21-05-2013, 04:50 PM
Wind Power on Weak Grids in California and US Midwest
INTRODUCTION
This article will discuss the early development of wind parks in California on remote weak
grids and subsequent later development on similar weak grid environments in the US
Midwest. There will be some comparison with contemporary development in strong grid
areas. Load flow, short circuit capacity, and stability issues will be discussed. Discussion
will include a perspective of the root causes and improvements to the weak grid conditions
including utility, regulatory, industry, and policy aspects.
The Tehachapi wind resource area is at an extreme end of the SCE Grid, approximately 70
KM from a major grid substation in the Antelope Valley and supplied over that distance at
only 66 KV. This grid has 80 MW maximum load demand spread among Residential,
Commercial, and Industrial users. Installed wind generation of 345 MW dominates the
load flow and generation is often exported from the Tehachapi region into the Antelope
Valley.
In 1989 a group of developers built their own 230 KV private transmission line from
Tehachapi to the Antelope Valley in order to connect 320 MW of additional Tehachapi
projects onto the grid. On occasion, total Tehachapi wind generation exceeds the load
demand of both the Tehachapi area and the Antelope Valley area, and some of the
generation is exported into the LA Basin.
Since 1986 the 66 KV Tehachapi Grid has experienced a shortage of capacity that has
caused periodic, sometimes severe curtailment of wind generation and repetitive grid
reliability problems. Incremental upgrades to the grid have provided a reasonably good
view of operation on weak grids, and other technical and policy insights. Repowering of
over 50% of the original turbine capacity has dramatically increased generation and
capacity factor. However, new development of one of the richest wind resources areas has
been largely stalled for years by the inability to get new transmission.
Early wind turbine controllers were largely of the analog type with limited capabilities and
of less than robust reliability. Early Power Purchase Agreements (PPA) were written by the
utilities with little understanding of wind turbines, less understanding of impacts from large
arrays of induction generators, and a great fear of self excited islanding and over voltage
conditions. PPAs and rule enforcements severely restricted the amount of VAR support
wind turbines were allowed to provide. The impact from poor early planning and
conceptual design has been incrementally worked through to provide marginally acceptable
grid conditions today. Significant lessons have been learned.
Early wind energy development in Tehachapi was spurred by the Tax Credit Gold Rush of
the early 1980s and the California Policy to encourage the development of Renewable
Energy. Utilities did not have that same Policy vision, quite the opposite, and stalled the
development of adequate grid facilities to support the large PPA capacity they had
contracted. Regulatory, Legislative, and Utility Policy emphasized economic evaluation
over the need to serve reliably, particularly involving generation. Rules centered on large
central station generation and stalled grid expansion suitable to serve wind energy
development. It was deemed more economic to pay to curtail wind generation, and to
import cheap Pacific NW Hydro power, than to build needed transmission. Recognition of
economic and long-term impacts was largely ignored. New Legislative and Policy changes
hold promise to reverse these problems and provide for orderly expansion of the grid with
costs for Renewable Energy generation spread into the rate base.