1. SPARC Course
Wind Turbine Principles and Modeling
for Transient Stability Analysis
March/April 2021
IIT D and IIT DH
1
Vijay Vittal
Regents’ Professor
Ira A. Fulton Chair Professor
Arizona State University
3. Wind energy conversion
• Wind energy has been in use for centuries
• Originally used for pumping water, grinding
grain, or for agricultural purposes
• In the past two decades technical
advancements have made it possible to use
wind energy for electricity production
3
4. Wind generation technology
• Wind turbine types
– Conventional induction generators
– Induction machines with wound rotor and static
power converter technologies
• Important factors for wind integration
– Steady state performance – P, Q, PF
– Dynamic response to perturbation
– Intermittency and time variation of P and Q
– Output quality
4
5. Wind generation technology
• Wind farm related capabilities
– Has great impact on aggregate plant performance
– Advanced function can be implemented
• VAr support using distributed resources
• Controlled ramp rate
5
8. Fixed-speed wind turbine
• For the fixed-speed wind turbine the induction
generator is directly connected to the electrical
grid as shown in the previous figure
• The rotor speed of the fixed-speed wind
turbine is in principle determined by a gearbox
and the pole-pair number of the generator
• The fixed-speed wind turbine system has often
two fixed speeds
8
9. Fixed-speed wind turbine
• Accomplished by using two generators with
different ratings and pole pairs, or it can be a
generator with two windings having different
ratings and pole pairs
• This leads to increased aerodynamic capture as
well as reduced magnetizing losses at low wind
speeds
• This system (one or two-speed) was the
“conventional” concept used by many Danish
manufacturers in the 1980s and 1990s
9
13. Variable speed wind turbine with
doubly fed induction generator (DFIG)
• This system, shown on the previous slide,
consists of a wind turbine with doubly-fed
induction generator
• This means that the stator is directly connected
to the grid while the rotor winding is
connected via slip rings to a converter
• The power electronic converter only has to
handle a fraction(20–30%)of the total power
13
14. DFIG
• For variable-speed systems with limited variable-
speed range, e.g. ±30% of synchronous speed, the
DFIG can be an interesting solution
• The power electronic converter only has to handle a
fraction(20–30%) of the total power
• The losses in the power electronic converter can be
reduced in comparison to a case where it has to
handle the total power
• This also reduces the cost of the converter
• The stator circuit of the DFIG is connected to the grid
while the rotor circuit is connected to a converter via
slip rings
14
16. DFIG
• The back-to-back converter consists of two converters,
i.e., machine-side converter and grid-side converter,
that are connected “back-to-back”
• Between the two converters a dc-link capacitor is
placed, as energy storage, in order to keep the voltage
variations (or ripple) in the dc-link voltage small
• With the machine-side converter it is possible to control
the torque or the speed of the DFIG and also the power
factor at the stator terminals, while the main objective
for the grid-side converter is to keep the dc-link voltage
constant
16
17. DFIG
• The speed–torque characteristics of the DFIG
system is shown on the next slide
• The DFIG can operate both in motor and
generator operation with a rotor-speed range of
±Δ around the synchronous speed, ω1
max
r
17
19. DFIG Operation
• As shown in the figure on the previous slide,
the DFIG machine can operate either as a
motor or as a generator at both
subsynchronous and supersynchronous speeds
• For the cases of a DFIG operating as a
generator at both subsynchronous and super
synchronous speeds the configurations of the
active power flows in the various circuits of
the machine are depicted on slide 20
19
20. DFIG Power Exchange
20
Vrotor
Vgrid Pgap
Rotor
m
Tem
Ploss,rotational
Pload
Pmech
s.Pgap
Rr
Protor-conv
Ploss,rotor
Stator
Pstator
Ploss,stator
DFIG power exchange at
subsynchronous speeds
Vrotor
Vgrid
Pgap Rotor
m
Tem
Ploss,rotational
Pload
Pmech
s.Pgap
Rr
Protor-conv
Ploss,rotor
Stator
Pstator
Ploss,stator
DFIG power exchange at
supersynchronous speeds
21. Principle of Operation
• The DFIG has a wound rotor and the speed at which the
machine operates can be varied by adjusting the frequency
fRotor of the ac currents fed into the rotor winding
• The primary reason for using a DFIG is to produce a three-
phase voltage whose frequency fStator is constant. This is to
make sure that the frequency fStator remains equal to the
frequency fNetwork of the ac power network to which the
generator is connected, in spite of the variability in the
generator rotor speed nRotor caused by the variability in the
wind speed
• The frequency fRotor of the ac currents that need to be fed into
the DFIG rotor windings to maintain the generator output
frequency fStator at the same frequency as fNetwork depends on the
speed of rotation of the rotor nRotor
21
23. Modeling of GE Wind Turbine
Generators for Grid Studies
• Paper from WTG Modeling Panel Session at
the 2003 General Meeting
23
V. Vittal and R. Ayyanar, Grid Integration
and Dynamic Impacts of Wind Energy,
Springer 2013.
