Heart of the Converter High-Voltage Power Semiconductor Devices

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ABB enjoys a unique position in HVDC systems technol- power semiconductors, cooling equipment, transformers, ogy, being the sole company able to supply all key compo- bushings and other DC and AC power products, and in nents. An HVDC substation includes converter valves with many cases, cable connections.

Heart of the converter High-voltage power semiconductor devices

MUNAF RAHIMO, SVEN KLAKA – Silicon- wide range of possibilities in terms of dependent on technologies devel- based high-voltage semiconductor controlling the way electrical energy is oped for lower power applications, devices play a major role in megawatt transported and used. At the heart of which were then scaled to handle power electronics conversion. In this revolution lies the power semicon- higher voltages and currents. In particular, advances in ultrahigh-volt- ductor device: This device performs HVDC applications today, the two age semiconductors have over the the actual task of modulating the main types of switching devices are past few decades led to tremendous energy flow to suit the demands of the the phase-controlled thyristor (PCT) improvements in high-voltage direct application. The main trend in the and the insulated-gate bipolar current (HVDC) transmission. The development of power devices has transistor (IGBT). Also important is power device is considered a main always been increasing the power the power diode found in a range of enabler for the growing demands of ratings while improving overall device applications spanning rectification, modern grid systems in terms of performance in terms of reduced snubber and freewheeling. increased power levels, improved losses, increased robustness and efficiency, greater control and better controllability, and improving Different power-semiconductor-based integration of renewable energy reliability under normal and fault circuit topologies such as current sources. conditions. source converters (CSCs) and voltage source converters (VSCs) are em- The power electronics revolution, The HVDC-applications market is ployed for the AC/DC conversion which over the past few decades has small but important for semiconduc- process. For long-distance and swept across the power delivery and tors. Progress in the domain of power multi-gigawatt power transmission, automation sectors, has opened up a devices has in the past largely been PCT-based CSC topologies are widely

44 ABB review special report 1 Power device evolution and basic structures

IGBT PCT/IGCT

Cathode (Metal) 1,E+08 PCT/Diode 150 mm Bipolar Insulator 125 mm 100 mm technologies GTO IGCT Bipolar IGCT gate layer BJT 1,E+07 n MOS p n MOSFET gate layer p GTO MOS based 1,E+06 technologies IGBT Switching power (VA) N-base N-base Ge Si

1960 19701980 19902000 2010 1,E+05 p p 1960 1970 1980 1990 2000 2010 2020 Year Anode (metal) Year Thyristor Insulated

Carrier concentration GTO/IGCT PressPack

IGBT

2 PCT evolution and 150mm PCT rated at 8.5 kV / 4,000 A for UHVDC The power electronics revolution has opened up 10,000 200 ) ETT 2 a wide range of 8,000 160 possibilities in 6,000 120 Wafer area (cm area Wafer Blocking rating (V) 4,000 80 terms of control-

2,000 40 ling the way elec-

0 0 1970 1975 1980 1985 1990 1995 2000 2005 2010 trical energy is Year transported and Blocking capability Wafer area used. used due to the overall low system low n-doped base region at the losses. For relatively shorter distances pn-junction to support the high and lower power levels, IGBT-based electric fields required for the high VSC conversion is becoming the voltage ratings. For current-carrying system of choice due to a number of capability with low losses, they advantageous integration and control require large active areas and highly features (especially when taking into doped contact regions to provide account the introduction of renewable high minority carrier (holes) injection energy into the grid). Despite the levels for modulating the low doped existence of optimized high-voltage n-base, and good ohmic contacts to devices with attractive electrical the outside world. Current normally characteristics, the main development flows in the vertical direction (per trend continues to seek higher power pendicularly to the wafer surface) and superior overall performance. in devices whose voltage range exceeds 1 kV. Today, silicon-based Power semiconductor devices for devices can be designed with good HVDC applications overall performance parameters up to High-voltage power semiconductors 8.5 kV for a single component. It is differ from their low-voltage counter- important to note here that power part in a number of structural as- semiconductors employed in grid pects. First, they include a wide and systems do not differ from those

A single-source supplierTopic 45 3 Evolution of IGBT technology and modern structures

Enhanced planar IGBT Trench IGBT Planar Cathode MOS cells Cell design Enhancement

Trench IGBT High SOA n-enhancement technologies temp. p p layer NPT Bulk design SPT/FS PT 6,500 V 4,500 V n-base n-base 3,300 V Ratings 2,500 V n-buffer (SPT) 1,700 V 1,200 V 600 V p p

