Control of an island Micro-hydropower Plant with Self-excited AVR and combined ballast load frequency regulator

Control of an island Micro-hydropower Plant
with Self-excited AVR and combined ballast
load frequency regulator
Guillermo Castillo, Leonardo Ortega, Marcelo Pozo, Xavier Domínguez
Departamento de Automatización y Control Industrial
Escuela Politécnica Nacional
Ladrón de Guevara, E11-253. Quito-Ecuador
{guillermo.castillom, leonardo.ortega, marcelo.pozo, xavier.dominguez}@epn.edu.ec
Abstract—This project describes the design and construction
of an Automatic Voltage Regulator (AVR) and an Electronic
Load Controller (ELC) for the voltage and the frequency
regulation in an island Micro-hydropower Plant (MHP). For the
frequency control, the speed regulation by ballast load method
has been used. To this approach, a combined binary-continuous
load regulation was employed. The implemented AVR is totally
self-excited by means of an energy transfer system which allows
an isolated operation of the MHP. The entire system has been
designed considering the current standard regulations of the
Ecuadorian Agency of Electricity Control and Regulation
(ARCONEL). The frequency and the voltage regulation were
properly achieved through the implementation of digital PI
controllers tuned based on mathematic models obtained from
experimental data of frequency and voltage. The control of the
system was validated by both, software simulations and field tests
performed.
Keywords— AVR,
hydropower Plant,
I.
digital
PI
controller,
ELC,
Micro-
simplification in turbine design, lower cost, easy operation and
maintenance, avoiding water pipe overpressure and faster
response to load variations [2]. The designed ELC uses ballast
load regulation with a mixed strategy which consists in using
both binary and continuous load regulation. In contrast, the
voltage regulation has been achieved through an AVR with a
self-excited voltage system. The AVR input ac voltage was
rectified by a single phase half controlled rectifier to obtain dc
current which was then applied to the generator field winding
[1]. The proposed control structure is illustrated in Fig. 1.
TURBINE
GENERATOR
USER LOAD
PG
PB
FIELD
WINDING
ELC
AVR
SET POINT
INTRODUCTION
Various methods for frequency and voltage regulation have
been studied and reported in the related literature. Frequency
regulation by water flow control is presented in [3], where
velocity of the turbine-generator group is controlled by
regulating the water flow by means of a valve operated by a
servomotor. Reference [2] presents a frequency regulation
strategy which uses a combined regulation of the velocity of
the turbine-generator unit by both, water flow and ballast load
control. A popular method consists in using auxiliary ballast or
dump loads to regulate the frequency [4]-[7]. In this approach a
constant water flow and a constant load connected to the
generator are required.
In the present project, to regulate the frequency of the MHP, an
Electronic Load Controller (ELC) has been implemented due to
the benefits compared to the other methods such as
978-1-5090-1629-7/16/$31.00 ©2016 IEEE
FREQUENCY
MEASUREM.
VOLTAGE
MEASUREM.
ACTUATOR
Voltage and frequency regulation are key aspects in Microhydropower Plants (MHP) operation because of a relative small
deviation of the control variables could cause permanent
damages or lifespan reduction. Therefore, the development of
low-cost, practical and robust control systems to regulate
MHPs is highly important, especially in countries where the
hydric resources are abundant and the conditions to install
isolated or grid-connected MHPs for public and private
projects are favorable [1]-[2].
PU
CONTROLLER SET POINT
Fig. 1.
CONTROLLER ACTUATOR
BALLAST
LOAD
Diagram of the proposed control system.
Regarding the ELC, the ballast or auxiliary load was
connected in parallel with the user load, thus the power
consumed by the total load (ballast and user) was kept constant
and equal to the power generated by the MHP (PG).
This relationship is shown in (1), where PU is the power
consumed by the user and PB is the power dissipated in the
ballast load. PB is controlled depending on the frequency of the
generated voltage.
PG = PB + PU = cte
(1)
For the modeling of the turbine-generator unit, the response
of the plant to a step input was employed. This method
demonstrated to be a convenient and useful approach when the
system parameters are unknown as in the case of the present
project.
II.
SYSTEM DESCRIPTION
A.
