A MATLAB/SIMULINK Model to study the performance of the VFT for the interconnection of Weak

and Strong AC Grids

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 hossamudn@hotmail.com mohamed_abdulla@ieee.org eng_mona_ibrahim@ieee.org

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 the paper accurately represents the VFT’s dynamic characteristics. Based on this model, some further simulations are conducted to study VFT’s 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 the VFT.

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 proposed model. 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

(angle δ). 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 sequence takes 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 by torque 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 d

Figure 6-The actual and reference power upon changing the power order

• 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 the VFT 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= 66 50 480rpm

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

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 connected to another source to simulate the VFT and test its operation under different conditions for power flow control.

We then implemented a simple setup to test the capability of the doubly fed induction machine to transfer power between two asynchronous systems without any control topologies to verify the idea of power transfer.

The experimental procedure goes as follows:

1. We connected the system according to the connection diagram shown in figure (17) and the pictures of the equipments are as shown in figures (18-21).

2. The speed of the dc motor driving the synchronous generator was used to adjust the frequency of the synchronous generator, to test the power flow under different frequencies.(49Hz, 50Hz and 51Hz)

3. At each frequency we adjusted the value of the variac voltage that represents the grid at a constant frequency of 50Hz. Hence the wound rotor induction machine is connecting two asynchronous systems to transfer the power between them.

The results of the experiment are as shown in Table 1.

We can notice from, the table that there is a change in the power flow with the change in frequency and the voltage applied to the rotor through the grid.

Figure16-The AC current at the rotor side of the VFTFigure12-the reference and actual speed upon

applying the frequency disturbance

Figure13-The actual and reference power upon frequency disturbance

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

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

Table 1-Experimental Resu

Figure 17 – The connection diagrexperimental setup

Figure 18-The Doubly fed inductio

Figure 19- DC Motor and the SynGenerator

Table 1-Experimental ResuTable 1-Experimental Results ults

01020304050

79

pow

er a

t th

e st

ator

sid

e

voltage at

ram of the

on machine

nchronous

Figure 20- The Power

Figure 21- The W

Figure 22- relation betweef=49

ults

95 105

t the stator side

r Pack and The Variac

Whole Connection

n the voltage and power at 9HZ

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 VFT system under faults and frequency disturbances. It shows the VFT’s outstanding capability in improving power stabilit. Also a simple test system was implemented showing the change in power flow through the single rotor machine with the change in voltage and frequency.

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 Frequency Transformer,” 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 Power Engineering 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 Power Engineering 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-Québec 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 Asynchronous Interconnection 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, "Mathematical Model 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 variable frequency 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 Induction Machine Analysis For Power Flow Control”

[16] P. Kundur, Power System Stability and Control, New York: McGraw-Hill, 1994.