IJESR/Feb 2013/ Volume-3/Issue-2/Article No-24/2509-2520 ... 24feb13esr.pdf · Compensator (DSTATCOM) in Power System (PS) for compensation of reactive power and harmonics. The results

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IJESR/Feb 2013/ Volume-3/Issue-2/Article No-24/2509-2520 ISSN 2277-2685

International Journal of Engineering & Science Research

A CASCADED H-BRIDGE 5-LEVEL DSTATCOM FOR POWER QUALITY

IMPROVEMENT USING LEVEL SHIFTED AND PHASE SHIFTED PWM

TECHNIQUES

A Srikanth*1, B Lalitha

2

1M.Tech Student, Department of Electrical and Electronics Engineering, PVP Siddhartha

Institute of Technology; A.P, India.

2Asst Prof, Department of Electrical and Electronics Engineering, PVP Siddhartha Institute of

Technology; A.P, India.

ABSTRACT

Static Compensators (STATCOMs) are based on shunt Flexible AC Transmission System

(FACTS) devices that can regulate line voltage at the Point of Common Coupling (PCC),

balance loads or compensate load reactive power by producing the desired amplitude and

phase of inverter output voltage which is connected to a DC capacitor. The STATCOM

connected in distributed system is called as DSTATCOM. There are many possible

configurations of Voltage Source Inverters (VSI) and consequently many different

configurations of DSTATCOM. With the advancement of power electronics and emergence

of new multilevel converter topologies, it is possible to work at voltage levels beyond the

classic semiconductor limits. The multilevel converters achieve high-voltage switching by

means of a series of voltage steps, each of which lies within the ratings of the individual

power devices. Among the multilevel Converters, the cascaded H-bridge topology (CHB) is

particularly attractive in high-voltage applications, because it requires the least number of

components to synthesize the same number of voltage levels. This paper presents an

investigation of five-Level Cascaded H – bridge (CHB) Inverter as Distribution Static

Compensator (DSTATCOM) in Power System (PS) for compensation of reactive power and

harmonics. The results are obtained through MATLAB/Simulink software package. The

proposed DSTATCOM is simulated using both phase shifted and level shifted pulse width

modulation techniques.

Keywords: DSTATCOM, Power Quality, Level shifted Pulse width modulation (LSPWM),

Phase shifted Pulse width modulation (PSPWM), Proportional-Integral (PI) control.

I. INTRODUCTION

Shunt compensation for medium voltage distribution systems requires higher rating for

voltage source converters (VSCs). Ratings of the semiconductor devices in a VSC are always

limited; therefore for higher rated converters it is desirable to distribute the stress among the

number of devices using multilevel topology [1]. Cascaded multilevel configuration of

inverter has the advantage of its simplicity and modularity over the configurations of diode-

clamped and flying capacitor multilevel inverters. The multilevel power conversion has been

receiving increasing attention in the past few years for high power application. Numerous


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topologies have been introduced and studied extensively for utility and drive applications in

the recent literature. These converters are suitable in high voltage and high power

applications due to their ability to synthesize waveforms with better harmonic spectrum and

attain higher voltage.

A DSTATCOM is a voltage source converter (VSC) based device. When operated in a

current control mode, it can improve the quality of power by mitigating poor load power

factor, eliminating harmonic content of load and balancing source currents for unbalanced

loads [3-4].A multilevel inverter can reduce the device voltage and the output harmonics by

increasing the number of output voltage levels. There are several types of multilevel inverter

topologies: cascaded H-bridge (CHB), neutral point clamped, flying capacitor [5]. In

particular, among these topologies, CHB inverters are being widely used because of their

modularity and simplicity. Various modulation methods can be applied to CHB inverters.

CHB inverters can also increase the number of output voltage levels easily by increasing the

number of H-bridges. 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. These types of arrangements have been widely used for PQ applications

due to increase in the number of voltage levels, low switching losses and higher order

harmonic elimination.

With the advancement of power electronics and emergence of new multilevel converter

topologies, it is possible to work at voltage levels beyond the classic semiconductor limits.

The multilevel converters achieve high-voltage switching by means of a series of voltage

steps, each of which lies within the ratings of the individual power devices. Among the

multilevel Converters [6], the cascaded H-bridge topology (CHB) is particularly attractive in

high-voltage applications, because it requires the least number of components to synthesize

the same number of voltage levels.

