Engineering Essay

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IEEE Industry Applications Mugazine November/Decernber 1996 U I

he European railway scene has undergone major changes in the last five years. The state-owned railway service traditional in

Europe is increasingly going to be reorganized into a production sector responsible for train operation managed industrially-and, further on, into a state-governed sector providing the right of way and fixed facilities to give railway transport a fair chance to compete economica.lly with individual road transport.

This forecast holds mainly for Western Europe. Eastern Europe is reconstituting its state-owned railways after the decay of the Communist reign and the formation of so many new countries.

The unfolding European unification gives rail- way societies common access to needed materials throughout the continent, though they must still confront national differences in current supply and train safety systems.

O n the technical side, there is a growing de- mand for locomotives and power headcars for H i g h Speed Trains (HSTs) capable ofcrossing the borders of the different current supply systems unim- peded-so-called multi-system vehicles with high power. Modern technology. including GTOs and microprocessors, offers economically viable solu- tions, but with increased complexity.

The manufacturers responded to these changes by a series of mergers. There are now four major groups: the Swedish-Swiss-German ABB Trans- portation, the Anglo-French GEC ALSTHOM Transport, SIEMENS A G Verkehrstechnik, and AEG Transportation Systems, both German (see Table 1). They absorbed not only most of the smaller suppliers of electrical railway equipment, but also the builders of the mechanical parts, which were often detached from their mother societies active in mechanical engineering. Leadership of the new system migrated inevitably to the electrical suppliers.

As the power demand has increased (now 1,750 k W and 2 1 metric tons per axle, or 1,000 k W and 17 tons for HSTs), an effective slip-slide control has become crucial. The high dynamic properties of individually fed induction motors with field-ori- ented control offer new solutions.

In light rail transportation, the low-floor tram

This survey o f the state o f the art and innovational trends of railway technology in Europe was prejiared for and agreed by the European Working Group of the IEEEIIAS Industrial Power Converter Committee, rep- resenting 15 countries and most application and research areas of power e1ectronic.r i n Europe. I t ujpeared i n its original f i r m a t the 1996 1EEEIlES International Symposium OPZ Industrial Electronics in Warsaw, Po- land. The author is head o f the Chair Jor Generation and Application o f Electvic Energy, Ruhr- University Bochum, 0 - 4 4 780 Bochum, Germany.

is easing access from the street level for elderly and handicapped travellers. I t has led to increasing demand for lightweight equipment consisting of induction motors and transistor inverters.

Railway Current Supply Systems Due to the early start of main-line electrification in different European countries soon after the turn of the century, there are now four dominant supply systems that have survived. It is not likely that existing systems will be changed even if there are massive technical drawbacks, due to the enormous costs.

After the introduction of the inductive-ohmic commutation pole shunt in 1903, the series-wound commutator motor allowed electrification with high tension. The frequency had to be decreased to 16-213 Hz to reduce induced voltage to safe values. Since 1912, German, Austrian, Swiss, Swedish, and Norwegian lines were electrified with 16-213 Hz, 15 kV.

The high initial cost of the separate generation and distribution system led to a desire to use the standard 50 Hz system of the electric utilities. After first steps with rotating split-phase conver- tors in Hungary, it was the introduction of phase- controlled rectifiers that permitted the use of DC motors. Based on trials i n southwest Germany in the OS, the French State Railways electrified their northeastern network with 50 Hz, 25 kV. From there the new system spread over all Europe, where electrification started anew, as in Great Britain, Ireland, Portugal, Denmark, Finland, and the majority of Southeastern and Eastern European countries.

Originating from suburban services in the first decades of the century, the 15OOV DC system is still going strong in the Netherlands and the south- ern and southwestern parts of France, though the power to be transferred to the train is limited to some 5 M W , which impedes high-speed traffic. Since the late 1 9 2 0 ~ , several countries have intro- duced the more powerful 3 kV D C system, based on U.S. experienc-e-examples are Belgium, Spain, Italy, Poland, the northern region of the former Czechoslovakia, and the former Soviet Union. Light rail, metro, and inter-urban services use 600- 1500 V DC, often fed by a third rail. A special situation occurs in southern England, as the Chan- nel Tunnel HSTs are urged to use this system on their approach to London.

Modern equipment using converters with self- turn-off devices controlled by microprocessors of- fers technically viable solutions for multi-system vehicles. A drawback is the great diversity of solu- tions ordered only in small numbers by the differ- ent railways.

Main Drive Concepts for Electrical Rail Vehicles This section will depict briefly the development and current status of the different technical solu-

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/E€€ industry Applicofions Mogozine November/December 1996

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tions that followed from the needs of the different supply systems and the change of the transporta- tion profile.

