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Development and experimental validation of a global simulation model of an high power electrical drive in a cement plant

VITTORIO PETTONATI (December 2000)

vittorio.pettonati@gmail.com

ABSTRACT : The analysis of the behavior of an high power drive needs an appropriate global simulation tool; this fact is due to the nonlinearities of the system that makes necessary the simulation both before making the prototype and especially before installing the drive in a plant to show how it could be interact with the others existing components. Until now just a lot of papers have treated this argument, however focusing their attention especially on low power applications; moreover only a few of this have validated the proposed results comparing them with the real waveforms. The aim of this paper is to present a model of an high power AC electrical drive in a cement plant, developed in the SIMULINK© and POWER SYSTEM BLOCKSET© environment using a global approach. The simulation’s results are referred to two different supply conditions : an “ideal” condition, concerning the drive supplied with three phase sinusoidal voltages, and a “real” condition that is referred to distorted voltage; for both this cases the model’s validity is confirmed by the comparison with the experimental acquisitions.

INTRODUCTION Energy costs have a substantial impact on overall cement production cost; one possibility for energy saving resides in the choice of optimized drive concept, for example for fan drives [1]. In fact the volumes of gas derived by the fans can be controlled in various ways and among these a control with variable speed, by means a frequency converter, is the best solution. This choice is therefore followed by the distortion of the voltage and of the current absorbed by the same drive [2]; the first because of the transitory short circuits that occur when there is the transition between one power device and another, the second because of the introduction of harmonic component; moreover the distorted current circulating on the cables’ impedances cause distorted voltage’s drops that change the voltage of the linking point of the same drive. This distortion can cause serious problems on the load and on the plant’s components like transformers, reactors, capacitors, cables, devices, relays, generators, turbines and motors; this problems if aren’t suitably resolved can cause bad operations of the same drive and of others devices correlated to it. The points of major importance, in large power plants in general and in a cement plant in particular, are then to analyze the interaction between converters and to

quantify the effect of the harmonics injected by these converters in the supply line. It’s therefore evident that it’s fundamental to have the possibility to predict the behavior of a drive and the consequences resulting from its installation in a plant; the simulation then represent an irreplaceable instrument for a correct design. To study such differential conducted emission, a functional analysis based on global simulation approach is necessary; unfortunately, when this concerns a large power plant, the simulation becomes difficult to achieve [3] [4]. Also dedicated power electronics software which permit a qualitative evaluation of the harmonic currents are very rare and however they tend to make simplified assumptions for the converter, by considering it as an ideal voltage or current sources. As it well known a basic distinction between circuit-oriented and equations solvers may be done for simulations programs [5]; the main advantage of the first type, like ATP, is that the user is focused on the circuit’s topology rather than the mathematical equations but the internally-built models of the component are seldom accessible to the user. The Power System Blockset, recently proposed, combines the advantages of both equation solver and circuit-oriented simulators approach [6]; it permit to model and simulate power electronics system and it represents the integration of the widely used MATLAB/Simulink computational engine, taking from this the advantage to represent a system by means blocks. This let to plan a global approach which takes into considerations the four subsystems (converter, motor, controls, load) of the drive, reaching to represent for each of them the exact operation’s algorithms. The present paper describes the model implemented with Simulink© and Power System Blockset© of an high power low voltage electrical drive installed in a cement plant to control a kiln exhaust gas duct fan; are also presented the obtained results, comparing them with the real waveforms in order to show how the model exactly simulate the behavior of the drive. With reference to the supply voltages two different cases have been considered to underline the different performances of the system. THE HIGH POWER ELECTRICAL DRIVE AND

ITS MODEL

The electrical drive considered in this paper is installed in a cement plant’s medium voltage neutral isolated system, with a capacitors’ battery for the compensation

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of the reactive power (this is a typical situation in industrial plants); it is principally constituted (Fig.1) by a dedicated three phase transformer , a frequency converter and an induction motor whose mechanical shaft is directly connected with the rotor of a exhaust fan.

Fig.1 – Basic scheme of the high power drive

The system data are reported in tab.1. Fig.2 shows the model implemented: with light gray are underlined the blocks necessary for the generation of the variables representative of the interesting parameters of the system, while in dark gray are the elements that constitute the drive [7]. In particular the block named “trasformatore” simulate the three phase saturable transformer by means three single phase saturable transformers, the block named “Silcovert” simulate the converter and implements its modulation technique, its control strategy, its pre charge logic and the inverting and rectifying stadium, the block “Motore” model the dynamics of a three phase asynchronous machine representing its electrical parts by a fourth-order state-space model and its

mechanical part by a second-order system, the block “Ventilatore” simulate the fan’s resistant torque varying with the square of the rotation speed.

Supply 6 kV

Capacitors 100 kVAR

Transformer Dyn11 550 kVA 50 Hz

6000/400 V/V

PO = 1393 W PCC= 6759 W

I I =52,92 A III = 793,85 A Vcc% = 6,72 %

Converter VSI IGBT

Technology Control

Scalar (V/Hz)

513 kVA 780 A 380 V

DC link Capacitance 29700 µF

Modulation THIPWM

digital

Modulation frequency

2 kHz P max motor

436 kW P max DC link 449 kW

Motor 3 phase

induction Squirrel cage

Poles 6 400 kW

400 V 50 Hz 690 A

993 rpm Rated torque 3845 N*m

Moment of inertia

30 kgm2

RM = 66 Ω RS =0,00834 Ω RR =0,0084 Ω

XM = 3,52 Ω XS= 0,0938 Ω XR= 0,0656 Ω

Tab.1 – The system data

Fig.2 – The model of the high power drive implemented with Simulink© and Power System Blockset

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RESULTS Two different simulations have been carried out with a perfectly sinusoidal three phase supply voltages’ system and a third harmonic distorted voltages’ system, respectively. In the first case, named “ideal conditions”, the results have been compared with the respectively waveforms acquired during the laboratory’s test, while in the second case, named “real conditions”, have been compared with the waveforms acquired when the drive has taken service in the cement plant. The following figures show the above mentioned results with reference to the motor’s starting; the harmonic spectrums have been evaluated with the MATLAB computational engine.

