Application of High Power Thyristors in HVDC and FACTS Systems

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Application of High Power Thyristors in HVDC and FACTS Systems Powered By Docstoc
					                   Application of High Power Thyristors
                     in HVDC and FACTS Systems
                                                 Hartmut Huang#1, Markus Uder#2
                                    Siemens AG, Fryerslebenstr. 1, 91056 Erlangen, Germany

                                                Reiner Barthelmess#3 , Joerg Dorn#4
                       Infineon Bipolar GmbH & Co. KG, Max-Planck-Str. 5, 59581 Warstein, Germany

       Abstract— Both HVDC and FACTS systems use power electronic converters for the power conversion and power
       quality control. High power thyristors have been serving as the key component in HVDC and FACTS converters for
       several decades now and are still being further developed for higher power rating nowadays. This paper describes the
       thyristor technology and its development in application in HVDC and FACTS. The fundamental features and
       characteristics of high power thyristors is discussed with particular reference to its application in high voltage and
       high current area. Many thyristors connected in series together with specially designed auxiliary mechanical and
       electronic systems build so called thyristor valves, which form the HVDC and FACTS converters. An overview of
       thyristor valve design is provided. Furthermore, the latest development in the thyristor and thyristor valve technology
       and its application in the ultra high voltage DC application (800 kV) is introduced. A summary of technical key
       parameters and design features of 6” thyristor valves are provided including the valve design date for the first
       UHVDC application.

                                                                    land and space for transmission lines requires higher
                       I. INTRODUCTION                              transmission voltage, which reduced the transmission losses as
   There is an increasing demand for high efficiency and high       well. During last decades most bulk HVDC transmission
quality of power transmission world wide. In this context the       schemes worldwide have been built with 500 kV as rated dc
modern High Voltage DC Transmission (HVDC) and Flexible             voltage. Recently years there are several large HVDC
AC Transmission Systems (FACTS) gains more importance               transmission schemes under planning in China, India and
and utilization in today’s power transmission system. Both          Brazil, which have a transmission distances between 1000 km
HVDC and FACTS systems use power electronic converters              and 2000 km. Ultra high dc voltage (UHVDC) in the range
for the power conversion and power quality control. Therefore       800 kV is the preferred dc voltage level for these applications.
the performance and quality of converter systems depend
much on the key component- high power thyristors. Since its           8
introduction in the HVDC application late sixties of last
century, thyristor technology has continuously further
developed to higher power rating over last decades (Fig.1).
The first thyristors used had a silicon wafer with a diameter of
33mm. They had a peak blocking voltage of 1600V and
supported a direct current of up to 1000 A. For higher current
ratings, thyristors were connected directly in parallel. Over the
last thirty years, the device ratings were permanently
increased. Today silicon wafers of 6 inch diameter can be
manufactured; the peak blocking voltage per device is 8000V
and a d.c. current of 4500A can be handled without parallel
                                                                      1970           1980           1990           2000          2010
   The increase of thyristor’s power rating goes hand in hand
with increased demand for larger HVDC power transmission            Fig. 1: Development of voltage rating (blue line) in kV and current rating (red
                                                                    line) in kA of power thyristors
schemes. Particularly the need to maximize the utilization of
                                                                         current (several mA) flows both in the forward direction and
While the first 800 kV HVDC project Yun-Guang has a power                in the reverse direction.
rating of 5000 MW, other 800 kV HVDC projects has a                         Also a non-ideal static behaviour of the thyristor is the on-
significant higher dc current. Xiangjiaba-Shanghai Project has           state voltage during conduction. The entire voltage drop of an
a power rating of 6400 MW (dc current =4 kA) and Jinping                 HVDC thyristor is of the order of two to three volts. This
UHVDC Project has the highest bipole rating of 7200 MW                   means that for typical currents several kA, considerable power
with rated dc current of 4.5 kA. In order to provide an                  losses must be dissipated.
optimized converter design to cover these high dc current and               Thyristors in press pack housings are ideal for both,
voltage application, new thyristors with larger diameters have           efficient cooling of the device and stacking for series
been developed.                                                          connection.


    In 1960 the development of thyristors (also called SCRs =
silicon controlled rectifier) was started; since that time many
development steps followed in order to increase the power
capability of the devices and to improve the reliability.

