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There are, however, significant differences between interrupting AC and DC that cannot be overcome by scaling or adapting existing AC solutions. ABB has overcome these challenges with a hybrid breaker hybrid because it uses both conventional switching technology and semiconductors. This technology has recently been recognized by the MIT Technology Review as one of top 10 innovations of The hybrid breaker is the topic of the lead article of this edition of ABB Review.
While on the topic of switches, this edition presents a variety of other switching articles, spanning different applications and power ranges from high-voltage AC to motor control, and even ABBs new semiconductor device: All HVDC lines realized so far have been point-to-point links.
The scope of application of the technology could be greatly increased if lines could be built with more than two terminals, enabling them to develop into HVDC grids. How-ever, the absence of a suitable breaker for the required voltages and speeds and with acceptable losses has hitherto prevented the advent of such topologies for HVDC.
Title pictureABBs hybrid circuit breaker is one of the greatest innovations in the companys history. Finally DC grids can become a reality. An HVDC grid is shown in 1a. A circuit with a mechanical HVDC breaker and an arrester is shown in 1b, and the tran-sients that occur during breaking in 1c. The current starts to rise when the fault occurs the rate at which it rises is deter-mined by the inductance of the line reac-tor. When the switch opens, the current is commutated to the arrester and starts to decrease.
The fault current in the arrester bank establishes a counter volt-age, and this reduces the fault current to zero by dissipating the energy stored both in the HVDC reactor and in the fault current path. The total time to clear the fault consists of: Both time intervals are important consid-erations in the design and cost of the HVDC breaker, as well as that of the line reactor.
The breaking time is governed by the response time of the protection and the action time of the HVDC switch. A short-circuit fault typically has to be cleared within 5 ms in order to not affect converter stations as far away as km. Because converter stations typi-cally rely on the DC voltage being at least 80 percent of its nominal value to assure normal operation, faults must be cleared within milliseconds.
A purely mechanical HVDC breaker can clear a line within several tens of millisec-onds, but this is too slow to fulfill the re-quirements of a reliable HVDC grid .
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Nevertheless, mechanical breakers are used for such purposes as the extin-guishing of fault currents. Further draw-backs of mechanical breakers are that they require additional components to generate the current zero crossing so that the current can cease to flow. C ompared to high-voltage AC grids, active power conduc-tion losses on HVDC lines are relatively low and losses related to reactive power are zero, mak-ing HVDC grids an attractive proposition for transmission over long distances  a topic that is of especial interest in view of the rapid growth in generation from renewables.
But the hybrid breaker does not need to await the full-scale emergence of HVDC grids to come into its own. Many present transmission proposals involve point-to-point HVDC links and hybrid breakers have a part to play here too.
Besides converting power, HVDC converter sta-tions can simultaneously contribute to the AC networks stability through reac-tive-power control. If the converter can be rapidly disconnected from the HVDC line in case of a fault, the converter station can go directly into stand-alone operation as a static compensation unit STATCOMand so continue to support the AC networks stability.
ABB Review 2|13
Technical demands on HVDC breakers are high. The time permitted to interrupt a current flow is shorter than for a com-parable AC application due to the lower impedance of the lines meaning the volt-age drop caused by a fault can spread A shorter fault clearance time implies reduced requirements for power dissipation in the arrester bank, but requires a higher voltage capability of the arrester. This also increases the energy handled by the arrester and cor-respondingly leads to a higher cost for the HVDC breaker.
It is therefore impor-tant to keep the breaking time as short as possible. When the breaking time and the maximum breaking current capability are given, the only remaining adjustable parameter is the inductance of the HVDC reactor which governs the rate of current rise. The time allowed for fault clearance will affect the required voltage capability of the arrester as well as that of pole volt-age protection.
A shorter fault clearance time implies reduced requirements for power dissipation in the arrester bank, but requires a higher voltage capability of the arrester. This spells a higher pole-to-pole voltage rating, and thus adds to the costs of the HVDC breaker. The following example provides a general impression of the relationship between the parameters mentioned.
