The projects of mass rapid transport in India have now taken up speed. The metro will be electrified at 25kV AC everywhere in India except Bangalore and Kolkata which will have 750V DC third rail system.

Both systems have got some merits and demerits. The strong points of both should be considered before recommending a system of electrification for any metro system. The system of electrification, once chosen will be final and cannot be changed once the metro system is commissioned. 25kV traction is suitable for the heavy haul trains and where high speeds are needed. The horse powers of the electric locomotives are up to 6000 HP and thus they need lot of power. Even with the locomotives of this size the current in the OHE of Indian Railways never exceeds 600A, the capacity of the conductors. It is for keeping the value of current low, 25kV AC is preferred over 1500V DC system. But in metros, the current will be nowhere near the maximum current carrying capacity. For example, Mumbai metro line one will have 84 coaches in six coach formations; including standby and the maintenance spares. The load current in metro railways may be less than 100 Amp. Central Railway has 60x12 car rakes and 54x9 car rakes. Four 25kV substations are planned to meet the demand of goods, passengers and EMU services between CST Mumbai and Kalyan. Metro will have one substation for 24 coaches as against one substation for every 300 coaches in railways.

The need of 25kV on railways is therefore apparent. With ever increase in demand of commuters, there is a plan to make 12 car rakes into 15 car rakes. There is absolutely no possibility of adding more coaches in metro system as extension of platforms in metro system after commissioning is ruled out. 1500V DC system in Mumbai served well till late when the traffic had not increased to demand 12-car and 15-car rakes. Conversion of 1500V DC to 25kV was required due to uniformity of traction with other parts of India. Therefore, what is suitable for India cannot be straightaway considered as suitable for the elevated metros.

There are few other areas which make an interesting comparison of both types of tractions.

A. DESIGN
Motor coaches: A motor coach of 25kV system needs a transformer. The axle weight of the AC motor coach is therefore more. For the same traction motor, the D.C. motor coach with less weight will accelerate faster. The journey time for dc rake will be less than the ac rake. The requirement of rakes in dc system for the same PPHPD will be less. The axle weight being less, the civil structure is subjected to less direct load. The bending movement due to wind load on OHE conductors will be absent in third rail system.

Earthing: The 25kV AC systems needs elaborate earthing. Since the concrete is non-conducting, the rail and the buried earth conductor are connected at an interval. A separate earthing rod runs through the pier and is also in contact with the reinforcement. Such elaborate arrangement in 750 VDC is not needed. The voltage in third rail is low; the negative rail is near the positive rail, the leakage current will not flow to the earth in third rail system.

Equipments for electrification: The equipments for the 750 DC third rail systems are simpler. 25kV AC system needs catenary wire, contract wire, overhead protection conductor and return conductor. In addition, the system needs over head structures, structure bonds, bracket assemblies, insulators, booster transformers etc. The 750V DC systems needs only additional rail which is supported on low voltage insulators.

Maintenance: The manpower required for maintaining the equipment with multiple conductors at height is more than that required for maintaining a single power rail at rail level. A special maintenance vehicle is needed to maintain the OHE. The AC motor coaches need additional maintenance facilities for transformer. The maintenance of 25kV AC systems is therefore, expensive.

Energy loss: The strong point in favour of 25kV AC is lower current in OHE resulting in less energy loss in conductors. This is true. But this is important when the load currents are heavy. The currents in metro system are small compared to the railway systems. The 25kV AC is therefore more useful for main lines. In third rail system, the energy loss can be reduced by the use of aluminium rail having less resistance than the normal rail.

While comparing the energy loss, the additional loss taking place in AC motor coaches due to transformers should be considered. Whether or not the train moves, the transformer will consume 5 per cent of the power due to iron loss and additional 2 per cent as copper loss when running. The weight of the motor coach in 750V DC is less due to absence of transformers. The motor coach therefore, has to carry less weight which results into less consumption of energy.

D.C. link voltage: 25kV systems can provide the higher dc link voltage to enable designing of motors high power to weight ratio. But, this problem can be taken care of by powering some or all axles of trailer coaches. Bangalore metro has got a traction motor of 190KW as against 220KW in Mumbai, but the acceleration and the maximum speed is same in both cases.

Use of non-standard voltage: If it is considered beneficial to use AC system for some reason, the use of non-standard voltage can be thought of. If the distribution network is at 33kV, use of 19kV in place of 25kV will eliminate the need of traction substations completely. In case the distribution is at 22kV, use of 12kV may be considered to get the similar advantage. It will be possible to get the same dc link voltage with 19kV or 12kV.

Underground system: Third rail provides unique advantage for underground system. The tunnel size will be reduced by about 19 per cent. The muck to be handled also gets reduced. The ramps leading to the tunnel are smaller lengths.

