Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
The use of an inverter makes PV similar to Type 4 wind turbine-generators in that the inverter
can provide real-time control of voltage, supporting both real and reactive power output. Given
the absence of performance standards for PV inverter modules, it is likely that actual
performance of PV inverter modules will vary from supplier to supplier.
PV plants with ratings on the order of hundreds of MW are being proposed throughout the North
America. It is unclear if the scale of these plants will limit the impact on ramping by virtue of
significantly greater land coverage.
PV connected at distribution levels, e.g. residential and small commercial installations are
subject to IEEE Standard 1547. This standard prohibits distributed generation, including PV,
from riding through grid disturbances involving significant voltage or frequency excursions, and
also prohibits providing voltage control. 38 Thus, widespread deployment of small distribution
connected variable generation has the potential to have adverse impacts on grid performance.
Evidence of this problem is starting to surface in some small grids now. Further evolution and
reconciliation of IEEE 1547 to take broader grid performance considerations into account is
needed.
2.4.3. Power Management
For variable generation to provide power plant control capabilities, it must be visible to the
system operator and able to respond to dispatch instructions during normal and emergency
conditions. Real-time wind turbine power output, availability, and curtailment information is
critical to the accuracy of the variable generation plant output forecast, as well as to the reliable
operation of the system. It is critical that the Balancing Area operator have real-time knowledge
of the state of the variable generation plant and be able to communicate timely instructions to the
plants. In turn, variable generation plant operators need to respond to directives provided by the
Balancing Area in a timely manner. The need for this information was clearly illustrated during
the restoration of the UCTE system following the disturbance of Nov. 9, 2006 when there was a
lack of communications between distribution system operators (DSOs) and transmission system
operators (TSOs) delayed the TSO’s ability to restore the bulk power system. 39
Therefore, as small variable generation facilities grow into significant plants contributing
significantly to capacity and energy, balancing areas will require sufficient communications for
monitoring and sending dispatch instructions to these facilities. 40 Further, Balancing areas and
38 See IEEE Standard 1547, “IEEE Standard for Interconnecting Distributed Resources with Electric Power
Systems,” page 7, 4.2, “Response to EPS Abnormal Conditions”
39 http://www.ucte.org/resources/publications/otherreports /
40 An international standard communications protocol has been prepared, IEC 61400-25, Wind turbines –
Communications for monitoring and control of wind power plants – Overall description of principles and models,
International Electrotechnical Commission, December, 2006.
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