24. 24
P. Pourbiek, et al., “Generic Dynamic Models for Modeling Wind Power
Plants and Other Renewable Technologies in Large-Scale Power System
Studies,: IEEE Transactions on Power Systems, Vol. 32, No. 3, pp. 1108
– 1116, September 2017.
M. Asmine, et al., “Model Validation for Wind Turbine Generator
Models,” IEEE Transaction on Power Systems, Vol. 26, No. 3, pp. 1769 –
1782, August 2011.
P. Pourbiek, et al., “Generic Stability Models for Type 3 & 4 Wind
Turbine Generators for WECC,” Power and Energy Society General
Meeting (PES), 2013 IEEE
WECC Type 3 and Type 4 Modeling Guides
https://www.wecc.biz/Reliability/WECC-Type-4-Wind-Turbine-Generator-
Model-Phase-II-012313.pdf
https://www.wecc.biz/Reliability/WECC-Type-3-Wind-Turbine-Generator-
Model-Phase-II-012314.pdf
25. Model development
• This is a simple model appropriate for bulk power
system dynamic studies
• This model is for positive sequence phasor time-
domain simulations
• The analysis is mainly focused on how the wind
turbine-generators (WTGs) react to disturbances,
e.g. faults, on the transmission system.
• The model provides for calculation of the effect
of wind speed fluctuation on the electrical output
of the WTG
25
26. Model development
• Very fast dynamics associated with the control
of the generator converter have been modeled
as algebraic (i.e. instantaneous)
approximations of their response
• This is justifiable because the fast transients
associated with the power electronic
converters would have settled in the time
frame of interest for the primary purpose of
analysis which is examining electro-
mechanical transients 26
27. Model development
• Representation of the turbine mechanical
controls has been simplified as well
• The model is not intended for use in short
circuit studies
27
28. Modeling WTGs for dynamic analysis
A number of components contribute to WTG dynamics
and should be represented
• Turbine aerodynamics
• Turbine mechanical controls (pitch control or
active stall control)
• Shaft dynamics
• Generator electrical characteristics
• Electrical controls
•Protection relay settings
28
31. Key issues
• Key dynamic issues to be investigated during
studies:
– Voltage stability/regulation
– Voltage ride-through
– Frequency control/Inertia Response
• These and other general stability concerns may
be investigated using positive sequence
programs and models
31
32. Key Issues
• Voltage Ride-Through
–For conventional Induction machines, inverse
“stalling” phenomenon, need for fast blade
pitching combined with UPS for controls and
possibly dynamic VAr support (SVC, STATCOM
etc.)
–For doubly-fed and full-converter units this is a
control/protection issue to prevent
overvoltage/currents on the rotor circuit that may
damage the rotor side converter IGBTs, typically
solved with active crow-bar or chopper circuits.
32
33. Key Issues
Voltage regulation–SVC, STATCOM, synchronous
condenser or inherent in machine (doubly-fed or 4-
quad full-converter) + switched shunt compensation
33
34. Fast Blade Pitching for Ride-Through on a Convention
Induction Generator Unit
34
35. Crowbar circuit
• A crowbar circuit is an electrical circuit used to prevent an
overvoltage condition from damaging critical elements. It
operates by putting a short circuit or low resistance path across
the voltage source to the critical element.
• An active crowbar is a crowbar that can remove the short
circuit when the transient is over thus allowing the device to
resume normal operation. Active crowbars use a transistor,
gate turn off (GTO) thyristor or forced commutated thyristor
instead of a thyristor to short the circuit. Active crowbars are
commonly used to protect the frequency converter in the rotor
circuit of the doubly fed generators against high voltage and
current transients caused by the voltage dips in the power
network. Thus the generator can ride through the fault and
quickly continue the operation even during the voltage dip.
