Three-Level Neutral Point Clamped Converter Based And Switching Level Modeling UPFC

8/12/2019 Three-Level Neutral Point Clamped Converter Based And Switching Level Modeling UPFC

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International Journal of Computer Trends and Technology (IJCTT) volume 4 Issue10 Oct 2013

ISSN: 2231-2803 http://www.ijcttjournal.org Page3744

Three-Level Neutral Point Clamped Converter Based

And Switching Level Modeling UPFCBhupesh Deshmukh

#1, Dhaneshwari Sahu

*2

#Author

Dept. of Electrical, RITEE, Chhattisgarh, India* Co- Aut hor

Dhaneshwari Sahu, Assistant Professor, Electrical Dept., RITEE, Chhattisgarh, India

Abstract in the recent years, due to economics

and environment problems, build of new power

planet and transmission line become more difficult.

Hence it is advisable to enhance power transfer

capability of the existing transmission lines up to

thermal limit instead of constructing new one. For

enhancing the power capability, FACTS controller

like SSC, TCSC, and SVC are developed. But these

controllers cannot compensate the real and reactivepower separately. For this a controller called,

Unified Power Flow Controller (UPFC) is

developed which uses both the series and shunt

controller with a common DC capacitor link. This

paper presents state space vector analysis for

Three-Level Neutral Point Clamped Converter

operating as unified power flow controllers

(UPFCs). It allows direct ac/ac power conversion

without dc energy storage links; therefore, It

reduces volume, cost, capacitor power losses,

together with higher reliability. The line active and

reactive power, together with ac supply reactive

power, can be directly controlled by selecting an

appropriate Three-Level Neutral Point Clamped

Converter switching state guaranteeing good

steady-state and dynamic responses. This advanced

control of Three-Level Neutral Point Clamped

Converter guarantee faster responses without

overshoot and no steady- state error, presenting no

cross-coupling in dynamic and steady-state

responses. Simulations are carried out, showing the

effectiveness of the proposed method in steady-state

and transient conditionsKeywords Direct power control, flexible ac

transmission control (FACTS), multilevel

converter, sliding mode control, unified power-flow

controller (UPFC).

I. INTRODUCTIONThe Ability to control power flow in an electric

power system without generation rescheduling or

topology changes can improve the power system

performance using controllable components, the

line flows can be changed in such a way that

thermal limits are not exceed, losses are minimized,

stability margins are increased and contractual

requirements are fulfilled without violating theeconomic generation dispatch. Flexible AC

Transmission systems (FACTS) technology is the

ultimate tool for getting the most out of existing

equipment via faster control action and new

capabilities. The most striking feature is the ability

to directly control transmission line flows bystructurally changing parameters of the grid and to

implement high gain type controllers based on fastswitching. The application of FACTS devices to

power system security has been an attractive

ongoing area of research. In most of the reported

studies, attention has been focused on the ability of

these devices to improve the power system security

by damping system oscillations and minimal

attempts have been made to investigate the effect ofthese devices on power system reliability.

Basically the FACTS controllers are four types:-1. Series controllers

2. Shunt controllers3. Combined Series-Series Controllers

4. Combined Series -Shunt controllersThe following are the benefits that are principally

derived by using the FACTS controllers.1. The flow of power is ordered. It may be asper the contract or as per the requirements

of utilities.

2. It increases the loading capability of thelines to their thermal capability.


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Overcoming their limitations sharing ofpower among lines can accomplish this.

3. It improves the stability of the system andthus makes the system secure.

A Unified Power Flow Controller (UPFC) is a

member of FACTS devices. It consists of two solid

state synchronous voltage source converterscoupled through a common DC link as shown in

Figure 1[2]. The DC link provides a path to

exchange active power between the converters. The

series converter injects a voltage in series with the

system voltage through a series transformer. The

power flow through the line can be regulated by

controlling voltage magnitude and angle of series

injected voltage. The injected voltage and line

current determine the active and reactive power

injected by the series converter. The converter has a

capability of electrically generating or absorbing

the reactive power. However, the injected activepower must be supplied by the DC link, in turntaken from the AC system through the shunt

converter. The shunt converter also has a capabilityof independently supplying or absorbing reactive

power to regulate the voltage of the AC system.When the losses of the converters and the

associated transformers are neglected, the overallactive power exchange between the UPFC and the

AC system become zero.

Figure 1 Configuration of UPFC.

However, both the series and shunt converters can

independently exchange reactive power [3]. UPFC

can improve both steady sate stability, dynamicstability and transient stability [4-5]. For the

convenience practical of application, the seriesvoltage angle of UPFC is kept in perpendicular with

a line current [6].

