The largest capacity single phase generator or DER (battery) operating in parallel with the gird is 100kW. Above that size, a balanced 3 phase system is required.
DERs are permitted to cause up to 2% voltage fluctuation at the Point of Interconnection and ½ the band width of any voltage regulator or ½ the net dead band of a capacitor bank. DERs in maximum output, are permitted to raise feeder voltage to the ANSI or state limit whichever is more conservative.
The aggregate limit of large (250 kW and over) generators running in parallel with a single, existing distribution circuit is 0.5 MWs on the 4kV, 3MWs on the 12 kV, 6 MWs on the 25 kV, and 10 MWs on the 34 kV.
Distributed generation installations which exceed the limit for an existing circuit require an express circuit. The maximum generator size for express circuits shall be:
The aggregate limit of large (250 kW and over) generator injection to a single distribution transformer of 22.5 MVA nameplate or larger is 10 MWs. Transformers with nameplate ratings lower than 22.5 MVA will be given lower ratings on an individual basis. If the transformer rating is significantly greater than 40 MVA it may be possible to consider a greater generation capacity.
Adding a new transformer will be considered if there is no availability on any of the existing transformers and space is available in an existing substation. Any proposed transformers would be PHI's standard distribution transformer (37 MVA nameplate rating.)
If there is no more injection capacity or space for an additional transformer at the closest substation, the next closest substation will be considered. The length of an express circuit is limited to 5 miles, or for the sake of the feasibility study, 3.8 straight line miles to the substation. This simplification is used because the feasibility study phase does not allow for the time and resources to examine routes in detail (including existing pole lines, easements, ROW, and environmental issues etc.)
If a distribution express circuit can’t be built from an existing substation for a project, it will be necessary to construct a new distribution substation with a standard ring bus design. It will be supplied by extending existing transmission lines. In NJ, it is the developer's responsibility to verify eligibility of this configuration for solar renewable energy certificates with New Jersey's Clean Energy Program if desired.
All limits, given above in MWs, are subject to more detailed study to ensure feasibility.
NOTE: To allow for more PV to be put on a secondary network, Pepco has come up with the following criteria that would accommodate larger systems that are greater than the minimum daytime load of a facility:
The limit is equal to 80% of the export (i.e. excess) generation of the customer’s system (at the grid minimum load condition), before causing reverse power to any of the network transformers/protectors in the grid network. The PV system/facility being evaluated is evaluated with all other active and pending generation represented in the network.
Voltage Fluctuation – This is a metric used to represent the DER’s impact on distribution feeder voltage. It quantifies the difference in feeder voltage between when the system is running at full output and then after the generation has been suddenly lost. Larger systems and systems connected further from a substation tend to have a higher voltage fluctuation value. If this criterion can’t be met with power factor mitigation, an impact study will be required to ensure that voltage can be maintained within applicable standards.
Steady State High Voltage – A simulation is performed which predicts how high the voltage will rise at a point in time when energy consumption is lowest on the feeder and the DER is injecting power. The system is simulated in a normal, steady state and abnormalities are not accounted for. In some cases, steady state high voltage can be mitigated by changing settings on voltage regulation equipment.
Reverse Power Flow – Some devices may require setting changes, a re-evaluation of their control scheme, or replacement. The lowest daytime (9am - 3pm) load going thru the lowest loaded phase of a voltage regulator or distribution power transformer must be 20% greater than the aggregate solar output downstream of the respective equipment or mitigation is required.
Restricted Circuits – any size – Given current technology, each distribution circuit will have a limit to the amount of distributed generation that can be accommodated. When the installed generation on a circuit has reached its maximum, (generally just before the point of voltage violations), no further applications can be accepted for DER’s, regardless of size, unless the customer is willing to pay for the needed upgrades. Potential DER owners may request, at their expense, to pay for upgrades that would allow them to install their system. In many cases, the required upgrade costs may make an installation cost prohibitive.
Restricted Circuits – over 250kW – Circuits which have active and/or pending generation that exceeds the amount that can be accommodated may be restricted to generators with AC ratings of 250 kW or less. Typically, this is done in the case where distributed generation requests exceed set criteria limits in order to avoid closing the circuit entirely. (See: 3. Existing Distribution Circuit Capacity Limits)