35
36. Active Crowbar for a Doubly-Fed Asynchronous Generator
36
0
Doubly-fed
induction generator
Gear box Unit transformer
Rotor side
converter
Grid side
converter
Capacitor
g
L
Grid
crow
I
crow
R
Crow bar
Fully controllable
semiconductor switch
(IGBT)
37. Full-Converter with Synchronous Generator
37
Permanent Magnet
Synchronous Machine or
Induction Generator
Gear Box
Transformer
Power Electronic
Converter
=
»
»
Grid
39. 39
Power flow modeling
WTG Type Model Shunt Compensation Generator
Transformer
Conventional IM P, Q bus; Q =
const.1
Explicitly model shunt capacitor Typically, xt = 6% on
transformer rating
Variable Rotor Resistance IM P, Q bus; Q =
const.1
Explicitly model shunt capacitor Typically, xt = 6% on
transformer rating
Doubly-fed asynchronous
Generator
P, V bus;
Qmin Q Qmax
Inherent in machine, typical
power factor +/- 0.95 (or better)
Typically, xt = 6% on
transformer rating
Full Converter P, V bus;
Qmin Q Qmax
2
Typically, inherent in inverter
capability (+/- 0.95 pf or better)
Typically, xt = 6% on
transformer rating
1. The reactive consumption of an actual induction generator will vary as the voltage drops, but to properly
model this aspect one would need an equivalent power flow model of an induction machine, which is not
available is some software packages.
2. Provided the converter is a voltage-source converter the unit will have reactive. power capability. However,
at its limit this is a constant current and not a constant VAr device. Thus, once at limit a P, V model is not
exactly correct – once again this could be a limitation of some simulation tools.
40. 40
Why Generic Models?
• Existing vendor specific wind turbine models typically
include technical details specific to a particular
manufacturer and design.
• These models are highly specific and in most cases
proprietary.
• User often articulate a need to achieve compatibility across
simulation platforms (PSS/E, PSLF, TSAT).
• After year of accumulated industry experience, wind turbine
generator performance and modeling requirements are
better understood.
• The idea is to create several types of generic models which
– Will not require an proprietary information
– Will be parametrically adjustable to any specific wind turbine
of the same type available on the market.
41. 41
Proposed generic models
• Four basic topologies based on grid interface
– Type 1 – conventional directly connected induction generator
– Type 2 – wound rotor induction generator with variable rotor
resistance
– Type 3 – doubly-fed induction generator
– Type 4 – full converter interface
42. 42
Technical issues
• Generic models are intended for transmission
planning studies, which focus on grid
disturbances and not wind disturbances
• Simulation of the aerodynamic conversion
should be simplified to avoid using proprietary
Cp curves information
• Too much simplification is not desired: a model
does not perform well if aerodynamics are
ignored (e.g., constant mechanical power)
44. Type 3 Generic Model
44
ref reg
ref gen
V /V or
Q /Q
At plant level
ref
Pref gen
Freq /Freq and
Plant /P
repc_a
Plant Level Control
reec_a
regc_a
wtgtrq_a
wtgpt_a wtgar_a wtgt_a
Torque
Control
Pitch-Control Aero Drive-Train
Q Control
P Control
Current
Limit
Logic
Generator/
Convertor
Model
gen
Q
ref0
P
e
P
ref
ω
ref
Ext
P
(or T )
spd
ord
P
flag
PQ
=1 P priority
=0 Q priority
θ m
P
qcmd
I
pcmd
I
qcmd
I
pcmd
I
t
V
ref
ext
Q
(or Q )
q
I
p
I
45. Model Set-up
• In setting up a typical time domain transient
stability study, the operating condition for the
wind generator under consideration is provided
