IEEE Power & Energy Magazine - May/June 2014 - 93

Frequency Droop
Control

Interconnection

Station

Collector

Equivalent

Transmission
Transformer
System
Pad-Mounted
the control system also proLine
Equivalent Transformer
vides frequency droop control to
handle unusual grid situations.
PV
For example, in case of abovekV, R, X, B
normal frequency, the controlkV, R, X, B
kV, MVA, R, X
kV, MVA, R, X PV System
Equivalent
ler will reduce the active power
POI or Connection to
Plant-Level
of the plant, as illustrated in Figure 8.
Transmission System
Reactive
Compensation
if the plant is under curtailment,
the power can also be increased
if the below-normal frequency is figure 11. A single-machine equivalent load flow representation.
detected. note that all the parameters illustrated in the figure are
configurable and are shown here for illustrative purposes only. Power Plant Controller Model
the plant controller model Repc_A is used when plant-level
control of active and/or reactive power is desired. A funcFault Ride-Through Capability
A significant benefit of utility-scale pv systems that incorpo- tional block diagram for Repc_A model is shown in Figure
rate fault ride-through capability is that they do not trip off 10. the model incorporates the following functionalities
during system disturbances but continue to provide power Renewable energy Modeling task Force (Wecc-ReMtF):
when the grid needs it. the ability to ride through specific low
✔ closed-loop voltage regulation at a user-designated
and high voltages or low- and high-frequency ranges is being
bus with the provisions for line drop compensation,
designed effectively into all modern variable generators. Most
voltage droop response, and a user-settable dead-band
utility-scale inverters have this capability. With proper design
on the voltage error signal.
practices, the pv plant is engineered to ensure that all com✔ closed-loop reactive power regulation on a user-desponents in addition to inverters also have the ability to ride
ignated branch with a user-settable dead-band on the
through short-term grid events.
reactive power error signal.
✔ A plant-level governor response signal derived from
frequency deviation from the nominal user-designated
Modeling and Validation
branch. the frequency droop response can be applied
A general structure of a pv power plant recommended by
to active power flow on a user-designated branch. Frethe Western electricity coordinating council (Wecc)
quency droop control is capable of being activated in
is shown in Figure 9. For the simplification of a solar pv
both over and under frequency conditions. the freplant, the dynamics related to the dc side of the inverter
quency deviation applied to the droop gain can be
(pv array dynamics, inverter dc link, and voltage regulator)
subject to a user-settable dead-band.
are ignored. the overall model structure shown in Figure 9
represents the complete pv plant and consists of an equivalent generator model "ReGc_A" to provide current injec- Test Model Description
tions into the network solution, an electrical control model this section demonstrates the outcome from a set of field tests
"Reec_B" for local active and reactive power control, and performed at a pv plant when operating at 90-MW active
a centralized plant controller model "Repc_A" to allow for power output. the test plant equivalent model is configplant-level active and reactive power control.
ured as shown in Figure 11, where multiple medium-voltage

90.00
9.63
0.328
1.011

1.000

90.0
1
9.6
PV Plant

2
89.75
8.35

89.75 89.58
8.35 13.20
37.74
1.007

1
12-MVar Cap Bank
89.58
89.54
9.58
13.20
1.000

5
89.54

9.57
525.0
1.050

89.54
9.55

89.54
1
9.54
525.0
Grid
1.050

34.62
1.003

figure 12. A PSLF equivalent model for a plant under test.
may/june 2014

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