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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
STATCOM-Based Controller for a Three-Phase
SEIG Feeding Single-Phase Loads
Bhim Singh, Fellow, IEEE, S. S. Murthy, Life Fellow, IEEE, and Raja Sekhara Reddy Chilipi
Abstract—This paper presents single-phase power generation
using a three-phase self-excited induction generator (SEIG) working in conjunction with a three-phase static synchronous compensator (STATCOM). The STATCOM is employed to compensate
the unbalanced currents caused by single-phase loads that are connected across the two terminals of the three-phase SEIG. Therefore,
the SEIG is capable of feeding single-phase loads up to its rated
power. Moreover, the STATCOM regulates the SEIG terminal voltage through reactive power compensation and also suppresses the
harmonics injected by consumer loads. A single-phase synchronous
D-Q frame theory-based control algorithm is used to generate gating pulses to the three-phase STATCOM. The proposed method
of single-phase power generation from the three-phase SEIG is
investigated experimentally on a 3.7-kW, 230-V, Y-connected induction machine. The performance of the SEIG–STATCOM system is evaluated for both linear and nonlinear single-phase loads.
Furthermore, the performance of the SEIG at different terminal
voltages is investigated and the terminal voltage corresponding to
the maximum power output is identified.
Index Terms—Self-excited induction generator (SEIG), singlephase synchronous D-Q frame theory, static synchronous compensator (STATCOM).
I. INTRODUCTION
N externally driven squirrel-cage induction machine with
its stator terminals connected to a reactive power source
(capacitance) is popularly known as a self-excited induction
generator (SEIG) [1] [2]. Due to the specific advantages of
the squirrel-cage induction machine over the conventional synchronous machine such as low cost, brush-less construction,
ruggedness, cheap, and inherent short-circuit protection, SEIGs
are being employed in remote and isolated power generating systems as mechanical energy conversion devices [3]–[5]. When
an SEIG is driven by prime movers such as biomass, biogas,
and biodiesel engines, the frequency of the generated voltage
is almost constant from no load to full load. But, poor voltage
regulation has been the major drawback of an SEIG in its applications. Hence, the terminal voltage of an SEIG needs to be
regulated during load perturbations. Several voltage regulating
schemes have been reported for SEIG-based autonomous power
generation systems [6]–[18]. The methods reported in [6]–[10]
A
Manuscript received January 31, 2013; revised May 28, 2013, August 1,
2013, and November 8, 2013; accepted December 30, 2013. Date of publication
January 29, 2014; date of current version May 15, 2014. Paper no. TEC-000592013.
The authors are with the Department of Electrical Engineering, Indian
Institute of Technology Delhi, New Delhi 110016, India (e-mail: bsingh@
ee.iitd.ac.in; [email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TEC.2014.2299574
have employed passive elements for voltage regulation. Bim
et al. [8] have proposed a voltage compensation method for
a three-phase SEIG using the long-shunt method. Shirdhar
et al. [9] have used the short-shunt compensation method for
a three-phase SEIG. The methods employing only passive elements are not capable of regulating the terminal voltage when
the nature of the load changes. Therefore, some attempts have
been made to maintain a constant terminal voltage using active
switches which use the combination of a fixed capacitor and
thyristor-controlled inductor known as static var compensator
(SVC) [11]. The SVC-based voltage regulating methods would
generate low-order harmonic currents caused by the switching
of line currents and they also involve large size and heavy weight
of passive elements. With the development of fast acting selfcommutating switches, pulse width modulated (PWM) voltage
source inverter (VSI)-based static reactive power compensators
(STATCOMs) [12]–[18] have been evolved. These STATCOMbased voltage regulators exhibit better dynamic performance
and their voltage regulation capability would not be affected by
nature of the load.
Since many of the isolated power generating systems feed
single-phase loads, single-phase induction generators can also
be used. A single-phase induction machine with a proper excitation capacitance can be used as a generator [19]. In general,
single-phase machines are limited to relatively small power outputs. For larger power ratings above 5 kW, three-phase machines
are more efficient, cheaper, and readily available. Hence, a threephase SEIG can be used in single-phase applications. A number
of attempts have been made to operate a three-phase SEIG in
a single-phase mode [20]–[23]. In all these methods, the SEIG
cannot be loaded up to its rated power, requires de-rating of
the machine. Moreover, the SEIG currents are not balanced and
unequal voltages across the generator windings are the major
drawbacks of the aforementioned single-phasing methods of a
three-phase SEIG.
In this paper, an alternative method of feeding single-phase
loads using a three-phase SEIG without de-rating the machine
is proposed. In this method, a three-phase SEIG works in conjunction with a three-phase STATCOM and the single-phase
loads are connected across two of the three terminals of the
SEIG. The benefit of integrating a STATCOM in an SEIGbased standalone power generation feeding single-phase loads
is threefold—first, generator currents balancing; second, voltage regulation; and third, mitigates the harmonics injected by
nonlinear loads. The STATCOM injects compensating currents
to make the SEIG currents balanced and regulates the system
voltage as well. Moreover, this method offers balanced voltages across the generator windings and ensures the sinusoidal
winding currents while feeding nonlinear loads.
0885-8969 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
Fig. 1.
321
Schematic diagram of the SEIG–STATCOM system feeding single-phase loads.
Although a number of publications have been reported on
the STATCOM-based controller for SEIG systems, all the functions of a STATCOM such as current balancing, voltage regulation, and harmonic currents suppression are not experimentally demonstrated. Singh et al. [14] have presented simulation
of the PI (Proportional plus Integral) controller-based control
scheme for voltage profile improvement using the STATCOM.