by the solved pre-disturbance power flow
• The active and reactive power output and the
voltage at the terminal bus from the power
flow are used to initialize the model, and the
outputs from the model provide the current
injection into the network at the bus the wind
generator is located
45
46. Model Set-up
• The electrical power output (Pe) of the wind generator is
used to calculate the reference rotor speed (ref) of the
wind turbine
• The reference rotor speed at rated wind speed is
dependent on the design of the wind turbine
• Typically it is about 1.2 pu at rated wind speed
• For lower power output levels, the reference speed bears a
nonlinear relationship with the electrical power output of
the machine and is given by ref = f (Pe)
• The nonlinear relationship shown above is manufacturer
dependent and only provided by the manufacturer
46
47. Model Set-up
• The other key components of the model are:
– Aerodynamic model
– Mechanical control and shaft dynamics
– Electrical generator characteristics
– Electrical control
• Slide 44 shows the generic model for the Type 3 wind
turbine generator
47
48. Type 3 WTG Generic Model Features
• The model obtains an input of the blade pitch
angle from the pitch control model and
provides the turbine mechanical speed as an
input to the pitch control model and to the
active power model
• It includes a simplified aerodynamic model
which calculates the mechanical power
developed by the wind turbine using the blade
pitch angle and the wind speed
48
49. Type 3 WTG Generic Model Features
• The pitch control model is shown on slide 57. The
actuators associated with the blade position are rated
limited, hence a time constant is also included to
represent the translation of the blade angle into
mechanical output
• Two PI controllers are represented in the blade pitch
angle control
• These controllers receive inputs of speed and power
errors to synthesize the control signal
49
50. Type 3 WTG Generic Model Features
• The Type 3 generator/convertor model is shown on
Slide 53. In this model the flux dynamics are
neglected to reflect the rapid response of the power
electronic converter to the higher level commands
arising from the electrical controls shown on Slide 54
• This model also includes a low-voltage power logic
(LVPL) which limits the active current command
during and immediately following sustained faults
• The electrical controls shown on Slide 54 include the
reactive and active power control modules
50
51. Type 3 WTG Generic Model Features
• These modules display the controls associated with
the converter that determine the reactive power and
active power that will be delivered to the system
• These quantities are determined by the signals
and respectively
• The synthesis of the signal , the reactive
power order Qord is computed by two separate control
modules – wind plant reactive power control
emulator and the power factor regulator
51
q command
I
p command
I
q command
I
52. Type 3 WTG Generic Model Features
• The active power order is obtained using the
wind turbine generator electrical output power and
shaft speed as shown in the Slide 54
• In obtaining the active power order, the speed
reference ref is derived from the turbine speed set
point versus the power output curve
52
p command
I
53. Type 3 WTG Generator/Converter Model
53
LVPL Logic
Power
Voltage
Low
Zerox Brkpt
V
Lvpl1
LVPLsw
0
1
g
1
1 sT
pcmd
I
LVPL & rrpwr
V
fltr
1
1 sT
p
I
1
0
gain
lvpnt0 lvpnt1
V
LOW VOLTAGE
ACTIVE CURRENT
MANAGEMENT
x
x
t
V
g
1
1 sT
qcmd
I
Σ
Σ
0lim
V
hv
K
0
0
t 0lim
