IEEE Power & Energy Magazine - May/June 2014 - 91

may/june 2014

Power (MW)

Commanded (MW)

calculates and sends the commands
Stopping and Starting One Block (30 MW)
for each inverter individually to
90
90
lower its output to achieve the
desired set point, using a closed85
85
Plant Stop Command
Low
loop control mechanism. note that,
Irradiance
in some cases the plant controller
80
80
will turn off certain inverters to
75
75
achieve this desired set point since
Inverters Are
the output of each inverter cannot
Started in
70
70
Inverters Are
Sequence
be lowered below a certain threshTurned Off in
65
65
old without causing a high dc voltSequence;
age operating condition.
Ramp Rates
60
60
Are Controlled
Figure 4 illustrates field data
Plant Start Command
from a pv plant operating at
55
55
around 90-MW power. the cur0
1
2
3
4
5
6
7
8
9
10
Time (min)
tailment limit is initially changed
Source: Agua Caliete PV Plant in Yuma, Arizona, USA
from 100 to 82.5 MW. the plant
13 March 2012 ~90 MW Online
controller turns down the inverters (and turns off some of them if
required) to achieve the new set figure 7. Plant shut-down and start-up controls.
point. note that the turndown of
power is gradual to meet the specified ramp-rate limit.
Frequency Droop Parameters
the curtailment limit is reduced
MW
again to around 75 MW, and the
controller responds as expected.
When the limit is raised, the controller adjusts the output of the
Droop
Deadband
inverters to increase the total plant
Low
Decrease Active
High Limits
Limits
output. Finally, when the limit is
Power in Case of
Hz
raised to 100 MW, the plant is no
High Frequency
longer curtailed since the plant is
Deadband
producing less than the limit.
Droop
in all the control actions, the controller's command to each inverter is
unique, given the specific conditions
each inverter is experiencing. For
example, when the plant is under
Frequency Deadband
0.036 Hz
curtailment, the plant controller
Frequency Droop
5.0 %
can release the power limit of indiHigh Limit 61.500 Hz
vidual inverters if the total output of
Low Limit 58.500 Hz
the plant starts falling below the set
Active Power High Limit
290.0 MW
point. So in case of a cloud passage,
Active Power Low Limit
10.0 MW
which results in a reduction of the
output of a part of the plant, the controller can make the adjustment to
increase the output of other inverters
that are not impacted.
the plant-level control strategy figure 8. The frequency droop function.
results in a capture of energy from
inverters that would have been otherwise unnecessarily cur- that are not impacted by the cloud cover to dynamically increase
tailed. this concept is illustrated in Figure 5. the left side of their previously curtailed limit. Since the total potential power
the figure represents the reduction in power output of some of of the plant is greater than the specified plant output limit (as
the inverters (grouped in blocks for illustration purposes) due illustrated on the right-hand side of Figure 5), the plant is able to
to partial cloud cover. the controller commands other inverters output the total power all the way to the limit.
ieee power & energy magazine

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

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