~
IEEE Transactions on Power Delivery, Vol. 10, No. 4,October 1995
2048
COMPARISON OF THE ATP VERSION OF THE EMTP AND THE NETOMAC PROGRAM FOR
SIMULATION OF HVDC SYSTEMS
P. Lehn
J. Rittiger
Member
Siemens AG
Box 3220,91052 Erlangen, Germany
ABSTRACT
This paper investigates the capabilities and limitations of the
EMTP and the NETOMAC program as applied to HVdc system
simulation. The fundamental differences between the two programs and their effect on simulation results are described. Consistency of the results obtained from these programs is examined
through simulation of a test HVdc network. As expected, a very
high degree of agreement between the two sets of simulation results proved to be achievable, but only when particular care was
taken to overcome intemal program differences. Finally, the
new advanced stability feature of NETOMAC is briefly discussed and then tested against the complex transient models established in the EMTP and in the NETOMAC transients program section.
Keywords: HVdc simulation, ATP, EMTP, NETOMAC.
INTRODUCTION
Particularly in the last 10 years, a wide variety of HVdc control
strategies have been tested and optimized with the help of various digital simulation programs. In 1985 the great interest in
HVdc system simulation led to the idea of establishing an HVdc
benchmark model [l]. In the following years a comparison of
one simulator and four digital models was carried out and then
documented by a Cigre Working Group [2]. Computed results
from the various simulation programs all agreed quite well with
the reference simulator results (although completely calibrated
NETOMAC results were first documented somewhat later [3]).
Since virtually every computer model was established at a different location, generally only discrepancies between results from
the simulator and a particular program model were addressed.
Variations between the results obtained from different simulation programs therefore received little attention.
A comparative study between the EMTP and the NETOMAC
program was therefore carried out by simulation of a common
HVdc system with the two programs. Here, in contrast to the
Cigre studies, particular attention was paid to minimizing all
possible differences and locating any remaining sources of error
which exist between the digital models. In addition, the study
provided an ideal opportunity to demonstrate and test the abilities of the new advanced stability feature of NETOMAC, which
was designed for the inclusion of complex HVdc networks in ac
stability simulations.
For the purpose of this study the ATP version of the E M V , supplied by the Leuven EMTP Center (LEC), was employed. The
95 WM 276-6 PWRD
A paper recommended and approved
by t h e IEEE Transmission and D i s t r i b u t i o n Committee
of t h e IEEE Power Engineering Society for p r e s e n t a t i o n a t t h e 1995 IEEE/PES Winter Meeting, January 29,
t o February 2, 1995, New York, NY. Manuscript submitted December 22, 1993; made a v a i l a b l e f o r p r i n t i n g November 30, 1994.
B. Kulicke
Member
Technische Universitat Berlin
Einsteinufer 11, 10587 Berlin, Germany
discussion presented however, is generally applicable to the
other available EMTP versions (such as the EPRI an BPA version numbers 2.x) as well.
THE EMTP AND THE NETOMAC PROGRAM
Both the EMTP and the NETOMAC program have been around
for over 2 decades. Although the programs were initially perhaps somewhat limited in their abilities, continual program development and the rapid advance of computational facilities has
transformed them into very powerful and flexible simulation
packages. Currently, both packages are capable of representing
an enormous variety of linear and nonlinear network elements,
as well as controls and the controlled switches (thyristors) which
are required for HVdc and SVC system modelling. They are also
both capable of load flow and transient calculations and they offer a wide variety of advanced features such as inclusion of macros, or "modules" in the EMTP, calculation of line constants, and
transmission line representation with the Marti Model. Of
course certain features are peculiar to the individual programs.
The most significant of these features in the EMTP are SPY, for
interactive execution, observation and control, as well as support
programs for conversion of supplied nonlinear transformer and
inductor data into EMTP input format. As for the NETOMAC
program, it offers a variable time step size, interpolation, and a
stability calculation feature which has recently been made even
more powerful.
When looking more closely at the two programs, 3 rather significant disadvantages of the present EMTP version can be seen
w'lien siinulatiuii of I-IVdc ail3 SVC devices is considered. Firs:,
the fixed time step of the EMTP causes errors in switching times
and can also cause the initiation of numerical oscillations [4][S].
