activation schemes of synthetic inertia controller on full
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Activation Schemes of Synthetic Inertia Controller on
Full Converter Wind Turbine (Type 4)
Francisco M. Gonzalez-Longatt
School of Electrical, Electronic and System Engineering
Loughborough University
Loughborough, United Kingdom
fglongatt@fglongatt.org
Abstract—One of the challenges in future energy systems is the
massive use of high power converters that decouple new energy
sources from the AC power grid, disabling natural frequency
response. This situation decreases the total system inertia
affecting the ability of power system to overcome system
frequency's disturbances. It has been established by the wind
power industry a controller to enable inertial response on wind
turbines generators (WTG) enabling the frequency response:
Artificial, Emulated, Simulated, or Synthetic Inertia. However,
there is a clear lack of knowledge about activation scheme used
for those controllers and how they work in practical manner.
This paper proposes three activation schemes for synthetic
inertia on WTG based on full converters: (i) Continuously
operating triggering, (ii) Under-frequency trigger and (iii)
Maximum-Frequency gradient trigger. Simulations over a test
system are used for a preliminary evaluation of the proposed
activation schemes. The main contribution of this paper is the
three schemes to activate the synthetic inertia controller and the
simulations results that demonstrate under-frequency trigger
provides good dynamic response.
Index Terms-- Frequency controller, frequency stability, power
system, protection scheme, wind turbine generator.
I. INTRODUCTION
Future energy systems will look completely different to
the power systems on nowadays [1]. High and low power
converters will be massively deployed almost everywhere
into on the electric network [2], [3] and for very different use:
(i) high power interfaces of the renewable energy produced
by highly variable generators, (ii) interface of several
technologies for energy storage, each one with very different
time constants, and (iii) interconnecting several synchronized
power systems, creating an Pan-European transmission
network which facilitate the massive integration of large-
scale renewable energy sources and the balancing and
transportation of electricity markets. The high/low power
converters typically tend to decouple energy sources from the
pre-existent AC power systems [3]. During a system
frequency disturbance (SFD) the generation/demand power
balance is lost, the system frequency will change at a rate
initially determined by the total system inertia (HT).
However, future power systems will increase the installed
power capacity (MVA) but the effective system inertial
response will stay the same nowadays [4]. The result is
deeper frequency excursions of system disturbances.
There are several good papers [1]-[3], [5], and technical
reports [6]-[7] dealing with theory [8]-[9], modelling [10] and
simulation [11] of inertial response of wind turbine
generators (WTG) and some of them provide general ideas
about possible impacts on power systems and there effects on
transient under-frequency response [12]-[13]. Even some
controls strategies have been proposed to mitigate the impact
of reduced inertia [14]. However, there is lack of knowledge
about control schemes used to activate the synthetic inertia.
This aim of this paper is to propose and to evaluate
activation schemes of synthetic inertia controller on full
converter wind turbine (FCWT) –Type 4. The paper is
organized as follows. Section II introduces the concept of
synthetic inertia and presents releasing “hidden” inertia
controller. Section III proposes three activation schemes for
the synthetic inertia: (i) Continuously Operating, (ii) Under-
frequency Trigger and (iii) Maximum-Frequency Gradient
Trigger. Section III the simulations results are used to assess
the impact of the proposed activation schemes on the system
frequency response and electro-mechanical variables on the
WTG. For illustrative purposes, the test system used in this
paper considers a large synchronous generator representing a
reduced model for a traditional power system and an
equivalent wind turbine representing the reduced equivalent
model of a small-size lossless wind farm. The main
contributions of this paper are: (a) proposing three schemes to
activate the synthetic inertia controller and (b) a preliminary
assessment of these schemes. Simulations results on a test
system demonstrates under-frequency trigger provide good
dynamic response. Finally, the advantages/disadvantages of
the activation schemes are discussed in Section IV.