Additionally, due to its modular structure, the hardware implementation is rather simple and

the maintenance operation is easier than alternative multilevel converters. The multilevel

voltage source inverter is recently applied in many industrial applications such as ac power

supplies, static VAR compensators, drive systems, etc. One of the significant advantages of

multilevel configuration is the harmonic reduction in the output waveform without increasing

switching frequency or decreasing the inverter power output [7]. The output voltage

waveform of a multilevel inverter is composed of the number of levels of voltages, typically

obtained from capacitor voltage sources. This multilevel is three levels. As the number of

levels reach infinity, the output THD approaches zero. The number of the achievable voltage

levels, however, is limited by voltage unbalance problems, voltage clamping requirement,

circuit layout, and packaging constraints.

II. DSTATCOM FOR THE PROPOSED SYSTEM

A D-STATCOM (Distribution Static Compensator), consists of a Voltage Source Converter

(VSC), a dc energy storage device, a coupling transformer connected in shunt to the

distribution network through a coupling transformer. The VSC converts the dc voltage across

the storage device into a set of three-phase ac output voltages. These voltages are in phase



and coupled with the ac system through the reactance of the coupling transformer. Suitable

adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective

control of active and reactive power exchanges between the D-STATCOM and the ac system.

Such configuration allows the device to absorb or generate controllable active and reactive

power.

Figure 1: Schematic Diagram of DSTATCOM

Distribution Static Compensator (DSTATCOM) has the capacity to overcome the above

mentioned drawbacks by providing precise control and fast response during transient and

steady state, with reduced foot print and weight. The majority of power consumption has

been drawn in reactive loads such as fans, pumps etc. These loads draw lagging power-factor

currents and therefore give rise to reactive power burden in the distribution system.

The following are the features of the DSTATCOM:

• It is modified form of STATCOM.

• It is having better control operational features

as compared to STATCOM.

• This device is connected to the line in shunt mode.

• This device is based on voltage source inverter (VSI).

• In this device there are no chances of resonance phenomenon.

• The device enhances the voltage profile of the system.

A. Cascaded H-Bridge Inverter Topologies:

1. Five Level Inverter topology:



Figure 2: Design of cascaded multilevel (5 level) inverter topology

A cascaded multilevel inverter is constructed by using conventional H-bridges. The three-

phase DSTATCOM comprises of 24-power transistors with diodes and each phase consists of

two-H-bridges in cascaded method for 5-level output voltage, shown in Fig 2. Each H-bridge

is connected to a separate dc-bus capacitor and it serves as energy storage elements to supply

a real-power difference between load and source during the transient period [8]. The

capacitor voltage is maintained constant using PI-controller. The 24- power transistors

switching operations are performed using triangular-sampling current controller and

harmonics is achieved by injecting equal but opposite current harmonic components at Point

of Common Coupling (PCC).

B. Reference Current control strategy

The PI controller tries to maintain the dc-bus voltage across the capacitor. This instantaneous

real-power compensator with PI-controller is used to extract reference value of current to be

compensated.

Figure 3: Reference current generator using instantaneous real-power theory

The reference currents isa*, isb *and isc * are calculated instantaneously without any time delay by

using the instantaneous α, β coordinate currents. The required references current derivate from the

inverse Clarke transformation and it can be written as



The reference currents (isa*, isb * and isc *) are compared with actual source current isa , isb and isc that

facilitates generating cascaded multilevel inverter switching signals using the proposed triangular-

sampling current modulator. The small amount of real-power is adjusted by changing the amplitude of

fundamental component of reference currents and the objective of this algorithm is to compensate all

undesirable components. When the power system voltages are balanced and sinusoidal, it leads to

constant power at the dc bus capacitor and balanced sinusoidal currents at AC mains simultaneously.

III. PROPOSED INSTANTANEOUS POWER THEORY

The proposed instantaneous real-power (p) theory derives from the conventional p-q theory

or instantaneous power theory concept and uses simple algebraic calculations. It operates in

steady-state or transient as well as for generic voltage and current power systems that allow

to control the DSTATCOM in real-time. The DSTATCOM should supply the oscillating

portion of the instantaneous active current of the load and hence makes source current

sinusoidal.

