International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
Comparative Study Between Power System Blockset and
PSCAD/EMTDC for Transient Analysis of Custom Power Devices Based on Voltage Source Converter
W. Freitas, and A. Morelato
Dept. of Electrical Energy Systems, State University of Campinas, Campinas, Sao Paulo, C.P. 6101,
13081-970, Brazil (e-mail: walmir@ieee.org)
in the next sections are the same presented in [5]. Such fact
become easier to validate the results obtained here.
Abstract – This paper presents a comparative study between
two commercial programs considering transient analysis of
custom power devices based on voltage source converters.
The programs investigated were the Power System Blockset
for use with Matlab/Simulink, which employs state-variable
analysis, and PSCAD/EMTDC, which is based on nodal
analysis. The objective is to determine the main differences
between them considering computation time, easiness of implementation of the necessary models, evaluation of the existent libraries and accurateness of results. The custom power
devices analyzed were the DSTATCOM and the DVR. In all
studies presented, such devices were simulated by using detailed models, i.e. the switching elements IGBTs/diodes and
the PWM signal generator were explicitly represented. In the
end of work, the main advantages and disadvantages of each
program are argued.
II. CUSTOM POWER DEVICES
Custom power concept has been proposed to ensure high
quality of power supply in distribution networks using
power electronics devices [1], [2]. Additionally, various
custom power devices are based on the voltage source
converter technology [6], [7]. Thus, two voltage source
converter-based devices were investigated in this work.
The chosen devices were the DSTATCOM and the DVR.
In distribution voltage level, usually, the employed switching element is the IGBT (Integrated Gate Bipolar Transistor), due to its lower switching losses and reduced size.
Moreover, the converter rating employed in these devices
is relatively low. Hence, the output voltage control can be
executed through PWM (Pulse Width Modulation) switching pattern, reducing the low order harmonic generation.
Furthermore, here, the converters are indirectly controlled,
i.e. only the output voltage angle is controlled and the
magnitude remains proportional to the dc voltage [2],[6].
Keywords – Custom power devices, DSTATCOM, DVR, Power
System Blockset, PSCAD/EMTDC.
I. INTRODUCTION
Recently, various power electronic devices have been
proposed especially to be applied to medium voltage networks, generally named Custom Power [1], [2]. Among
these new devices, special attention has been given to the
family based on the voltage source converter technology
due to several attractive features, such as faster response,
output little influenced by network variables and possibility of utilization together with energy storage devices, allowing active and reactive power compensation simultaneously. Two devices belong to this kind of equipment are
the DSTATCOM (Distribution Static Synchronous Compensator) and the DVR (Dynamic Voltage Restorer).
Generally, such devices have been used to improve
power quality and reliability aspects. Whatever the application being considered, it is necessary to carry out electromagnetic studies to predict the dynamic behavior and to
project suitably these devices. Nowadays, these transient
studies are usually accomplished through digital simulation.
In this context, various approaches have been developed
and implemented for the formulation and solution of the
network equations. Broadly, such approaches can be classified on methods based on state-variable analysis or nodal
analysis. Power System Blockset (PSB) for use with Matlab/Simulink employs state-variable analysis [3], whereas,
PSCAD/EMTDC is based on analysis nodal [4]. Thus, in
this work, such tools were tested during transient analysis
of the previously mentioned custom power devices. It is
important to declare that all models and cases investigated
A. DSTATCOM
A DSTATCOM (Distribution Static Synchronous Compensator), which is schematically depicted in Fig. 1, consists of a voltage source converter connected in shunt to
the distribution network through a coupling transformer
[1]- [3]. Such configuration allows the device to absorb or
generate controllable active and reactive power. The
DSTATCOM has been utilized mainly for regulation of
voltage, correction of power factor and elimination of current harmonics [2], [5].
Vi
Vj
Zij
Ztr
converte
Vdc
Fig 1 DSTATCOM structure.
1
International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
A. Power System Blockset
Here, such device is employed to provide continuous
voltage regulation using an indirectly controlled converter
[6]. The controller is shown in Fig. 2. The controller input
is an error signal obtained from the reference voltage and
the value rms of the terminal voltage measured. Such error
is processed by a PI controller and the output is the angle δ,
which is provided to the PWM signal generator. This controller is the same employed in [5]. It is important to note
that in this case, indirectly controlled converter, there is
active and reactive power exchange with the network simultaneously [2], [5]-[7].
