topology for medium voltage drive products al- ... (DTC) as developed for low voltage drives the MV .... commutated thyristor drives and in HVDC transmission.
A reliable, interface-friendly Medium Voltage Drive based on the robust IGCT and DTC technologies P. K. Steimer, J. K. Steinke, H.E. Gruning
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R&D Drives and Power Electronics ABB lndustrie AG CH-5300 Turgi Switzerland Phone: +41-56-299'38'88 E-mail: peter.steimer @ch.abb.com
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Abstract - The use of voltage source converter topology for medium voltage drive products allows to realize the utility interface compatible with IEEE 519-1992 by means of a series 12-pulse (or optionally a 24-pulse) diode rectifier without any harmonic filters or power factor correction equipment. On the motor side the three-level voltage source inverter, also known as the neutralpoint clamped (NPC) inverter, is the preferred one. In combination with the newest snubberless IGCT (Integrated Gate-Commutated Thyristor) technology it.is possible to build MV inverters without series connection up to 4kV line-to-line voltage and higher. This results in a highly efficient and reliable MV converter system with a minimum parts count. By using a LC-filter between inverter and motor terminals, feeding of standard motors without derating becomes possible. With the use of the direct torque control (DTC) as developed for low voltage drives the MV drives applications benefit from the synergies with low voltage drives. Specific add-ons for the NPC inverter and the loss-less resonance control of the LC-filter are needed. The DTC and the IGCT are in commercial operation for several years, showing their expected very high level of reliability and robustness. Both technologies reach the today's known limits of physics either in silicon or in torque control dynamics.
cases it is important to keep the payback time of the investment short and to minimize the standstill of production. The selection of a medium voltage converter, which - is compatible with IEEE 519-1992 in regards of the utility interface, - has a high availability, - allows leaving the existing motor within the installation - can damp optionally any resonances in the mechanical train due to its excellent torque control and - only takes a small amount of floor space should be the optimum investment. II. THE MEDIUM VOLTAGE DRIVE PRODUCT This paper describes a new developed Voltage Source Converter (VSC) product, which is based on - an input isolation transformer - a conventional diode rectifier in a 12-pulse series arrangement (optionally 24-pulse), - a Voltage Source Inverter (VSI) based on the concept of the Neutral-Point Clamped (NPC) Inverter - a LC output filter in order to feed the motor with a sinusoidal output voltage. Because of the sinusoidal motor voltage, existing motors can be fed without any derating and without additional motor noise. This new VSC product is very compact in regards of its footprint due to the use of the low loss IGCT power semiconductor as in [ l ] . The necessary input transformer can be chosen in its preferred technology and can be placed where it is most convenient with respect to available space.
I . INTRODUCTION Energy savings in combination with an optimized process control are today major reasons for considering to replace an existing fixed speed drive by a variable-speed drive (VSD). Especially in retrofit
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F,ig. 1 VSC with 12-pulse rectifier, three-level inverter.with IGCTs and LC filter Looking at low voltage variable-speed drives, the PWM voltage source converter with a 6-pulse diode rectifier and a voltage source inverter o/Sl) in a twolevel topology is the dominating topology. The basic driving forces for this topology have been the power semiconductor trend, which supports asymmetric switching devices, i.e. IGBTs, together with the demand for high dynamics for smaller markets. The trade-off is today in the interface of the inverter and motor, which has asked for additional R&D for its optimisation in regards of voltage reflection issues, common mode stresses and bearing, currents. Increased requirements in regards of the utility interface issue will drive future R&D efforts. In the medium voltage drive market a similar topology based with the NPC voltage source inverter has found increased market acceptance. This toplopogy has the following advantages: - solves the utility interface issue without additional filter components according IEEE 519-1992 by means of series multi-pulse diode rectifiers (12pulse or optionally 24-pulse) - uses asymmetric (medium voltage) power semiconductors, which have inherently the lowest losses i.e. IGCTs - can supply sinusoidal motor currents - can utilize the control and application know-how from low voltage ,drives. The R&D investment for medium voltage ,drive product and systems can now be concentrated on the development of medium voltage specific parts and the implementation of the low voltage drives knowhow, where appropriate.