978-1-4673-8040-9/15/$31.00 ©2015 IEEE
II. SYNTHETIC INERTIA
Modern WTGs use power electronics converters to enable
variable speed operation in order to capture wind energy over
a wide range of speeds. However, power converter isolates
the rotational speed from the system frequency so WTG
based on back-to-back AC/DC/AC converters offer no
natural response to system frequency [15], [10]. The WT
industry has created several controllers for modern WTG’s in
order to provide inertial response (and governor response on
some cases) for large frequency deviation for, short-duration:
Artificial, Emulated, Simulated, or Synthetic Inertial.
Examples of synthetic inertia controlled commercially
available for WTG are: General Electric WindINERTIA™
[16], ENERCON Inertia Emulation [17].
The objective of the synthetic inertia control is “to extract
the stored inertial energy from the moving part on WTGs”
[18]. There are several versions of synthetic inertia
controllers; however they can be classified in two main
approaches: (a) Releasing “hidden” inertia and (b) Reserve
capacity in pitch. In this paper the hidden inertia approach is
considered and it is named synthetic inertia from here.
Synthetic inertia concept allows a controller to the take the
kinetic energy from a WT rotating mass. This controller is
well-explained in several publications [11], [8]. It is control
loop that increases electric power output during the initial
stages of a significant downward frequency event. The active
power (inertial power, P) of the control is achieved by:
2 sys
syn sys
dfP H f
dt (1)
where Hsyn express the synthetic inertia (sec) and fsys system
frequency (p.u). Implementation of synthetic inertia
controller is depicted on Figure 1.
d
dK
dtsysf 2Hsyn
measP,r ref
,r meas
rPI
MPPTP
Conterter
Synthetic Inertia Controller
r
P
MPPT
refP
HsynP
System
Frequency
[p.u]
P
1 2
f
f f
K
s T s T
min
max
min
max
Figure 1. Representative diagram of Maximum Power Point Tracking
(MPPT) controller and Synthetic Inertia Controller (shadowed) [1].
Several publications relate the main aspects about
synthetic inertia [19], [18]; however, there is not a paper that
formally discusses the trigger mechanism to activate the
synthetic inertia controller. Three activation schemes are
presented and discussed in this paper:
A. Scheme I: Continuously Operating
This is the approach assumed in several publications [9],
[5], however is unrealistic. Many publications assume there is
not a triggering mechanism for the synthetic inertia
controller, in fact, it means the inertia controller receives
continuously a system frequency measurement (fmeas) signal
from the AC system and it is used to derivate the inertial
power (P) using (1) –see Figure 1. This is an unrealistic
control scheme because kinetic energy is taken from rotating
mass continuously and wind turbine is not allow the recover
its kinetic inertia in typical normal operation. However, this
scheme is included in this paper only for comparison
purposes.
B. Scheme II: Under-frequency Trigger
This activation scheme uses a trigger controller that
produces a trigger signal (ts) based on a comparator. The
controller compares the system frequency measurement (fmeas)
with a frequency threshold (fact), the output signal is
generated to activate the synthetic inertia controller if system
frequency measured is below the action frequency (fact). The
activation function of this controller is as follow:
0Trigger Signal:
1
meas act
meas act
ts f f
ts f f
(2)
C. Scheme III: Maximum-Frequency Gradient Trigger
This activation scheme uses a controller that is similar to
the typical logic control observed in ROCOF relays. It
measures the frequency and calculates df/dt, once the rate of
change of frequency exceeds the pre-determined setting
(df/dtact), a trip signal is initiated. The activation function of
the maximum-frequency gradient trigger for synthetic inertia
is defined by:
0
Trigger Signal:
1
meas
act
meas
act
df dfts
dt dt
df dfts
dt dt
(3)
The df/dtact is threshold that activates the synthetic inertia
controller. This approach has been used for years on ROCOF
relays and it is used on [20] to activate the synthetic inertia,
there is not discussion about its implication on that reference.
