVDC can therefore never become overloaded. An AC network connected
with neighboring grids through HVDC links may as the worst case loose
the power transmitted over the link, if the neighboring grid goes down - the
HVDC transmission will act as a firewall against cascading disturbances.
Benefits:
•The networks can retain their independence
The Scandinavia - Northern Europe HVDC interconnections
Links:
•HVDC transmission for controllability of power flow
•HVDC transmission for asynchronous connection
•Applications in Power Systems: Interconnection
•ABB HVDC Portal
•An HVDC link can never be overloaded
•HVDC transmission will act as a firewall against cascading disturbances.
more about HVDC & Asynchronous Connection
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V. 1.1
Factors / Phenomena: Bottlenecks
Technology / System: HVDC and HVDC Light®
Example of application: Interconnection of power systems
Bottlenecks may be relieved by the use of an HVDC or HVDC Light link in
parallel with the limiting section of the grid. By using the inherent
controllability of the HVDC system the power system operator can decide
how much power that is transmitted in the AC-link and how much by the
HVDC system.
Longer AC lines tend to have stability constrained capacity limitations as
opposed to the higher thermal constraints of shorter lines. By using the
inherent controllability of an HVDC system in parallel with the long AC
lines, the power system can be stabilized and the transmission limitations
on the AC line can be increased.
Benefits:
•Increased Power Transfer Capability
Links:
•Additional flexibility in Grid Operation
•HVDC transmission for controllability of power flow
•Improved Power and Grid Voltage Control
•Applications in Power Systems: Interconnection
•An HVDC link can never be overloaded!
•ABB HVDC Portal
.
more about HVDC & Bottlenecks
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V. 1.1
Phenomena / Factor: Long lines
Technology / System: HVDC
Example of application: Expressway for power
A HVDC transmission line costs less than an AC line for the same
transmission capacity. However, the terminal stations are more expensive
in the HVDC case due to the fact that they must perform the conversion
from AC to DC and vice versa. But above a certain distance, the so-called
"break-even distance", the HVDC alternative will always give the lowest
cost. Therefore many long overhead lines (> 700 km) particularly from
remote generating stations are built as DC lines.
Benefits:
•Lower investment cost
•Lower losses
•Lower right-of-way requirement for DC lines than for AC lines
Links:
•HVDC does not contribute to the short circuit current
•HVDC transmission for lower investment cost
•HVDC transmission has lower losses
=> Go to Long Submarine Cables
•Applications in Power Systems: Connection of
generation
•ABB HVDC Portal
.
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V. 1.1
Phenomena / Factor: Long submarine cables
Technology / System: HVDC
Example of application: long distance water crossing
In a long AC cable transmission, the reactive power flow due to the large
cable capacitance will limit the maximum possible transmission distance.
With HVDC there is no such limitation, why, for long cable links, HVDC is
the only viable technical alternative. There are HVDC and HVDC Light
cables from 40 km up to 580 km in operation or under construction with
power ratings from 40 to 700 MW.
Benefits:
•Lower investment cost
•Lower losses
Links:
•HVDC submarine cables
•ABB HVDC Portal
.
more about HVDC & Long Submarine Cables
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Factors / Phenomena: Loop Flow
Technology / System: HVDC and HVDC Light
Example of application: Interconnected power systems
Loop Flows, or Parallel Path Flows, may be alleviated by the use of HVDC
or HVDC Light. In interconnected power systems, the actual path taken by
a transaction from one area to another may be quite different from the
designated routes as the result of parallel path admittance, thus diverting
or wheeling power over parallel connections.
The figure shows how parallel path flow can be avoided by replacing an
AC line with a HVDC/HVDC Light link between area A and area C
Benefits:
•HVDC can be controlled to transmit contracted amounts of power and
alleviate unwanted loop flows.
•An HVDC link can alternatively be controlled to minimize total network
losses
•An HVDC link can never be overloaded!
Links:
HVDC transmission for controllability of power flow
·
Applications in Power Systems: Interconnection
ABB HVDC Portal
more about HVDC & Loop Flow
.