The major drawback of the PI controller-based control scheme
is its instability under transients as it does not consider load
currents while estimating the amplitudes of active and reactive
components of reference source currents. The outputs of the
STATCOM’s dc-bus voltage PI controller and the system ac
voltage PI controller are directly considered as the amplitudes
of active and reactive components of reference source currents,
respectively. Therefore, the control algorithm presented in [14]
requires adjustment of PI controller gains under transients, otherwise it causes severe peak overshoot and undershoot of the
dc-bus voltage of the STATCOM. Dastagir and Lopes [15] have
presented the experimental illustration of SEIG voltage control employing a battery-assisted STATCOM using conventional
three-phase D-Q frame theory-based control. The transient response of this scheme is found to be sluggish as the system is
gradually reaching steady state after disturbance. Moreover, the
additional important features of the STATCOM such as current
balancing and current harmonics suppression have not been explored. Chen et al. [17] have proposed a control algorithm for
the STATCOM in frequency perturbed systems to avoid loss of
synchronization under random varying loads. Its performance
is also demonstrated only for three-phase balanced linear loads.
Therefore, this paper proposes a single-phase synchronous
D-Q frame [24]–[27] based control algorithm for a three-phase
STATCOM system, which is capable of producing balanced
source currents even under heavy unbalance currents and voltages caused by single-phase loads. Unlike the control algorithm
presented in [14], the proposed control takes the load current
into account, the average active and reactive power components
of load current are added/subtracted to the output of the dc-bus
voltage PI controller and ac voltage PI controller, respectively.
Therefore, in the proposed control algorithm, adjustment of PI
controller gains is not required under transients. Since the system under investigation is an isolated power generating system
feeding local loads, the generated voltages are presumed to be
balanced. The proposed method of feeding single-phase loads
from a three-phase SEIG is tested experimentally and the benefits of incorporating a STATCOM to balance SEIG currents and
system voltage regulation are demonstrated with experimental results. Moreover, the performance of the SEIG at different
terminal voltages is investigated and the terminal voltage corresponding to the maximum power output is identified.
II. SYSTEM CONFIGURATION AND PRINCIPLE OF OPERATION
Fig. 1 shows the schematic diagram of the STATCOMcompensated three-phase SEIG feeding single-phase loads. The
system consists of an SEIG driven by renewable energy-based
prime mover. The single-phase consumer loads are connected
across “a” and “c” phases of the SEIG. A two-level, three-leg
insulated-gate bipolar transistor (IGBT)-based VSI with a selfsustaining dc-bus capacitor is used as a STATCOM. The STATCOM is connected at point of common coupling (PCC) through
filter inductors as shown in Fig. 1. The STATCOM regulates the
system voltage by maintaining equilibrium among the reactive
power circulations within the system. Moreover, the STATCOM
suppresses harmonics injected by nonlinear loads and provides
load balancing while feeding single-phase loads.
The unbalanced load currents in a three-phase system can be
divided into two sets of balanced currents known as positive
sequence components and negative sequence components. In
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Fig. 2.
IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
Block diagram of the single-phase synchronous D-Q theory control algorithm for the STATCOM.
order to achieve balanced source currents, the source should be
free from the negative sequence components of load currents.
Therefore, when the STATCOM is connected across PCC, it supplies the negative sequence currents needed by the unbalanced
load or it draws another set of negative sequence currents which
are exactly 180◦ out of phase to those drawn by unbalanced
load so as to nullify the effect of negative sequence currents of
unbalanced loads.
III. CONTROL ALGORITHM OF THE STATCOM
Fig. 2 shows the block diagram of the proposed single-phase
synchronous D-Q frame theory-based control algorithm for the
three-phase STATCOM. The reference source currents (i∗sa , i∗sb ,
i∗sc ) for regulating the terminal voltage and current balancing are
computed using a single-phase synchronous D-Q frame theory
applied to the three-phase SEIG system.
A. Single-Phase Synchronous Rotating D-Q Frame Theory
It is simple to design a controller for a three-phase system in
synchronously rotating D-Q frame because all the time-varying
signals of the system become dc quantities and time-invariant. In
case of a three-phase system, initially, the three-phase voltages
or currents (in abc frame) are transformed to a stationary frame
(α − β) and then to synchronously rotating D-Q frame. Similarly, to transform an arbitrary signal “x(t)” of a single-phase
system into a synchronously rotating D-Q frame, initially that
variable is transformed into a stationary α − β frame using the
single-phase p-q theory [28]–[30] and then to a synchronously
rotating D-Q frame. Therefore, to transform a signal into a stationary α − β frame, at least two phases are needed. Hence, a
pseudo second phase for the arbitrary signal x(t) is created by
giving 90◦ lag to the original signal. The original signal represents the component of α-axis and 90◦ lag signal is the β-axis
component of stationary reference frame.
Fig. 3. Stationary α − β frame and synchronously rotating D-Q frame representation of vector x(t).
Therefore, an arbitrary periodic signal x(t) with a time period
of “T ” can be represented in a stationary α − β frame as
T
xα (t) = x(t); xβ (t) = x t −
.