Development and procurement of vehicles with 16-213 Hz commutator motors controlled by tap- changer transformers is now finished; the order for 90 locomotives Class 1 1 2 by t h e Deutsche Reichsbahn in 1991 was given primarily to combat unemployment in East Germany. The charac- teristic feature of these locomotives is the use of a thyristor AC controller, which allows continuous voltage control among a reduced number of trans- former taps.

Nearly the same holds true for locomotives with DC motors fed by phase-controlled line converters (for AC lines) or choppers (for D C lines). N o major new developments are expected, so serious are t h e constraints of the maximal product ofpower x speed (now 1 , 2 5 0 k W , 1 , 1 0 0 rpm) and the manufacturing and maintenance costs of the commutator. Outstanding examples ofrecent years include the Austrian C1. 1044 El], the Swedish Rc5, and the British C1. 9 1 for AC; and the Italian E652 or the “Pendolino” tilting HST CI. 4 5 0 for 3 kV DC-or the French family of Cl. 1 5 0 0 0 for 50 Hz, 25 kV AC, CI. 7400 for 1 5 0 0 V D C , and C1. 22500 for both systems. Though the G T O chopper was used in light rail application, i t was not introduced in locomotive applications any further.

T h e rotating-field machine eliminates t h e power limitation and permits high-power light- weight drives. There is no constraint on the dura- tion of operation, especially at standstill, and maintenance is reduced markedly. This is even more important in new construction of high-speed applications with self-steering axles or tilting equipment, or low-floor tram single-wheel drives. There are two main types, both widely used in traction now: the synchronous machine and the induction machine.

After early, less successful prototypes in the former Soviet Union, the French achieved success with the combination of the synchronous machine and the current source inverter commutated by the over-excited machine. There are two forms: the

The rotating-field machine eliminates the power limitation and permitj h igh-power lightweight drives.

high-power dual-system locomotive “Sybic” (syn- chronous-two-current) uses a double star “mono- moteur” bogie drive fed by a 12-pulse converter with an auxiliary starting circuit. The HST TGV-A employs standard three-phase, carbody-mounted motors [2]; starting is accomplished by an auxiliary star-point commutation circuit (see Fig. 1). The line converter is an integrated combination of an AC half-controllable bridge and a chopper. Espe- cially for 16-213 Hz lines, there are severe limita- tions by weight and losses of the DC link reactor of the current-impressed converter and line inter- ference (psophometric current > 10 A, bulky line filter necessary). The motor costs about 20-30% more than a comparable induction machine. Paral- lel feeding of motors is absolutely impossible. O n the other hand, the system is very robust and reliable, using medium-fast high-voltage thyris- tors. GTOs are not expected to be as useful for the current-impressed system as for the voltage-im- pressed one.

Synchronous machines with permanent (rare- earth) magnets have not yet been introduced due to higher costs ( + 2 0 - 3 0 % ) and the difficult field- weakening that is indispensable for an economic layout. As field-weakening calls for injection of high reactive current, the rating of the self-com- mutated inverter has to be increased substantially. Until now, switched-reluctance motors had been employed only in a British experimental tram. It is not clear whether the advantages of the simpler inverter are valid in the high power range, too, and compensate for operational drawbacks such as noise and low dynamics.

Now there seems to be a general accordance that the most simple and robust machine, the squirrel- cage induction machine, fed by voltage source inverters, offers the best performance and the best potential for the future. This is the solution with

IFFE Industry Applicotions Muguzine NovemberDecember I996

P

- T C l 1

WE1

TC12

f f SM2 f TC21 I I -----I I

F i g , I , Main crrcuit of TGV-A (one bogie). SM-: synchronous traction motors. WE-: excitation windings. Tc, Cc-: commutation thyristors, capacitors. Rs, Sn : braking resistor and contactor. Ld: DC link inductw. TCH: thyristor chopper. F: line filters.

which the “history” of modern three-phase drives started in 197 1. In the review three “generations” can be discerned (the “catchwords” are explained more in detail in the following):

