Path 27

Path 27, or the Intermountain Power Project DC Line (IPP DC Line), or the Southern Transmission System is a HVDC transmission line that carries electricity from the Intermountain Power Plant near Delta, Utah to the city of Los Angeles via the Adelanto Converter Station in Adelanto, California. It is owned and operated by the Intermountain Power Agency, a large cooperative primarily owned by the Los Angeles Department of Water and Power (50%) along with five other Los Angeles area cities (25%) and 30 small Utah municipalities (remaining 25%).
It is a bipolar overhead power line long, and can transfer a maximum power of 2,400 megawatts at ±500 kV. The part of the line that travels through the Mojave Desert is followed by many other AC 500 kV lines and Interstate 15.
The Pacific DC Intertie is the other HVDC line serving Los Angeles. HVDC lines such as the Intermountain line and the Pacific DC Intertie can be distinguished by having two transmission wires, rather than the three necessary for transmission of three phase AC power.
History.
ABB Group, a Swiss-Swedish multinational corporation headquartered in Zürich, Switzerland, was the primary entity commissioned for this turnkey project in the mid-1980s. ABB also had responsibility for the converter stations, which were commissioned in April 1986.
In 2008, ABB received the order for an additional upgrade. This upgrade included delivery of the MACH2 control and protection system along with additional AC filters and cooling systems. This upgrade has allowed the link to reach a transmission capacity of 2400 MW (2.4 GW).
Solar Development.
Energy Capital Group, LLC (ECG) is developing ECG Utah Solar 1, LLC, a 300 MW-AC PV solar plant strategically sited to utilize existing interstate transmission infrastructure on 1754 acres leased from the Utah School and Institutional Trust Lands (SITLA) less than one mile from the Intermountain Power Project (IPP) north of Delta, Utah.
The project's planned interconnection point is the IPP switchyard which is a point of delivery for the Los Angeles, Anaheim, Riverside, Pasadena, Glendale and Burbank utilities. The switchyard connects to the Southern Transmission System (STS) the 500 HVDC line (Path 27) that travels 488 miles directly to a switchyard in Adelanto, Ca (see map). LADWP is a California Balancing Authority and operates the switchyard which makes this project as though it is on California soil for the state Renewable Portfolio Standard (RPS).
Route.
From the south, the Intermountain DC circuit starts at the Adelanto substation and static inverter plant. The line heads east for a short distance, until two 500 kV and one 287 kV powerlines (all carrying LADWP Path 46 power) meet the DC line from the Victorville substation. Together, the four lines head generally northeast, paralleling the Interstate 15 corridor and crossing the highway at two places in the California High Desert region. An additional line, called Path 64, but also part of Path 46, joins the four lines for a short distance near Newberry Springs before turning eastward and running closer to the Interstate 15 until the California-Nevada state line.
The DC line, two 500 kV lines, and the 287 kV line cross over the Interstate 15 again north of Primm and rejoin Path 64 and other powerlines northeast of the community before heading into Eldorado Valley. There, the AC lines terminate at McCullough, Marketplace, and Mead substations while the DC line bypasses these facilities. Two 500 kV AC lines originating from McCullough and Mead substations then join the DC line to head further north. They cross U.S. Route 95 in Henderson with one of 500 kV AC lines on double-circuit towers. These lines run east of the Las Vegas Valley. Just short of crossing I-15 for another time north of the Las Vegas region, one 500 kV line splits off for the Harry Allen Generating Station. One 500 kV line comes back from Harry Allen plant and rejoins the other 500 kV line at Crystal substation. Another 345 kV line from the Harry Allen Plant also runs parallel with the lines to head into Utah.
The lines then run northeast together somewhat north of I-15, until about 10 miles northwest of Mesquite, where the 500 kV AC (Path 21) line abruptly turns east, spanning the Virgin River to head to the Navajo Generating Station. The DC line continues north-northeast as the line crosses high mountains and large desert basins until it turns east in west-central Utah and terminates at the Intermountain Power Plant.
Electrodes and Electrode Lines.
The electrode line from Adelanto Static Inverter Station is carried for 84.5 kilometres on the shield wire positions of the pylons. South of Coyote Lake at the electrode line branches on steel tube poles northward and ends at .
The electrode line from Delta Static Inverter Station is carried for 44 kilometres on the shield wire positions of the pylons. At the electrode line branches westward to the grounding electrode situated at .
Both electrode lines consist of two 908 mm2 Falcon conductors, mounted on ANSI Class 52-5 porcelain insulators.
Each electrode consists of 60 deep wells with a depth of 87 metres in Delta and 60 metres in Coyote Lake covering an area of approximately 0.65 km2.


HVDC converter

An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. HVDC converters capable of converting up to two gigawatts (GW) and with voltage ratings of up to 900 kilovolts (kV) have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel to achieve total system DC voltage ratings of up to 1,100 kV.
Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC ("rectification") or from DC to AC ("inversion"). A complete HVDC system always includes at least one converter operating as a "rectifier" (converting AC to DC) and at least one operating as an "inverter" (converting DC to AC). Some HVDC systems take full advantage of this bi-directional property (for example, those designed for cross-border power trading, such as the Cross-Channel link between England and France). Others, for example those designed to export power from a remote power station such as the Itaipu scheme in Brazil, may be optimised for power flow in only one preferred direction. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency.
Types of HVDC converters.
HVDC converters can take several different forms. Early HVDC systems, built until the 1930s, were effectively rotary converters and used electromechanical conversion with motor-generator sets connected in series on the DC side and in parallel on the AC side. However, all HVDC systems built since the 1940s have used electronic (static) converters.
Electronic converters for HVDC are divided into two main categories. "Line-commutated converters" (HVDC classic) are made with electronic switches that can only be turned on. "Voltage-sourced converters" are made with switching devices that can be turned both on and off. Line-commutated converters (LCC) used mercury-arc valves until the 1970s, or thyristors from the 1970s to the present day. Voltage-source converters (VSC), which first appeared in HVDC in 1997, use transistors, usually the Insulated-gate bipolar transistor (IGBT).
As of 2012, both the line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are needed, and voltage-source converters used mainly for interconnecting weak AC systems, for connecting large-scale wind power to the grid or for HVDC interconnections that are likely to be expanded to become "Multi-terminal" HVDC systems in future. The market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind power, with one particular type of converter, the Modular Multi-Level Converter (MMC) emerging as a front-runner.
Electromechanical converters.
