IEEE Power & Energy Magazine - May/June 2014 - 94

Reactive Power (MVar)

Reactive Power Command (MVar)

in the test system, a 12-Mvar
capacitor
bank, located at the 34.5Field Test Versus Simulation for 12-MVar Capacitor Bank Switching
15.0
kv collector bus, is engaged as an
external stimulus. Figure 13 illus12.0
trates that, when the capacitor bank
9.0
Cap Bank Switching
Qcmd_actual
is engaged at "t = 0" s relative time,
6.0
Fast Initial Response
Qcmd_simulation
the power plant controller sends
3.0
control signals to each individual
0.0
inverter to immediately address
-3.0
this switching event, and eventually
-6.0
inverters reactive power contribuSlower Plant Level Response
-9.0 Capacitor Bank Switched Online
tion to the plant drops. First Solar's
-12.0
power plant controller reactive
-1.10
0.40
1.90
3.40
4.90
6.40
7.90
power command (Qcmd_actual)
Relative Time (s)
distributed to the individual inverters is shown in green in Figure 13.
figure 13. The Qcmd response field test versus simulation.
Both field measured reactive power
command (Qcmd_actual) and simulated model command (Qcmd_
simulation) are shown in Figure
Field Test Versus Model Simulation for 12-MVar Capacitor Bank Switching
13. the figure illustrates the initial
15.0
quick response by the pv plant,
12.0
which took almost 100 ms, followed
9.0
Inverter Vars (Measured)
by the dominance of the power plant
(Simulated)
Inverter
Vars
6.0
Cap Bank Switching
controller in the order of seconds, to
3.0
maintain post-disturbance stability.
0.0
Figure 14 shows the detailed
-3.0
plant-reactive power response to
-6.0
capacitor switching. this figure
Capacitor Bank Switched Online
-9.0
illustrates that the simulation model
-12.0
performance adequately mimics
-5.90 -4.40 -2.90 -1.40 0.10 1.60 3.10 4.60 6.10 7.60
actual plant behavior. the response
Relative Time (s)
closely matches actual behavior,
with a difference immediately folfigure 14. Inverter VARs during a 12-MVar cap bank switching test.
lowing the switching operation that
could be attributed to the lower sampling
rate
in
the
field
measurement
than in pSlF simulation.
feeders are equivalenced at one collector, a 34.5-kv bus at
the low side of the plant substation transformer. load-flow
model equivalencing is performed using the method sug- Daily Grid Operation
gested in the Wecc modeling guide for solar pv plants Another grid integration concern, especially for a grid
Modeling and validation Work Group/technical Studies operator, is daily load balancing. Solar generation is a type
Subcommittee.
of variable power generation that is not fully dispatchthis plant uses SMA 800-kvA inverters controlled by able since the energy source is influenced by the presence
First Solar's power plant controller. the power plant control- of solar radiation and by atmospheric conditions. Reliable
ler allows for coordination of all online inverters for plant- power-system operation requires the continuous balance of
level voltage regulation at the poi, located at the 500-kv supply and demand. to successfully manage a variable gensubstation bus. the purpose of testing at this plant is for the eration source like solar, grid operators treat pv generation
model validation of First Solar's controller against Wecc's as "negative" load, and they utilize short-term forecasts to
proposed power plant controller. data captured from the field schedule and dispatch compensatory controllable resources.
tests were filtered and then compared to simulation results the operators are already familiar with a certain amount of
obtained from a plant model built in Ge's positive Sequence variability and uncertainty, particularly with system load (or
load Flow (pSlF) simulation software. A single-line dia- demand). they have successfully utilized a variety of tools
gram of the plant model in pSlF is shown in Figure 12. such as generator and transmission flexibility, ancillary serA 12-Mvar capacitor bank switching test is performed to vices, and demand-side resources to achieve reliable sysexamine the plant controller performance.
tem operation. the growing sophistication and accuracy of
94

ieee power & energy magazine

may/june 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2014