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

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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



19

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.



20

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.



21

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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enhancement of power quality by … , 2s v d anil kumar 1electrical and electronics engineering...

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