Accommodating High Levels of Variable Generation - NERC

Transmission Planning & Resource Adequacy
Another significant consideration is the influence of high levels of variable generation on the
distribution system. As the penetration of distributed resources grows, 81 their influence on bulk
system supply and delivery planning, including their variable generation characteristics (e.g.
ramping), cannot be ignored. For example, to maintain bulk power system reliability,
distribution system designs may need to be enhanced to accommodate reactive power control
requirements, 82 coordinated system restoration, visibility of and communication with distributed
variable resources by bulk power system operators, as well as system protection and safety
concerns. In addition, the NERC Functional Model may need to be enhanced in the future to
recognize owners and operators of distributed generation.
In some areas of North America, it is possible that very high penetrations of distribution system
connected variable generation could be achieved in the future, as has occurred in some regions of
Denmark and Germany. 83 As mentioned earlier, under these circumstances, the requirement for
bulk power system voltage ride-through capability can be in conflict with the anti-islanding
voltage drop-out requirements of distribution connected generation which comply with IEEE
Standard 1547. 84 A study is needed to reconcile bulk power system voltage ride-through
requirements and IEEE Standard 1547 in order to maintain the reliability of the bulk power
system (e.g. tripping of local generation during distant faults, tripping of generation during
under-frequency load shedding, complications with system restoration).
Distributed variable generators, individually or in aggregate (e.g. small scale photovoltaic), can
impact the bulk power system and need to be treated, where appropriate, in a similar manner to
transmission connected variable generation. The issues of note are forecasting, restoration,
voltage ride-through, safety, reactive power, observability and controllability. High levels of
distributed generation may require new network design. Further, distributed variable generation
units may fall below the MW size requirements which might require a Generation Owner or
Generator Operator to register and therefore to be held to NERC’s standards. The NERC
registry criteria 85 may need to be broadened to include smaller generators not covered by the
81 For example, the U.S. Department of Energy’s Energy Information Administration’s definition of Distributed
Generator is “A generator that is located close to the particular load that it is intended to serve. General, but nonexclusive,
characteristics of these generators include: an operating strategy that supports the served load; and
interconnection to a distribution or sub-transmission system (138 kV or less)”
82 The Danish Cell Project - Part 1: Background and General Approach; Per Lund, Energinet.dk, Denmark. IEEE
PES GM, Tampa, 2007
83 Holttinen H., et al 2007, Design and Operation of Power Systems with Large Amounts of Wind Power: State of
the Art Report, VTT Working Paper 82, IEA Wind.
84 http://grouper.ieee.org/groups/scc21/1547/1547_index.html
85 See page 8 of http://www.nerc.com/files/Statement_Compliance_Registry_Criteria-V5-0.pdf
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