(1)
4
For a single-phase system, the concept of the stationary α − β
frame and synchronously rotating D-Q frames relative to an arbitrary periodic signal x(t) is illustrated in Fig. 3. The signal x(t)
is represented as vector x, and the vector x can be decomposed
into two components xα and xβ . As the x vector rotates around
the center, its components xα and xβ which are the projections
on the α − β axes vary in time accordingly. Now, considering
that there are synchronously rotating D-Q coordinates that rotate with the same angular frequency and direction as x, then the
position of x with respect to its components xD and xQ is same
regardless of time. Therefore, it is clear that the xD and xQ do
not vary with time and only depend on the magnitude of x and
its relative phase with respect to the D-Q rotating frame. The
angle θ is the rotating angle of the D-Q frame and it is defined
as
t
θ=
ωdt
(2)
0
where ω is the angular frequency of the arbitrary variable x.
SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
The relationship between stationary and synchronous rotating frames can be derived from Fig. 3. The components of the
arbitrary single-phase variable x(t) in the stationary reference
frame are transformed into the synchronously rotating D-Q
frame using the transformation matrix “C” as
xD
xQ
=C
xα
(3)
xβ
where,
C=
cos θ
sin θ
− sin θ
cos θ
.
(4)
B. Reference Source Currents Estimation Using Single-Phase
Synchronous Rotating D-Q Frame Theory
The main objective of employing a three-phase STATCOM in
a three-phase SEIG-based standalone power generating system
feeding single-phase consumer loads is to balance the generator
currents so that the generator can be loaded to its full capacity
without derating. The control structure of the STATCOM employs an ac voltage PI controller to regulate the system voltage
and a dc bus voltage PI controller to maintain the dc bus capacitor voltage constant and greater than the peak value of the
line voltage of PCC for successful operation of the STATCOM.
The PCC voltages (va , vb , vc ), source currents (isa , isb , isc ),
load current (il ), and dc bus voltage (Vdc ) are sensed and used
as feedback signals. Considering PCC voltages as balanced and
sinusoidal, the amplitude of the PCC voltage (or system voltage)
is estimated as
2 2
v + vb2 + vc2 .
(5)
Vt =
3 a
Consider one of the three phases at a time and then transform the voltages and currents of that particular phase into a
stationary α − β frame, then the PCC voltages and load current
in stationary α − β frame are represented as
vaα (t) = va (t); vbα (t) = vb (t); vcα (t) = vc (t) (6)
T
vaβ (t) = va t −
(7a)
4
T
(7b)
vbβ (t) = vb t −
4
T
(7c)
vcβ (t) = vc t −
4
T
.
(8)
ilα (t) = il (t); ilβ (t) = il t −
4
The sinusoidal signal filters based on a second-order generalized integrator [31] or a sinusoidal signal integrator (SSI) [32]
can be used for creating β-axis signals which are lagging the
original signals. In the present investigation, a filter based on SSI
is used. The SSI filters generate quadrature signals using system
323
frequency information. Since the system frequency fluctuates
under load perturbations, a PLL [31] is used to continuously estimate the system frequency, and the estimated frequency is fed
to SSI filters which makes the proposed control adaptive to frequency fluctuations, thereby avoids the loss of synchronization
of the STATCOM.
Now consider a synchronously rotating D-Q frame for phase
“a” which is rotating in the same direction as va (t), and the
projections of the load current il (t) to the D-Q axes give the
D and Q components of the load current. Therefore, the D-axis
and Q-axis components of the load current in phase “a” are
estimated as
sin θa
cos θa
ilα
IlaD
=
(9)
IlaQ
− sin θa cos θa
ilβ
where cos θa and sin θa are estimated using vaα and vaβ as
follows:
vaα
cos θa
1
=
.
(10)
sin θa
vaβ
2 + v2
vaα
aβ
IlaD represents the active power component of the load current
as the signals belong to the same axis are multiplied and added
to estimate the D-axis component, whereas IlaQ represents the
reactive power component of the load current as the orthogonal
signals are multiplied and added to derive the Q-axis component.
Similarly, the D-axis and Q-axis components of the load current in phase “c”are estimated as
sin θc
cos θc
−ilα
IlcD
=
.
(11)
IlcQ
− sin θc cos θc
−ilβ
The negative sign of currents in (11) indicates that the load
current in phase “c” is equal to phase “a” but 180◦ out of phase.
As the single-phase load is connected across the phases “a”
and “c,” D-axis and Q-axis components for phase “b” are not
estimated.
The D-axis components of the load current in phases “a” and
“c” are added together to obtain an equivalent D-axis current
component of total load on the SEIG as
IlD = IlaD + IlcD .
(12)
Similarly, an equivalent Q-axis current component of total
load on the system is estimated as
IlQ = IlaQ + IlcQ .
(13)
The equivalent D-axis and Q-axis current components of total
load are decomposed into two parts namely fundamental and
oscillatory parts as
IlD = IlD + I
lD
(14)
IlQ = IlQ + I
lQ .
(15)
The reason for the existence of the oscillatory part is due to
the nonlinear and single-phase nature of connected loads in
the system. Even if the connected loads are linear in nature,
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
the D and Q components estimated in (12) and (13) would
still contain oscillatory parts due to the unbalance caused by
single-phase loads. To ensure the power quality, the reference
D-axis and Q-axis components of source currents must be free
from these oscillatory components. Hence, the signals IlD and
IlQ are passed through low-pass filters (LPFs) to extract the
fundamental (or dc) components as shown in Fig. 2.
To maintain the dc-bus capacitor voltage of the STATCOM at
a reference value, it is sensed and compared with the reference
value and then the obtained voltage error is processed through a
PI controller. The dc-bus voltage error of the STATCOM Vdcer
at k th sampling instant is expressed as
Vdcer (k) = Vdcref (k) − Vdc (k)
(16)
where Vdcref (k) and Vdc (k) are the reference and sensed dc-bus
voltages of the STATCOM at k th sampling instant, respectively.