V V
t 0lim
V V
0lim
I
qrmax
I
qrmin
I
HIGH VOLTAGE REACTIVE CURRENT LIMIT
q
I
INTERFACE
TO
NETWORK
MODEL
+
+
-
-
Upward rate limit on Iq active when Qgen0 > 0
Downward rate limit on Iq active when Qgen0 < 0
0
s
1
s
2
s
54. Type 3 WTG Electric Control Model
54
pfref
Pe
p
gen
Q
Σ
+
min
Q
max
Q
-
Σ
Σ
+
+
+
0
1
0
1 1
0
Σ
0
1
2
+
-
+
+
Current
Limit
Logic
0
1
tan
p
1
1 sT
ext
Q
From
repc_a
Flag
pf
Flag
P
ref
P
t
V
Freeze State if
Voltage_dip =1
If (Vt < Vdip) or
(Vt > Vup) then
Voltage_dip = 1
else
Voltage_dip = 0
end
t
V _ filt
qh1
I
qfrz
I
qinj
I
Flag
Q
Flag
V
t
V _ filt
ref0
V
ref1
V
t
V _ filt
VDL1
VDL2
pmax
I
0
max
dP
min
dP
ord
P
Thld2
0 – Q priority
1 – P priority
PQFlag
s0
s1
s2
s4
s5
(s0)
ql1
I
rv
1
1 sT
Σ
dbd1, dbd2
qv
K
0.0
iq
1
1 sT
Iqmax
Iqmin
vi
vp
K
K +
s
s3
qmax
I
qmin
I
qcmd
I
max
V
min
V
qi
qp
K
K +
s
min
V
max
V
Freeze State if Voltage_dip =1
N
D
N
D
N
D
p
spd
(speed)
From
wtgt
pord
1
1+sT
Freeze State if
Voltage_dip =1
max
P
min
P
0.01
0.01
N
D
pcmd
I
To
regc_a
To
wtgp_a
Voltage_dip
-
To
regc_a
From
wtgtq_a
Switch
SW
ref
V
55. Type 3 WTG Logic for Switch SW
55
If Voltage_dip = 0
qinj qfrz
I = I
0
2 1
qinj qv
I = I
qinj
I = 0
If Voltage_dip = 1
If Thld > 0 & Voltage_dip = 0
go to State 2
If Thld , 0 & Voltage_dip = 0
stay in State 1 for Thld
seconds
After Thld seconds, go to
State 0
56. Type 3 WTG Aerodynamic Model
56
mech
P
From
wtgpt_a
θ Σ Σ
p a
K
0
θ m0
P
To
wtgt_a
+
+
-
-
P
57. Type 3 WTG Pitch Control Model
57
+
-
From
wtgt_a
t
ω
+
ord
P
ref0
P
+
+
pi
1
1+sT
θ
max
max
PI
& PIRAT
min
min
PI
& PIRAT
From
reec_a
+
+
+
+
max
PI
max
PI
max
PI
max
PI
min
PI
min
PI
min
PI
min
PI
cc
K
-
iw
K
ic
K
pw
K
pc
K
1
s
1
s
To
wtgar_a
s0
s1
pi10
pi20
s2
ref
ω
+
58. Type 3 WTG Torque Control Model
58
speed
e
f P
P
1 1
P , spd
2 2
P , spd
1 1
P , spd
4 4
P , spd
e
P
e
f P
P
1
1 sT
ref0
P
g
ω
(from drive train model)
ref
ω
ref
ω
1
1+sT
ip
K
s
pp
K
flag
T
1
0
ref
P
(To reec_a)
emax
T
emax
T
emin
T
emin
T
Freeze State Upon
Voltage Dip
+
+
+
+
-
-
0
s
1
s 2
s
59. Type 3 WTG Plant Control Model
59
branch
I
reg
V
branch
Q
branch
Q
reg c c branch
V - R + jX I
fltr
1
1 sT
cmpFlg
V
1
0
P
1
1 sT
fltr
1
1 sT
0
0
0
ref
P
1
c
K
0
1
flg
Ref
ref
Q
ref
V
dbd max
e
min
e
max
q
min
q
i
p
K
K +
s
ft
fv
1+sT
1+sT
ext
Q
branch
P
Pref
Plant
Freq
ref
Freq
fdbd1 fdbd2
dn
D
up
D
max
fe
min
fe
ig
pg
K
K +
s
max
P
min
P
lag
1
1+sT
flag
Frq
To
reec_a
s0
s1
s2 s3
s4
s5 s6
To
wtgtq_a
+
+
+
+
+
+
+
+
+
-
-
-
-
ref frz
2
2
If V < V then
freeze state s
ds dt = 0
60. Type 3 Generic Model
The generic model has seven modules
1. The renewable energy generator/convertor (regc_a) model,
which has inputs of active (Ipcmd) and reactive (Iqcmd)
current command and outputs of active (Ip) and reactive (Iq)
current injections into the grid model.
2. The renewable energy electrical controls (reec_a) model,
which has inputs of real power reference (Pref) that can be
externally controlled, reactive power reference (Qref) that can
be externally controlled and feedback of the reactive power
generated (Qgen). The outputs of this model are the real
(Ipcmd) and reactive (Iqcmd) current command.
60
61. Type 3 Generic Model
3. The emulation of the wind turbine generator drive-train
(wtgt_a) for simulating torsional oscillations. The output of
this model is speed (spd). In this case speed is assumed to be
a vector spd = [ωt ωg], where ωt is the turbine speed and ωg
the generator speed. The mechanical power/torque can be
varied through the action of the connected aero-dynamic
model.
4. A simple model of the wind turbine generator aerodynamics
(wtgar_a).