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V. 1.1
Factors / Phenomena: Power Oscillations
Technology / System: HVDC and HVDC Light®
Example of application: Steady State and Transient Stability
Improvement
Long AC lines tend to have stability constrained capacity limitations as
opposed to the higher thermal constraints of shorter lines. By using the
inherent controllability of an HVDC system in parallel with the long AC
lines, the power system can be stabilized and the transmission limitations
on the AC line can be increased.
The HVDC damping controller is a standard feature in many HVDC
projects in operation. It normally takes its input from the phase angle
difference in the two converter stations.
Benefits:
•Increased Power Transfer Capability
Links:
HVDC transmission for controllability of power flow
Applications in Power Systems: Interconnection
•Improved Power and Grid Voltage Control
HVDC Light System Interaction Tutorial.
•An HVDC link can never be overloaded!
ABB HVDC Portal
more about HVDC & Power Oscillations
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V. 1.1
Factors / Phenomena: Flicker
Technology / System: MiniCap
Example of application: Installation of a MiniCap to reduce flicker
during large motor starting
Voltage flicker can become a significant problem for power distributors
when large motor loads are introduced in remote locations. Installation of
a series capacitor in the feeder strengthens the network and allows such
load to be connected to existing lines, avoiding more significant
investment in new substations or new distribution lines.
The use of the MiniCap on long distribution feeders provides selfregulated reactive power compensation that efficiently reduces voltage
variations during large motor starting.
Benefits:
•Reduced voltage fluctuations (flicker)
•Improved voltage profile along the line
•Easier starting of large motors
•Self-regulation
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V. 1.1
Factors / Phenomena: Long lines & cables
Technology / System: MiniCap
Example of application: Improved voltage profile of long distribution
lines by adding a MiniCap
The voltage profile on a radial circuit depends on the circuit parameters
and the load characteristics. The voltage profile can be significantly
improved by installing a MiniCap along the line. A typical voltage profile
for a radial circuit with and without a series capacitor is shown below.
Note that the voltage profile curve has a jump at the location of the series
capacitor which represents a large voltage rise downstream of the series
capacitor.
The use of the MiniCap on long distribution feeders provides improved
voltage profile for all loads downstream of the installation.
Benefits:
•Increased power transmission capability through decreased total line
reactance
•Improved voltage profile along the line
•Reduced line losses
more about MiniCap and Long lines & cables
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V. 1.1
Factors / Phenomena: Reactive Power Factor
Technology / System: MiniCap
Example of application: Improved power factor at the utility source
with a MiniCap
The reactive power produced by the series capacitor is proportional to the
capacitor impedance and the line current. With the series capacitor
supplying a significant portion of the reactive power requirements of the
distribution line and of inductive motor loads, much less reactive power is
drawn from the utility source, resulting in a greatly improved power factor
at the sending end of the line.
The use of the MiniCap on a distribution feeder provides self-regulated
reactive power for improved power factor at the utility source.
Benefits:
•Increased power factor at the utility source
•Easier starting of large motors
•Improved voltage regulation and reactive power balance
•Self-regulation
more about MiniCap and Reactive Power Factor
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V. 1.1
Factors / Phenomena: Bottlenecks
Technology / System: PSGuard Wide Area Monitoring System
Example of application: Phase angle monitoring
The phase angle monitoring application facilitates the monitoring of
network stresses caused by heavily loaded lines. It provides operators
with real-time information about voltage phase angle deviations – a crucial
issue e.g. for the successful reclosing of transmission lines.
Its main function is to supply sufficient information to the power system
operator to evaluate the present angle difference between two locations.
Upon detection of an extraordinary status, PSGuard alerts the operator by
giving an early warning or, in critical cases, an emergency alarm.
PSGuard display: Phase angle monitoring with
early warning and emergency alarm
The present version provides monitoring functionality, and its outputs are
intended as mature decision support for operators in taking stabilizing
measures. Actions that the operator may take to improve grid stability
range from generation rescheduling or actions on the reactive power
compensation, blocking of tap changers in the load area and load
shedding in extreme cases.
Benefits:
•Improved system stability, security and reliability
•Safe operation of power carrying components closer to their limits
•Optimized utilization of transmission capacities
•Enhanced operational and planning safety
Other applications:
more about: PSGuard Wide Area Monitoring System
•Line Thermal Monitoring (LTM)
and Bottlenecks
•Voltage Stability Monitoring (VSM)
•Power Oscillation Monitoring (POM)
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Factors / Phenomena: Long lines and cables
Technology / System: PSGuard Wide Area Monitoring System
Example of application: Line thermal monitoring
Loading of power lines or HV cables is in many cases constrained by thermal limits
rather than by voltage instability concerns. A thermal limit of a line is usually set
according to conservative and stabile criteria, i.e. high ambient temperature and calm
air. This yields assumptions of very limited cooling possibilities and thus low loadability.
