Modelling, Simulation and Experimental Transient Over Volta

8/10/2019 Modelling, Simulation and Experimental Transient Over Volta

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Abstract-- The PWM Inverters are widely used in electricpower system to accomplish speed control of motors in various

process industries, power station auxiliaries, etc. The usage of fast

switching devices like IGBT PWM Inverters has led to the failure

of many motors in different field of applications. This has

aroused interest to study the impact of these PWM Inverters on

the performance of induction motors. In the recent past many

authors have presented their investigations on various issues

associated with the motor failure due to over-voltage that appears

at the motor terminals due to the impedance mismatch between

the power cable and the motor as well as non-uniformdistribution of voltage in stator winding.

This paper describes the modeling of the system consist of

PWM Inverter, Cable, induction motor and their interaction.

The system is simulated in MATLAB and experimental study is

carried out for validating the modeling and simulation of the

drive system. Also frequency responses the motor is obtained to

identify the surge impedance of the motor. At the end Sensitivity

analysis is made to study the influence of system parameters Viz

cable length, motor rating and rise time on motor terminal

overvoltage. Finally simulated are compared with experimental

results and found to be in good agreement.

I.INTRODUCTION

The growing use of induction motors for high power

adjustable speed applications is essentially due to the quick

technological evolution of fast switching electronic devices,

such as the insulated gate bipolar transistors (IGBT), which

are nowadays widely adopted in medium voltage, medium

power converters, for their performances in terms of driving,

switching behavior, etc.

While the high switching speeds and advanced PWM

schemes significantly improve the performance of the PWM-

inverter-fed induction motors, the high rate of voltage rise

(dv/dt) of 0650 V in less than 0.1 s has adverse effects on

the motor insulation. These steep rising and falling pulses leadto an uneven distribution of voltages within the motor,

especially during switching transitions. This contributes to

insulation deterioration and subsequent failure of the motor.

In addition, the dv/dt contributes to damaging bearing

Basavaraja Member,IEEE,,Research Scholar,NIT Warangal/Associate

Professor SREC,Warangal,AP,India( [email protected])

D.V.S.S.Sivasarma Member, IEEE, Assistant Professor, EED, NITWarangal

Andhra Pradesh, India ([email protected])

currents and electromagnetic interference (EMI). If a long

cable is employed between the inverter and the motor,damped high frequency ringing at the motor terminals occurs

resulting in excessive over voltage, which further stresses the

motor insulation. Also, the motor impedance, which is

dominated by the winding inductance, presents an effectiveopen circuit at high frequencies at the end of the long cable.

This produces a reflected voltage at the end of the cable

approximately equal in magnitude and with the same sign,

resulting in twice the magnitude of the incident voltage at the

motor terminals as shown in fig 2(b).Hence the problems

associated with PWM fed A.C Drives are

1. Terminal over voltages due to PWM Inverter and cables

2. Surge propagation within winding

3. Bearing currents

4. EMI

This paper describes the modeling of the system consist

of PWM Inverter, Cable, induction motor and their

interaction. The system is simulated in MATLAB and

experimental study is carried out for validating the modeling

and simulation of the drive system. Also frequency responses

the motor is obtained to identify the surge impedance of themotor. At the end Sensitivity analysis is made to study the

influence of system parameters Viz cable length, motor rating

and rise time on motor terminal overvoltage. Finally simulated

are compared with experimental results and found to be in

good agreement.

II. System representation

Fig. 1. PWM inverter driving an induction motor using long cable leads

Modelling, Simulation and Experimental

Analysis of Transient Terminal Overvoltage in

PWM-Inverter fed Induction Motors

B.Basavaraja1,Member,IEEE, D.V.S.S.Siva Sarma2,Member,IEEE

1-4244-1298-6/07/$25.00 2007 IEEE.


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Fig. 2. Inverter output voltage and Over voltage at motor

Fig.1.shows schematic of the PWM inverter (ASD) driving

an induction motor using long cable. An adjustable speed

drive basically consists of a rectifier and an inverter section,

which converts the dc to ac of a selected frequency.

Fig.2 shows the experimental results of inverter output

voltage and motor terminal voltage waveform.

Difference in impedances of the cable and motor leads to

voltage reflection and hence over voltage appears at motor

terminal [3].

III. THEORY OF OVERVOLTAGES (INVERTER-CABLE-

MOTOR INTERACTION)

Cause of overvoltage at the motor terminals is due to

mismatch between cable surge impedance and motor surge

impedance.