Prototypes and first small series for mainly industrial haulers (roughly 1 9 7 1 - 1 9 7 9 ) : Forced-commutated thyristor inverters, “sca- lar” characteristic control of the induction machine, widely analog control realized with integrated circuits. Heavy electric shunting locomotives RAG E 1200 or Swiss Ee 6/6II. Basic development of net-friendly line con- verter, the “four-quadrant converter.” First series for main-line service (1979- 1987): Continuing use of forced-commu- tated thyristor inverters. Series Deutsche Bundesbahn C1. 120, first batch of German HST ICE; parallel development of voltage source inverter (VSI) mainly for locomotives and of autosequentially commutated current source inverter (ASCI), mainly for LRVs and metros, e.g. Berlin “S-Bahn” 4 8 0 or French double-deck commuter trains 2 2 0 5 0 0 . In-

creased use of digital control, introduction of microprocessors. Procurement of big series for high-perform- ance applications (HST, multi-system vehi- cles, low-floor t r a m s ) : G T O / t r a n s i s t o r equipment, concentration on VSI circuit fa- vored especially by these devices, general ac- c e p t a n c e of f o u r - q u a d r a n t c o n t r o l l e r . Advanced “vectorial” control of induction machine using microprocessors and digital signal processors. Introduction of field bus systems replacing conventional relay technol- ogy. Worthy of mention: Second batch of ICE with G T O equipment [ 51, Spanish dual-sys- tem loco S252 [ 61, Swiss “Bahn 2000” loco C1. 460 71, Swedish HST X2000, ABB G T O retrofits of DB 120 0 0 4 and 1 2 0 005, Anglo-French Channel Tunnel HST “Euro- star” TMST [ 81, and Siemens prototype loco “Eurosprinter” [ll]. General tendency to- ward single-axle drives promising better con- trollability under slip-slide-condition and to dispense with motor series inductors neces- sary with thyristor inverters.

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ud

‘d

n P

h

(a) Basic Equivalent Circuit c 5

(b) Phasor diagram (cos 4, = + I )

(c) Voltage and Current at the DC-Side

J U u u u u (d) Voltage and Current at the AC-Side

Fig. 2. Four-q~a~rant-converter ( 4 Q - C ) .

F o u r - Q u a d v a n t Converter As all traction inverters were and are of the indirect type with DC interlink, special attention has t o be paid to the line converter. DC-fed LRVs and metros are more and more fed directly (via filter), dispens- ing with the pre-choppers useful with forced-com- mutated equipment. Half-controllable bridges and LC filters are viable with minor power demand, but not ifthe request on low line current distortion i s dominant, as in 16-213 Hz networks; they are used, for example, in the Channel tunnel HST “TMST” {SI.

The self-comrnutated line-side pulse converter, the “four-quadrant converter” ( 4 Q - C ) was invented in Germany in 1972 141 and is now standard for high-performance applications; see Fig. 2 . The main transformer secondary winding is connected to the outputs of two self-commutated inverter pairs of arms (same as those of the invtrter-see Fig. 2a). Appropriate PWM control generates such a voltage at the transformer secondary (Figs. 2b,d) that the line current-limited by the stray induc- tance-has the wanted amplitude and phase (pref- erably O0 or 180°) and only small harmonic content (psophometric current 1-2 A). A tuned tank circuit parallel to the DC link capacitor takes u p the power pulsation of the monophase line with twice the line frequency (Fig. 2c).

Diesel-Electric Locomotives In addition to the “classical” approach with alter- nator, diode bridge rectifier, and D C traction mo-

l€€€ Industry Applicutions Muguzine m Novernber/Decernber I996

tors (similar to U S . practice), the diesel-electric (DE) locomotive with VSI-fed induction motor is rather popular in Western Europe. For heavy duty it is economically competitive with the diesel-hy- draulic locomotive that has been favored in Ger- many for a long time. Outstanding orders have included the 120 Class DE 6400 locomotives of Netherland Railways (1 120 k W , “second genera- tion”) and the first prototypes of the third genera- t i o n , s u c h as t h e R A G D E 1 0 0 3 a n d t h e high-power MaK DE 1024 (2650 k W , the first locomotive with 2.8 kV DC link voltage and 4.5 kV-GTOs).

The capability of GTOs to turn themselves off permits the DC link voltage to be governed only according to alternator speed (and thus diesel en- gine power), freeing it from dependence on the motor currents, as necessary with forced-commu- tared VSI equipment. This gives an additional degree of freedom for optimizing the overall per- formance. W i t h the order of over 1,000 3,600 HP D E locomotives for E M D a n d Siemens by Burlington Northern and other Class I railroads, the European-type diesel-electric locomotive with VSI-fed induction motors seems finally to have been accepted in the U S . market. Economic com- petitiveness could be reached by bogie feeding accepted by U.S. customers, since self-steering ax- les in the bogie minimize tire and flange wear.

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GTO Inverter Circuit Technology The standard GTO for high-power applications is the 4.5 kV, 3-4 kA device; besides that the 2.5 kV, 2 kA device plays still an important role in Europe. Both are of the asymmetric (anode-shorted), now alloy-free type.

All high-power minority carrier semiconductor devices need-at least beyond a certain level of blocking voltage-a turn-off “switching relief or “snubber” limiting dvidt to avoid excessive turn- off losses and-even more critical-the danger of dynamic avalanche. Similarly, turn-on diidt must be limited to reduce turn-on losses and protect free-wheeling and snubber diodes against a turn-off that is too hard [ 9 ] .

