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2. European interconnected power grids.
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.
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.
European interconnected power grids.
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3. 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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4. 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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5. 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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6. Occur when the supply voltage drops below 10% of the nominal value
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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7. Another factor to consider is the land use
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.
Go to Long Cables
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8. 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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9. 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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10. 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 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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11. 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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12. Short duration decrease/increase (sag/swell) in supply voltage
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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13. 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-to-neutral 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 three-phase 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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14. 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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15. 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:
Shunt Capacitors
more about Harmonic Filters and Harmonics
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16. 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:
Shunt capacitors
more about Harmonic Filters and Reactive Power Factor
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17. The Scandinavia - Northern Europe HVDC interconnections
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
An HVDC link can never be overloaded
HVDC transmission will act as a firewall against cascading disturbances.
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
more about HVDC & Asynchronous Connection
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18. 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
Additional flexibility in Grid Operation
Improved Power and Grid Voltage Control
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 & Bottlenecks
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19. 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
HVDC does not contribute to the short circuit current
=> Go to Long Submarine Cables .
Links:
HVDC transmission for lower investment cost
HVDC transmission has lower losses
Applications in Power Systems: Connection of generation
ABB HVDC Portal
more about HVDC & Long Lines
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20. 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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21. 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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22. 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
Improved Power and Grid Voltage Control
An HVDC link can never be overloaded! .
Links:
HVDC transmission for controllability of power flow
Applications in Power Systems: Interconnection
HVDC Light System Interaction Tutorial.
ABB HVDC Portal
more about HVDC & Power Oscillations
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23. 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 self-regulated 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
more about MiniCap and Flicker
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24. 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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25. 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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26. 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.
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:
Line Thermal Monitoring (LTM)
Voltage Stability Monitoring (VSM)
Power Oscillation Monitoring (POM)
PSGuard display: Phase angle monitoring with early warning and emergency alarm
more about: PSGuard Wide Area Monitoring System
and Bottlenecks
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27. PSGuard display: Line temperature pattern computed by PSGuard
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 on-line 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
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 GPS-synchronization 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.
Benefits:
Improved power flow control
Safe operation of power carrying components closer to their limits
Other applications:
Power Oscillation Monitoring (POM)
PSGuard display: Line thermal monitoring with early warning and emergency alarm
PSGuard display: Line temperature pattern computed by PSGuard
more about: PSGuard Wide Area Monitoring System
and Long Lines & Cables
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28. 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)
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
Measurements by PSGuard WAMS: The loss of a power plant in Spain (1000 MW) initiated Wide Area Oscillations Measurement by PSGuard
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29. Example: Estimation of relative frequency and damping
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:
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).
Example: Estimation of relative frequency and damping
more about: PSGuard Wide Area Monitoring System
and Power Oscillations
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30. PSGuard display: Voltage stability monitoring P-V Curve
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.
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
Safe operation of power carrying components closer to their limits
Optimized utilization of transmission capacities
PSGuard display: Voltage stability monitoring P-V Curve
more about: PSGuard Wide Area Monitoring System
and Voltage Instability
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31. 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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32. 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
more about Series Compensation and Loop Flows
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33. 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
Other applications:
Harmonic Filters
more about Shunt Capacitor and Reactive Power Factor
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34. 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 compensation from a lower voltage level.
No harmonics created which may require filter banks.
more about Shunt Capacitor and Voltage Instability
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35. 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:
Power Oscillation Damping
Power Quality (Flicker Mitigation, Voltage Balancing)
more about Static Var Compensator and Bottlenecks
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36. 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
more about Static Var Compensator and
Power Oscillations
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37. 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:
Power Oscillation Damping
Power Quality (Flicker Mitigation, Voltage Balancing)
more about Static Var Compensator and
Voltage Instability
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38. Factors / Phenomena: Flicker Technology / System: SVC (Industry)
Example of application: Power Quality Improvement, Flicker Mitigation
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
Grid Voltage Support
more about SVC (Industry) and Flicker
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39. 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
more about SVC (Industry) and
Reactive Power Factor
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40. 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
more about SVC (Industry) and
Unbalanced Load
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41. 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 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:
Power Quality (Flicker Mitigation, Voltage Balancing)
more about STATCOM and Bottlenecks
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42. Factors / Phenomena: Flicker Technology / System: STATCOM®
Example of application: Power Quality Improvement, flicker mitigation
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:
Grid Voltage Support
Power Quality Improvement
more about STATCOM and Flicker
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43. 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
more about STATCOM and Unbalanced Load
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44. 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 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:
Power Quality (Flicker Mitigation, Voltage Balancing)
more about STATCOM and Voltage Instability
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45. 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
Subsynchronous Resonance Mitigation
more about TCSC and Bottlenecks
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46. 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
Subsynchronous Resonance Mitigation
more about TCSC and Loop Flow
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47. 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:
Transient Stability Improvement
Interconnections between grids
Subsynchronous Resonance Mitigation
more about TCSC and Power Oscillations
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