ENHANCEMENT OF POWER QUALITY BY … , 2S V D ANIL KUMAR 1Electrical and electronics engineering...
Transcript of ENHANCEMENT OF POWER QUALITY BY … , 2S V D ANIL KUMAR 1Electrical and electronics engineering...
International Journal of Advances in Applied
Science and Engineering (IJAEAS)
ISSN (P): 2348-1811; ISSN (E): 2348-182X
Vol. 2, Issue 4, Dec 2015, 17-22
© IIST
International Journal of Advances in Engineering and Applied Science (IJAEAS) Vol-2 Iss-4 , 2015
17
ENHANCEMENT OF POWER QUALITY BY CASCADED MULTILEVEL
CONVERTER BASED DSTATCOM
1G.NAGAGOPIRAJU ,
2S V D ANIL KUMAR
1Electrical and electronics engineering department, SACET, Chirala, AP, India
2Associate professor & HOD, Electrical and electronics engineering department, SACET, Chirala AP, India
Email: [email protected], [email protected]
ABSTRACT: This paper presents the enhancement of power quality in power systems by Distribution Static Compensation (DSTATCOM) with
cascaded multilevel converter for compensation of reactive power and harmonics. The advantages of Cascaded H-Bridge multilevel (CHM)
inverter for improvement of DSTATCOM performance, it helps to compensation of reactive power and harmonics. D-Q theory is used for
DSTATCOM reference compensating currents while proportional and integral (PI) control is used to capacitor dc voltage regulation. A five
level CHB inverter with phase shifted pulse width modulator (PWM) techniques are adopted to investigate the performance of DSTATCOM
for power quality. Finally the results are obtained through MATLAB/SIMULINK software by proposed DSTATCOM linear and nonlinear
loads.
KEYWORDS: DSTATCOM, d-q theory, Proportional integral (PI) control.
I. INTRODUCTION
Reactive power plays a vital role on the security and
stability of power system, therefore, the reactive power
compensation device has a very wide range of
application in power system. Non-linear loads more in
now a days due to these loads present more harmonics in
power systems [1-2]. Therefore, we need compensation
of reactive power and harmonics in power systems, but
technology of power electronics, especially flexible
alternating current transmission system has a rapid
development. As a part of it, DSTATCOM has good
performances of slightly capacity, high efficiency, fast
dynamic response, good control stability and so on, and it
has gradually become one of the representative
techniques in the field of reactive power compensation
[1-4].
At present, DSTATCOM hasn’t been widely applied in
the power grid of high voltage and large capacity, which
is due to the limit of withstand voltage level and capacity
of power electronic switching devices. Because of this,
multiple technology and multi-level technology have
been widely used [5]. Compared with multiple
technology, which contains a bulky, high loss and high
cost coupling transformer, multi-level technology is not
only more simple but also more efficient. Therefore, it
represents the direction of development of large capacity
DSTATCOM.
There are three kinds of structure of the commonly used
multi-level converter, which are flying-capacitor
multi-level converter [6], diode-clamp multi-level
converter [7], and H-bridge cascade multi-level
converter. Cascaded multi-level converter has a lower
output of waveform harmonic content, save more
switching devices and has a more simple control system
than other kinds of multi-level converter. Moreover its
H-bridge modules have a compact structure, flexible
configuration and reliable performance [5]. So the
cascaded multi-level structure is used as converter
topology of DSTATCOM in this paper.
Level shift PWM and phase shift PWM [12] are two
main methods for multi-level carrier pulse width
modulation. Carrier phase shift PWM makes several
triangular carriers, which are same in amplitude and
frequency, separate a certain angle in phase, and compare
them with the modulation wave to generate PWM
waveforms. Carrier phase shift PWM is generally used
for H-bridge cascaded converter. This is because that it
has many advantages compared to other PWM control
methods. This paper presents a DSTATCOM with a
proportional integral controller based CHB multilevel
inverter for the harmonics and reactive power mitigation
of the nonlinear loads.
II. MODELING OF DSTATCOM
The Cascaded multi-level converter is in parallel with
power line through a connected reactor. By adjusting the
phase and amplitude of AC output voltage of the
converter appropriately, the circuit can absorb or sent out
reactive current that meets the requirements, and achieve
the purpose of dynamic reactive power compensation.
This is the basic principle of DSTATCOM [6]. Voltage
drop of the connected reactor has generated the
compensation current, which is generated by the power
grid voltage of access point of DSTATCOM and the
Enhancement Of Power Quality By Cascaded Multilevel Converter Based Dstatcom
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output voltage of converter on both sides of the reactor,
and then the connected reactor can also filter out some of
the high harmonics generated by the converter.
Fig.1 shows the block diagram of the system with
DSTATCOM connected in shunt configuration. The
output voltage of converter will lag a small angle
compared to the power grid voltage in phase, so the
converter can absorb a small amount of active power
from the power grid side to compensate the internal loss
of STATCOM, and stable the DC bus voltage.
DSTATCOM is modeled as a three-phase IGBT
(Insulated Gate Bipolar Transistor) bridge based VSI
(voltage source inverter) with dc bus capacitor at the DC
link.
Fig. 1 Cascaded Multilevel inverter DSTATCOM.
III. REACTIVE POWER COMPENSATION CONTROL
The role of this control method is to maintain fixed
voltage magnitude at the point where a high sensitive
load under system disturbances is connected. The control
system only Submit measures the rms voltage at the load
point, i.e., no reactive power measurements are required.
The voltage source converter VSC switching strategy is
based on a sinusoidal PWM technique which offers
simplicity and good response. Since custom power is a
relatively low-power application, PWM methods offer a
more flexible option than the fundamental frequency
switching methods favored in FACTS applications.