5. A simplified representation of the wind turbine generator
pitch-controller (wtgpt_a). This is similar to the existing type
3 pitch-control model, with the addition of one parameter
Kcc.
61
62. Type 3 Generic Model
6. A simple emulation of the wind turbine generator torque
control (wtgtrq_a).
7. A simple renewable energy plant controller (repc_a), which
has inputs of either voltage reference (Vref) and
measured/regulated voltage (Vreg) at the plant level, or
reactive power reference (Qref) and measured reactive power
(Qgen) at the plant level. The output of the repc_a model is a
reactive power command that connects to Qref on the reec_a
model. The model representation of the simplified plant
controller is shown on Slide 59
62
63. Type 1 Generic Model
63
Qgen
Pgen
Wind
Turbine
Model
Simple
Pitch-Controller
Model
Generator
Model
WT1A
WT1G
Shaft Speed
Pgen
Pm
Vt
64. Type 1 Generator Model
• The generator model used for this type of
WTG is very similar to the model for the
induction machine done in class
• Various transient stability packages have this
machine model included as the default model
for induction machines.
64
65. Type 1 Turbine Model
• Typically the two-mass representation of the wind
turbine shaft drive train is used (Slide 66). It
calculates the speed deviations of the rotor on the
machine and on the blade sides.
• By setting the turbine inertia fraction to zero the
model can be converted to a conventional single mass
model.
65
66. Two-Mass Model
66
Pm
Pe
t
t
g
g
g
t
tg
tg
0
0
Tm
Te
÷
÷
Σ
Σ
Σ
Σ
Σ K
Dshaft
t
1
2H
1
2Hg
1
s
1
s
1
s
d tg
+
+
+
+
+
+
+
-
-
-
-
+
From
Governor
Model
From
Generator
Model
0
s
1
s
2
s
67. Type 1 Pseudo-Governor Model
• The simple pitch controller model is shown on slide
68
• The model was designed and developed after a
thorough investigation of the aero-dynamic
characteristics and pitch control of several vendor
specific wind turbines.
• The arrangement shown on the next slide was
developed based on this exercise
67
68. Type 1 Simple-Pitch Model
68
t
V
r
1
1+sT
t
T=F V Flag1
min
P
0
P
1
s 1
1
1+sT
mech
P
max
r
min
r
Rate limit
1
0
+
-
69. Type 2 Generic Model
69
Qgen
Pgen
Wind
Turbine
Model
Simple
Pitch-Controller
Model
Generator
Model
WT2A WT2T
WT2G
Shaft Speed
Pgen
Vt
Rotor Resistance
Control Model
Pgen
RRotor
Aero Torque
70. Type 2 Rotor Resistance Control Model
70
Pgen
From
Generator
Model
Speed
from
Turbine
Model
p
p
K
1+sT
w
w
K
1+sT
Power-Slip Curve
+
ip
pp
K
K +
s
Rmax
Rmin
Rrotor
To
Generator
Model
71. Type 4 Generic Model
• The structure of the Type 4 generic model is shown in the
figure on Slide 72
• Several of the component models in this generic model are
identical to the ones used for the Type 3 generic model
71
74. Type 4 Generic Model
The Type 4 generic model has four key components:
1. The renewable energy generator/converter model
(regc_a), which has inputs of real (Ipcmd) and reactive
(Iqcmd) current command and outputs of real (Ip) and
reactive (Iq) current injection into the grid model.
2. The renewable energy electrical controls model (reec_a),
which has inputs of real power reference (Pref) that can
be externally controlled, reactive power reference (Qref)
that can be externally controlled and feedback of the
reactive power generated (Qgen). The outputs of this
model are the real (Ipcmd) and reactive (Iqcmd) current
command.
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75. Type 4 Generic Model
75
3. The emulation of the driven-train (wtgt_a) for
simulating drive-train oscillations. The output of this
model is speed (spd).
4. A simple renewable energy plant controller (repc_a),
which has inputs of either voltage reference (Vref) and
measured/regulated voltage (Vreg) at the plant level, or
reactive power reference (Qref) and measured (Qgen) at
the plant level. It also has inputs of reference power
(Plant_pref) and measure generated power (Pgen), and
reference frequency (Freq_ref) and measured frequency
(Freq). The outputs of the repc_a model are the reactive
power and real power command that connect to Qref and
Pref on the reec_a model.