III. SIMULATIONS AND RESULTS
This section presents simulations and results over a Test
System. An equivalent synchronous generator (GS) and loads
are used as representative equivalent model of a traditional
power system and a small transmission system is included
considering two voltage levels. VSWT using an Electrically
Excited synchronous generator (EESG), Type 4-C, is used on
the simulation for demonstrative purposes. The output of the
generator is passed through the full rated power converter to
the grid. In this paper, an equivalent model of a cluster of
304.5MW direct-drive EESG is considered (similar
characteristic of the Enercon E-112). Figure 2 depicts the
general structure of a VSWT the model for the direct-drive
EESG. This model uses a back-to-back converter, details of
all models used can be found on [21]-[22]. The parameters
used for these models are escalated to simulate an equivalent
10 4.5 MW wind farm. DIgSILENT PowerFactoryTM [23]
is used for time-domain simulations and DIgSILENT
Simulation Language (DSL) is used for dynamic modelling
[24]. Figure 3 to 5 show the DSL models created for the
activation schemes considered in this paper.
Equivalent Windfarm10x4.5MW
Event
Hwt = 2.33 sec
Hgs = 10.0 sec
Rec Gen3.29 kV1.00 p.u.-5.18 deg
DC Bus7.26 kV1.10 p.u.0.00 deg
HV110.00 kV1.00 p.u.-0.00 deg
Gen3.30 kV1.00 p.u.-0.00 deg
MV20.00 kV1.00 p.u.
-150.63 deg
LV 3.31 kV1.00 p.u.
-147.54 deg
G~
WT
45.00 MW4.08 Mvar
7.91 kA
Tr1
-5.00 MW0.05 Mvar
0.14 kA
5.00 MW0.00 Mvar
0.03 kA
Load 50 MW
50.00 MW-0.00 Mvar
1.44 kA
Series Reactor
-45.00 MW0.00 Mvar
7.91 kA
45.00 MW4.08 Mvar
7.91 kA
Load 40 MW
40.00 MW-0.00 Mvar
0.21 kA
G~
GS
45.00 MW0.00 Mvar
0.24 kA
DC Capacitor
0.00 MW0.00 Mvar0.00 kA
Tr2
-45.00 MW-0.05 Mvar
1.30 kA
45.00 MW2.48 Mvar
7.87 kA
PWM Generator-Side
-45.00 MW0.00 Mvar7.91 kA
45.00 MW0.00 Mvar6.20 kA
PWM Grid-Side
45.00 MW2.48 Mvar
7.87 kA
-45.00 MW0.00 Mvar-6.20 kA
DIg
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Figure 2. Test System.
A SFD is applied to the test system and the response of the
main electromechanical variables on the power system
(equivalent synchronous generator -GS) side and wind farm
(WF) are shown on Figure 6. Simulations include three
activation schemes and base case (No control label on Figure
6) where no frequency support is provided by the wind farm.
The main impact of all considered activation schemes is
momentary reduce the active power on the synchronous
generator (GS).
Hidden Inertia: Continuous Operation
Inertia1st Order FilterDerivate
Synthetic Inertia Control
-
{Kp+Ki/s}Kp,Ki
Lmax
Lmin
-
1/(1+sT)Tfilter
Sink
2HH
1/Kfn
s
Hidden Inertia: Continuous Operation
0
1
2
3
fglongatt
PowerFactory 14.1.3
Evaluation of Trip Schemes for Synthetic Inertia
Francisco M. Gonzalez-Longatt Hidden Inertia:Continuous Operation
December 2012
Project:
Graphic: Graphic
Date: 12/6/2012
Annex:
Dwr
dwsys_dtFmeas wsys dwdt
Pin
P
wr
Delta_P
wr_ref
Pref
P_
mp
pt
DIg
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Figure 3. DSL Model Scheme I Hidden Inertia Trigger f: Trigger by Under Frequency
InertiaDerivate
Synthetic Inertia Control
1st Order Filter
Detectorfmin
1/(1+sT)Tfilter
Limit..
y_max
y_min
s 2HH
{Kp+Ki/s}Kp,Ki
Lmax
Lmin
-
Sink
1/Kfn
Hidden Inertia Trigger f: Trigger by Under Frequency
0
1
2
3
fglongatt
PowerFactory 14.1.3
Evaluation of Trip Schemes for Synthetic Inertia
Francisco M. Gonzalez-Longatt Hidden Inertia Controller. Trigger: dfdt
December 2012
Project:
Graphic: Graphic
Date: 12/6/2012
Annex:
DELTA_..
