Transcript Control Principles and Dynamic Fault Analysis of a Fully Rated

International Conference on Renewable Energies and Power Quality (ICREPQ0 14)
Cordoba (Spain), 8th to 10th April, 2014
Renewable Energy and Power Quality Journal (RE & PQJ)
ISSN 2172-038 X, No.12, April 2014
Control Principles and Dynamic Fault Analysis of a Fully Rated Converter based
Wind Turbine Driving a Synchronous/Induction Machine
T. Abuaisha and J. Hanson
Institute for Electrical Power Supply with use of Renewable Energies (E5)
Technische Universit¨at Darmstadt
Landgraf-Georg-Straße 4, 64283 Darmstadt, Germany
Email: {tareq.abuaisha, jutta.hanson}@e5.tu-darmstadt.de
Abstract.
Integration of renewable energies in general and
wind energy in particular is increasing all over the globe.
Due to its flexible performance, good decoupling characteristics
and better controllability, Wind Turbine Generator based on
a Fully Rated Converter (WTG-FRC) stands as an attractive
choice among other wind turbine conversion systems. Throughout this paper, dynamic modelling and performance analysis
of the generic model of this particular WTG are achieved
using PSCAD/EMTDC. A comparison through simulation results
between WTG-FRC driving a permanent magnet synchronous
machine (PMSM) and WTG-FRC driving a wound rotor induction machine (WRIM) in terms of control principles and dynamic
fault analysis is achieved.
Keywords
fully rated converter, wind turbine generator, type-4 WTG,
permanent magnet synchronous machine (PMSM), wound
rotor induction machine (WRIM)
1. Introduction
For transmission and/or distribution system operators to
be able to play their role in the grid, they need to carry
out proper simulations and emulations on the concerned
power system. These simulations do vary from the very
complex and detailed types (e.g. electromagnetic transient
simulation) to the very basic and preliminary types (e.g.
load flow simulation).
In order to carry out these tasks, system operators
need to model the physical behaviour of the equipments
existed in the grid. As a result, efficient models of wind
turbines and wind farms which are capable of describing
the physical behaviour in a proper manner is a substantial
requirement [1].
Generally manufacturer-specific models are available.
However, these models usually contain much proprietary
information which is being represented by a black-box.
Access to these models requires a non-disclosure agreement between the dynamic model user and the turbine
manufacturers [2].
In response to this challenge, generic wind turbine
generator models for all the different types of wind turbine generators (WTGs) have been developed [3]. The
main purpose of generic models is to have worldwide
manufacturer-independent WTG models that can be distributed, implemented and improved without any restrictions due to intellectual proprietary [2].
Throughout this research work, the generic model of
type-4 WTG based on a fully rated converter (FRC)
has been implemented and simulated in PSCAD/EMTDC.
The control principles and dynamic fault behaviour of
a permanent magnet synchronous machine (PMSM) are
compared with those of a wound rotor induction machine
(WRIM).
In order to analyse the dynamic behaviour of type-4
generic model, a three-phase fault in which the voltage
drops to around 50% of its initial value is studied as an
example.
2. Type-4 WTG Technology
Type-4 WTG utilizes the power electronics to provide
high controllability and efficiency. It is typically equipped
with forced commutated pulse width modulated (PWM)
inverters and rectifiers, or completely with PWM voltage
source converters (VSCs) in new designs (adopted in this
research), to provide a fixed voltage and fixed frequency
and apply pitch control of the turbine blades [4], [5].
Type-4 WTG consists of a converter connected in series
between the generator and the grid. This converter must
withstand the full power rating of the generator, and thus
this configuration is known as fully-rated converter (FRC)
based wind turbine. The typical structure of this WTG is
described in Fig. 1.
The inductor L shown in Fig. 1 is known as coupling
inductor, it is used to connect the grid-side VSC to the
grid. As shown, FRC-based WTG consists of a generator,
two pulse width modulated (PWM) voltage source converters (VSC) with a back-to-back DC link, wind turbine
and a control system.
