Journal of Energy & Environment
Journal homepage: www.uniten.edu.my/jee
Novel C2R-R Configuration for Micro-Hydro Plants Used in Islanded Systems
T.S. Weerakoo1*, R.P.S. Chandrasena1, A. Arulampalam1
1
University of Peradeniya, Faculty of Engineering, Dept. of Electrical and Electronic Engineering, Peradeniya, 20400 Sri Lanka
KEYWORDS
ABSTRACT
Micro-Hydro
C-2C
C2R-R
ELC
Ballast Load
This paper presents a novel strategy used to obtain single phase power from three phase
power generated through a three phase induction generator. In pico and micro hydro power
generations in rural areas, the traditional C-2C configuration is used with the three phase
induction generator. The above traditional method reduces the utilization of the generator
to 50% of its rating while operating at 0.5 power factor. This paper discusses the
drawbacks of the conventional C-2C method in micro hydro power generation and how
they can be overcome by introducing the proposed new method. This method is preferable
to called as C2R-R method based on its configuration. A clear theoretical analysis is given
on the C-2C and C2R-R methods. The result revealed that machine utilization can be
increased to 86.6% without over loading the machine when the proposed method is used.
Further simulation study was carried out using EMTDC/PSCADTM software on a 27.7
kW induction generator. Both traditional C-2C method and proposed C2R-R method have
been studied along with its Electronic Load controller to maintain the generator output
voltage and frequency. Stability of the machine operations is also checked for input torque
and output load power variations. The simulation results confirmed that, with the proposed
method, the machine utilization is increased while having more stable operation for sudden
input torque or output load changes.
© 2009 Universiti Tenaga Nasional. All rights reserved.
1. INTRODUCTION
Nowadays, electricity has become an essential commodity
in day-today human life. Hence, most of the developing
countries are struggling to meet the ever increasing demand
for electricity. The major challenges for achieving the
electricity demand are adding new generations to the system
and expanding the existing transmission network such that all
have access to the grid connected electricity. The hydro
depended countries face constraints on increasing the
generation capacity due to exhaust usage of the hydro
potentials. The environmental and economic concerns on the
fossil fuel block electricity generation from them. Extending
the grid to sparsely populated remote areas will be highly uneconomical. In Sri Lanka, about 70% of the population had the
access to the electricity from the national electricity grid [1].
Because of those reasons, the off-grid electrification has
drawn an increased attention in the recent past.
There are various options for off grid electricity
generation. They can be fossil fuel generation such as diesel
generator or renewable energy source based generation such
as hydro, solar, wind, dendro and bio-gas. According to the
green energy concept, the renewable energy source based
generation got its popularity. Sri Lanka, being a country closer
to the equator, has lots of water stream and has a good
potential for small scale micro-hydro power development. As
*Corresponding author
E-mail address: T.S. Weerakoo <tharindu@ee.pdn.ac.lk>.
an example about 350 villages in Sri Lanka are electrified by
village micro hydro schemes.
In micro hydro schemes, either synchronous or induction
machine can be used as the generator. Due to the simple
construction, robust operation, wider availability and cheap in
cost, induction machines are much popular in micro-hydro
industries. Generally three phase induction generators are used
with well-known C-2C arrangement to obtain a single phase
output while maintaining the balance operation in the machine
[2-3]. However, no literature to be found on optimum
configuration for obtaining the maximum single phase power
from a three phase induction generator while maintaining the
balance operation of the machine. It can be shown that C-2C
can generate 50% of the rated power without over loading the
machine. Therefore it does not utilize the machine at its
maximum.
Normally, mechanical governor system was used to match
the input power to the turbine according to the load demand.
However, due to its complexity a new mechanism of
Electronic Load controller (ELC) came into the industry and
became very popular. In ELC, the load is controlled using
electronic devices. This maintains a constant electrical load on
the generator in spite of change in user loads. ELC uses ballast
load to damp the extra power that is not required by the users,
so that it maintains the constant load on the generator. This
permits the use of a turbine without governor control system if
38
T.S. Weerakoo et al./ Journal of Energy & Environment, Vol. 1 (2009), No. 1, 38-44
it is supplied roughly with constant head and water flow.