In the present investigation, the dc-bus voltage reference is set
to 400 V.
The output of the PI controller for maintaining a constant
dc bus voltage of the STATCOM at k th sampling instant is
expressed as
Iloss (k) = Iloss (k − 1) + Kpd {Vdcer (k) + Vdcer (k − 1)}
+ Kid Vdcer (k)
(17)
where Iloss is the active power component of the current (or
D-axis current component) that must be supplied to meet the
losses in the STATCOM. Kpd and Kid are the proportional and
integral gain constants of the dc-bus voltage PI controller, respectively. The source should supply the power loss component
of the current (Iloss ) along with the filtered equivalent D-axis
current component of the single-phase load estimated in (14).
In order to ensure balanced and sinusoidal source currents, the
D-axis component of source currents after compensation must
be equal for all the phases and it should not contain any ripple.
Therefore, IlD is added to Iloss and distributed among all the
phases equally to obtain the D-axis component of the reference
source current in each phase which can be expressed as
∗
IsD
ph
IlD + Iloss
.
=
3
(18)
∗
IsD
ph also indicates the active power component of the current
that should be supplied by the source after compensation.
For regulating the system voltage (i.e., PCC voltage), the
STATCOM has to inject the reactive power component of the
current to meet the reactive power demands of both the load
and SEIG. The amount of the reactive power component of
the current to be injected by the STATCOM is estimated by
an ac voltage PI controller. The amplitude of the PCC voltage
computed in (5) is compared with the reference voltage. The
PCC voltage error Ver (k) at k th sampling instant is given as
Ver (k) = Vtref (k) − Vt (k)
(19)
where Vtref is the amplitude of the reference PCC voltage and
Vt (k) is the amplitude of sensed three-phase ac voltages at the
PCC terminals, at k th instant. The reference voltage is selected
to maintain the PCC line voltage at 220 V.
The output of the PI controller for maintaining the PCC voltage at the reference value in k th sampling instant is expressed
as
IQ (k) = IQ (k − 1) + Kpa {Ver (k) + Ver (k − 1)}
+ Kia Ver (k)
(20)
where Kpa and Kia are the proportional and integral gain constants of the PI controller, Ver (k) and Ver (k − 1) are the voltage
errors at k th and (k − 1)th instants, respectively. IQ (k) is the
equivalent Q-axis component (or reactive power component) of
the current to be supplied by the STATCOM to meet the reactive power requirements of both the load and SEIG, thereby it
maintains the PCC voltage at the reference value.
The per phase Q-axis component of the reference source
current required to regulate the system voltage is defined as
∗
IsQ
ph =
IQ − IlQ
.
3
(21)
∗
IsQ
ph indicates the magnitude of the reactive power component
of the current that should be supplied to each phase of the source
(i.e., SEIG) to achieve the reference terminal voltage. The value
∗
of IsQ
ph can be either positive or negative based on loading
conditions.
Using the D-axis and Q-axis components of currents derived
in (18) and (21), the phase “a,” α- axis and β-axis components
of the reference source current can be estimated as
−1 ∗
∗ cos θa
IsD ph
sin θa
isaα
=
.
(22)
∗
i∗saβ
− sin θa cos θa
IsQ
ph
In the above matrix, the α-axis current represents the reference
source current of actual phase “a,” and the β-axis current represents the current that is at π2 phase lag which belongs to the
fictitious phase.
Therefore, one can have
∗
∗
i∗sa = IsD
ph cos θa − IsQ ph sin θa .
(23)
Similarly, reference source currents for phases “b” and “c” are
estimated as
∗
∗
i∗sb = IsD
ph cos θb − IsQ ph sin θb
(24)
i∗sc
(25)
=
∗
IsD
ph
cos θc −
∗
IsQ
ph
sin θc .
Three-phase reference source currents (i∗sa , i∗sb , and i∗sc ) are
compared with the sensed source currents (isa , isb , and isc ) and
the current errors are computed as
iaerr = i∗sa − isa
iberr =
i∗sb
− isb
icerr = i∗sc − isc .
(26a)
(26b)
(26c)
These current error signals are fed to the current-controlled
PWM pulse generator for switching the IGBTs of the
SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
325
STATCOM. Thus, the generated PWM pulses are applied to
the STATCOM to achieve sinusoidal and balanced source currents along with desired voltage regulation .
IV. EXPERIMENTAL IMPLEMENTATION
Fig. 1 shows the STATCOM-compensated SEIG system feeding single-phase loads. A 3.7-kW, 230-V, 50-Hz, Y-connected
induction generator has been used to investigate the performance
while feeding single-phase loads. A Δ-connected 4-kVAR capacitor bank is connected across the SEIG terminals to provide
self-excitation. A 3.7-kW, 415-V, 50-Hz, Y-connected induction motor fed from a variable frequency drive is used to realize
the prime mover for the SEIG. A three-phase two-level IGBTbased VSI has been used as the STATCOM. The estimation of
kVA rating of the STATCOM is presented in Appendix A. The
STATCOM is connected across the PCC through filter inductors
Lf . Both linear and nonlinear loads are considered for testing
the system. A single-phase uncontrolled diode bridge rectifier
feeding a series R–L load is used as a nonlinear load. The Hall
effect current sensors (LEM-25 A) are used to sense the source
currents of phases “a” and “c.” The current in phase “b” is
estimated under the assumption that the sum of instantaneous
currents in three phases is zero. Three voltage sensors have
been used to sense the phase voltages va , vb , and the dc-bus
capacitor voltage of the STATCOM (Vdc ). A dSPACE-based
digital controller (DS1104 DSP) has been used to implement
the control algorithm and to generate the switching pulses to the
STATCOM. A fixed step sampling time of 55 μs has been used
for processing the control algorithm. Detailed data of system
parameters are given in Appendix B.
