Getting to the heart of Propulsion Failures of Three Phase Locomotives

Last updated on January 2nd, 2014 at 02:35 am

Research on high ambient temperatures have led to premature failure of electronic components on Indian Railways’ latest high-power electric locomotives class WAG9,WAP5,WAP7.

By: R.N.Lal and Sandeep Srivastava (Research, Designs & Standards Organisation)

The performance of electronic components in Indian Railways’ high-power electric locomotives has been far below expectation, particularly in comparison with the European countries from which they were derived.  With IR now committed to a significant expansion of its three-phase traction fleet, RDSO has been undertaking extensive studies over the past three years to get to the bottom of the problem.

IR currently operates 350 high-power electric locomotives, and is planning to build around 150 per year in future, whilst at the same time raising their continuous rating from the current 6000 hp to as much as 12000 hp.

The loco designs have their origins in the 22 WAG9 freight locomotives and 11 WAP5 passenger locomotives imported from ABB of Switzerland in 1995-96.  Under a technology transfer agreement, manufacturing was subsequently indigenized; the locos are assembled at Chittaranjan Loco Works, using components supplied by BTIL, Nelco and BHEL.  But right from the outset the failure of electronic cards for the traction converter, auxiliary converter and vehicle control units has been a source of concern, affecting both the reliability and availability of the locomotives.

Since December 2007, RDSO has been working with IR’s locomotive shed managers and the suppliers to collect and analyse data about all failures since 2003-04.  This showed that the main reason for the problems lay in the

premature failure of components such as capacitors, optic-fiber transmitters and receivers, and EPROMs.  The same components did not show similar failure rates in Europe.

European benchmark

In European countries using a similar traction package, electronic card failures are typically observed at intervals of nine or ten years.  By comparison in Indian conditions our failure figures suggest that cards are prematurely failing within 5-6 years.

Suspicions pointed to higher ambient temperatures in the locomotive machine rooms, so we conducted tests comparing the WAG9s with the WAG 6A, 6B and 6C classes of locomotives which are employing thyrstor converters.

According to the manufacturers, the cards are designed to withstand temperatures of 85°C, giving a substantial safety margin in countries where the ambient temperature rarely exceeds 40°C.  But in India, ambient temperatures can regularly reach 50°C in summer, and the higher machine room temperatures lift the working conditions for the electronics to 65°C or more.

Practical observation of locos left standing in direct sunlight found temperatures above 70°C near the key components.  This comes close to breaching the recommended 10% safety margin for the 85°C limit.  In such conditions, pre-cooling using the machine room blower is required before the locomotive can be started.

Contributory sources

We found that power dissipation by the cards themselves contributed very little to the temperature rise, around 1.5 kW compared to total power losses of 32 kW in the machine room.  However, in the WAG9 the electronics for the traction and auxiliary converters are placed above the converters themselves, which generate heat.  By contrast the electronics for the WAG6A are mounted on the back wall of the cab, where temperatures are comparatively lower; and on the WAG6B or 6C they are in the cab, where the temperatures are even less than ambient.

The electronics of the WAG9 are very close to the loco roof, and therefore subject to solar radiation of up to 46 kW in the summer.  Temperature in the machine room near electronics was found to be 10^oC more in the mid noon than before sun rise.

Pressurization of the machine room to 1.2 bar, which is intended to reduce dust ingress, increases the temperature inside the locomotive by up to 8°C. Staff at Waltiar shed confirmed that equipment failures were higher for the WAG6A, which has a pressurized machine room like the WAG9, WAP5 and WAP7, than for the WAG6B or 6C variants. On the WAG9, the cooling air from the converters is used to pressurize the machine room before exiting through roof vents, dissipating heat from the auxiliary converter power losses estimated at around 6.5 kW

Remedial measures

As an initial step RDSO recommended that key components such as electrolytic capacitors, EPROM and fiber optic transmitter and reciever should be changed after six years to prevent failures in service, and this has been incorporated into IR’s strategy for rehabilitation of electronic cards. We have also developed an aluminium heat sink for the traction converter electronics in place of steel, and this is now being adopted after trials confirmed that it reduced the temperature rise by around 6°C.

One locomotive was equipped with a pair of 3-tonne air-conditioning units to cool the incoming air to the machine room blower. This loco is on test at Ghaziabad loco shed, demonstrating a net reduction of internal temperature of around 8°C. We have recommended  to fit air-conditioning to several other locos for a more extensive evaluation.

Another loco at Tughlakabad shed was fitted with a passive cooling module using the Peltier effect. This reduced the temperature around the traction converter cards by 8°C, but the heat was only transferred elsewhere inside the loco whereas with air-conditioning it was rejected out completely. Nevertheless IR has decided lo fit five more WAG9s with passive cooling for extended trials.

Dust trap

Dust could also be a factor, as this is known to affect the reliability of electronic components. It was found during periodic examination of WAG9s that up to 6 kg of dust had accumulated in the air ducts. This is probably due to insufficient filtration of the incoming air, as unlike the WAG6A, the WAG9 does not have a secondary stage filter. One WAG9 at Gomoh shed was fitted with a secondary mesh filter and modified cyclonic filter; at the same time increasing the impeller size of the blower and running it at a higher frequency has increased air delivery by 40%.

However, increasing the airflow actually had a negative impact on temperatures, as running the blower harder and increasing the machine room pressurisation raised the temperature of the outlet air from 4°C to 6°C above ambient. So another locomotive is planned to be fitted with a modified cyclonic filter and secondary mesh similar to the WAG6A but with no blower changes.

RDSO has been working with Central Railway to address the effect of the sun, and the roof of one loco at Ajni loco shed has been covered with heat-reflective paint. This helped to reduce internal temperatures by 8 or 9°C during daytime, but saw an increase of 6 to 8°C at night because heat could no longer be dissipated through the roof Nevertheless, it did keep internal temperatures below 55^oC on hot sunny days and 50°C at other times. We have identified a paint capable of reflecting sunlight but without the thermal insulation properties which is under evaluation.

Dissemination

RDSO has been holding regular meetings with Railway shed managers and the technology transfer companies since December 2008 to provide feedback from the study. Working with IR’s management team, we have developed an action plan and special maintenance instructions which have now been adopted.

The suppliers have developed functionally-equivalent power supply cards to replace those that suffered the most failures, and these have been running successfully for the past one year. Higher-rated electrolytic capacitors with a potentially longer life has also been identified, and this will be introduced progressively as the electronic cards are rehabilitated.

The net result of these changes has been a steady reduction in the failure rates of the various components.

Another challenge will come with the adoption of IGBT propulsion technology in the next generation of WAG9 and WAP7 locomotives now under development.  Although the heat losses from the IGBTs will be around half of those from the current GTOs, this is actually a very small contribution to the internal heat problems.  As well as adopting the enhanced maintenance action plans, we believe that Indian Railways needs to undertake a complete review of the cooling arrangements for future builds of high powered locomotives ensuring the electronics work in an environment where the temperature always remains below 45^oC.