   Power thyristors are manufactured from highly pure
monocrystaline silicon. They are so called NPNP
semiconductors. This means that they consist of four layers
which 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 is
located on the P base; J1-J3 designate the junctions between
individual zones. The off-state voltage in the reverse direction
is blocked at junction J1 between P-emitter and N-base. The
off-state voltage in the forward direction is blocked at junction
J2 between P-base and N-base.
                                                                             Fig. 3: Schematic illustration of stresses on press pack power thyristor

                                                                            Next to the static non-ideal behaviour, thyristors have also
                                                                         dynamic restrictions:
                                                                            Limited di/dt-capability after turning on, as well as the
                                                                         reverse recovery behaviour including turn-off time has to be
                                                                         considered in the design of the powers stack.

Fig. 2: Schematic cross section illustration of a high power thyristor

   Thyristors are fast but not ideal switches. Several of the
imperfections of the thyristor in comparison with the ideal
switch can be recognized in the static V/I-characteristic of the         Fig. 4: High power thyristors made of 4”, 5” and 6” silicon wafer
thyristor. In the presence of off-state voltage, an off-state
                                                                        Despite the high blocking capability of modern thyristors
                                                                    still a series connection of thyristors is necessary to compose a
                                                                    valve with the required high voltage withstand capability.

                                                                       The number of thyristors that have to be connected in series
                                                                    varies – depending on the application- between e.g. 10
                                                                    thyristors per valve rated 8kV in a typical SVC application
                                                                    and up to 120 thyristors in a typical HVDC valve in an 800kV

                                                                    A. Electrical valve components

                                                                       Due to the fact that a thyristor is not an ideal switch and to
                                                                    properly perform their function in the series connection under
                                                                    all steady state and transient conditions, the thyristors need to
                                                                    be complemented by auxiliary components: snubber
Fig. 5: Photgraph of a 6 inch thyristor
                                                                    capacitors, snubber resistors, non linear reactors, d.c. grading
                                                                    resistors, and grading capacitors.
   There is a trend towards higher transmission current
capability of long-distance HVDC systems. With this trend,                                       CK          grading capacitor
the requirements for higher current capabilities arise. On the
other hand, the blocking voltage of about 8kV per thyristor
was derived as an optimum of overall operational losses.                                  CB           RB
   As a consequence, a 6 inch thyristor with a blocking
voltage of 8 kV (repetitive blocking voltage) was developed.
This thyristor is capable to be utilized for dc transmission with                               RDC
currents up to 4500 A. Due to the joining of the silicon wafer                                                          LVD
with a molybdenum carrier disc, the required surge current
capability could be reached with an excellent high safety
margin. These immense current capabilities make the thyristor                                                        saturable
                                                                                          thyristor level             reactor
also interesting for other applications with high current
requirements and high blocking voltage needs.
                                                                    Figure 6: Main circuit components and their circuit arrangement in HVDC
                                                                    thyristor valves; valve used as a dc switch

                                                                                                CB            RB

   Since the first commercial use of high voltage thyristor                                            RDC
valves in HVDC-transmission systems in the early seventies,
there has been a constant enhancement of performance
concerning the thyristors blocking as well as current carrying
                                                                                                  thyristor level
   That improvement of the thyristor characteristics results in
a drastic decrease of components in a thyristor valve: to
transmit the same amount of power as in the beginning of the
thyristor-era in HVDC-technique, only about 5% of the
thyristors (and snubber circuits) are necessary today.              Figure 7: Main circuit components and their circuit arrangement in SVC
                                                                    thyristor valves; valve used as an ac switch
   Thus the reliability of the valves was considerably
increased and the way was pathed to the advantageous design
of modern thyristor valves resulting in a clear structured and      1) Snubber capacitors CS
compact valve setup comprising easy assembly, easy                     Snubber capacitors are required in parallel to each thyristor
accessibility and easy maintainability                              to handle the voltage overshoot during turn off. In a modern
thyristor valve, they are single, SF6 filled units rated for the    stressed) at low frequency phenomena but linearize the
full blocking capability of the thyristor.                          voltage distribution for high frequency (steep) wave shapes.
                                                                    They are filled with SF6 gas to achieve a high voltage
   2) Snubber resistors RS
                                                                    withstand without the use of oil as a dielectric.
   To damp oscillations caused by the combination of snubber
capacitor and circuit inductance, a resistor is connected in
series to the capacitor. The resistor is subjected to the full      B. Thyristor Gating and monitoring
snubber capacitor current. Therefore, it has to be designed for         Because of the high voltage environment of the thyristors,
high losses.                                                        it is absolutely necessary to electrically separate the triggering
   To dissipate these losses the deionized water available in       and monitoring unit at ground potential (referred to as valve
the valve is used due to its good heat removal capability. The      base electronics VBE) from the thyristor at high voltage
resistive material is directly placed into the water (wire-in-      potential. Therefore, the trigger command for the thyristor is
water technology). A resistor of this type can dissipate from       transmitted as a light pulse via a fibre optic cable irrespectible
4.5 kW to 7 kW at moderate flow rate.                               of the thyristor type used: electrically triggered thyristors
   3) DC grading resistors RDC                                      (ETT) or direct light triggered thyristors (LTT).
   When the valve is blocked and is subjected to d.c. voltage,
the voltage distribution along the series connection is
determined by the leakage current of the thyristors which is
subject to manufacturing tolerances. With an appropriate
valve cooling design (see below) part of the d.c. grading is
achieved by the water circuit. In addition, a self cooled
resistor of about 0.5MΩ is connected in parallel to each