All these snubbers consist of energy-storing devices (capacitors and inductors), which have to be discharged or demagnetized periodically. The losses of the snubbers of an inverter leg can be summarized by

where C, = capacity of turn-off snubber capacitor, (Cs = ITQ / dv/dtadmissibIe) ITQ = actual turn-off current

Irr = recovery current of the FW diode Vd = DC link voltage Ls = inductivity of turn-on inductor (Ls = Vd / dildtadrnissible) f, = switching frequency In steady-state operation with sinusoidal load

current, the rms value of this enters into the equa- tion for ITQ.

Before discussing the three snubber configura- tions in use and the factor k, it is worthwhile to point out two different “design philosophies” pre- dominating today in Europe:

A. In the “German-speaking camp” (Germany and Switzerland), the aim has been to develop high-power G T O inverters first, matching with the existing thyristor inverters as those of C1. 120 with Vd = 2.8 kV. Thus a nominal DC link voltage Vdnom -- 60% VDRM was the goal, with VDRM the admissible repetitive blocking voltage, necessitat- ing elaborate snubber schemes and a relatively low average switching frequencyfcofonly 200-250 Hz. There were problems with long-time DC stability.

B. In Great Britain (and similarly in Japan), metro and e.m.u. applications stood in the fore- ground, with a reduced power demand but severe restriction on AC components in the DC line cur- rent. Thus, V&om < 40% VDRM was chosen, al-

Fig. 3. GTO muhber urrungements (one pair o f urms): U . Symmetricul RCD-snubber. b. Undelund-Murquardt unsymmetrical snubber. c. Wugnei.-McMurruy symmetrical snubber.

l€€f Industry Applicufions Muguzine Lvember/December 1996 I

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lowing switching frequencies of u p to 500 Hz. Recuperative snubber networks were discussed {X] but not yet realized.

M a i n S n u b b e r C o n f i g u r a t i o n s The oldest and most widely used snubber circuit is the RCD snubber (e.g., [ 8 , 91), Fig. 3a. As each device has its own snubber capacitor, the limitation of spike voltage is comparably easy, but losses are high as there are unnecessary unloadings of the capacitor opposite to the device turning off. Thus the factor k amounts to 3. The clamping capacitor Cc i s necessary for a layout according to A above; Cc = 2-5 Cs. It was, for example, employed in the first full-electric locomotive with GTOs, the Swiss BT Re 414 and the similar Zurich S-Bahn locomo- tive SBB C1. 450.

The unsymmetrical snubber according to Un- deland Cl01 and Marquardt [SI has only one snub- ber capacitor (Fig. 3b). G T O Vz i s snubbed indirectly via the series connection of Cs, v 6 and a storage capacitor Cc, 5-10 times bigger than Cs. This mesh must be designed very carefully to keep stray inductance small. The factor k i s reduced to 1 so that the overall losses of such an inverter including semiconductor devices are only 50% of that with RCD snubbers.

In spite o f the advantages mentioned, both the clamping and storage capacitor cause some prob- lems. A t t h e c o m m u t a t i o n of t h e s n u b b e r chargeidischarge current from Cs to Cc, weakly damped oscillations occur, leading to raised turn-

on losses at light load. A commutating inverter-leg lifts the common plus-potential, as there i s an inevitable common inductance toward the D C link capacitor. The diodes Vs and v6 are then flooded in the non-commutating legs, as the mesh Ls-Cc- Vs-Vg has a low impedance. If the GTO turns on at such a moment, the snubber diodes Vs and v 6 will be turned off with minimum stray inductance against V(Cc) - Vd, which means a very high stress that only few diode types will withstand at v d = 2800 V.

An alternative snubber circuit with good elec- trical qualities was proposed originally by Wagner and again by McMurray [ I l l ; see Fig. 3c. A pre- requisite is that the total inductivity of the snubber resistor Rs(-0.25 n) plus that of the D C link capacitors, including the leads to the inverter-leg, are kept low enough, e.g., below 100 n H . Then at turn-off of V I the “remote” capacitor Cs2 i s con- nected in parallel to Csi. Then Csi, Cs2 can be chosen as the half value given by admissible dvidt; the spike voltage is not increased substantially, and the snubber losses are the same as in the unsym- metrical circuit, k =I with Cs = Csi + Cs2. If the source loop has such a low inductivity, a clamping capacitor is not necessary. One inverter-leg i s no longer influenced by the switching o f another.