Secondly, undesirable time delays occur between the solution of
the control equations (in TACS) and the network equations [SI.
Finally, because TACS was originally designed for the representation of analog controls, limitations become evident when mode m digital controls are to be modelled 161. While the newer
MODELS option of the ATP version of the EMTP overcomes
the problems of modelling digital controls and even reduces
some of the time delays between the controls and network solutions, a great many users still only employ TACS to ensure their
data files remain compatible with other EMTP versions.
In the case of the NETOMAC program, delays between the network and controls solutions are eliminated for HVdc applications, and switching operations employ an interpolation technique [8,9]. The delays in thyristor firing that occur with TACS
and NETOMAC are shown graphically in fig. 1. (It should be
noted that not necessarily all the EMTP versions function quite
identically [7] and the figure only shows the typical delays that
are to be expected.) As can be seen from the figure, the delay in
the EMTP when TACS is employed is one time step plus errors
in zero crossing detection plus errors resulting from the fact that
the time a is not an exact multiple of the time step. The accuracy
of the NETOMAC firing time is only limited by the precision of
the linear interpolation routine used.
The modelling of controls is quite different in the two programs.
Both NETOMAC and MODELS in ATP employ a much larger
library of special functions than TACS and they also accepts true
FORTRAN input offering if ...then ...else structures and the like.
0885-8977/95/$04.00 0 1995 IEEE
2049
true zero
crossing
desired firing
instant
TACS \vaveform
synchronized to
true zero
tlcriretl firiiig
firing signal realized in TACS
firing implement by EMTP
realized in TACS
firing time desired
by TACS
firing liiiic. itiiplciiieiited i n nct\vork
using interi,olation’.
realized using
interpolation“
”accurate to within the liinitatims of liilcar interpolalioll
Fig. 1: Thyristor firing in EMTP (left) and NETOMAC (right)
cable section. These include changes in the controller gains, inThey are therefore better suited to the representation of digital
corporation of a VDCL characteristic at both the rectifier and incontrols than T A U . All three environments, TACS, MODELS,
verter and logic to ensure gradual recovery after engagement of
and NETOMAC are equally capable in their abilities to model
the VDCL.
analog controls.
THE EMTP AND NETOMAC MODELS
SYSTEM CONFIGURATION
Since both the EMTP and the NETOMAC program are capable
of simulating virtually any conceivable electrical network, no
constraints were imposed on the configuration of the network to
be used in the comparison study. In order to rigorously test the
capabilities of the new advanced stability model however, a system with a relatively complex dc circuit was chosen. The system
model selected for the study is shown in fig. 2. Both ac systems
operate at 50 Hz,and the networks are both strong with a SCR
of 5.0. Each converter is compensated to about 80% of its reactive power demand by two ac filters producing 240 Mvar. The
HVdc monopole is rated at 600 M W and 400 kV. The dc line
consists of 2 transmission line sections separated by 66 km of
submarine cable. The two transmission line sections are modelled using T-sections that are about 20 km in length, while the
cable requires 13 T-sections, each approximately 5 km long.
Two rejection filters tuned to 50 and 100 Hz are included in the
dc circuit to eliminate resonances of the combined dc network at
these frequencies. The highly capacitive submarine cable section and the first and second harmonic resonances of the dc circuit make the system very demanding on the advanced stability
model.
Since the controls were all to be modelled in TACS, an analog
controller, based on the controller used by the FGH in their simulation of the HVdc benchmark model [lo], was chosen. Certain
modifications were, however, made necessary by the more complex dc circuit which included a highly capacitive submarine
As long as the electrical network consists of only linear elements,
creation of identical models in the two programs is possible. Minor discrepancies between the two models however, start to arise
as soon as saturable transformers, time controlled switches and
thyristors are introduced. The least problematic of these elements is the time controlled switch where, switching events must
occur at the same point on the voltage waveform, which is not
necessarily at exactly the same instant in real time. Although
ideally the conditions are synonymous, in reality there can be a
small phase shift between the two program outputs (in the order
of several time steps). Also, since the EMTP does not interpolate to open switches precisely at zero current, the resulting
chopped current can have a substantial effect on the system response. For the EMTF’ and NETOMAC models to respond similarly to a given switching action, the elimination of these errors
through careful examination and correction of the switching time
and adjustment of the EMTP switch current margin to control
the chopped current is required.
