Energy and Power Engineering, 2013, 5, 527-533
doi:10.4236/epe.2013.54B101 Published Online July 2013 (http://www.scirp.org/journal/epe)
The User-defined Modeling Method of Power System
Components Based on RTDS-CBuilder
Yi Wang1, Sha Li2, Yulan Hu1, Ranran An1, Jiang Wu2, Jiaman Li2, Zexiang Cai2
1Electric Power Research Institute, Guangdong Power Grid Corporation, Guangzhou, Guangdong, China
2South China University of Technology, Guangzhou, Guangdong, China
[e-mail], [e-mail]
Received March, 2013
ABSTRACT
This paper puts forward a method to design the user-defined component based on the user-defined modeling environ-
ment CBuilder of RTDS simulator. And also develops the user-defined component model with algorithm described by
C language, visual graphics appearance, and the component function. And it generates the dynamic link library which
has the same execution efficiency as that of the included model of RTDS. This paper takes the IEEE type EXST1 static
excitation system as an example to build the user-defined component. The closed-loop tests on the user-defined com-
ponent and the included one of RTDS are performed to examine the accuracy of the proposed method. By comparison,
the test results show that the external characteristics of the user-defined component and the included model of RTDS
are basically the same in the initialization process, the step process of the terminal voltage reference value and the case
of the large disturbance.
Keywords: Real Time Digital Simulator; CBuilder; User Define Component; Control Component; Closed-loop Test
1. Introduction
New components and system control technique are con-
stantly applied to power system in recent years. The
power system simulation technique should be able to
flexibility provides various models of system devices,
including new regulating, protective devices, etc [1]. The
model of power system components in simulation tools
was coded according to specific algorithms and then
packaged, so that the user is not able to alter it [2]. Con-
sequently, when the exiting components in software
component library are unable to meet the user’s actual
simulation requirements for primary and secondary
equipments or control strategy, it is necessary for the
simulation tools to provide the user with a uniform us-
er-defined component modeling platform. Thus it can
enrich the models of simulation tools and improve the
ability and efficiency of simulations. Reference [3],
modeled on the Line traveling wave protection of an ac-
tual DC transmission project, and presents a user-defined
modeling method based on PSCAD/EMTDC and the
application in DC line protection simulation. In order to
solve the problem that the default DC transmission mod-
el is unable to describe the actual DC transmission fea-
ture in electromechanical transient simulation of
large-scale AC/DC power system, reference [4] put for-
wards the concept of DC system user-defined modeling
using PSASP. Reference [5] presented the user-defined
modeling method and the procedures of PSS/E in con-
ventional and expanded transient simulation, and a user-
defined model of excitation system was then developed.
Among various kinds of power system simulation tools,
RTDS is the most general power system real-time digital
simulator. Its user-defined modeling function component
CBuilder satisfies users’ demands for specific models.
Reference [6, 7] developed the electromechanical tran-
sient simulation model and the real-time simulation
model of electronic current transducer with the air-core
coil respectively. In this paper, the CBuilder modeling
principles and process was analyzed and introduced, and
the user-defined component model was also built based
on the previous work.
2. RTDS-CBuilder User-defined Modeling
Developing Environment
CBuilder implement the user-defined component func-
tion by taking C language-like program code as program
code. And the CBuilder platform interfaces automatically
with RTDS simulation program and user model library.
The user-defined C-like codes implant into the RTDS
main function via components, with no need to compile