However, the ambient conditions are often much better in terms of possible cooling and
would allow higher loading of a line with a minimal risk. This can be achieved if an online tool for line temperature assessment is available. One of the algorithms of PSGuard
serves this purpose. However, its functionality and applicability on the real power
systems should be tested in the practice.
The algorithm works as follows
PSGuard display: Line thermal monitoring with early
warning and emergency alarm
•The voltage and current phasors measured at both ends of a line are collected (the
phasors have to be measured at the same instant, which is possible through the GPSsynchronization of the phasor measurement units, PMUs)
•Actual impedance and shunt admittance of a line are computed.
•Resistance of the line/cable is extracted
•Based on the known properties of the conductor material (reference temperature and
dependency coefficient are usually supplied by the manufacturer), the actual average
temperature of the line is determined.
The obtained temperature is an average, not the spot one. The relation between them
shall be verified, i.e. through consideration of the impact of the various weather
conditions along the line at a given time.
PSGuard display: Line temperature pattern computed by PSGuard
Benefits:
•Improved power flow control
more about: PSGuard Wide Area Monitoring System
•Safe operation of power carrying components closer to their limits
and Long Lines & Cables
Other applications:
•Power Oscillation Monitoring (POM)
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Factors / Phenomena: Oscillations
Technology / System: PSGuard Wide Area Monitoring System
Example of application: Power oscillation monitoring
Power oscillation monitoring is the algorithm used for the detection of
power swings in a high voltage power system. The algorithm processes the
selected voltage and current phasor inputs and detects the various power
swing (power oscillation) modes. It quickly identifies the frequency and the
damping of swing modes. The algorithm deploys adaptive Kalman filtering
techniques.
Displayed results
•Damping of the dominant oscillatory mode (time window, i.e. trend display)
•Frequency of the dominant oscillatory mode (time window, i.e. trend display)
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Measurements by PSGuard WAMS: The loss of a power
plant in Spain (1000 MW) initiated Wide Area Oscillations
Measurement by PSGuard
•Amplitude of the oscillation (time window, i.e. trend display)
Optional
•Damping of other oscillatory modes (all in one time window, distinguished by different
colors)
•Frequencies of other oscillatory modes (all in one time window, distinguished by different
colours
Alarms
When the damping of any oscillation mode decreases to below a predefined value (in two
steps, first is alert, the second emergency alarm)
Read more
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Factors / Phenomena: Oscillations
Technology / System: PSGuard Wide Area Monitoring System
Example of application: Power oscillation monitoring
Benefits:
•Increased power transfer
•Enhanced security
Short-term operation benefits:
Example: Estimation of relative frequency and damping
•Immediate awareness of the power system state in terms of the presence of oscillations,
thus an operator sees the urgency of the situation
•Indication of the frequency of an oscillation which may then be associated with the known
existing mode of the power system, i.e. the operator may distinguish if a local or inter-area
mode is excited
Long-term benefits:
•With the help of the stored data, long-term statistics can be collected and, based on their
evaluation, the system reinforcements can be performed (such as retuning of Power
System Stabilizers (PSS) to damp the frequencies appearing most often as dangerous
ones).
more about: PSGuard Wide Area Monitoring System
and Power Oscillations
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Factors / Phenomena: Voltage instability
Technology / System: PSGuard Wide Area Monitoring System
Example of application: Voltage stability monitoring
The voltage stability monitoring application facilitates the monitoring of the
grid’s dynamic behavior and provides stability calculations for steady state
situations as well as stability predictions in contingency cases. It builds on
and extends the basic functionality of PSG830 with functions related to the
monitoring of voltage stability for a transmission line / corridor.