As early as the 1880s, the advantages of DC long-distance transmission were starting to become evident and several commercial power transmission systems were put into operation. The most successful of these used the system invented by René Thury and were based on the principle of connecting several motor-generator sets in series on the DC side. The best-known example was the 200 km, Lyon–Moutiers DC transmission scheme in France, which operated commercially from 1906 to 1936 transmitting power from the Moutiers hydroelectric plant to the city of Lyon.
Kimbark reports that this system operated quite reliably; however, the total end to end efficiency (at around 70%) was poor by today's standards. From the 1930s onwards, extensive research started to take place into static alternatives using gas-filled tubes – principally mercury-arc valves but also thyratrons – which held the promise of significantly higher efficiency.
Very small mechanical rotary convertors remained in use for niche applications in adverse environments, such as in aircraft and vehicles, as a power conversion method from batteries to the high voltages required for radio and RADAR, until the 1960s and the transistor era.
Line-commutated converters.
Most of the HVDC systems in operation today are based on line-commutated converters (LCC). The term "line-commutated" indicates that the conversion process relies on the line voltage of the AC system to which the converter is connected in order to effect the "commutation" from one switching device to its neighbour. Line-commutated converters use switching devices that are either uncontrolled (such as diodes) or that can only be turned on (not off) by control action, such as thyristors. Although HVDC converters can, in principle, be constructed from diodes, such converters can only be used in rectification mode and the lack of controllability of the DC voltage is a serious disadvantage. Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves (until the 1970s) or thyristors (to the present day).
In a line-commutated converter, the DC current does not change direction; it flows through a large inductance and can be considered almost constant. On the AC side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the AC network. For this reason, a line-commutated converter for HVDC is also considered as a "current-source converter". Because the direction of current cannot be varied, reversal of the direction of power flow (where required) is achieved by reversing the polarity of DC voltage at both stations.
Line Commutated Six-pulse bridge.
The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or "six-pulse bridge", containing six electronic switches, each connecting one of the three phases to one of the two DC terminals. A complete switching element is usually referred to as a "valve", irrespective of its construction. Normally, two valves in the bridge are conducting at any time: one to a phase on the top row and one (from a different phase) on the bottom row. The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals. Thus, the DC output voltage at any given instant is given by the series combination of two AC phase voltages. For example, if valves V1 and V2 are conducting, the DC output voltage is given by the voltage of phase 1 minus the voltage of phase 3.
Because of the unavoidable (but beneficial) inductance in the AC supply, the transition from one pair of conducting valves to the next does not happen instantly. Rather, there is a short "overlap" period when two valves on the same row of the bridge are conducting simultaneously. For example, if valves V1 and V2 are initially conducting and then valve V3 is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. During this period, the DC output voltage is given by the average of the voltages of phases 1 and 2, minus the voltage of phase 3. The "overlap angle" μ (or u) in an HVDC converter increases with the load current, but is typically around 20° at full load.
During the overlap period, the output DC voltage is lower than it would otherwise be and the overlap period produces a visible "notch" in the DC voltage. An important effect of this is that the mean DC output voltage decreases as the overlap period increases; hence the mean DC voltage falls with increasing DC current.
The mean DC output voltage of a six-pulse converter is given by:
 formula_1
Where:
The firing angle α represents the time delay from the point at which the voltage across a valve becomes positive (at which point a diode would start to conduct) and the thyristors being turned on. From the foregoing equation, it is clear that as the firing angle increases, the mean DC output voltage decreases. In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter. Firing angle control is used to regulate the DC voltages of both ends of the HVDC system continuously in order to obtain the desired level of power transfer.
The DC output voltage of the converter steadily becomes less positive as the firing angle is increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages. However, the firing angle cannot be extended all the way to 180°, for two reasons. Firstly, allowance must be made for the overlap angle μ, and secondly for an additional "extinction angle" γ which is needed for the valves to recover their ability to withstand positive voltage after conducting current. The extinction angle γ is related to the "turn-off time" tq of the thyristors. A typical value of γ is 15°. α, γ and μ are inter-related thus:
formula_2 (in degrees)
Line Commutated Twelve-pulse bridge.
With a phase change only every 60°, considerable harmonic distortion is produced at both the DC and AC terminals when the six-pulse arrangement is used. Large filtering components are needed to restore the waveforms to sinusoidal. An enhancement of the six-pulse bridge arrangement uses 12 valves in a "twelve-pulse bridge". A twelve-pulse bridge is effectively two six-pulse bridges connected in series on the DC side and arranged with a phase displacement between their respective AC supplies so that some of the harmonic voltages and currents are cancelled.
The phase displacement between the two AC supplies is usually 30° and is realised by using converter transformers with two different secondary windings (or "valve windings"). Usually one of the valve windings is star (wye)-connected and the other is delta-connected. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30°, and the levels of low frequency harmonics are considerably reduced, considerably simplifying the filtering requirements. For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves make provision for temporary operation with one of the two six-pulse groups bypassed.
Mercury arc valves.
Early LCC systems used mercury-arc valves, with designs that had evolved from those used on high power industrial rectifiers. A number of adaptations were needed to make such valves suitable for HVDC, in particular the use of anode voltage grading electrodes to minimise the risk of "arc-back" at the very high reverse voltages experienced in HVDC. Much of the pioneering work in this area was performed in Sweden by Dr Uno Lamm, widely considered the “Father of HVDC” and in whose name the IEEE introduced the “Uno Lamm Award” for outstanding contributions in the field of HVDC. The very long anode columns needed for high voltage applications limited the current which could safely be carried by each anode, so most mercury-arc valves for HVDC used several (most often, four) anode columns in parallel per valve.
Usually, each arm of each six-pulse bridge consisted of only one mercury-arc valve, but two projects built in the former Soviet Union used two or three mercury-arc valves in series per arm, without parallel connection of anode columns.
Mercury arc valves for HVDC were rugged but required high maintenance. Because of this, most mercury-arc HVDC systems were built with bypass switchgear across each six-pulse bridge so that the HVDC scheme could be operated in six-pulse mode for short periods of maintenance.
Mercury arc valves were built with ratings of up to 150 kV, 1800 A. The last (and most powerful) mercury arc system installed was that of the Nelson River DC Transmission System in Canada, which used six anode columns in parallel per valve and was completed in 1977. The last operating mercury arc system (the HVDC Inter-Island link between the North and South Islands of New Zealand) was shut down in 2012. Mercury arc valves were also used on the following HVDC projects:
Thyristor valves.
The thyristor valve was first used in HVDC systems in 1972 on the Eel River Converter Station in Canada. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a defined instant. Because thyristors have breakdown voltages of only a few kilovolts each, HVDC thyristor valves are built using large numbers of thyristors connected in series. Additional passive components such as grading capacitors and resistors need to be connected in parallel with each thyristor in order to ensure that the voltage across the valve is shared uniformly between the thyristors. The thyristor plus its grading circuits and other auxiliary equipment is known as a "thyristor level".
Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different (high) potential with respect to earth. The command information to turn on the thyristors therefore cannot simply be sent using a wire connection – it needs to be isolated. The isolation method can be magnetic (using pulse transformers) but is usually optical. Two optical methods are used: indirect and direct optical triggering. In the indirect optical triggering method, the low-voltage control electronics sends light pulses along optical fibres to the "high-side" control electronics, which derives its power from the voltage across each thyristor. The alternative direct optical triggering method dispenses with most of the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs), although a small monitoring electronics unit may still be required for protection of the valve.
As of 2012, thyristor valves had been used on over 100 HVDC schemes, with many more still under construction or being planned. The highest power rating of any single HVDC converter (twelve-pulse bridge) in operation was 2000 MW in 2010, on the ±660 kV Ningdong–Shandong scheme in China. Two such converters are provided at each end of the scheme, which is of conventional bipolar construction. Since 2007 the highest voltage rating of a single HVDC converter has been the ±450 kV NorNed scheme linking Norway to the Netherlands, which has only a single converter at each end in an arrangement that is unusual for an LCC HVDC scheme.
Voltage-source converters.
Because thyristors (and mercury rectifiers) can only be turned on (not off) by control action, and rely on the external AC system to effect the turn-off process, the control system only has one degree of freedom – when in the cycle to turn on the thyristor. This limits the usefulness of HVDC in some circumstances because it means that the AC system to which the HVDC converter is connected must always contain synchronous machines in order to provide the timing for the commutating voltage – the HVDC converter cannot feed power into a passive system. This is not a problem supplying additional power to a grid that is already live, but cannot be used as the sole source of power.
With other types of semiconductor device such as the insulated-gate bipolar transistor (IGBT), both turn-on and turn-off timing can be controlled, giving a second degree of freedom. As a result, IGBTs can be used to make "self-commutated converters" which are closer to a large inverter in operation. In such converters, the polarity of DC voltage is usually fixed and the DC voltage, being smoothed by a large capacitance, can be considered constant. For this reason, an HVDC converter using IGBTs is usually referred to as a "voltage-source converter" (or "voltage-sourced converter"). The additional controllability gives many advantages, notably the ability to switch the IGBTs on and off many times per cycle in order to improve the harmonic performance, and the fact that (being self-commutated) the converter no longer relies on synchronous machines in the AC system for its operation. A voltage-sourced converter can therefore feed power to an AC network consisting only of passive loads, something which is impossible with LCC HVDC. Voltage-source converters are also considerably more compact than line-commutated converters (mainly because much less harmonic filtering is needed) and are preferable to line-commutated converters in locations where space is at a premium, for example on offshore platforms.
In contrast to line-commutated HVDC converters, voltage-source converters maintain a constant polarity of DC voltage and power reversal is achieved instead by reversing the direction of current. This makes voltage-source converters much easier to connect into a "Multi-terminal" HVDC system or “DC Grid”.
HVDC systems based on voltage-source converters normally use the six-pulse connection because the converter produces much less harmonic distortion than a comparable LCC and the twelve-pulse connection is unnecessary. This simplifies the construction of the converter transformer. However, there are several different configurations of voltage-source converter and research is continuing to take place into new alternatives.
Two-level converter.
From the very first VSC-HVDC scheme installed (the Hellsjön experimental link commissioned in Sweden in 1997) until 2012, most of the VSC HVDC systems built were based on the "two level converter". The two-level converter is the simplest type of three-phase voltage-source converter and can be thought of as a six pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes, and the DC smoothing reactors have been replaced by DC smoothing capacitors. Such converters derive their name from the fact that the voltage at the AC output of each phase is switched between two discrete voltage levels, corresponding to the electrical potentials of the positive and negative DC terminals. When the upper of the two valves in a phase is turned on, the AC output terminal is connected to the positive DC terminal, resulting in an output voltage of +½ Ud with respect to the midpoint potential of the converter. Conversely when the lower valve in a phase is turned on, the AC output terminal is connected to the negative DC terminal, resulting in an output voltage of -½ Ud. The two valves corresponding to one phase must never be turned on simultaneously, as this would result in an uncontrolled discharge of the DC capacitor, risking severe damage to the converter equipment.
The simplest (and also, the highest-amplitude) waveform that can be produced by a two-level converter is a square wave; however this would produce unacceptable levels of harmonic distortion, so some form of Pulse-width modulation (PWM) is always used to improve the harmonic distortion of the converter. As a result of the PWM, the IGBTs are switched on and off many times (typically 20) in each mains cycle. This results in high "switching losses" in the IGBTs and reduces the overall transmission efficiency. Several different PWM strategies are possible for HVDC but in all cases the efficiency of the two-level converter is significantly poorer than that of a LCC because of the higher switching losses. A typical LCC HVDC converter station has power losses of around 0.7% at full load (per end, excluding the HVDC line or cable) while with 2-level voltage-source converters the equivalent figure is 2-3% per end.
Another disadvantage of the two-level converter is that, in order to achieve the very high operating voltages required for an HVDC scheme, several hundred IGBTs have to be connected in series and switched simultaneously in each valve. This requires specialised types of IGBT with sophisticated "gate drive" circuits, and can lead to very high levels of electromagnetic interference.
Three-level converter.
In an attempt to improve on the poor harmonic performance of the two-level converter, some HVDC systems have been built with "three level converters". Three-level converters can synthesize three (instead of only two) discrete voltage levels at the AC terminal of each phase: +½ Ud, 0 and -½ Ud. A common type of three-level converter is the "diode-clamped" (or "neutral-point-clamped") converter, where each phase contains four IGBT valves, each rated at half of the DC line to line voltage, along with two clamping diode valves. The DC capacitor is split into two series-connected branches, with the clamping diode valves connected between the capacitor midpoint and the one-quarter and three-quarter points on each phase. To obtain a positive output voltage (+½ Ud) the top two IGBT valves are turned on, to obtain a negative output voltage (-½ Ud) the bottom two IGBT valves are turned on and to obtain zero output voltage the middle two IGBT valves are turned on. In this latter state, the two clamping diode valves complete the current path through the phase.
In a refinement of the diode-clamped converter, the so-called "active neutral-point clamped" converter, the clamping diode valves are replaced by IGBT valves, giving additional controllability. Such converters were used on the Murraylink project in Australia and the Cross Sound Cable link in the United States. However, the modest improvement in harmonic performance came at a considerable price in terms of increased complexity, and the design proved to be difficult to scale up to DC voltages higher than the ±150 kV used on those two projects.
