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Application of High Power Thyristors in HVDC and FACTS Systems
Hartmut Huang#1, Markus Uder#2
Siemens AG, Fryerslebenstr. 1, 91056 Erlangen, Germany [email protected] [email protected]
Reiner Barthelmess#3 , Joerg Dorn#4
Infineon Bipolar GmbH & Co. KG, Max-Planck-Str. 5, 59581 Warstein, Germany [email protected]
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.
I. INTRODUCTION There is an increasing demand for high efficiency and high
quality of power transmission world wide. In this context the modern High Voltage DC Transmission (HVDC) and Flexible AC Transmission Systems (FACTS) gains more importance and utilization in today’s power transmission system. Both HVDC and FACTS systems use power electronic converters for the power conversion and power quality control. Therefore the performance and quality of converter systems depend much on the key component- high power thyristors. Since its 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 connection.
The increase of thyristor’s power rating goes hand in hand
with increased demand for larger HVDC power transmission schemes. Particularly the need to maximize the utilization of
land and space for transmission lines requires higher transmission voltage, which reduced the transmission losses as well. During last decades most bulk HVDC transmission schemes worldwide have been built with 500 kV as rated dc voltage. Recently years there are several large HVDC transmission schemes under planning in China, India and Brazil, which have a transmission distances between 1000 km and 2000 km. Ultra high dc voltage (UHVDC) in the range 800 kV is the preferred dc voltage level for these applications.
0
2
4
6
8
1970 1980 1990 2000 2010
Fig. 1: Development of voltage rating (blue line) in kV and current rating (red line) in kA of power thyristors
While the first 800 kV HVDC project Yun-Guang has a power rating of 5000 MW, other 800 kV HVDC projects has a significant higher dc current. Xiangjiaba-Shanghai Project has a power rating of 6400 MW (dc current =4 kA) and Jinping UHVDC Project has the highest bipole rating of 7200 MW with rated dc current of 4.5 kA. In order to provide an optimized converter design to cover these high dc current and voltage application, new thyristors with larger diameters have been developed.
II. STATE OF ART OF MODERN THYRISTOR TECHNOLOGY 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. 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 thyristor. In the presence of off-state voltage, an off-state
current (several mA) flows both in the forward direction and in the reverse direction.
Also a non-ideal static behaviour of the thyristor is the on-state voltage during conduction. The entire voltage drop of an HVDC thyristor is of the order of two to three volts. This means that for typical currents several kA, considerable power losses must be dissipated.
Thyristors in press pack housings are ideal for both, efficient cooling of the device and stacking for series connection.
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. 4: High power thyristors made of 4”, 5” and 6” silicon wafer
Fig. 5: Photgraph of a 6 inch thyristor
There is a trend towards higher transmission current
capability of long-distance HVDC systems. With this trend, 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.
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 currents up to 4500 A. Due to the joining of the silicon wafer 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 also interesting for other applications with high current requirements and high blocking voltage needs.
III. HIGH POWER THYRISTOR VALVES
Since the first commercial use of high voltage thyristor 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 capability.
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.
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 compact valve setup comprising easy assembly, easy accessibility and easy maintainability
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 converter.
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 capacitors, snubber resistors, non linear reactors, d.c. grading resistors, and grading capacitors.
CK
CB RB
LVD
thyristor level
grading capacitor
RDC
saturable reactor
Figure 6: Main circuit components and their circuit arrangement in HVDC thyristor valves; valve used as a dc switch
CB RB
thyristor level
RDC
Figure 7: Main circuit components and their circuit arrangement in SVC thyristor valves; valve used as an ac switch
1) Snubber capacitors CS
Snubber capacitors are required in parallel to each thyristor to handle the voltage overshoot during turn off. In a modern
thyristor valve, they are single, SF6 filled units rated for the full blocking capability of the thyristor.
2) Snubber resistors RS 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 snubber capacitor current. Therefore, it has to be designed for high losses.
To dissipate these losses the deionized water available in the valve is used due to its good heat removal capability. The resistive material is directly placed into the water (wire-in-water technology). A resistor of this type can dissipate from 4.5 kW to 7 kW at moderate flow rate.
3) DC grading resistors RDC 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 thyristor.
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 in series with the thyristor string which have to meet conflicting requirements: a high inductance at the beginning of current flow but a low inductance as soon as the thyristor is turned on safely, so as not increase the commutating reactance. The valve reactors are therefore designed with a saturating iron core.
Without further provisions, the valve reactor would form an oscillating circuit of low damping with the stray capacitances of the converter. This can result in a high oscillating discharge current that extinguishes the turn on current in the thyristor. The reactor is therefore provided with a damping resistor that is coupled via a secondary winding and thus is not effective when the reactor core has saturated.
5) Grading capacitors CK The various components in the valve, being at different
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 voltage distribution between thyristor levels would result. To control this unbalance, grading capacitors of a few nF are connected in shunt to the series connection of thyristor levels and valve reactors. They are not required (and only little
stressed) at low frequency phenomena but linearize the voltage distribution for high frequency (steep) wave shapes. They are filled with SF6 gas to achieve a high voltage withstand without the use of oil as a dielectric.
B. Thyristor Gating and monitoring Because of the high voltage environment of the thyristors,
it is absolutely necessary to electrically separate the triggering and monitoring unit at ground potential (referred to as valve base electronics VBE) from the thyristor at high voltage potential. Therefore, the trigger command for the thyristor is transmitted as a light pulse via a fibre optic cable irrespectible of the thyristor type used: electrically triggered thyristors (ETT) or direct light triggered thyristors (LTT).
