DTE Energy Technologies 

With Detroit Edison Co. and Kinectrics Inc. 

Distributed Resources Aggregation Modeling and Field Configuration Testing 

Background

In the 20th century, the United States built an electric 

power system that became the envy of the world for 

its reliable, low-cost power. Today, that system 

consists primarily of large central-station power 

plants intertied via a high-voltage transmission 

system. The transmission system feeds lowervoltage, local distribution networks that provide oneway power flow to end-users. 

Interest in distributed resources (DR), or local energy 

generation and storage, has increased substantially 

because of their potential to improve reliability and 

lower costs by providing power at or near the load. 

To realize these benefits on a larger scale, utilities 

and generating customers need quantitative data and 

analytical tools to assess interactions between DR 

and distribution lines. 

Aggregation Effects 

The effects of low concentrations of DR on the 

distribution system are generally known. However, 

the effects of larger generators and larger 

concentrations of generators are not well understood. 

A number of utility grid coordination issues—including 

the performance of utility fuses, reclosers, and 

protective relays as well as limits to uphold system 

quality and reliability standards—are involved. 

Goals 

DR can provide many benefits, but local electric 

distribution systems traditionally have not been 

designed to operate in parallel with interconnected 

distributed power systems. As a result, there 

are concerns about compatibility, reliability, power 

Substation

Device 

Fault A 

DR 

System Fault Current - IfS 

DR Fault Current - IfDR 

CB-1 

Breaker First Sectionalizing

Current sensitivity example 

quality, system protection, voltage dynamics, and 

safety. The goals of this study are to address 

selected system integration issues and determine the 

DR system penetration limits imposed by the local 

grid because of utility coordination issues. 

System Protection Issues 

• Improper coordination – protective device 

operates for fault on adjacent circuit 

• Improper coordination – reduced fault detection 

sensitivity 

• Nuisance fuse blowing because of DR fault 

current 

• Faults within the DR zone 

• Isolating DR for upstream fault 

• Fuse blowing because of upstream single-phase 

fault 

Voltage and Stability Issues 

• Harmonics 

• Voltage regulation malfunctions 

• Steady-state stability 

• Dynamic stability during fault conditions 

• Loss of exciters causes low voltage 

Overview 

This research is an example of ongoing efforts to 

develop the data and analytical tools necessary to 

assess the reliability and performance of the 

transmission and distribution system and foster the 

deployment of new technologies. 

The study examined two typical distribution feeder 

circuits in the Ann Arbor, Michigan, area. One was a 

radial distribution line, and one was a ring 

configuration. DTE developed three-phase circuit 

models of the feeder lines, protective devices, and 

DR. It used comparisons with simplified analytical 

results and short test simulations to verify the 

accuracy of the models. Later, it used ASPEN, 

MATLAB, PTI PSS/E, and EMTP software to 

generate detailed simulations that represent how 

aggregated DR affect system characteristics such as 

voltage, harmonics, phase and ground overcurrent, 
fault conditions, islanding detection, loss of excitation, 

transient stability, and coordination between circuit 

breakers, fuses, and reclosers. 

DTE determined the maximum size of aggregated 

DR possible at locations on the sample feeders 

without disrupting the line performance or the 

behavior of protective devices. In general, the study 

should help other engineers determine DR 

penetration limits by providing validated methods, 

algorithms, and device charts that can be adapted to 

specific cases. 

Findings

This study provided detailed modeling, simulations, 

and validated analyses to ensure sound conclusions 

and recommendations concerning DR penetration 

limits and equipment requirements. It established 

generic methods and procedures to evaluate DR 

effects. 

The conclusions include: 

• The DR aggregation limits were typically on the 

order of 1–10 MVA. 

• The system voltage has a significant effect on the 

maximum DR size (or aggregated size) that can be 

connected to a circuit. Doubling or tripling the 

system voltage would roughly double or triple the 

maximum aggregated size of DR. 

• The type of fault influences the size of DR. 

• Nuisance fuse blowing tends to limit DR sizes to 

less than 2 MVA for compact circuits fed from 15-

MVA substation transformers (high system fault 

current = 7,600 A at substation). 

• Harmonic analysis may be required for inverters 

because of the wide range of acceptable DR sizes 

(820 kVA–9.2 MVA). 

• The location of the DR’s circuit is very important in 

determining the voltage limits for loss of excitation 

DR limits. 

• If critical clearing time is 0.1 seconds or less, 

stability should be maintained. The larger the 

machine inertia, the more stable the unit. Multiple 

machines on a feeder may increase stability by 

tending to support local voltage during upstream 

disturbances. 

Too much distributed generation in the wrong 

location can create or aggravate problems with 

voltage, harmonic content, system stability, or 

protective device coordination. However, DTE clearly 

presents how DR, far from being a detriment, can 

significantly benefit the grid—with proper evaluation 

of the local power grid and careful DR selection and 

placement. 

Issue 1 Maximum DG Current With No Recloser / Fuse Operation 

1200 

1000 

800 

600 

400 

200 

0 

40k 

65k 

80k 

100k 

140Drecl 

DR Contribution Ratio = 11 

0 2000 4000 6000 8000 10000 12000 14000 

System Fault Current 

Maximum DR fault current versus system fault current 

Publications 

Davis, M.; Costyk, D.; Narang, A. “Distributed and 

Electric Power System Aggregation Model and Field 

Configuration Equivalency Validation Testing.” 

NREL/SR-560-33909. July 2003. 

Publications are available on the NREL publications 

database at http://www.nrel.gov/publications/. 

Contacts 

NREL Technical Monitor 

Tom Basso – (303) 275-3753 

National Renewable Energy Laboratory 

1617 Cole Blvd.

Golden, CO 80401 

NREL DEER Technology Manager 

Richard DeBlasio – (303) 275-4333 

National Renewable Energy Laboratory 

1617 Cole Blvd.

Golden, CO 80401 

DOE Program Manager 

Eric Lightner – (202) 586-8130 

U.S. Department of Energy 

EE-2D/Forrestal Building 

1000 Independence Ave. SW 

Washington, DC 20585 

Additional Distributed Power Information 

http://www.electricity.doe.gov/ 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, CO 80401-3393 

NREL is a U.S. Department of Energy National 

Laboratory 

Operated by Midwest Research Institute ● Battelle ●

Bechtel 

NREL/FS-560-35039 October 2003 

Printed on paper containing at least 50% wastepaper,

including 20% postconsumer waste.

Maximum DR Fault Current