Another type of three-level converter, used in some adjustable-speed drives but never in HVDC, replaces the clamping diode valves by a separate, isolated, "flying" capacitor connected between the one-quarter and three-quarter points. The operating principle is similar to that of the diode-clamped converter. Both the diode-clamped and flying capacitor variants of three-level converter can be extended to higher numbers of output levels (for example, five), but the complexity of the circuit increases disproportionately and such circuits have not been considered practical for HVDC applications.
Modular Multi-Level Converter (MMC).
First proposed for HVDC applications in 2003 by Marquardt and first used commercially in the Trans Bay Cable project in San Francisco, the "Modular Multi-Level Converter" (MMC) is now becoming the most common type of voltage-source converter for HVDC.
Like the two-level converter and the six-pulse line-commutated converter, a MMC consists of six valves, each connecting one AC terminal to one DC terminal. However, where each valve of the two-level converter is effectively a high-voltage controlled switch consisting of a large number of IGBTs connected in series, each valve of a MMC is a separate controllable voltage source in its own right. Each MMC valve consists of a number of independent converter "submodules", each containing its own storage capacitor. In the most common form of the circuit, the "half-bridge" variant, each submodule contains two IGBTs connected in series across the capacitor, with the midpoint connection and one of the two capacitor terminals brought out as external connections. Depending on which of the two IGBTs in each submodule is turned on, the capacitor is either bypassed or connected into the circuit. Each submodule therefore acts as an independent two-level converter generating a voltage of either 0 or Usm (where Usm is the submodule capacitor voltage). With a suitable number of submodules connected in series, the valve can synthesize a stepped voltage waveform that approximates very closely to a sine-wave and contains very low levels of harmonic distortion.
The MMC differs from other types of converter in that current flows continuously in all six valves of the converter throughout the mains-frequency cycle. As a result, concepts such as “on-state” and “off-state” have no meaning in the MMC. The direct current splits equally into the three phases and the alternating current splits equally into the upper and lower valve of each phase. The current in each valve is therefore related to the direct current Id and alternating current Iac as follows:
Upper valve: formula_3
Lower valve: formula_4
A typical MMC for an HVDC application contains around 300 submodules connected in series in each valve and is therefore equivalent to a 301 level converter. Consequently, the harmonic performance is excellent and usually no filters are needed.
A further advantage of the MMC is that PWM is not necessary, with the result that the power losses are much lower than those of the 2-level converter, at around 1% per end. Finally, because direct series-connection of IGBTs is not necessary, the IGBT gate drives do not need to be as sophisticated as those for a 2-level converter.
The MMC has two principal disadvantages. Firstly, the control is much more complex than that of a 2-level converter. Balancing the voltages of each of the submodule capacitors is a significant challenge and requires considerable computing power and high-speed communications between the central control unit and the valve. Secondly, the submodule capacitors themselves are large and bulky. A MMC is considerably larger than a comparable-rated 2-level converter, although this may be offset by the saving in space from not requiring filters.
As of 2012 the largest-capacity MMC HVDC system in operation is still the 400 MW Trans Bay Cable scheme but many larger schemes are under construction, including an underground cable interconnection from France to Spain consisting of two 1000 MW links in parallel at a voltage of ±320 kV.
MMC variants.
A variant of the MMC, proposed by one manufacturer, involves connecting multiple IGBTs in series in each of the two switches that make up the submodule. This gives an output voltage waveform with fewer, larger, steps than the conventional MMC arrangement. This arrangement is referred to as the "Cascaded Two Level" (CTL) converter. Functionally it is exactly equivalent to the conventional half-bridge MMC in every respect except for the harmonic performance, which is slightly inferior – although still claimed to be good enough to avoid the need for filtering in most instances.
Another alternative replaces the "half bridge" MMC submodule described above, with a "full bridge" submodule containing four IGBTs in an H bridge arrangement, instead of two. The full-bridge variant of MMC allows the submodule capacitor to be inserted into the circuit in either polarity. This confers additional flexibility in controlling the converter and allows the converter to block the fault current which arises from a short-circuit between the positive and negative DC terminals (something which is impossible with any of the preceding types of VSC). Furthermore, it allows the DC voltage to be of either polarity (like a LCC HVDC scheme), giving rise to the possibility of hybrid LCC and VSC HVDC systems. However, the full-bridge arrangement requires twice as many IGBTs and has higher power losses than the equivalent half-bridge arrangement.
Other types of voltage-source converter.
Various other types of converter have been proposed, combining features of the two-level and Modular Multi-Level Converters. These "hybrid" VSC systems aim to achieve the low losses and high harmonic performance of the MMC with a more compact design and greater controllability, but these concepts are still at the research stage.


Tetrode

A tetrode is a vacuum tube (called "valve" in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called "anode" in British English). There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.
The tetrode was developed in the 1920s by adding an additional grid to the first amplifying vacuum tube, the triode, to correct limitations of the triode. During the period 1913 to 1927, three distinct types of tetrode valves appeared. All had a normal control grid whose function was to act as a primary control for current passing through the tube, but they differed according to the intended function of the other grid. In order of historical appearance these are: the space-charge grid tube, the bi-grid valve, and the screen-grid tube. The last of these appeared in two distinct variants with different areas of application: the screen-grid valve proper, which was used for medium-frequency, small signal amplification, and the beam tetrode which appeared later, and was used for audio or radio-frequency power amplification. The former was quickly superseded by the rf pentode, while the latter was initially developed as an alternative to the pentode as an audio power amplifying device. The beam tetrode was also developed as a high power radio transmitting tube.
Tetrodes were widely used in many consumer electronic devices such as radios, televisions, and audio systems until transistors replaced valves in the 1960s and 70s. Beam tetrodes have remained in use until quite recently in power applications such as audio amplifiers and radio transmitters.
How it works.
The tetrode functions in a similar way to the triode, from which it was developed. A current through the heater or filament heats the cathode, which causes it to emit electrons by thermionic emission. A positive voltage is applied between the plate and cathode, causing a flow of electrons from the cathode to plate through the two grids. A varying voltage applied to the control grid can control this current, causing variations in the plate current. With a resistive or other load in the plate circuit, the varying current will result in a varying voltage at the plate. With proper biasing, this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, providing voltage gain. In the tetrode, the function of the other grid varies according to the type of tetrode; this is discussed below.
Space charge grid tube.