Trip_Signal
dwdtLI..dwdt
Pin
Pref
P_
mp
pt
Fmeas
Dwr
P
wr
wr_ref
DIg
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Figure 4. DSL Model Scheme II
Hidden Inertia Trigger dfdt: Trigger by Rate of Change of Frequency
Inertia
Synthetic Inertia Control
Derivate 1st Order Filter
s
Detectormax_dfdt
-
{Kp+Ki/s}Kp,Ki
Lmax
Lmin
-
1/(1+sT)Tfilter
Sink
2HH
1/Kfn
Hidden Inertia Trigger dfdt: Trigger by Rate of Change of Frequency
0
1
2
3
fglongatt
PowerFactory 14.1.3
Evaluation of Trip Schemes for Synthetic Inertia
Francisco M. Gonzalez-Longatt Hidden Inertia Controller. Trigger: dfdt
December 2012
Project:
Graphic: Graphic
Date: 12/6/2012
Annex:
dwdt
FILdwdt
Dwr
Fmeas
Pin
P
wr
wr_ref
Pref
P_m
ppt
DIg
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Figure 5. DSL Model Scheme III
9.99987.97995.95993.93991.9200-0.1000 [s]
1.0061
0.9996
0.9931
0.9866
0.9800
0.9735
[-]
No Control
Scheme II: Red
Scheme III: Pink
Seheme I: Blue
9.99987.97995.95993.93991.9200-0.1000 [s]
0.8803
0.7902
0.7000
0.6098
0.5197
0.4295
[p.u.]
No Control
Scheme I: Blue
Scheme II: Red
Scheme III: Pink
GS: Active Power (p.u.)
System Frequency (p.u.)
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9.99987.97995.95993.93991.9200-0.1000 [s]
1.0005
0.9990
0.9975
0.9960
0.9945
0.9930
[p.u.]
Scheme I
Scheme III
Scheme II
No Control
3.492 s 0.994 p.u.
3.580 s 0.995 p.u.
3.939 s 0.996 p.u.
9.99987.97995.95993.93991.9200-0.1000 [s]
0.5145
0.4986
0.4828
0.4669
0.4510
0.4352
[p.u.]
2.014 s 0.486 p.u.
1.726 s 0.482 p.u.
1.375 s 0.474 p.u.
6.277 s 0.440 p.u.
6.284 s 0.443 p.u.
6.284 s 0.442 p.u.
No Control Scheme I
Scheme III
Scheme II
WT: Rotor Speed (p.u.)
WT: Active Power (p.u.)
fglongatt Evaluation of Trip Schemes for Synthetic Inertia Results WT
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Figure 6. Simulation Results: (a) Grid Side and (b) Wind farm Side.
The activation scheme based on continuously merriment
signal (Scheme I) is the only scheme that includes a
negligible time delay on the activation of synthetic inertia, it
is an expected result because the control scheme is acting
continuously to any frequency change. The small time delay
found on the response is provided by the first order filter used
to remove the noise amplification on the derivate of the
measured frequency. Activation Scheme II and III require
activation condition must be satisfied before activate the
synthetic inertia controller, and it depends on the whole
system frequency response where the characteristics of the
traditional power system (equivalent synchronous generator)
has impact on the dynamic performance. Simulation results
show the under-frequency trigger activation (Scheme II)
produces the second faster activation time (ta ~ 2.0 ms) after
(a) Grid Side Results
(b) Wind Farm Results
the frequency disturbance detection (fact = 0.998 p.u.). The
activation time for maximum-frequency gradient trigger is the
longest (ta ~ 450 ms), it is because the df/dt depends on the
total system inertia. In this case the system’s inertia is large
compared with the system inertia provided by the controller.
Details of the wind turbine frequency response are shown
on Figure 7. The active power (PTW) values of the secondary
peak following the activation action are shown on Figure 7
and numeric values of the primary peak are presented on
Table I. 3.92643.31482.70332.09171.48020.8686 [s]
1.0062
0.9996
0.9931
0.9866
0.9800
0.9735
[-]
3.92643.31482.70332.09171.48020.8686 [s]
0.8597
0.8489
0.8380
0.8272
0.8164
0.8055
[p.u.]
