IEEE Power & Energy Magazine - November/December 2015 - 37

35,000

Desert Southwest

Wind
PV
CSP
Gas+

Hydro
Nuclear
Coal

25,000
20,000

Northeast
(of the West)

15,000
10,000

Coal

Generation Production (GW)

30,000

Coal

5,000
0
Light
Spring
Base

Light
Spring
High Mix

Light
Spring
Extreme
Sensitivity

Light
Spring
Base

Light
Spring
High Mix

Light
Spring
Extreme
Sensitivity

figure 6. Wind and solar displacement of thermal.

thermal problems did occur and were addressed with a few
conventional transmission reinforcements. These two high
renewable cases represent conditions far from those experienced in present operation of the Western Interconnection.
The system transient stability under these conditions was
tested for a 500-kV transmission fault in the heart of the
high-wind area of Wyoming. Both the base case and highmix case resulted in acceptable dynamic behavior for this
limited test. Thus, a more than 80% reduction of coal generation in the Northeast region in the high-mix case did not
cause systemic problems.
Decommitment in the extreme-sensitivity case further stresses the system, with a more than 90% reduction
of coal generation from the light spring base case. This
case is unstable, exhibiting a rapid voltage collapse and
system separation during the fault. This is one characteristic of so-called "weak grid" issues. Systems with very
high levels of inverter-based generation are challenged
to provide fast, confident control during faults and other
disturbances. Control of wind generation for weak grids
is constantly improving, and test cases using the most
advanced presently available wind plant controls stabilized this particular case.
Alternatively, the grid can be strengthened by providing
higher short circuit strength. Traditional reinforcements such
as building new lines does this well. But there are alternatives.
One tested in the study was the conversion of three coal units
to synchronous condensers. This reinforcement, along with
the addition of mechanically switched shunt compensation,
was sufficient to stabilize this fault, even with the conservative
november/december 2015

load and wind plant modeling used. The synchronous condenser conversion (see "Back to the Future: Rise of the Condensers") works well to stabilize the system, which recovers
in an orderly fashion when the fault is cleared.

Consequences of
the Rooftop Solar Revolution
Solar PV generation is being added at an astonishing rate
in many place in the United States and around the world.
PV systems on rooftops and other locations deep within the
traditionally passive distribution systems have the potential to alter the interaction between the loads and the bulk
power system. The many GW of rooftop PVs visible as
light blue in Figure 3 and Figure 4 were included in thousands of sophisticated "composite load" models throughout the Interconnection. These models capture some of the
potential effects that PVs residing close to the load may
have much better than the common practice of netting the
PVs and the load.
One aspect of particular concern is that DG of all types,
but especially inverter-based devices like PVs, can stop
abruptly when there is a disturbance on the grid. This tripping can be deliberate, in order to avoid the risk of inadvertent islanding, as mandated by the old IEEE standard
1547. But it can also be inadvertent, occurring in cases
where inverters are insufficiently robust to continue operation when the voltage at their terminals departs from either
nominal magnitude or frequency. In either case, widespread tripping of DG in response to a large disturbance
has the potential to exacerbate a disturbance.
ieee power & energy magazine

37



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015

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