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Vol.05,Issue.34
October-2016,
Pages:7158-7165
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A Three Phase Five Level Inverter Based STATCOM using Modular
Multi-Level Converter (MMC) Topology
MUDDANA HEMASRI1, V. SIVANAGARAJU2
1
2
PG Scholar, Chirala Engineering College, Chirala, AP, India.
Assistant Professor, Chirala Engineering College, Chirala, AP, India.
Abstract: In this paper a novel single-phase MMC-based inverter with STATCOM capability for grid connection is proposed.
The proposed inverter is designed for grid-connected loads in the mid-sized range. The proposed algorithm enhances the transient
performance of the closed-loop system with only proportional controller and minimizes the STATCOM reactive current ripples..
The function of the proposed inverter is to transfer active power to the grid as well as keeping the power factor of the local grid
constant at a target power factor regardless of the incoming active power from the renewable energy source, especially from a
wind turbine. Generally, the main goal of this paper is to present a new inverter with FACTS capability in a single unit without
any additional cost. The simulations have been done in MATLAB/Simulink for a 5-level inverter.
Keywords: Carrier-Based Pulse Width Modulation (CBPWM), Cascaded H-Bridge Inverter, Power Factor (PF) Correction,
Reactive Power (VAR) Compensation, Static Synchronous Compensator (STATCOM).
I. INTRODUCTION
Power quality and efficiency issues arising from unmanaged
power flow, which include low power factor (PF), voltage
collapse, unbalance, excessive harmonics, transients and
oscillations, have been a major concern in power transmission
and distribution systems. Reactive loads, which naturally
possess low PF, draw excessive reactive power (VAR)
restricting the maximum active power transfer and moreover,
adding losses to the power transmission and distribution
systems [1]. Furthermore, voltage variations or disturbances
such as voltage sags/swells, which is caused by low PF loads,
hard switching, lightning, and sudden increase/decrease in the
loading conditions, will challenge the tolerance level of
electrical equipment in terms of stability and reliability [2].
Therefore, it is essential to improve the voltage stability of
power system networks under both contingency and normal
operating conditions. This led to the development of flexible
ac transmission system controllers such as VAR
compensators to enhance neighboring utilities and regions
with more economical and reliable exchange of power. The
rapid development of the power electronics industry has
opened up opportunities for improving the operation and the
management of power system networks [3]. The conventional
voltage-source inverter (VSI)-based static VAR compensators
such as STATCOM has been the most effective solution for
providing VAR compensation due to its ability to compensate
for a wider range of VAR in fraction of cycle [3]. Briefly,
FACTS devices are power electronic-based devices that are
used to improve the power quality issues. One of the most
important power quality issues is power factor (PF) of the
grid. It is mostly desired to keep the PF of the grid near unity
in order to be able to use the maximum capacity of the power
systems.
Fig.1. Block diagram of the STATCOM with the
associated proposed control scheme.
In common wind applications, an inverter is used to
connect the wind turbine to the grid to transfer the active
power coming from the wind turbine to the main grid. To
improve the PF of the grid, a FACTS device is used to act as a
sink or source of reactive power. The distribution static
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MUDDANA HEMASRI, V. SIVANAGARAJU
synchronous compensator (D-STATCOM) is a well-known
is proportional to the voltage difference between the grid
member of the FACTS family, mostly used for distribution
voltage at PCC, vpcc and the STATCOM ac output voltage vc
systems. In this paper, the proposed single-phase inverter acts
. This can be achieved by controlling the overall magnitude of
not only as a regular inverter, but it is also able to act as a
the dc-link capacitor voltages VDC, hence the voltage vc and
DSTATCOM to keep the PF of its local grid at a target PF
its phase angle δ with respect to the grid voltage vpcc as given
regardless of the wind speed. In other words, this inverter is
by the following equation:
placed between the wind turbine and the main grid, such as a
normal inverter, in order to not only convert DC power
coming from DC link to a suitable AC power for the main
(1)
grid, but also control the PF of the grid by injecting enough
where δ is the phase difference between vpcc and vc , Zf is
reactive power to the grid. In this paper, the concepts of the
the coupling impedance, ϕ (i.e., PF angle) is the phase
inverter and D-STATCOM have been combined to a make an
difference between ic and vpcc, icd is the active or real
inverter which possesses the D-STATCOM option. Replacing
current used to charge/discharge the dc-link capacitors, and
conventional inverters with this inverter will eliminate the
icq is the reactive current flowing through Zf .
