Commuter Rail AC Electrification Load-Flow Simulation Report - RTD

Commuter Rail AC Electrification Load-Flow Simulation Report Revision 1
3.3.4 Paralleling Stations
Paralleling stations are an integral part of the autotransformer-fed system. Since substation to
switching station spacing is often large, each section of the distribution system may be equipped
with one or more paralleling stations. The paralleling stations are installed either between the
substation and the switching station or between the substation and end of the line in order to
improve the voltage profile along the system by transforming the feeder/catenary voltage to
catenary/rail voltage using autotransformers. Further, the paralleling stations parallel the
catenary and feeder circuits of the two tracks and provide electrical sectioning points within the
system.
Each paralleling station is equipped with medium voltage indoor switchgear or outdoor circuit
breakers configured in a similar arrangement as in the switching stations. However, since the
autotransformer feeder and the catenary voltages on either side of the paralleling stations are
always of the same phase and magnitude, there is no need for bus-tie circuit breakers. For the
same reason, overlaps or section insulators are used in paralleling stations instead of phase
breaks.
3.3.5 Benefits of Paralleling Stations and Switching Stations
A key advantage of this arrangement is that the switchgear in the switching and the paralleling
stations enables sections of the distribution system to be disconnected following a fault or for
routine maintenance. The switchgear is configured to permit paralleling of the overhead
distribution system conductors in multiple track areas. The conductor paralleling decreases the
effective system impedance between substations and trains which improves the train voltage
profile along the system. The paralleling also provides for current sharing between conductors
of adjacent tracks and improves system fault detection.
3.3.6 Autotransformers
In the autotransformer system, the catenary-rail voltage is delivered by the feeder-catenary
distribution system via autotransformers. Autotransformers are installed at each paralleling
station and at each switching station. The autotransformer winding ratio must correspond to the
distribution voltage (feeder-to-catenary) and the traction voltage (catenary-to-rail) ratio.
The autotransformer-fed system enables power to be distributed along the system at higher
than the train utilization voltage. For example, in the 2x25 kV autotransformer system, power is
distributed at 50 kV (line-to-line) while the trains operate at 25 kV (line-to-ground). This
arrangement results in a system with lower voltage drop along the alignment than is possible
with 25 kV direct-fed system, resulting in an improved train voltage profile along the line.
Depending on the train position along the system, the current in the feeder may flow in the
opposite direction than the current in the catenary. In this event, certain electromagnetic field
cancelation occurs. This field cancelation mitigates, to some degree, the effects of
electromagnetic interference on other wayside equipment as well as communications and
signaling circuits.
02/27/2009 FRSC Page 10 of 250