1990 1995 2000 2005 2010

Anode

employed in other power electronics bipolar device, which is mainly For relatively applications such as traction and characterized by its favorable excess industrial drives. carrier distribution for low on-state shorter distances losses in conduction mode. The IGBT, Nevertheless, for HVDC applications on the other hand, is a MOS (metal- and lower power operating in the hundreds of kilovolts oxide semiconductor)-controlled levels, IGBT- range, devices are normally connect- device with a bipolar effect for ed in series to support the total achieving low on-state losses. based VSC DC-line voltage. The choice of the single device voltage rating employed The PCT conversion is in these systems depends largely on In contrast to the IGBT, the PCT is becoming the the performance/cost calculation for not a turn-off device. It is nevertheless a given topology and operational the device of choice for line-commu- system of parameters. Devices rated for lower tated CSC HVDC systems due to its voltages normally have favorably exceptionally low on-state losses and choice. lower overall loss figures but imply very-high-power handling capability. that a larger number of components Until very recently, state-of-the-art are needed. single devices were rated at 8.5 kV with a total diameter of 125 mm. The basic structure of a PCT and With the increase in demand for even IGBT is shown in ➔ 1 as is its evolu- higher power HVDC system ratings, tion over the years along with other larger area 150 mm, 8.5 kV PCTs with current ratings up to The PCT is the device of 4,000 A were developed choice for line-commutated for the latest Ultra HVDC CSC HVDC systems due to systems its exceptionally low on-state operating at ± 800 kV with losses and very-high-power total transmission power handling capability. exceeding 7 GW. The device concepts such as the integrat- PCT evolution, including the new ed gate-commutated thyristor (IGCT). 150 mm PCTs and the UHVDC system The historical increase in the switching valves consisting of series connected power levels of high-voltage devices is PCTs, is shown in ➔ 2. also shown. The PCT is a thyristor

46 ABB review special report 4 High-voltage IGBT technologies on-state losses Vce(sat) 5 IGBT application from single chip to system valve

7 Wafer 6

5 System valve

4 Today Sub-module section (V)

ce(sat) 3 V 2

1 85 A/cm2 75 A/cm2 60 A/cm2 50 A/cm2 40 A/cm2 30 A/cm2 0 IGBT 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 V (V) ce Module Planar NPT Enhanced trench SPT limit

Planar SPT BiMOS thyristor

Enh. planar or trench SPT BiMOS thyristor limit or IGCT

Enhanced trench SPT

Further technology developments are on an enhanced-planar cell design underway to increase current ratings (EP-IGBT), which has enabled the Today, high-power to exceed 6,000 A while lowering establishment of a new technology conduction losses. curve benchmark over the whole IGBT press-pack IGBT voltage range from 1,200 V up and insulated The IGBT to 6,500 V as shown in ➔ 4. Future The IGBT is a MOS-controlled bipolar higher power densities and loss modules have rat- switch and presents the inherent reductions are possible by implement- advantages of that technology including Enhanced Trench ET-IGBT cell ings ranging from ing a controlled low-power driving designs, Emitter Switched Thyristor requirement and short-circuit self-limit- EST structures as shown in ➔ 4 and 1,700 V / 3,600 A ing capability. The IGBT has experi- integration solutions such as the to 6,500 V / 750 A. enced many performance break- bi-mode insulated-gate transistor throughs in the past two decades ➔ 3. (BIGT).

The fast progress in IGBT cell designs A customized press-pack module (Stak- (planar, trench, enhancement layers) Pak) was developed for series connec- and bulk technologies such as punch- tion of IGBT and diode chips to be used through (PT), non-punch-through (NPT) in grid applications. The mechanical and soft-punch-through (SPT) has led design is optimized to clamp the press to their widespread deployment in packs in long stacks. The module many high-voltage applications. Today, remains fully functional due to its design high-power IGBT press-pack and with individual press-pins for each chip insulated modules have ratings ranging as shown in ➔ 5. Furthermore, the from 1,700 V / 3,600 A to 6,500 V / choice of materials is optimized to 750 A. The most recent trends have achieve high reliability. targeted lower losses by using thinner n-base regions (SPT) and plasma enhancement layers and/or trench cell designs as shown in ➔ 3 accompanied by higher SOA and higher operating temperature (HT) levels. Similar development efforts targeted an improved diode design to match the latest IGBT performance. The free- Munaf Rahimo wheeling diodes play a very important Sven Klaka role in the application during normal ABB Semiconductors switching and under surge current Lenzburg, Switzerland conditions. The current IGBT platform [email protected] employed in grid applications is based [email protected]

A single-source supplierTopic 47 Crossing land and sea

High-voltage 1 Laying the Gotland cable in 1954 DC cables

THOMAS WORZYK, TORBJÖRN SÖRQVIST, OLA HANSSON, MARC JEROENSE – It is notable that Gotland 1, the very first modern HVDC project of the 1950s, featured an underwater link. Following this 2 An extruded HVDC cable for submarine application, equipped with optical fibers for 1954 breakthrough, further HVDC temperature monitoring cables were installed across the world. ABB has as of today delivered 2,300 km of mass impregnated DC cables.