Hence the ELC maintains the machine speed. However, it has
been reported that in micro hydro systems, ELC fails very
frequently and required frequent maintenance. Hence, in order
to develop a robust ELC system, a thorough understanding of
the effects of the variations of the various parameters on the
system is a necessity.
This research paper presents a new technique to generate
two phase power from a three phase generator using C2R-R
arrangement. This becomes more attractive as it utilizes the
machine up to 86.6%. Also this study analyses the effect on
machine operation due to varying output and input power.
This help to develop a robust algorithm for ELC. The ELC
control is also fully described.
39
According to the phasor diagram shown in Fig. 2, the
line currents are leading the phase voltage by 600. From Eq.
(2) it is clear that by decreasing the power factor angle to 30 0,
we can increase the amplitude of the load active power. To
make the different phase angle, it is compulsory to introduce
some modification for above C-2C configuration.
2. CONVENTIONAL TECHNIQUE TO GET SINGLE
PHASE OUTPUT FROM THE THREE PHASE POWER
Generally to obtain single phase power from three phase
induction generators, C-2C configuration has been used. It can
be shown that only half of the machine capacity can be
harnessed, if the above technique is used. The traditional C-2C
arrangement of the three phase induction generator and the
associated phasor diagram are shown in Figs. 1 and 2
respectively.
Fig. 2. Phasor Diagram of the C-2C Arrangement
3. PROPOSED METHOD TO ENHANCE THE
UTILIZATION OF THE GENERATOR CAPACITY
In most of the small hydro power generation schemes all
over the world, traditional C-2C arrangement is used to obtain
single phase power. However, as it was shown, this method
can deliver only 50% power from the generator. A novel
configuration with C2R-R, as shown in Fig. 3, is proposed and
it can be shown that with the new configuration, it is capable
of delivering more power than that with the traditional C-2C
method. Two phase loads can be connected with common
delta point as neutral terminal and thus the load power can be
increased.
Fig. 1. Traditional C-2C Arrangement
The single phase output power to the resistive load can be
written as:
Ptotal  vab  iRab
(1)
ia is the resultant of the load current iRab and the capacitor
current iCab. It can be proved that in order to make the load on
the machine to be balance, the angle between the i a and the iCab
should be 600. Then the angle between the ia and iRab should be
300. Then iRab can be written in terms of ia.
o
(2)
Ptotal  v  i  cos 30
ab a
Hence, the power delivered by the machine becomes 50% of
its ratings, as shown in Eq. (3) below, when C-2C
configuration is used.
1
Ptotal  Ptotal _ 3
2
Fig. 3. Load Distribution of the Proposed C-2R Method
(3)
T.S. Weerakoo et al./ Journal of Energy & Environment, Vol. 1 (2009), No. 1, 38-44
40
In the proposed method only one capacitor is needed.
This capacitor current and the current through the load
resistance Rcb determine the line current ic for balanced
situation. Therefore the necessary capacitance has to be
properly calculated. Full line current flows through the load
Rab. The total single phase power generation can be calculated
as bellow:
Ptotal  vab  iRab  vcb  iRcb
Fig. 5. Circuit Arrangement with C-2C Configuration
Considering the line voltage and line current, the total power
can be expressed as:
3
Ptotal 
P3 _ rated
2
(4)
Hence, it is clear that the proposed method increases the
total output power to 87% while enabling facility to have two
phase power. This is very useful when the micro hydro plant is
situated in the centre of a village and the power is need to be
distributed in two directions from the plant.
The simulation results are shown in Figs. 6(a) and (b).
Fig. 6(a) shows the instantaneous three phase voltages and
currants when the machine operates under steady state
conditions. It clearly shows that the machine’s terminal
voltages currents are balanced. The rms phase voltage is 212
V and the line current is 38.2 A. The long run of the
simulation confirmed the stable operation of the C-2C system.
Fig. 6(a). Three-Phase Voltage and Current at the Generator
Terminal with the C-2C Configuration
Fig. 4. Phasor diagram of the proposed C2R-R method
Variations of active and reactive power are shown in Fig.
6(b). As figure shows, the active power generated is 12.1 kW
whereas the reactive power absorbed by the generator is 20
kVAr. The active power generation is about 43.6% of three
phase rated power of the generator.
