A simulation of the actual installed controller will be carried
out and an analysis of the adjustment of their respective
parameters will be made, the manufacturer was based on
the recommendations defined in the “IEEE Recommended
Practice for Excitation System Models for Power System Stability Studies [4], [5], IEEE Std 421.5TM -2005”, specifically
the ST6B model. The identification and validation process in
hydroelectric plants usually concentrates on two subsystems:
governor-turbine and exciter. Standard models GGOV1 and
STB6 are preferred for the dynamical structures of governorturbine and exciter, respectively [6].
II. M ODELLING METHODS
First we will proceed with a theoretical review of the most
important parts and characteristics of the voltage regulator,
then we will proceed with the simulation of the model to
know the maximum variations of the parameters of the control
loop of the Voltage Regulator, with which we will obtain a
range of the generation unit operability. This paper follows
the IEEE descriptive material intended to assist in the planning
for design, development, and operation of small hydroelectric
power plant control systems [7].
A. Baba Hydroelectric Power Plant description
The Baba Hydroelectric Power Plant is located in the
province of Los Rios and is part of the Baba Multipropurpose
project, the Power Plant has two generation units with a total
power of 42 MW. Figure 1 shows the inside and outside Baba
plant where the two generators are located.
Fig. 1. Baba Hydroelectric Power Plant
The generators are of the synchronous type, vertical poles
projecting out from the surface of the rotor, and the turbine
is of the Kaplan type. The voltage regulation system consists
of a Static Type Excitation (ST) system whose manufacturer
is Voith. Table 1 describes the main characteristics of the
generator.
TABLE I
M AIN PARAMETERS OF THE GENERATOR
Turbine type
Constructive shape
Apparent Power
Nominal Active Power
Nominal Voltage
Nominal current
Power Factor
Velocity
Number of poles
Frecuency
Vertical Kaplan
2 X W1- IM8015
S = 23,40 MVA
P = 21,06 MW
V = 13,8 KV
I = 979 A
fp = 0,9
n = 225 rpm
p = 32
f = 60 Hz
B. Description of the functionality of the excitation system
The excitation system provides field current to a synchronous generator, additionally it contains control and protection functions that guarantee a stable dynamic response
in the Generation Unit. Among the main control functions
are the voltage of the generator terminals, the reactive power
and the ability to improve the stability of the power system
[8]. The protection functions include the capacity limits of
the synchronous machine, such as the regulation of low and
high excitation, Volt Hertz limiter and minimum field current
limiter.
The voltage regulator has as its main parts:
• Controller
• Rectifier bridge
• Collector Rings
• Excitation Transformers
The functional blocks of the excitation control in connection
with the synchronous generator are shown in Figure 2.
TABLE II
- E XCITATION SYSTEM DATA
Description
Excitation current MAX
Excitation current nominal
Vacuum excitation current
Excitation current in the air gap
Excitation voltage maximum
Excitation voltage nominal
Excitation voltage in vacuum
Excitation tension in the air gap
Field resistance
Ifm
Ifn
Ifo
Ifag
Ufm
Ufn
Vfo
Ufag
Rf
Units
959 Amps dc
854 Amps dc
466 Amps dc
379 Amps dc
169.4 Volts dc
143 Volts dc
—- Volts dc
58.2 Volts dc
119.6 mΩ
D. Excitation system Model
The excitation system at the Baba control panel is of the
self-excited type with completely static excitation, none of the
elements of the voltage regulator are rotating within the unit.
The field voltage is obtained by means of the terminal voltage
in the stator, which is conditioned by means of the excitation
transformer, the rectification of this voltage is carried out
through a fully controlled bridge rectifier, the angle The trigger
variable of the rectifier thyristors is the actuator variable to
obtain the appropriate field voltage and current at the unit
operating point.
The most traditional and practical structure of the synchronous machine that is suitable for the stability analysis of
power systems was developed by the standard IEEE 421.5
[10], which has developed flexible models applied to most of
the excitation systems found in synchronous generators. These
models have a bandwidth of at least 3 Hz and a frequency
deviation of 5% and is oriented to obtain the controller
parameters suitable for field tests [11]. The model used by
the manufacturer was the ST6B Static excitation system with
field current limiter.
The AVR shown in Figure 3 consists of a PI voltage
regulator with feedback from the internal loop field voltage
regulator. The field voltage regulator implements a proportional and integral control. If the field voltage regulator is not
implemented, the corresponding parameters KFF and KG are
set to 0. VR represents the limits of the power rectifier. The
current upper limit IFD is included in this model. The power
of the rectifier, VB, can be supplied from the terminals of the
generator or from an independent source [12].
Fig. 2. Functional scheme of the excitation control system of a synchronous
machine [9].
C. Excitation system data
Next, Table 2 shows the excitation system data.
Fig. 3. Model ST6B according to the IEEE421.5 standard - Static excitation
system with field current limiter [11].
III. M ETHODOLOGY
The application of generation schemes to hydroelectric
power plants offers a series of advantages, based essentially
on the greater flexibility of the controllers [13]. This section
explain the methodology.
A. General Description
A general description of all the Matlag-Simulink is following in the next sections.
1) Simulation system modelling: According to document
No. VOITH 299411 provided by the manufacturer of the static
excitation system, the model to be simulated is that shown in
Figure 4. This comprises the stator voltage control loop in
which the PSS is included and a field current feedback.
Fig. 5. Stator voltage control loop [12].
TABLE III
- D ESCRIPTION OF THE PARAMETERS OF THE EXCITATION VOLTAGE
CONTROL LOOP
Parameters
Ug sp max(VHz)
Ug sp min
Tu
Tf
Kp
Ti
Kd
Td
PSS after UEL
Kbr
Description
Maximum AVR reference allowed
Minimum AVR reference allowed
Transducer Time Constant
Field Current Time Constant
Proportional gain
Integral time
Derivative gain
Derivative time constant
PSS after UEL
Rectifier Bridge Gain
Tbr
Rectifier bridge time constant
Interval
0.9 - 1.2
0.7 - 1
20 ms
20ms
0.1 - 40
0.1 - 10s
-40 - 0
0.1 - 10s
0o1
6.25
1.4ms o
1.7ms
the PSS and the AVR droop compensator was included, but
their influence on the analyses was not carried out
Fig. 4. Full AVR model implemented by Voith
2) Simulation considerations: For the analysis and simulation of our model, the current limiter protections of the SCL
stator, overexcited OEL, underexcited UEL, volt hertz VHZ
and limiter of minimum field current MFCL, shall not be
taken into account. We will explain them didactic but will
not influence the simulation. In the control loop shown, the
reference signal is the Set point generator voltage and the
output variable is the unit field voltage, to be able to feedback
the process the transfer function of the synchronous generator
in vacuum was included. Load response was no included
because the object of analysis is the response of the voltage
regulator to the variation of the parameters of the controller.
The values used are per unit. For simulation purposes, a step
input of magnitude 1 p.u. The simulation and the entry of the
system parameters is carried out in Matlab, the simulation of
the model will be carried out in the Simulink application and a
sweep of the influential parameters in the voltage regulator will
be carried out to know the operating limits of the Generation
Unit.
3) Stator voltage control loop: Figure 5 shows the control
loop without the transfer function of the open circuit generator.
Table 3 shows the description of the parameters of the
excitation voltage control loop.
B. MatLab - Simulink simulation
The model shown in Figure 6 was used. The vacuum generator constants data were obtained with the nominal current
and field voltage and in vacuum. Illustratively, the location of
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