The space charge grid tube was the first type of tetrode to appear. In the course of his research into the action of the "audion" triode tube of Lee de Forest, Irving Langmuir found that the action of the heated thermionic cathode was to create a space charge, or cloud of electrons, around the cathode. This cloud acted as a virtual cathode. With low applied anode voltage, many of the electrons in the space charge returned to the cathode, and did not contribute to the anode current; only those at its outer limit would be affected by the electric field due to the anode, and would be accelerated towards it. However, if a grid bearing a low positive applied potential (about 10V) were inserted between the cathode and the control grid, the space charge could be made to extend further away from the cathode. This had two advantageous effects, both related to the influence of the electric fields of the other electrodes (anode and control grid) on the electrons of the space charge. First, a significant increase in anode current could be achieved with low anode voltage; the valve could be made to work well with lower applied anode voltage. Second, the transconductance (rate of change of anode current with respect to control grid voltage) of the tube was increased. The latter effect was particularly important since it increased the voltage gain available from the valve.
Space-charge valves remained useful devices throughout the valve era, and were used in applications such as car radios operating directly from a 12V supply, where only a low anode voltage was available. The same principle was applied to other types of multi-grid tubes such as pentodes. As an example, the Sylvania 12K5 is described as "a tetrode designed for space-charge operation. It is intended for service as a power amplifier driver where the potentials are obtained directly from a 12V automobile battery." The space-charge grid was operated at +12V, the same as the anode supply voltage.
Another important application of the space-charge tetrode was as an electrometer tube for detecting and measuring extremely small currents. For example, the General Electric FP54 was described as a "space-charge grid tube ... designed to have a very high input impedance and a very low grid current. It is designed particularly for amplification of direct currents smaller than about 10 amperes, and has been found capable of measuring currents as small as 5 x 10 amperes. It has a current amplification factor of 250,000, and operates with an anode voltage of 12v, and space-charge grid voltage of +4V."
 The mechanism by which the space-charge grid lowers control-grid current in an electrometer tetrode is that it prevents positive ions originating in the cathode from reaching the control grid.
Note that when a space-charge grid is added to a triode, the first grid in the resulting tetrode is the space-charge grid, and the second grid is the control grid.
Bi-grid valve.
In the bi-grid type of tetrode, both grids are intended to carry electrical signals, so both are control grids. The first example to appear in Britain was the Marconi-Osram FE1, which was designed by H. J. Round, and became available in 1920. The tube was intended to be used in a reflex circuit (for example the single-valve ship receiver Type 91) where the same valve performed the multiple functions of RF amplifier, AF amplifier, and diode detector. The RF signal was applied to one control grid, and the AF signal to the other. This type of tetrode was used in many imaginative ways in the period before the appearance of the screen-grid valve revolutionised receiver design.
One application is shown in the illustration. This is recognisable as an AM telephony transmitter in which the second grid and the anode form a power oscillator, and the first grid acts as a modulating electrode. The anode current in the valve, and hence the RF output amplitude, is modulated by the voltage on G1, which is derived from a carbon microphone.
A tube of this type could also be used as a direct conversion CW (radiotelegraphy) receiver. Here the valve oscillates as a consequence of coupling between the first grid and the anode, while the second grid is coupled to the antenna. The AF beat frequency is audible in the headphones. The valve acts as a self-oscillating product detector.
Another, very similar application of the bi-grid valve was as a self oscillating frequency mixer in early superhet receivers
One control grid carried the incoming RF signal, while the other was connected into an oscillator circuit which generated the local oscillation within the same valve. Since the anode current of the bi-grid valve was proportional both to the signal on the first grid, and also to the oscillator voltage on the second grid, the required multiplication of the two signals was achieved, and the intermediate frequency signal appeared in an appropriately tuned circuit connected to the anode. In all of the applications mentioned, the bi-grid tetrode acted as an analogue multiplier (analog multiplier) which multiplied together the signals applied to the two grids.
The super-sonic heterodyne (superhet) receiver principle was invented in France by Lucien Levy in 1917 (p 66), though credit is usually also given to Edwin Armstrong. The original reason for the invention of the superhet was that before the appearance of the screen-grid valve, there was no type of valve which could give good gain at radio frequencies (i.e. frequencies much above 100 kHz), so a technique was applied whereby the incoming RF signal was "mixed" (i.e. multiplied) with a locally generated oscillatory voltage (the local oscillator) so as to produce a beat frequency of about 30 kHz. This intermediate frequency represented the incoming signal in all important respects, but at a significantly lower frequency which could be successfully amplified by the triode amplifiers available then. This was a complicated technique. It went out of use when screen-grid valves appeared which could act as satisfactory RF amplifiers capable of amplifying the incoming signal without lowering its frequency (see Screen grid valve, below). Superhet receivers reappeared in the early 1930s when, because of the proliferation of transmitting stations, their greater selectivity became an important advantage; almost all receivers operate on this principle today, albeit with a higher IF frequency.
Screen grid valve.
In the screen grid class of tetrode, the main function of the second grid is to act as an electrostatic screen between the anode and the control grid (i.e. the first grid) in order to reduce the internal capacitance between control grid and anode. The first true screen-grid valve, with a screen grid designed for this purpose, was patented by Hiroshi Ando in 1919, and the first practical versions were built by N. H. Williams and Albert Hull at General Electric and Bernard Tellegen at Phillips in 1926.
This type of tetrode was developed to correct deficiencies in the triode tube which became apparent when attempts were made to use triodes as small-signal radio-frequency amplifiers. In the triode, the control grid was next to the plate. Capacitance between these two electrodes caused instability and oscillation when both anode and grid were connected into tuned resonant circuits, as was the case in early radios, or in any application where the anode circuit presented an inductive load to the valve. Oscillation could only be avoided by using very small stage gain at frequencies above about 100 kHz, and at frequencies above 1 MHz, triodes are virtually useless in tuned amplifiers in which both anode and grid circuits are tuned to the same frequency. A second advantage of the tetrode, which also arose from reduced anode-grid capacitance, was that, when the anode voltage was in a certain range, changing its value had much less effect on the anode current than was the case with triodes. This corresponds to an increased anode slope resistance, and allowed much higher voltage gain by permitting a larger external load resistance. At the time of the introduction of screen-grid valves (around 1927) a typical small triode used for small-signal amplification had an anode slope resistance of 20 kilohms or less, and a grid-anode capacitance of 1 to 5 pF, while the corresponding figures for a typical screen grid valve were 1 megohms and 0.004 pF - in some cases much less capacitance.
Screen-grid valves exhibited greater voltage gain and higher frequency capability than triodes, and permitted the development of the first true RF amplifiers in the MF and HF frequency ranges in radio equipment. They were commonly used as the first stage of radio-frequency amplification in domestic radio receivers in the period 1927 to 1930, after which they were superseded in this application by RF pentodes. Triodes were ill-suited to this type of application on account of their relatively high anode-grid capacitance and low anode resistance.
