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PT.11: THE QUEENSLAND 25kV 50Hz AC SUBURBAN SYSTEM THE EVOLUfION OF ELECTRIC RAILWAYS While Sydney and Melbourne had electrified suburban rail systems in the 1920s, Brisbane held off until the 1970s. The city then leap-frogged the rest of Australia by installing high voltage electrification. By BRYAN MAHER Up until the 1950s, all locomotive power in Queensland was traditional steam, even for Brisbane's suburban services. Then the State government undertook a bold venture to provide long distance air conditioned diesel-electric mainline trains. First to run was the Brisbane to Cairns " Sunlander". Electrification of the Brisbane suburban rail system, first mooted as far back as 1915, had a shortlived start during 1947. At the time, electric trams had been running in Brisbane city since 1887. In 1952 the tramway system reached peak performance in terms of the number of tramcars, with nearly OVERHEAD WIRE 25kV 50Hz SINGLE PIIASE TRANSFORMER SILICON CONTROLLED RECTIFIERS FIELDS RAILS Fig.1: the 25kV AC overhead wire feeds the primary winding of an onboard power transformer, with the return circuit via the wheels and rails. The two secondary windings feed thyristor bridges which control DC traction motors. 88 SILICON CHIP 200km of track. The total service then had 325 tramcars, including drop-centre and corridor types. Against this background the electrification of the city and suburban rail system seemed natural. Planning for a 1500V DC railway proceeded and civil engineering works were completed in 1947-1957. Elections then brought a change of government and a reduction in loan funds. The new government overruled the electrification program, opting instead for a gradual introduction of diesel-electric locos for the suburban service. Later, the Brisbane tramway system suffered a major setback. In September 1962, 68 trams stored for the night in the Paddington depot were caught in a disastrous fire. As the inferno raged the few night shift maintenance men managed to drive three cars out before the depot roof partially collapsed. This short-circuited the 600V DC on the trolley wires and tripped the circuit breakers at the substation. Without traction power, the workers could only stand (powerless!) and watch as 20% of the tramcar fleet was destroyed. Though a handful of new .trams were built, by the mid 1960s diesel buses gradually took over the city and inner suburban service. The end to Brisbane's electric trams and electric trolley buses came in April 1969. Twenty-two trams were acquired by the Brisbane Tramway Museum Society and can be seen operating today at Ferny Grove. Electric suburban railway Back on the suburban railway scene, the same State Government revived the idea of electrifying the whole suburban railway system in These are the new 3-car sets which are used in Brisbane and its suburbs. They are powered from 25kV AC via the overhead line and each 3-car set has eight 135kW DC traction motors, giving a total power of 1.08MW. the late 1970s. The big day came in November 1979 when electric trains were inaugurated. The electric system ran from Darra, via Roma Street and Central stations, to Ferny Grove, a distance of 34km, serving a total of 26 stations. Progressively extended, electrification has now reached Beenleigh, using the new Merrivale Bridge across the Brisbane River. It presently reaches east to Moreton Bay suburbs, west to Ipswich and north to Caboolture. The suburban electric cars, made by Walkers/ ASEA Ltd in their Maryborough workshops, are constructed of stainless steel and fully air-conditioned. They are 23 metres lorig, 2.72 metres wide and 3.87 metres high. They are normally run as 3-car sets which can be coupled up to form six or 12-car trains. The three-car sets are semipermanently coupled to form one unit, 72.42 metres long and weighing 150.2 tonnes fully loaded. Three-car sets are used for off-peak periods and 6-car trains run during peak hours, with specials of 12 cars used regularly. A 6-car train seats 496 passengers, and can carry a maximum of 1000 passengers. Designed for a maximum speed of 100km/h, a fully loaded train can be brought from full speed to standstill in a distance of 425 metres. 25kV AC 50Hz supply The Brisbane railway electrification scheme was the first in Australia to use high voltage 50Hz AC. The overhead catenary wire runs at 25kVAC 50Hz. As Fig.1 shows, the high voltage overhead wire feeds via a lightweight pantograph and main circuit breaker to the primary winding of the onboard transformer, with th'e return circuit via the wheels and rails. The on-board transformer is mounted under the floor of the middle car of each 3-car group. Two 690V secondary windings on the transformer feed thyristor bridges, phase-controlled by timing trigger circuits as indicated in Fig.2. This provides up to 1100V DC for the armatures of the four traction motors of this car. A third secondary winding on the transformer supplies 136V AC (via an intermediate transformer) to another controlled thyristor bridge supplying the field windings of the traction motors. All secondary circuits also pass to the leading car where a further two controlled thyristor bridge rectifiers supply armature current to the four DC traction motors of this car. Yet another thyristor bridge rectifier supplies the separately excited motor field windings. The trailing car has no traction motors but is equipped with a driver's cabin and controls (so the 3-car set can be driven in either direction), An auxiliary converter mounted under the trailing car provides a 415VAC 3-phase · 50Hz 135kVA supply to all auxiliaries including oil pump motors, air conditioning, fluorescent interior lighting and headlights. A separate single phase rectifier bridge supplies the DC motor driven main air compressor for door operation and air brakes. A 110V DC battery provides for marker and emergency lighting, emergency ventilation, emergency air compressor and also the 50V DC SEPTEMBER 1988 89 OVERHEAD WIRE 25kV 50Hz I I I I I I I c56RAIL I -!