Thyristors forHVDC, HVDC
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//-->Application of High Power Thyristorsin HVDC and FACTS SystemsHartmut Huang#1, Markus Uder#2Siemens AG, Fryerslebenstr. 1, 91056 Erlangen, Germany1hartmut.huang@siemens.com2markus.uder@siemens.comReiner Barthelmess#3, Joerg Dorn#4Infineon Bipolar GmbH & Co. KG, Max-Planck-Str. 5, 59581 Warstein, Germany3reiner.barthelmess@infineon-bip.com4joerg.dorn@infineon-bip.comAbstract—Both HVDC and FACTS systems use power electronic converters for the power conversion and powerquality control. High power thyristors have been serving as the key component in HVDC and FACTS converters forseveral decades now and are still being further developed for higher power rating nowadays. This paper describes thethyristor technology and its development in application in HVDC and FACTS. The fundamental features andcharacteristics of high power thyristors is discussed with particular reference to its application in high voltage andhigh current area. Many thyristors connected in series together with specially designed auxiliary mechanical andelectronic systems build so called thyristor valves, which form the HVDC and FACTS converters. An overview ofthyristor valve design is provided. Furthermore, the latest development in the thyristor and thyristor valve technologyand its application in the ultra high voltage DC application (800 kV) is introduced. A summary of technical keyparameters and design features of 6” thyristor valves are provided including the valve design date for the firstUHVDC application.I.INTRODUCTIONThere is an increasing demand for high efficiency and highquality of power transmission world wide. In this context themodern High Voltage DC Transmission (HVDC) and FlexibleAC Transmission Systems (FACTS) gains more importanceand utilization in today’s power transmission system. BothHVDC and FACTS systems use power electronic convertersfor the power conversion and power quality control. Thereforethe performance and quality of converter systems dependmuch on the key component- high power thyristors. Since itsintroduction in the HVDC application late sixties of lastcentury, thyristor technology has continuously furtherdeveloped to higher power rating over last decades (Fig.1).The first thyristors used had a silicon wafer with a diameter of33mm. They had a peak blocking voltage of 1600V andsupported a direct current of up to 1000 A. For higher currentratings, thyristors were connected directly in parallel. Over thelast thirty years, the device ratings were permanentlyincreased. Today silicon wafers of 6 inch diameter can bemanufactured; the peak blocking voltage per device is 8000Vand a d.c. current of 4500A can be handled without parallelconnection.The increase of thyristor’s power rating goes hand in handwith increased demand for larger HVDC power transmissionschemes. Particularly the need to maximize the utilization ofland and space for transmission lines requires highertransmission voltage, which reduced the transmission losses aswell. During last decades most bulk HVDC transmissionschemes worldwide have been built with 500 kV as rated dcvoltage. Recently years there are several large HVDCtransmission schemes under planning in China, India andBrazil, which have a transmission distances between 1000 kmand 2000 km. Ultra high dc voltage (UHVDC) in the range800 kV is the preferred dc voltage level for these applications.864219701980199020002010Fig. 1: Development of voltage rating (blue line) in kV and current rating (redline) in kA of power thyristorsWhile the first 800 kV HVDC project Yun-Guang has a powerrating of 5000 MW, other 800 kV HVDC projects has asignificant higher dc current. Xiangjiaba-Shanghai Project hasa power rating of 6400 MW (dc current =4 kA) and JinpingUHVDC Project has the highest bipole rating of 7200 MWwith rated dc current of 4.5 kA. In order to provide anoptimized converter design to cover these high dc current andvoltage application, new thyristors with larger diameters havebeen developed.II.STATE OFART OF MODERNTHYRISTORTECHNOLOGYIn 1960 the development of thyristors (also called SCRs =silicon controlled rectifier) was started; since that time manydevelopment steps followed in order to increase the powercapability of the devices and to improve the reliability.Power thyristors are manufactured from highly puremonocrystaline silicon. They are so called NPNPsemiconductors. This means that they consist of four layerswhich are doped alternately with P and N (Fig. 2). The outer,highly doped zones are the emitting zones; the weakly doped,inner layers are the base zones. The control connection G islocated on the P base; J1-J3 designate the junctions betweenindividual zones. The off-state voltage in the reverse directionis blocked at junction J1 between P-emitter and N-base. Theoff-state voltage in the forward direction is blocked at junctionJ2 between P-base and N-base.current (several mA) flows both in the forward direction andin the reverse direction.Also a non-ideal static behaviour of the thyristor is the on-state voltage during conduction. The entire voltage drop of anHVDC thyristor is of the order of two to three volts. Thismeans that for typical currents several kA, considerable powerlosses must be dissipated.Thyristors in press pack housings are ideal for both,efficient cooling of the device and stacking for seriesconnection.Fig. 3: Schematic illustration of stresses on press pack power thyristorNext to the static non-ideal behaviour, thyristors have alsodynamic restrictions:Limited di/dt-capability after turning on, as well as thereverse recovery behaviour including turn-off time has to beconsidered in the design of the powers stack.Fig. 2: Schematic cross section illustration of a high power thyristorThyristors are fast but not ideal switches. Several of theimperfections of the thyristor in comparison with the idealswitch can be recognized in the static V/I-characteristic of thethyristor. In the presence of off-state voltage, an off-stateFig. 