V. RESULTS AND DISCUSSION
A prototype of the proposed SEIG–STATCOM system has
been developed and tested experimentally at different loads.
The experimental results presented in Figs. 4–9 demonstrate the
performance of the developed system under steady state as well
as dynamic conditions.
A. Steady-State Performance of the SEIG–STATCOM System
Fig. 4 shows the steady-state performance of the SEIG–
STATCOM system feeding a single-phase resistive load of 3.35
kW. Fig. 4(a) shows the SEIG current (ig a ) along with the PCC
line voltage vab . Fig. 4(b) shows the harmonic spectra of the
SEIG current in phase “a.” The total harmonic distortion (THD)
of generator currents is found less than 5% meeting IEEE-519
standard requirements on current harmonics. Fig. 4(c) shows
the RMS (root mean square) value of the PCC line voltage (vab )
and its harmonic spectrum. Fig. 4(d) shows the single-phase
load current under rated loading. Fig. 4(e) and (f) shows the
generated power (Pgen ) and the power consumed by singlephase loads (Pload ) which are connected across the phases “a”
and “c.” It has been observed that the generator is feeding a
single-phase load of 3.35 kW which is almost the rated capacity
of a generator without any unbalance in voltages and currents.
Fig. 4. Steady-state performance of the SEIG–STATCOM system feeding
single-phase linear loads. (a) v a b and ig a . (b) THD of ig a . (c) THD of v a b .
(d) v a b and il . (e) P g e n . (f) P lo a d .
B. Performance of the SEIG at Different Terminal Voltages
In Fig. 4(a)–(f), it can be observed that the SEIG terminal
voltage is maintained at 220 V which is slightly lower than the
rated voltage of the SEIG. The reason for maintaining a slightly
lower terminal voltage is to operate the SEIG close to optimum
voltage point [33], [34]. An optimum voltage is the allowable
terminal voltage of the SEIG at which the SEIG is able to
generate power more than the rated capacity without exceeding
the rated winding current. Fig. 5(a) shows the waveforms of
phase “a” SEIG current and the line voltage vab and their RMS
values. The RMS values shown in Fig. 5(a) represent the rated
voltage and rated current of the SEIG. The SEIG output power
and load power at the rated voltage and rated current are shown
in Fig. 5(b) and (d), respectively. The generator output power
and load power at the rated voltage (which is 230 V) are found to
be less when compared to those at 220 V and the rated currents
which are shown in Fig. 4(e) and (f), respectively. It can be
explained as follows; the current carried by the SEIG winding at
any instant can be divided into reactive power and active power
components. The reactive power component of the current is
responsible for the generator terminal voltage, and active power
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
TABLE I
PERFORMANCE OF THE SEIG AT DIFFERENT TERMINAL VOLTAGES AND RATED
WINDING CURRENT
Fig. 5. SEIG output power and load powers at the rated voltage. (a) v a b and
ig a . (b)P g e n . (c) v a b and il . (d) P lo a d .
Fig. 6.
Magnetizing characteristics of a three-phase SEIG.
component represents the active power delivered. For a constant
generator current, if the reactive power component decreases,
the active power component can be increased. Any reduction in
the reactive power component of the current can be utilized in
increasing the active power component keeping the generator
current constant.
It is well known that the reactive power component of the
current reduces with the reduction in the terminal voltage. However, the reactive power component of current requirement of
the SEIG not only depends on the terminal voltage but is also
affected by its magnetizing reactance. The variation of the magnetizing reactance (Xm ) with the terminal voltage is obtained
from synchronous speed test and it is shown in Fig. 6. It is seen
that the value of Xm is less at higher terminal voltage and it increases as the terminal voltage reduces. Therefore, the reactive
power component of the current required by the SEIG does not
vary proportionally with the terminal voltage.
The performance of the SEIG at different terminal voltages
is illustrated in Table I. As the voltage is varied from 240 to
200 V, the SEIG output power is seen to be increased from
3.48 to 3.91 kW, and it started decreasing when the terminal
voltage is further reduced below 200 V. The variation of power
generation is also reflected on the power available to loads.
When the SEIG is operating at 240 V which is slightly higher
than the rated voltage, the SEIG requires more reactive power
due to the cumulative effect of the high terminal voltage and less
value of Xm . Hence, for fixed and rated values of the winding
current, the active power component of the current must be
decreased. Therefore, the power output of the SEIG is less at
higher voltages. Although the terminal voltage is more than the
rated value, the power output of the SEIG is still reduced because
the percentage reduction in an active component of the current
is more than the percentage increase of the terminal voltage. It is
observed that the SEIG is delivering the rated power at the rated
voltage of 230 V. When the terminal voltage is reduced slightly
below the rated value, the reactive power demanded by the SEIG
reduces due to the reduction in the voltage and increase in Xm ,
leading to the reduction of the reactive power component of
the SEIG winding current. Since the reactive power component
of current requirement is reduced, the active power component
of the current can be increased for a fixed value of the winding
current which in turn improves the SEIG operating power factor.
Therefore, the power output of the SEIG is increased at slightly
reduced voltages because the percentage increase in the power
factor is more than the percentage reduction in the terminal
voltage.
The terminal voltage of the SEIG is further reduced till 170 V.