It’s main function is to provide the operator of the power system with
sufficient information to evaluate the present power margin with respect to
voltage stability, that is, the amount of additional active power that can be
transported on a transmission corridor without jeopardizing the voltage
stability. The present version provides monitoring functionality, and its
outputs are intended as mature decision support for operators in taking
optimizing resp. stabilizing measures. Actions that the operator may take to
improve voltage stability range from generation rescheduling or actions on
the reactive compensation, blocking of tap changers in the load area and to
load shedding in extreme cases.
PSGuard display: Voltage stability monitoring P-V Curve
Applied directly, the application is assigned to a single line or cable.
However, on a case-by-case basis, the method can be applied also to
transmission corridors with more complex topologies.
Benefits:
•Improved system stability, security and reliability
•Reduced cost and greater functionality of Protection & Control systems
more about: PSGuard Wide Area Monitoring System
and Voltage Instability
•Safe operation of power carrying components closer to their limits
•Optimized utilization of transmission capacities
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Factors / Phenomena: Bottlenecks
Technology / System: Series Compensation
Example of application: Transient Stability Improvement
Bottlenecks may be relieved by the use of Series Compensation. Longer
lines tend to have stability-constrained capacity limitations as opposed to
the higher thermal constraints of shorter lines. Series Compensation has
the net effect of reducing transmission line series reactance, thus
effectively reducing the line length. Series Compensation also offers
additional power transfer capability for some thermal-constrained
bottlenecks by balancing the loads among the parallel lines. Figure shows
a two-area interconnected system where the power transfer from area A to
area B is limited to 1500MW due to stability constraints. Additional
electricity can be delivered from area A to area B if Series Compensation is
applied to increase the maximum stability limits.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Grid Voltage Control
Other applications:
•Power Flow Control
more about Series Compensation and Bottlenecks
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Factors / Phenomena: Loop Flows
Technology / System: Series Compensation
Example of application: Power Flow Control
Loop Flows, or Parallel Path Flows, may be alleviated by the use of Series
Compensation. In interconnected power systems, the actual path taken by
a transaction from one area to another may be quite different from the
designated routes as the result of parallel path admittance, thus diverting or
wheeling power over parallel connections.
Figure shows parallel path flow alleviation by the use of Series
Compensation. With a reduction in the direct interconnection impedance
between area A and area C, the Parallel Path Flow which is routed through
area B is decreased.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Lower Transmission Losses
•Improved Transient Stability
•Improved Grid Voltage Control
Other applications:
•Transient Stability Improvement
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Factors / Phenomena: Reactive Power Factor
Technology / System: Shunt Capacitor
Example of application:
Regulation of the power factor to increase the transmission capability and
reduce transmission losses
Shunt capacitors are primarily used to improve the power factor in
transmission and distribution networks, resulting in improved voltage
regulation, reduced network losses, and efficient capacity utilization. Figure
shows a plot of terminal voltage versus line loading for a system that has a
shunt capacitor installed at the load bus. Improved transmission voltage
regulation can be obtained during heave power transfer conditions when
the system consumes a large amount of reactive power that must be
replaced by compensation. At the line surge impedance loading level, the
shunt capacitor would decrease the line losses by more than 35%. In
distribution and industrial systems, it is common to use shunt capacitors to
compensate for the highly inductive loads, thus achieving reduced delivery
system losses and network voltage drop.
Benefits:
•Improved power factor
•Reduced transmission losses
•Increased transmission capability
•Improved voltage control
•Improved power quality
more about Shunt Capacitor and Reactive Power
Factor
Other applications:
•Harmonic Filters
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Factors / Phenomena: Voltage instability
Technology / System: Shunt Reactor
Example of application: Extra/Ultra High Voltage air insulated
transmission line and cable line voltage stability
The primary purpose of the shunt reactor is to compensate for capacitive
charging voltage, a phenomenon getting more prominent for increasing line
voltage. Long high-voltage transmission lines and relatively short cable
lines (since a power cable has high capacitance to earth) generate a large
amount of reactive power during light power transfer conditions which must
be absorbed by compensation. Otherwise, the receiving terminals of the
transmission lines will exhibit a “voltage rise” characteristic and many types
of power equipment cannot withstand such overvoltages. Figure shows at
top level voltage at the receiving end when transmission line is loaded with
rated power. Then shunt reactor is not needed. Next figure shows a voltage
increase when line is lightly loaded and bottom figure shows what happens
when a shunt reactor is connected. The voltage stability is kept due to the
inductive compensation from the reactor.
