On Line Simulation Models of Electric Drives

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Proceedings of the 2008 International Conference on Electrical Machines

Paper ID 1061

On Line Simulation Models of Electric Drives
Shlomi Eitan and Raul Rabinovici, IEEE Senior Member
Department of Electrical and Computer Engineering Ben-Gurion University of The Negev Beer-Sheva, Israel Tel: 972-8-6461582, Fax: 972-8-6472949 [email protected]
Abstract- The paper presents an online simulation model of a dc drive supplied by a single phase full wave thyristor controlled rectifier and by a single phase cycloconverter. The model was created in Matlab to shorten the simulation running time. It is intended for real-time and rapid prototyping of dc power electronics and electrical drives. The first simulated hardware consists of a two quadrant dc controlled rectifier while the second consists of a four quadrant operation. The online simulation was compared to a conventional simulation in Simulink using Simpowersystem Blockset.

I.

INTRODUCTION

The need for extensive simulations is more and more actual with the ever evolving use and complexity of power electronic systems, and with increased pressure for reduced time-to-market and costs. Real-time simulation of electric systems and drives, including hardware-in-the-loop testing, becomes an increasingly important requirement for the rapid prototyping and testing new circuit topologies and control [15]. Power systems are constantly evolving to include new technologies for controlling the flow of power using power electronics and for improving the reliability of networks using advanced protection strategies. Real-time simulation provides a solid framework to test the new control/protection concepts so as to detect, analyze, and correct any potential problems before commissioning [6]. Furthermore, the real-time simulations could be of worth with virtual laboratories, teleexperiments, and quasi-authentic learning environment [7]. Real-time simulations are performed generally by software means. However, FPGA technologies could also be used to get very short simulation steps [8-10]. The first part of the paper presents an implementation of online simulation of a single phase full wave thyristor controlled converter that supplies a separately excited dc motor drive and the second part presents an implementation of single phase cycloconverter operating without circulating current using online model based on the previous one. It is possible to operate the cycloconverter only one converter at any instant, where switching from one converter to the other would be carried out after a small delay. It is a step down frequency conventional cycloconverter that supplies separately excited dc motor drive in a four quadrant operation.

The present model was validated by numerical simulations on Simulink. Matlab is a high-level technical computing language and interactive environment for algorithmic development, data analysis and numerical computation. Simulink is a software package for modeling and simulating dynamic systems. It provides a graphical design environment that allows designers to build models as block diagrams. It also provides an interactive graphical environment and a customizable set of block libraries that let you accurately design, simulate, implement, and test control of time varying systems. Section II describes the online simulation algorithm. Section III presents simulation results and Section IV conclusions. II. ALGORITHM DESCRIPTION The online simulation algorithm consists of three main parts: • Main program - contains the menu list of all dc drive circuits for which the user will have to launch. • Offline simulation program - which deals with solving differential equations, calculating coefficients values, and is responsible for delivering them to the online simulation program. • Online simulation program – contains the model of the controlled drive systems. Processing time is based on this model. The paper presents simulation results of a single phase full wave thyristor controlled rectifier, in the continuous and discontinuous modes and single phase cycloconverter with a step down frequency [11, 12]. The online models of the controlled drive systems were developed within Matlab environment. They were validated on Simulink Simpowersystems Blockset. The differential equations describing the drive behavior in its different modes of operations are integrated successively until a steady-state solution over the control period is obtained. During integration the final conditions for a certain mode of operation are considered as the initial conditions for the next mode. In order to save processing time, the coefficients of the differential equations were computed in the offline program model using 'dsolve' command from Matlab. They were then transferred together with circuit parameters to

978-1-4244-1736-0/08/$25.00 ©2008 IEEE

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Proceedings of the 2008 International Conference on Electrical Machines the online program. The dc drive model used in the simulations is shown in fig 1.

Pulse Generator

g A

+ TL A+ F+ m

+ v -

armature voltage ia Ea Ia

Vin

B

Universal Bridge

dc

AF-

Te

Te w Ke Eg

DC Machine

Load Step

Vf

Continuous powergui

Fig. 1. Single phase full wave thyristor controlled rectifier with separately excited dc motor drive simulated by Simulink and Simpowersystems Blockset.