The function and operation of these components are explained as follows [5]:
• Generator: it can have a wound field, wound rotor
synchronous generator (WRSG) or can use permanent
magnets, permanent magnet synchronous generator
(PMSG) to provide the rotating magnetic field. Or it
can even be an induction generator. A purpose of this
Wind
Turbine
Fully Rated Converter (FRC)
z
}|
{
Machine-side Converter
Grid-side Converter
(MSC)
(GSC)
PMSM or
WRIM
Optional
PE
PM
+
AC
UDC
G
CH
UR
Gear
box
AC
URS
Pitch Control (β ◦ )
Grid
PE : Electrical Power
PM : Mechanical Power
CH : Chopper
UGS
PWM MSC
Transformer
L
US
−
Machine
Terminal
3 Phase fault at
low voltage side of Transformer
(WTG Terminal)
PWM GSC
Subscripts
UDC , Q, US
{P , Q} or {P , UR }
Control System
R : Rotor, S: Stator
RS : Rotor Side, GS : Grid Side
Fig. 1. Detailed structure of a FRC-based wind turbine.
UDC
UL1
(abc to dq)
L1
d
UL2
L2
UL3
L3
Park
Transform
q
Y
0.5
N/D
M
P
X
Y
(dq to abc)
L1
d
M
P
X
q
L2
Inverse
L3
Park
Transform
UL1, ref
UL2, ref
UL3, ref
Fig. 2. Calculation of modulation references in FRC-WTG generic model as implemented in PSCAD/EMTDC.
1
research work is to compare between the performance
of the synchronous and induction machines in terms
of control principles and dynamic fault analysis.
• Voltage Source Converters (VSCs): are two backto-back pulse width modulated (PWM) converters
based on multi-chip IGBT modules, inter-connected
through DC-link capacitors. The DC-link capacitor
decouples the two unsynchronized AC systems.
• Control System: it is employed to insure the desired
functionality of the FRC-based wind turbine as follows:
– Pitch angle (β) command is used to insure
maximum wind power extraction by the turbine
blades.
– Voltage signal (URS ), which is intended to control
the rotor-side VSC.
– Voltage signal (UGS ), which is intended to control
the grid-side VSC.
Note that the role of these two signals in controlling
the VSCs depends on the type of the employed
machine i.e. synchronous or induction machine.
The variable magnitude, variable frequency output of the
generator (UR ) is converted to a fixed magnitude, fixed
frequency voltage (US ) by the fully-rated converter consisting of a rotor-side VSC, a grid-side VSC and a DC
link capacitor. Both converters are designed to handle the
full rated power of the wind turbine.
One of the modern and most efficient algorithms to
control the rotor-side and grid-side VSCs is the PWM
technology [6]. The converters adjust the phase and mag-
Table I. Parameters used in the synchronous and induction
machines
Parameter Definition
Unit
PMSM
WRIM
Rated complex power
Rated voltage
Rated frequency
Stator resistance
Mechanical damping
Leakage reactance
MVA
V
Hz
p.u.
p.u.
p.u.
1.5
600
50
0.017
0.055
0.064
1.5
600
50
0.02
0
0.1
nitude of the injected phase voltages to achieve the abovedescribed requirements.
3. Modelling and Simulation of Type-4
WTG
A wide variety of FRC-WTGs exists, either by using
synchronous machines wound rotors, permanent magnets
or induction machines. Either with gearbox or with a
higher number of poles and a reduced gearbox or even
without gearbox [7]. Within this section a FRC-WTG
driving a permanent magnet synchronous machine and
an induction machine is implemented and simulated.
Afterwards, based on the simulation results the control
principles used in voltage source converters of both WTGs
are distinguished. Table I shows the parameters which are
used to simulate the synchronous and induction machines.
Fig. 2 shows the procedure for the calculation of
the modulation references inside the FRC-WTG generic
model. Thereafter, the modulation references are used to
create the interpolated firing pulses which are needed to
UL2
L2
UL3
L3
Clarke
Transform
sTk
G. 1+sT
k
β
sTk
G. 1+sT
k
Uα
1
sTk
|Φ| : Flux Magnitude
Φα
X
Y
P
Uβ
1
sTk
Y
M
M
P
X
|Φ|
θΦ
θΦ : Flux Estimated
Angle
Φβ
Fig. 3. Estimation of stator flux position in FRC-WTG generic
model.