need to use a separate capacitor bank or STATCOM device to
fix the PF of the main grids. Obviously, depending on the size
The steady-state operating range of the STATCOM in all
of the power system, multiple inverters might be used in order
four quadrants of the PQ plane is shown in Fig. 3, where the
to reach the desired PF.
active and reactive variables are defined in cosine and sine
functions, respectively. From Fig. 1, PLL determines the
reference phase angle θ of the grid voltage vpcc, which is
used to transform the load current ilq , the STATCOM output
voltage vc and current ic into dq constant vectors using the
Park transformation. Then, the controller performs feedback
control and generates a set of
Fig. 2. Phase-leg of a five-level CHI.
II. MULTILEVEL CASCADED INVERTER-BASED
STATCOM
Fig. 1 shows the single-line block diagram of the
STATCOM along with the proposed control scheme. The
STATCOM is implemented by a five-level inverter (see Fig.
2), whose phase voltages vc are synthesized by the
summation of output voltages (i.e., +VDC, 0, −VDC) from
each individual H-bridge inverter. Each leg of an H-bridge
inverter is formed by two seriesconnected switching devices,
which switched on/off complementarily to prevent short
circuiting the dc link. This can be achieved by appropriate
dead time between each switching device to ensure that either
one of them is completely off before switching on the other
one. From Fig. 1, the STATCOM is paralleled to the power
system via a series coupling inductor Zf at the PCC [22]. The
fundamental objective of the shuntVAR compensation is to
reduce the voltage drop across the uncertain source
impedance Zs, hence increases the transmittable power along
the transmission line. The amount of the reactive current ic
flowing through the coupling impedance Zf with an
impedance ratio equals to 10 (i.e., tangent−1 (jωLf /Rf )) [46]
Fig. 3. STATCOM in four-quadrant operational area.
switching signals through a dedicated modulation technique
to drive the semiconductor switches of the multilevel inverter.
Based on (1), the transfer function of the STATCOM in
dqcoordinates is defined by
(2)
By integrating (2) between current samples k and k + 1
and then dividing it by the selected rates (i.e., Tid and Tiq for
dand q-axes current vector controller, respectively), the
average magnitude of dq-voltage vectors from the sample
periods k to k + 1 was then derived as follows:
International Journal of Scientific Engineering and Technology Research
Volume.05, IssueNo.34, October-2016, Pages: 7158-7165
A Three Phase Five Level Inverter Based STATCOM using Modular Multi-Level Converter (MMC) Topology
Hence, the desired STATCOM output voltage magnitude
vc and its phase angle δ with respect to vpcc are given as
follows:
(3)
(16)
(17)
(4)
Since fast and optimal current controller response is
always of prime importance in STATCOM applications,
therefore, the STATCOM output currents at the next sample
(i.e., icd (k + 1) and icq (k + 1)) are set to track the current
references at the current sample (i.e., icd (k) and ilq (k)) as
follows:
(5)
Fig4 shows the implementation of the resulted STATCOM
dq-voltage reference values given by (16) and (17). In Fig. 1,
each separated dc-link capacitor is treated as an energy
storage element to store the rectified energy via each Hbridge
rectifier circuit. The sum of the dc voltage levels (VDC) is
regulated according to the system requirement determined by
the active current reference icd in the voltage control loop
using a P-controller with a gain given by
(6)
In order to make the variation of (5) and (6) occur linearly
between the two samples k and k + 1 during one sampling
period (i.e., k to k + 1)
(18)
where IDC is the current that flows through the MCHI and C
is the total dc capacitance of the two H-bridges.
(7)
(8)
The grid voltage vpcc and the STATCOM output voltage
vc are assumed to be constant and equal to its voltage
reference within one sampling period (i.e., k to k + 1) as
follows:
(9)
(10)
Fig.4. Block diagram of current decoupling control with
P-controllers.
(11)
(12)
By substituting (5)–(12) into (3) and (4), the resultant dq
voltage reference values are obtained as follows:
Fig. 5. Block diagram of dc voltage feedback control with
P-controller.