Regeneration: 25kV system has been favored due to regeneration done resulting into saving of energy. But it may be noted that regeneration is not limited to 25kV system. The regeneration is made possible by the use of three phase traction motor. Both third rail system and 25kV system use three phase traction motor. In third rail system, another train absorbs the regenerated power. In case there is no train to absorb it, the power is dissipated in the resistances called AARU. In Kuala Lampur monorail in Malaysia where AARU has been provided, the need to waste energy in AARU rarely arises.

Substations: The third rail needs more number of substations as compared to the 25kV system. Though the size of dc substation is small, getting space for the substations in metro cities is always a challenge. But with use of dry type transformers, it is possible to erect elevated substations at the end of the platform.

B. SAFETY
Parallelism of transmission lines: The metro cities can have high voltage transmission lines. These lines can run parallel to the metro alignment and will induce considerable voltage in 25kV AC lines. The transmission lines may have to be made underground from safety considerations. For third rail system, such lines need not be made underground.

Ground fault: Earth is a good conductor of electricity but the concrete is not. The resistance of the concrete is much higher than the earth. The resistance will also depend upon the cracks in the concrete, moisture content, age of concrete and the point at which the conductor comes in contact with concrete.

If the parted 25kV AC conductor falls on the concrete and the fault is not cleared by the circuit breaker, dangerous condition will arise.

In another scenario, if the parted conductor hangs below the viaduct, it will endanger the safety of the road users. In yet another scenario, if the parting takes place above the railway crossing and the power is not switched off, wrong phase coupling can result. The substations of metro and railways will have to be interconnected through SCADA to avoid for wrong phase coupling. Such need will not arise with third rail system.

Pantograph entanglement: The pantograph entanglement often results into breaking of pantograph parts like nut bolts, copper strip hoses and sometimes entire pantographs get uprooted. These parts often get thrown off. If these parts fall outside the via duct, they can endanger the safety of road users.

Electromagnetic and electrostatic induction: 25kV can induce voltage in internet cables, TV cables and metallic structures like grills etc. Adequate study in this area is needed.

Crossing of TV cables: Large number of cables for the TV connections runs on the roads, some parallel, some across. The cables running parallel to the alignment will have an induced voltage. In case of breakage of TV cable crossing tracks, it will fall on the 25kV line and create unsafe condition for the people whose houses are served by these cables. Since these cables are without any protective system, the fault will not get cleared till the wire burns out. A legislation to ban such wires crossing the 25kV AC may be necessary.

Use of OPC and BEC: Overhead protection conductor (OPC) and buried earth conductor (BEC) are required for 25kV AC traction for elevated lines. They are not needed for railways since the railway line runs at grade. The OPC is an ACSR conductor. This conductor can break at the support point. For the transmission line, double string insulators are used at the road and railway crossing to reduce the probability of the conductors falling down. The OPC if falls down on road where the road traffic is heavy will create unsafe condition.

The buried earth conductor is supported on sides of the viaduct. BEC, rail and OPC run in parallel. Since the joint between rail and the ACSR conductor (BEC) will be a bimetallic contact, these joints are expected to give problems sooner or later. There have been some theft cases of BEC reported. In the absence of this conductor, the return current may flow to the ground through rain water. Rainwater is pure, but accumulated dust on the viaduct will make it conducting.

C. COST COMPARISON OF PRESENT PROJECTS
The comparison of costs in metro projects is difficult since the funding structure of the projects differs. The problems vary from project to project. Since the length of the lines are small, the per kilometer costs varies significantly. There are some hidden costs and hidden benefits. The comparisons purely on the basis of the cost will be misleading (See table.)

Metro Rail Cost Comparison
Metro	Total (`crore)	Length (km)	Cost/km (`crore)
Kolkata	4,676	13.8	340
Hyderabad*	16,375	71.2	230
Chennai	2,598	8.6	302
Mumbai-Line 1	2,356	11.4	207
Mumbai-Line 2	8,250	32.0	258
Bangalore	8,158	42.3	193
*Include real estate development cost

The costs tabulated for all the projects are the overall costs of the Projects which includes civil engineering cost, rolling stock, signaling cost and other usual costs. The other conditions remaining same, the cost of third rail system will be less.

D. KNOWN AREAS OF COMPARISON
In addition to the above points, there are some points often indicated while comparing the two systems.

Visual Appeal: The third rail system does not need overhead lines and therefore the skyline does not get affected. In many places, the third rail was recommended primarily to avoid unsightly poles and overhead wires.

Robustness: The third rail systems are more robust than overhead lines. The conductor rail is able to take higher mechanical forces than the over-line conductors. There are many current collecting shoes. Even if one of the shoes is damaged, the train can continue its journey.

Flyovers: Occasionally, flyovers have to be constructed over the existing metro lines. The height of the flyovers for 25kV AC systems will be more.

E. CONCLUSION
While choosing 25kV traction, attention should be given to safety. For the underground application, third rail will be desirable.

(S.P. Khade is Director—Technical, Mumbai Metropolitan Region Development Authority)