Fig. 2. One-wire schematic of the transmission line

with UPFC.A one-wire schematic of a transmission-line system

equipped with a UPFC is given in Fig. 2. A UPFC

is connected to the transmission line by coupling

transformers, both with a shunt and with a seriesconnection, consists of two ac/dc converters, the acsides connected to the shunt and series connection

with the transmission line, and the dc sidesconnected back to back. UPFCs are typically built

with voltage-sourced converters, having a capacitoras (limited) dc energy storage.

A. MODELING OF THE UPFC POWERSYSTEM

During model construction and controller design,

power Sources VS, VR is assumed to be infinite

bus. We assume series transformer inductance andresistance negligible compared to transmission-line

impedance. Connection transformers of series and

shunt converters of the UPFC as in Fig. 1 are not

explicitly included in the mathematical model used

for controller design. Under these assumptions, we

can simplify the grid as experienced by the UPFC

to Fig. 2. Sending and receiving end power sourcesVS, VR are connected by transmission line r, L.

The total current drawn from the sending endconsists of the current flowing through the line iS

and the current exchanged with the shunt converteriP Shunt transformer inductance and resistance are

represented by LP and rP The series inductance andresistance are commonly accepted as a model for

overhead transmission lines of lengths up to 80 km

[6], [7].The UPFC shunt converter model is similar


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and is not described in this paper; its functions andcontrol are well described in literature [8], [9], [10]

and the performance of the shunt converter is onlyof secondary influence on the control system

described in this paper, as demonstrated in previouswork [11]. Effects of dc bus dynamics are eligible

in the control bandwidth of the power flow. For all

simulations and experiments in this paper, the shuntconverter is only used to satisfy active power flow

requirements of the dc bus. Using the model of Fig.2, differential equations that describe the current iS

in three phases can be formulated. Voltages

are used for notation

simplicity. The differential equations for the UPFC

model are given as

Applying the Clarke and Park transformationresults in differential equations in dq space.

Voltages

and are introduced fornotation simplicity. It is assumed that the pulsation

of the grid is known and varies withoutdiscontinuities. Applying the Laplace

transformation and with substitution between thetwo dq space transfer functions, (2) is obtained,

where currents , , are given in function of

voltages and

The active and reactive power of the power line is

determined only by the current over the line and the

sending end voltage.

The simulation is based on a full three-phase model

of the UPFC and the power lines constructed withMatlab Simulink. It is performed on a balanced

model of the experimental setup.

Fig. 3 Three-Level Neutral Point Clamped

Converter Based UPFC.

The discussed controller in fig 3 is demonstrated in

Simulink and the results are shown in fig 6, 7, and 8.

The simulation is based on a full three-phase model

of the UPFC and the power lines constructed with

Matlab Simulink. It is performed on a balanced

model of the experimental setup.UPFC shuntconverter and dc capacitor dynamics are included in

the system model. The shunt converter is set tocontrol the total dc voltage level of the converter dc

bus. No reactive power transfer between the shuntconverter and the sending end bus is set the sending

unit.

II. PROPOSED MODELAn infinite bus is a source of constant frequency

and voltage either in magnitude or angle. Single

Machine Infinite Bus System (SMIB) equipped

with a UPFC is connected to the remote system

through a transformer and a transmission line

having two section models as shown in Fig.

4(a). A UPFC is placed in the transmission line at

point m (between middle of two line sections m-n)in the system. The reactance of various components

of the system is shown in Fig. 4(b). The phasor of

Series injected voltage and shunt injected current ofthe UPFC is shown in Fig. 4(c).


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Fig.4 (a): Single line diagram (b) reactance diagram

(c) Phasor diagram of UPFC

This paper describes the switching level

modelling of UPFC using IGBT. The

performance of UPFC is demonstrated on SMIB

system & real and reactive power flow tracings

are obtained. The UPFC is composed of two

back to back PWM Converters connected by a

common DC link. This modeling is done with

Simulink blockset and simulation is carried out

in MATLAB environment as shown in Fig.4

A. Simulation ModelThe synchronous generator is connected to the

linear load through the Power transformer and

section model of transmission line. The

UPFC is located at the middle of the

transmission line. The shunt device of UPFC

consists of three phase IGBT converter with

linear angle controller. The shunt converter is

connected to the transmission line in parallel

through a three phase transformer.The series

device of the UPFC consists of three phaseIGBT inverter with SPWM controller. The seriesconverter is connected to the transmission line in

series through three single phase transformers.The IGBT firing pulses are generated for shunt

& series converters as described earlier. Byvarying the firing angle () to DC voltage is

controlled accurately. The inverter outputvoltages are effectively controlled by varying the

modulation index (M).

Fig. 5 Switching level model of Unified Power

Flow controller (UPFC)

Fig. 5 describes the switching level modeling of

UPFC using IGBT. The performance of UPFC is

demonstrated on SMIB system & real and reactive

power flow tracings are obtained. The UPFC is

composed of two back to back PWM Converters

connected by a common DC link. This modeling isdone with Simulink blockset and simulation is

carried out in MATLAB environment as shown inbelow fig 9, 10 and 11.


