Rail meters are digital DC voltmeters capable of measuring high voltages between the overhead wire and the running rails. Rail traction systems are either DC (direct current) or AC (alternating current), the former being used for many years and is simpler for railway traction purposes, the latter being better over long distances and cheaper to install but off late is seen to be more complicated to control at train level.
While AC traction systems always use overhead wires, three(3) different DC power transmission techniques are common across the world – using an overhead wire (overhead catenary), using an extra third rail at the ground level (third rail system), and using two extra rails (fourth rail system). Since the article focuses on Rail meters, discussion hereafter is limited to the overhead catenary transmission method.
Overhead catenary systems require at least one collector attached to the train to always be in contact with the AC/DC power source. Overhead current collectors use a “pantograph,” so-called because that was the shape of most of them until about 30 years ago. The return circuit is via the running rails back to the substation. The running rails act as the earth potential and are connected to the substation to complete the transmission circuit.
A three-phase rectifier system with six (6) pulse is used for DC traction systems to introduce harmonics in the ac side and distortion in the dc voltage. Most modern systems use twelve (12) pulse systems (two sets of six-pulse rectifying circuits with AC Input voltage 30◦ apart, connected in series or parallel) to reduce harmonics.
Depending on the operating DC voltage between the overhead wire and running rails, rail meters are selected. Three DC voltage systems are predominantly used in most parts of the world – 1.5 kV DC, 3 kV DC and 25 kV AC at 50 Hz. But due to unique challenges in power measurements, several aspects are also to be considered before selecting the appropriate rail meter:
Most rail meters have a specific DC voltage measurement range and must be selected based on the traction supply voltage. For example, the Bierer Make, RCDC1000 Digital DC Resistive Voltmeter has a single range of 0 – 2kV. A typical Rail Meter unit consists of the following components:
The following is the standard procedure for measurements with Bierer Voltmeters:
Step 1: Magnetic rail plate is attached to the Meter probe, and hook attachment is connected to the second probe.
Step 2: The Second probe and the meter are connected using interconnect cable
Step 3: The meter probe and second probe are attached at an appropriate length on the live line tool for the tested voltage.
Step 4: The Rail meter measures the DC voltage between the negative return rail and the overhead positive catenary conductor.
Readings are recorded at two different periods for accurate power quality assessment and to troubleshoot overhead catenary power systems.
Though DC power rail measurements using Rail Meters seem straightforward, the procedure has become increasingly difficult due to several factors acting on the Rail meters, such as:
In addition to the factors mentioned above, voltage interruptions, hysteresis voltage, voltage Dip, and Swell at the power supply also affect the performance and accuracy of rail meters. Therefore, annual calibration of rail meters is mandatory according to the EURAMET calibration standards to maintain repeatability and accuracy of Rail meter measurements.
Calibration of Rail meters also determines the calibration uncertainty value and the deviation from the actual reading. These readings are used as correction factors or to decide if the Rail meter is fit for use.
e2b calibration offers industry-leading ISO-certified Rail meter calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your rail meters. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.