Y. WANG ET AL.
528
or call the external subprogram frequently [8].
CBuilder user-defined modeling platform mainly in-
clude the editing methods such as graphics, parameters,
IO Points and C File Associations. These editing meth-
ods define the appearance, function parameters, IO
Points and mathematical models of user-defined compo-
nents. Model.c file program code compiled by the C
FILE Associations is the core of the user-defined com-
ponent. Codes of areas like STATIC、RAM、CODE
define and realize the relative component calculation of
the s by calculation. It will generate executable files au-
tomatically for the user to call after the file was compiled
successfully.
3. RTDS-CBuilder User-defined Modeling
Method
In order to build the user-defined component concisely
and normatively, the developing process of user-defined
component designed in this paper is shown in Figure 1,
according to the characteristics of the user-defined mod-
eling platform editing environment.
CBuilder function module can build two kinds of
models, the power system component and the control
component. In most of the simulation test scenes, the
existing power system components in RTDS component
library can basically satisfy the test requirements. How-
ever, with the booming of the new control equipment, the
demands to model the control user-defined component in
RTDS system gradually increase. Dispensed with net-
work solving, the program code of the control component
is much easier in comparison with that of the power sys-
tem component, and it affects the real-time much lesser.
Therefore, this paper takes the IEEE type EXST1 static
excitation system of the generator control component as
an example and introduces the basic approach for RTDS
modeling.
3.1. Appearance Design of Component Model
The logic block diagram of the IEEE type EXST1 static
excitation system is shown in Figure 2.
Figure 1. Development process of CBuilder UDC.
According to Figure 2, the excitation system has 3
input variables, including per unit voltage of generator
bus VPU, PSS input
s
V, exciting urrent c
f
I, and an
output variable of excitation voltage
f
E, etc. It can select
by condition whether we need the PSS input valuable or
not according to requirements of the test. On the basis of
the exterior pattern of excitation system in the RTDS
component library, in the editing environment of the
IEEE type EXST1 static excitation system designed in
this paper is shown in Fi g u r e 3.
3.2. Parameters Design of Component Model
The excitation system mainly consists of components
such as difference adjustment unit, amplifier unit and
amplitude limitation unit [9]. As Figure 2 shows, the
main parameters inside the IEEE type EXST1 static ex-
citation system is shown in Table 1.
As the reference voltage at the generator terminal ref
is an adjustable variable, it should change in line with
various power systems operation modes. Therefore, a
slider control variable should be set in the C File Asso-
ciations editing environment, so that it can be called and
installed on the RTDS user-defined real time operating
and monitor interface RUNTIME. Meanwhile, we set the
above internal parameter besides ref in the parameters
editing environment of CBuilder. Moreover, we set the
parameters including system name, PSS selected variable,
monitor internal variable resource allocation of the proc-
essor board, etc. Details are shown in Figure 4.
V
V
Figure 2. Logic diagram of IEEE type EXST1.
Figure 3. Appearance design of IEEE type EXST1.

Y. WANG ET AL. 529
Table 1. Parameters of IEEE type EXST1.
Parameter Description
f
T Time constant of the filter
ref
V Referenced operating voltage at generator terminal
maxi
V Maximum voltage inside the system
mini
V Minimum voltage inside the system
a
K
Gain magnification of the system
a
T Time constant of the amplifier
b
T,,
c
Tf TTime constant of the stable loop
f
K
Gain of the stable loop
c
K
Load factor of commutation reactance rectifier
max
V Maximum output amplitude limit
min
V Minimum output amplitude limit
Figure 4. Parameter design of IEEE type EXST1.
After the above parameters were designed and stored,
the parameters will be stored in the C File Associations
editing environment of this user-defined component au-
tomatically, so that the results can be edited and called by
the model.c file and the model.h file.
3.3. Code Design of Component Model
As Figure 2 shows, the IEEE type EXST1 static excita-
tion system consist of the amplitude limit unit, the trans-
fer function of inertial element, the transfer function
actual differentiation element, the add and subtract unit,
etc. Two types of these transfer functions can be de-
scribed as simultaneous of differential equation and al-
gebraic equation in their mathematical models. Yet, solu-
tion of the differential equation is particularly significant
for the execution efficiency of the component program.
For the simulation with microsecond calculation step, it
can maintain the stability of the value by using explicit
solution. In addition, there is no need to solve the equa-
tion, so it reduces the amount of calculation [10].
Therefore, this paper takes the inertial element 1/ (1)
s
T

as an example to program it using three explicit solutions.
These three solutions for differential equations are shown
in Table 2, where R and C are input and output of the
equations, T is the time constant, and is the simula-
tion step.
t
In this section, we build the user-defined component
model of inertial element 1/(1 )
s
T. In the RTDS/Draft
file, for the triangular of the included model in RTDS
component library and the 3 user-defined component
models, which have the input frequency 50 Hz and the
amplitude 0.8 pu, we uniformly set the time constant of
each component model as 2 s, set the simulation step as
50 μs, and set the relative simulation time as 4 s. We ana-
lyze the output data of each model with the simulation
time in the vicinity of 4s, as Table 3 shows.
We can see from the above table, while the simulation
step is set as 50 μs and the simulation time is set as 4 s,
the first 4 significant digits of the simulation results from
these three algorithms are the same, and the simulation
results are relatively close. Comparatively speaking,
Table 2. Differential equation’s solution of inertia link.
Solutions for
differential equations Explicit solution formula
EMTDC 






T
t
T
tetRettCtC 1)()()(
Euler
T
t
ttCttRttCtC

 )]()([)()(
Mod-Euler
 2/)()()(
)]()([)()(
)]()([)()(
tCtCtCT
t
tCtRttCtC
T
t
ttCttRttCtC
ba
ab
a





