Case Details

Rail Transit February 28, 2025

A rail transit group metro vehicle traction system current monitoring program

The technical challenge
Metro traction systems need to withstand constant vibration and strong electromagnetic interference, under-vehicle installation requires IP65 protection.
prescription
FL-2 splitter, reinforced mounting structure, shielded twisted pair signal transmission, IP65 protection design
Project results
Accuracy ±0.8%, passed EN 61373 railroad test, equipped with 320 motors, cost reduction of 30%

Project Background

A large rail transportation group is responsible for the operation and maintenance of subway lines in several cities. The traction system of subway vehicles adopts AC frequency conversion speed control technology, which requires accurate monitoring of traction motor current for control system feedback, energy consumption statistics and fault diagnosis.

The traditional solution uses current transformers (CTs), but suffers from large size, high cost, and poor low-frequency response. The customer wanted to trial the shunt solution on new line vehicles to obtain higher measurement accuracy and lower system cost.

Technical specification requirements:

  • Traction motor power: 180kW × 4 units/vehicle
  • DC bus voltage: DC 750V/1500V
  • Maximum current of single motor: 400A
  • Current measurement accuracy: ±1%
  • Working temperature: -25℃ ~ +70℃.
  • Protection class: IP65 (under-vehicle equipment)

Technical challenges faced

1. High voltage isolation

The DC bus voltage of the traction system is as high as 750V or even 1500V. Although the shunt has no high-voltage isolation problem for low-side measurements, it needs to be considered for the insulation fit of the overall system.

2. Vibration environment

Subway cars operate with constant vibration and shock, and the equipment underneath is exposed to an even harsher vibration environment. Shunts and their connections must be able to withstand long-term vibration.

3. Electromagnetic interference

The high-speed switching of the traction inverter IGBTs generates strong electromagnetic interference, which may affect the accurate transmission of the shunt signal.

4. Protection requirements

The under-vehicle equipment is exposed to rain, dust, oil and other harsh environments and needs to be IP65 rated.

Technical Solutions

1. Shunt sizing

Each traction motor is equipped with one FL-2 shunt, key parameters:

  • Resistance value: 150µΩ ±0.5%
  • Rated current: 500A
  • Rated voltage drop: 75mV @500A
  • Temperature coefficient:<25ppm>
  • Power loss: 24W @400A

2. Design of mechanical reinforcement

  • Splitter with reinforced mounting structure, 4 M8 bolts fixing
  • Mounting base with vibration damping design to reduce vibration transmission
  • Electrical connections are made with crimp terminals to avoid loose threaded connections.
  • Passed EN 61373 vibration shock test for railroad vehicles

3. EMC design

  • Voltage sampling using shielded twisted-pair cable, shielding and body equipotential connection
  • Signal conditioning module mounted in a metal shielded box
  • Common mode filtering and TVS protection added to module inputs

4. Protective design

  • Shunt surface nickel-plated anti-corrosion treatment
  • Signal Conditioning Module in IP65 Housing
  • The connecting cable adopts the weather-resistant type for railroad

Project results

  • Measurement accuracy:Current measurement accuracy of ±0.8%, better than ±1% design requirements
  • Reliability:Passed EN 61373 vibration shock test and a full set of railroad environmental tests.
  • Application Scale:Has been tried in 2 metro lines of new cars, a total of 80 trains equipped with 320 sets of motors
  • Cost advantage:System cost reduction of approximately 30% compared to conventional CT solutions
  • Running performance:Trial operation for over 2 years, no faults reported, customer plans to roll out to more routes