Vset
+
Power System Blockset for use with Matlab/Simulink is
based on state-variable analysis and employs either variable or fixed integration-step algorithms [3]. In this software, the dynamics of the linear part of the electrical circuit are represented by continuous or discretized timedomain state-space equations. Additionally, the non-linear
part of electrical circuit is solved separately using predefined models, and combined with the solution of the
linear part. In this simulation package there are various
electrical system apparatus, electrical machines and power
electronics components. Furthermore, such tool has a
graphical interface very friendly. The version 2.2 was
adopted in this work.
δ
PI
B. PSCAD/EMTDC
VT
Fig 2 Indirect controller.
PSCAD/EMTDC employs the well know and established nodal analysis together with trapezoidal integration
rule with fixed-step algorithms [4]. The version 3.0, which
is employed in this work, is written using Fortran 90, it has
allowed a speed-up of the computing time comparing to
older versions. Such software present various electrical
system apparatus, electrical machines and power electronics components. The graphical interface is also very well
developed.
B. DVR
A DVR (Dynamical Voltage Restorer) also consists of a
voltage source converter connected to the distribution network through a transformer. On the contrary to the
DSTATCOM, the transformer is connected in series with
the distribution line. In general, the DVR has been employed to protect critical and sensitive loads against short
duration voltage dips and swells as well as to eliminate
voltage harmonics [2], [5]. The controller structure employed here is the same presented in Fig. 2. The objective
is to avoid voltage sags during short-circuits in the network.
Moreover, it is considered that the DVR acts only during
fault period, on the contrary, it is considered by-passed.
Vi
Ztr
IV. COMPUTER SIMULATIONS
A. DSTATCOM
The test system employed to carry out the simulations
concerning the DSTATCOM actuation is shown in Fig. 4,
which is the same system presented in [5]. Such system is
composed by a 230 kV, 50 Hz transmission system, represented by a Thévenin equivalent, feeding a distribution
network through an 3-winding transformer connected in
Y/Y/Y, 230/11/11 kV. To verify the working of a
DSTATCOM, a variable load is connected at bus 2 and a
three-phase capacitor bank at bus 1. During the simulation,
in the period from 300 to 600 ms, the switch 1 is closed
and from 900 to 1200 ms the switch 2 is closed, which
remain closed until of the end of the simulation
Vj
converter
Vdc
Vs
2
1
Zs
Fig 3 DVR structure.
S1
S2
III. ELECTROMAGNETIC TRANSIENT ANALYSIS
Several electromagnetic transient analysis computational
tools have been developed during the last decades. Consequently, there are various approaches for the formulation
and solution of the network equations. In a simplified way,
such approaches can be divided in state-variable analysis
and nodal analysis. Based on such fact, two programs are
analyzed here. Power System Blockset for use with Matlab, which employs state-variable analysis, and
PSCAD/EMTDC, which is based on nodal analysis.
L1
3
L2
converter
capacitor
(Y)
vdc
Fig 4 One-line diagram of the test system 1.
The implementation of this system using PSB and
PSCAD/EMTDC are shown in Fig. 5 and 6 respectively.
The main components are described to follow.
2
International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
Brk
1
A
A
19 kV
powergui
A
+
750.0
B
2
C
B
B
C
Vrms
Vrms
Discrete system
Ts=1e-005
B
Switch.
ABC
3
C
Brk
Brk3
1
C
B
pulses
C
A
DSTATCOM
A
-
z
a2
A
b2
A
N
B
Pulses Signal(s)
c
Vinv _ref
4
Vrms
Vrms
Control
System
Discrete
PWM Generator
Load 1
(b) DSTATCOM.
C
Measures
A
A
A
B
Switch.
B
ABC
B
C
C
A
B
B
C
A
B
C
C
1
C
Brk1
Vinv f
PI
delta
Mag
Vinv_ref
Load 2
1
abc
Reference Converter
Voltage
C
A
B
C
c
100 MVA
230/11/11 kV
C
B
c3
Switch.
ABC
Brk2
B
C B
b3
C
A
A
B
a3
C
A
A
c2
B
230 kV
A
Vrms
Phase
1
Vref
C
B
A
3.0
(c) indirect controller.
1
(a) test system.
Mag
1
pi/180
sin
delta
degrees -> rad
-120*(pi/180)
12:34
2*pi*50
1
sin
120*(pi/180)
Vinvf
sin
Gain
(d) phase-modulation of the control angle δ..