Table 3. Simulation results of inertia link.
Time (s)RTDS EMTDC Euler Mod-Euler
4.00000 0.346104 0.346104 0.346108 0.346101
3.99975 0.346129 0.346129 0.346133 0.346127
3.99950 0.346152 0.346152 0.346156 0.346150
3.99925 0.346173 0.346173 0.346176 0.346171
3.99900 0.346191 0.346191 0.346194 0.346189
3.99875 0.346206 0.346206 0.346209 0.346205
3.99850 0.346219 0.346219 0.346222 0.346218
3.99825 0.346230 0.346230 0.346232 0.346229
3.99800 0.346238 0.346238 0.346240 0.346237

Y. WANG ET AL.
530
compared with the user-defined component of Mod- Eu-
ler and Euler method, the user-defined component using
EMTDC algorithm has the simulation results which is
closer to that of the transfer function module RTDS
comes with. Therefore, in this paper, we solve these 3
types of transfer functions contained in the IEEE type
EXST1 static excitation system by using the solutions for
differential equations provided by EMTDC. The solution
formulas are shown in Table 4 shows, where R and C are
input and output of the equation, T is the time constant,
and is the simulation step.
t
Moreover, the programed methods of the amplitude
limit unit and add and subtract unit are comparatively
easier, and unnecessary details were given here. Accord-
ing to the format requirements of model.c file, we edit
code in areas such as STATIC, RAM and CODE. As the
code in CODE is executed in real time in RTDS, the
programming language we wrote should be as efficient
as possible [11]. For instance, the parameters which do
not need to be re-computed can be computed in RAM
area, division should be avoided in CODE area, the vari-
able that only be used in CODE area do not need to be
declared in STATIC area, etc. Components can right
away be called in the RTDS library after compiling.
4. RTDS-CBuilder User-Defined Model
Testing
Static excitation system model in RTDS has already
passed the engineering validation and it meets the appli-
cation requirements of engineering facts. By doing com-
parison testing using the user-defined model and the
RTDS included model, the correctness of the user-de-
fined modeling method presented in this paper can be
verified. While there are differences between the user-
defined component model and the RTDS included model
in terms of the choosing of differential equation algo-
rithm, the processing modes of each unit, etc [12]. As the
source code of the existing component in the component
library is unknown, this paper compares in terms of the
differences of external performance between the two
Table 4. Differential equation’s solution of EMTDC.
Transfer
function Explicit Solution Formula
1
1ST







T
t
T
tetRettCtC1)()()(
1
ST
ST

T
t
T
t
ettRtRettCtC

 )()()()(
1
2
1
1
ST
ST



2
22
)()(
1)()()(
2
1T
t
T
t
T
t
ettRtR
T
T
etRettCtC











models. Closed-loop tests were perform to user- defined
component model and RTDS included model by setting
up specific scenes of the power grid, to examine whether
the external characteristics are identical or not.
In this paper, we use the power system with the typical
connection mode of 500 kV auto transformers in the dy-
namic model test standard of the DL/T871-2004 power
system relay protection product as the closed-loop test
system. The system have 6 nodes by all, including gen-
erator MACH1, infinite source SCR1, infinite source
SCR2, 500 kV terminal bus BUS1, high voltage side bus
of the main transformer BUS2, medium voltage side bus
of the main transformer BUS3, low voltage side bus of
the main transformer BUS4, 500 kV terminal bus BUS1
and high voltage side bus of the main transformer BUS2
connected via the 200 km single line TL1. The network
topological graph of the closed-loop testing system is
shown in Figure 5.
The typical data of IEEE type EXST1 static excitation
system of generator MACH1 is shown in Table 5.
Figure 5. Typical wiring of 500 kV autotransformer.
Table 5. Typical parameters of IEEE type EXST1.
Parameter Value Parameter Value
f
T 0.0 b T 10.0
ref
V 1.01 c T 1.0
maxi
V 1.2 f T 1.0
mini
V -1.0 f
K
0.03
a
K
200 max V 5.1
a
T 0.4 min V -4.0
c
K
0.0

