Maximum domestic grid tied PV systems sizes

maximum inverter sizes April 2015

Recently numerous articles have been circulated showing very good looking residential installations where the contractors have installed fair sized pv systems of which most have been grid tied.

In the absence of standards most contractors have been installing systems sized according to the consumption patterns of the consumer. It should be noted that there is a standard that guides installers and companies selling these systems as to the maximum size systems that can be installed.

The reason for limiting the system size is due to the infrastructure. When reference is made to infrastructure in this post, we are talking about the utility supply transformer and cable capacity for a certain area. In most residential areas, the transformer is shared between a number of properties. Lets consider that a grid tied system has been installed in a residential area and the consumers for the property in question aren’t home, with no loads being drawn by the specific consumer.

Technically this poses no problem as the other consumers situated on the same side of the transformer would consume whatever has been generated by the grid tied pv system and therefore the PV system would have little or no impact on the existing infrastructure. In the event where 50% or more of the consumers connected to the same transformer has grid-tied systems, the area could experience a rise in voltage potential as a result of the combined PV systems all feeding back into the same network. This is potentially where complications could arise with the current infrastructure not being able to maintain its integrity under the new operating conditions.

According to NRS097-2-3 of 2014, the combined PV system supply should

  • not exceed 25% of the shared transformer capacity.
  •  any generator greater than 4,6 kVA should be balanced across
    phases.
  •  If the maximum individual generation limit is exceeded, the customer could potentially be
    connected through a dedicated LV feeder, or
  • apply for a larger utility supply circuit breaker

In the case of non-compliance with the above mentioned criteria, where additional generation
does not meet the simplified connection criteria; it cannot be connected to the network
without further detailed studies.

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