4: High power thyristors made of 4”, 5” and 6” silicon waferDespite the high blocking capability of modern thyristorsstill a series connection of thyristors is necessary to compose avalve with the required high voltage withstand capability.The number of thyristors that have to be connected in seriesvaries – depending on the application- between e.g. 10thyristors per valve rated 8kV in a typical SVC applicationand up to 120 thyristors in a typical HVDC valve in an 800kVconverter.A. Electrical valve componentsDue to the fact that a thyristor is not an ideal switch and toproperly perform their function in the series connection underall steady state and transient conditions, the thyristors need tobe complemented by auxiliary components: snubbercapacitors, snubber resistors, non linear reactors, d.c. gradingresistors, and grading capacitors.CKgrading capacitorFig. 5: Photgraph of a 6 inch thyristorThere is a trend towards higher transmission currentcapability of long-distance HVDC systems. With this trend,the requirements for higher current capabilities arise. On theother hand, the blocking voltage of about 8kV per thyristorwas derived as an optimum of overall operational losses.As a consequence, a 6 inch thyristor with a blockingvoltage of 8 kV (repetitive blocking voltage) was developed.This thyristor is capable to be utilized for dc transmission withcurrents up to 4500 A. Due to the joining of the silicon waferwith a molybdenum carrier disc, the required surge currentcapability could be reached with an excellent high safetymargin. These immense current capabilities make the thyristoralso interesting for other applications with high currentrequirements and high blocking voltage needs.CBRBRDCLVDthyristor levelsaturablereactorFigure 6: Main circuit components and their circuit arrangement in HVDCthyristor valves; valve used as a dc switchIII.HIGHPOWERTHYRISTORVALVESSince the first commercial use of high voltage thyristorvalves in HVDC-transmission systems in the early seventies,there has been a constant enhancement of performanceconcerning the thyristors blocking as well as current carryingcapability.That improvement of the thyristor characteristics results ina drastic decrease of components in a thyristor valve: totransmit the same amount of power as in the beginning of thethyristor-era in HVDC-technique, only about 5% of thethyristors (and snubber circuits) are necessary today.Thus the reliability of the valves was considerablyincreased and the way was pathed to the advantageous designof modern thyristor valves resulting in a clear structured andcompact valve setup comprising easy assembly, easyaccessibility and easy maintainabilityCBRBRDCthyristor levelFigure 7: Main circuit components and their circuit arrangement in SVCthyristor valves; valve used as an ac switch1) Snubber capacitors CSSnubber capacitors are required in parallel to each thyristorto handle the voltage overshoot during turn off. In a modernthyristor valve, they are single, SF6 filled units rated for thefull blocking capability of the thyristor.2) Snubber resistors RSTo damp oscillations caused by the combination of snubbercapacitor and circuit inductance, a resistor is connected inseries to the capacitor. The resistor is subjected to the fullsnubber capacitor current. Therefore, it has to be designed forhigh losses.To dissipate these losses the deionized water available inthe valve is used due to its good heat removal capability. Theresistive material is directly placed into the water (wire-in-water technology). A resistor of this type can dissipate from4.5 kW to 7 kW at moderate flow rate.3) DC grading resistors RDCWhen the valve is blocked and is subjected to d.c. voltage,the voltage distribution along the series connection isdetermined by the leakage current of the thyristors which issubject to manufacturing tolerances. With an appropriatevalve cooling design (see below) part of the d.c. grading isachieved by the water circuit. In addition, a self cooledresistor of about 0.5MΩ is connected in parallel to eachthyristor.Due to huge dimensions of high voltages resp. ultra highvoltage HVDC thyristor valves additional components arenecessary to limit the impact of the large -converter inherent-stray capacitances on the thyristors.4) Valve reactors LVDTo limit the di/dt stress of the thyristors at turn on and thedv/dt during transients in the off state, reactors are connectedin series with the thyristor string which have to meetconflicting requirements: a high inductance at the beginningof current flow but a low inductance as soon as the thyristor isturned on safely, so as not increase the commutating reactance.The valve reactors are therefore designed with a saturatingiron core.Without further provisions, the valve reactor would forman oscillating circuit of low