4. SIMULATION RESULTS
In order to check the performance, both the conventional
C-2C and the proposed C2R-R configurations were modeled
and simulated using EMTDC/PSCADTM. For both cases, the
study was carried out for the same input torque applied to the
induction generator. The phase voltage of the generator was
adjusted to 230V while delivering rated rms current of 40A.
The delta connected induction generator has been considered
to avoid the necessity of generator transformer to connect load
to the generator as this is a micro scale hydro power
generation scheme.
4.1 Simulation results of the traditional C-2C system
Fig. 5 shows the system with the C-2C configuration
modeled in PSCAD. In this simulation model, theoretical
capacitors and load resistors values were used.
4.2 Simulation results of the proposed C-2R system
Fig. 7 shows the circuit configuration of the proposed
C2R-R arrangement, modeled in PSCAD. The capacitor and
load resistors values were set according to the theoretically
calculated values in order to enhance the machine’s utilization.
Fig. 8(a) shows the instantaneous voltage and current of the
system obtained in the simulation. At the balanced condition,
the rms voltage and current values were 219 V and 39 A
respectively. The power generation of the generator is shown
in Fig. 8(b). As Fig. shows, the active power delivered to the
load is 22.5 kW and reactive power consumption by the
generator is 13.17 kVAr. The total power delivered by the
generator is about 81.1% of the generator’s rated power.
T.S. Weerakoo et al./ Journal of Energy & Environment, Vol. 1 (2009), No. 1, 38-44
41
Pgen
Active power (kW)
24.0
20.0
16.0
12.0
8.0
4.0
0.0
Reactive power (kVAr)
0.0
-4.0
-8.0
-12.0
-16.0
-20.0
-24.0
1.20
Rotor Speed (pu)
Qgen
Speed
1.00
0.80
0.60
0.40
0.20
0.00
3.0
Fig. 6(b). Power Variation and Rotor Speed Variation
1.005
W
S
StoT
T
IM
Vga
A
Vgb
B
Vgc
C
B Igb
Motor
1
TIME
B
C Igc
Torque
C
StoT
Cap
+
+
Load2
+
Load1
BRK
Timed
Breaker
Logic
Closed@t0
Current (A)
Voltage (V)
Fig. 7. Circuit Arrangement Modeled with Proposed C2R-R
Configuration for Micro Hydro Scheme
400
300
200
100
0
-100
-200
-300
-400
80
60
40
20
0
-20
-40
-60
-80
Vgen_a
Vgen_b
Vgen_c
Igen_a
Igen_b
Igen_c
Vgen
rms voltage (pu)
1.20
Fig. 8(a).
Three-Phase Instantaneous Voltage and Current at
1.00
the Machine Terminals
0.80
0.60
0.40
0.20
0.00
1.840
1.860
1.880
1.900
1.920
1.940
1.960
1.980
5.0
6.0
7.0
8.0
9.0
10.0
...
...
...
Fig. 8 (b). Power Variation and Rotor Speed Variation
5. COMPARISON OF THE PERFORMANCE OF THE
PROPOSED C2R METHOD WITH THAT OF THE
C2C METHOD
BRKA
A Iga
4.0
...
...
...
According to the Section 2 and 3, it has been proved that
the C-2C configuration can deliver only 50% of the capacity
of the generator whereas the proposed C2R-R method can
deliver 87%. Further to investigate their limitations, the
simulation study was carried out with different operating
conditions. This was done by varying input torque while
keeping load same and then varying the load while keeping
input torque the same. In all operating cases, the machine
overloading was carefully prevented. This was done in both C2C and C2R-R methods to understand the steady state stability
operation of both systems when a disturbance come either
from input torque or output load power. Figs. 9 and 10 show
the variation of voltage and power for different torque levels
of C-2C and C2R-R methods respectively.
As Fig. 9 shows, it is clear that the generator can only
deliver up to 16 kW out of its rated power within its maximum
allowable voltage limits when the C-2C scheme is employed.
According to the results of C2R-R configuration, shown
in Fig. 10; the power output is considerably higher than that of
the traditional method. It is capable of producing almost its
rated power without violating the voltage and current ratings
of the generator, if the machine is operated at its C2R-R
arrangement. However, if the machine does not have enough
magnetization then it must be exited at its startup operation.