It has been observed that the SEIG is delivering maximum power
of 3.91 kW at 200 V. When the voltage is reduced below 200 V,
the output power of the SEIG has started decreasing because the
percentage reduction in the terminal voltage is more significant
than the percentage increase in the active power component of
the current. Moreover, after 200 V, percentage variation of the
magnetizing reactance becomes less significant. Even though
some voltages below 220 V are giving more output power, due
to the limitations imposed by electricity standards they are not
considered for investigation.
SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
327
Fig. 7. Steady-state performance of the SEIG–STACOM system feeding single-phase nonlinear loads. (a) va b , ig a , ig b , and ig c . (b) v a b , is a , is b , and is c .
(c) v a b , V d c and V t R M S and il . (d) v a b , ic o m a , ic o m b and ic o m c .
C. Steady-State Performance of the SEIG–STATCOM System
Feeding Single-Phase Nonlinear Loads
The steady-state performance of the SEIG–STATCOM system feeding nonlinear loads is shown in Fig. 7. The generator
currents (ig a , ig b , and ig c ) and the PCC line voltage vab are
shown in Fig. 7(a). Fig. 7(b) shows the balanced three-phase
source currents isa , isb , and isc . The steady-state waveforms of
the PCC voltage vab , the load current il , the dc-bus voltage Vdc ,
and the RMS value of the PCC line voltage VtRM S are given in
Fig. 7(c). The dc-bus voltage and the RMS PCC line voltage are
seen to be quite stable in steady state. The compensation currents injected by the STATCOM to balance the source currents
are shown in Fig. 7(d). From Fig. 7(a), it is observed that the
SEIG currents are balanced and sinusoidal which demonstrates
the harmonics suppression and load balancing capabilities of
the STATCOM.
D. Dynamic Performance of the SEIG–STATCOM System
Feeding Single-Phase Nonlinear Loads
Fig. 8(a)–(c) shows the dynamic performance of the SEIG–
STATCOM system feeding single-phase nonlinear loads under
load reduction. Fig. 8(a) shows the line voltage vab , the SEIG
current ig c , the STATCOM current icom c and load currents il be-
fore and after the load reduction. Reduction in the magnitude of
the SEIG current and STATCOM currents can be observed during the load change. Moreover, the SEIG current is observed
to be sinusoidal, which clearly demonstrates the harmonics
mitigation capability of the STATCOM. Fig. 8(b) shows the
three-phase STATCOM currents icom a , icom b and icom c along
with the PCC line voltage vab and it can be noticed that the
STATCOM currents are decreasing as the load is reduced.
Fig. 8(c) shows the dc-bus voltage of the STATCOM, the phase
“c” current of the STATCOM, and the load current. A small rise
in the dc bus voltage Vdc is observed during the load change,
due to the PI controller action, it has been brought back to the
reference value.
Dynamic performance of the SEIG–STATCOM system during load application is depicted in Fig. 9(a)– (c). The SEIG
current ig c , the STATCOM current icom c , and the load current
il along with the PCC line voltage vab are shown in Fig. 9(a).
Increase in the amplitude of the SEIG current and STATCOM
current can be observed during the load change. Fig. 9(b) shows
the STATCOM currents and PCC line voltage vab . When the
load increases, the compensation current required increases and
hence, the STATCOM currents have started increasing during
the load change. In Fig. 9(c), the transient behavior of the
STATCOM dc bus voltage, the STATCOM current icom c and
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
Fig. 8. Dynamic performance of the SEIG–STATCOM during load removal.
(a) v a b , ic o m c , ig c and il . (b) v a b , ic o m a , ic o m b and ic o m c . (c) v a b , V d c ,
ic o m c and il .
the load current il are given along with the PCC voltage vab
under load application. During the sudden application of load,
the dc-bus voltage Vdc has reduced a little and then settled to
the reference value. The PCC voltage is found very stable and
well regulated during the load perturbation.
VI. CONCLUSION
The proposed method of feeding single-phase loads from a
three-phase SEIG and STATCOM combination has been tested,
Fig. 9. Dynamic performance of the SEIG–STATCOM during load application. (a) v a b , ic o m c , ig c and il . (b) v a b , ic o m a , ic o m b and ic o m c . (c) v a b , V d c ,
ic o m c and il .
and it has been proved that the SEIG is able to feed singlephase loads up to its rated capacity. A single-phase synchronous
D-Q frame theory-based control of a three-phase STATCOM
has been proposed, discussed, and experimentally implemented
for current balancing of the SEIG system. Experimental results have demonstrated effective current balancing capability
of the proposed single-phase synchronous D-Q frame-based
control using the STATCOM. In addition to current balancing,
the STATCOM is able to regulate the terminal voltage of the
SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
generator and suppresses the harmonic currents injected by nonlinear loads. The performance of the SEIG at different voltages
has been investigated experimentally to identify the terminal
voltage corresponding the maximum power output. It has been
observed that when the SEIG is operated at 200 V instead of the
rated voltage, the generator is able to deliver 3.91 kW without
exceeding the rated winding current. The satisfactory performance demonstrated by the developed the STATCOM–SEIG
combination promises a potential application for isolated power
generation using renewable energy sources in remote areas with
improved power quality.
APPENDIX A
ESTIMATION OF STATCOM KVA RATING
The capacitor bank supplies the reactive power demanded by
the SEIG to maintain the rated voltage at the rated active power.