The problem lies in the demanding construc- tion. Ten years ago it was still regarded as critical to achieve an inductivity of 100-150 n H in the indirect snubber loop of the unsymmetrical circuit. Now this demand is extended to the whole mesh via

Fig. 4. Three-level znverter (one leg; RCD snubber).

IEEE indusfry Applitoiions Moguzine a Novemher/Decemher 1996

the source capacitor and the snubber resis- tor-which, by the way, has to take up - 8 kW! The “Eurosprinter” is already equipped with such an inverter (1600 k W , water-cooled), as described in 1111.

Three-Level Inverters All inverter circuits mentioned up to now have been of the standard two-level design, connecting either the positive or the negative potential of the DC link to the output.

In the 1970s an inverter circuit was proposed connecting additionally the center-tap of the DC link capacitor to the load, enabling three-level voltage forms to reduce the load current distortion at low switching frequency. But not until the introduction of the neutral point clamped concept [12] did the three-level inverter become important for traction (see Fig. 4). The main advantage was the higher DC link voltage, with devices with given VDRM enabling inverters suited for direct connection to the 3 kV DC supply, with available 4.5 kV GTOs.

The reasons why acceptance is now wavering include the following:

R No standard design, and comparatively low demand Series connection of quasi-standard two-level inverters is now possible and preferred

B Available power with standard 4.5 kV-3kA devices is suited for bogie drives, but uneco- nomical for the single-axle drives that are now preferred

i?: Control is rather demanding Thus, the three-level inverter is regarded now

as a singular solution, not representing the main- stream of development. Though 11 5 locomo- tives-with bogie drives-have been delivered for Swiss Federal Railways for high-speed and Alp shuttle service [7], the next generation with single- axle drive and a power of 7 MW is equipped with two-level inverters.

Multi-System Equipment Consider a basic unit of a bogie with two individu- ally controlled VSI-fed induction motors, nominal DC link voltage of 2400-2800 V, and equipped with two or three four-quadrant controllers for AC line operation. The available 4.5 kV GTOs do not permit the direct connection of the DC link to the 3 kV mains because of transients up to 4.2 kV.

In principle it is possible to operate these in- verters directly connected to the 1.5 kV DC line or in series connection at the 3 kV DC line. But then the maximum power is reduced significantly, espe- cially at high speed limited by the breakdown torque of the induction motor proportional to V:. This is not viable for high-power applications.

With a line voltage of 750-1500 V, the 4Q-C inverter-leg can be used by regrouping as a step-up

chopper. The problem lies in 3 kV operation. To- day three proposals (Fig. 5) are discussed in Europe, two already realized:

Not until the introduction o f the neutrul point clamped concept

did the three- level inverter become important for truction.

A. The input filter capacitors ofthe choppers are connected in series, only one branch shown. Then

holds. Vdl, Vd2 = Vdnom(AC); Vi can be substan- tially smaller than Vdnom. The dotted lines depict the connection of the transformer secondary used in AC operation; the evident 1.5 kV DC parallel connection is not shown for better clarity.

B. Each step-up chopper originating from the 4Q-C pair of arms is connected in series with the opposite DC link capacitor so that

vi = vdl + a2 ‘ Vd2 = v d 2 + a1 Vdl

holds (Fig. 5b). By that relation, input voltage must always be greater than Vdnom. The circuit is used in the Spanish (RENFE) dual-system locomo- tive S 2 5 2 [b]. W i t h 1.5 kV DC, the choppers are directly connected to the opposite bus bar.

C. Both circuits need additional chopper induc- tors Ld1,2, increasing weight and losses. The circuit proposed by ABB in 1993 and applied in Class E 41 2 of Italian State Railways employing double- star motors, replaces series/ parallel connection of input choppers with that of DC links (Fig. 5c). The three-phase 4Q-Cs are regrouped as motor inverters feeding the second motor half in DC operation. In AC operation, both motor windings systems are connected in series.

Common to all proposals is that the resonant tank filter capacitor is to be used as DC line input filter capacitor Ci.

Gate Drive Units for GTOs, Inverter Protection The gate drive is a crucial part for the reliable operation of a GTO. It has to generate turn-on pulses of 30-50 A with 20-40 ps duration, 2-8 A permanent gate current (depending on tempera- ture), and turn-off pulses with 40-70 AIps and 700-1000 A amplitude. Average power demand is about 50 W, supplied either by DC (internal DCiDC chopper) or by medium-frequent AC (e.g., 16 kHz). The signal is transmitted by glass (or

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Iff€ Industry Applicafions Magazine November/December 1996

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MI

(s Vi? I =k $? m ‘ I t ”I M2

-7- vi = 3kV DC

M1

7 V, = 3kV DC

Fig. 5. Multi-system equipment (JOY one bogie): a. Series connection ofinput&hrs of step-up choppers. b. Series connection of step-up chopper and D C link. c. Series connection d D C links, double-star motor. Switches shown only for 3 kV D C and AC; D C position

plastic) fibers, and the actual state is monitored by an additional fiber to enable “active” interlocking of both GTOs of one inverter-leg.