The highest priority goal in the design of an industrial converter is high availability. This means high reliability, short maintenance times and short repair times. A good base for all three items is a design with a minimum number of components.
A. Rectifier For a “passive” rectifier 12 6kV-Diodes have to be used, if the motor voltage is 4kV. The 12-pulse-diodebridge is an optimized solution in regards of cost, efficiency, reliability and harmonics as long as no regeneration of energy to the line is necessary. The harmonics on the line side can be kept below the IEEE 5191992 limits with a properly designed transformer, as long as the short circuit power of the feeding line is at least about 30 times higher than the rated drive power. In case of a weak feeding network or more strict harmonic limits, a 24-pulse-diode rectifier can fulfill the requirements.
B. Inverter The I GCT (Int egrat ed Gate-Comm utat ed Thyristor) is available for a peak blocking capability of 6 kV with a dc blocking capability of 3.3 kV [ I ] . Converters built with these IGCTs can have more than 1000 A rated current. With an inverter based on 12 91mm IGCTs with integrated diode an output voltage of 4kV and an output power of more than 5MVA can be obtained at an average switching frequency of 500Hz (corresponding to a carrier frequency of 1kHz). For the inverter the three-level configuration (see Fig.1) is the choice with the biggest advantages. By adding 6 NPC-diodes the voltage-sharing problem of the “series” IGCTs can be solved. By utilizing the pos-
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sibilities of the three-level inverter the resulting pulse frequency can be doubled compared to the two-level inverter with the same individual average switching frequency of the semiconductor switches. At the same time, the step height of the output voltage is only one half of that of the two-level inverter. Even without output filter, the three-level inverter has up to 4 times lower content of output harmonics than a two-level inverter. In Figure 12, a measured curve of the inverter output before the filter is shown. Rated motor voltage is 2.3 kV.
C LC-Filter Although the three-level inverter offers a much lower content of output harmonics than the two-level inverter, additional motor losses and additional stress of motor insulation compared to a motor connected directly to the line is still a problem. Even a multilevel inverter does not solve this problem completely. The only possible solution is an output filter. For a PWMVSI, it can be realized by a three-phase LC-filter. Fig. I shows the principal converter circuit diagram with LCfilter. In order to be able to feed a standard industrial motor the motor voltage has to be almost sinusoidal. The necessary performance is to have a lower THD than IEEE 519-1992 gives as a limit for the line voltage. Fig. 12 shows measured curves from the line-to-line output voltage on the inverter side and on the motor side of the filter. The total harmonic voltage distortion is only 1.7%, which is well below the IEEE 519-1 992 limit of 5%. Rated motor voltage was 2.3 kV. More details concerning the design of the LC filter for medium voltage drive converters can be found in [ l I ] .
D Common Mode Circuit
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The output voltage of every inverter type is not a pure positive or a pure negative phase sequence system due to its switched operation. A zero phase sequence system is typically always present, if not avoided by a special modulation scheme with other drawbacks. In a symmetrical three-phase load like the LC filter or the motor, a zero phase sequence voltage system (also often called common mode voltage) does not result in a current. But the real system is a 4-wire system. The 4'h wire is the ground connection. The input and output of the converter is connected by its parasitic capacitors representing the cable's, the motor's and transformer's capacitance against ground. By grounding the star point of the output capacitor bank, the motor can be protected from common mode voltages. This avoids the risk of bearing currents resulting from the inverter zero phase sequence voltage. An optional common mode choke, which can be built-in
most convenient into the dc-link, reduces the amplitude of the charging and discharging current of the capacitance to ground, which may be a problem if the cable to the input transformer is long. A measurement of the ground current at. 300 meters of cable between transformer and rectifier showed, that the peak instantaneous earth current is reduced by the common mode choke by a factor of 5.