The Gotland 1 link ➔ 1 did not only feature the first submarine MI (mass- impregnated) HVDC cable, but it also heralded an era of onshore connections. A new link was built at Gotland in 1999 using an 80 kV extruded installed more than 5,000 km of HVDC cable system. Three years later The Murray Link extruded HVDC cable ➔ 2 – 3. ABB installed the Murray Link at the antipodes using almost twice the is still today the Almost all modern HVDC cables are voltage (150 kV) and more than double single-core. The simplicity of the the length (180 km). The Murray Link longest land- design is one of the reasons for the is still today the longest land-based based high-volt- significant reduction in required high-voltage cable in the world. The investment compared with HVAC. contracting of a number of extruded age cable in the HVDC cables can be manufactured in HVDC cable projects for offshore many varieties. Round copper or wind farms at ± 320 kV is a clear world. aluminum conductors of large sizes indication for the confidence in this can be used. There are two different technology. As of 2015, the energy section, NordBalt will be the longest insulation types used for HVDC supply of Lithuania will be supported extruded high-voltage cable in the cables: Mass-impregnated cables (MI) by the record-breaking NordBalt world. Once the DolWin2 cable have a layered insulation made of project. With its 400 km submarine system is complete, ABB will have special paper and impregnated with a

48 ABB review special report 3 Cable history

HVDC and HVDC cables have always had a were expected to grow after the war. At the – Solid 90 mm2 solid copper conductor flavor of fascination – from the War of Currents same time, the work of Dr. Lamm in Ludvika in order to reduce disadvantageous between AC and DC at the end of the 19th resulted in mercury-arc valves with acceptable conductor expansion under cable torsion century to the German “Energiewende,” reliability. in manufacturing or submarine laying engineers and even the public continue to – 7 mm mass-impregnated insulation, discuss the fantastic possibilities of HVDC As so often happens in technical development, comfortably exceeding physical require- cables to transport large amounts of power not only ingenuity but its combination with ments over virtually unlimited distances. During the pioneering spirit and a certain market situation – Extra-dense insulation paper in order to first half of the 20th century, HVDC research in lead to the breakthrough – and to the Gotland avoid partial discharges or ionization countries such as Sweden, the United States project. The Swedish State Power Board as it was called at that time and Germany focused on converter stations. decided in 1951 to connect the Swedish Island – Double lead sheath. Liljeholmens AB later The development of high-voltage cables of Gotland electrically to the mainland – a developed a continuous lead sheathing principally served the widespread AC tech- distance of 100 km. No other technology but machine, which improved the quality and nology. Although some of the peculiarities a submarine HVDC cable could solve the task. emerged as a virtual standard in high-volt- surrounding HVDC cables were understood It was a bold decision – nobody had done this age cable sheathing early on, manufacturing experience remained before. – Double armoring in the landfall cables scarce. An HVDC cable system was built during WW II in Germany but was never commissioned Bjurström, the mastermind of Liljeholmens AB The cable design was so successful that the due to the turmoil of war. It was later re-erected at that time, presented the Gotland cable original Gotland cable link could be upgraded in Russia as an experimental line. A submarine at the 1954 Cigré conference, in a paper from 100 to 150 kV when power demand HVDC connection between Norway and the humbly titled “A 100 kV DC cable.” The increased. A piece of the original cable Netherlands was proposed as early as 1933, Gotland cable was a masterpiece of its time, was analyzed after decommissioning a few 75 years before ABB realized this link. engineered to be reliable. While it is quite decades later and showed no signs of aging. easy to produce a few hundred meters of ABB’s predecessor cable factory (Liljeholmens cable, the production and installation of Today, only the double-lead sheath would be AB) performed HVDC tests on mass-impreg- extreme lengths is a very different business. manufactured differently. nated cable in the 1940s, maybe sensing the Bjurström and his colleagues put all the best upcoming interest in the market where grids into the Gotland cable:

high-viscosity compound. The other type, the extruded insulation, com- The simplicity of the HVDC prises a polymeric insulation made by continuous extrusion of ultra-clean cable design is one of the polymeric material. MI cables are a reasons for the significant good choice for the highest ratings but extruded cables are catching up. reduction in required investment Both insulation types are solid, ie, no harmful oil is spilled should the cable when compared with HVAC. be damaged.

