In our TerraBlog series on microgrids, we are exploring some of the key market and technical considerations for evaluating and deploying solar PV and battery energy storage integrated systems that can provide back up power in the event of an electric utility outage. In this second installment, we will explore some of the technical considerations.

When designing a microgrid  there are a number of factors to consider:

  • Load – are you trying to back up the entire site or just a subset of loads (critical loads). Providing backup power for an entire site will mean larger generation and energy storage systems to meet load requirements.
  • Duration – how long do you want your selected load to be able to stay operational during a Utility power outage
  • Generation
    • If there are existing generation source(s) installed:
      • how are the generation source(s) interconnected;
      • who owns the generation source(s);
      • is the size of the generation source(s) large enough to provide sufficient energy for the loads selected in step 1 and during what time frame?
    • If existing generation is not installed, can generation source(s) be installed?
  • Energy Storage – based on the supported loads, duration requirements, and generation source(s) what size of energy storage system is required?

Typical microgrid configurations include installation of a critical load electrical panel where specific loads are selected to receive power during a Utility power outage and are thus isolated from the rest of the everyday loads. During a Utility power outage a transfer switch would be used to isolate the critical load electrical panel, along with the generation and energy storage systems, from the grid thus creating a “microgrid” of loads and energy sources – all controlled by a microgrid controller to ensure that the isolated microgrid operates reliability. Selecting a subset of a site’s loads to receive power during a Utility power outage allows for more economic sizing of the generation and energy storage systems required to adequately provide power to those loads during a Utility power outage. Although use of a critical load electrical panel can create a cost effective solution when only a subset of a sites loads need to maintain power during Utility grid outage, there are other potential challenges with this configuration, particularly when a line-side tap has been used to interconnect the generation source.

Typical interconnection methods include end of bus breakers and line-side taps, which are illustrated in Figure 1 below.

A line-side tap is frequently used when interconnecting generation sources (such as solar PV systems) that are sized to offset the majority of the electrical energy consumption on a site. In this situation, because the solar PV system size (kW AC) is typically large relative to the ratings of the main electrical panel, options for connecting the solar PV system on the load side of the main breaker are limited and these options would result in significant additional costs (new switchboard) or load limiting. An alternative approach is to consider adding a new switchboard section on the Utility side of the line-side tap. The new switchboard section would contain the relocated Utility meter as well as a new main breaker as illustrated in Figure 2. In this way, the new main breaker can serve as the isolation point during a Utility power outage while allowing the solar PV system to remain as a generation source to power the site even when the site is isolated from the grid. This option is particularly beneficial when the existing generation source is owned by a third-party, given that this configuration would require limited, if any, physical reconfiguration of the existing generation source.

In this arrangement, the entire main switchboard, and all the loads connected to it, would be part of the microgrid during a Utility power outage versus having a separate critical load electrical panel. Depending on the final size of the generation source(s) and the energy storage system installed, load controls, such as controllable breakers that could be operated by the microgrid controller, would need to be installed on all loads in the main switchboard that would overload the microgrid during a Utility power outage.

While there are many considerations that go into what the best microgrid configuration is, proper sizing of microgrid components based on the loads served during a grid power outage is a critical part of designing a reliable microgrid and should be given careful consideration. This microgrid configuration maintains the flexibility to size generation sources for optimal financial benefits while getting the added benefit of a microgrid during a Utility power outage.

We hope you enjoyed this “part-two” of our TerraBlog series on microgrids. Up next, we will take a closer look at some of the market and technical variables that will influence the speed and scale of energy resiliency programs. Stay tuned.

As independent advisors, TerraVerde is supporting CCAs and Public Agencies in evaluating and deploying energy resiliency programs. Over the past 10 years, we have developed over 100 MW and over $400M worth of solar & battery programs. To learn more about our feasibility analysis, program design, and project development services, reach out to us at hello@terraverde.energy.

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