E. Protection Each extra part like fuses reduces the reliability of a converter and adds costs to the equipment. DC fuses for several thousand volts are expensive devices. The new converter is equipped with two semiconductor switches, called "Protection IGCT, instead of dc fuses. These semiconductor switches disconnect immediately the rectifier from the dc link in case of an inverter failure. The reaction is so fast that the line current does not rise more than some percent above its normal level. Only the energy of the dc-link capacitor is dissipated into the failed inverter, no additional energy from the line goes to the failed device. Therefore any mechanical destruction is avoided. On the line side of the rectifier no fuses are needed as long as the transformer has a high enough impedance. In case of a diode failure, which has a low probability, a typical MV breaker with its ovrcurrent protection acts fast enough to protect also the rectifier from mechanical destruction. This fuseless protection concept is comparable to the already proven protection schemes in large loadcommutated thyristor drives and in HVDC transmission systems. F. Core Technologies In the presented medium voltage converter product the implemented core technologies, which differentiate this solutions from its competition, are - the IGCT (integrated gate-commutated thyristor), which has been developed especially for the medium voltage converter market and which uses important material volume synergies with all other low-cost bipolar power semiconductor like GTOs and thyristors - the DTC (direct torque technology), which is a major reuse form the low voltage technology and which uses the voltage source converter and its motor at its physical limits. In the medium voltage drive product it is for the first time combined with a NPC inverter and a LC-filter, which both asked for medium voltage specific add-ons.
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Ill. IGCT TECHNOLOGY
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Fig. 2: Snubberless 3kA 4.5kV GCT turn-off.
-Fig. 3: On-state of 4kA / 4.5kV GTO and
,I - A. htegrated gate Commutated Thyristor -
Dramatic improvements in the traditional field of the bipolar GTO (gate-turn-off) -thyristor (see [2] and [3]) have created the IG%T (Integrated Gate Commutated and [6]). The IGCT is Thyristor) technology (see [I] today the perfect answer to the optimum combination of the proven, low-loss thyristor- technology and the snubberless, cost-effective gate turn-off for demanding medium and high-voltage power electronic applications
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-To reach the desired IGCT characteristics several development steps, some of them quite similar to the steps within the IGBT development, have been successfully completed in the last years: - improved turn-off switching characteristics to obtain operation without dv/dt snubbering at high current density (Fig. 2), -- drastically reduced on-state and turn-off-losses by minimized silicon thickness due to the used-buffer layer design (Fig.$3), - reduced gate-driye requirements especially during conduction by m,eans of transparent anode technology - development of I anti-parallel diodes capable of' snubberless turn-off at high di/dt,
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Fig. 4: 51mm IGCT with monoltythically integrated antiparallel diode, with gate drive mounted on a aircooler power.
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integration of'main switches (GTO and diode) in one semiconductor package, especially at low
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14.00 Pclamp in [kW/MVA] 12.00
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3.3kV IGBT Module 50% load NPC, fc=840Hz
2.5kV PP IGBT 100% load NPC, fc=840Hz
3.3kV IGBT Module 100%load NPC, fc=840Hz
Fig 5: Loss comparison of IGCTs and HVlGBTs During the past few years several IGCT suppliers have addressed and solved all of these issues. The IGCT has moved in the mean time into multiple applications like - high power interties up to 100MVA (see [8] and [9]) - medium voltage drives up to 20 MVA - dynamic voltage restorer - dynamicUPS - solid state switches and - high power traction. The main advantage of the IGCT in comparison with other competing high power semiconductors are - low component count due to dynamic blocking voltages up to 6kV (in the future up to 10kV) and inherent high reliability - low losses due to the thyristor conduction mode (fig. 5). - presspack housing, which allows a explosion-proof design and a design for limited fault currents - shoot-through current limitation by means of the di/dt limiting reactor (turn-on snubber) and - low inverter costs due to optimum utilization of silicon and the volume synergies with other bipolar thyristor products.
B. Sniibberless” high power circuitry
The IGCT device can be operated without a turn-off
Fig. 6: Undeland circuit for GTOs. snubber due to its homogenous turn-off in a transistor mode as explained in Fig. 2. A turn-on snubber is needed for the turn-off of the antiparallel diode with a defined di/dt. In Fig. 6, the classical Undeland circuit [4] is shown as it is used with GTO thyristors: during turn-off of the positive rail GTO GI, snubber capacitor C1 is active; during that of the negative rail GTO G2 ,a series con-
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nection of C1 and C2 is active. C2 then is chosen several times larger than C1, and consequently it can act as a DC-link clamp as well. Parasitic inductance in the main DC capacitor bank thereby becomes less important, making design for high voltages (large bus-bar distances) and high currents (large bus cross-sections) a lot easier.