- I I I FOUR MOTORS I I I I I I I I I I I I I I I I I I I I I I '----"~-¾------~1 \ I FLEX CABLES I JOIN CARS I I I I I I I I I I I I II I TO AUXILIARIES I I I ..___ _ _ _ _ _ __,, I ------- OM-CAR _______ L __________ J M-CAR _____________ I J (a) Fig.2(a): This diagram shows the electrical system of Brisbane's 3-car set in more detail. Note the 1.290 resistors which are switched across each pair of traction motors during regenerative braking. Each of these resistors dissipates several hundred kilowatts during braking. circuits for the driver's control systems and all car door operation. A large iron-cored reactor helps in smoothing the rectified DC supply for the traction motor armatures. Even though the motor field windings are separately supplied by DC, the motor field yoke is made of laminated steel to minimise eddy currents caused by 100Hz ripple current. The main transformer is rated at 1.635MVA, of which 1.34MVA is for traction power. With careful distribution of the heavy loads such as the main transformer, reactor and air compressors over all three cars, the loading is kept to a low 15.25 tonnes per axle. Traction motors This 3-car set makes quite a complex electrical unit, driven by eight ASEA 480V 310-amp DC traction motors, each rated continuously at 135kW. These are connected in series pairs across the controlled 90 SILICON CHIP 1100V DC supply, giving a total power of 1.0BMW for the 3-car unit. On level track and with a full passenger load, the train can briskly accelerate to 48km/h within 60 seconds. Top speed is lO0km/h. Motor bogies Each bogie of the leading and middle cars is equipped with two traction motors, each motor driving one axle. The drive is through a 5. 7: 1 traction gear mounted on the axle. The motor top speed is 3 780 RPM at a train speed of lO0km/ hour. The motors are hung on roller bearing suspension tubes on each axle. Such a mounting allows the motor drive pinion to remain in mesh with the driving axle gear as the wheels rise and fall with track variations. All motor armatures and train axles run in roller bearings. The primary suspension takes the form of rubber bonded Chevron spring elements, suspending each axle box horizontally and vertically. Air bag secondary suspension units transmit body weight to the bogie frames. The air bags have a control system designed to keep the body at a nominated height above the bogie, even with changing passenger loads. At the same time, the air pressure within the suspension bags is continually sensed by an electropneumatic transducer. The electrical signal so produced is used to modify motor current during acceleration (to prevent wheel slip) and braking effort (to prevent wheel skid when stopping). Thus, if a car is lightly loaded, it will have less braking effort applied than a more heavily laden car in the same 3-car set. Traction rods, torsion bars, and vertical and horizontal hydraulic shock absorbers combine to provide smooth riding conditions under ac- celeration, braking or negotiation of curves. Brakes The brake system uses electrical dynamic braking blended with electrically controlled air brakes. A back-up compressed air brake is in readiness at all times, to fully control the train should the electrical brake be insufficient. The changeover is automatic and smooth in action. The dynamic brake acts by varying the current to the field windings of the traction motors while a heavy duty 1.29 ohm braking resistor is connected across the armatures. The motors then act as DC generators, with the current generated being dissipated in the braking resistor. This electrical load on the motors (now acting as generators) smoothly slows the train. Because this regeneration process depends on motor armature speed, the braking control system must continually sense train speed and automatically apply more field current to the motors as the train slows down. The resulting system is sufficiently accurate, as Fig.3 shows, to provide constant deceleration of one m/sec2 when slowing from 90km/hour to 40km/hour. Below 40km/hour, this deceleration rate cannot be provided by dynamic braking alone as this would demand too much field current. Below 40km/h, the air brakes steadily take over to bring the train to a complete stop. There are four brake cylinders on each bogie, actuating composition brake blocks for each wheel. The braking action is in three modes, all controlled automatically without the driver having to be concerned about which mode is operating at any one moment. In mode 1, the electropneumatic brake system is automatically modified for passenger load and graduated application/release. This is