The online algorithmic model accepts the coefficients and the parameters of the circuits and is responsible for computing both continuous and discontinuous (if necessary) current mode of operation. The design of online model enables to integrate this model with any offline model representing different dc drive circuit via its appropriate differential equations. The armature circuit of a dc motor is represented by the back emf voltage, the armature inductance and the armature resistance. The state equations describing the dc motor are the conventional ones. The back emf voltage is proportional to the machine speed. The electromechanical torque developed by the dc machine is proportional to the armature current. Fig. 2 shows the electrical model of the dc motor used in the simulations. The state equations describing the dc motor are used in the offline part of the algorithmic by using the “dsolve” command. It is very doubtful if the armature current will be continuous at high values of the firing angle, high speed, and light loads. The armature current would be discontinuous for these operating conditions while the speed regulation will be significantly poor in the region of discontinuous current armature. The motor performances deteriorate and the dynamic response will be sluggish. Therefore, it would be better to operate the motor, from control points of view, in the continuous conduction mode. Fig. 3 describes the flow chart used to simulate the discontinuous current mode of operation. Detecting and computing discontinuous current in online model is much more complicated than computing continuous current mode. The discontinuous current, as soon as it starts, has several forms for every half cycle. Each current shape has

its own algorithmic implementation as is shown in Fig. 3. The cycloconverter of Fig. 4 consist of two controlled rectifiers that can provide a four-quadrant source for the armature circuit of a separately excited motor. The equivalent circuits for such an arrangement are shown in Fig. 4a, 4b, and 4c. Control of the gating signals is such that those of rectifier n are blanked out whenever the armature current is positive, while those of rectifier p are blanked out whenever the armature current is negative. Under these circumstances each rectifier presents effectively infinite impedance to the output of the other and only one rectifier acts as an energy source or sinks at any one time.
ia

Ra La

Varmature
iLOAD
R1
c
eg

Fig. 2. Electrical model of the dc motor. The current source represents the load torque, the resistance R1 the friction torque, and the capacitor C the inertia. The armature current is proportional to the motor torque, while the capacitor voltage is proportional to the dc motor speed.

The cycloconverter of Fig. 4 consist of two controlled rectifiers that can provide a four-quadrant source for the armature circuit of a separately excited motor. The equivalent

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Proceedings of the 2008 International Conference on Electrical Machines circuits for such an arrangement are shown in Fig. 4a, 4b, and 4c. Control of the gating signals is such that those of rectifier n are blanked out whenever the armature current is positive, while those of rectifier p are blanked out whenever the armature current is negative. Under these circumstances each rectifier presents effectively infinite impedance to the output of the other and only one rectifier acts as an energy source or sink at any one time. Since it is desired that the waveform of the actual output voltage shall be such as to produce as nearly as possible the effect of a sinusoidal output voltage, the delay angle at which each thyristor in the rectifiers is turned on is controlled with reference to an ideal voltage wave.

Fig. 3. Flow chart of discontinuous current mode simulation of Single phase full wave thyristor controlled rectifier

The online model of the cycloconverter is basically the same as with the full wave thyristor controlled converter such that the calculation of the variable states are computed between two gating pulses and as soon as the current changes direction for

example from positive to negative, rectifier p is disabled and rectifier n starts working. The differential equations are the same as the Single phase full wave thyristor controlled rectifier when working with dc motor. The reducing factor of

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Proceedings of the 2008 International Conference on Electrical Machines processing time is much bigger in cycloconverter circuit than in the single phase with one controlled rectifier as we shall see later. Subsystems 1 and 2 used to generate pulses for rectifier p and n respectively while subsystem3 is used to detect the load zero current and invoke the desired rectifier.

Continuous
g a

pow ergui

k

k

g

Th1
g a g

Th2
a

Th5
k

Th6
k

ZERO DETECT OR
OUT1 CURRENT

DC Machine subsysyem1 100Vp-p 50Hz Step
TL A+ F+ k m

EMF LOAD CURRENT

u1

if (u1 == 0) elseif (u1 == 1) else

alpha 1 & 3

dc

AF-

a

100Vp-p 50Hz

If

g

a

g

Vfield

g

a

Th7
k

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k

g

alpha 2 & 4

a

a

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k

T h4

Scope2
alpha1 In1 Vin alpha3 VOLTAGES alpha_1 alpha_3 In1 In2 if { } Out1 Out2 alpha1 alpha3

Vin=V*sin(Wo*t)

Subsystem3
alpha4 In1 alpha2 VOLTAGES alpha_4 alpha_2 In1 elseif {Out1 } In2 Out2 alpha2 alpha4

scope1

LOAD CURRENT

Subsystem2

Fig. 4a. Single phase cycloconverter with separately excited dc motor drive simulated by Simulink and Simpowersystems Blockset.
vin (wanted) V*sin(wo*t) 1 In1
2*V/pi * cos(Ws*t)