C. Reliability of IGCT technology Based on extensive field data of IGCT-converters and GTO gate drivers, it is predicted to reach an
Fig. 7: 3-phase inverter circuit using one single Without the necessity for a turn-off snubber C, and Cp are omitted, and the circuit simplifies, especially if optimized for a three-phase inverter as shown in fig. 7. The clamp capacitor Cd will have to clamp fast transients through Dd only. As a consequence, Cd is much smaller than C2 in the Undeland circuit, typically it comes close to the value of a GTO's snubber capacitor (C, in Fig. 6). The parasitic capacitance of the DC-bus therefore does not effect the turn-off transient of the GCTs and the freewheel diodes. Due to small silicon wafer thickness and transparent anode design, IGCTs turn-on faster than GTOs. With = 3...4kA) the dl/dt limit is 91mm wafer diameter (ITGQ~ pushed from the GTO's 500A/ps to more than 3kA/ps. But high voltage diodes, due to comparably large storage charges, still recommend a limitation in the range of dVdt = 0.5 ... 2 W p . The external inductor Ls with freewheel circuit Dd, Rs, thus has to be kept in the circuit. Ls, Rs, Dd and Cd in fact form a multi-purpose clamp: under normal operation Ddand C d limit transient voltages on the switching devices, Ls limits dl/dt at diode reverse recovery, and Ddr Rs form the freewheel path for the energy stored in inductor Ls. Additionally Ls will limit the fault current under shoot-through. Without additional components, a GCT phase leg thereby is fully protected even under worst case conditions. In NPC topology for medium voltage applications, only two such clamps are required to serve all 3 phases with a total of 12 GCTs.
Fig. 8: 3-phase water-cooled NPC-inverter excellent reliability below 400 FIT per IGCT position (includes GCT, gate driver, gate driver supply and multi-purpose clamp). The actual field experience with the high power interties is below 200 FIT per IGCT position (GCT, gate driver, gate driver supply and clamp). Clamp
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Fig. 9: Predicted reliability distribution per IGCT position
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Fig. 11: DTC control principle
The control variables of the DTC are the instantaneous values of torque and stator flux of the motor. Both values are calculated in a motor model software from measured stator currents and stator voltages. DTC has proven its robustness and torque stability in many "simple" industrial applications like pumps and fans. It has shown to be a universal solution for motor control at a very competitive cost level.
Fig. 10: Overview speed-sensorless DTC control VI. DTC TECHNOLOGY The advanced motor control, called Direct Torque Control (DTC) due to its operation principle, has been developed in a first step especially for the low voltage drive market [lo]. The main benefits and driving forces for this technology have been - the optimized process control due to the high dynamic torque control working at the physical limits of the inverter and motor - improved sensorless control operation due to its very robust torque control loop - the realization of low cost control hardware. SA
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The synergy with low voltage drives can be found to a major part in the core part of the control .and its customer and application engineering interfaces. Utilization of the development effort made for low voltage drives and of the experience from thousands of drives installed all over the world and in almost all types of applications gives a big advantage for a medium voltage drive application. Most parts of the DTC control software were taken from the existing LVD DTC. This includes the motor model and torque and flux control as well as other functions like the ride through of short outages of the main power. This ride through function keeps the converter in operation for main power fails with duration of up to 5 seconds. During this time, the stored mechanical energy of the drive system is converted to electrical energy and utilized to keep the motor magnetized. This allows to immediately accelerate the motor back to its reference speed as soon as the main power is back. The main additional software parts for the medium voltage converter product are (see [II]and [12]): - Modulation of a three-level inverter - Neutral-Point potential control - Filter resonance damping control - Enhancement of the machine model with an LC filter model - Protection of the Medium Voltage Converter.
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V. CONCLUSIONS
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The new medium voltage drive product is based on the state-of-the-art medium voltage power semiconductor i.e. the IGCT. Traditional press pack design has been turned back into a strength, which supports modular converter design, easy handling and highest reliability. In combination with the best in class control the DTC - the product is using the technology inverter and the motor at their own dynamic limit.