In the case of the saturable transformer model, and similar nonlinear elements, care must obviously first be taken to ensure that
the given data is correctly converted for input into each of the
two programs. While this process is simplified by support programs in EMTP, it remains somewhat tedious in NETOMAC. It
should be noted however, that even once data is correctly converted and entered, slight discrepancies will still be evident in the
operation of the device. For example, in the case of a piecewise
linear characteristic, the variable time step of NETOMAC al-
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lows interpolation to the point of discontinuity and ensures that
the characteristic entered is strictly obeyed. Since this is not possible in the EMTP, a slight overshoot at the discontinuity occurs
[5].The resulting discrepancy decreases with a decreasing time
step size in the EMTP.
Finally there is the difficulty associated with the implementation
of controllable switches, i.e. thyristors. Simply put, it is impossible for a thyristor modelled in the EMTP and in I ‘ETOMAC to
function identically. First, there are the inhere1 ! time delays
which occur between the controls and network solutions in
EMTP and are avoided in NETOMAC by interpolation to the
exact switching time specified by the controls. This, however, assumes that the output of the controls themselves are identical,
which is also not the case. Although the controls simulation in
NETOMAC and TACS in EMTP perform the same function,
they have very little else in common. It is therefore exceedingly
difficult for even simple blocks such as a PI controller to be identically represented in the two programs. In fact, even the PI controller must be broken down into its component P and I parts, if
the limit handling in the two programs is to be carried out identically. In many cases, due to the different control function libraries, a single block in one program has to be replaced by a set
of interconnected blocks in the other just to mimic the same response.
NETOMAC ADVANCED STABILITY MODEL
While the EMTP, previous NETOMACversions, and other similar programs have been employed for calculation of transients in
combined ac-HVdc systems, the small time step required when
modelling the thyristor bridges have generally limited such simulations to perhaps a few seconds in length. System stability calculations have therefore been left to stability programs in which the
dc system is extremely simplified and the entire dc network must
be represented as a transfer function [2][11]. Such modelling
however, becomes very difficult and restrictive in systems with
complex dc circuits, especially in cases where the effects of dc
line faults and switching events are to be investigated [3]. These
considerations have led to a new and unique NETOMAC program development which allows simultaneous calculation of the
ac network in the program’s stability section and calculation of
the dc network in the program’s transient section. This hybrid
model represents the ac network by a single line diagram employing complex admittances, as is standard for stability calculations, but simultaneously also solves the differential equations
representing the dc circuit. Since the dc circuit is represented by
a standard transient model, all options normally available in a
transient simulation can be made use of in the ac circuit. These
include, but are not limited to, representation of mutual coupling between the transmission lines of the two poles, representation of lines using a Marti Model, simulation of any types of
faults and complete modelling of dc filters, stray capacitances,
and so on. As for the ac system, the generators are of course
modelled using differential equations in order to simulate their
dynamic response. The interface between the ac and dc networks is the converter, which is modelled using the quasi-stationary converter equations as per [12], with additional logic for
handling fault conditions.
Control of the converter employs the same controller input data
set as used in the NETOMAC transient model. In fact, since the
input format for the NETOMAC stability and transient sections
is basically the same, the majority of the network input data for
the stability model can also be directly copied from the equivalent transient model. Exceptions to this include single phase and
phase to phase branches which must be eliminated, as well as the
thyristor bridges and converter transformer which are already
represented using the quasi-stationary equations.
