BACKGROUND TO ELECTROMAGNETIC INTERFERENCE
Electromagnetic interference (EMI) and
compatibility (EMC) as identified by EU EMC directive 2004/108/EC, is a
major concern for any high power electrical sector employing power electronics
with switching devices and microelectronics. The requirements for science and
engineering applications are described in the basic standard EN55011 for radiated
emissions, while EN55022 specifies RF conducted limits. Since April 2000 this
resulted in the creation of product standard 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), all requiring testing for compliance with the European Standard.
LONGITUDINAL VOLTAGE AND
Most EMC problems have a common background
irrespective of the application sector. Therefore basic and generic 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
unrecognised 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
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 signalling equipment.
- The capacity of the installed equipment to remain unaffected by the worst case
environment (including weather effects) in which it operates.
To fully appreciate the background and
applicability of the EN50121 standard the following web-cast is essential viewing.
EMC in Railways Speaker: Gary Crawshaw, Project Engineering Manager,
12:00:00.0 Transport Channel
>> PLAY WEBCAST>> recommend to friend
To ensure compliance and to satisfy Health and
Safety requirements, it is necessary that rail vehicles and railway equipment are
rigorously designed and EMC tested prior to service introduction. This guarantees
that specified limits (as define by the standards) are satisfied in terms of
generated electromagnetic emissions and measured susceptibility to external EMI.
In the UK this testing constitutes a mandatory part of the EMC assessment required
and performed by the NRAP (Network Rail Acceptance Panel) or one of its approved
subsidiary Vehicle Acceptance Bodies (VAB). The NRAP ultimately approve all tested
equipment as suitable (or not) for operation on defined NR routes within the UK
rail network. For example, concerning immunity to external electromagnetic
radiation (mobile phones), EN50121 requires compatibility of functionality at a
field strength of 20V/m in 800kHz to 1GHz band, 10V/m for 1.4GHz to 2.1GHz, and
5V/m for 2.1GHz to 2.5GHz. The specified EN50121 radiated levels from a moving
vehicle (worst case) is split into frequency bands to prevent excessively onerous
limits throughout. 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 national Railway Standards and not included in
EN50121-2, but will be within the scope of EN50500 in the future, when
standards for this section of the railway spectrum are also harmonised. The
individual EMC tests that represent aspects of EN50121 compatibility include at
least the following sections from basic standards (those covering all electrical
and electromagnetic disciplines), and possibly more depending the class of vehicle
- EN55011 (RF Limits and Measurement Methods - Class A Industrial, 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 Electrical Fast Transient / Bursts
- EN61000 part 4-6 Conducted RF Immunity
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 (pictured above) 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. We offer 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.