0.5
0
0.5
-1
4.1
d
L2
q
L3
Park
Transform
0
Uq
≈0
d
L2
q
L3
Park
Transform
MW
P (t) =
Q(t) =
L1
0
4.14
4.16
4.18
4.2
Ud
θΦ
Ampere
I
L1


IL2


IL3
4.12
(a) Three-phase line voltages and DC-link voltage
3
2 . Vd (t)Id (t) + Vq (t)Iq (t)
3
2 .(Vq (t)Id (t) − Vd (t)Iq (t))
Id
Iq
Pm
⇒ pu
1
Pnom
Mvar
Qm
m: Measured
≈0
1
3 Phase line currents in kA
L1
UL1
UL2
UL3
UDC
1
θΦ
V olt
U
L1


U
L2


UL3
PMSM, during steady state, measured at WTG terminal
1.5
3 Phase line voltages in kV
DC Voltage in kV
UL1
(abc to αβ)
L1
α
4.12
4.14
4.16
4.18
800
4.2
P
Q
Active Power in kW
Reactive Power in kvar
600
400
200
0
-200
4.1
4.12
4.14
4.16
4.18
4.2
(c) Active and reactive power
Fig. 5. Simulation results of the FRC generic model with a
permanent magnet synchronous machine (PMSM) measured at
WTG terminal.
WRIM, during steady state, measued at WTG terminal
1
3 Phase line voltages in kV
The implemented voltage source converters (VSCs)
consist of two fully controlled IGBT bridge circuits, thus
they are referred to as six pulse voltage source converters.
Fig. 5 shows the simulation results of the FRC-WTG
when driving a permanent magnet synchronous machine
recorded at the WTG terminal (see Fig.1), whereas Fig. 6
shows the simulation results when a wound rotor induction
machine is employed measured at the same point.
While Fig. 5a shows the three-phase line voltages along
with the DC-link voltage for the PMSM, Fig. 6a shows
the three-phase line voltages for the WRIM. The DC-link
reference voltage is 1100 V. Fig. 5b shows the three-phase
currents for the PMSM topology. From these figures it can
be seen that through controlling the modulation indices,
the pulse width modulation (PWM) provides control over
voltage and frequency and thus the desired output AC
voltage can be accomplished.
On one side, Fig. 5c shows the active and reactive power
for the PMSM measured at the WTG terminal (see Fig.1).
The reference active and reactive power are assumed to be
600 kW and 0 kvar respectively.
On the other side, Fig. 6b shows the active and reactive power for the WRIM under the same operating
conditions. From this figure it can be realized that the
induction machine absorbs reactive power to produce the
required magnetic flux, and hence power factor correction
capacitors are often placed at the generator terminals
-0.5
UL1
UL2
UL3
0.5
0
-0.5
-1
4.1
4.12
4.14
4.16
4.18
4.2
(a) Three-phase line voltages
800
P
Q
600
Active Power in kW
Reactive Power in kvar
A. Simulation Results during Steady-state Situation
0
(b) Three-phase line currents
400
200
0
-200
-400
4.1
4.12
4.14
4.16
4.18
4.2
(b) Active and reactive power
1.00845
Rotor Speed in pu
4. Exemplary Results
0.5
-1
4.1
Fig. 4. Active and reactive power calculation in dq frame.
trigger the IGBTs in the grid side and machine side voltage
source converters.
In order to perform the required alignment between the
synchronous rotating frame and the stator magnetic flux,
the instantaneous angular position of the stator magnetic
flux (θφ ) has to be estimated. The procedure used for
estimating the angle is shown in Fig. 3. A possible
approach to calculate the instantaneous active and reactive
power is through dq reference frame [8]. This approach is
explained in Fig. 4.
Section 4 will present the simulation results of the FRCWTG generic model.
IL1
IL2
IL3
ωm
1.00845
1.00844
1.00844
1.00844
1.00843
4.1
4.12
4.14
time [sec]
4.16
4.18
4.2
(c) Machine rotor speed
Fig. 6. Simulation results of the FRC generic model with
a wound rotor induction machine (WRIM) measured at WTG
terminal.
when induction machines are employed in wind turbine
generators.
Fig. 6c shows the rotor speed of the WRIM under
stable operating conditions, and thus it should be around
1 p.u. For all the simulations performed throughout this
research, the wind speed is assumed to be constant during
the simulation time period which is around 10 sec. This
presents an appropriate time-frame for short-term voltage
stability studies which typically considers a time-frame
between 5 and 30 sec [9].
are set to 0.01 Ω and 1016 Ω respectively.