   Due to huge dimensions of high voltages resp. ultra high
voltage HVDC thyristor valves additional components are
necessary to limit the impact of the large -converter inherent-
stray capacitances on the thyristors.
   4) Valve reactors LVD
   To limit the di/dt stress of the thyristors at turn on and the
dv/dt during transients in the off state, reactors are connected    Figure 8: gating and monitoring of light triggered thyristors (LTT) and
in series with the thyristor string which have to meet              electrical triggered thyristors (ETT).
conflicting requirements: a high inductance at the beginning
of current flow but a low inductance as soon as the thyristor is       Associated to each thyristor a printed circuit board
turned on safely, so as not increase the commutating reactance.     monitors the state of the thyristor and generates check back
The valve reactors are therefore designed with a saturating         signals also transmitted via fibre optical cables to the VBE.
iron core.                                                             The check back signals of all thyristor levels are processed
   Without further provisions, the valve reactor would form         in the VBE and communicated to the converter control unit.
an oscillating circuit of low damping with the stray                The main task of that valve monitoring system is to check the
capacitances of the converter. This can result in a high            availability of the thyristor valve resp. the converter.
oscillating discharge current that extinguishes the turn on            To enhance the reliability of the thyristor valves redundant
current in the thyristor. The reactor is therefore provided with    thyristor levels are incorporated in the series string of levels.
a damping resistor that is coupled via a secondary winding             Due to the fact that even a defective press pack thyristor is
and thus is not effective when the reactor core has saturated.      able to handle the full load current, the valve could remain in
   5) Grading capacitors CK                                         operation without restriction as long as the number of
                                                                    defective levels in one valve does not exceed the number of
   The various components in the valve, being at different
                                                                    redundant levels.
electrical potentials and at different distances with respect to
ground and to other components, represent a complex network
of stray capacitances. For steep voltage transients, an uneven      C. Valve cooling
voltage distribution between thyristor levels would result. To
control this unbalance, grading capacitors of a few nF are             In HVDC thyristor valves, more than 95% of the heat
connected in shunt to the series connection of thyristor levels     losses are produced in the thyristors, snubber resistors, and
and valve reactors. They are not required (and only little          valve reactors, requiring forced cooling. Due to its good
thermal capability water is used as cooling medium in
thyristor valves. To serve as an effective insulating medium,
and to limit electrolytic currents, the conductivity of the water
is maintained at or below about 0.2µS/cm at maximum water
inlet temperature. Also, the cross-section of all piping is kept
as 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 water
pipes connecting to the thyristor heat sinks can be made to
have the same electrical potential throughout avoiding
electrolytic currents between the water cooled components of
a thyristor stack.
                                                          grading electrode

  water out

                                                                               Figure 10: modular unit used in SVC applications

  water in

Figure 9: piping configuration for the cooling circuit of a thyristor stack.

