a novel model to study the vft performance when controlling power transfer between weak and strong...

8/10/2019 A Novel Model to Study the VFT Performance When Controlling Power Transfer Between Weak and Strong AC Grids Using MATLAB

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A Novel Model to study the VFT performance when

controlling power transfer between Weak and Strong AC

Grids using MATLAB/SIMULINK

Prof.Dr.Ahmed Hossam El Din Dr.Mohamed Ashraf Abdullah Eng. Mona Ibrahim

Department of Electrical Engineering Department of electrical engineering Department of Electrical Engineering

University of Alexandria University of Alexandria University of Alexandria

Alexandria, Egypt Alexandria, Egypt Alexandria, Egypt

[email protected] [email protected] [email protected]

Abstract: This paper represents a new model of the

Variable Frequency Transformer (VFT) using

MATLAB/SIMULINK. The VFT is used to connect

two power systems. The simulations shown in thepaper accurately represents the VFTs dynamic

characteristics. Based on this model, some further

simulations are conducted to study VFTs

characteristics under fault conditions and its roles in

preventing the spread of faults into the other area. The

simulation results show that the VFT effectively

suppresses the power oscillations between the two

interconnected power systems and thus prevents the

faults from spreading.

I. INTRODUCTION

The variable frequency transformer (VFT) is a

controllable, bi-directional transmission device that can

transfer power between asynchronous networks.

Functionally, the VFT is similar to a back-to-back

HVDC converter. The technology is based on a rotary

transformer (continuously variable phase-shifting

transformer) with three-phase windings on both rotor and

stator. A drive system adjusts the VFT rotor position in

order to control the phase shift between the two networks

through the action of a fast power controller. The VFT

controls power transfer up to 100 MW in both directions.network with and without the VFT was discussed by D.

Nadeau [10]. A comparison between the performance of

a back-to-back HVDC system with series compensation

external to the converter transformers, and a variable

frequency transformer for power transfer power between

asynchronous AC systems and flow control feeding or

supplying a weak AC network was introduced by B.

Bagen, D. Jacobson, G. Lane, and H. M. Turanli in

reference [11]. The steady state and dynamic simulations

show that both technologies are able to control power

flow accurately. The variable frequency transformer

consumes less reactive power than a back-to-back

HVDC system, provides faster initial transient recovery.

II. VFT MODELING

Figure (1) illustrates a conceptual system diagram of theVFT.

The VFT model is constructed using the

MATLAB/SIMULINK software to study the dynamic

performance of the VFT when connecting a weak AC

grid to a strong AC grid. We used the MATLAB

software package because other research papers used

PSCAD/EMTDC to build the model and hence we

decided to use a new software package which is equally

reliable and accurate. Figure 2 shows the proposedmodel. In the proposed model, the VFT is modeled as a

doubly-fed induction machine, where the stator is

connected to system 1 and the rotor is connected to

system 2.

System 1:

Voltage= 220V (line-to-line RMS voltage)

Frequency= 60 Hz

System 2:

Voltage= 220V (line-to-line RMS voltage)

Frequency= 50 Hz

The variable frequency transformer is used to control the

power flow between the two systems by means of

changing the rotor position with respect to the stator

Figure1-System diagram of the variable

frequency transformer
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(angle ). To control the rotor position a DC motor is

used coupled with the rotor of the asynchronous

machine, thus controlling the rotor position according to

the ordered power. Figure (3) shows the controller of the

DC motor.

Upon starting the simulation the following sequencetakes place:

During starting the rotor will be open-circuit, and

the speed controller is applied on the dc motor

which is mechanically coupled with the doubly

fed IM to control the speed of the shaft to be

equal to the reference speed (the difference

between stator and rotor rotating fields)

After 5s from starting a check for the phase angle

differences across the circuit breaker is

performed, and when the difference is zero the

breaker is closed.

At t=10s the speed controller is replaced bytorque controller to control the shaft torque (+ve

value or ve value or zero) to control the power

flow (from stator to rotor or from rotor to stator

or zero power transfer) respectively.

Below we will discuss the response of the VFT under the

following conditions:

a) Normal operating condition.

b) The change in the power order.

c) Applying a single line to ground fault.

d)

Change in the frequency.

A. Normal operating condition:

Below are the output waveforms of the VFT model

during the normal operating condition where the power

order is 5000W and the reference speed is 300rpm

B. Change in power order:

We will study two cases of the change in the power order

The power is changed from 5000W to 10000W

at 25secs and the figures below show the

response of the VFT under the change in the

power order.

Figure 4-The actual and reference power

Figure 2- VFT Proposed Model

Figure 3-The Controller

Figure5-The actual and reference

Figure 6-The actual and reference power upon

changing the power order


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The power is changed from 5000W to 0W at

t=40sec and then to -5000W at t=60sec.and the

figures (8) and (9) show the VFT response upon

changing the power order polarity.

C. Applying a single line to ground fault:

A single line to ground fault is to be applied to the

system at t=20s to t=22s. The waveforms showing theVFT response are shown in figure 10. The simulations

show clearly that the VFT can significantly improve the

power system stability, restrain power oscillations, and

thus prevent faults from spreading into the neighboring

power systems.

D. The VFT Response to Frequency Disturbances:

We will investigate here the behavior of the VFT in case

of change in frequency in the AC grid to which the VFT

is connected.

When the frequency of any of the two sources changes

(overfrequency or underfrequency) this change is

automatically sensed and hence the reference speed is

changed. The VFT accurately tracks the new reference

speed and transfers power between the two sources.

For instance if the frequency of system 1 changes from

60Hz to 66Hz, hence the change in the frequency is

sensed and the reference speed is changed.