In VSCs the angle or frequency and amplitude of
the imposed generated waveforms are controlled through
controlling the modulation indices (mGS and mRS ) of the
PWM as explained in equation 1.
U GS, L1L2L3
mGS = kGS .
,
UDC
U RS, L1L2L3
,
mRS = kRS .
UDC
B. Simulation Results during a Three-phase Fault
In order to highlight the differences in control principles
and dynamic fault behaviour, a three-phase to ground fault
was caused at the low voltage side of the transformer.
Afterwards, the performance of the FRC-WTG driving a
PMSM is analysed during fault and then compared with
the case when driving a WRIM. The complete simulation
time period is 10 sec, whereas the fault time period is 2
sec (from 4 - 6 sec). The ON and OFF fault resistances
PMSM, under three-phase fault, measured at WTG terminal
Fault duration
Approach stability
Pre-fault region
mGS and mRS are the modulation indices of the grid
and machine side converters respectively
kGS and kRS are the proportionality constants
U GS, L1L2L3 are the three-phase complex voltages
measured at the grid side converter
U RS, L1L2L3 are the three-phase complex voltages
measured at the rotor side converter
•
•
•
•
2
1
0.5
0
UL1
UL2
-1
UL3
3 Phase fault
UDC
2
3
4
5
Fault cleared
6
7
Post-fault region
0.5
Post-fault region
3 Phase fault
-1
2
8
3
4
5
6
Fault cleared
7
8
(a) Three-phase line voltages and DC-link voltage
4
4
Approach stability
Pre-fault region
3 Phase line currents in kA
Fault duration
2
0
Post-fault region
IL1
-2
-4
WRIM, under three-phase fault, measured at WTG terminal
Fault duration
Approach stability Pre-fault region
0
(a) Three-phase line voltages and DC-link voltage
3 Phase line currents in kA
UL1
UL2
UL3
UDC
1.5
1
-2
where:
3 Phase line voltages in kV
3 Phase line voltages in kV
2
(1)
3 Phase fault
IL3
3
4
5
Fault cleared
6
7
Post-fault region
IL1
IL2
IL3
-4
2
8
3
(b) Three-phase line currents
Active Power in kW
Reactive Power in kvar
Active Power in kW
Reactive Power in kvar
5
Fault cleared
6
7
8
500
0
-500
-1000
-1500
2
4
5
6
7
-500
-1000
P
Q
3
0
8
P
Q
-1500
2
(c) Active and reactive power at WTG terminal
3
4
5
6
7
2000
Active Power in kW
Reactive Power in kvar
600
400
P
Q
200
8
(c) Active and reactive power at WTG terminal
800
Active Power in kW
Reactive Power in kvar
3 Phase fault
4
(b) Three-phase line currents
500
P
Q
1000
0
-1000
0
-200
Approach stability Pre-fault region
0
-2
IL2
2
Fault duration
2
2
3
4
5
time [sec]
6
7
8
-20002
3
4
5
time [sec]
6
7
8
(d) Active and reactive power at machine terminal
(d) Active and reactive power at machine terminal
Fig. 7. Results of the FRC generic model driving a permanent
magnet synchronous machine (PMSM) under a three-phase balanced fault.
Fig. 8. Simulation results of the FRC generic model driving
a wound rotor induction machine (WRIM) under a three-phase
balanced fault.
Fig. 7 shows the simulation results of the FRC-WTG
generic model when it is driving a PMSM. Whereas, Fig.
8 shows the simulation results of the FRC-WTG when it
is driving a WRIM.
The results are measured at the low voltage side of the
transformer (WTG terminal) except for Fig. 7d and Fig.
8d, where they are measured at the machine terminal (refer
to Fig.1).
Subsequently after the fault; the voltage falls down and
the current rises up for both machines. This can be seen
in Fig. 7a and Fig. 7b for the PMSM and in Fig. 8a and
Fig. 8b for the WRIM topology, respectively.
As a result to the increased rotor currents, the DC-link
voltage tends to increase (see magnified parts of Fig. 7a
and Fig. 8a). Thus, the grid side VSC tries to balance the
DC-link voltage by forcing it to decrease again, and at the
same time it supports the AC voltage during fault.
Moreover, the machine side converter in the PMSM will
control the active and reactive power by forcing them to
stabilize at their reference values i.e. 600 kW and 0 kvar
respectively, this control action is obvious in Fig. 7d.