For a given rated line current Besides converting power, HVDC converter stations can simultaneously contribute to the AC networks stability through reactive power control. This branch consists of a semiconductor-based load commutation switch 2c connected in series with a fast mechanical discon-nector 2b.
During normal operation, the current only flows through the bypass 2a.
With the branch 2a no longer carrying current, the disconnector 2b opens, thus protecting the load commutation switch 2c from the primary voltage that builds up across the main HVDC breaker. The required voltage rating of the load commutation switch is thus significantly reduced in comparison to a component that remains in the main current path throughout the switching cycle.
On account of this reduced load-blocking voltage, the on-state voltage of the load commutation switch is typically in the range of several volts only.
The on-state losses of the hybrid HVDC breaker are thus reduced to a percentage of the losses incurred by a pure semiconductor breaker, ie, 0. This avoids major disturbances in the HVDC grid, and keeps the required current-breaking capability of the back-up breaker at reasonable values. Prototype designThe hybrid HVDC breaker prototype is designed to achieve a current breaking capability of 9.
The maximum cur-rent breaking ca-pability is indepen-dent of the current rating, depending on the design of the main HVDC breaker only. The fast disconnector and main HVDC breaker are designed for switching voltages exceeding 1. The main HVDC breaker 2d consists of several HVDC breaker cells with individu-al arrester banks limiting the maximum voltage across each cell 2e to a spe-cific level during current breaking.
Two stacks are required to break the current in either current direction. The maximum duration of the current limiting mode depends on the energy dissipation capa-bility of the arrester banks 3d.
Over-currents on the line or higher-level switchyard protection can activate the current transfer from the bypass into the main HVDC breaker or possible backup breakers prior to the trip signal of the backup protection. In case of a breaker failure, backup break-ers can be activated almost instanta-The main semiconductor-based HVDC breaker 2d is separated into several sec-tions with individual arrester banks 2f dimensioned for full voltage and current breaking capability.
After fault clearance, a disconnecting circuit breaker 2g inter-rupts the residual current and isolates the faulty line from the HVDC grid to pro-tect the arrester banks from thermal over-load. The mechanical switch 2b opens at zero current and with low voltage stress, and can thus be realized as a disconnec-tor with a lightweight contact system. The fast disconnector will not be ex-posed to the maximum pole-to-pole volt-age defined by the protective level of the arrester banks until after having reached the open position.
Thomson drives  result in fast opening times and a com-pact disconnector design using SF6 as insulating medium. Proactive control of the hybrid HVDC breaker allows it to compensate for the time delay of the fast disconnector if the opening time of the disconnector is less than the time required for selective pro-tection. Proactive current commutation is initiated by the hybrid HVDC breakers built-in overcurrent protection as soon as the HVDC line current exceeds a cer-tain overcurrent level 3a.
The main HVDC breaker delays current breaking until a trip signal is received or the faulty line current is close to the maximum breaking current capability of the main HVDC breaker 3b. Application of press pack IGBTs with 4. For the design of the load commutation switch 2c, one IGBT HVDC breaker module for each current direction is suffi-cient to fulfill the requirements of the volt-age rating.
The mechanical switch opens at zero current and with low voltage stress, and can thus be realized as a disconnector with a lightweight contact system. A cooling system is required due to the switchs continuous exposure to the line current. Test resultsA scaled-down prototype of the main breaker cell with three series connected IGBT modules and a common arrester bank was used to verify the current-breaking capability of 4.
A fourth IGBT module was connected in the opposite primary current direction to verify the functionality of the incorporated anti-parallel diode. Discharge of a capaci-tor bank by a thyristor switch, limited only by a minor DC reactor, represented pole-to-ground faults in the HVDC grid.