Reference speed =

P= number of poles of the asynchronous machine

f1= frequency of system 1

f2= frequency of system 2

hence,

New reference speed=

rpm

Below are the waveforms of the VFT under frequency

disturbance:

Figure7- Actual and reference speed upon

changing the power order

Figure 10 - Actual and Reference power upon

applying a single line to ground fault

Figure 8- The actual and reference power

Figure 9-The Actual And Reference Speed When

Changing The Power Order Polarity

Figure 11-The actual and reference speed upon

applying the single line to ground fault


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From the above waveforms we can conclude that the

VFT system has a very good performance under

frequency disturbances. That is although the VFT may

be connecting weak AC grids however power could still

flow from the sending to the receiving end.

III. EXPERIMENTAL SETUP

In the above model of the VFT we used a wound rotor

induction machine with its rotor ends left open to

simulate the VFT and test its operation under different

conditions for power flow control.

We then implemented a simple setup to test thecapability of the doubly fed induction machine to

transfer power between two asynchronous systems

without any control topologies for the sake of verifying

the idea of power transfer using DFIM.

Figure16-The AC current at the rotor side of the VFT

Figure12-the reference and actual speed upon

applying the frequency disturbance

Figure13-The actual and reference power upon

fre uenc disturbance

Figure14-The AC current at the stator side of the VFT

Figure15-The AC current at the rotor side of the VFT


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IV. CONCLUSIONS:

This paper presents a complete and comprehensive

model of the VFT system using MATLAB/SIMULINK.

The model shows the dynamic performance of the VFTsystem under faults and frequency disturbances. It shows

the VFTs outstanding capability in improving powerstability, suppressing power oscillations and preventingfaults from spreading into the neighboring systems.

V. REFERENCES:

[1] Robert Gauthier, A World-First VFT Installation in

Quebec, Transmission and Distribution World, Nov

2004.Available

http://tdworld.com/mag/power_worldfirst_vft_installation

[2] E. Larsen, A Classical Approach to constructing a

power Flow Controller, IEEE Power Engineering

Society Summer Meeting, 1999.Volume: 2, pp 1192

1195, 18-22 July 1999

[3] M. Dusseault, J. M. Gagnon, D. Galibois, M. Granger,

D. McNabb, D.Nadeau, J. Primeau, S. Fiset, E. Larsen, G.

Drobniak, I. McIntyre, E.Pratico, C. Wegner, First VFT

Application and Commissioning, presented at Canada

Power, Toronto, Ontario, Canada, September 28-30, 2004.

[4] P. Doyon, D. McLaren, M. White, Y. Li, P. Truman,

E. Larsen, C. Wegner, E. Pratico, R. Piwko,Development of a 100 MW Variable FrequencyTransformer, presented at Canada Power, Toronto,

Ontario, Canada, September 28-30, 2004.

[5] E. Larsen, R. Piwko, D. McLaren, D. McNabb, M.

Granger, M. Dusseault, L. P. Rollin, J. Primeau,

Variable-Frequency Transformer A New Alternative

for Asynchronous Power Transfer, presented at Canada

Power, Toronto, Ontario, Canada, September 28-30, 2004.

[6] E. R. Pratico, C. Wegner, E. V. Larsen, R. J. Piwko,

D. R. Wallace, and D. Kidd, "VFT Operational Overview

- The Laredo Project," presented at 2007 IEEE PowerEngineering Society General Meeting, Tampa, FL, USA.

[7] R. J. Piwko, and E. V. Larsen, "Variable Frequency

Transformer FACTS Technology for Asynchronous

Power Transfer," presented at 2005 IEEE PES T&D

Conference and Exposition, New Orleans, LA, USA.

[8] J. J. Marczewski, "VFT Applications between Grid

Control Areas," presented at 2007 IEEE PowerEngineering Society General Meeting, Tampa, FL, USA.

[9] P. HassinkP. E. MarkenR. O'Keefe, and G. R. Trevino,

"Improving Power System Dynamic Performance in

Laredo, TX," presented at 2008 IEEE PES T&D

Conference and Exposition, 21-24, April2008

[10] D. Nadeau, "A 100-MW Variable Frequency

Transformer(VFT) on the Hydro-Qubec TransEnergie

Network The Behavior during the Disturbance,"

presented at 2007 IEEE Power Engineering Society

General Meeting, Tampa, FL, USA

[11] B. Bagen, D. Jacobson, G. Lane, and H. M. Turanli,

"Evaluation of the Performance of Back-to-Back HVDC

Converter and Variable Frequency Transformer for Power

Flow Control in a Weak Interconnection," presented at

2007 IEEE Power Engineering Society General Meeting,

Tampa, FL, USA

[12] G. Chen, and X. Zhou, "Digital Simulation of

Variable Frequency Transformers for AsynchronousInterconnection in Power System," presented at 2005

IEEE PES T&D Conference and Exhibition: Asia and

Pacific Proceedings, Dalian, China.

[13] Y. Chen, G. Chen, and R. Yuan, "MathematicalModel and Simulation Analysis of Variable Frequency

Transformers, Power System Technology, vol. 32, no.

17, pp73-77, 2008. (In Chinese)

[14] Rongxiang Yuan, Ying Chen, Gesong Chen, Yong

Sheng Sch. of Electr. Eng., Wuhan Univ., Wuhan, China,

Simulation model and characteristics of variablefrequency transformers used for grid interconnection,

Power & Energy Society General Meeting, 2009. PES '09.

IEEE

[15] Brian C. Raczkowski and Peter W. Sauer University

of Illinois at Urbana-Champaign, Doubly-Fed InductionMachine Analysis For Power Flow Control

[16] P. Kundur, Power System Stability and Control, New

York: McGraw-Hill, 1994.
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