Powering Freight & Transportation - Power Systems Design

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COVER STORY
LEM’s new DV voltage transducer
magnetic circuit providing the isolation
and to integrate only electronic components
(amplifiers, resistors, capacitors,
A/D and D/A converters, micro-controllers,
etc.) The large heatsinks usually
installed on Hall effect and Fluxgatebased
voltage transducers have been
removed by reducing losses.
With electrical drives for railway locomotives
supplied from networks up to
3kV, the measurement signal needs to
be transmitted to electronic circuits at
low voltage for control and/or display
purposes. The transmission of power
and signals between a high voltage en-
vironment to a low voltage environment
requires specific insulation features.
DV’s description
To achieve this, LEM has designed a
new range of voltage transducers based
on a new patented technology which is
different to the traditionally used closed
loop Hall effect technology. The result is
the DV series voltage transducers that
cover nominal voltage measurements
up to 4200 VRMS. To operate, they only
need to be connected to the measuring
voltage, without inserting additional
resistors on the primary side, and a
standard DC power supply range of ±
13.5V to ±26.4 V.
With a primary voltage higher than
zero, the transducer consumes a
maximum of 23mA (maximum internal
consumption), plus the output current
(typically 50mA at nominal value), when
programmed with current output.
In comparison to the other methods
used to measure high voltages, this
provides considerable energy savings
on customer supply - for example, a
Fluxgate based voltage transducer
consumes between 35-50mA with no
primary voltage.
Based on LEM’s long experience in
current and voltage transducers for traction
application, the DV covers customer
requirements for nominal voltage
measurement to 4.2kV RMS. It features
a combination of all the advantages of
previous LEM products and fulfilment of
all new EMC requirements. This product
has been developed according to IRIS
standards:
• Low consumption of about 19-23mA
• Frequency bandwidth 12kHz
• Safety insulation 18.5 kV
This is followed by a digital encoder
producing a single serial signal enabling
data to be transmitted via one single,
isolated channel. Thereafter, an amplifier
feeds the signal to the primary side
transformer, transformer required to
Figures 1: Starting from the left of the diagram at the primary side, where input voltage might typically be ±4.2kV, the first
stage is a voltage divider that reduces the supply down to a few volts, and is able to withstand high dv/dt while having low
thermal drift. Then a sigma delta modulator converts the signal from analogue to digital as a 16-bit output.
Power Systems Design October 2008
provide the desired galvanic isolation.
Due to the high voltage environment, a
double core transformer is considered,
limited in size thanks to the considered
high digital working frequency. Windings
are wound into a PCB, similar in layout
to a planar transformer, which affords
product repeatability and assures component
behaviour.
The product is primarily designed for
onboard traction and stationary traction
substation applications. Within a
substation the insulation test voltage is
above 18kV for one minute for a working
voltage of 4.2kV, while onboard, the
insulation test voltage is max 13kV.
The transformer therefore needs to
withstand such a high test voltage,
while at the same time the lifetime of the
insulation can be guarantee by a partial
discharge test, where a voltage is applied
between the primary and secondary
to determine if there is a discharge,
which should measure less than 10 Pico
coulombs.
On the secondary side the bit-stream
is decoded and filtered by a digital filter.
Because the primary signal square wave
is distorted by the transformer, there is
a Schmitt trigger on the secondary side
of the transformer to restore it to square
wave. This is then fed into a decoder
and digital filter, the function of which
is to decode the data bit stream into a
standard digital value that can be used
in digital to analogue conversion within
the microcontroller. The recovered
output signal is completely insulated
against the primary (high voltage), and
is an exact representation of the primary
voltage.
The transducer can be easily adapted
for different ranges by modifying the
gain programmed by the microcontroller.
This does not require changes in the design
of the transformer or in the design
of the assembly of the circuit boards in
the housing. The microcontroller cancels
offsets and adjusts the gain by software,
and then converts the signal from digital
to analogue output. The micro-controller
transfers data from the digital filter to a
12-bit D/A converter with a transfer time
of around 6 μs. The analogue output
voltage is then filtered and converted
into a current (75mA full scale) using
www.powersystemsdesign.com
a current generator protected against
short-circuits.
The microcontroller also regulates a
DC/DC converter that creates internal
secondary regulated supply voltages
supplied by customer DC supply which
will typically be ±24V or ±15V, while
also supplying ±5V and ±3.3V to the
primary side sigma delta converter and
digital encoder. The additional circuitry
is shown as a group at the top of the
circuit schematic, with the frequency
of the DC to DC converter given by the
microcontroller.
The last block to the right of the
microcontroller is a voltage to current
converter for customers who prefer current
output, typically 50mA, in order to
comply with electromagnetic compatibility
(EMC) regulations. The lower impedance
current output is less prone to
interference from external electromagnetic
fields. A voltage output version
to 10V is also available, for example,
where the transducer is to be used with
shielded cable or with short connections
to customer electronics.
Main characteristics
Providing excellent overall accuracy
with ±0.3% of VPN at ambient temperature
and over its operating temperature
range from -40°C to 85°C, the DV shows
a low temperature drift resulting in an
overall accuracy of only ±1 % of VPN.
Initial offset at 25°C is 50μA max with a
maximum possible drift of ±100μA over
the operating temperature range. Sensitivity
error at 25°C is ±0.2%. The microcontroller,
used among other things for
D/A conversion, is also useful for offset
and gain adjustment during production,
enabling these parameters. Linearity is
only ±0.1%.
The DV transducer’s typical response
time (defined at 90% of VPN) against a
voltage step at VPN has a delay of 48μs
(Max 60µs). Other closed Loop based
on Hall effect voltage transducers have
a response delay of several hundred
microseconds. As a result of the fast
response time, a large bandwidth has
been verified at 12kHz at -3 db (Fig. 3).
Mechanical and standards
The DV’s modular approach allows
easy adaptation with various connec-
COVER STORY
tions available for the primary side, e.g.
terminals or isolated cable, and any kind
of connection for the secondary side like
connectors, shielded cables, terminals
(threaded studs, M4, M5, UNC etc.) according
to customer specifications.
The DV models have been designed
and tested according to latest recognised
worldwide standards for traction
applications. The EN 50155 standard
“Electronic Equipment used on Rolling
stock” in railway applications is the
standard of reference for electrical, environmental
and mechanical parameters.
It guarantees the overall performances
of products in railway environments.
LEM’s main production centres for
traction transducers are IRIS certified -
essential for companies supplying the
railway market. DV transducers are CE
marked as a guarantee of compliance
to the European EMC directive 89/336/
EEC and low voltage directive. They
also comply with the derived local EMC
regulations and with the EN 50121-3-2
standard (railway EMC standard) in its
latest update, with EMC constraints
higher than that of the typical industrial
application standards.
The EN 50124-1 “Basic requirements
- clearances and creepage distances for
all electrical and electronic equipment”
standard has been used as a reference
to design the creepage and clearance
distances for the DV transducers versus
the required insulation levels (rated
insulation voltage) and the conditions of
use. Clearance is the shortest distance
in air between two conductive parts and
creepage is the shortest distance along
the surface of the insulating material
between two conductive parts. Pollution
degree is application specific and is a
way to classify the micro-environmental
conditions having an effect on the insulation.
Overvoltage category is also application
specific and characterises the
exposure of the equipment to overvoltage.
Partial discharge (PD) is the dissipation
of energy caused by the buildup
of localised electric field intensity.
Electric discharges partially bridge the
insulation. Failure is by gradual erosion
or ‘insulation, leading to puncture or
surface flashover. The partial discharge
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