Fig 5 DSTATCOM and associated controllers implemented in PSB.
Timed
Breaker
Logic
Closed@t0
BRK
Brk
GCn
GBn
GAn
delta
750.0
GCp
GBp
GAp
Timed
Breaker
Logic
Open@t0
Brk3
A
B
GAp
GAp
GAn
GAn
PWM GBn
Signals
GBn
GBp
GCp
GCp
GCn
GCn
C
Vpu_rms
GBn
Control
System
delta
Brk3
Vpu_rms
Brk
3 Phase
RMS
Load 1
A
A
A
R=0 0.1
0.758
0.1
0.758
B
C
B #1
B
C
C
0.1
A
A
#3 B
B
C
C
100.0 [MVA]
230.0 11.0 [kV]
11.0
0.758
Brk1
Timed
Breaker
Logic
Open@t0
12.1
0.1926
12.1
0.1926
12.1
0.1926
Load 2
A
Vpu_rms
B
C
C
Timed
Breaker
Logic
Open@t0
Brk2
Brk2
Graphs
3.0
3.0
B
A
3.0
Brk1
0.05
0.0059
0.05
0.0059
0.05
0.0059
(a) test system.
Vpu_rms
Vpu_rms
ma
delta
delta
P
1
Vref
D
+
*
*
−
I
F
Vpu_rms
CrtlA
delta
Phase
Control System
delta
delta
Mag Sin
CrtlA
Freq
CrtlB
Freq
delta
delta
A
Compar−
ator
GAp
B
GAn
A
GBp
Compar−
ator
B
GBn
A Compar−
ator
B
GCp
D
COMP
−120
−1
F
+
TIME
A
0.1
B
Compar−
ator
COMP
CrtlC
Sin
Mag
Freq
CrtlB
Freq
(b) indirect controller.
1
50
ma
120
Freq
ma
F
delta
D
+
+
Phase
Mag Sin
Freq
CrtlC
Freq
(b) PWM signal generator.
Fig 6 DSTATCOM and associated controllers implemented in PSCAD/EMTDC.
3
GAn
GBp
GBn
GAp
GAn
GBp
GBn
+
Phase
ma
GAp
GCn
GCp
GCn
GCp
GCn
International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
mention that there is no filtering equipment in this simulation. Moreover, the results obtained using PSB and
PSCAD/EMTDC are very close and the results obtained
here are very similar to the results presented in [5]
All necessary elements to implement the network; for
example, three-phase transformers, circuits breakers, capacitors; are existent in the PSB library, Fig. 5(a). Furthermore, a three-phase converter model is also included,
Fig. 5(b). Such converter can employ various kinds of
switches, e.g. GTO, IGBT, MOSFET. The PWM signal
generator is also available in the PSB library, which can be
used to generate signals for 6 and 12 pulses converters, Fig.
5(b). The implementation of the indirect controller is well
easy using the Simulink blocks, Fig. 5(c). Moreover, the
phase-modulation of the control angle δ is presented in
Fig. 5(d).
PSCAD/EMTDC has also all necessary components to
represent the network, Fig. 6(a). Regarding the
DSTATCOM and associated controllers, PSCAD/EMTDC
does not have a pre-defined converter model in its library.
However, such component can be implemented very easily,
Fig. 6(a). The implementation of the indirect controller is
also very simple using only existent components, Fig. 6(b).
There is a PWM signal generator in the PSCAD/EMTDC
library, however, it has not been used in this work. The
PWM signal generator adopted here is shown in Fig. 6(c).
In Fig. 7, the voltage response of bus 2 for the events
previously described is shown. In this case, there is no
DSTATCOM into the network. It can be verified that the
results obtained through the PSB and PSCAD/EMTDC are
very similar. The difference is mainly due to methodology
employed to calculate the three-phase rms voltage. In PSB,
the three-phase rms voltage is calculated using Fourier
analysis over a sliding window of one cycle of the threephase instantaneous voltage measured [3]. On the other
hand, in PSCAD/EMTDC, the three-phase rms voltage is
calculated transforming the three-phase ac instantaneous
voltage measured in dc and after smoothing it through a
filter [4].
1.6
1.6
PSB
EMTDC
1.4
1.2
Vrms (pu)
1
0.6
0.4
0.2
0
0.2
0.8
time (s)
1.4
Fig 8 Voltage response of the test system 1 using PSB and
PSCAD/EMTDC – case with DSTATCOM.
