3.3 Distributed Energy Resources (DER) Impacts on Distribution Systems

3.3.1 Types and Characteristics of DER Systems

Distributed Energy Resource (DER) systems include generation devices and storage devices, and often includes “controllable load”. 

DER systems are being interconnected to the distribution grid in increasing numbers, and changing the operational characteristics of the basic distribution grid design that has been in use for decades (see Figure 9). As opposed to the larger bulk generators, these DER devices are typically smaller and are (by definition) interconnected to grid at the distribution level or sometimes at the subtransmission level.

Figure 9 : Distribution System with DER systems

The DER systems which generate electrical power may have renewable sources of power or may be driven (directly or indirectly) by fossil fuels. Some of these DER devices are synchronous generators, such as the diesel generators often used for backup in hospitals and business complexes.  Most of the more recent DER systems are inverter-based generators or storage devices, including (non-renewable) microturbines, most combined-heat-and-power (CHP) systems, and fuel cells, as well as (renewable) energy DER systems.

Non-synchronous DER systems include one or more inverters to convert non-60-cycle current to 60-cycle alternating current (60 Hz) . For instance, photovoltaic systems convert direct current (dc) to alternating current (ac) via inverters, while a microturbine which generates 400 Hz output would first convert the 400 Hz to direct current, and then convert the direct current to 60 Hz alternating current.

Common renewable DER generation systems include photovoltaic systems, wind power systems, small hydro plants, geothermal, and biomass systems.

Energy storage DER systems have inverters which are used in both directions: to convert to 60 Hz ac when “discharging” or generating power, and, in reverse, to convert from 60 Hz typically to dc when “charging” or storing energy. The more common energy storage technologies include batteries, flywheels, pumped hydro, compressed air, super-capacitors, and superconducting magnetic energy storage.  Another potential storage technology is the electric vehicle (i.e. vehicle-to-grid) if permitted technically and by regulations. One trend today is to combine a renewable energy DER with a storage device (e.g. a PV system directly combined with a battery) so that fluctuations can be smoothed out.

Controllable load can be seen as the inverse to generation. Increasingly controllable loads can be considered in the mix for managing energy.  Controllable loads can often just involve shifting the load to a different time, such as running washing machines or water pumping stations during off-peak times. Another example is pre-cooling large plenums during off-peak and then cycling off air conditioners during on-peak.