As part of a modernization project to ensure the continued reliability of their rapid transit system, Port Authority Trans Hudson (PATH) in New Jersey undertook a capital improvement project to replace an existing traction power substation near Jersey City, NJ.
As part of this project, our engineers performed AC & DC short circuits, coordination, and arc flash studies for the proposed electrical facilities.
circuit and coordination study
For any rail transit authority, the main goal of a traction power substation is to facilitate the movement of trains in a timely and reliable manner. From an operating standpoint, the traction power substation equipment must determine the difference between allowable heavy-duty overloads associated with train starting and damaging power flow due to equipment failure or a track fault.
Thus, a properly modeled short circuit and coordination study contributes to the complete understanding of the rate of rise of DC and suggests the proper setting of “rate of rise” relays, which is vital for the reliable operation of the transit system.
operating criteria
To meet the desired functionality of the traction power substation, their engineers ascertained a protection scheme to maximize functionality for PATH based on their operating criteria.
The extent of the analysis and studies included:
- Equipment duty evaluation and protective device settings for the AC Switchgear.
- Circuit modeling of the Extra Heavy Duty Rectifier Transformers with coupling factors and commutating reactance parameters, as applicable to ANSI Circuit #31 with 12-pulse rectifiers.
- AC-side backup protection for Extra Heavy Duty Rectifier operation (formerly NEMA RI-9), to supplement integral equipment protection provided by the manufacturer.
- DC Switchgear and circuit breaker evaluation with the determination of DC overload and “rate of rise” overcurrent protection settings.
- DC Traction System service voltage comparison between existing 650V DC operation and future upgrade to 750V DC.
- Computation of L/R Time Constants for distant and close-in track faults.
DC protective devices
To establish an effective electrical distribution system for transit facilities, it is important to consider all aspects associated with the intended application. When performing a study involving an AC supply to a Rectifier System and DC distribution network, the differences in behavior may appear counterintuitive.
Whereas high fault currents reduce the service life of AC protective devices, lower fault currents with high L/R ratios to the point of fault present difficulties for DC protective devices. Whereas the modeling of AC distribution components has become straightforward given the aid of commercial software evaluation tools, the same software requires scrutiny and engineering oversight to accurately depict the DC network.