A better fine tuning of the reactive power can be made by the use of a tap
changer in the shunt reactor. It can be possible to vary the reactive power
between 50 to 100 % of the needed power.
Benefits:
•Simple and robust customer solution with low installation costs and
minimum maintenance
•No losses from an intermediate transformer when feeding reactive more about Shunt Capacitor and Voltage Instability
compensation from a lower voltage level.
•No harmonics created which may require filter banks.
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Factors / Phenomena: Bottlenecks
Technology / System: Static Var Compensator (SVC)
Example of application: Grid Voltage Support
Static Var Compensators are used in transmission and distribution
networks mainly providing dynamic voltage support in response to system
disturbances and balancing the reactive power demand of large and
fluctuating industrial loads. A Static Var Compensator is capable of both
generating and absorbing variable reactive power continuously as opposed
to discrete values of fixed and switched shunt capacitors or reactors.
Further improved system steady state performance can be obtained from
SVC applications. With continuously variable reactive power supply, the
voltage at the SVC bus may be maintained smoothly over a wide range of
active power transfers or system loading conditions. This entails the
reduction of network losses and provision of adequate power quality to the
electric energy end-users.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Grid Voltage Stability
•Improved Grid Voltage Control
•Improved Power Factor
Other applications:
more about Static Var Compensator and Bottlenecks
•Power Oscillation Damping
•Power Quality (Flicker Mitigation, Voltage Balancing)
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Factors / Phenomena: Power Oscillations
Technology / System: Static Var Compensator (SVC)
Example of application: Power Oscillation Damping
Static Var Compensators are mainly used to perform voltage and reactive
power regulation. However, when properly placed and controlled, SVCs
can also effectively counteract system oscillations. A SVC, in effect, has
the ability to increase the damping factor (typically by 1-2 MW per Mvar
installed) on a bulk power system which is experiencing power oscillations.
It does so by effectively modulating its reactive power output such that the
regulated SVC bus voltage would increase the system damping capability.
Figure shows power oscillation prompted by a disturbance on a
transmission system. The uncompensated system undergoes substantial
oscillations following the disturbance while the same system with SVC
experiences much improved response.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Dynamic Stability
Other applications:
•Power Quality (Flicker Mitigation, Voltage Balancing)
•Grid Voltage Support
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Power Oscillations
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Factors / Phenomena: Voltage instability
Technology / System: Static Var Compensator (SVC)
Example of application: Grid Voltage Support
Static Var Compensators are used in transmission and distribution
networks mainly providing dynamic voltage support in response to system
disturbances and balancing the reactive power demand of large and
fluctuating industrial loads. A Static Var Compensator is capable of both
generating and absorbing variable reactive power continuously as opposed
to discrete values of fixed and switched shunt capacitors or reactors.
Further improved system steady state performance can be obtained from
SVC applications. With continuously variable reactive power supply, the
voltage at the SVC bus may be maintained smoothly over a wide range of
active power transfers or system loading conditions. This entails the
reduction of network losses and provision of adequate power quality to the
electric energy end-users.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Grid Voltage Stability
•Improved Grid Voltage Control
•Improved Power Factor
Other applications:
more about Static Var Compensator and
•Power Oscillation Damping
Voltage Instability
•Power Quality (Flicker Mitigation, Voltage Balancing)
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V. 1.1
Factors / Phenomena: Flicker
Technology / System: SVC (Industry)
Example of
Mitigation
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application:
Power
Quality
Improvement,
Flicker
SVC is used most frequently for compensation of disturbances generated
by the Electrical Arc Furnaces (EAF). With a well-designed SVC,
disturbances such as flicker from the EAF are mitigated. Figure shows the
flicker mitigation effect of a SVC installed at a steel making plant.
Flicker, the random variation in light intensity from incandescent lamps
caused by the operating of nearby fluctuating loads on the common electric
supply grid, is highly irritating for those affected. The random voltage
variations can also be disturbing to other process equipment fed from the
same grid. The proper mitigation of flicker is therefore a matter of power
quality improvement as well as an improvement to human environment.