Since the insulation must be pro- Characteristics of HVDC cables temperature. A loaded cable gener- tected from water, submarine HVDC At first sight there is not much ates heat in the conductor, which will cables always feature a metallic difference between an HVAC and an typically result in a temperature drop sheath serving as a water block HVDC cable. Both contain a conduc- across the insulation. The tempera- and path for short-circuit currents. tor, insulation, a water barrier and in ture distribution will have a corre- Extruded underground HVDC cables the case of the submarine cable, sponding effect on the resistivity can instead use a lightweight metallic armor. The main difference lies in the distribution. As a result, the electrical laminate sheath and a copper wire electrical high-voltage insulation. field distribution may be inverted in a screen. loaded HVDC cable compared with From an electrical point of view, HVAC (where the electric field is The HVDC cable can be dressed with HVDC insulation behaves differently always the highest at the conductor tough outer plastic sheaths, or wire than its HVAC counterpart. While screen and decreases toward the armoring for, eg, subsea applications. the electrical field – an important insulation screen). The effect of field The extruded HVDC cable can be design and operational parameter – inversion is a well-known peculiarity installed in the most challenging is defined by the permittivity in an of both mass-impregnated and environments, such as the deep sea HVAC cable, it is (also) controlled extruded cable. and vertical shafts ➔ 4. by the resistivity in the HVDC case. The resistivity of the insulation material depends in part on the

A single-source supplier 49 4 Selection of ABB’s groundbreaking HVDC cable projects 5 The PEA technique presents a reliable method to quantify space charge Project Location DC voltage Insula- Year Remarks name tion Cable screen Guard ring Cu-foil Ferrite First submarine HVDC Semicon Gotland 1 Sweden 100 - 150 kV MI 1954 cable ever

Baltic Sweden- Highest Voltage, longest 450 kV MI 1994 Cable Germany length at that time

Longest underground UA Murray Link Australia ± 150 kV XLPE 2002 cable system of all times. PEA 580 km. Longest Norway- NorNed ± 450 kV MI 2008 power cable system, Netherlands all categories

First extruded HVDC SouthWest Sweden ± 320 kV XLPE 2015 system with combined Link cables and OH line.

Longest HVDC Light Pulse generator Sweden- system, 400 km submarine NordBalt ± 300 kV XLPE 2015 Lithuania cable route with cost- saving Al conductor

The development of the extruded (such as lightning and switching The main differ- cable took time: The physics of the impulses). DC field control is achieved polymer insulation under DC electric by a continuous layer of nonlinear field ence between stress was a new area, and proper grading material (FGM) connecting full non-destructive measurement potential to ground. HVAC and technology was not available. In the HVDC cable lies 1960s, theorists had predicted that so-called space charges would impact in the electrical the electric field and therefore possibly have a negative effect on the perfor- high-voltage mance of the cable. Space charges, insulation. as the term suggests, are charges that are present inside the insulation. These charges will affect the local field strength and “distort” the distribution of the electrical field. In the 1980s Tatsuo Takada in Japan presented a reliable measurement technology to quantify space charge in both magni- tude and space. The measurement principle is called the pulsed electro- acoustic (PEA) method. ABB was one of the first companies to use it ➔ 5.

Applying this tool as well as a wide technology base in physics, chemistry and high-voltage engineering, ABB was able to develop 80 kV extruded Thomas Worzyk cable systems. The technology has Ola Hansson since been boosted to 320 kV. Marc Jeroense ABB Power Systems Cable accessories for HVDC Light Lyckeby, Sweden extruded cables utilize a combination [email protected] of geometric and refractive field [email protected] grading for capacitive field distribu- [email protected] tions and nonlinear resistive field grading for DC field distributions. The Torbjörn Sörqvist accessories are designed to ensure an ABB Power Products appropriate electric-field level when Alingsås, Sweden subjected to time-varying voltages [email protected]

50 ABB review special report Topic 50 Conversion

HVDC valves 1 Valve suspended from the ceiling of the valve hall

JONATAN DANIELSSON, BAOLIANG SHENG – The heart of the converter is the valve, which comes in two basic designs for HVDC Classic and HVDC Light®, based on thyristors and IGBT semiconductors, respectively.

Two different designs of thyristor- based line-commutated valves have been developed by ABB: a conventional base-mounted design with support insulators standing on the floor, and another (more frequently For the sake of reliability they have a ing voltages and currents, while also used) design with the valve suspended minimum of electrical components reducing conducting and switching from the ceiling of the valve hall ➔ 1. and connections. The main compo- losses. The first HVDC valves designed The latter design is particularly suitable nents are thyristors and their voltage by ABB were for the upgrading of the for withstanding seismic stresses, but dividers, control units and heat sinks. Gotland transmission – they used also offers cost advantages. thyristors with an area of The design uses a modular concept The design uses a modular 5 cm2. The to ensure high reliability while retaining next valve the flexibility to design valves accord- concept to ensure high design (used ing to customer requirements and for the preferences. A valve is built up with reliability while retaining the Skagerrak one or several layers depending flexibility to design valves transmission) on the required voltage withstand featured capability ➔ 2. Each layer consists of according to customer require double-sided series-connected thyristor modules cooling of the with intermediate current-limiting ments and preferences. thyristors, reactors, corona shields and piping which had for the cooling liquid. The individual The modules are of compact design an area of 8 cm2. Today thyristors have parts are supported by a mechanical for easy access during maintenance. an area of up to 130 cm2 capable of structure with a central shaft running The design allows components to be withstanding continuous currents up vertically through the structure with exchanged without opening the water to 4,500 A and short-circuit current ladders and working platforms for cooling circuit ➔ 3. up to 50 kA, eg, for the Xiangjiaba- easy access. Shanghai ± 800 kV Ultra-HVDC Each module contains a number project [1]. Thyristor modules of thyristors connected in series. Thyristor modules are of a mechani- Continuous thyristor development cally standardized compact design. enables them to handle ever-increas-

