Distributed vs. Centralized Grid-Tie Inverter for a Campus Power System 1

[MCSEng]

We are currently in the concept stage of a design involving the deployment of Solar PV System in an existing Electrical Power Distribution System on a Campus and are inviting different views on certain issues. A Diesel Standby Electrical Power Generator is also existing.

Two options are being considered: (1) Connect Many Smaller Gird-Tie Inverters Locally to Various Sub-Distribution Panelboards which are closer to the Photovoltaic panels; (2) Connect Fewer Larger Capacity Inverters to the Main Switchboard, which require cables to be laid over longer distances.

Key advantages of Option 1 are that the PV supply is closer to the loads and cables would not have to be laid over long distances, back to the main switchboard and could be the more feasible option. However, disadvantage is that this would require upgrading more sub-distribution boards since the prospective short circuit current at these points could be significantly increased; these panelboards are potential solar PV supply points for the rest of the power system and could carry increased loads under steady state conditions. Isolating the power system for maintenance would also be more complicated. These are amongst other disadvantages.

For Option 2, the advantages include simplicity in technical design and operation. Disadvantages include: high cost of laying cable from distributed generation points; electrical losses in longer cable running back to the main switchboards from remote PV Panels or Inverters.

Has anyone analyzed such a scenario? What were the other factors that were considered? Are there any technical articles, publication, standards etc. that provide guideline for various power system network configuration for Solar PV distributed on large acreage commercial and industrial sites? How is the matter of the Standby Diesel Generator handled during a power outage by the utility company?

Sorry for the long post. I saw a similar question but the focus was more on high voltage distribution.

 

[ScottyUK]

Generally power electronic sources aren't especially good at delivering high fault currents. The electronics can shut down or enter a protective limit condition faster than most protection schemes can detect. Your concern about higher fault levels in Option 1 may be misplaced unless the inverters are quite large. A short term limit at about 3x continuous rating is fairly common for electronic sources.

In my part of the world the rules about connecting 'small' inverters to the utility are different from those which apply to 'large' inverters. I'm not certain that the rules really consider a scenario where there are multiple 'small' inverters so it may be easier from a legislative perspective to go with Option 1.

Regarding on-site generation, most utilities would require an approved break-before-make transfer switch. The utility will be concerned about a generator connected to their system which they don't have the ability to inter-trip off their system when necessary. If your generator can operate in parallel with the utility then you need to speak to the utility because most will have their own set of specific requirements.

 

[FreddyNurk]

ScottyUK is pretty much correct in the assertions about inverters and fault current, its generally low enough that the opposite problem starts to arise, lack of current to trip equipment.

Some factors that may be relevant to installation of inverters and possible integration of diesel are below:

If its a break before make backup generator the easiest (relatively...) thing to do is shutdown the inverters on loss of mains, rather than allowing the inverters to resynch to the diesel​

Multiple smaller units can be more beneficial, possibly cheaper, if the overall distribution is spread out such that cabling all the solar to one place is problematic. Not such an issue for higher voltage arrays these days, but still relevant.​

The relevant utility may get more excited about multiple smaller units if anti-islanding protection is relevant, as there is a perception that multiple smaller units can indeed unintentionally create their own grid.​

A single larger unit is easier to trip off than multiple smaller distributed units, but may require its own infrastructure changes.​

Voltage rise on subboards may become a problem with a distributed inverter scheme, compared to a single large inverter.​

Integration of solar and diesel (even for a standby generator) is not impossible, but not likely to be worth it unless the infrastructure for diesel synchronising is already there, and an easy ability to shut the solar down is present, as otherwise the solar may drive the diesel into reverse power, eventually resulting in loss of power anyway.

Isolation for maintenance is also a very good point, not so fun with multiple smaller sources of generation that also need isolation.