Y. WANG ET AL. 531
The major functions of the excitation system are to
provide the excitation source for the generator, stabilize
the terminal voltage, demagnetize when the generator
shed the load, rapidly and forcibly excite in the case of
system disturbances, demagnetize automatically in the
case of internal fault of the generator , control the reac-
tive power output of the generator, etc [13,14]. Closed-
loop tests were then perform to the user-defined compo-
nent and the Module in this paper in 3 conditions, in-
cluding voltage initialization, the step process of the ter-
minal voltage of generator, large system disturbance, etc.
The major parameters of the test consist of terminal vol-
tage of the generator, excitation voltage
f
E, the reactive
power output of the generator 1
M
ACH
1) The voltage initialization process
Q, etc.
In the process the generator from the end of initializa-
tion to the stable state, we need to inspect the establis-
ment of excitation voltage, the stabilizing process of the
terminal voltage, etc. The recorded experimental data
diagram Figure 6 is excitation voltage, Figure 7 is ter-
minal voltage, and the recording time is 20s. The gray
curves and is the test results to the
user-defined component, and the black curves
and is the test results to the Module.
()
f
EC
()
pu
VM
()
pu
VC
()
f
EM
As Figure 6 and Figure 7 show, the variation trends
of these two parameters of the user-defined component
and the Module, excitation voltage and terminal voltage,
are generally the same.
2) The step process of terminal voltage reference value
The terminal voltage reference value was stepped to a
certain multiple of the reference value after the system
Figure 6. Contrast of excitation voltage in initialization.
Figure 7. Contrast of terminal voltage in initialization.
entering the stable state. At this moment, examine the
rise time and the overshot of the terminal voltage, and the
control condition of the reactive power output of the ge-
nerator. The recorded experimental data diagram Figure
8 is the excitation voltage, and Figure 9 is the reactive
output with the wave recording time 10 s. The gray
curves 、1 of these figures are the test
results to the user-defined component, and the black
curves 、 are the test results to the
Module.
()
f
EC
(
f
EM
()
MACH
QC
1( MACH
Q ))M
As Figure 8 and Figure 9 show, the variation trend of
the two parameters, the excitation voltage and the reac-
tive power output, are generally the same.
3) The large system disturbance situation
When faults occur to the system, there is a shock to the
system voltage, so the voltage stability problem is likely
to occur. When faults occur to the AC system in the vi-
cinity of the generator, a disturbance is applied to the bus
voltage at the generator terminal. As the input of the ex-
citation system, the disturbance signal make the excita-
tion system adjust dynamically, and stabilize the system
voltage.
This paper designs the following experimental scenar-
ios. After the system entering the stable state, a
three-phase fault with the fault duration time 0.1 s was
applied to K1 point which was at the main transformer
side of line TL1 and then large disturbance occurred in
the system. The recorded experimental data diagram
Figure 10 is the excitation voltage and Figure 11 is the
Figure 8. Contrast of terminal voltage in initialization.
Figure 9. Contrast of reactive power in step process.

Y. WANG ET AL.
532
Figure 10. Contrast of excitation voltage in large disturbance.
Figure 11. Contrast of terminal voltage in large disturbance.
terminal voltage, with the recording time 5 s. The gray
curves , in these figures are test results
to the user-defined component, and the black curves
, are test results to the Module.
()
f
EC
VC
()
pu
VC
()()
f
EM pu
As Figure 10 and Figure 11 show, the variation trend
of these two parameters of the user-defined component
and the Module, excitation voltage and terminal voltage,
are generally the same.
4) Result analysis of simulation test
In this section, external characteristic closed-loop tests
were performed to the user-defined component and the
Module of the IEEE type EXST1 static excitation system.
The variation trends of the user-defined component and
the Module parameters, including generator terminal
voltage pu , the excitation voltage V
f
E and the gen-
erator reactive power output 1
M
ACH , are basically the
same, when the system is in the initialization process,
the step process of the terminal voltage reference value,
the condition of large disturbance, etc. The error should
be in an acceptable regime, in the consideration of the
difference of selection of transfer function algorithms,
the logical unit processing, etc. Therefore, the user- de-
fined component of the IEEE type EXST1 static excita-
tion system basically meets the test requirements.
Q
5. Conclusions
This paper introduce the modeling method application of
the user-defined modeling module CBuilder using power
system real time simulation software RTDS. The model
was designed by the user according to the external ap-
pearance of power system control component, the input
and output and the component parameters. Meanwhile,
we edit the internal transfer function and the logic unit
using C-like code on the basis of the system logic block
diagram. Thus we obtain the user-defined model of the
component, and take the IEEE type EXST1 static excita-
tion as an example to perform closed-loop simulation
verification. The test results indicate that the component
model built on the basis of this method, can well imple-
ment the controlling function of the component and sat-
isfy the users’ simulation demands for the specific model.
Meanwhile, the user-defined modeling module possesses
fine man-machine interaction interface, and the user-
defined component module it built has the advantage of
easily-extension, conciseness, efficiency, etc.
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