A single-source supplierTopic 51 2 Single layer of a valve 3 Thyristor module

Reactor

Thyristor module

4 Voltage across thyristor level

Service platform Current busbar

Aluminum shields Water pipes

Light guide channel

Protected area

A switching position is built up from IGBT-based voltage source type mainly depends on the appli a thyristor with two parallel circuits, converter valves cation. each consisting of a damping circuit The use of IGBT-based voltage source and a DC grading circuit, as well a converters (VSCs) in HVDC power A switch-type valve has a close TCU (thyristor control unit). The TCU transmission was a breakthrough. apparent resemblance to conventional converts the optical firing pulses Its first application was in a 3 MW thyristor valves: A large number of from the control system to electrical HVDC Light test installation in Hällsjön series-connected IGBT devices are signals to trigger the gate that fires in 1997. Since then, 20 VSC HVDC switched simultaneously. Pulse-width the thyristor. The TCU includes modulation state-of-the-art built-in functions (PWM) is used that protect it against overvoltages The liquid in the closed to achieve a during the reverse recovery period good approxi- (after a thyristor turns off) as well as valve cooling system is mation of a from high voltages in the forward continuously passed through sinusoidal blocking state ➔ 4. output voltage a deionizing system to keep (AC volt- By using this hybrid technique, ABB age) ➔ 5. is able to provide a very compact its conductivity low. TCU. ABB’s service record is also A CTL unbreakable: Of the more than 19,000 converter thyristors installed since 2000, only power transmissions have been integrates the DC capacitors into the four thyristor failures have been installed or are under construction by valve. The valve consists of series- reported. This demonstrates the supe- ABB alone [2]. connected voltage cells that can rior design of electrically triggered produce a sinusoidal voltage ➔ 6. thyristors. Two types of VSCs have been devel- The greater the number of cells con- oped for HVDC transmission: the nected in series, the more sinusoidal “switch” type and the cascade two- the waveform. A CTL converter valve level (CTL) or “controllable voltage for the DolWin1 HVDC transmission source” type. The choice of converter project is shown in figure 3 page 25.

52 ABB review special report 5 Voltage and current in a switch-type VSC valve with PWM 6 Voltage output of a CTL-type converter with seven voltage cells in series connection

200 (kV)

ac(t) 100

-100 1 + ⁄2U (kV) and U dc v(t)

U -200 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 time (s) 4 U

(kV) 0 2 - ⁄ Udc v(t) I -2

-4 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 time (s)

7 StakPak IGBT used in ABB’s VSC valves 8 Overview of a typical valve cooling system 9 Typical skid-based modularized redundant cooling system Main pumps Shunt valves Deaeration vessel M

Outdoor Expansion coolers vessel Con- verter Mechanical filters valve

Deionizer filters Replenishment system

Mechanical filters

In this project, one valve consists of tors. This efficiently transports heat thirty-six voltage cells. away from the device to be cooled through heat exchangers using either The modular design concept for air or a secondary circuit. The liquid in valves is found in both switch and the closed-valve cooling system is CTL types. ABB’s VSC valves use continuously passed through a StakPak IGBTs ➔ 7. These switching de-ionizing system to keep its modules create an internal short conductivity low. circuit should they fail, making them similar to thyristors. This function A typical valve cooling system is Jonatan Danielsson allows a current to continue to con- shown in ➔ 8 and ➔ 9. ABB Power Systems, Grid Systems duct through a faulty IGBT device Raleigh, NC, United States without calling for an external bypass [email protected] circuit (as is the case for other IGBT types). The decreased deployment Baoliang Sheng of components at high potential can ABB Power Systems, Grid Systems therefore augment the valve’s avail- Ludvika, Sweden ability, reliability and compactness. [email protected] StakPak follows the conventional press-pack design advantages such as better cooling and robust mechani- References cal module structure [3]. [1] J. Waldmeyer and W. Zhengming, “Six Inch Thyristors for UHVDC,” in Proc. 2006 International Conference on UHV Transmission Valve cooling Technologies, Nov. 27–29, 2006, Beijing, China. The purpose of the cooling system is [2] G. Asplund, “Application of HVDC Light to to dissipate the power losses gener- Power System Enhancement,” IEEE/PES Winter Meeting, Singapore, January 2000. ated in the valves. Coolant fluid is [3] B. Jacobson, et al., “VSC-HVDC Transmission circulating through the heat sink in with Cascaded Two-Level Converters,” B4-110, close contact with the semiconduc- CIGRE 2010, Paris, France.