The maximum current breaking capabil-ity of the IGBT HVDC breaker cell is determined by the saturation current of the IGBT modules 6 rather than the safe operation area as is typical in voltage source converter applica-tions. Zero voltage switching reduces the instantaneous switching losses and ensures equal voltage distribution independent of the Snubber current DC breaker voltage DC breaker current Position voltageVoltageincreaseNosaturationVoltage kV or current kA Voltage kV or current kA Time s Time s Position failureProper 12 ABB review 2 13A typical test result is shown in 8.
A maximum breaking current of over 9 kA is verified. Furthermore, equal volt-age distribution with a maximum voltage drop of 3. Test resultsThe main breaker test setup was ex-panded to verify the complete hybrid HVDC breaker concept. A second capacitor bank and large reactors were installed to limit the rate of line current rise to typical HVDC grid values. The ultra-fast disconnector and load commu-tation switch are included in the system configuration.
Since only one of the IGBT modules failed during the test, the fault could still have been cleared by the two other modules. Due to the high voltage level, the second test setup required significantly more space.
The test circuit for the hybrid HVDC breaker concept is shown in 7. The short-circuit fault was initiated by the triggered spark gap Q5. Successful verifi-cation testing at device and compo-nent level demon-strated the per-formance of the components. Introduction of bimode insulated gate transistor BIGT technolo-gies will double the current breaking capability of press-pack modules.
Successful verification testing at device and component level demonstrated the performance of the components. A breaking event with a peak current of 9 kA and 2 ms delay time for opening the ultra-fast discon-nector in the branch parallel to the main breaker is shown in 9.
The maximum rated fault current of 9 kA is the limit for the existing generation of semiconduc-tors. The next generation of semicon-ductor devices will allow breaking per-formance of up to 16 kA. The purpose of the tests was to verify switching perfor-mance of the power-electronic parts, and the opening speed of the mechani-cal ultra-fast disconnector.
The assem-bly under test consisted of one 80 kV unidirectional main breaker cell, along with the ultra-fast disconnector and load commutation switch. The higher voltage rating is accomplished by con-necting several main breaker cells in series.
Tests have not only been carried out for normal breaking events, but also for situations with failed components in the breaker. OutlookIntroduction of bimode insulated gate transistor BIGT technologies  incor-porating the functionality of the reverse conducting diode on the IGBT chips will double the current breaking capability of existing presspack modules see also The two-in-one chip on pages of this edition of ABB Review.
The next step is to test such a breaker in a real HVDC transmission line. In particular, operating voltages are being increased, mostly to minimize transportation losses.
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This places higher demands on the critical elements that control and protect these networks the circuit breakers. At the heart of the circuit breaker lies the interrupter the chamber where the switching physically takes place.
The changing technical and market conditions, as well as new international standards, have brought about the need to develop a new generation of interrupter. A circuit breaker with the capacity to switch 15 large power plantsBreaking new ground 15Breaking new groundAdditional requirements were: Small bay size it should be possible to put a complete bay into a standard container Full-short line fault switching capabil-ity without needing a line-to-ground capacitor Reduction in SF6 gas volume Lowest possible reaction forces impact on buildings and foundations Small, standard drive Two-cycle interrupt timeCircuit breakers A circuit breaker is a remarkable piece of equipment.
It has to cope with a range of currents from 1 A up to several tens of kA; it has to withstand a large range of voltage scenarios, eg, very fast voltage rises and long-term AC stresses; it must perform mundane daily switching op-erations as well as emergency inter-ruption of short-circuit currents; it may be inactive for a long period but must then be capable of emergency interruption of faults within a few milli-seconds.
Designing a new breakerMany very different factors need to be considered when designing a new switching device and deciding on a new technology. T he networks that keep crucial electrical power flowing to so-ciety are being run at ever-higher voltages to minimize transportation losses and reduce envi-ronmental impact. This higher voltage, and other demands, means that a key element for the protection and control of power networks, the circuit breaker, also has to evolve.
Of critical importance is the availability of the circuit breaker, as this directly impacts the reliability of the electrical network itself. Reduced breaker component count and low operating energy lead to lower risk of unexpected outages.