Furthermore, (18) is defined in dq-coordinates as follows:
(13)
(14)
where the proportional gainKp i(d,q) of the P-controller is
given by
(19)
Since only the d-axis current component is required for
charging and discharging the dc-link capacitors, hence all the
q-axis components (i.e., IDC q and VDCq) in (19) are
neglected. By integrating (19) between the current samples k
and k + 1 and then dividing it by the preferred sampling rate
(Tvd for d-axis voltage vector controller), the average
magnitude of active current vector from the sample periods k
to k + 1is therefore given by
(15)
International Journal of Scientific Engineering and Technology Research
Volume.05, IssueNo.34, October-2016, Pages: 7158-7165
MUDDANA HEMASRI, V. SIVANAGARAJU
maximum overshoot and high settling time (i.e., longer
response and oscillation period). For instance, in [49], PIcontrollers were considered to provide voltage and current
(20)
regulations for STATCOM based on the nine-level cascaded
The sum of the dc voltage VDC d across the dc-link
H-bridge inverter. Although good dynamic response was
capacitors at the next sample (i.e., VDC d (k + 1)) is set to
achieved, yet oscillation of reactive current icq which led to
trace the voltage reference (i.e., VCD d (k)) as follows:
poor transient response and hence, instability of the proposed
(21)
system still can be obviously seen in steady-state operation. It
has been also shown in [50] that the PI-controller may not be
The current is assumed to be constant and equal to its
robust enough due to the variations of parameters and
current reference within one sampling period (i.e., k to k + 1)
operating points, which may block the STATCOM due to the
overcurrent caused by the dynamic overshoot.
(22)
By substituting (21) and (22) into (20), the resultant
current reference value is then obtained as follows:
(23)
where the dc voltage reference VDC d is 1 per unit (p.u.) and
the proportional gain Kp vd of the P-controller is given by
(24)
Fig5 shows the block diagram of the dc voltage feedback
control described by (23).
Since P-controllers exhibit a rapid correction response,
therefore, they have been selected and employed in this paper
to achieve both fast and robust control of the reactive current.
The proposed algorithm basically provides an oppose signal
to the controller in response to the variation of the
STATCOM reactive current icq during the steady-state
condition, hence enhancing the transient response aswell as
the steady-state errorwithout an integral function in the
feedback loop. The proposed algorithm is derived based on
the difference between the commanded voltage v∗ cc , the
grid voltage vpccd , and the voltage drop vL across the
coupling inductor Lf . Assuming that the STATCOM is
operating in quadrant I region (see Fig. 3) and reached steady
state, the d-axis current components in (3) and q-axis current
components in (4) can be neglected as shown in Fig. 6. Based
on Fig. 6, the arithmetic sequence to derive the proposed i∗
cq is first illustrated as follows
(25)
(26)
(27)
Fig. 6. STATCOM operation in first quadrant.
where v∗ ccd defines the resultant d-axis STATCOM voltage
of an operating point based on the desired signal ωLilq ,
vpccd defines the d-axis grid voltage, v∗ ccq defines the
resultant q-axis STATCOM voltage of an operating point
based on the resultant current reference value obtained by
(23), and finally, v∗ cc defines the commanded voltage
magnitude of the vector sum of v∗ ccd and v∗ ccq . The
command reference voltage v∗ cc is set to 1 p.u. to represent
the ideal STATCOM voltage vc (i.e., without variation) in the
steady-state operating point. By referring to (27), the v∗ ccd
in (25) can be redefined as follows:
III. PROPOSED EXTERNAL REACTIVE CURRENT
REFERENCE “i∗ cq ” ALGORITHM
As shown by Fig. 1 and (6), the load reactive current ilq is
extracted in order to correct the PF of the power system. In
this paper, another algorithm based on the reactive current
reference icq is introduced to the STATCOM current vector
controller to tackle the variation of the STATCOM output
reactive current icq in dc vector quantity under the steadystate condition. This current variation/oscillation may be
caused by the inverter’s switching noise, nonideality of the
(28)
components [47], unconstrained of inverter switching event
[34], STATCOM output voltage vc waveform distortion at
By substituting (26) into (28) and then equating (25) with
low switching frequency [35], [48], the tolerance of the dc(28), one can simply define i∗ cq as follows:
link capacitors, the ripple or noise content on the nonbalanced
dc voltage, or poor transient response in the adaptive
controllers. A well-tuned PI-controller could achieve zero
steady-state error response with a minimal rise time.