3.854 s 0.850 p.u.
2.015 s 0.814 p.u.
1.369 s 0.826 p.u.
1.739 s 0.818 p.u.
1.144 s 0.847 p.u.
3.92643.31482.70332.09171.48020.8686 [s]
1.0062
0.9995
0.9927
0.9860
0.9792
0.9725
[-]
2.084 s 0.974
2.122 s 0.975
2.039 s 0.975
2.071 s 0.976
2.038 s 0.975 2.125 s
0.975
3.92643.31482.70332.09171.48020.8686 [s]
0.4955
0.4840
0.4726
0.4611
0.4496
0.4381
[p.u.] 2.007 s 0.486 p.u.
1.713 s 0.482 p.u.
1.362 s 0.474 p.u.
GS: Active Power (p.u.)
WT: Rotor Speed (p.u.)
WT: Active Power (p.u.) System Frequency (p.u.)
Scheme III
Scheme II
Scheme I
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2.57902.38432.18961.99491.80021.6055 [s]
1.0061
0.9996
0.9931
0.9866
0.9800
0.9735
[-]
2.57902.38432.18961.99491.80021.6055 [s]
0.8803
0.7902
0.7000
0.6098
0.5197
0.4295
[p.u.]
2.57902.38432.18961.99491.80021.6055 [s]
0.9844
0.9823
0.9801
0.9780
0.9759
0.9737
[-]
2.084 s 0.974
2.071 s 0.976
2.038 s 0.975
2.125 s 0.975
2.57902.38432.18961.99491.80021.6055 [s]
0.4955
0.4839
0.4723
0.4607
0.4491
0.4375
[p.u.]
GS: Active Power (p.u.)
WT: Rotor Speed (p.u.)
WT: Active Power (p.u.) System Frequency (p.u.)
Scheme III
Scheme II
Scheme I
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Figure 7. Details of wind farm active power production and system
frequency.
Inertial power provided by the activation Scheme III is the
largest and it is intrinsically related with the threshold df/dtact.
The initial peak of inertia power (PTW,max) after the activation
process is large in all cases and it is caused by the df/dt,
however, Scheme II exhibit a larger peak than Scheme I. This
initial peak indicates a quick response on releasing large
amount of kinetic energy in the rotating masses on the WT
caused by the power electronic converter. However,
discharging that kinetic energy to the grid is only for a short
period available, and potential dangerous consequences on
the mechanical parts must be seriously evaluated. A more
retailed impact of the activation schemes on the system
frequency response is evaluated considering the changes on
the frequency and power indicators shown on Table I.
TABLE I. MAIN INDICATORS OF FREQUENCY RESPONSE
No Control Scheme I Scheme II Scheme III
PWT,max (p.u) - 0.48172 0.48940 0.49292
tmax - 1.71217 1.01842 1.65683
fmin (p.u) 0.97404 0.97581 0.97500 0.97533
tmin (s) 2.08617 2.08417 2.12142 2.03583
df/dt (p.u/s) -0.02924 -0.03962 -0.03330 -0.03181
The effect of the activation scheme on the rate-of-change-
of-frequency and frequency nadir (fmin) is very important on
the system frequency stability. The positive effect of all
schemes is shown on Figure 8. The synthetic inertia modifies
the df/dt, however, activation Scheme II causes the slower
change (fmin ~ 0.975 p.u @ 2.1214 s) and Scheme I and
Scheme III produces almost the same change on fmin but
Scheme III reach the frequency nadir first (tmin = 2.035 s)
compared with Scheme II. The activation Scheme II, under-
frequency trigger, the best dynamic response in terms of
lowest frequency nadir and delaying the time where the nadir
is reached.