As already mentioned, the WRIM absorbs reactive
power depending on its characteristics and operating point.
Thus when a reactive power control that does not match
the machine requirement is forced, the machine can enter
an unstable situation.
Due to this fact, the machine side VSC can not perform
control on reactive power, as a result the active and reactive
power at the machine terminal will not stabilize at their
reference values during the fault as seen in Fig. 8d.
In the PMSM when the speed and hence the frequency is
set to a reference point, the machine terminal voltage will
be determined thereupon. And because of the permanent
magnet there will be no voltage control. However, by
performing reactive power control at the machine side
VSC, the voltage will be adjusted to keep the reactive
power at the set point i.e. reactive power and voltage
control accordingly.
5. Conclusion
Due to its flexible performance, good decoupling characteristics and better controllability, FRC-based WT represents an attractive choice among other wind turbine
conversion systems. In this paper, the type-4 (FRC)
generic model has been analysed and simulated in
PSCAD/EMTDC.
In order to distinguish the control principles and to
compare the dynamic fault behaviour, a three-phase to
ground fault was caused at the WTG terminal. Afterwards,
the performance of the FRC-WTG driving a PMSM is
analysed during fault and then compared with the case
when driving a WRIM.
The simulation results proved that when a WRIM is
employed, in this case the machine side VSC can not
perform control on reactive power otherwise the machine
can go unstable. Whereas reactive power control, and
thus an indirect voltage control, on machine side VSC is
possible for the FRC-WTG driving a PMSM.
The grid side converter for both machine topologies
controls the DC-link voltage and also AC voltage of the
grid. While in PMSM topology the machine side converter
controls active power and possibly reactive power, in
WRIM it can control the voltage or active power but not
the reactive power.
Acknowledgment
The authors would like to thank Manitoba HVDC
Research Center for allocating the fully rated converter
generic model which served as benchmark for the analysis
and simulations throughout this research.
References
[1] I. Erlich, F. Shewarega, and O. Scheufeld, “Modeling wind
turbines in the simulation of power system dynamics,” in
Scientific Journal of Riga Technical University, Power and
Electrical Engineering, vol. 27, 2010, pp. 41–47.
[2] NERC, “Standard models for variable generation,” NERC,
116-390 Village Blvd., Princeton, NJ 08540-5721, USA,
Tech. Rep., August 2010, North American Electric Reliability Corporation.
[3] A. Ellis, Y. Kazachkov, E. Muljadi, P. Pourbeik, and
J. Sanchez-Gasca, “Description and technical specifications
for generic wtg models − a status report,” in IEEE/PES
Power Systems Conference and Exposition (PSCE), May
2011, pp. 524–529, WECC Working Group on Dynamic
Performance of Wind Power Generation & IEEE Working
Group on Dynamic Performance of Wind Power Generation.
[4] A. Dadhania, “Modeling of doubly fed induction generators for distribution system power flow analysis,” Master’s
thesis, Ryerson University, Toronto, Ontario, Canada, 2010.
[5] J. Machowski, J. W. Bialek, and J. R. Bumby, Power System
Dynamics: Stability & Control, 2nd ed. UK, Poland: John
Wiley & Sons Ltd, 2008, pp. 265–297.
[6] J. Holtz, “Pulsewidth modulation-a survey,” IEEE Transactions on Industrial Electronics, vol. 39, no. 5, pp. 410–420,
December 1992.
[7] H. Polinder, D.-J. Bang, H. Li, and Z. Chen, “Concept
report on generator technologies, mechanical & electromagnetic optimization,” TUD & AAU, Mekelweg 4, 2628CD
Delft, Netherlands & Aalborg East DK-9220, Denmark,
Tech. Rep., 2007, d 1B2.b.1.
[8] K. J. Faria, “Doubly-fed induction generator based wind
power plant models,” Master’s thesis, The University of
Texas at Austin, Austin, Texas, USA, 2009.
[9] A. Perdana, “Dynamic models of wind turbines,” Ph.D. dissertation, Chalmers University of Technology, Goeteborg,
Sweden, 2008, a Contribution towards the Establishment of
Standardized Models of Wind Turbines for Power System
Stability Studies.
[10] I. S. Wander, “Modeling of synchronous generator and fullscale converter for distribution system load flow analysis,” Master’s thesis, Ryerson University, Toronto, Ontario,
Canada, 2011.