In the initial state of development, the “shoot- through” protection as known from forced-com- mutated thyristor inverters was chosen to protect against inverter-leg and output terminal shorts. “Crow-bar” thyristors in the DC link o t in the

inverter-leg modules themselves are necessary to prevent damage to devices.

As there are no commutation failures (the prin- cipal reason for inverter-leg shorts) and with inter- locking of gate drive units, there is in general no longer any need for a “shoot-through” protection, which stresses the motor severely by effecting a three-phase short. Now a staggered strategy of voltage and current control thresholds and an ef-

IFF€ Indusfry Applications Magazine November/December 1996

fective limititation of DC link voltage by GTO- controlled dump resistors is considered sufficient.

A l t e r n a t i v e GTO Devices Reverse-conducting GTOs (with integrated free- wheeling diode) are used in several medium-power LRV choppers and inverters, but not in high-power inverters and 4Q-Cs.

In the end of the ‘80s, great expectations were held for MOS-controlled GTOs (MCTs) promising “snubberless” operation with raised frequency, at lower control expense. As the development stag- nates, mainly through problems with the upscaling to turn-offcurrents needed for traction, interest has decayed. The insulated gate bipolar transistor is the candidate that most ofthe applications community prefer.

Bipolar and Iizsulaated-Gak-Bipolar Transistors In the medium power range up to 200 k W , the bipolar transistor, especially that with insulated gate (IGBT), has superseded the G T O due ro its ability to turn off the current even in short-circuit situations and due to the simpler gate drive. IGBT inverters are now dominant, with auxiliary con- verters for locomotives and railcars, for passenger car comfort equipment, and for low-floor LRV main drives. Locomotive main drives are to be expected within the next three years.

W i t h present DC link voltages of 600-750 V dvidt snubbers are not necessary, making the con- struction less demanding. Inverters for 750 V DC lines have until now been equipped with step- down pre-chopper or built as three-level inverters as necessary. 1700 V GTOs have not yet been available in series production numbers. These problems have been overcome now; inverters with 3.3 kV devices for Vd = 1500 V are under devel- opment.

A major problem is the limitation of output dvidt causing EM1 and jeopardizing the motor winding insulation.

Construction Aspects All inverters with self-turn off devices need a con- densed design to minimize all stray inductances; devices must be protected from dust and moisture safely. In spite of high efficiency (> 9 8 % ) , the concentration of losses puts a high demand on the cooling system.

Cooling Air cooling is suited for minor power; it is rugged and less expensive. The cooling air stream must strictly be separated from the electrical part, e.g., by introduction of an intermediate cooling airflow.

Fluid cooling (Fig. 6) is inevitable for high- power applications and increases freedom to ar- range the inverters within the car body, avoiding

bulky airducts. Both can cooling (where the devices are clamped between cans through which the cool- ing medium runs) and immersed cooling (in which all devices are held in a large vessel) are applied.

Evaporative cooling (Fig. 6a) has a very high heat transfer density and needs no pumps. The final heat exchange to the ambient air takes place either at the finned surface of the vessel IS}, in separate coolers mounted on top of the vessel, or through the introduction of an additional intermediate water circuit. Freon R 11 3 has been replaced by non-fluoridized carbohydrogenes such as FC 72. The snubber resistor is placed outside the evaporat- ing vessel.

Mineral oil (Fig. 6b, c) is a well-known medium, rugged and powerful, ensuring isolation demands {7, 81. Pumps are needed to transport the oil to a central heat exchanger integrated into either the transformer oil cooler or the diesel engine cooler. The risk of inflammability can be reduced by using silicone oil or esters.

Water cooling offers the best cooling properties, but problems arise due to the conductivity of nor- mal water. W i t h cans on high potential, de-ionized water can be substituted for normal water. W i t h normal water, the cans must be grounded and the devices must be isolated by aluminium-nitrite plates (Fig. 6d) C11 f or the cans themselves must be made from aluminium-nitrite. Snubber resistor, diidt-chokes, and gate-units (G.U.) may be mounted on cooling plates, too.

M o d u l a r i t y In forced-commutated thyristor inverters, the thyristors of one arm (with their appertaining an- tiparallel diodes and snubbers) formed a single module [ 3 } . W i t h G T O , an almost complete in- verter-leg forms one module, including snubber capacitors, inductors, and resistors, provided the cooling system is efficient enough {5, 8 , 11). ABB offers oil-cooled “large modules” comprising two or three inverter-legs in one common vessel.