(29)
However, PI-controllers have some drawbacks such as
International Journal of Scientific Engineering and Technology Research
Volume.05, IssueNo.34, October-2016, Pages: 7158-7165
A Three Phase Five Level Inverter Based STATCOM using Modular Multi-Level Converter (MMC) Topology
And from Fig. 6, the voltage drop vL can then be
IV. SIMULATION RESULTS
represented as follows:
The STATCOM is first studied through various simulation
examples using MATLAB/Simulink software package to
investigate the effectiveness and the performance of the
(30)
proposed reference current (i*cq) algorithm.The STATCOM
is investigated with the system parameters tabulated in Table
Hence, by substituting (30) into (29) yields the proposed
I. The fundamental switching time (i.e., 0.02 s) is selected for
reactive current reference i∗ cq as follows:
Tvd and Tid in (13) and (24), respectively, while Tiq is set to
be 0.002 s to avoid large resultant gain which can lead to
system instability. The STATCOM system’s dynamics,
(31)
steady-state, and transient scenarios are analyzed with
The outline of (31) can be illustrated by ignoring the
different loading conditions as shown in Figs.8 to 11. The
parameters in the denominator. For instance, the proposed
simulation study was carried out with linear single-phase
algorithm is equal to zero or bypassed when the grid voltage
reactive load which changes from RL to RC characteristic
vpccd is unity (i.e., PF correction provided by STATCOM
(i.e., from lagging to leading PF) at the time of 1 s.
based on load reactive current iq ). However, due to the
inverter nonlinearity [34], [35], [37], [47], there is always a
marginal voltage difference between the grid voltage vpcc
and the STATCOM output voltage vc (i.e., vL is not equal to
zero) specifically during the steadystate operation. To
guarantee tracking of the demand signal (i.e., ilq ), the
proposed algorithm is employed to trace this voltage
difference and then feed-forward toward the decoupling
control scheme (i.e., added on top of the commanded load
reactive current ilq ) for providing an appropriate
counterreaction accordingly (i.e., minimizes the STATCOM
reactive current ripples). Fig. 7 illustrates the block diagram
of the proposed reactive current reference algorithm, i∗ cq
which is represented by (31). Finally, the resultant reactive
current reference i∗ cq is added to the load reactive current
ilq in (8) to form the final dq-voltage references as follows:
(32)
(33)
where the proportional gainKp i(d,q) of the P-controller is
given in (15).
Fig. 7. Block diagram of the proposed external reactive
current reference i*cq .
Fig.8. Three phase non linear load voltage and three phase
non linear load current during step change in load.
The dynamic and transient response of the total dc-link
capacitor voltage VDC controller in response to inductive and
capacitive VAR generations is demonstrated in Fig. 11(a)
while Fig. 11(b) presents the three-level ac output voltages
generated from each H-bridge inverter level and the overall
five-level output voltage. Furthermore, Fig. 11(a) shows that
the dc voltage level across each dc-link capacitor is
maintained constant using a rotated switching swapping
scheme. This ensures equal switching stresses, conduction
losses, and power handling between the H-bridges. Moreover,
the scheme assists the control system to response quicker
during the step changes of reactive currents. According to (1),
it is revealed that when the dc peak voltage level is higher
than the peak voltage vpcc, the STATCOM is operating in a
capacitive mode to deliver reactive current to the power
system. In contrast, when the STATCOM operates in an
International Journal of Scientific Engineering and Technology Research
Volume.05, IssueNo.34, October-2016, Pages: 7158-7165
MUDDANA HEMASRI, V. SIVANAGARAJU
inductive mode, the dc peak voltage level is lower than the
V. CONCLUSION
peak voltage vpcc to absorb reactive current from the power
A simulation model for the hybrid multilevel inverter is
system. From Fig. 11(b), the switching patterns are swapped
developed in Simulink. The proposed system demonstrates
between the H-bridge inverters at every two fundamental
the application of a new inverter with FACTS capability in a
frequency cycle to resolve the current imbalance stress [52].
single unit without any additional cost. Clearly, depending on
However, the five level output voltage waveform still can be
the size of the compensation, multiple inverters may be
correctly obtained on the ac side of the cascaded inverter.
needed to reach the desired PF. The proposed controller
system adjusts the active power by changing the power angle
(delta) and the reactive power is controllable by the
modulation index (m). The performance of the proposed
method was investigated through simulation and the
algorithm has demonstrated its ability to achieve good
response, reliability, and transient response for VAR
compensation and PF correction under different loading
conditions.
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