A. DVR
The test system employed to carry out the simulations
concerning the DVR actuation is shown in Fig. 9, which is
the same system presented in [5]. Such network is composed by a 13 kV, 50 Hz generation system, represented by
a Thévenin equivalent, feeding two transmission lines
through an 3-winding transformer connected in Y/∆/∆,
13/115/115 kV. Such transmission lines feed two distribution networks through two transformers connected in ∆/Y,
115/11 kV. To verify the working of a DVR employed to
avoid voltage sags during short-circuit, a fault is applied at
point X via a resistance of 0.66 Ω. Such fault is applied
from 300 to 600 ms.
PSB
EMTDC
1.4
0.8
S1
1.2
Vrms (pu)
Vs
1
2
1
X
0.8
Xtr
3
L1
converter
0.6
5
0.4
vdc
0.2
0
0.2
4
Zs
0.4
0.6
0.8
1
1.2
6
1.4
time (s)
L2
Fig 7 Voltage response of the test system 1 using PSB and
PSCAD/EMTDC – case without DSTATCOM.
Fig 9 One-line diagram of the test system 2.
In Fig. 8, the voltage response of bus 2 in the presence
of a DSTATCOM is presented. It can be verified that the
DSTATCOM can keep the terminal voltage approximately
constant during the transients. The capacity of the
DSTATCOM dc element was adopted equal to 19 kV, and
such capacitor is previously charged through the circuit
breakers shown in Fig. 5(b) and 6(a). It is important to
The implementation of this system using PSB and
PSCAD/EMTDC are shown in Fig. 10 and 11 respectively.
The mains components have already been described previously. In such figure, the insertion transformer connection
is explicitly shown.
4
International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
A
a
B
b
C
c
A
A
B
B
C
C
A
B
a2
A
b2
A
B
B
a3
C
b3
C
A
B
B
C
c2
N
A
c3
Fault
ABC
C
C
A
A
B
B
C
C
1
A
-
A
powergui
A
a
B
B
b
BC
c
C
B
2
C
B
pulses
3
A
A
V
A
A
+
B
B
1
C
C
z
C
Pulses Signal(s)
C
Vinv _ref
4
Vrms (pu)
Vrms (pu)
Ia
Control
System
Discrete system
Ts=1e-005
(a) DVR.
A
B
C
Vabc (pu)
DVR
(a) test system.
Fig 10 DVR and associated controllers implemented in PSB.
A
0.05
100.0 [MVA]
Vpu_rms
20.0
0.4806
C
0.05
20.0
#2 B
B #1
0.05
A
0.4806
115.0
C
11.0
Va_inj
20.0
0.4806
Graphs
P_DVR
Q_DVR
ABC
Timed
Breaker
Logic
Closed@t0
C
FAULTS
Logic
Fault
Timed
BRK
B
A
3 Phase
RMS
A
A
A
0.1
B
B
C
B
C
A
100.0 [MVA]
A
0.001
C
#1
#3
13.0 115.0 [kV] 115.0
BRK
A
100.0 [MVA]
0.005
B
B
0.001
0.005
0.001
0.005
C
#1
C
115.0
BRK
B
#2
BRK
GBn
#1
#1
#1
1.0
GBp
A
0.001
A
B
0.001
#2
#2
#2
B
C
0.001
C
CrtlB
GAp
−120
P_DVR
1
Vpu_rms
D +−
F
delta
TIME
I
0.3
TIME
0.6
A
ma
Mag
ma
*
Freq
Compar−
ator
F
0.1926
120
Compar−
ator
B
CrtlC
Sin
A
A
Compar−
ator
GBp
GBn
Compar−
ator
GCp
GCn
B
CrtlB
GAp
GAn
B
Freq
F
delta
D
+
+
delta
Phase
Phase
ma
CrtlC
Sin
50
Compar−
ator
*
B
A
delta
D
+
+
Phase
Q_DVR
1
P
A
B
A
B
Power
P Q
Vref
0.1926
0.1
GAn
CrtlA
GCp
0.1926
0.1
C
Va_inj
GCn
0.1
B
C
11.0
Vpu_rms
A
Freq
Freq
Mag
Freq
ma
Freq
Sin
Mag
Freq
CrtlA
Fig 11 DVR and associated controllers implemented in PSCAD/EMTDC.
the DVR is capable of keeping approximately constant the
terminal voltage. In this case, the DVR coupling transformer is connected in delta in the DVR side and the leakage reactance is equal to 10%. Additionally, the capacity of
the dc element was adopted equal to 5 kV [5]. The other
data can be obtained in [5].