Control Electronics o f Truction lnverfers

Control o f the Inductioiz M a c h i n e Development started in the late ‘60s with slip frequency current characteristic control, IS] which is still widely in use. The slip frequency and the current amplitude necessary to keep the magnetic flux linkage on the optimal precalculated value and to deliver the wanted torque are stored in charac- teristic fields (EPROMS) dependent on speed and set value of tractive effort. The slip frequency is added to measured rotor speed, giving stator fre- quency. The current set value is compared in PI- controllers with the measured value. PWM forms the valve-firing patterns; with a low ratio ofswitch- ing frequency over stator frequency, i t must be synchronized to avoid disturbing torque subhar-

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IEEE Industry Applications Magazine Novemher/Decemher I 996

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Fig. 6. Fluid cooling of high-power GTO inverters: a. Evaporation cooling. h. Can coolzng. c. Immersed oil cooling, d. Water cooling. Devzces isolated by AlWp1ate.r.

monics. This scalar control i s exact in steady state, but the stability i s poor in the field-weakening range. The dynamics are limited to rise times sig- nificantly greater than rotor time-constant.

In rotor-field oriented control schemes, stator current is decomposed into a “flux-determining” component parallel and a “torque-determining” component perpendicular to rotor flux linkage (vector control). Both components are controlled and ensure accurate operation in the dynamic case, too. Direct field orientation measures motor termi- nal voltages and currents and calculates the rotor flux and the pertaining current components in a real-time machine model [ 1 4 ] . As this cannot work at low frequency, the control scheme is aided by indirect field orientation detecting rotor position. Switching patterns are generated by optimized synchronous PWM. The dynamic behavior is as good as that of the current control loop, provided the components are decoupled well by appropriate circuits. Modern controllers incorporate 32-bit mi- croprocessors and digital signal processors (DSPs) for the machine model and the necessary coordinate transformations.

Another very powerful approach that has be- come feasible with the high computational power of DSPs is the stator-flux oriented direct self con- trol (DSC, [lS]). The stator flux space vector i s guided on a given trajectory, preferably a hexagon needing the fewest switchings. Torque is control- led by adjusting the track speed of the stator flux space vector by pulsing. This is done by simple

two-level comparation of the torque calculated in the (non-linear) machine model and the set value, which can easily be performed as the complete calculation cycles takes only 40-50 ps. DSC as an asynchronous scheme uses the restricted switching frequency of high-power inverters best, producing a smooth torque with no low-order harmonics and a DC link current with reduced low-order harmonics in comparison with optimized synchronous P W M schemes with same low frequency and optimal torque response even in the field-weakening range.

W i t h a high ratio of switching frequency vs. stator frequency, the stator-flux-oriented approach can as well be combined with P W M , e.g., for IGBT inverters, where direct two-level control is not possible any more. In indirect stator-quantities control (ISC) the-here circular-trajectory and the average tracking speed are calculated synchro- nously in each pulse period of PWM.

Control of the Four-Quudrunt Converter The basic control scheme is that the DC link voltage deviation determines the active line current component. A precontrol signal i s generated from measured line voltage and the calculated voltage drop over transformer stray impedance and resis- tance; a PI-controller corrects errors. Synchronized P W M i s used to obtain a well-defined line current spectrum, avoiding disturbing injection of fre- quency portions with track signal frequencies. Adding displaced current set values, the line volt- age can be raised by leading current or lowered by

IEEE Industry Applicotians Magazine November/December 1996

lagging (braking) current in the case of a weak net. Fully digitalized controllers are standard now E14'J.

Slip-Slide Control As the projected exploitation of adhesion increased up to 40%, slip-slide-control had to be enhanced substantially. Conventional systems evaluate speed differences between axles and the derivative of speed and reduce the tractiveibraking effort set value in an open-loop control.

New systems incorporat'e speed closed-loop control. The set value is led always to find the point of maximum actual adhesion, that is, where the tangent at the tractive effort YS. wheel creep curve is horizontal; slip-stick oscillations can be canceled. To damp resonant oscillations of the cardan quill, a high motor torque dynamic (rise time 5 10 ms) seems to be necessary. A major problem is still the quality of the digital speed pick-up, especially at low speed.