Figure 4: α-β coordinates transformation

The p-q theory performs a Clarke transformation of a stationary system of coordinates a b c

to an orthogonal reference system of coordinates α β. In a b c coordinates axes are fixed on

the same plane, apart from each other by 120o that as shown in Fig 4. The instantaneous

space vectors voltage and current Va , ia are set on the a-axis, Vb , ib are on the b axis, and Vc

, ic are on the c axis. These space vectors are easily transformed into α β coordinates. The

instantaneous source voltages vsa, vsb, vsc are transformed into the α β coordinate’s voltages

as vα , vβ by Clarke transformation as follows:

(2)

Similarly, the instantaneous source current isa, isb, isc also transformed into the α β

coordinate’s currents as i α , i β by Clarke transformation that is given as;

(3)

Where α and β axes are the orthogonal coordinates. The Vα, iα are on the α-axis, and Vβ, iβ are

on the β-axis.



Real-Power (p) calculation:

The orthogonal coordinates of voltage and current vα, iα are on the α -axis and vβ, iβ are on the

β -axis. The instantaneous real-power calculated from the α -axis and β -axis of the current and

voltage respectively are given by the conventional definition of real-power as:

(4)

This instantaneous real-power pac is passed through high pass filter (HPF) for eliminating the

fundamental component. The HPF indicates ac component of the real-power losses and is

denoted as pac.

The DC power loss is calculated from the comparison of the dc-bus capacitor voltage of the

cascaded inverter and desired reference voltage. The proportional and integral gains (PI

Controller) determine the dynamic response and settling time of the dc-bus capacitor voltage.

The DC component power losses can be written as

(5)

The instantaneous real-power (p) is calculated from the AC component of the real-power loss

Pac and the DC power loss P DC (Loss); it can be defined as follows;

(6)

The instantaneous current on the αβ coordinates of Icα and icβ are divided into two kinds of

instantaneous current components; first is real-power losses and second is reactive power

losses, but this proposed controller computes only the real-power losses. So the

α,β coordinate currents icα, icβ are calculated from the vα, vβ voltages with instantaneous real

power p only and the reactive power q is assumed to be zero. This approach reduces the

calculations and shows better performance than the conventional methods. The α,β coordinate

currents can be calculated as

(7)

From this equation, we can calculate the orthogonal coordinate’s active-power current. The

α component of the instantaneous active current is written as:

(8)

Similarly, the β component of the instantaneous active current is written as:

(9)



Let the instantaneous powers p(t) in the α -axis and the β -axis is represented as pα and

pβ respectively. They are given by the definition of real-power as follows:

(10)

From this equation (10), substituting the α and β components of active powers of (8) and (9);

we can calculate the real-power p(t) as follows:

(11)

The AC and DC component of the instantaneous power p(t) is related to the harmonic

currents. The instantaneous real power generates the reference currents required to

compensate the distorted line current harmonics and reactive power.

IV. MATLAB/SIMULINK MODELING AND SIMULATION RESULTS

Here MATLAB/Simulink model is developed for two cases with nonlinear load. In case one,

the model is simulated without the DSTATCOM and in case two it is simulated with the

DSTATCOM using both phase shifted and level shifted PWM techniques.

I. Without DSTATCOM

The circuit diagram shown in fig.6 consists of DSTATCOM. For without DSTATCOM

operation the DSTATCOM block is removed and operated. It produces output voltage with

non sinusoidal source currents as shown in fig.5.

Fig 5: Source voltage, current and load current without DSTATCOM

Fig 6: Phase-A source voltage and current without DSTATCOM



Fig. 6 shows the phase-A source voltage and current without DSTATCOM operation. Here as

the load is non linear, the source power factor is not unity.

Fig. 7 shows the harmonic spectrum of Phase –A Source current without DSTATCOM. The

THD of source current without DSTACOM is 28.28%.

Fig 7: Harmonic spectrum of Phase-A Source current without DSTATCOM

II. With DSTATCOM:

Fig. 8 shows the MATAB/Simulink power circuit model of DSTATCOM. It consists of four

blocks named as load block (non linear load), control block, DSTATCOM block and

measurements block.

Fig 8: MATLAB/Simulink power circuit model of DSTATCOM

A. Level shifted:

The DSTATCOM model is simulated using level shifted PWM technique.