damping with the straycapacitances of the converter. This can result in a highoscillating discharge current that extinguishes the turn oncurrent in the thyristor. The reactor is therefore provided witha damping resistor that is coupled via a secondary windingand thus is not effective when the reactor core has saturated.5) Grading capacitors CKThe various components in the valve, being at differentelectrical potentials and at different distances with respect toground and to other components, represent a complex networkof stray capacitances. For steep voltage transients, an unevenvoltage distribution between thyristor levels would result. Tocontrol this unbalance, grading capacitors of a few nF areconnected in shunt to the series connection of thyristor levelsand valve reactors. They are not required (and only littlestressed) at low frequency phenomena but linearize thevoltage distribution for high frequency (steep) wave shapes.They are filled with SF6 gas to achieve a high voltagewithstand without the use of oil as a dielectric.B. Thyristor Gating and monitoringBecause of the high voltage environment of the thyristors,it is absolutely necessary to electrically separate the triggeringand monitoring unit at ground potential (referred to as valvebase electronics VBE) from the thyristor at high voltagepotential. Therefore, the trigger command for the thyristor istransmitted as a light pulse via a fibre optic cable irrespectibleof the thyristor type used: electrically triggered thyristors(ETT) or direct light triggered thyristors (LTT).Figure 8: gating and monitoring of light triggered thyristors (LTT) andelectrical triggered thyristors (ETT).Associated to each thyristor a printed circuit boardmonitors the state of the thyristor and generates check backsignals also transmitted via fibre optical cables to the VBE.The check back signals of all thyristor levels are processedin the VBE and communicated to the converter control unit.The main task of that valve monitoring system is to check theavailability of the thyristor valve resp. the converter.To enhance the reliability of the thyristor valves redundantthyristor levels are incorporated in the series string of levels.Due to the fact that even a defective press pack thyristor isable to handle the full load current, the valve could remain inoperation without restriction as long as the number ofdefective levels in one valve does not exceed the number ofredundant levels.C. Valve coolingIn HVDC thyristor valves, more than 95% of the heatlosses are produced in the thyristors, snubber resistors, andvalve reactors, requiring forced cooling. Due to its goodthermal capability water is used as cooling medium inthyristor valves. To serve as an effective insulating medium,and to limit electrolytic currents, the conductivity of the wateris maintained at or below about 0.2µS/cm at maximum waterinlet temperature. Also, the cross-section of all piping is keptas small as possible to provide for a high effective resistance.By choosing a proper geometry of the physical layout (fig.11) and by placing electrodes at strategic locations, the waterpipes connecting to the thyristor heat sinks can be made tohave the same electrical potential throughout avoidingelectrolytic currents between the water cooled components ofa thyristor stack.grading electrodewater outFigure 10: modular unit used in SVC applicationswater inFigure 9: piping configuration for the cooling circuit of a thyristor stack.On the other hand, due to the conductivity of the water, thepiping of the cooling circuit in a thyristor valve functions as aresistive network. By appropriate layout of the pipe work suchas the parallel circuit in fig.9 this effect is used to advantage toprovide resistive voltage grading of the thyristor levels andvalve sections, assuming part of the duty of the d.c. gradingresistors.D. Valve mechanical designTo easily adapt the thyristor valves to the HVDC orFACTS application and to standardize the valve design astrictly modular design is used to compose a customizedthyristor valve resulting in a cost optimized design.The thyristor modules (Figs. 10, 11) are self-supportingunits with a frame of aluminium profiles, which mechanicallysupports all components within the modules.Figure 11: modular unit used in HVDC applicationsThe arrangement of the thyristors and heat sinks in thestack and their associated equipment is a straightforwardimage of the electric circuit diagram. A uniform voltagegrading and ease of testing are advantages of this design.The mechanical arrangement of a valve depends on theapplication and the number of series connected thyristor levels.A typical valve design used in a Static VAR CompensatorIn HVDC thyristor modules the frame also serves as a consists of three modular units -each one associated to a phasecorona shield; its electrical potential is that of the centre cross in a three-phase system- arranged on top of each other thusbeam so that the module is divided into two symmetrical areas. forming a three-phase ac switch.The tower stands on the valve hall floor. The fibre opticalEach area accommodates a complete valve section, consistingof thyristor stack, snubber circuits, valve reactors, monitoring cables and the cooling water tubes are supplied from theboards, grading capacitor, water circuit and the routing of the bottom side of the toweroptical fibres.
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