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REFERENCES
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S.Linder, S.Klaka, M. Frecker, E. Carroll, H. Zeller, "A new range of reverse conducting gate-commutated thyristors for high voltage, medium power applications", EPE, Trondheim, Norway, 1997
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H. Gruning, "Low-inductive GTO terminal", Patent EP 0588026 and US 5345096, (1992)
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S. Eicher, F. Bauer, H.R. Zeller, A. Weber, W. Fichtner, "Design considerations for a 7kV/3kA GTO with transparent anode and buffer layer", Conf. rec. of PESC, Baveno, 1996
Fig. 12: Inverter and motor voltage at steady-state The resonance of the filter has to be damped by control in order to avoid additional motor losses caused by current components with resonance frequency. For damping a current with a frequency of e.g. 400 Hz, a very fast control is needed. Simulations showed that a resulting modulation frequency of at least 1000 Hz is necessary. In the described converter, the damping control algorithm is an integrated part of the motor control. The inner control loop of this control is calculated each 25c(s, which is a good enough resolution for stabilizing the filter (see fig. 12). The state of the LCfilter as it is shown in Fig. 1 can be determined by two additional measurements. A combined model of filter and motor has to be calculated in order to be able to control motor state as well as filter state. Looking at the torque dynamics, the LC filter has no major infjuence. Without the filter, the stray inductance of the motor and the voltage difference between inverter output voltage and the induced motor voltage determine, how fast a torque producing current can be built-up. Typical for a medium voltage machine is a value of 4 -6 ms for building up rated torque. Looking at the dynamics with LC filter, .the inductance of the filter choke has to be addedl to the stray inductance of the motor. The inductance of the filter choke is designed to be about 25% of th-e motor's stray inductance. Therefore the physically minimum time to built-up a dedicated torque value is only 25% longer than without filter. For the typical case the value would therefore increase from 4 - 6 ms to 5 7.5 ms. The new value is by far fast enough for all industrial processes.
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T. M. Undeland, "Snubber for pulse width modulation bridge converters with power transistors or GTO". Int. Power Electr. Conf. (IEPCD), Tokyo, 1983, pp313-323 H. Gruning. B. 0degard, "High Performance low cost MVA inverters realized with integrated Gate comutatetatedthyristors (IGCT)" , EPE. Trondheim, Noway, 1997 P. K. Steimer, H. Gruening, J. Werninger, E. Carroll, S.Klaka, S. Linder, "IGCT - A new emerging technology for high hower low cost inverters", IEEEllAS conference, New Orleans, 1997 H. Gruening, J. Voboril, ,,Field Controlled Thyristors - a new Family of Power Semiconductors with Advanced Circuitry", in Conf. Rec. 1988 IEEE PESC, pp. 1311 - 1318. P.K. Steimer, H. Gruning, J. Werninger, P. Dahler, G. Linhofer,
R . Boeck ,,Series connection of. GTO thyristors for high-power
static frequency converters", ABB Review 5/96, pp. 14 - 20.
R. Boeck, 0. Gaupp, P. Dahler, E. Barlocher, J. Werninger, P. Zanini, "Bremen's 100 MW static frequency intertie", ABB Review, vol. 6, 1996 P. Pohjalainen, P. Tiitinen and J. Lalu, "The Next Generation Motor Control Method - Direct Torque Control, DTC," EPE Chapter Symposium on Electrical Drive Design and Application, Lausanne, Switzerland, 1994, pp. 115 - 120 J.K.Steinke, P.A.Pohjalainen and Ch.A.Stulz; "Use of a LC Filter to Achieve a Motor-friendly Performance of the PWM Voltage Source Inverter," IEEE-IEMDC'97, Milwaukee (WI, USA), May 18-21, 1997, pp. TA2-4.1 - TA2-4.3
F. Springmeier, J.K. Steinke, "Control of the DC-link Potential of a Three-level GTO Inverter as Part of the Direct .Self Control (DSC), PEMC'SO, Budapest (Hungary), Oct. 1 -3, 1990, v01.2, pp. 479 483