In normal operation the screen grid is connected to a positive DC voltage slightly less than the plate voltage, and bypassed to the cathode with a capacitor, so it was at AC ground. To take full advantage of the very low internal grid-anode capacitance, the valve must be used in circuits in which the shielding between anode and grid is continued externally. In the case illustrated (S625), the valve was intended to be inserted into a hole in an external, grounded, sheet-metal shield aligned to correspond with the position of the internal screen-grid. The input, or control-grid circuit was on one side of the shield, while the anode, or output circuit was on the other. In the case of the Osram Music Magnet, each entire stage of the 2-stage rf amplifier, as well as the tuned detector stage, was enclosed in an individual large aluminium screening box. These boxes have been removed in the illustration, but the up-turned edges of the bases of the boxes can be seen.
Anode characteristic of screen-grid valves.
The reason for the limited applicability of the screen-grid valve, and its rapid replacement by the RF pentode (introduced around 1930) was the peculiar anode characteristic (i.e. variation of anode current with respect to anode voltage) of the former type of tube.
In normal applications, the anode voltage was about 150 V, while that of the screen-grid was about 60 V (Thrower p 183). As the screen grid is positive with respect to the cathode, it collects a certain fraction (perhaps a quarter) of the electrons which would otherwise pass from the grid region to the anode. This causes current to flow in the screen grid circuit. Usually, the screen current due to this cause is small, and of little interest. However, if the anode voltage should be below that of the screen, the screen grid can also collect secondary electrons ejected from the anode by the impact of the energetic primary electrons. Both effects tend to reduce the anode current. If the anode voltage is increased from a low value, with the screen grid at its normal operating voltage (60V, say) the anode current initially increases rapidly because more of those electrons which pass through the screen-grid are collected by the anode rather than passing back to the screen grid. This part of the tetrode anode characteristic resembles the corresponding part of that of a triode or pentode. However, when the anode voltage is increased further, the electrons arriving at the anode have sufficient energy to cause copious secondary emission, and many of these secondary electrons will be captured by the screen, which is at a higher positive voltage than the anode. This causes the anode current to fall rather than increase when the anode voltage is increased. In some cases the anode current can actually become negative (current flows out of the anode); this is possible since each primary electron may produce more than one secondary. Falling positive anode current accompanied by rising anode voltage gives the anode characteristic a region of negative slope, and this corresponds to a negative resistance which can cause instability in certain circuits. In a higher range of anode voltage, the anode voltage sufficiently exceeds that of the screen for an increasing proportion of the secondary electrons to be attracted back to the anode, so the anode current increases once more, and the slope of the anode characteristic becomes positive again. In a yet higher range of anode voltages, the anode current becomes substantially constant, since all of the secondary electrons now return to the anode, and the main control of current through the tube is the voltage of the control grid. This is the normal operating mode of the tube.
The anode characteristic of a screen-grid valve is thus quite unlike that of a triode. It includes a range of anode voltages, where the anode voltage is less than that of the screen grid, in which there is a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is usually undesirable, although it can be exploited as in the dynatron oscillator (Eastman, p 431). The approximately constant-current region of low slope at high anode voltage is also a distinctive characteristic of the tetrode. This is highly desirable, since it corresponds to a high source resistance in the anode circuit, and greatly enhances the voltage gain which the device can produce. Early screen-grid valves had amplification factors (i.e. the product of transconductance and anode slope resistance) ten times that of comparable small-signal triodes. The high anode resistance (i.e. low slope) in the normal operating range is a consequence of the electrostatic shielding action of the screen grid, since it prevents the electric field due to the anode from penetrating to the control grid region, where it might otherwise influence the passage of electrons, increasing the electron current when the anode voltage is high, reducing it when low.
In practice the negative-resistance kink of the anode characteristic limits the usefulness of the screen-grid valve to applications where only small signals are amplified, so that the variations in anode voltage are also correspondingly small, and the operating conditions never depart from the region of high positive impedance (low positive slope) at large anode voltage. The secondary emission can be prevented from contributing to the screen current (and thus detracting from the anode current) by adding a suppressor grid, making a pentode, or beam plates to make a beam tetrode/kinkless tetrode, which can be used in power amplifiers where the variations of anode voltage, due to the presence of a large signal voltage, are much greater. The anode characteristics of these tubes are very similar to pentodes. In both cases the anode current rises rapidly to an approximately constant value when the anode voltage rises to as little as a few tens of volts, and the region of high positive anode resistance (low slope) extends from this low value up to the maximum anode voltage which could be several hundred volts or more.
The negative resistance operating region of the tetrode is exploited in the dynatron oscillator, which is an example of a negative resistance oscillator. (Eastman, p431)
Beam tetrode.

The high value of the anode slope resistance of tetrodes (mentioned above) makes them capable of high voltage and power gain, and is also potentially a cause of high anode efficiency which, if it could be exploited, would make tetrodes superior to triodes as power amplifying devices in applications such as audio power amplifiers, and the output stages of radio transmitters. For a triode power amplifier working with a transformer or inductive load in Class A, the maximum theoretical efficiency is 25%. This low figure is in part a consequence of the low anode slope impedance (R) of this type of tube; the low value of a triode R is almost always much less than the optimal anode load impedance in a power amplifier. For a pentode or tetrode, however, R is usually sufficiently high for the optimal load impedance to be achieved, and under these circumstances the maximum theoretical efficiency rises to 50%. This gives tetrodes and pentodes an important practical advantage over triodes, which is of particular value when high power outputs are required.
However, the tetrode kink limits the permissible variation of anode voltage, and restricts the use of screen-grid valves to small-signal applications. The suppressor grid of the pentode eliminates the kink in the anode characteristic by preventing secondary electrons, which originate in the anode, from reaching the screen grid, and thus permits a wider excursion of anode voltage, as is required for power amplification. The same effect can be produced in the case of a tetrode by introducing two modifications. Firstly, the wires of the screen grid are aligned with those of the control grid so that the former lie in the electron 'shadow' created by the latter. This reduces the screen grid current, thereby giving greater efficiency, and also concentrates the electrons into dense beams in the space between the screen grid and the anode. The intense negative space charge of these beams prevents secondary electrons from the anode from reaching the screen grid, thereby eliminating the tetrode kink. Secondly, in small valves whose electrode structure is supported in the conventional way with vertical wire rods and mica spacers, it was found to be necessary to introduce sheet-metal beam-forming electrodes between the screen grid and the anode. The purpose of these beam-plates is to constrain the electron beams into parts of the electrode system which are sections of a cylinder. (See sectional view, right). Successful creation of the electron beam between screen grid and anode required for a kinkless anode characteristic depends on the details of the geometry of the electrode structure of the beam tetrode. In the cases where the electrodes have complete cylindrical symmetry, a kinkless characteristic can be achieved without the need for beam-plates, alignment of the screen grid wires with those of the control grid being sufficient. This form of construction is usually adopted in larger tubes with an anode power rating of 100W or more. The Eimac 4CX250B (rated at 250W anode dissipation) is an example of this class of beam tetrode. Note that a radically different approach is taken to the design of the support system for the electrodes in these types (see illustration). The 4CX250B is described by its manufacturer as a 'radial beam tetrode', drawing attention to the symmetry of its electrode system.