Apart from this, high switching frequencies can be used
to improve on the efficiency of the converter, without
incurring significant switching losses.
The controller input is an error signal obtained from the
reference voltage and the rms terminal voltage measured.
Such error is processed by a PI controller; the output is
the angle 0, which is provided to the PWM signal
generator. It is important to note that in this case, of
indirectly controlled converter, there is active and
reactive power exchange with the network
simultaneously. The PI controller processes the error
signal and generates the required angle to drive the error
to zero, i.e. the load rms voltage is brought back to the
reference voltage.
Fig.2 Reactive power compensation by PI controller.
IV. HARMONICS COMPENSATION CONTROL TECHNIQUE
The Modified Synchronous Frame method is presented
in [7]. It is called the instantaneous current component
(idiq) method. This is similar to the Synchronous
Reference Frame theory (SRF) method. The
transformation angle is now obtained with the voltages of
the ac network. The major difference is that, due to
voltage harmonics and imbalance, the speed of the
reference frame is no longer constant. It varies
instantaneously depending of the waveform of the
3-phase voltage system. In this method the compensating
currents are obtained from the instantaneous active and
reactive current components of the nonlinear load. In the
same way, the mains voltages V(a,b,c) and the available
currents i1 (a,b,c) in α-β components must be calculated as
given by (4), where C is Clarke Transformation Matrix.
However, the load current components are derived from
a SRF based on the Park transformation, where 'θ'
represents the instantaneous voltage vector angle (5).
Fig.3 Block diagram SRF method.
Fig. 3 shows the block diagram SRF method. Under
balanced and sinusoidal voltage conditions angle θ is a
uniformly increasing function of time. This
transformation angle is sensitive to voltage harmonics
and unbalance; therefore dθ /dt may not be constant over
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a mains period. With transformation given below the
direct voltage component is
V. CASCADED MULTILEVEL INVERTER AND PWM
TECHNIQUE
A cascaded multilevel inverter is constructed by the
conventional of H-bridges. Each phase consists of
two-H-bridges in cascaded method for 5-level output
voltage, shown in Fig 4. Each H-bridge is connected a
separate dc-bus capacitor and it serves as an energy
storage element.
Fig.4 Five Level inverter per phase.
The most popular PWM techniques for CHB inverter
are 1. Phase Shifted Carrier PWM (PSCPWM), 2. Level
Shifted Carrier PWM (LSCPWM). In this level shifted
PWM technique, three carrier signals (triangle
waveform) and one reference single (Sinusoidal Positive
waveform) are used, Fig.5 shows the Phase shifted
carrier pulse width modulation. Each cell is modulated
independently using sinusoidal unipolar pulse width
modulation and bipolar pulse width modulation
respectively, providing an even power distribution
among the cells. A carrier phase shift of 180°/m (No. of
levels) for cascaded inverter is introduced across the
cells to generate the stepped multilevel output waveform
with lower distortion.
Fig.5 Phase shift PWM Technique.
The required capacitance for each cell depends on the
allowable ripple voltage and the load current. The rms
ripple current flowing into the capacitor can be written as
follows and the ripple current frequency is double the
load current frequency.
IGBT loss can be calculated by the sum of switching loss
and conduction loss [8]. The conduction loss can be
calculated by, Here VDC is the actual DC-Link voltage
and Vnom is the DC-Link Voltage at which Esw is given.
Switching losses are calculated by summing up the
switching energies.
VI. MODELING AND SIMULINK RESULTS
Simulink results are on nonlinear and linear loads of
proposed concept and modeling circuit present in
section. The system parameters for simulation study are
source voltage of 11kv, 50 hz AC supply, DC bus
capacitance, Inverter series inductance, Source
resistance and inductance, nonlinear load and linear load.
Fig. 6 show the MATAB/SIMULINK power circuit
model of DSTATCOM. It consists of five blocks named
as source block, nonlinear load block, control block,
DSTATCOM block and measurements block.
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Fig.6 Proposed DSTATCOM modeling circuit.
Fig.7 Proposed circuit results for source voltage, source
current and load current.
Fig.8 At nonlinear load, inverter output voltage
waveform.
Fig.9 DC voltage for nonlinear load.
Fig.10 Linear load results.
Fig.11 Inverter output voltage for linear load.
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Fig.12 Dc voltage wave for linear load.
Fig.7 shows the three phase source voltages, three phase
source currents and load currents respectively DST
ATCOM with nonlinear load. It is clear that with
DSTATCOM even though load current sinusoidal source
currents are sinusoidal. Fig.10 shows the three phase
source voltages, three phase source currents and load
currents respectively DST ATCOM with linear load. Fig.
9 & 8 shows the DC bus voltage. The DC bus voltage is
regulated to 11kv by using PI controller. Five level
inverter voltage wave. Fig. 11 & 12 shows the DC bus
voltage. The DC bus voltage is regulated to 11kv by
using PI controller. Five level inverter voltage wave.
Fig.13 Source current THD% for Nonlinear load.
Fig.14 source current THD% for linear load.
Fig.13 shows the harmonic spectrum of Phase - A Source
current with non - linear load DSTATCOM. The THD of
source current with DSTATCOM is 5.03%. Fig.14
shows the harmonic spectrum of Phase - A Source
current with DST ATCOM. The THD of source current
without DST ACOM is 2.41%.
VII. CONCLUSION
This paper has addressed enhancement of power quality
for compensation of reactive power and harmonics by
cascaded multilevel inverter based DSTATCOM. From
THD% of nonlinear load clears that and results of
proposed five level multilevel inverter phase shift PWM
DSTATCOM. This control strategy has no limitation on
the cascade number of the single-phase H-bridge
converters and can also be easily expanded to a higher
number of voltage levels.
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