T 0.001 s Monostable1
Enable1 - 0<alpha<pi

>=

2/pi*V

Scope8
2/pi*V*cos(ws*t) 0<alpha<pi
In1 Out1 alpha1 Enable1 - 0<alpha<pi

Constant J-K Flip-Flop Set bit 1 1

1 alpha1

-2/pi*cos(ws*t)

Enabled Subsystem >= T 0.001 s Monostable2
Enable2 - pi<alpha<2*pi

Q_TAG 1

Q_TAG Q_TAG

Q

J CLK

OR

<= 0.002

CURRENT 1

!Q

K

>= -0.002
-2*V/pi * cos(Ws*t) alpha3 Enable2 - pi<alpha<2*pi -2/pi*V

2
Out1

pi<alpha<2*pi

In1

alpha3 3 VOLT AGES
alpha1 alpha3

Scope4

Fig. 4c. Subsystem1 which detects Load zero current

Fig, 4b. Subsystem3 that generates pulses for the p rectifier and Subsystem 2 that generates pulses for the n rectifier.

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Proceedings of the 2008 International Conference on Electrical Machines

CURRENT OF SINGLE-PHASE-CONVERTER 4-THYRISTOR SIMULINK CIRCUIT
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DISCONTINUOUS CURRENT OF SIMULINK 2 1.5

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time

1.9 time

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CURRENT OF SINGLE-PHASE-CONVERTER 4-THYRISTOR MATLAB CIRCUIT
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DISCONTINUOUS CURRENT OF MATLAB 2 1.5 1 0.5 0 -0.5 1.85

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(a)
EMF VOLTAGE OF SINGLE-PHASE-CONVERTER 4-THYRISTOR SIMULINK CIRCUIT
80 EMF VOLTAGE 60

Fig. 6. Simulation results of the discontinuous conduction mode (up-Simulink, down-Matlab).

40

v o lta g e

20

0

Figs. 7 and 8 show simulation results on the single phase cycloconverter circuit of Fig. 4a, obtained by the conventional Matlab and Simulink Tool Box and by the present model.

-20

-40

-60

Vout* & Vin & Iout* Fin = 50Hz and Fout = 5.7Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

100

time

V o lta g e & c u rre n t

50

EMF VOLTAGE OF SINGLE-PHASE-CONVERTER 4-THYRISTOR MATLAB CIRCUIT
80 EMF VOLTAGE 60

0

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v o lta g e

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0

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time Output Load Voltage
150 100
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

-20

-40

-60

time

(b) Fig. 5. Simulation results in Matlab (present online approach) and Simulink (conventional simulation), as a response due to a step change in the load: (a) dc motor output current; (b) dc motor back emf voltage.

V o lta g e

50 0 -50

-100 -150 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

time

III. SIMULATION RESULTS For the full wave thyristor controlled converter circuit, the response to a step change in load is shown in Fig. 5: the back emf voltage (equivalent to the dc motor speed) and motor armature current (equivalent to the dc motor electromechanical torque) both in Matlab (present online simulation) and in Simulink (conventional simulation) environment. The online model was configured to use 200 samples per cycle of the 50 Hz ac voltage. Other parameters are as follows: Ra=10Ω, La=0.1H, f=50Hz, amplitude of the ac input voltage Vin=100V, R1=50Ω, c=0.02F, firing angle α=400, iLOAD= 10U(t)-U(t-1). The results in processing time are: Simulink conventional model, the processing time is 6.29 sec; Matlab present online model, the processing time is 0.1 sec. Fig. 6 shows simulation results in the discontinuous conduction mode.
C u rr e n t

Output Load Current
3 2 1 0

-1 -2

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

time

Fig. 7. Simulation results for 0.2 sec running time in cycloconverter circuit.

The current shown in dash line in the fig. 7 was multiplied by a gain factor of 17 to illustrate the transition between positive in negative current. In order to avoid short-circuit of the source, the interval between the instant at which the current from rectifier n becomes zero and the instant of application of signal p must exceed the turn-off time of the thyristors in

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Proceedings of the 2008 International Conference on Electrical Machines rectifier n like shown in Fig. 7 at t = 0.125sec . A converse rule governs the application of the gating signal to rectifier n. The parameters of the cycloconverter circuit are as follows: Ra = 10Ω , La = 0.8 H , V = 100v , f s = 50 Hz , R1 = 50Ω ,
* * c = 0.02 F , Vin = 50v , f in = 5.7 Hz (parameters of desired voltage and frequency in the load circuit). The results in the processing time are: Simulink conventional model, the processing time is 14.5 sec; Matlab present online model, the processing time is 0.12 sec. These models are implemented in a PC-cluster of Pentium IV 2.8Ghz processor.