Benefits:
•Reduced Flicker
•Harmonic Filtering
•Voltage Balancing
•Power Factor Correction
•Furnace/mill Process Productivity Improvement
Other applications:
•General Reactive Power Compensation at Steelworks
more about SVC (Industry) and Flicker
•Grid Voltage Support
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V. 1.1
Factors / Phenomena: Reactive Power Factor
Technology / System: SVC (Industry)
Example of application: Reactive Power Compensation at Steelworks
Static Var Compensators provide dynamic voltage support to balance the
reactive power demand of large and fluctuating industrial loads. A Static
Var Compensator is capable of both generating and absorbing variable
reactive power continuously as opposed to discrete values of fixed and
switched shunt capacitors or reactors. With continuously variable reactive
power supply, the voltage at the SVC bus may be maintained smoothly
over a wide range of operating conditions. This entails the improved power
factor and sufficient power quality, leading to better process and production
economy.
Benefits:
•Power Factor Correction
•Furnace/mill Process Productivity Improvement
•Harmonic Filtering
Other applications:
•Power Quality Improvement, Flicker mitigation
•Power Quality Improvement, Voltage Balancing
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Reactive Power Factor
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Factors / Phenomena: Unbalanced Load
Technology / System: SVC (Industry)
Example of application: Railway Feeder connected to the Public Grid
The traction system is a major source of unbalanced loads. Electrification
of railways, as an economically attractive and environmentally friendly
investment in infrastructure, has introduced an unbalanced and heavy
distorted load on the three-phase transmission grid. Without compensation,
this would result in significant unbalanced voltage affecting most
neighboring utility customers.
The SVC can elegantly be used to
counteract the unbalances and mitigate the harmonics such that the power
quality within the transmission grid is not impaired. Figure shows a typical
traction substation arrangement with a load balancer (an asymmetrically
controlled SVC). The load balancer transfers active power between the
phases such that the balanced voltage can be created (seen from the grid).
Benefits:
•Voltage Balancing
•Harmonic Filtering
•Power Factor Correction
Other applications:
•Power Quality Improvement, Flicker Mitigation
•Grid Voltage Support
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Unbalanced Load
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Factors / Phenomena: Bottlenecks
Technology / System: STATCOM®
Example of application: Grid Voltage Support
STATCOM, when connected to the grid, can provide dynamic voltage
support in response to system disturbances and balance the reactive
power demand of large and fluctuating industrial loads. A STATCOM is
capable of both generating and absorbing variable reactive power
continuously as opposed to discrete values of fixed and switched shunt
capacitors or reactors. With continuously variable reactive power supply,
the voltage at the STATCOM bus may be maintained smoothly over a wide
range of system operation conditions. This entails the reduction of network
losses and provision of sufficient power quality to the electric energy endusers.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Grid Voltage Stability
•Improved Grid Voltage Control
•Improved Power Factor
Other applications:
•Power Quality (Flicker Mitigation, Voltage Balancing)
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Factors / Phenomena: Flicker
Technology / System: STATCOM®
Example of
mitigation
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application:
Power
Quality
Improvement,
flicker
STATCOM® is an effective method used to attack the problem of flicker.
The unbalanced, erratic nature of an electric arc furnace (EAF) causes
significant fluctuating reactive power demand, which ultimately results in
irritating electric lamp flicker to neighboring utility customers. In order to
stabilize voltage and reduce disturbing flicker successfully, it is necessary
to continuously measure and compensate rapid changes by means of
extremely fast reactive power compensation. STATCOM® uses voltage
source converters to improve furnace productivity similar to a traditional
SVC while offering superior voltage flicker mitigation due to fast response
time. Figure shows the flicker mitigation effect of an STATCOM® installed at
a steel making plant.
Benefits:
•Eliminated Flicker
•Harmonic Filtering
•Voltage Balancing
•Power Factor Correction
•Furnace/mill Process Productivity Improvement
Other applications:
more about STATCOM and Flicker
•Grid Voltage Support
•Power Quality Improvement
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Factors / Phenomena: Unbalanced Load
Technology / System: STATCOM®
Example of application: Railway Feeder connected to the Public Grid
Modern electric rail system is a major source of unbalanced loads.
Electrification of railways, as an economically attractive and
environmentally friendly investment in infrastructure, has introduced an
unbalanced and heavy distorted load on the three-phase transmission grid.
Without compensation, this would result in significant unbalanced voltage
affecting most neighboring utility customers.