Voltage Regulation
The regulation of the generator output voltage is achieved
by means of the variation of the voltage applied to the field
winding. An ac-dc converter with half controlled bridge
configuration was used to control the field voltage. Fig. 2
shows the implemented circuit to control the field excitation
voltage. This scheme also includes a diode bridge to supply
enough field voltage for the machine start-up process using its
own excitation system.
circuit that allows the load connection only when the
instantaneous voltage is zero. Thus, the noise caused by current
transients produced when load is not connected at zerocrossing is eliminated.
Fig. 3. Diagram of mixed regulation for ballast load
III.
Fig. 2. Energy transfer circuit for generator field winding.
Once the generator output voltage has reached a suitable
value to power the control system and assure an appropriate
performance of the electronic circuits as well of the power
converters, the control panel is turned on. At that instant the
main contactor of the system is connected and the voltage and
frequency controllers initiate the operation. Simultaneously, the
transfer relay is operated to switch the energy transfer from the
diode bridge to the ac-dc converter. Since then, the voltage
regulation is performed by the AVR.
B. Frequency Regulation and load connection system for the
ballast load.
The proposed frequency regulation system is based in the
ballast load regulation method with a combined strategy which
uses three loads in discrete steps and one load in continuous
regulation. Fig. 3 shows the diagram of this mixed regulation.
Firstly, a continuous regulation is employed to connect a
percentage of the ballast load RL4 (continuous load) so that the
ballast load dissipates the surplus power from the generator
which is not required by the load. When the power dissipated
over RL4 is increased and a value equal to the nominal power
of RL1, RL2, or RL3 (ballast loads controlled by discrete steps
regulation) is reached, the power consumed by RL4 is
transferred to the discrete loads. Therefore, if the power
dissipated in the ballast load keeps increasing, the continuous
regulation is used on RL4 until the power is enough to activate
another discrete ballast load. The process could be repeated
again until all the discrete loads have been connected.
For the connection of both, discrete and continuous loads
TRIACs were used. For the continuous load, a single phase acac converter was implemented to control the power transferred
to the load by means of the trigger angle alpha (α). Whereas for
the discrete loads, the TRIACs were used as solid state relays
to implement an ON-OFF control with a zero-crossing detector
MODELING AND CONTROL
A. Plant Modeling
To obtain the model of the plant, experimental data for
voltage and frequency were obtained for step variations of the
control variables. The AVR and the ELC were designed for a
nominal power of 1600 watts in the MHP. Two models were
obtained, one for the frequency plant and the other for the
voltage plant.
Frequency Plant Model: This model establishes the
relationship between the frequency as output variable and the
percentage of load connected to the generator as input variable.
The test results are shown in Fig. 4. The variation range of the
step input (75% to 100%) was selected considering a
frequency response near the operation point of the plant (60
hertz) within the allowed range of variation. As it can be seen,
the response of the plant corresponds to the behavior of a first
order system. Thus, the transfer function of the system can be
expressed by (2).
G(s) =
K
τ s +1
(2)
Where,
Κ= Gain of the process or steady state gain.
τ= Time constant of the process.
Mathematically, the gain K is defined by (3) [8].
K=
ΔO
ΔI
=
Variation of the output variable
Variation of the input variable
(3)
For the step response showed in Fig. 4, the transfer function of
the plant can be obtained. This procedure was repeated to
obtain several transfer functions for both positive and negative
variations of the input variable. Then, the average of the gain
and the time constant from all transfer functions were
calculated and the transfer function which represents the
model of the plant was obtained (4).
G(s) =
−0.145
1.697 s + 1
(4)
Fig. 6. Response of the voltage plant to an alpha step input.
Fig. 4. Response of the frequency plant to a percentage load step input
Finally, the reached model was validated by means of
comparison of the response of the system from the mathematic
model versus the response obtained from experimental tests.
This comparison is expounded in Fig. 5. As can be observed,
the mathematical model of the plant is highly similar to the
experimental behavior of the plant. The maximum error was
1.5 hertz, which represents a 2.5% of the frequency nominal
value.
Fig. 7 shows the response of the system obtained from
both, the mathematic model and the experimental test. For this
case, the maximum error was 2 volts, which is equivalent to
0.9% of the MHP’s nominal voltage.