Fig. 3 gives an overview of the solution technique used in the advanced stability model. This technique allows simulation of any
variety of dcline faults or switching events as well as symmetrical
ac phenomena. Due to the single line representation of the ac
system, unsymmetrical faults can only be roughly approximated
by the model. Harmonic studies and representation of internal
converter faults are not possible with the simplified model.
ac network 1 :
coiii plex ad iiii 1.
model
dc Ilctwork:
true trnnsieiit
Illode1
iic net\\
7rk
2:
coiiipley idiiiit
I11o d e I
Fig. 3: The NETOMAC Advanced Staliility
Model Solution Technique
While, as the name implies, the advanced stability model was
primarily developed for the purposes of stability calculations, its
remarkable performance may make it a topic of interest for transient simulation program users as well. Although there are limitations to the accuracy of the transient system response calculated with the hybrid model, an impressive improvement in
computation time is achieved (Table 2). This is particularly true
in cases where large ac systems are to be simulated. The fast calculation speed and the reasonable accuracy displayed by the advanced stability model in transient simulations makes it a useful
tool for screening which HVdc fault cases are critical and should
be more closely investigated using a detailed transient model.
The stability model also proves useful in the basic control design
for complex HVdc systems, such as multi-terminal links.
SIMULATION RESULTS
The system of fig. 2 along with the modified FGH benchmark
cont:ols [lo] was simiilated with the EXITP and thc NETOXZAC
program. Single phase and three phase faults to ground at both
the rectifier and the inverter stations, as well as a dc line fault occurring between the cable and transmission line 2, were simulated. Figures 4 , 5 , 6 , 7 and 8 show a comparison of the simulation results obtained from the different programs/methods f o r
the above mentioned fault scenarios. The traces shown here give
the dc rectifier current, the dc inverter voltage at the dc filters as
well as the rectifier firing angle and the inverter extinction angle.
A numerical comparison of the prefault steady state operating
point calculated with the 3 models is given in Fdble 1.
Quantity
EMTP
a rectifier
y inverter
a inverter
Id rectifier
14.7
18.0
145.2
1.000
NETOMAC
transient
advanced
stability
15.0
18.0
145.2
1.ooo
15.3
18.0
145.2
1.000
Table 1: Comparison of the prefault steady state qhantities
As can be seen from figs. 4 through 8, and also from Table 1, extremely good agreement between the two transient programs is
achievable. It can also be seen that the advanced stability model
closely approximates the transient simulation results for all symmetrical ac disturbances and dc fault cases tested. In contrast to
conventional HVdc stability models [ 2 ] ,the advanced stability
~
205 1
AC transient
Inverter
Voltage
(P.U.1
Rectifier
Current
(P.4
!iMn
w
'i\
n
L
0l
c
0
I
ALPHA
Rectifier
(degrees)
I
GAMMA
Inverter
(degrees)
04
100
0
200
300 0
F
l
100
200
I
300 0
100
200
300
200
300
Fig. 4 Three phase rectifier fault
Inverter
Voltage
(P.U.1
0
A
Rectifier
Current
(P.U.1
0
ALPHA
Rectifier
(d=grf=)
GAMMA
Inverter
(degr=)
0
&
0
100
200
300 0
I
100
200
300 0
'
I
100
I
Fig. 5: Three phase inverter fault
1
Inverter
Voltage
(P.U.1
'
I
0
Rectifier
Current
(P.U.1
ALPHA
Rectifier
(degrees)
GAMMA
Inverter
(degrees)
04
0
100
200
I
3000
100
200
Fig. 6 Single phase rectifier fault
3000
100
200
300
2052
NETOMAC tra nsient
NETOMAC stability
EMTP
Inverter
Voltage
(P.4
Rectifier
Current
(P.4
ALPHA
Rectifier
(degrees)
Inverter
(degrees)
0
100
200
3000
100
m
M o o
Fig. 7: Single phase inverter fault
0
100
200
3000
i00
200
300
100
200
300
Voltage
(P.U.)