Similar to SVC, the
STATCOM can elegantly be used to restore voltage and current balance in
the grid, and to mitigate voltage fluctuations generated by the traction
loads. Figure shows a conceptual diagram of STATCOM application for
dynamic load balancing for traction.
Benefits:
•Voltage Balancing
•Harmonic Filtering
•Power Factor Correction
Other applications:
•Power Quality Improvement, Flicker Mitigation
•Grid Voltage Support
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Factors / Phenomena: Voltage instability
Technology / System: STATCOM®
Example of application: Grid Voltage Support
STATCOM, when connected to the grid, can provide dynamic voltage
support in response to system disturbances and balance the reactive
power demand of large and fluctuating industrial loads. A STATCOM is
capable of both generating and absorbing variable reactive power
continuously as opposed to discrete values of fixed and switched shunt
capacitors or reactors. With continuously variable reactive power supply,
the voltage at the STATCOM bus may be maintained smoothly over a wide
range of system operation conditions. This entails the reduction of network
losses and provision of sufficient power quality to the electric energy endusers.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Grid Voltage Stability
•Improved Grid Voltage Control
•Improved Power Factor
Other applications:
•Power Quality (Flicker Mitigation, Voltage Balancing)
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V. 1.1
Factors / Phenomena: Bottlenecks
Technology / System: TCSC
Example of application: Transient Stability Improvement
Bottlenecks may be effectively relieved by the use of entirely or partially
thyristor controlled series compensation. As with conventional SC
technology, TCSC can improve stability of power transmission, reactive
power balance, and load sharing between parallel lines, thus mitigating the
impact of transmission bottlenecks. Figure shows a two-area
interconnected system where the power transfer from area A to area B is
limited to 1500MW due to stability constraints. Additional electricity can be
delivered from area A to area B if series compensation is applied to
increase the maximum stability limits. High degree of series compensation
level is permitted with the controlled series compensation achieving further
improved transmission capacity utilization.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improve Dynamic Stability
•Improved Grid Voltage Control
•Immunity against Subsynchronous Resonance
Other applications:
•Power Oscillation Damping
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•Subsynchronous Resonance Mitigation
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V. 1.1
Factors / Phenomena: Loop Flows
Technology / System: TCSC
Example of application: Power Flow Control
Loop Flows, or Parallel Path Flows, may be effectively alleviated by the use
of entirely or partially thyristor controlled series compensation.
In interconnected power systems, the actual path taken by a transaction
from one area to another may be quite different from the designated routes
as the result of parallel path admittance, thus diverting or wheeling power
over parallel connections. Controlled series compensation is a useful
means for directing power flows along contracted paths under various
loading and network configurations. Figure shows parallel path flow
alleviation by the use of controlled series compensation. With a reduction
in the direct interconnection impedance between area A and area C, the
Parallel Path Flow which is routed through area B is decreased.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Lower Transmission Losses
•Improved Transient Stability
•Improved Grid Voltage Control
Other applications:
•Power Oscillation Damping
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•Subsynchronous Resonance Mitigation
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V. 1.1
Factors / Phenomena: Oscillations
Technology / System: TCSC
Example of application: Power Oscillation Damping
Thyristor Controlled Series Capacitors may be used to damp bulk power
system oscillations. A TCSC, in effect, has the ability to increase the
damping torque (or damping power) on a bulk power system which is
experiencing angular oscillations between the two terminals of the
compensated transmission line. It does so by effectively modulating the
amount of power that flows through the line. When an angular increase
occurs between the two terminals of a line during an oscillation, the TCSC
will increase power flow in order to oppose the increase in angle; likewise,
the TCSC will decrease power flow through the line during the angular
decrease portion of the oscillation cycle. Figure shows angular oscillation
prompted by a temporary short circuit on a transmission system. The
uncompensated system undergoes substantial oscillations following the
short circuit while the same system with TCSC experiences much improved
response.
Benefits:
•Increased Power Transfer Capability
•Additional flexibility in Grid Operation
•Improved Transient Stability
•Improved Grid Voltage Control
•Immunity against Subsynchronous Resonance
Other applications:
more about TCSC and Power Oscillations
•Transient Stability Improvement
•Interconnections between grids
•Subsynchronous Resonance Mitigation
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