The overall effect of the original developments was to produce a highly effective power amplifier tube, whose anode characteristic is very much like that of a pentode, but which has greater efficiency as a result of reduced screen current. A further bonus was that third harmonic distortion was much reduced relative to a comparable pentode (Terman pp 198–9). Beam tetrode audio power output valves were introduced in 1937, and quickly replaced conventional pentodes in this application. Later developments produced beam power tubes which were capable of high-power output at frequencies extending into the UHF region.
The beam tetrode, patented in 1933, was invented in Britain by two EMI engineers, Cabot Bull and Sidney Rodda, as an attempt to circumvent the power pentode, whose patent was owned by Philips. Although the beam-plates (when present) could be counted as a fifth electrode (as in a pentode), this type of tube is nevertheless classified as a tetrode, perhaps to underline the difference in principle from that employed in true pentodes, which rely upon the effect of a suppressor grid. Beam tetrodes were widely used as audio power amplifying tubes in consumer items such as radios and televisions and in industrial electronic equipment until the 1960s when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. Low power consumer beam tetrodes are still used in a few legacy and specialty vacuum tube audio power amplifier devices such as tube guitar amplifiers; the KT66 and KT88 are popular examples in audio equipment, while QY4-400 is an example having 400W anode dissipation, capable of applications in radio transmitters up to 100 MHz. The 4CX250B, mentioned above can be operated at full 250W anode dissipation up to 500 MHz. Many other types abound.
Critical-distance tetrode.
An alternative approach to the problem of eliminating the tetrode kink was introduced by Hivac in 1935. It was found by J.H Owen Harries that if the separation of the anode from the screen grid were varied, a critical separation could be found (about 3 cm) where the kink in the tetrode's anode characteristic disappeared, and the amplification of the valve became particularly distortion-free. Both fidelity and efficiency exceeded those of the available pentodes of the time. A range of tetrodes of this type were introduced, aimed at the domestic receiver market, some having 2V directly heated filaments, intended for low-power battery-operated sets, others with 4V or higher, indirectly heated cathodes for mains operation. Output powers ranged from 0.5W to 11.5W . Confusingly, several of these new valves bore the same type number as existing pentodes with almost identical characteristics. Examples include Y220 (0.5W, 2V filament), AC/Y (3W, 4V heater), AC/Q (11.5W, 4V heater), etc.


Acceptance angle (solar concentrator)

Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index in which the receiver is immersed. Maximizing the acceptance angle of a concentrator is desirable in practical systems and it may be achieved by using nonimaging optics.
For concentrators that concentrate light in two dimensions, the acceptance angle may be different in the two directions.
Definition.
The "acceptance angle" figure illustrates this concept.
The concentrator is a lens with a receiver "R". The left section of the figure shows a set of parallel rays incident on the concentrator at an angle "α" < "θ" to the optical axis. All rays end up on the receiver and, therefore, all light is captured. In the center, this figure shows another set of parallel rays, now incident on the concentrator at an angle "α" = "θ" to the optical axis. For an ideal concentrator, all rays are still captured. However, on the right, this figure shows yet another set of parallel rays, now incident on the concentrator at an angle "α" > "θ" to the optical axis. All rays now miss the receiver and all light is lost. Therefore, for incidence angles "α" < "θ" all light is captured while for incidence angles "α" > "θ" all light is lost. The concentrator is then said to have a (half) acceptance angle "θ", or a total acceptance angle 2"θ" (since it accepts light within an angle ±"θ" to the optical axis).
Ideally, a solar concentrator has a transmission curve "c"I as shown in the "transmission curves" figure. Transmission (efficiency) is "τ" = 1 for all incidence angles "α" < "θ"I and "τ" = 0 for all incidence angles "α" > "θ"I.
In practice, real transmission curves are not perfect and they typically have a shape similar to that of curve "c"R, which is normalized so that "τ" = 1 for "α" = 0. In that case, the real acceptance angle "θ"R is typically defined as the angle for which transmission "τ" drops to 90% of its maximum.
For line-focus systems, such as a trough concentrator or a linear Fresnel lens, the acceptance angle is one dimensional, and the concentration has only weak dependence on off-pointing perpendicular to the focus direction. Point focus systems, on the other hand, are sensitive to off-pointing in both directions. In the general case, the acceptance angle in one direction may be different from the other.
Acceptance angle as a tolerance budget.
The acceptance angle "θ" of a concentrator may be seen as a measure of how precisely it must track the sun in the sky. The smaller the "θ", the more precise the tracking needs to be or the concentrator will not capture the incoming sunlight. It is, therefore, a measure of the tolerance a concentrator has to tracking errors.
However, other errors also affect the acceptance angle. The "optical imperfections" figure shows this.
The left part of the figure shows a perfectly made lens with good optical surfaces "s"1 and "s"2 capturing all light rays incident at an angle "α" to the optical axis. However, real optics are never perfect and the right part of the figure shows the effect of a badly made bottom surface "s"2. Instead of being smooth, "s"2 now has undulations and some of the light rays that were captured before are now lost. This decreases the transmission of the concentrator for incidence angle "α", decreasing the acceptance angle. Actually, any imperfection in the system such as:
contributes to a decrease in the acceptance angle of the concentrator. The acceptance angle may then be seen as a "tolerance budget" to be spent on all these imperfections. At the end, the concentrator must still have enough acceptance to capture sunlight which also has some angular dispersion "θ""S" when seen from earth. It is, therefore, very important to design a concentrator with the widest possible acceptance angle. That is possible using nonimaging optics, which maximize the acceptance angle for a given concentration.
Figure "angular aperture of sunlight" on the right shows the effect of the angular dispersion of sunlight on the acceptance angle.