Addition-ally, if the size of the breaker can be reduced, cost and space requirements will also fall. With this in mind, ABB began develop-ment of a new, single-chamber breaker for kV networks. Since both the nominal and the short-circuit currents that are to be handled are expected to increase in the future, a rated nominal current of 5 kA and a rated short-circuit current of 63 kA based on 50 Hz and 60 Hz were targeted.
Title picture Power lines at ever-higher voltages are driving new developments in high-voltage technology. How do the latest circuit breakers deal with the new challenges? Capacitive switching capabilityThis duty is characterized by relatively small currents but high voltages across circuit breaker contacts, so a high dynamic voltage withstand capability is required.
The voltage withstand capabil-ity needs to be greater than the rising network voltage during the opening operation of the circuit breaker. This is best characterized as a race between the opening contacts and the transient voltage buildup. It is vital that the breaker wins this race since no voltage break-downs can occur as these can lead to a voltage escalation that stresses substa-tion components and overhead lines. In other words, this new breaker has to have a high contact speed so that a high dielectric withstand capability is reached in a very short time.
Additionally, smaller breakers reduce cost and real-estate requirements. Exhaust volumemoving contact sideExhaustshieldsTankArcing zoneExhaust volumefixed contact side 16 ABB review 2 13In international standards, this aspect is covered by a very detailed test proce-dure and an extensive test program.
Full-short line fault interrupting capabilityThis requires a high gas pressure in the volume between the breaker contacts in order to provide enough cooling power to quench the arc so interruption will be successful. This pressure buildup is one key value for fast fault-clearing capability. A single-chamber interrupter designed for high short-circuit interrupt-ing capability requires a high clearing pressure. Terminal fault interrupting capabilitySince one of the requirements is to stay within a two-cycle interrupting time, a short opening time is required, which leads to higher asymmetrical require-ments than for earlier breakers.
Interrupt-ing at high asymmetry levels leads to high-pressure buildups that must be han-dled by the drive as well as the exhaust and nozzle system. For this new breaker, this means that high energy inputs into the arcing zone as well as the exhaust system need to be safely handled. Transformer-limited fault requirementsThis special requirement, which has to be met at some locations, comes up when a fraction 7 to 30 percent of the rated short-circuit current is present together with a very high rate of rise of the recovery voltage the voltage that appears across the terminals after cur-rent interruption.
In order to with-stand such severe stress, it is neces-sary to build up a high dynamic volt-age withstand ca-pability very quickly after current inter-ruption. This means the hot gas between the arcing contacts needs to be replaced by cold gas as swiftly as possible.
Deciding on a switching technologyCircuit breakers currently come in several varieties, all of which have their own merits: Puffer breakers Advanced puffer breakers Puffer-assisted self-blast breakers Pure self-blast breakers Self-blast breakers with linear double moving system Self-blast breakers with nonlinear double moving systemThe virtues of several of these concepts were combined when developing the new breaker, which has been designated as an advanced puffer breaker with a nonlinear double moving system.
Such an approach has advantages: High and adjustable contact speed. Low moving masses, leading to low reaction forces. New materials and production techniques were evaluated to help identify a product with costs comparable to conven-tional offerings. Low ratio between no-load pressure buildup and maximum pressure buildup leading to low temperatures of the extinguishing gas during power interruption. Low mechanical stress on moving parts due to reduced speed of certain parts. Even for higher asymmetry levels, maximum pressure buildup does not overstress the arcing unit parts mechanically since it is possible to limit the maximum pressure gener-ated.
The development relied heavily on simu-lation software to mimic different physical effects, like flow, pressure buildup and electric fields, during current interrup-t ion 1 2. Flickr has finally created a single page with all these settingsthis is much easier to do now and is the preferred method - it will shut down all API driven sites. Google has a bad habit of keeping out-of-date links and thumbnails in their search results, I can try to help you remove them.
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