Fig 9: Source voltage, current and load current with DSTATCOM



Fig.9 shows the three phase source voltages, source currents and load currents. The source

current is sinusoidal in nature with DSTATCOM.

Fig 10: Phase-A source voltage and current

Fig. 10 shows the phase-A source voltage and current, even though the load is non-linear, the

source power factor is unity.

Fig 11: 5- level LSPWM output of D-STATCOM

Fig.11. shows the 5-level output of level shifted multilevel inverter.

The harmonic spectrum of phase-A source current is shown in Fig.12. The THD of the source

current with the DSTATCOM is 7.02%.

Fig 12: Harmonic spectrum of Phase-A Source current with DSTATCOM (level shift)

B. Phase shifted:

Fig. 13 shows the phase-A voltage of five level output of phase shifted carrier PWM inverter.



Fig 13: five levels PSCPWM output

Fig 14: Source voltage, current and load current with DSTATCOM

Fig. 14 shows the three phase source voltages, three phase source currents and load currents

respectively with DSTATCOM when phase shifted PWM is used. It is clear that with

DSTATCOM even though load current is non sinusoidal, source currents are sinusoidal.

Fig. 15 shows the DC bus voltage. The DC bus voltage is regulated to 12kv by using PI

regulator.

Fig 15: DC Bus Voltage



Fig. 16 shows the phase-A source voltage and current. Here also even though the load is non

linear, the source power factor is unity.

Fig 16: Phase-A source voltage and current

Fig. 17 shows the harmonic spectrum of Phase –A Source current with DSTATCOM. The

THD of source current with DSTACOM is 4.89% when the phase shifted PWM technique is

used.

Fig 17 Harmonic spectrum of Phase-A Source current with DSTATCOM (phase shift)

V. CONCLUSION

A DSTATCOM is a fast response, solid-state power controller that provides flexible voltage

control at the point of common coupling (PCC) for power quality (PQ) improvements. It can

continuously generate reactive power by varying the amplitude of the inverter voltage with

respect to the line bus voltage so that a controlled current flows through the tie reactance

between the DSTATCOM and the distribution network. This enables the DSTATCOM to

provide voltage regulation, power factor correction and harmonics compensation. This paper

studied a five level inverter used in a DSTATCOM. Finally MATLAB/Simulink based model

is developed and simulation results are presented for non linear loads using level shifted and

phase shifted PWM techniques and it can be said that phase shifted PWM technique is better

than level shifted PWM technique.



REFERENCES

[1] Corzine KA, Familiant YL. A New Cascaded Multi-level H-Bridge Drive. IEEE Trans.

Power.Electron 2002; 17(1): 125-131.

[2] Lai JS, Peng FZ. Multilevel converters: A new bread of converters. IEEE Trans. Ind.

Appli 1996; 32(3): 509-517.

[3] Maynard TA, Fadel M, Aouda N. Modelling of multilevel converter. IEEE Trans.

Ind.Electron 1997; 44: 356-364.

[4] Bhagwat P, Stefanovic VR. Generalized structure of a multilevel PWM Inverter. IEEE

Trans. Ind. Appln 1983; 1A-19(6): 1057-1069.

[5] Rodriguez J, Lai J-S, Zheng peng F. Multilevel Inverters; A Survey of Topologies,

Controls, and Applications. IEEE Trans. Ind. Electron 2002; 49(4): 724-738.

[6] Naderi R, Rahmati A. Phase-shifted carrier PWM technique for general cascaded

inverters. IEEE Trans. Power.Electron 2008; 23(3): 1257-1269.

[7] Singh B, AlHaddad K, Chandra A. A Review of Active Filter for Power Quality

Improvements. IEEE Trans on Industrial Electronics 1999; 46(5): 960970.

[8] Angulo M, Lezana P, Kouro S, Rodr´ıguez J, Wu B. Level-shifted PWM for Cascaded

Multilevel Inverters with Even Power Distribution. IEEE Power Electronics specialist

conference, 17-21 june 2007; 2373-2378.

[9] McGrath BP, Holmes DG. Multicarrier PWM strategies for multilevel inverters. IEEE

Trans. Ind. Electron 2002; 49(4): 858–867.

IJESR/Feb 2013/ Volume-3/Issue-2/Article No-24/2509-2520 ... 24feb13esr.pdf · Compensator (DSTATCOM) in Power System (PS) for compensation of reactive power and harmonics. The results

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