In Fig 12, the voltage response of bus 4 is presented; in
this case there is no DVR into the network. It can be verified that during the fault occurrence the voltage is very
affected. Furthermore, again, the simulation results obtained through PSB and PSCAD/EMTDC are very similar.
The voltage response of bus 4 when there is a DVR into
the network is shown in Fig. 13. It is possible to note that
5
International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA
The main difference between the results obtained from
PSB and PSCAD/EMTDC occurs when the DVR comes in
and out. Such fact is mainly due to different methodologies
employed for each software to calculate the three-phase
rms voltage, as has already been previously discussed. This
has large impact on the controller performance. However,
it can be considered that the results are very similar. It is
important to remember that different measures may also
occur using real electric measurement apparatus.
•
•
PSB
EMTDC
1.2
•
Vrms (pu)
1
0.8
•
0.6
0.4
0.2
0
0.1
0.2
0.3
0.4
0.5
time (s)
0.6
0.7
0.8
0.9
In general terms, both programs are suitable for transient
analyses of custom power devices and very easy to use.
The main advantage of the PSB is it be developed into
Matlab/Simulink environment, such fact become possible
to utilize it together with several other control design tools.
On the other hand, the main advantage of the
PSCAD/EMTDC is computing time. In this software the
simulations run very fast.
Fig 12 Voltage Response of the test system 2 using PSB and
PSCAD/EMTDC – case without DVR.
PSB
EMTDC
1.2
Vrms (pu)
1
0.8
ACKNOWLEDGMENTS
0.6
The authors would like to acknowledge the financial
support of FAPESP, Brazil (Proc. 01/05938-0). The authors would also like to acknowledge Mr. Gilbert Sybille
from IREQ for his help in the development of the
DSTATCOM model using the Power System Blockset.
0.4
0.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
REFERENCES
time (s)
[1] N.G. Hingorani, “Introducing Custom Power”, IEEE Spectrum,
Vol. 31, pp. 41-48, 1995.
[2] E Acha, V.G. Agelidis, O. Anaya-Lara, T.J. Miller, “Power Electronic Control in Electrical Systems”, 1st edition, Newnes, 2002.
[3] TEQSIM International Inc., “Power System Blockset User's
Guide”, 2001.
[4] Manitoba HVDC Research Centre, “EMTDC User's Manual”,
1992.
[5] O. Anaya-Lara, E. Acha, “Modeling and Analysis of Custom
Power Systems by PSCAD/EMTDC,” IEEE Trans., Power Delivery, Vol. PWDR-17 (1), pp. 266-272, 2002.
[6] N.G. Hingorani and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems”, 1st
edition, The Institute of Electrical and Electronics Engineers,
2000.
[7] Y.H. Song, and A.T. Johns, “Flexible ac transmission systems
(FACTS)”, 1st edition., The Institution of Electrical Engineers,
1999.
Fig 13 Voltage Response of the test system 2 using PSB and
PSCAD/EMTDC – case with DVR.
V. CONCLUSIONS
Considering the applicability of the PSB and
PSCAD/EMTDC for transients analysis of custom power
devices and based on results presented and discussed in
previous sections, the following conclusions can be obtained:
•
to improve the PSB performance. In this case a C
code is generated. In both programs an integration
step equal to 50 µs was utilized.
Easiness of implementation of the models: both
program have all necessary elements to implement
the DSTATCOM and DVR models easily. Moreover, the graphical interfaces of these programs are
very friendly.
Evaluation of the existent libraries: in this case the
two programs shown to be very complete. Additionally, the non-existent models can be very easily
implemented.
Accurateness of results: it can be verified that the
results obtained from PSB and PSCAD/EMTDC
are very close. Therefore, it is possible to conclude
that both programs present the same kind of accurateness.
Accomplishment sequential studies for parameter
adjustments: both programs have tools to carry out
multiple-simulations for determination of parameters; e.g. PI constants. However, PSB can be utilized with several toolboxes and blocksets existent
in Matlab/Simulink environment, for example
Nonlinear Control Design Blockset.
Computation
time:
in
this
case
the
PSCAD/EMTDC is quicker because it is implemented in Fortran 90 and the PSB is based on a
script language. However, it is possible to use the
Simulink Accelerator and the Real-Time Workshop
6