Fig. 7. Comparison between the mathematic model and the actual behavior
of the voltage plant
B. Frequency Digital Controller
Fig. 5. Comparison between the mathematic model and the experimental
behavior of the frequency plant
Voltage Plant Model: To obtain the voltage plant model,
variations in the firing angle (alpha) of the half controlled
rectifier (input variable) were performed to observe the MHP
voltage (output variable).
Fig. 6 shows a firing angle variation from 79.2° to 72°,
which achieves a voltage variation near to the operation point
of the plant (220Hz) and within the allowed range of variation.
Equation (5) details the obtained model of the voltage plant.
G(s) =
1.143
0.65s + 1
(5)
The frequency closed-loop block diagram is exhibited in
Fig. 8. The Control Digital Action (CDA) Delay block
represents the delay introduced by the instrumentation and the
digital controller. The value of Ts corresponds to the time that
the controlled rectifier takes to update its trigger signal [9],
this is at every new period of the sine voltage signal (16.67
milliseconds).
PI Controller
E(s)
R(s) +
CDA Delay
Plant
U(s)
-
Fig. 8. Frequency Closed Loop Block Diagram
C(s)
To achieve a successful regulation of the system and
avoid steady state errors, PI controllers were used. To define
the proportional and integral gains, the current standard
regulations regarding to the Dispatch and Operation
Procedures for Generators in Backup Systems from the
Ecuadorian Electricity Regulator (ARCONEL) were
considered [10] (See Table I).
TABLE I
Admissible frequency ranges for generators
Operation conditions
Admissible frequency
range
Without
action
of
generator’s
57.5 - 62 Hz
instantaneous disconnection relays
For 10 seconds maximum period
57.5 - 58 Hz; y 61.5 - 62 Hz
For 20 seconds maximum period
58 - 59 Hz; y 61 - 61,5 Hz
Without time limit
59 y 61 Hz.
C (s) =
C (s) =
11.33s + 22.35
s
U [ k ] = U [ k − 1] + 11.42 * E[ k ] − 11.23 * E[ k − 1]
IV.
(8)
(9)
EXPERIMENTAL RESULTS TO LOAD VARIATION TESTS
The practical implementation was performed in Quillán
Micro-hydropower Plant, which is located in the Province of
Tungurahua-Ecuador and generates 1600 watts. The nominal
parameters of the MHP are shown in Table II. Several test
procedures were performed to validate an appropriate
operation of the implemented AVR and ELC.
In order to meet with the mentioned regulation, a
Maximum Percentage Overshoot (MPO) of 4.17% and a
settling time (ts) of 200 milliseconds were chosen. For the
design of the PI frequency controller, the symmetrical
optimum PI technique was used to find the proportional and
integral regulator parameters (Kp=-236.85 and Ki=-82.93).
Thus, the controller’s transfer function is presented in (6).
−236.85 s − 82.93
transfer function and the difference equation can be expressed
as (8) and (9) respectively.
(6)
s
TABLE II
Parameters of Quillán MHP
Parameter
Nominal Value
Power
1600 watts
Voltage
220 volts
Current
9.09 amperes
0.8
Cos θ
Fig. 10 shows the external and internal view of the control
panel including all the sensors, electronic boards, protections
and operation elements.
Once the controller transfer function was obtained, the
difference equation can be expressed as in (7).
U [ k ] = U [ k − 1] − 238.23 * E[ k ] + 235.46 * E[ k − 1]
(7)
C. Voltage Digital Controller
To obtain the digital controller to perform the voltage
regulation, the same procedure detailed for the frequency
controller was used. For this case, the closed-loop block
diagram is shown in Fig. 9. Ts is 8.33 ms.
PI Controller
E(s)
R(s) +
CDA Delay
Plant
U(s)
C(s)
Fig. 10. Control panel implemented for the tests.
-
A. Load Variation Tests
Fig. 9. Voltage Closed Loop Block Diagram.
To define the PI controller parameters, the regulation
regarding to the Quality of Electrical Distribution Service [11]
from the ARCONEL was considered. The maximum
admissible voltage variation in urban areas is ±8% of the
nominal voltage (220 volts), which corresponds to an
acceptable range from 202.4 volts to 237.6 volts. For this
controller, a MPO of 4% and a settling time (ts) smaller than
200 ms where selected.
