PV

System

Utility

Customer 

Load

Secondary

Network

Distribution

System

PCC

PV

System

Customer 

Load

PCC

ConEd

Network

Protector

ConEd

Network

Protector

ConEd

Network

Protector

ConEd

Network

Protector

Customer

Load

Customer

Load

Customer

Load

Customer

Load

0

20

40

60

80

100

120

MW

Time

0:00 4:00 8:00 12:00 16:00 20:00 0:00

Load

Reduced Load

PV Generation

0

40

80

120

MW

Time

0:00 4:00 8:00 12:00 16:00 20:00

-40

PV Generation Reduced Load

Load

New York City has set a goal to increase its installed 

PV capacity from 1.1 MW in 2005 to 8.1 MW by 

2015. A key barrier to reaching this goal, however, is 

the complexity of the interconnection process on 

secondary network distribution systems (networks).

Networks use coordination and protection schemes 

that are based on the assumption that power flows 

in one direction—from the utility distribution 

system to the customer point of interconnection. 

With traditional power plants, this is true. When 

power sources like PV are connected to the 

distribution system at the utility customer’s site, 

however, power has the potential to flow in the 

opposite direction—from the customer toward the 

utility distribution system. This reverse power flow 

has the potential to disrupt network protection 

schemes and threaten network reliability.

Networks use coordination and protection schemes that are based on 

the assumption that power flows in one direction—from the utility 

distribution system to the customer point of interconnection. With 

traditional power plants, this is true. When power sources like PV are 

connected to the distribution system at the utility customer’s site, 

however, power has the potential to flow in the opposite direction— 

from the customer toward the utility distribution system. This reverse 

power flow has the potential to disrupt network protection schemes 

and threaten network reliability.

NETWORK POWER FLOW CONSIDERATIONS

HIGH PENETRATION PV CONSIDERATIONS

NREL worked with Con Edison through the DOE Solar America Cities 

partnership program to evaluate the maximum PV deployment capability 

in several areas served by secondary networks in New York City. While many 

areas within New York City had significantly greater daytime loads than full 

PV deployment, some areas were found to have significantly less daytime 

load than full PV deployment (based on actual Con Edison network load 

profiles compared to PV generation calculations). 

A one-line example of 

the sensing and control used for a typical 

MIR system. The MIR 

relay and breaker 

arrangement are 

nearly identical to an 

RPR system. While an 

MIR relay would be set 

at an agreed upon 

value between system 

owner and the utility 

company, an RPR 

would be set at a lowlevel of reverse power 

close to zero amps.

DCI system which 

senses the building 

load and can control 

the output of the PV 

system.

SYSTEM INTEGRATION CONSIDERATIONS

FOCUS ON FOUR KEY SOLUTIONS

1. Minimum Load – Evaluate 12 to 24 months of daytime load demand data 

for the site under consideration and size the PV system so that it will not 

produce more energy than the facility consumes at any instant in time. 

2. Reverse Power Relay (RPR) – If there is a possibility that the PV system 

might export power to the utility, and that is unacceptable, then an RPR 

may be installed to disable the PV system in case of a backfeed situation. 

Some utilities require an RPR, as well as other methods, to insure the PV 

systems do not backfeed into the network. This system is a simple approach to ensuring zero backfeed and is based on real-time monitoring.

3. Minimum Import Relay (MIR) – Similar to the RPR, the MIR may be 

installed to disable the PV system in case of a backfeed situation. 

However, the MIR is typically set at a threshold above zero current, as 

the RPR is typically set, and will disable the inverter once power flow 

into the customer meter drops below a predetermined level.

4. Dynamic Controlled Inverter system (DCI) – At least one inverter manufacturer is offering an option for a dynamic controlled inverter which, 

unlike the RPR, ramps down the power level of the inverter if the energy 

being consumed at the site drops below a specific level. The energy flow 

is monitored at the main disconnect (or at the utility revenue meter) and 

a control signal is sent to the inverter(s) which effects a reduction of 

output if required. This type of inverter system may be the most costeffective and logical approach of all options being considered at this time.

As the number of PV systems in the New York City area increase in number and 

size, the following issues will grow in importance: 

PV systems that are non-exporting will be the least problematic for the electric 

utility. Non-exporting PV systems should be moved to a fast-track in permitting 

and approval as the level of studies and controls should be minimal.

There are several means to insure that a system is non-exporting, and those include;

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PV systems tied to spot networks must be non-exporting, and the load at the 

spot network PCC should be greater than zero. 

PV systems that are exporting must have a higher level of attention given to 

them when the utility evaluates or studies the application. 

Monitoring, information exchange, and control systems will need to be 

expanded and will be critical for the utility in order to meet their obligation to 

serve electric loads. 

A concerted effort will be required to model the response of secondary networks 

with a high level of PV system deployment in order to understand system limitations. 

Smart grid technologies will become the essential tool for the utility system as more 

PV systems, and other DG systems, come online. 

NETWORK PROTECTORS

A unique feature of a secondary network distribution system, 

compared to a radial distribution system, is the addition of the 

network protector (NP). The NP is a relay and breaker pair that 

senses reverse-current flow toward the utility and is designed to 

prevent current from flowing back toward the utility system or to 

ensure the given transformer disconnects from its low-voltage 

side. This is an important design feature that ensures reliability 

and continuous operation in the event one or more feeders are 

lost due to a device failure or when the electric utility has 

planned maintenance activities. 

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

SOLUTIONS FOR DEPLOYING PV SYSTEMS IN NEW

YORK CITY’S SECONDARY NETWORK SYSTEM

Michael H. Coddington, Benjamin D. Kroposki, 

and Kate H. Anderson

Michael.Coddington@nrel.gov

http://www.nrel.gov/docs/fy10osti/49789.pdf

Block diagram of a non-exporting system

Power flow within a secondary grid network 

that contains utility customers with PV systems.

Cooper Network maximum export day Fox Hill Network maximum export day

MAINTAINING NETWORK RELIABILITY – CONCERNS AND SOLUTIONS

Presented at the 4th International Conference on the Integration of Renewable andDistributed Energy Resources, 6-10 December 2010, Albuquerque, New Mexico.

This work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory. The authors 

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