A single-source supplierTopic 53 Converter 1 An early converter transformer transformers

A 60-year journey

MATS BERGLUND – ABB’s HVDC- 2 The function of the HVDC converter transformer transmission transformer technology has evolved from the 1954 Gotland Voltage and dielectric separation integral part of the control of the HVDC In all HVDC systems (regardless of the system, both from an active and reactive link ➔ 1. ABB’s predecessor com- converter topology) the HVDC converter power perspective. Compared with other pany, ASEA, broke new ground with transformer serves as an interconnection transformers, the HVDC converter its 400 kV DC technology used in the between the AC grid and the converter transformer often displays a large regulation 1,000 MW CU project (Coal Creek valve. Besides converting AC voltage and range – a consequence of its role as an current to a level suitable for the converter integrated regulating function of the Station to Underwood, ND, United valve, the HVDC converter transformer fulfills HVDC system. States) in 1977. The company additional roles in the HVDC system. For asserted its undisputed technology many converter topologies (HVDC Classic Short-circuit current limitation and leadership with the Itaipu (Brazil) and asymmetric HVDC Light®) the most system properties important function of the transformer is to In the early days of converter valves, a key project of 1982 (3,150 MW / 600 kV keep the DC-voltage offset created by the feature of the transformers was to provide an DC). This was to be a cornerstone in converter valve out of the AC network. impedance to limit the short-circuit current HVDC converter-transformer history. This results in AC and DC stress being to the semiconductor valve. As the capability To realize this record-breaking superimposed in the winding of the of semiconductors grew, this property transformer. lessened in importance. The transformer still milestone, ASEA built on the fulfills basic functions in enabling correct knowledge it accumulated delivering Power and voltage regulation interaction between the HVDC transmission high power transformers to the The HVDC converter transformers is an and the AC network. booming AC-networks during the 1970s. 3 Design aspects for the HVDC-converter transformer It was not until the end of the 1990s that anything close to Itaipu would be High power, high voltage and DC Reactive loading attempted in terms of transmission HVDC converter transformers are among For HVDC transformers, reactive power the largest transformers in terms of power, capability of the complete transmission is power. The next records were set by voltage and complexity. On top of this come translated into the transformer’s ability to transmission projects in China: The the requirements dictated by the need to handle the reactive load. first two major HVDC links were from handle DC voltages. the Three Gorges hydropower station Short-circuit-like duty Impact from current harmonics The mercury-arc valves of the early HVDC to the load centers in eastern China, For some applications, the load current seen days sometimes “backfired,” subjecting for which ABB was a key supplier. by the transformers is not perfectly smooth. the transformers to a condition close to This implies extra care for the thermal short circuit. Apparently, the ABB design was A diversification of HVDC-transmis- dimensioning of the HVDC converter sufficiently robust as there is no history of transformer as well as its accessories, short-circuit incidents – but this was not sion configurations into the Light and bushings and tap changers. formally proven until 2007 when ABB Classic variants started at the end of short-circuit tested an HVDC converter the 1990s. The thyristor-based HVDC transformer for the first time. Classic has built on its strengths –

54 ABB review special report 4 HVDC transformer configurations The ultrahigh All HVDC systems need transformers, but direct consequence of HVDC-transmission how these are arranged can vary quite power capability and the transformer DC-transmission substantially between projects depending on topology. the specification of the project. In their basic voltage is highly forms, each pole of an HVDC Classic system For HVDC Classic, the transformer needs to be fed by six phases of transform- alternatives are shown below. challenging to ers while HVDC Light needs a three-phase supply. Typically, more transformers are needed the insulation when transmission power and transmission The HVDC-transformer configuration of an voltage are high. The spare transformers abilities of the individual project is unique to the project and strategy in the converter station also plays determined by the sheer size of transformers an important role in the selection of converter in relation to the limitations in the transport transformer solution. infrastructure between the place of manu- transformers. facture and the HVDC converter station. For HVDC Light, the transformer solutions The main driver behind size of the converter possible often are limited to the choice of transformer is the rated power, which is a single-phase and three-phase transformers.

transmission voltage of 1,100 kV DC. 3-phase, 3-winding 3-phase, 2-winding 1-phase, 3-winding 1-phase, 2-winding This means that the transformers transformers transformers transformers transformers must have a very high power rating, Y Y Y Y and thus a substantial physical size Y Y – weighing in excess of 600 t ➔ 6.