The controller parameters which fulfill the previous
conditions were Kp=11.33 and Ki=22.35.Thus, the controller
The AVR and the ELC were tested for the nominal power
of the MHP (1600 watts). However, to consider a safety band,
a ballast load of 2000 watts was selected. From tests
performed to obtain the model of the plant, it was determined
that a load variation up to 500 watts produced frequency
variation within the ranges allowed according to Table I.
Therefore, four 500 watts loads were used, one of them was
controlled by continuous regulation by means of phase control
and the other three with discrete ON-OFF control.
During the tests, different types of user loads (See Table
III) were connected and disconnected to evaluate the response
of the frequency and voltage controllers.
TABLE III
Data sheet of the employed user loads
Load Type
Voltage [V]
Power [W]
Incandescent Lights
210-230
100
Cell phone
100-240
5
Laptop
100-240
120
Blender
120
600
Iron
110
1000
B. Response of the Frequency Controller
When the power consumed by the user was higher, the
frequency deviation was also higher as in the case of the
electric iron connection (Fig 11). Table IV presents the
frequency response when connecting and disconnecting
different user loads.
TABLE IV
Response of frequency controller to load variations
Max.
Relative
Response
Event
Frequency
Error
time (s)
Variation (Hz)
(%)
Blender connection
0
0.93
1.5
8 lights connection
3.7
1.73
2.9
Electric iron connection
4.5
1.95
3.3
Electric iron disconnection
4.3
1.85
3.1
Full load disconnection
8
2.91
4.9
Fig. 12. Results obtained during the frequency controller test
C. Response of the Voltage Controller
Fig. 13 presents the voltage response of the MHP when an
electric iron was connected. As can be seen when a voltages
deviated from the reference value occur, the voltage controller
changed the firing angle of the half-controlled bridge to
correct the generated voltage in less of 0.4 seconds. The
voltage response to different load variations is detailed in
Table IV.
Fig. 11. Response of the controller to electric iron connection and
disconnection
Fig. 12 shows the achieved results for the frequency
controller during a 45 minutes test. At the beginning, the MHP
started up without any user load connected, and the frequency
increased up to the nominal frequency value. Then, the MHP
was subjected to load connections and disconnections for
different power rated loads. When a frequency variation was
detected, the controller responded immediately and changed
the percentage of ballast load to correct the frequency to the
reference value. Although several load variations where
performed during the whole test, the frequency kept within the
admissible range established by ARCONEL in [3].
Fig. 13. Response of the voltage controller
TABLE IV
Response of voltage controller to load variations
Event
Response
time (s)
Max. Voltage
Variation (V)
Blender connection
Blender disconnection
Electric iron connection
Electric iron disconnection
Full load disconnection
0.3
1
0.4
1.5
1.5
2
2
4
4
6
Relative
Error
(%)
0.9
0.9
1.81
1.81
2.73
Finally, the results obtained during the whole test time of
the voltage controller operation are shown in Fig. 14. The test
lasted 30 minutes and proved that voltage remained within the
admissible range established by ARCONEL in [5] when
different user loads where connected or disconnected,
independently of their power. Furthermore, the voltage
controller output remained in an almost constant value, which
is characteristic of the ballast load frequency regulation
method.
Fig. 14. Results obtained during the voltage controller test for differents
disturbances
V.
CONCLUSIONS
The achieved results for the obtained mathematic models
where highly similar to the real behavior of the plants, which
demonstrates the effectiveness and reliability of the modeling
procedure used. The maximum error obtained in the validation
of the voltage plant model was 2 volts, which corresponds to
0.9% of the reference voltage. The maximum error obtained in
the validation of the frequency plant model was 1.5 hertz,
which corresponds to 2.5% of the reference frequency.
The control strategy for the proposed AVR topology
allows the MHP to start up without an external power supply,
which turns the MHP in an isolated and autonomous
generation system.
The ELC implemented in this project reduces the amount
of loads used for discrete steps and decreases the power
managed by the AC-AC converter used for continuous
regulation related with classical frequency controllers.
The AVR and ELC implemented keep the voltage and
frequency within the ranges stablished in the current standard
regulations of the ARCONEL for load variations up to 62.5%
respect to the nominal generation power. The maximum
voltage deviation registered was 2.73% and the maximum
frequency deviation was 3.33%, which demonstrated an
effective and alternative voltage and frequency control.
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