T.S. Weerakoo et al./ Journal of Energy & Environment, Vol. 1 (2009), No. 1, 38-44
Fig. 9. Power and Voltage Variation for Different Torques of
C2C Configuration
Fig. 10. Power and Voltage Variation for Different Torques of
C2R Configuration
The Fig. 11 shows the basic schematic diagram of a
single controller and the ballast loads arrangement. As it
shows, ballast loads consist of three switched resistors
connected in parallel and the controller consist three
comparators. The outputs of the comparators give the control
signals for the switches of the ballast resistors.
The PI controller determines the control signals for the
three comparators. The rms terminal voltage is compared with
the reference and the error is processed through a PI controller
to obtain V_PI. V_PI and reference voltage for first comparator
(V_1) is compared to derive the control signal for the first
ballast load (S_1). Similarly, reference voltage for the second
comparator (V_2) is compared with V_PI in order to obtain the
control signal for the second ballast load (S_2). Third
comparator generates the switching signal for the third ballast
load by comparing the saw tooth wave form of twice the
power frequency with control voltage V_control. The control
voltage V_control is obtained by adding V_pi with –V_3 where
V_3 is defined such that it reset the V_control to its initial value
when either of the ballast is ON.
The Fig. 12 explains how the controller is functioning.
When the V_control cuts the sow tooth signal, the comparator 3
generates pwm signal and it determines the firing angle for the
TRIAC connected to the third ballast load. As Fig. illustrates
when the V_control reachs to zero (which is identical to zero
firing angle), ballast loads are switched and V_control is reset
(identical to 180 degrees firing angle).
Fig. 13 shows the performance of the full ELC for a
sudden load change. At 2.2 S, the output load is decreased to
12 KW from 18 kW. Then the voltage tries to increase, but it
is maintained fairly at constant by the ballast load. At 10 S, the
load disturbance is removed. Then the ballast loads are
disengaged from the system and the system comes back to its
normal operation. The frequency of the output voltage has
been estimated by using the PLL block available in the master
library of the PSCAD software.
6. ELECTRONIC LOAD CONTROLLER FOR C2R-R
ARRANGEMENT
Electronic loads controller (ELC) is used to maintain the
generator output voltage and frequency by matching its input
and output. There are two separate ballast loads connected in
parallel with the two loads of the machine. In order to switch
the ballast loads, two independent controllers are used.
Fig. 11. Schematic Diagram of the ELC with the Ballast
Loads
42
Fig. 12. Controller Output Signals of the ELC Modeled in
PSCAD/EMTDCTM
T.S. Weerakoo et al./ Journal of Energy & Environment, Vol. 1 (2009), No. 1, 38-44
7. ADVANTAGES
CONFIGURATION
OF
C2R-R
SYSTEM
The existing method of C-2C configuration is used to
convert three phase generated power to single phase power.
Since the single phase power generation capability of the C2C method is about 50%, the proposed C2R-R method has an
advantage of generating 86% of the machine’s rated power
without violating its rating limits. In the C2R-R configuration
two phase loads can be connected with single neutral terminal.
Therefore, single phase power can be delivered into to two
directions to cover wider area of distribution without putting
up any transformers. Also the ballast load will be distributed
in two phases. This increases the system reliability and its
smooth operation by reducing per unit switching load. For
example a real test site (Memura village, Sri Lanka) is shown
in Fig. 14 along with a micro hydro scheme with the proposed
configuration.
43
8. CONCLUSION
This paper presented a novel method of operation of the
three phase induction generator under single phase power
distribution in micro-hydro generation. Proper selection of
capacitor and the loads bring the system in balanced operation
condition. Furthermore this paper discussed the benefits and
the advantages of the proposed C2R-R method. To get the
maximum efficient from the generator, correct distribution of
loads is necessary. Smooth and proper switching of ballast
loads always ensures the balance operation of the system.
By considering the other methods of converting three
phases to single phase power, this C2R-R method shows a
good performance. Full analytical designing of proposed
method is discussed in this paper. Operation of the C2R-R
system has been demonstrated with EMTDC/PSCADTM
simulation.
Fig. 14. Load Distribution of the C2R-R Configuration in A
Real Test Site
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