BACKGROUND TO ELECTROMAGNETIC INTERFERENCE
Electromagnetic interference (EMI) and compatibility (EMC) is a major concern for any high power electrical sector employing power electronics with switching devices and microelectronics. The general requirements for science and engineering applications are described in EN55011.
Since April 2000 this has consequently resulted in the EN50121 mandatory requirements for new railway applications, which includes traction rolling stock (e.g. diesel and electric locomotives),
mainline vehicles (e.g. intercity passenger and freight trains) and urban transit (e.g. underground trains, trams, LRVs), require testing for compliance with the European Standard.
LONGITUDINAL VOLTAGE AND CURRENT
Most EMC problems have a common background irrespective of the application sector. Therefore general standards and codes of practice can be applied across many industries. However, an EMC hazard peculiar to railways is "Interference from Leakage Currents circulating under the Train". The hazard is manifested in two forms:
- A longitudinal voltage developed along a train that can affect an adjacent track and its track circuits, either with or without the presence of infrastructure track faults.
- A leakage current circulating under the train (longitudinal current) when the train is straddling an insulated rail joint that has a broken side-bond.
Jointless track circuits avoid the problem created by longitudinal current, but for many legacy signalling systems it remains an unrecognized or under-estimated hazard. Both these hazards are mitigated by consideration of the systems and components in place to minimize the risk of stray currents at critical track circuit frequencies entering the running rails. The hazard analysis is concerned with the risk that the train propulsion control or auxiliary system fails in such a manner that excessive levels of leakage current can enter the running rails. This would be identified by the false activation of track circuit relays from excessive longitudinal voltage or current at the signalling frequencies. If this hazard occurred it could lead to a false signal indication (a wrong side) and potentially result in collision.
Proving these risks are acceptably small requires a careful analysis of the return current and earthing strategies employed by the rolling stock. Current harmonics flow the length of the train when faults to traction or auxiliary cabling permit parallel current paths in the vehicle body or rails. The question to answer is how large could this current be and what is the probability of the fault condition? Equally parasitic paths can exist that propagate return harmonic currents via bogie earth brushes and the rails. When this current is significant it develops a voltage along the length of the train. In the case of a systematic design (common cause) error these longitudinal voltages sum to create a train length voltage capable of a wrong side failure event on a neighbouring track.
The function of EMC risk assessment, often performed by fault tree analysis, is to consider all possible mechanisms that could create longitudinal voltage or current, and then to prove that every scenario is a negligible risk. Longitudinal current can be very difficult to reliable measure in test conditions, so greater dependence is placed on the theoretical assessment.
EMC REQUIREMENTS
The EN50121 standard addresses three main electromagnetic interference or EMC railway system consequences.
- The effect on the environment surrounding the railway.
- The effect on communications signaling equipment.
- The capacity of the installed equipment to remain unaffected by the worst case environment (including weather effects) in which it operates.
ELECTROMAGNETIC INTERFERENCE TESTING
To ensure compliance and to satisfy Health and Safety requirements, it is essential that rail vehicles and railway equipment are rigorously EMC tested prior to service introduction. This guarantees that specified limits (as standards define) are met by generated electromagnetic emissions and measured susceptibility to external EMI.
This testing constitutes a mandatory part of the EMC assessment required and performed by the NRAB (Network Rail Acceptance Body) or one of its approved subsidiary Vehicle Acceptance Bodies (VAB). The Vehicle Safety Review Panel (VSRP) ultimately approve all tested equipment as suitable (or not) for operation on defined NR routes within the UK rail network. For immunity to external electromagnetic radiation, EN50121 requires compatibility of functionality at a field strength of 20V/m in 150kHz to 1GHz band. The specified EN50121 radiated levels from a moving vehicle (worst case) is split into frequency bands. The most critical band for railway infrastructure compatibility purposes is the lowest, spanning 9kHz to 59kHz, with a limit of 300dBµV/m. The range from DC to 9kHz is covered by Railway Standards and not included in EN50121-2.
The individual EMC tests that normally represent components of EN50121 compatibility include at least the following sections, and possibly more depending the class of vehicle under test:
- EN55011 (Class B)
- EN61000 part 4-2 Immunity to Electrostatic Discharge
- EN61000 part 4-3 Radiated Emissions Immunity to Radiated Fields
- EN61000 part 4-4 Immunity to Fast Transient Bursts
- EN61000 part 4-6 Conducted Immunity
COMPATIBILITY IMPLICATIONS
EMC testing can only be conducted on a completed vehicle or equipment, so the commercial consequences of a significant test failure can be catastrophic.
To prevent the possibility of this scenario arising, extensive design calculations assisted by circuit modelling of conducted currents and radiated fields are essential.
One of the most demanding EMC projects was the Channel Tunnel and the Eurostar trains operating though it.
Cecube was employed during this project to perform traction equipment modelling to resolve compatibility anomalies. This was achieved by implementing a real-time software modification.
Cecube offers proven experience of identifying the main areas of risk prior to equipment build, and then tailoring theoretical deductions to resolve these issues in practice before testing commences.
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