$25. 002009 ieee 52 ieee

58 

IEEE

 

power & energy magazine

 

march/april 2009

Several PHEVs or PEVs plugged into a secondary circuit, 

or a larger number of cars in a parking garage connected to a 

lateral feeder, could cause a localized overload on the distribu-

tion circuit and transformers. Many distribution circuits have 

been operating close to their operating limits, and the additional 

load may push them above their emergency operating limits. 

For example, a 25-kVA or 50-kVA secondary transformer on 

a  single-phase lateral may not be able to sustain the charging 

loads of several plug-in vehicles while it’s subjected to varia-

tions in demand due to normal customer activities. Further-

more, the potential unbalanced conditions created by such loads 

could cause problems on main feeders and other laterals.

Shopping malls, big-box stores, and offi ce  buildings 

are  considering  offering  fast  charging  capability  to  their 

 customers and employees. This will result in signifi cantly 

higher loading on the facility’s transformer. Figure 6 shows 

the additional loading caused by charging plug-in vehicle 

batteries as a function of the number of cars.  

Such distribution circuit overload scenarios are conceptually 

similar to the congestion conditions on the transmission system 

due to excessive fl ows on a given transmission line. Changes 

in power fl ows on a distribution circuit have similar effects as 

changes in interchange fl ows on the transmission system.

Managing  distribution   system  overloads  may  be  akin  to 

the congestion management in the transmission system that 

was addressed in the wake of opening the transmission sys-

tem as a result of electric industry deregulation in the mid-

1990s. This resulted in the need for transmission reservation 

and scheduling (the open access same-time information sys-

tem (OASIS), interchange distribution  calculator (IDC), and 

E-tagging) and the use of locational pricing to manage con-

gestion. Perhaps similar concepts need to be considered for 

distribution capacity reservation and/or the use of locational 

retail pricing to manage the loading of the distribution facili-

ties. Other concepts developed on the transmission system, 

e.g., fi rm and curtailable schedules, may also need to be ex-

trapolated to the distribution system, or concepts like distribu-

tion loading relief (DLR), in contrast to transmission loading 

relief (TLR), may emerge.  But most likely, these issues will 

be addressed through DR and distributed resource manage-

ment strategies. In addition, mandating the use of time-of-use 

meters, coupled with improved rates and tariffs that would 

provide additional granularity and accurately refl ect the cost 

of distribution system congestions at the point of sale, would 

encourage natural demand-side control and would promote 

proper scheduling of plug-in vehicle charging.