Sunlight is not a set of perfectly parallel rays (shown in blue), but it has a given angular aperture "θ""S", as indicated by the green rays. If the acceptance angle of the optic is wide enough, sunlight incident along the optical axis will be captured by the concentrator, as shown in the "angular aperture of sunlight" figure. However, for wider incidence angles "α" some light may be lost, as shown on the right. Perfectly parallel rays (shown in blue) would be captured, but sunlight, due to its angular aperture, is partially lost.
Parallel rays and sunlight are therefore transmitted differently by a solar concentrator and the corresponding transmission curves are also different. Different acceptance angles may then be determined for parallel rays or for sunlight.
Concentration acceptance product (CAP).
For a given acceptance angle "θ", for a point-focus concentrator, the maximum concentration possible, "C"max, is given by
where "n" is the refractive index of the medium in which the receiver is immersed. In practice, real concentrators either have a lower than ideal concentration for a given acceptance or they have a lower than ideal acceptance angle for a given concentration. This can be summarized in the expression
which defines a quantity CAP (concentration acceptance product), which must be smaller than the refractive index of the medium in which the receiver is immersed.
For a linear-focused concentrator, the equation is not squared
The Concentration Acceptance Product is a consequence of the conservation of etendue. The higher the CAP, the closer the concentrator is to the maximum possible in concentration and acceptance angle.


Chandrapur back-to-back HVDC converter station

The Chandrapur back-to-back HVDC station is a back-to-back HVDC connection between the western and southern regions in India, located close to the city of Chandrapur. Its main purpose is to export power from the Chandrapur Super Thermal Power Station to the southern region of the Indian national power grid. It is owned by Power Grid Corporation of India.
The converter station consists of two independent poles, each with a nominal power transmission rating of 500 MW. Both poles were built by GEC-Alstom between 1993 and 1997 and have nominal DC voltage and current ratings of 205 kV, 2475 A.
The converter station is located from the eastern terminal of the Chandrapur–Padghe HVDC transmission system. The close proximity of the two converter stations meant that the control systems needed to be carefully coordinated, a task made more challenging by the fact that the two stations were built by different manufacturers. To address this problem a series of joint simulation studies, involving the control equipment from both converter stations connected to a common simulator, was performed.
On 31 December 2013, the Northern, Eastern and Western grids were synchronised with the Southern regional grid, creating a single synchronous AC grid over the whole of India. As a result, the converter station is no longer required for its original purpose of asynchronously linking the Western and Southern grids, although it can still be used as an embedded power flow device to help control power flow within the AC system. The stations could potentially be shifted to elsewhere to export/import power from other countries. Sometimes the excess power fed to the southern grid by this HVDC link is flowing back to Western region through the 765 kV AC lines between Southern grid and the Western grid with futility.
Arrangement and main equipment.
Overall arrangement.
Each of the two poles is identical and consists of a back-to-back connection of two 12-pulse bridge converters. Unusually, no DC smoothing reactors are used.
Each pole is grounded at one of the two DC terminals.
Converter transformers.
Each of the two poles uses three, single-phase, three-winding converter transformers on each side.
Thyristor valves.
The scheme comprises 48 thyristor valves (12 at each end of each pole) and with each thyristor valve including 54 thyristor levels in series. The thyristors are of 100 mm diameter and are rated at 5.2 kV. The thyristor valves are floor-mounted even though the station is located in a seismically active area.
AC filters and reactive power.
Each side of the station is equipped with a total of 848 MVAr of AC harmonic filters. Each side of each pole is equipped with three, 106 MVAr double-damped filters (tuned to 12th and 24th harmonics) and one, 106 MVAr "C-type" filter (tuned to 5th harmonic). On the Southern side there are also two, 50 MVAr shunt reactors.


Katsuhiko Ishibashi

 is a professor in the "Research Center for Urban Safety and Security" in the "Graduate School of Science" at Kobe University, Japan and a seismologist who has written extensively in the areas of seismicity and seismotectonics in and around the Japanese Islands. He recently coined the term "genpatsu-shinsai" (原発震災), from the Japanese words for "nuclear power" and "quake disaster".
Katsuhiko Ishibashi has said that Japan's history of nuclear accidents stems from an overconfidence in plant engineering. He was a member of a 2006 Japanese government subcommittee charged with revising the national guidelines on the earthquake-resistance of nuclear power plants, published in 2007. His proposal that the committee should review the standards for surveying active faults was rejected and, at the committee's final meeting he resigned claiming that the review process was "unscientific" and the outcome rigged to suit the interests of the Japan Electric Association, which had 11 of its committee members on the 19-member government subcommittee and that among other problems the guide was "seriously flawed" as a consequence because it underestimated the design basis earthquake ground motion.
In May 2011, he said: "If Japan had faced up to the dangers earlier, we could have prevented Fukushima".


Slip factor

In turbomachinery, the slip factor is a measure of the fluid slip in the impeller of a compressor or a turbine, mostly a centrifugal machine. Fluid slip is the deviation in the angle at which the fluid leaves the impeller from the impeller's blade/vane angle. Being quite small in axial impellers(inlet and outlet flow in the same direction), slip is a very important phenomenon in radial impellers and is useful in determining the accurate estimation of work input or the energy transfer between the impeller and the fluid, rise in pressure and the velocity triangles at the impeller exit.
A simple explanation for the fluid slip can be given as: Consider an impeller with "z" number of blades rotating at angular velocity ω. A difference in pressure and velocity during the course of clockwise flow through the impeller passage can be observed between the trailing and leading faces of the impeller blades. High pressure and low velocity are observed at the leading face of the impeller's blade as compared to lower pressure with high velocity at the trailing face of the blade. This results in circulation in the direction of ω around the impeller blade which prevents the air from acquiring the whirl velocity equivalent to impeller speed with non-uniform velocity distribution at any radius.
This phenomenon reduces the output whirl velocity, which is a measure of the net power output from a turbine or a compressor. Hence, the slip factor accommodates for a slip loss which affects the net power developed which increases with increasing flow-rate.
Mathematical Formulae for Slip factor.
Mathematically, the Slip factor denoted by 'σ' is defined as the ratio of the actual & ideal values of the whirl velocity components at the exit of the impeller. The ideal values can be calculated using an analytical approach while the actual values should be observed experimentally.
where,
Usually,σ varies from 0-1 with an average ranging from 0.8-0.9.
The Slip Velocity is given as:
VS = Vw2 - V'w2 = Vw2(1-σ)
The Whirl Velocity is given as:
V'w2 = σ Vw2
Slip Factor correlations.
The above explained models clearly states that the Slip factor is solely a function of geometry of Impeller. However, later studies proved that Slip factor depends on other factors as well namely 'mass flow rate',viscosity etc..