Y Y Y Y Y Y Y Y The ultrahigh DC-transmission voltage Y Y is highly challenging to the insulation Y Y Y abilities of the converter transformers. Y These are subjected to extreme test 1 unit per 2 units per 3 units per 6 units per 12-pulse bridge 12-pulse bridge 12-pulse bridge 12-pulse bridge voltages before delivery. Besides the voltage stresses often appearing in HVDC Classic, these transformers often claim superlatives in terms of 5 Project execution power rating as well. The largest HVDC converter transformers built

Technical and scientific proficiency is one China are examples where ABB has set have single-phase power ratings in important aspect of HVDC converter industry records. In the project between excess of 600 MVA – the largest in the transformers, but equally important is the Hami and Zhengzhou ABB delivered all world of transformers ➔ 7. way projects are executed and how HVDC converter transformers to the technology is implemented. HVDC projects 8,000 MW, 800 kV DC mega-project in often need a long series of HVDC converter 17 months. This should be compared with HVDC Light transformers being manufactured in a timely the delivery times of the Three Gorges HVDC Light systems share certain manner to complete the supply for an projects where the complete time schedule features with HVDC Classic systems, individual project. As lead times for delivery ranged over 36 months. These improve- while others are less prominent. of projects are gradually reduced, this aspect ments could very well stem from the Depending on the system configura- becomes more and more important. abundance of projects delivered: ABB was responsible for more than 50 percent of all tion, the converter transformers are ABB has taken the lead in this important the projects realized – far more than any of sometimes exposed to a DC stress development. The deliveries to SGCC in its competitors. similar to that of the HVDC Classic systems, while in some system configurations they are not. The expanding into ultrahigh transmission To understand what contemporary further the capability of HVDC Light voltages enabling very efficient power HVDC transmission means for the advances, the greater the DC stress transmission over long distances. HVDC converter transformers a that the converter transformer must HVDC Light, which started off as a complete view of the technologies and face ➔ 8. moderate power application, has their applications are given below. grown into a medium-power transmis- A clear contrast to the HVDC Classic sion system that nowadays can do HVDC transformers today systems is found in the load current what HVDC Classic could do at the HVDC Classic has in recent years that the converter transformers for end of the 1990s ➔ 2 – 5. seen rapid expansion in terms of HVDC Light are subject to. For HVDC performance. HVDC Classic can now Light, the load current is in most transmit up to 11,000 MW at a cases free from current harmonics,

A single-sourcesingle source supplierTopic 55 6 800 kV UHVDC (ultrahigh voltage direct current) converter transformer 7 Power rating references

Project Rated power Year

Itaipu 314 MVA 1982

Quebec-New 404 MVA 1987 England

Sylmar 625 MVA 2007

Rio Madeira 630 MVA 2011

Celilo upgrade 785 MVA 2014

The thyristor- based HVDC Classic has built on its strengths – expanding into ultrahigh transmission voltages enabling very efficient power 8 Converter transformers and the spare transformer at Woodland HVDC Light® station in Ireland. transmission over long distances.

HVDC converter transformers will need to connect HVDC systems directly to AC networks at even higher voltages than are used today. An HVDC-transformer technology for interconnecting the highest voltage DC transmissions to 800 kV AC systems already exists and there will soon be commercial applications. Beyond this, interconnections to even higher AC voltages, such as the 1,000 kV AC networks in China, are a possibility.

whereas in HVDC Classic the trans- reactive, something that needs to formers must cope with the thermal be carefully considered in the trans- stress originating from the extra losses former design. incurred by current harmonics. The future Mats Berglund One consequence of the reactive It is not only the DC voltage level on ABB Power Products power capability of HVDC Light is that the HVDC transmission itself that is Ludvika, Sweden a large portion of the load can be being increased. Another trend is that [email protected]

56 ABB review special report Capacitors

A key component 1 Capacitor bank at Porto Velho HVDC station in Brazil in HVDC systems

BIRGER DRUGGE, PETER HOLMBERG – Capacitors have multiple functions in HVDC systems ➔ 1. They cover aspects such as reactive power compensation, DC filtering, AC filtering and power-line carrier coupling. Capacitor technology for HVDC has gone through the various shifts in technology from impregnated mixed dielectrics (paper/film) to full-film dielectrics with highly specialized capacitor fluids. The latest technology used in HVDC Light® applications features dry solutions based on special metallized film dielectrics.

ABB’s technology is based on its long experience and deep understanding of specific DC phenomena such as space charges. An example of ABB’s prowess is the internally fused capacitor used to maintain high levels of power quality and availability.