Therefore, at the rated unity power factor load, the STATCOM
does not supply any reactive power to the SEIG, it provides only
current balancing, and hence, the source currents operate at unity
power factor. If the connected load demands reactive power
along with the rated active power, the STATCOM provides the
load reactive power along with current balance. At lighter loads,
the power factor of source currents deviate from unity to lagging,
because the STATCOM absorbs excess reactive power supplied
by the capacitor bank. In order to estimate the STATCOM rating,
following cases are considered.
1) A single-phase load of 3.8 kW unity pf.
2) A single-phase load of 3.8 kW 0.8 lagging pf.
3) A single-phase load of 3.8 kW 0.8 leading pf.
It has already been mentioned that the SEIG is able to deliver
3.8 kW at 220 V and rated current. Therefore, the rated load is
considered 3.8 kW instead of 3.7 kW.
Case 1: Neglect the losses in the STATCOM and consider
that the total generated power is delivered to the load. At rated
load, the source currents are always at unity power factor and
they are be computed as
Pgen
∠ 0◦ = 9.97 ∠ 0◦ A
Isa = √
3 × VL
(27a)
Isb = 9.97 ∠ −120◦ A
(27c)
Isc = 9.97 ∠ 120◦ A
(27c)
Pload
∠ − 30◦ = 17.27 ∠ − 30◦ A
VL × pf
Applying KCL at PCC, the STATCOM currents are computed
from the load current and source currents as
Icom a = Isa − Il = 9.97 ∠ 120◦ A
(29a)
Icom b = Isb = 9.97 ∠ −120◦ A
(29b)
◦
Icom c = Isc + Il = 9.97 ∠ 0 A.
(28)
where pf is the load power factor which is unity in this case. The
load current is in-phase with the line voltage Vac as the load is
connected across the phases “a” and “c.” Therefore, it lags the
phase “a” voltage by 30◦ .
(29c)
Now, it can be understood that the current in all the legs of the
STATCOM are the same and equal to the rated source current.
From the estimated STATCOM currents, the kVA rating of
the STATCOM is estimated as
√
(30)
Scom = 3 × VL × Icom a = 3.87 kVA
where Icom a is the RMS value of the STATCOM phase “a”
current. Therefore, when the SEIG feeds rated unity power factor
single-phase loads, the STATCOM rating is almost equal to
SEIG kW rating.
Case 2: In this case, a single-phase load of 3.8 kW, 0.8 lag pf
is considered, and the load current is estimated as
Il = 21.58 ∠ − 66.87◦ A.
(31)
Using the source currents given in (27) and the load current
from (31), the STATCOM currents are computed as
Icom a = 19.90 ∠ 85.7◦ A
(32a)
Icom b = 9.97 ∠ −120◦ A
(32b)
◦
Icom c = 11.74 ∠ 72.7 A
(32c)
From (32), it is observed that the phase “a” of the STATCOM
would carry the maximum current at 0.8 lag pf rated load connected across the phases “a” and “c.”
Based on currents in individual phases, the kVA rating of the
STATCOM is computed as
VL
Scom = √ × (Icom a + Icom b + Icom c ) = 5.285kVA. (33)
3
Case 3: In the similar way as in Case 2, in this case also the
STATCOM currents can be estimated.
The load current in this case is
Il = 21.58 ∠ 6.87◦ A.
where VL is the RMS value of the PCC line-to-line voltage and
Pgen is the power output of the SEIG. For all the variables,
phase “a” voltage has been considered as reference. Hence,
all the mentioned phase angles are with respect to phase “a”
voltage.
The load current for 3.8 kW, unity pf load is estimated as
Il =
329
(34)
The STATCOM currents for 3.8 kW, 0.8 leading pf load are
computed as
Icom a = 11.74 ∠ −164.3◦ A
(35a)
Icom b = 9.97 ∠ −120◦ A
(35b)
Icom c = 19.90 ∠ 34.3◦ A.
(35c)
In this case, phase “c” of the STATCOM carries the maximum
current instead of phase “a,” but the RMS value of the maximum
current is same in both the cases.
Using the currents given in (35), the kVA rating of the STATCOM is computed as
Scom = 5.285kVA.
(36)
The summary of kVA rating and currents in the individual phases
of the STATCOM for different loads is presented in Table II.
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 29, NO. 2, JUNE 2014
TABLE II
SUMMARY OF STATCOM RATINGS FOR DIFFERENT LOADS
APPENDIX B
SYSTEM PARAMETERS
1) Parameters of 3.7-kW, 230-V, 50-Hz, Y-Connected, FourPole Induction Machine Used as the SEIG
Rs = 0.39 Ω, Rr = 0.47 Ω, Lls = 0.00633 H, Llr =
0.00789 H, Lm = 0.2408 H at the rated voltage.
2) STATCOM Parameters:
Three-Leg IGBT VSI, Lf = 3 mH, Rf = 0.1Ω, and Cdc
= 1650 μF.
ac voltage PI controller: Kpa = 0.2, Kia = 0.3
dc voltage PI controller: Kpd = 1, Kid = 0.65.
3) Load Parameters:
A single-phase resistive load of a resistance 14.5 Ω connected across phases “a” and “c” is used as linear load.
Nonlinear loads: Single-phase bridge rectifier feeding R–L
load are used as nonlinear loads.
R = 14Ω, L = 250 mH.
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SINGH et al.: STATCOM-BASED CONTROLLER FOR A THREE-PHASE SEIG FEEDING SINGLE-PHASE LOADS
Bhim Singh (SM’99–F’10) received the Bachelor of
Engineering (Electrical) degree from the University
of Roorkee, Roorkee, India, in 1977, and the M.Tech.
degree in power apparatus and systems and Ph.D. degree from the Indian Institute of Technology (IIT),
New Delhi, India, in 1979 and 1983, respectively.
