3.3 Distributed Energy Resources (DER) Impacts on Distribution Systems

3.3.2 Challenges Associated with High Penetrations of Renewable DER

Increased numbers of DER systems interconnected to distribution systems pose both challenges and benefits to distribution operations.  The challenges include the variability inherent in some renewable DER systems (loss of output power when the sun stops shining or the wind stops blowing), but also include the variable needs of the DER owners to meet their own energy requirements rather than just providing energy to utilities (as most bulk generators are designed to do).

Adding significant amounts of generation sources to a distribution feeder can also change its operational characteristics in many fundamental ways. These changes could require mitigation techniques if penetration levels start to impact the power quality or reliability of the feeder.  Determining if and when to mitigate certain DER system impacts varies significantly, depending on feeder characteristics, the profiles of the DER generation and customer loads over time (time of day, day of week, season, etc.), and expected future growth of both generation and load.

Some of these DER challenges include:

• Intermittent or fluctuating power output. Solar and wind power are clearly driven by sources that can change in strength frequently and rapidly. Run-of-the-river hydro can also fluctuate in output although more slowly. In addition to the unpredictable short term fluctuations, there are better anticipated but still extreme changes in power output, for instance as PV systems rapidly decrease their output during the late afternoons. This has led to concerns about how to supply compensating power equally rapidly from bulk power or other sources. California ISO has a famous “Duck Curve” illustrating this concern of rapid changes in sources of generation, which not only stress the bulk power generators that must pick up the load, but also the distribution feeders and even the transmission circuits which must rapidly accommodate the shift in generation sources.
• Unreliable availability. The output from DER systems can vary not only because of fluctuating renewable power sources, but because providing power to the grid may be only a secondary purpose from a customer’s perspective. DER systems installed in commercial and industrial sites may create large swings of their exported power and in absence of an agreement with the utility to do so they may not necessarily have additional power available if the grid needs it during peak times.
• Impacts on power quality. In part due to the change in load profiles for customers with DER systems and in part due to the swings in power output from DER systems, the power quality on feeders may change in ways not anticipated during the design of the feeder, potentially causing power harmonics, excess reactive power, and voltage spikes and sags. These problems may require the utility to add compensating equipment or to upgrade the feeder before it might otherwise need upgrades. In some cases, such as in Hawaii, the excessive generation on feeders can cause damaging high voltage levels, power outages, and other serious grid problems. If the DER systems have appropriate voltage ride-through settings, they can at least avoid unnecessary outages. If they have power factor management capabilities, they can also minimize any reactive power problems.
• Steady-state over voltage.  Typical feeder design takes into account a voltage drop along the length of the circuit, with the voltage level leaving the substation set high enough and with voltage regulators judiciously placed along the feeder to ensure the end-of-line voltage is adequate.  But with the introduction of DER such as inverter-based PV generation along the feeder, voltages will not necessarily decrease at the rate assumed in the planning process. In some cases the voltage can actually increase in areas of high penetration.  Making the problem worse in some locations is the reduction in conductor size along the feeder, which can exacerbate the over voltage problem.  Mitigation techniques include reconductoring, adding more voltage regulation equipment, or adjustment of the fixed power factor of DER. If the DER systems have the advanced volt-var capability, they can dynamically modify the voltage levels up or down to bring them closer to the nominal voltage.
• Transient overvoltage.  Transient overvoltage can occur if a circuit is experiencing reverse flows through the substation transformer and an event causes the substation circuit breaker to open.  If the anti-islanding settings of the DER systems on the feeder are not correctly established according to existing regulations and standards, those DER systems could over-generating and cause the voltage to increase to excessively high voltages for a short period of time, possibly as high as 200%, and potentially damage customer and utility equipment. Therefore it is very important that DER systems are configured with the correct anti-islanding and voltage ride-through settings.
• Reverse power flows in substations.  Distributions systems have been designed with one-way flow of power out from a substation to the customers on a feeder. However, if large amounts of DER power are located on a feeder, it could become larger than the customer load and therefore actually change the direction of the power flow (see Hawaii’s Lock Ness curve). For radial distribution feeders, there is no fundamental physics problem with power being exported from one feeder to serve other feeders or provide supply to subtransmission or transmission voltage networks.  However, issues can arise if existing protection and control equipment is not configured to support reverse flows.  Legacy LTC controls often do not have the capability of sensing the direction of power flows and may operate improperly during times of reverse flows.  Mitigation methods for coping with this situation can be implemented, with the most common being the limiting of the amount of DER generation that is permitted to interconnect on any one feeder. For instance, some utilities limit the total DER generation to 15% or 30% of the minimum load on the feeder. Other solutions are to install LTCs that can handle reverse power flows and to reconfigure substation protection schemes that may be affected.
• Reverse power flows in secondary networks. Reverse power flows on secondary networks are more difficult to mitigate, and significant work is still going on to determine the most effective methods other than just limiting the amount of DER generation allowed on any secondary network segment.
• Impacts on reliability. There is no question that DER systems add to the number of factors that must be understood, planned for, and operated on in order to maintain power reliability. Along with the renewable energy causing fluctuations, the changing requirements of customers in using DER systems to support their own loads as well as feed back to the grid can cause disruptions. Utilities do not as yet have very good distribution planning software, power flow contingency analysis, or access to real-time information that could help assess situations with reliability issues.