automatically blended with mode 2, the dynamic brake effort. As the driver applies brakes, the electropneumatic system applies air to the brake cylinders until the brake shoes touch the running wheels. At This photograph shows the lightweight catenary for the single-phase 25kV supply. Note the negative return wires on the mast. the same time the traction motors controls, emergency lighting, conare switched to dynamic regen- trol circuits for the air conditioning, erative mode which provides most emergency fresh-air ventilation systems and the control of the elecof the braking effort. If the rate of decrease in speed is tropneuma tic brakes. less than that demanded by the Because thei r ope ra tion is driver, the system automatically in- critical, the traction controls are creases the air pressure in the powered by 50V DC obtained from brake cylinders to increase the rate. a 1.2kW voltage regulator mounted of retardation. Thus the change- .on each car and powered by the over from dynamic to air braking is ll0V DC battery. These voltage smooth, automatic and unnoticed stabilisers also provide a regulated AC supply of ± 50V peak at 200Hz by the passengers. for control of the traction thyristor Train controls rectifier bridges. All control and emergency functions are powered by a 48-cell 110V Driver's controls DC lead-acid battery slung under Control signals for the accelerathe leading car of each 3-car set, tion and braking are transmitted giving adequate control in the event throughout the train from the driver's end via a 3-wire PWM of loss of the 25kV supply. The 1 lOV DC systems include the (pulse-width modula ted) signal train communication radio and the derived form a solid state chopper public address system, car door circuit in the driver's cabin. One ":~ <lC\C\[\L\[\ TIME TIME Fig.2(b) & (c): these waveforms show the thyristor bridge rectifier output at full power (b) and at two-thirds power (c). SEPTEMBER1 988 91 switched across the braking resistors, DC current is applied to the separately excited fields as demanded by the PWM signal. Simultaneously, the dynamic brake voltage generated by the rotating armatures returns a signal indicating the extent of electric braking actually achieved. These two demand and response signals are compared in an analog difference circuit to determine the air pressure applied to the braking cylinders. In this way, dynamic and air braking is automatically blended. Automatic warning system Close-up view of the thyristor control gear mounted under the trailing car of the 3-car set. Thyristors are far more efficient than the resistive controllers used in older electric train sets. wire is active when acceleration is called for, another wire becoming active when braking effort is demanded by the driver. The degree of acceleration or braking demanded is determined by the signal pulse width; 100% pulse width corresponding to either maximum traction power or maximum braking. Minimum pulse width would mean a train coasting under momentum or downgrade with no traction force nor brake applied. A pulse width of 50% would demand medium acceleration or medium braking, depending on the third wire selected. The PWM coded signal is fed to a decoder circuit mounted in each car. The analog signal so derived is modified separately in each car by the weight of passengers in that car, as indicated by the air pressure transducer in each bogie air-bag suspension. In this way, if a train carries unevenly distributed passenger loading, a packed motor car would have more traction current applied to its traction motors than a lightly loaded motor car on the same train. The same applies to braking, as noted above. The automatic blending of electric dynamic brake with the pneumatic brake is achieved by a differential measurement. For electric dynamic braking, with the traction motor armatures 100~-----------+----------"'"'"'/ The Westinghouse automatic warning system consists of magnetic transmitters mounted on track sleepers between the rails ahead of electric colour-light signals, and magnetic receivers mounted under the train. The signal circuit state (green or otherwise) is conveyed to the stationary sleeper-mounted electromagnet, changing its magnetic polarity which is sensed by the train-mounted magnetic receiver. Thus, the state of each signal being approached, as well as being visible to the driver, is indicated audibly by a bell in the driver 's cabin in the case of a clear signal, or in the case of a red or amber signal by an air horn. Automatic brake application follows the air horn if the driver does not respond within three seconds. Results Compared to the diesel-hauled suburban trains which they replaced, these "state-of-the-art" electric trains have resulted in a 25 % faster trip as well as a much more enjoyable ride. This has successfully attracted many more travellers to the suburban service, significantly reducing the peak-hour traffic crush on suburban main roads. Next month we will further investigate high voltage "industrial frequency" electric railways. Acknowledgements TIME REQUIRED TO STOP TRAIN Fig.3: relative stopping times for air braking and dynamic braking. In practice, the two systems are automatically blended by on-board sensors. 92 SILICON CHIP Grateful thanks to Queensland Railways and Walkers/ASEA for technical data and photographs. ~