Vehicle Bus Systems Around 1980, microprocessors also became avail- able for the severe environment of rail vehicles. This enabled the traction equipment manufacturers to introduce microprocessor-based vehicle control systems where complicated control algorithms could easily be implemented in software. The in- troduction of new, inexpensive, and powerful mi- croprocessors, memory, and communication chips, etc., enabled the manufacturers to introduce serial buses on the vehicles. These are today referred to as "train communication networks," normally con- sisting of a vehicle data bus, interconnecting the equipment within one vehicle, and a train data bus interconnecting the equipment in different vehi- cles in a train set. W i t h these serial data buses a huge amount of information can be exchanged over just a twisted wire pair. This is not only saving a lot of vehicle wiring but also supporting new func- tions such as vehicle diagnosis.

Train communication net works (TCNs) have been in revenue service since the mid-'80s, and today more or less all new vehicles are equipped with a TCN. At this writing there are no interna- tional standards in this area, but in 1989 an IEC working group (IEC Technical Committee 9, Working Group 22) was started with the task of defining an IEC T C N standard. A first committee draft was distributed to all IEC National Commit- tees for comments in 1992. An implementation

of the proposed IEC T C N standard has been tested by UIC on a train in revenue service starting in July 1994.

Acknowledgment The author gratefully acknowledges the contribu- tion to the section on vehicle bus systems by P. Astrom, ABB Traction AB, Sweden.

References 11) F. Kuehrer and K . Mojzis, Die oesterreichische Thyristor-

lokomotive OeBB Reihe 1044. EiJenbuhntechnik 10 (1975), no. 3,pp. 71-83.

121 G. Coget, "The New Generation of SNCF High-Speed Rolling Stock: T h e TGV-Atlantique Train," Ruil Engzneer- in~lnternationul, 1986, no. 3, pp. 15-18.

[ ? I J. Brenneisen, E. Futterlieb, E. Mueller, and M. Schulz, "A New Converter Drive System for a Diesel-Electric Locomo- tive with an Asynchronous Traction Motor,"lEEE Trans. on lnd. Appl., vol. IA-9(1973), no. 4, p p . 482-491.

I41 H. Kehrmann, W. Lienau, and K. Nill, "Vierquadranten- t e I I e r-E i n e n e t z f r e u n d I i c 11 e E i n s p e i s u ng f u e r

Triebfahrzeuge m i t Drehstromantrieb." Elektrirche Bahnen 4 5 (1974), no. 6, pp. 135-142.

151 R . Marquardt, "High Power G T O Converters for the New German High-speed Train ICE," Prui-. 3rd Europ, Conb o n Power Electronics, Aachen 1989, pp. 583-588.

161 V. Distelrath and A. Martin, "The S 252 Dual-System AC Electric Locomotive w i t h Three-phase Drive for Spanish Railways,"~le~trirrheBahnen88(1990), no. 5,pp. 224-235.

[7] M. Gerber, E. Drabek, and R . Mueller, "Die Lokomotiven 2 0 0 0 Serie 4 6 0 der Schweizerischen Bundesbahnen." Schweizer Ezdenhuhn-lievne 1011991, pp. 321-377.

[SI J.A. Tanfiq, "Advanced Inverter Drives For Traction," Proc. 5th Eurup. Con5 on Power Electmnics, Brighton 1991, pp. 224-228.

[9] W . Runge and A. Sreimel, "Some Aspects of the Circuit Design of I-Iigh-Power GTO Converters," P m . 3rd Euruji. Cunc on Power ElectronicJ-, Aachen 1989, pp. 1555-1560.

I101 T . Undeland, b'. Jenset, A. Steinbakk, T . Rogne, and M. Hernes, "A Snubber Configuration for Both Power Transis- tors and GTO P W M Inverters," I Rei.ord 1984, pp. 4 2 - 5 3 .

1111 W . M . Fuehrer, R . Marquardt, and G. Papp, "Water- Cooled H i g h Power G T O Coiiverters for Electric Traction," Proc. 5th Europ. Cord on Power Eleitroniir, Brighton 1991, pp, 294-298.

[ 121 A. Nabae, I . Takahashi, and H. Akagi, "A New Neutral- Point-Clamped P W M Inverter," I E E E Truns. Ind. Appl. 17 (1981), no. 5 , pp. 518-523.

1131 K . - H . Ketteler, "Multisystem Propulsion Concept on the Basis of the Double Star Circuit," Proi-. 6th Europ. Con$ on Power Electronii~, Sevilla 1995, pp. 2.159 - 2.166.

[14] G . Gedeon, K . Klausecker, and W. Lang, "Mikrocom- puter-Ansteuerung fuer ICE," Elektrz- srhe Bahnen 8 6 (1988), no. 7 , pp. 229-235.

I151 M. Depenbrock, "Direct Self-Control (DSC) of Inverter- Fed Induction Machinc," I E E E Tram. on Poww Eleitronii-x, vol. 3 (198S), no. 4, pp. 420-429.

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IEEE Industry Applications Mugazine rn November/December 1996