HVDC’s increasing voltage levels have introduced special mechanical challenges in the design of the capaci- As a technology leader, ABB also tor stacks, not the least of which drove the technology shift to dry concern seismic requirements. HVDC capacitors for HVDC Light, required to handle unit voltage ratings Capacitors are part of the output filter above 150 kV. ABB’s dry DC capacitor that reduces harmonics. One chal- technology is based on metallized film Birger Drugge lenge faced is that these harmonics with self-healing properties. With Peter Holmberg generate sound in the components. ABB’s dry technology, customers ABB Power Products, Special patented sound-attenuating benefit from higher availability and High Voltage Products solutions have been developed to reduced footprint of the link, also Ludvika, Sweden keep the sound level low and meet improving the eco-aspect of the [email protected] environmental requirements. solution. [email protected]

A single-source supplierTopic 57 1 ABB’s HVDC arresters 2 Comparison of gapped SiC arrester and gapless ZnO surge arrester

6- to 12-pulse valve AC bus

Neutral bus HVDC AC filter arrester applications

DC breaker

DC filter DC bus

2a XDL 280 A surge 2b ZnO surge arrester (1970s) arrester (1980s) Surge arresters

60 years of HVDC protection

LENNART O. STENSTROM, JAN-ERIK The various arresters normally used in A short history of ABB (and predeces- SJÖDIN – An HVDC station features an HVDC project are shown in ➔ 1. sor company ASEA) HVDC surge different types of surge arresters to arresters is shown in ➔ 3 and ➔ 5. protect the equipment from overvol- Historically HVDC stations were first tages caused by lightning and/or protected by gapped SiC surge ABB has a number of designs for switching events. In some equipment arresters until the late 1970s when HVDC arresters optimized for thermal, such as DC breakers, surge arresters gapless ZnO surge arresters were electrical and mechanical stresses, also may form an integral part of the introduced. The last generation of the safety requirements and cost depend- apparatus. gapped arresters were quite complex ing on the application. and comprised a vast number of The stresses the arresters have to be components such as spark gaps, To date, tens of thousands of ABB designed for, compared with normal SiC blocks, grading capacitors and HVDC arresters have been delivered AC arresters, are characterized by, for grading SiC resistors. The design worldwide with an excellent service example: became even more complex when record. – Special voltage waveforms compris- parallel columns had to be used to ing frequency components ranging meet high energy demands or low Instrument transformers – from pure DC, and power frequency protection levels ➔ 2 shows how the reliable metering and protection to several kHz introduction of the ZnO material ABB has been producing instrument – Very low protection levels required remarkably simplified the internal transformers by the thousands for – High energy demands – eg, for valve design of the surge arresters com- more than 70 years ➔ 4. Their applica- arrester, neutral-bus arresters and pared with the previous gapped tions range from revenue metering, arresters for DC breakers HVDC arrester. control, indication and relay protec- – High ambient temperature, eg, for tion. One field of application in HVDC valve arresters is the use of capacitor voltage

58 ABB review special report 3 Surge arrester 4 420 kV cap. 5 The first gapless HVDC arrester and the first 500 and 1,100 kV DC bus arresters with history at ABB volt. transf. polymer housings

Pre-1979: Gapped SiC arrester 1979: First gapless ZnO surge arrester (probably the world’s first) delivered to a 250 kV HVDC station in Denmark ➔ 5a 2000: First 500 kV arrester with polymer-housing delivered to the 3GS project in China ➔ 5b 2006: First 800 kV arrester type tested and energized for a China project 2012: First 1,100 kV arrester fully type tested ➔ 5c

5a Gapless HVDC line 5b 500 kV DC bus arrester 5c 1,100 kV DC bus arrester (1979) (2000) arrester (2012)

of a capacitor voltage transformer ABB has been incorporates an inductive reactor, connected in series between the producing instru- capacitive voltage divider and the ment transform- high-voltage end of the primary winding, to compensate for the shift ers by the thou- in phase angle caused by the capacitive reactance of the capaci- sands for more tor’s voltage divider. This type of than 70 years. compensation allows simple construction to obtain high accuracy for high loads. However, the reactance transformers to obtain the source and capacitance form a tuned circuit voltage for converter control, as well that gives relatively low accuracy at as to obtain signals for the protec- frequencies outside the nominal tion of the converter station. The frequency. output from the capacitor voltage transformers is used to trigger the In an HVDC application, the capacitor thyristor valves at a certain time, voltage transformer that is used to based on the previous zero crossing obtain the source voltage for special of the AC voltage. Because even requirements in converter control for minor disturbances can be disas- transient and frequency response trous to the operation of the HVDC uses an electromagnetic unit without station, special requirements must be a separate compensation reactor. For met for transient and frequency this special type, the function of the response. compensation reactor and the primary winding of the intermediate Lennart O. Stenstrom The first capacitor voltage transform- transformer are combined into one Jan-Erik Sjödin ers were produced by ABB in the device. This arrangement gives a ABB Power Products, 1950s for delivery to the 1,000 km, substantially increased operating High Voltage Products 400 kV AC transmission line from frequency range, and highly improved Ludvika, Sweden Harsprånget to Hallsberg in Sweden. transient response. [email protected] The electromagnetic unit in the base [email protected]

A single-source supplier 59