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III.MATLAB/SIMULINK RESULTS

Fig 6 input voltage and current

Fig 7 input Power

Fig 8 output Power

A. ENHANCEMENT UPFCRESULTS

Fig 9 input Power

Fig 10 output Power

Fig 11 across fault


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IV. CONCLUSIONThree-Level Neutral Point Clamped Converter

Based Unified Power-Flow Controllers connected

to power transmission lines as UPFCs. Presented

simulation & experimental results show that activeand reactive flow will be advantageously controlled

by using the proposed DPC. Results show nosteady-state errors, no cross coupling, insensitivity

to non -modeled dynamics and fast response times,thus confirming the expected performance of the

presented nonlinear DPC methodology. Despiteshowing a suitable dynamic response, the PI

performance is inferior when compared to DPC.Furthermore, the PI controllers and modulator take

longer times to compute. Obtained results show that

DPC is a strong nonlinear control candidate for line

active and reactive power flow.

REFERENCES

[1]. N.G. Hingorani, Understanding FACTS-

Concepts and Technology of Flexible AC

Transmission Systems IEEE Power Engineering

society (standard publishers, IEEE press, 2001).

[2] Y.H. Song and A.T. Johns, Flexible ac

transmission systems (FACTS), The Institute of

Electrical Engineers, London, 1999.

[3] L. Gyugyi, Dynamic compensation of ac

transmission line by solid-state synchronous voltage

sources, IEEE Trans. Power Delivery, Vol. 9, pp.

904-911, Apr. 1994.[4] M. Noroozian, L. Angquist, M. Ghandhari, and

G. Andersson, Use of UPFC for optimal power

flow control, IEEE Trans. on Power Delivery, Vol.

12, No. 4, pp. 1629-1634, 1997.

[5] M. Lyapunov functions for series devices,IEEE Trans. on Power Delivery, Vol. 16, No. 4,

2001, pp. Ghandhahi, G. Adersson and I.A.Hiskens,Control 689-694.

[6] E. Gholipour and S. Saasate, Improving ofTransient Stability of Power Systems Using UPFC,

IEEE Trans. on Power Delivery, Vol. 20, No. 2, pp.

1677-1682, 2005.[6] P. Kundur, Power System Stability and Control,

N. J. Balu andM. G. Lauby, Eds. New York:

McGraw-Hill, 1994.

[7] J. J. Grainger and D.W. Stevenson, PowerSystem Analysis, A.B. Akay and E. Castellano, Eds.

New York: McGraw-Hill, 1994.

[8] L. Gyugyi, Unified power-flow control

concept for flexible ac transmission systems,

Proc. Inst. Elect. Eng., Gen., Transm. Distrib., vol.139, no. 4, pp. 323331, Jul. 1992.

[9] L. Gyugyi, C. Schauder, S.Williams, T.Rietman, D. Torgerson, and A. Edris, The

unified power flow controller: A new approach to

power transmission control, IEEE Trans. Power

Del., vol. 10, no. 2, pp.10851097, Apr. 1995.

[10] X. Jiang, J. Chow, A.-A. Edris, B. Fardanesh,and E. Uzunovic, Transfer path stability

enhancement by voltage-sourced converter- based

facts controllers, IEEE Trans. Power Del., vol.

25, no.2, pp. 10191025, Apr. 2010.

[11] S. Jiang, A. Gole, U. Annakkage, and D.

Jacobson, Damping performance analysis of ipfcand upfc controllers using validated small-signal

models, IEEE Trans. Power Del., vol. 26, no. 1,

pp. 446454, Jan. 2011

BIODATA

Author: Bhupesh Deshmukh received his

BE (Electronics & Telecommunication) degree

from Pandit Ravi Shankar Shukla University

Raipur in 2008. He is

currently an M.E. student inthe Electrical Engineering

specialization in power

electronics from

Chhattisgarh Swami

Vivekananda Technical

University Bhilai. His

research interests are in theareas of power electronics, power quality and

power system.


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Co-Author: Dhaneshwari

Sahu received her M.Tech inElectrical Engineering

Specialization in Controlsystem from VJTI Mumbai,

Mumbai University in 2010.

She completed her BE inElectrical Engineering from Govt Engg College

Bilaspur, Guru Ghasidas University in 2008. She isAssistant Professor in Raipur Institute of

Technology, Raipur, Chhattisgarh SwamiVivekananda Technical University Bhilai. Her

research interests are power quality, control system,mobile robotics, power system and controller based

application.

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Three-Level Neutral Point Clamped Converter Based And Switching Level Modeling UPFC

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