An important aspect about the impact of activation
schemes is the active power production of the traditional
synchronous generator. The long activation time on the
synthetic inertia controller caused by the Scheme III imply
traditional generator must quickly react to cope with the
system frequency disturbance, that situation makes this
activation scheme unpractical in a future electricity network
with low inertia. Scheme II produce a fast response and
initially reduce the active power solicitation form the
traditional generator, however, Scheme II provide the best
performance in term of release the requirements of active
power from the traditional generators.
3.92643.31482.70332.09171.48020.8686 [s]
1.0062
0.9996
0.9931
0.9866
0.9800
0.9735
[-]
3.92643.31482.70332.09171.48020.8686 [s]
0.8597
0.8489
0.8380
0.8272
0.8164
0.8055
[p.u.]
3.854 s 0.850 p.u.
2.015 s 0.814 p.u.
1.369 s 0.826 p.u.
1.739 s 0.818 p.u.
1.144 s 0.847 p.u.
3.92643.31482.70332.09171.48020.8686 [s]
1.0062
0.9995
0.9927
0.9860
0.9792
0.9725
[-]
2.084 s 0.974
2.122 s 0.975
2.039 s 0.975
2.071 s 0.976
2.038 s 0.975 2.125 s
0.975
3.92643.31482.70332.09171.48020.8686 [s]
0.4955
0.4840
0.4726
0.4611
0.4496
0.4381
[p.u.]
GS: Active Power (p.u.)
WT: Rotor Speed (p.u.)
WT: Active Power (p.u.) System Frequency (p.u.)
Scheme III
Scheme II
Scheme I
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Figure 8. Details of frequency response of the traditional power system side
considering the activation schemes.
The changes on the response of the inertial power and the
system frequency for several Hsyn is depicted in Figure 9.
Increased values of Hsyn increase inertial power contribution
and delay and reduce fmin. However, Scheme II is highly
affected by changes in Hsys and high values can cause loss of
synchronism of the EESG.
3.96413.34502.72592.10681.48770.8685 [s]
0.9786
0.9775
0.9765
0.9754
0.9743
0.9733
[-]
3.96413.34502.72592.10681.48770.8685 [s]
0.4592
0.4553
0.4515
0.4476
0.4437
0.4398
[p.u.]
1.702 s 0.458 p.u.
1.683 s 0.457 p.u.
1.708 s 0.453 p.u.
1.701 s 0.452 p.u.
1.659 s 0.450 p.u.
1.424 s 0.450 p.u.
3.96413.34502.72592.10681.48770.8685 [s]
1.00
0.80
0.60
0.40
0.20
0.003.96413.34502.72592.10681.48770.8685 [s]
1.00
0.80
0.60
0.40
0.20
0.00
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2.41382.29882.18382.06881.95381.8388 [s]
0.9760
0.9756
0.9752
0.9748
0.9744
0.9740
[-]
2.087 s 0.97403
2.086 s 0.97405
2.082 s 0.97413
2.082 s 0.97422
2.087 s 0.97441
2.082 s 0.97545
2.41382.29882.18382.06881.95381.8388 [s]
0.4592
0.4553
0.4515
0.4476
0.4437
0.4398
[p.u.]
2.41382.29882.18382.06881.95381.8388 [s]
1.00
0.80
0.60
0.40
0.20
0.002.41382.29882.18382.06881.95381.8388 [s]
1.00
0.80
0.60
0.40
0.20
0.00
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9.99947.97955.95973.93981.9199-0.1000 [s]
1.0113
1.0034
0.9954
0.9874
0.9795
0.9715
[-]
9.99947.97955.95973.93981.9199-0.1000 [s]
0.5204
0.5017
0.4830
0.4642
0.4455
0.4268
[p.u.]
1.38042 s0.48901 p.u.
1.38042 s0.47925 p.u.
1.35542 s0.46951 p.u.
1.40142 s0.45969 p.u.
Unstable
9.99947.97955.95973.93981.9199-0.1000 [s]
1.00
0.80
0.60
0.40
0.20
0.009.99947.97955.95973.93981.9199-0.1000 [s]
1.00
0.80
0.60
0.40
0.20
0.00
DIg
SIL
EN
T
3.03382.70832.38272.05721.73161.4061 [s]
0.9863
0.9834
0.9804
0.9774
0.9745
0.9715
[-]
2.13542 s0.97334
2.09017 s0.97404
2.09642 s0.97443
2.11642 s0.97481
2.12942 s0.97519
2.14542 s0.97556
3.03382.70832.38272.05721.73161.4061 [s]
0.5204
0.5017
0.4830
0.4642
0.4455
0.4268
[p.u.]
