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Challenges
Personnel needed a reliable, portable instrument to accurately measure and record the current on each feeder over a selected time period.
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Overview
The Southeastern Pennsylvania Transportation Authority (SEPTA) operates the transit system throughout the city of Philadelphia, extending into the surrounding counties in Pennsylvania and across the Delaware River into New Jersey.
A major component of this system is the network of subways and commuter trains. Power for the operation of these trains is provided by an array of substations distributed throughout the region. One such substation identified as 20-Sansom is located proximate to the busy Jefferson Station on East Market Street in downtown Philadelphia (Figure 1.)
The subway system is powered by a series of “third rail” feeders supplying 700 VDC at currents up to 4000 A. The power for this system is derived from a 4160 VAC, 3-phase input via transformers and 12-pulse rectifier array. The power is then distributed to individual segments of the system via a series of feeders (Figure 2.)
Measurement of the current is performed utilizing a low resistance direct shunt producing a 0-50 mV signal proportional to 0-4000 A (Figure 3a) that is then displayed on the front panel (Figure 3b).
The substation electrical personnel needed a reliable, portable instrument to accurately measure and record the current on each feeder over a selected time period in order to evaluate load profiles, particularly during peak operating periods.
After attempting to use a number of different manufacturers’ portable power analyzers without success, they ultimately selected the Vitrek XiTRON brand model XT2640. While many of these other instruments are well suited for analyzing AC power systems, the Vitrek analyzer was uniquely capable of handling this application. The unit has the accuracy and dynamic range required to measure these rapidly changing DC current loads and also has the capability to handle the very high DC common mode voltage. It turns out that making an accurate 0-50 mV measurement floating on a 700 VDC bus requires the exceptional isolation only found in Vitrek power analyzers (Figure 4.)
The information gathered from the XT2640 is stored in the device and can be displayed on the instrument’s high-resolution front panel color screen. The data can also be exported via rear panel Ethernet or RS-232 or front-panel USB drive access. In this particular case, the data is typically transferred via USB and then displayed on a large monitor in the substation (Figure 5.) This display depicts a particular feeder over a period of time to confirm that the current remains within expected levels.
TROUBLESHOOTING A FEEDER PROBLEM
Type 43 DC Overcurrent Relays are the most commonly used protective overcurrent devices used in the transit industry. The overcurrent relay provides maximum protection for trolley wires, third rails, feeder and substations from intermediate or remote overload conditions such as bolted faults, arcing faults and service overloads. Type 76 relays are deployed throughout the SEPTA system.
The XT2640 was pressed into service to determine why the Type 76 on one particular feeder was tripping off. After measuring and recording the currents as shown in Figure 5, it was determined that when a tripping event occurred, the actual currents were well within accepted operating ranges. The relay was taken out of service and repaired.
CONCLUSION
The Vitrek power analyzer has provided this metropolitan transit authority with a valuable tool to evaluate and manage the distribution of high-voltage/current DC feeders powering its subway system. An especially important benefit is the Vitrek instrument’s ability to accurately measure current data from a 0-50 mV shunt riding on 700 VDC common-mode voltage, something with their ease-of-use and superior operational performance provided SEPTA engineers with the right tool for the task.
ABOUT VITREK
Vitrek has provided innovative global solutions for high voltage test and measurement since 1990. All Vitrek products are designed and manufactured in the USA. Our products are used worldwide to provide testing solutions for industries including calibration, R & D and testing labs, photovoltaic, lighting, appliance, machinery, medical equipment, power conversion, electrical component, metrology, automotive, military/aerospace and energy industries.
For a FREE product demonstration, contact vitrek at (858) 689-2755 or email info@vitrek.com.
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