In 1983, he joined the Department of Electrical Engineering, University of Roorkee, as a Lecturer. He
became a Reader there in 1988. In December 1990,
he joined the Department of Electrical Engineering,
IIT, as an Assistant Professor, where he became an
Associate Professor in 1994 and a Professor in 1997. He has guided 45 Ph.D.
dissertations, 139 ME/M.Tech theses, and 60 BE/B.Tech. projects. He has been
granted one U.S. patent and filed 12 Indian patents. He has executed more than
60 sponsored and consultancy projects. His research interests include power
electronics, electrical machines, electric drives, power quality, flexible ac transmission systems, high-voltage direct current transmission systems, and renewable energy generation.
Dr. Singh is a Fellow of the Indian National Academy of Engineering, the
National Science Academy, the Indian National Science Academy, the Institute
of Engineering and Technology, the Institution of Engineers (India), the Institution of Electronics and Telecommunication Engineers, and the World Academy
of Sciences. He is also a life member of the Indian Society for Technical Education (ISTE), the System Society of India, and the National Institution of Quality
and Reliability. He received the Khosla Research Prize of the University of
Roorkee in the year 1991, and the J.C. Bose and Bimal K. Bose Awards of the
Institution of Telecommunication Engineers for his contributions in the field
of power electronics in the year 2000. He also received the Maharashtra State
National Award of the ISTE in recognition of his outstanding research work in
the area of Power Quality in the year 2006. He received the PES Delhi Chapter
Outstanding Engineer Award for the year 2006. He was the General Chair of
the IEEE International Conference on Power Electronics, Drives and Energy
Systems (PEDES’2006) and (PEDES’2010) held in New Delhi.
S. S. Murthy (SM’87–LSM’03–LF’12) received the
B.E. degree from Bangalore University, Bengaluru,
India, the M.Tech. degree from the Indian Institute of
Technology (IIT) Bombay, Mumbai, India, and the
Ph.D. degree from IIT Delhi, New Delhi, India.
He is currently the Vice-Chancellor of the Central University of Karnataka, Gulbarga, India, since
January 2013. He started his professional career in
1969 as a Lecturer at BITS Pilani and moved to IIT
Delhi as faculty in 1970, where he became Professor
in 1983. He was Head of the Department of Electrical Engineering, IIT Delhi, during 1998–2001 and CEA Chair Professor
during 2000–2003 and 2006–2011. He superannuated on Dec. 31, 2011, after
serving the Institute perhaps for the longest period of 41 years. He has held
visiting assignment at the University of New Castle, New Castle, U.K., University of Calgary, Canada, Ryerson University, Toronto, University of Waterloo,
Canada, Indian Institute of Science, Bengaluru, Kirloskar Electric, Bengaluru,
GE Global research Centre, Bengaluru, Trident, Bengaluru, and the Central
Power Research Institute, Bengaluru. He was the Head of the Research Institute
(ERDA, Baroda) during 1990–1992 and Technical University (National Institute
of Technology Karnataka, Surathkal) during 2003–2005. He was the Coordinator (and instrumental) for several international collaborations with U.K., Japan,
Korea, Canada, and Australia. He initiated several new academic programs at
IIT Delhi and was instrumental in establishing state-of-the-art energy audit and
energy conservation facilities at IIT under World Bank funding. He has published nearly 300 research papers in refereed journals and conferences, guided
more than 100 PG student theses/dissertations and completed more than 90
sponsored research and industrial consultancy projects dealing with electrical
machines, drives, and energy systems and sponsored by major electrical industries and government agencies. In addition, he has 40+ Technical reports,
seven manuals/conference proceedings to his credit. He holds 11 patents on
self-excited induction generator and control, renewable energy applications for
off-grid power, and a novel-braking scheme for motors. His research interests
include electric machines, drives, special machines, power electronic applications, renewable energy systems, energy efficiency, and conservation.
331
Dr. Murthy has received many awards notable being the Indian Society for
Technical Education (ISTE)/Maharashtra Government Award for outstanding
research, Institution of Electronics and Telecommunication Engineers/Bimal K.
Bose Award for contribution in power electronics and IEEE/ Power Engineering
Society Delhi Chapter Outstanding Engineer Award. He has made significant
contributions to Professional Societies, such as IEEE, the Institution of Engineering and Technology (IET), and Institution of Engineers (India) IE (I). He
was the General Chair of the IEEE International Conference on Power Electronics, Drives and Energy Systems 1996 held in New Delhi and the Indian
National Academy of Engineering (INAE) Conference on “Research Policy for
Sustainable Energy (2009),” in New Delhi. He is a Fellow of the INAE, a Fellow of the Institution of Electrical Engineers/IET, a Life Fellow of the IE (I), a
Life Member of IST,E and a Fellow of Institution of Electronics and Telecommunication Engineers. He is the member of “Energy Forum” of the INAE to
formulate global energy policies jointly with academies of other countries.
Raja Sekhara Reddy Chilipi received the B.Tech.
degree in electrical engineering from the PBR Visvodaya Institute of Technology and Science, Kavali,
India, in 2006. He is currently working toward the
Ph.D. degree in the Department of Electrical Engineering, Indian Institute of Technology, Delhi, India.
From 2006 to 2008, he worked as a Lecturer in
Bapatla Engineering College, Andhra Pradesh, India.
His research interests include self-excited induction
generators, power electronics applications in renewable energy systems, electric drives, power quality,
and control of custom power devices.