   On the other hand, due to the conductivity of the water, the
piping of the cooling circuit in a thyristor valve functions as a
resistive network. By appropriate layout of the pipe work such
as the parallel circuit in fig.9 this effect is used to advantage to
provide resistive voltage grading of the thyristor levels and
valve sections, assuming part of the duty of the d.c. grading

                                                                               Figure 11: modular unit used in HVDC applications
D. Valve mechanical design
    To easily adapt the thyristor valves to the HVDC or
FACTS application and to standardize the valve design a                           The arrangement of the thyristors and heat sinks in the
strictly modular design is used to compose a customized                        stack and their associated equipment is a straightforward
thyristor valve resulting in a cost optimized design.                          image of the electric circuit diagram. A uniform voltage
                                                                               grading and ease of testing are advantages of this design.
   The thyristor modules (Figs. 10, 11) are self-supporting
units with a frame of aluminium profiles, which mechanically           The mechanical arrangement of a valve depends on the
supports all components within the modules.                         application and the number of series connected thyristor levels.
                                                                        A typical valve design used in a Static VAR Compensator
   In HVDC thyristor modules the frame also serves as a             consists of three modular units -each one associated to a phase
corona shield; its electrical potential is that of the centre cross in a three-phase system- arranged on top of each other thus
beam so that the module is divided into two symmetrical areas. forming a three-phase ac switch.
Each area accommodates a complete valve section, consisting            The tower stands on the valve hall floor. The fibre optical
of thyristor stack, snubber circuits, valve reactors, monitoring    cables and the cooling water tubes are supplied from the
boards, grading capacitor, water circuit and the routing of the bottom side of the tower
optical fibres.
                                                                     Figure 14: 500kV HVDC pole consisting of 3 twin   towers each
                                                                  containing one quadruple valve
Figure12: 3phase SVC thyristor valve tower

   The mechanical design of an HVDC converter is based on            The modules are suspended from the valve hall roof. This
an arrangement of multiple valve towers for one twelve pulse      design is very flexible and reduces seismic forces acting on
group. In a typical 500kV converter each valve twin-tower         the modules.
comprises four valves, each valve is made up of three thyristor      Water circuit connections and optical fiber routing to the
modules. These twelve modules are arranged in six tiers           valve tower is done at its top (ground potential).
within the suspended twin structure of the tower. The high
voltage end is at the bottom and includes separate corona             IV. LATEST DEVELOPMENT OF POWER THYRISTOR
shields (fig. 14 ).
                                                                  A. Technology of 6” Thyristors

                                                                      In power transmission and distribution applications, such
                                                                  as HVDC systems of FACTS, highest reliability of the
                                                                  thyristors is required. On the other hand economic
                                                                  considerations ask for high power thyristors with high
                                                                  continuous current, surge current capability and optimized
                                                                  blocking voltage capability. The maximum diameter of the
                                                                  silicon wafer of an HVDC thyristor is currently six inch. The
                                                                  thickness of a wafer with a blocking capability of 8000 Volts
                                                                  is about 1.5 millimetre.
                           twelve pulse group

                                                                      Stable manufacturing processes and outstanding
                                                                  technologies are the key for an economic production of high
                                                                  power thyristors. Different measures, processes and
                                                                  technologies have been introduced in these mature power
                                                                  semiconductors achieving an unrivalled performance and