IDEAL CONDITIONS

2.475 2.48 2.485 2.49 2.495

-500

-400

-300

-200

-100

0

100

200

300

400

500

[s]

[V]

Fig.3 – Motor’s voltage

Fig.4 - Motor’s stator current

0 50 100 150 200 250 300 350 0 10 20 30 40

(A)

Fig.5 – Harmonic spectrum of the motor’s current

Fig.6 – Motor’s voltage (500 V/div) and current (400 A/div)

0.275 0.28 0.285 0.29 0.295 0.3 0.305 0.31 0.315-600

-400

-200

0

200

400

[s]

[V]

Fig.7 – Phase to phase transformer’s secondary voltage

0 100 200 300 400 500 600 700 0 20 40 60

(V)

Fig.8 – Harmonic spectrum of the secondary voltage

1.3 1.305 1.31 1.315 1.32 1.325 1.33 1.335-500

-400

-300

-200

-100

0

100

200

300

400

[s]

[V]

Fig.9 – THIPWM modulation references

2.19 2.2 2.21 2.22 2.23 2.24-1000

-800

-600

-400

-200

0

200

400

600

800

[s]

[A]

4

0 0.5 1 1.5 2 2.5-4000

-2000

0

2000

4000

6000

8000

[s]

[N*m

]

Fig.10 – Motor’s torque

0 0.5 1 1.5 2 2.50

500

1000

1500

2000

2500

[s]

[N*m

]

Fig.11 – Fan’s resistant torque

0 0.5 1 1.5 2 2.5-200

0

200

400

600

800

1000

1200

[s]

[giri

/prim

o]

Fig.12 – Motor’s speed

REAL CONDITIONS

Fig.13 – Distorted voltage (1500 V/div)

1.885 1.89 1.895 1.9 1.905 1.91 1.915 1.92-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

[s]

[V]

Fig.14 – Plant’s voltages

Fig.15 – Distorted current (10 A/div)

1.88 1.89 1.9 1.91 1.92 1.93 1.94 1.95 1.96-60

-40

-20

0

20

40

60

[s]

[A]

Fig.16 – Plant’s current

2.2 2.25 2.3 2.35 2.4 2.45

-800

-600

-400

-200

0

200

400

600

800

[s]

[A]

Fig.17 – Motor’s stator current

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Fig.18 – Motor’s stator current (200 A/div)

0 50 100 150 200 250 300 350 0 10 20 30 40

(A)

Fig.19 – Harmonic spectrum of the motor’s current

1.37 1.38 1.39 1.4 1.41 1.42 1.43

-400

-200

0

200

400

600

[s]

[V]

Fig.20 – Phase to phase transformer’s secondary

voltage

0 50 100 150 200 250 300 350 0 10 20 30

(V)

Fig.21 – Harmonic spectrum of the secondary voltage

CONCLUSIONS

This paper present the model, implemented in the Simulink and Power System Blockset environment, of an high power electrical drive developed using a global approach. The model permits not only to test the dynamic behavior but also the steady state behavior of the system, providing the opportunity to quantify the harmonic distortion introduced and the changes in the waveforms that occur when the same drive is installed in a plant with distorted voltages and currents. In this last case it’s clearly evident that the distortion of the waveforms is more marked; the model’s validity is confirmed by the comparison with the experimental acquisitions. It also permits to design an appropriate filter verifying its validity with a simulation; it

substantially allows to reduce the typical mistakes of the planning. Moreover, by changing the values of the parameters of the singles elements constituting the drive, it’s possible to study systems from low power to high power ranges.

ACKNOWLEDGEMENT

The author would like to thank the CTG Italcementi Group - Engineering Central Management - Plants & Installations Department - Electrical Installations Service and similarly to thank the Ansaldo Sistemi Industriali S.p.A. - Engineering Department - Power Electronics Operation.

REFERENCES

[1] A. Godichon – “Variable speed drives on large centrifugal fans” – IEEE Cement Industry Conference, 1992 [2] P. Williams, “Problems associated with electrical variable speed driver”, IEE Colloquium on energy efficient environmentally friendly drive systems principles, problems application, Digest n° 1996/144 [3] V. Rajagopalan – “Computer-aided analysis of power electronic system”, Marcel Dekker inc.,New York 1997 [4] A. Ba-Razzouk, A. Pittet, A. Cheriti, V. Rajagopalan – “Simulink based simulation of power electronics system”, IEEE 4th Workshop on Computers in Power Electronics, 1994 [5] Mohan, N. – “Modelling Power Electronics in power Systems using EMTP: A short course, Center for Electric Energy”, University of Minnesota, August 1994. [6] Haddad, K.A., Dessaint, L.A. “The Power System Blockset: a new and Powerful tool for Simulation of Power Electronics, Electric Drives and Power Systems”, JTEA'98 Tunisie November 1998 [7] “High-performance numeric computation and visualization software” User’s manual of MATLAB

program.