1.38042 s0.48901 p.u.
3.03382.70832.38272.05721.73161.4061 [s]
1.00
0.80
0.60
0.40
0.20
0.003.03382.70832.38272.05721.73161.4061 [s]
1.00
0.80
0.60
0.40
0.20
0.00
DIg
SIL
EN
T
3.56893.00452.44011.87571.31130.7469 [s]
1.0013
0.9956
0.9899
0.9842
0.9785
0.9727
[-]
3.56893.00452.44011.87571.31130.7469 [s]
0.4622
0.4577
0.4532
0.4487
0.4441
0.4396
[p.u.]2.02283 s0.45931 p.u.
2.01883 s0.45746 p.u.
2.02483 s0.45374 p.u.
2.03483 s0.45187 p.u.
1.96383 s0.45037 p.u.
1.65483 s0.46100 p.u.
1.01717 s0.45379 p.u.
3.56893.00452.44011.87571.31130.7469 [s]
1.00
0.80
0.60
0.40
0.20
0.003.56893.00452.44011.87571.31130.7469 [s]
1.00
0.80
0.60
0.40
0.20
0.00
DIg
SIL
EN
T
2.32342.22292.12232.02171.92121.8206 [s]
0.9757
0.9754
0.9750
0.9747
0.9743
0.9740
[-]
2.07283 s0.97439
2.07683 s0.97432
2.08183 s0.97418
2.08083 s0.97411
2.08283 s0.97405 2.08417 s
0.97404
2.32342.22292.12232.02171.92121.8206 [s]
0.4622
0.4577
0.4532
0.4487
0.4441
0.4396
[p.u.]2.02283 s0.45931 p.u.
2.32342.22292.12232.02171.92121.8206 [s]
1.00
0.80
0.60
0.40
0.20
0.002.32342.22292.12232.02171.92121.8206 [s]
1.00
0.80
0.60
0.40
0.20
0.00
DIg
SIL
EN
T
Figure 9. Details of frequency response of the inertial power and system’s
frequency considering the activation schemes and changes on Hsyn.
IV. CONCLUSION
This paper proposes three activation schemes for synthetic
inertia controller on WTG based on full rated power
converters: (i) continuously operating triggering, (ii) under-
Scheme I
Scheme II
Scheme III
frequency trigger and (iii) maximum-frequency gradient
trigger. Time-domain simulations over a simple test system
are used to evaluate the system frequency response
considering the activations schemes proposed. The main
electromechanical variables related to the frequency response
on the power system side and wind farm side have been
evaluated. Simulation results demonstrate the activation
scheme based on under-frequency trigger, provide the best
dynamic response in terms of lowest frequency nadir and
delaying the time where the nadir is reached. The inertial
power provided by the under-frequency scheme is lower than
using ROCOF, however, the most beneficial point is delay
the point of minimum frequency allowing a to governors in
traditional synchronous generators activate its response and
recover the system’s frequency. The value of the synthetic
inertia (Hsyn) must be carefully selected, beyond the physical
value, because it has important impact on the electro-
mechanical dynamic of the electrically excited synchronous
generator and the system frequency response.
The main contribution of this paper is the development of
three schemes to activate the synthetic inertia controller and
its assessment. Results demonstrate an outstanding system
frequency response when the synthetic inertia is activated
using the under-frequency trigger. The author is proposing
the under-frequency trigger and maximum-frequency gradient
trigger as main activation scheme considering the inertia
values on the system to be used. However, further evaluations
are required: (i) defining the optimal value of the trigger
frequency as a function of total system inertia, (ii)
determining the impact of this activation scheme on
frequency control and protection schemes, (iii) determining
the impact of activation schemes of synthetic inertia in a
detailed model of wind farms on system frequency response
considering
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