Figure 8: gating and monitoring of light triggered thyristors (LTT) and electrical triggered thyristors (ETT).
Associated to each thyristor a printed circuit board
monitors the state of the thyristor and generates check back signals also transmitted via fibre optical cables to the VBE.
The check back signals of all thyristor levels are processed in the VBE and communicated to the converter control unit. The main task of that valve monitoring system is to check the availability of the thyristor valve resp. the converter.
To enhance the reliability of the thyristor valves redundant thyristor levels are incorporated in the series string of levels.
Due to the fact that even a defective press pack thyristor is able to handle the full load current, the valve could remain in operation without restriction as long as the number of defective levels in one valve does not exceed the number of redundant levels.
C. Valve cooling In HVDC thyristor valves, more than 95% of the heat
losses are produced in the thyristors, snubber resistors, and 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.
water in
water out
grading electrode
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 resistors.
D. Valve mechanical design To easily adapt the thyristor valves to the HVDC or
FACTS application and to standardize the valve design a strictly modular design is used to compose a customized thyristor valve resulting in a cost optimized design.
The thyristor modules (Figs. 10, 11) are self-supporting
units with a frame of aluminium profiles, which mechanically supports all components within the modules.
In HVDC thyristor modules the frame also serves as a
corona shield; its electrical potential is that of the centre cross beam so that the module is divided into two symmetrical areas. Each area accommodates a complete valve section, consisting of thyristor stack, snubber circuits, valve reactors, monitoring boards, grading capacitor, water circuit and the routing of the optical fibres.
Figure 10: modular unit used in SVC applications
Figure 11: modular unit used in HVDC applications
The arrangement of the thyristors and heat sinks in the
stack and their associated equipment is a straightforward image of the electric circuit diagram. A uniform voltage grading and ease of testing are advantages of this design.
The mechanical arrangement of a valve depends on the application and the number of series connected thyristor levels.
A typical valve design used in a Static VAR Compensator consists of three modular units -each one associated to a phase in a three-phase system- arranged on top of each other thus forming a three-phase ac switch.
The tower stands on the valve hall floor. The fibre optical cables and the cooling water tubes are supplied from the bottom side of the tower
Figure12: 3phase SVC thyristor valve tower
The mechanical design of an HVDC converter is based on an arrangement of multiple valve towers for one twelve pulse group. In a typical 500kV converter each valve twin-tower comprises four valves, each valve is made up of three thyristor modules. These twelve modules are arranged in six tiers within the suspended twin structure of the tower. The high voltage end is at the bottom and includes separate corona shields (fig. 14 ).
quadruple valve
twelve pulse group
Figure 13: single line diagram of one 500kV HVDC pole
Figure 14: 500kV HVDC pole consisting of 3 twin towers each
containing one quadruple valve The modules are suspended from the valve hall roof. This
design is very flexible and reduces seismic forces acting on the modules.
Water circuit connections and optical fiber routing to the valve tower is done at its top (ground potential).
IV. LATEST DEVELOPMENT OF POWER THYRISTOR
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.
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 reliability.
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 high charge carrier lifetimes and homogeneous charge carrier
distributions on the wafer. The optimization of the trade-off between on-state voltage
on the one hand and reverse recovery charge and turn-off time on the other hand is achieved by electron irradiation, dependent on the application specific requirements.
In many applications a high-efficient thermal coupling of a
thermal capacity to the silicon wafer is desired in order to achieve a high surge current capability on the one hand and reasonable and manageable clamping forces for the thyristor on the other hand. A technology which is called low-temperature sintering allows joining of a molybdenum carrier disc to the silicon wafer even in case of large diameters like six inch wafers. The sintering process is performed at a process temperature of about 220°C. This results in a good thermal coupling between the molybdenum disc and the whole diameter of the silicon wafer. Thus also the edge of the thyristor has an effective cooling, which is important for high voltage devices utilized with high junction temperatures and high blocking requirements. Therefore an excellent high temperature high voltage blocking stability and high surge current capability with reasonable clamping forces are achieved by applying this technique.
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.
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 devices manufactured by the above mentioned technology have excelent long term stability and high reliability of electrical and thermal properties.
B. Application of 6” Thyristors in UHVDC Project
The first application of 6” thyristors is the 6400 MW
UHVDC transmission scheme connecting hydropower station Xianjiaba and the metropolitan area of Shanghai. Due to the extra high voltage and power rating the converter is arranged with two valve groups in series for each pole. Each valve group is consisting of 6 double valve MVU´s side by side forming a 12-pulse group for the 1600 MW valve group and 3200 MW per pole.
The converter valve at sending station Fulong has
following design parameters: Converter Station Fulong Thyristor type 6“ ETT No. of thyristors per valve 60 No. of redundant thyristors 2 No. of thyristors per valve section 15 No. of reactors per valve 8 No. of reactors per valve section 2 No. of valve sections per valve 4 Tower arrangement single MVU arrangement double valves Insulation levels across single valve (switching impulse/lightning impulse)
456/456 kV
Insulation level across MVU structure (switching impulse/lightning impulse)
1600/1800 kV
Figure 15 : schematic diagram of an 800kV UHVDC pole made up of two
series connected 400kV twelve pulse bridges The converter will be composed using 200kV twin valve
towers. Each valve will be equipped with 60 series connected 6” thyristors and 8 valve reactor units physically arranged in two modular units per valve.
Thus one twin valve tower is made up of 4 modular units suspended from the valve hall ceiling.
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
n = 2 n = 15
n = 1
valve section
snubber circuit
valve reactor
grading capacitor
6"/8kV-ETT
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).
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.