Rectifier
Current
I
100
200
3000
Fig. 8: DC line fault located at the junction of the cable and transmission line 2
model offers more accurate representation ofthe dc line current
As for the two sets of transient results, the high degree of agreeand voltage fluctuations during both the fault and the recovery.
ment is, unfortunately, not simply obtained by ensuring that the
Due to the single line model of the ac system, unsymmetrical
physical system is modelled as accurately as possible. Even once
model differences are minimized and care is taken to match fault
fault cases can only be roughly approximated with the advanced
switching instants, results will agree only if the time step is apstability model.
propriately selected. While selection of an unusually small time
Comparison of the stability model with the transient model restep (around 5 w)might prevent time step size dependant dissults shows that the stability model overestimates the residual
crepancies from arising beween the E M P and NETOMC
of the d c h e in the case ofthe three Phase rectifier faulttransient simulation results, the associated long computation
This is a consequence of us@ the simplified representation of
time required would general!y prove unacceptable as well as unthe converter without actual representation of any ofthe valves.
necessary for most studies. The time step used in both programs
at the rectifier -ent
in figs- and5 it can a b b e seen
was therefore increased until the quality of the transient simulathat the
somewhat m ~ r rapidly
e
than it
tion results started to degrade. For the HVdc system under study
The reason for the more rapid recovery is the fact that
this occurred when the EMIFP time step size went beyond 20 ps.
the ac system and the WnverterS assume balanced three Phase
Due primarily to the interpolaticn technique used by NETOoperation. Thus the instant the fault is switched off,a balanced
MAC however, its time step a a s raised to 100 ps without any parthree Phase voltage appears on the terminals of the m ~ e r t e r . ticularly noticeable degrz.'?.tinr: in its transient system response.
In the transient simulations however, the three phases must return sequentially since the fault current flowing in each phase
In contrast to the transier.: mode!s, the NETOMAC advanced
can only be interrupted at a zero crossing. Fig. 8 compares the
stability model requires the inpu? of' two time step sizes: one for
stability and transient results for a dc line fault. As can be seen,
the ac network stability soliiton m d another, possibly different
the advanced stability model has no difficulties handling a fault
one for the dc network transient so!ution. Here the optimal time
in the dc network.
step was found to be 500 11s for borh sections. Although a 500 ps
2053
time step for a stability calculation is quite small, it is only required for the duration of the fault. Once the fault is over and the
HVdc system starts to approach its normal operating point, the
time step for the ac system can be substantially increased. Since
the ac network model was small and only 300 ms of simulation
were desired, the time step size was left unaltered for the purpose of this study.
Table 2 gives a comparison of the time step sizes used in the
EMTP and the two NETOMAC models to achieve the simulation results of figs. 4, 5, 6 and 7. The associated computation
time required to simulate 640 ms of real time on an 80486, 33
MHz IBM compatible computer is also given. As can be seen,
the EMTP is in fact not as much slower than the NETOMAC
transient simulation as is suggested by the large time step difference since it is not slowed by the complicated interpolation routine. What is much more noticeable however is the large computation time advantage of the advanced stability model. In
cases where the dynamic performance of the system over a
longer time spans is of interest, the time step for the ac system in
this model can be dramatically increased after the higher frequency transient phenomena resulting from the applied fault
have passed. For large ac systems with many generators this can
mean another factor of 10 increase in computation speed.
System model
EMTP
(ATP20 Version 6)
NETOMAC
transient model
NETOMAC
advanced stability
Time step size
Computation time
mcls
765 seconds
100 ps
193 seconds
500 ws
21 seconds
Table 2: Time step size and computation time comparison
CONCLUSIONS
In the past, calibration and testing of digital simulation programs
has generally been carried out by comparing simulator and computer results. Such a study was performed on a large xale by the
Cigre Working Group 14-02 and it demonstrated that various
modem simulation programs could reproduce simulator results
with good accuracy. While all test programs approximated the
simulator responses, close scrutiny of the curves showed a whole
host of small discrepancies between the results from the various
programs.
Here an attempt was made to determine how large a degree of
discrepancies must be simply accepted as a result of internal program differences and how other discrepancy can be eliminated.
The resulting transient responses from the ATP version of the
EMTP and from NETOMAC demonstrated that virtually identical results can in fact be obtained from the two programs, in spite
of the many program differences. To overcome time delays and
the lack of interpolation in the EMTP however, the best agreement of results was achieved onlywhen the EMTP time step was
selected to be about 5 times smaller than the one used in NETOMAC.
Once two sets of comparable results were obtained from the
EMTP and the NETOMAC transients models, the capabilities
of the new advanced stability feature in NETOMAC were tested.