Real-time simulation results have been presented. The processing time in both circuits illustrate the rapid prototyping in designing online models for power electronics circuit with two quadrant and four quadrant DC variable speed drives under real-time environment. They demonstrate the rapid prototyping of the online model and the differences in processing time between the present simulation model implemented by Matlab and the conventional simulation model implemented by Simulink. The present approach would be able to reduce engineering efforts and costs. V. REFERENCES

CURRENT GRAPH OF MATLAB
2.5 2 1.5 1

c u rre n t

0.5 0

-0.5 -1 -1.5 -2 -2.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time

CURRENT GRAPH OF SIMULINK
2.5 2 1.5 1

c u rre n t

0.5 0

-0.5 -1 -1.5 -2 -2.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time

Fig. 8. Cycloconverter simulation results for 2 sec running time in Maltab (up, present online approach) and Simulink (down, conventional simulation)

IV. CONCLUSIONS This paper describes an online simulation algorithm of power electronics and electrical drives. An important point in the algorithm concept is the calculation of parameters that would accelerate the online simulation itself by an offline program. The present algorithm was implemented in Matlab on a Pentium-class processor with very short online processing time (100 msec running time for 2 sec in full wave thyristor controlled converter, and 120 msec in cycloconverter - real operation time in our example). In other electrical drive circuits, the user only needs to integrate the online model with the offline model and to change the appropriate differential equations related to electric drive.

[1] J. Parle, E. Acha, and C. R. Fuerte-Espquivel, “Real-Time digital simulation of electromagnetic transient phenomena in power transmission lines”, Proc. Int. Conf. Advances in Power System Control, Operation, and Management, vol. 2, Nov. 1997, pp 563-568 [2] Simon Abourida, Christian Dufour , "Real-Time PC Based Simulator of Electric Systems and Drives", International Conference on Power Systems Transients – IPST 2003 in New Orleans, USA, Web Site www.ipst.org/TechPapers/2003/IPST03Paper13-3.pdf [3] V. Dinavahi , R. Iravani and R. Bonert “Design of a real-time digital simulator for a D-STATCOM system,” IEEE Trans. Ind. Electron., vol. 51, pp. 1001, Jun. 2004. [4] G. Sybille and Le-Huy Hoang, A comparative study on real-time simulation methods for PWM power converters, IEEE International Symposium on Industrial Electronics, 2006, vol. 3, pp. 2571-2578 [5] Lu Bin, Wu Xin, H. Figueroa, and A. Monti, A low-cost real-time hardware-in-the-loop testing approach of power electronics controls, IEEE Trans. on Industrial Electronics, vol. 54, issue 2, April 2007, pp. 919-931 [6] R. Bettendorf, Winder software testing with real-time dynamic simulation, IEEE Trans. on Industrial Electronics, vol. 52, issue 2, April 2005, pp. 489-498 [7] M. Huba and M. Simunek, Modular approach to teaching PID control, IEEE Trans. on Industrial Electronics, vol. 54, issue 6, December 2007, pp. 3112-3121 [8] P. Le-Huy, Guerette, L. A. Dessaint, and Le-Huy Hoang, Dual-step realtime simulation of power electronic converters using an FPGA, IEEE International Symposium on Industrial Electronics, July 2006, vol. 2, pp. 15481553 [9] J. C. G. Pimentel and Le-Huy Hoang, Hardware emulation for real-time power system simulation, IEEE International Symposium on Industrial Electronics, July 2006, vol. 2, pp. 1560-1565 [10] C. Dufour, S. Abourida, J. Belanger, and V. Lapointe, Real-time simulation of permanent magnet motor drive on FPGA chip for highbandwidth controller tests and validation, 32nd Annual Conference on IEEE Industrial Electronics, IECON 2006, November 2006, pp. 4581-4586 [11] Muhammad H. Rashid, "Power Electronics", Prentice Hall, 3rd Edition 2003 [12] Thomas H. Barton, "Rectifiers, Cycloconverters, and AC Controllers", Calarendon Press, 1st Edition 1994

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