                         quadruple valve                             High purity diffusion processes, i.e. a low amount of
                                                                  undesired atoms within the silicon wafer are the basis for the
                                                                  production of high power thyristors, resulting in sufficient
   Figure 13: single line diagram of one 500kV HVDC pole          high charge carrier lifetimes and homogeneous charge carrier
distributions on the wafer.                                        UHVDC transmission scheme connecting hydropower station
   The optimization of the trade-off between on-state voltage      Xianjiaba and the metropolitan area of Shanghai. Due to the
on the one hand and reverse recovery charge and turn-off time      extra high voltage and power rating the converter is arranged
on the other hand is achieved by electron irradiation,             with two valve groups in series for each pole. Each valve
dependent on the application specific requirements.                group is consisting of 6 double valve MVU´s side by side
                                                                   forming a 12-pulse group for the 1600 MW valve group and
   In many applications a high-efficient thermal coupling of a     3200 MW per pole.
thermal capacity to the silicon wafer is desired in order to
achieve a high surge current capability on the one hand and           The converter valve at sending station Fulong has
reasonable and manageable clamping forces for the thyristor        following design parameters:
on the other hand. A technology which is called low-
temperature sintering allows joining of a molybdenum carrier          Converter Station                                 Fulong
disc to the silicon wafer even in case of large diameters like        Thyristor type                                    6“ ETT
six inch wafers. The sintering process is performed at a              No. of thyristors per valve                       60
process temperature of about 220°C. This results in a good            No. of redundant thyristors                       2
thermal coupling between the molybdenum disc and the whole            No. of thyristors per valve section               15
diameter of the silicon wafer. Thus also the edge of the              No. of reactors per valve                         8
thyristor has an effective cooling, which is important for high       No. of reactors per valve section                 2
voltage devices utilized with high junction temperatures and          No. of valve sections per valve                   4
high blocking requirements. Therefore an excellent high               Tower arrangement                                 single
temperature high voltage blocking stability and high surge
                                                                      MVU arrangement                                   double valves
current capability with reasonable clamping forces are
                                                                      Insulation levels across single valve             456/456 kV
achieved by applying this technique.
                                                                      (switching impulse/lightning impulse)
                                                                      Insulation level across MVU structure             1600/1800 kV
                                                                      (switching impulse/lightning impulse)
    A stable passivation of the bevelled edge region of the
silicon device is necessary to realise a long-term stability for
device life time requirements up to forty years. Semi-
insulating amorphous hydrogenated carbon layers are the key
to achieve high reliable blocking stability of the device.
    Due to the high density of states (DOS) of the semi-
insulating electroactive passivation layer, surface charges are
compensated effectivly by induction of mirror charges at the
interface of the semi-insulating layer to the reverse biased
silicon substrate. With an appropriate adjustment of the DOS-
distribution the induced charges at the interface of passivation
layer to the silicon effectively reduces the electrical field
strength at the surface of the blocking junction. On the other
hand this electroactive passivation layer shields the device
against surface charges and guarantees long-term stability of
the potential distribution at the semiconductor’s surface and
thus avoids a long-term drift of blocking characteristics of the
semiconductor.                                                         Figure 15 : schematic diagram of an 800kV UHVDC pole made up of two
                                                                   series connected 400kV twelve pulse bridges
    The experience from the field applications shows, that the
failure rate of thyristors in HVDC and FACTS converters is
below 10 fit (1 fit = 1 failure per 109 hours). This shows that       The converter will be composed using 200kV twin valve
devices manufactured by the above mentioned technology             towers. Each valve will be equipped with 60 series connected
have excelent long term stability and high reliability of          6” thyristors and 8 valve reactor units physically arranged in
electrical and thermal properties.                                 two modular units per valve.
                                                                      Thus one twin valve tower is made up of 4 modular units
                                                                   suspended from the valve hall ceiling.
B. Application of 6” Thyristors in UHVDC Project

   The first application of 6” thyristors is the 6400 MW
                                                                                                           V. CONCLUSION

                                                                                        Modern power electronics gain increased importance for
                                                                                     the power transmission and distribution applications.
                                                                                     Particularly the power thyristors play a key role in the modern
                                                                                     HVDC and FACTS systems. During last decades the
                                                                                     technology of thyristors has been contentiously developing
                                                                                     both in performance and rating. The design of converter
                                                                                     valves shall fully utilize the capability of thyristors on one
                                                                                     side and meet various challenging requirements of
                                                                                     transmission systems on other side. Long time design and
                                                                                     manufacturing experience ensure the high quality of these
                                                                                     important products. Increased power rating, especially in
                                                                                     connection with ultra high transmission voltage of 800 kV,
                                                                                     requires new type of thyristors with larger diameter. Therefore
                                                                                     new thyristors based on 6” Si-wafer have been successfully
                                                                                     developed. Thyristor valves using this new type 6” thyristor
                                                                                     will be used for the 6400 MW UHDVC Project XiangJiaBa –
                                                                                     Shanghai in China.

   Figure 16: drawing of a twin valve tower related to a 600kV valve base

    Figure 17 : drawing of a thyristor modular unit designed to contain 6”-
thyristors and the associated equipment

                                                        grading capacitor

                                                   snubber circuit
                                                                     valve reactor

             n=2          n = 15
                                                       valve section
 Figure 18: Schematic diagram of a thyristor modular unit consisting of two
identical valve sections with 15 thyristor levels and 2 valve reactor units each.

    One thyristor level consists of a 6”/8kV press pack
thyristor, a single snubber resistor and capacitor as well as the
dc grading resistors and the thyristor monitoring boards. The
latter comprises the electronic logic for individual thyristor
monitoring as well as for the conversion of optical control
signals received via fiber optics from the valve base
electronics (VBE).

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