The advanced stability model proved to be able to closely approximate the transient simulation results for all symmetrical ac
and all dc fault cases tested. The new model also offered a calculation speed which was 10 times faster than that of the transient
models.
Future work should include rigorous testing of the new and improved EPRI version 3 EMTP against other digital and/or analog models.
REFERENCES
[l] J. D. Ainsworth, "Proposed Benchmark Model for Study of
HVdc Controls by Simulator or Digital Computer", presented at
the CIGRE SC-14 Colloquium on HVDC with Weak AC Systems, United Kingdom, Sept. 1985.
[2] M. Szechtman et. al., "First benchmark model for HVdc control studies", Electra, No. 135, April 1991, pp. 54-67.
[3] J. Rittiger, "Digital Simulation of HVDC Transmission and
its Correlation to Simulator Studies", IEE Conference Publication Number 345, pp. 414-416.
[4] LEC, Alternative Transients Program Rule Book, 1987.
[5] H. W. Dommel, Electromagnetic TransientsProgram Reference
Manual (EMTP Theory Book), 1992.
[6] L. X. Bui, S. Casoria, G. Morin, J. Reeve, "EMTP TACSFORTRAN Interface development for Digital Controls Modeling", IEEE Trans. on Power Systems, Vol. 7, No. 1, Feb. 1992, pp.
314-319.
[7]A. E. Araujo, H. W. Dommel, J. R. Marti, "Converter Simulations with the EMTP: Simultaneous Solution and Backtracking
Technique", Paper APT 286-18-26, IEEE/NTUA Athens
Power Tech Conference, Athens, Greece, Sept. 1993,
pp.941-945.
[8] B. Kulicke, "Simulationsprogramm NETOMAC: Differenzenleitwertverfahren bei kontinuierlichen und diskontinuierlichen Systemen", Siemens Forsch. - U.Enhvick. -Ber, Bd. 10, Nr.
5,1981, pp. 299-302.
[9] R. H. Lasseter, K. H. Kruger, "HVDC Simulation using NETOMAC", IEEE Montech '86 - Conference on HVDC Power
Transmission, Sept. 1986.
[lo] T Wess, H. Ring, "Simulator study of HVDC controls with
a proposed Benchmark model", FGH report presented to
CIGRE WG 14-02, Oct. 1988.
[ l l ] R. Proulx, A. Valette, J. P. Gingras, D. Soulier, "User-Oriented Simulation of HVdc Control in a Transient Stability Program",IEEE Trans., PAS-104, No. 7, July 1985, pp. 1609-1613.
[12] D. Braunagel, L. Kraft, J. Whysong, "Inclusion of DC Converter and Transmission Equations directly in a Newton Power
Flow", IEEE Trans., PAS-95, No. 1, Jan./Feb. 1976, pp.76-88.
BIOGRAPHIES
€! Lehn - was born in Winnipeg, Canada on Nov. 27,1968. He
received both his B.Sc. and M.Sc. degrees in Electrical Engineering.from the University of Manitoba in 1990 and 1992 respectively. In 1992he joined the HVdc Systems Planning Group
at Siemens AG in Erlangen, Germany. His current interests include HVdc system modelling and control. Mr. Lehn is a member of the IEEE.
J. Rittiger - received his Dip1.-Ing. degree in Electrical Engineering from the University of Erlangen in 1987. After finishing
his studies he joined Siemens AG in Erlangen, Germany. Since
1988 he is working in a HVdc Systems Planning Group. His special fields are computer simulation of HVdc, control systems for
HVdc and problems concerning interconnected dc and ac systems.
B. Kulicke - was born in Wernigerode, Germany on Nov. 13,
1944. He received the M.S. degree from the Technical University Berlin in 1970 and the PhD degree in Power Engineering
from the University of Darmstadt in 1975. From 1970 to 1983 he
was with Siemens AG, working in the High Voltage Power Engineering Department. He was responsible for the development
of the NETOMAC program and was mainly involved in performing system studies including electromechanical and magnetic transients and stability problems. In 1984he was appointed
Professor and Director of the Department of High Voltage
Power Engineering at the Technical University Berlin. Prof. Kulicke is a member of the IEEE Power Engineering Society.