Cable Surge Impedance

Zo

L

C

---------1(a)

L = inductance per unit length

C= capacitance per unit length

Zo Varies with wire gauge and cable constructionZo Range is 80 to 180 ohms

Motor Size Z-load Impedance

Range

< 5 HP 2000 5000 ohms

125 HP 800 ohms

500 HP 400 ohms

Difference in impedances of the cable and motor leads to

voltage reflection and hence over voltage appears at motor

terminal.Fig3 shows the Voltage Reflection Analysis due to

long motor leads. To better understand the repeated

reflections on a finite length of cable with infinite dv/dt, onereflection of an incident wave will be considered

Fig. 3 shows the different steps involved in the voltage

reflection analysis of over voltages at the motor terminal. Fig.

3(a) shows the PWM pulse input at the sending end of the

cable. The motor terminal is considered as open circuit for

high frequencies. Fig3 (b) shows the incident voltage wave

and the associated current waves when the voltage input is

switched on. Fig (c) shows the reflected voltage pulse and

associated current pulse at the motor end of the cable. As the

motor terminals are open circuited, current must be zero.

Hence the reflected component of the current will have same

amplitude but with opposite sign, where as the incident

voltage wave will be reflected with positive coefficient

towards the sending end. The voltage at motor terminals

which is the sum of incident and reflected voltage will be

nearly double that of incident voltage. The reflected

component at the motor terminals travels towards inverter.

However, at the sending end inverter output voltage is E and

hence a traveling wave of E reflects and travel towards themotor. This voltage is associated with a current waveform.

This second incident wave soon reaches the receiving end and

is reflected again.Fig3 (d) and (e) shows these reflected

waveforms.

Fig. 3. Voltage Reflection analysis

During the second reflection at the motor terminal, voltage

waveform will be reflected with negative pulse and the current

pulse be reflected with positive pulse so that the current at the

motor terminals remains zero. These second reflected current

and voltages will travel towards the inverter and the cycle

repeats.

If Zm is the motor impedance and Zc is the cable surgeimpedence, reflection coefficient at the motor terminals

m= (Zm Zc)/( Zm+ Zc)------1(b)

Zc= SQRT (Lc/Cc) ----------- (2)

Where Lc and Cc are cable inductance and capacitance

per unit length.

Reflected coefficient at the inverter terminal

s=(Rs Zc)/(Rs+ Zc)------(3)

Where Rs is the source resistance.


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A. Over voltage at motor terminal

At the inverter, the reflected forward-traveling wave has

the same shape as the incoming backward-traveling wave but

with corresponding points reduced by s. Due to the

dominating winding inductance, the characteristic impedance

of the motor can be ten to one hundred times that of the

characteristic impedance of the cable connecting the drive to

the motor. Therefore, the incident wave voltage will be

reflected back towards the inverter as a function of eqn.

(1)(b), and the voltage amplitude at the terminals of the motor

will approximately double [4][5], as shown in Fig. 4

simulation and experimental results

Fig. 4(a). Simulation waveforms showing PWM Inverter output and motor

terminal voltage

Fig. 4(b). Experimental waveforms showing PWM Inverter output and motor

terminal voltage

Fig.4 (b) shows the experimental waveforms of PWM

Inverter output and motor terminal voltages for 14m cable,

200V input systems.

B. Effect of PWM Rise Time (dv/dt)

Peak voltage at the motor terminal can be determined by

using wave propagation theory and voltage reflection analysis.

The traveling time (tt in s) for the inverter output pulse to

travel from the inverter terminals to the motor terminals can

be expressed as:

tt

lc

v------------ (4)

where v is the pulse velocity and is given by

v=1

LcC

c

---------(5)

lc= cable length in feet

Lc= inductance per foot

Cc= capacitance per foot

tt= time for pulse to transit the length of the cable once

Forward traveling inverter output pulse will be reflected

at the motor terminals after tttime and the resulting backwardtraveling wave, moving towards the inverter, will have an

magnitude of:

Et(tt) =

r

mdct

t

Et for tt< tr (6)

and

Et(tt) = Edc* m for tttr (7)

whereEdc= dc bus voltage

m= reflection coefficient at the motor (typically 0.9 for

motors less than 20hp)

tr= inverter output pulse rise time (in s)

Hence when ttt r equation (7) applies and no reflection

and the peak motor terminal voltage will be reached after the

pulse travels the length of the cable. Usually cable length of

15mt or less will result in tt< tr, and therefore eqn. (6) would

apply.

The backward traveling wave will then be reflected at theinverter terminals in the same manner, however, now as a

function of the reflection coefficient of the inverter (or

source), s. From eqn. (3), it can be seen that for a typical low

impedance source, s will approach -1, and therefore, theresulting reflected wave traveling back towards the motor will

be negative in amplitude.

Therefore, after three transitions of the cable, the

increasing motor terminal voltage will be reduced by this

negative reflected wave, after it has traveled back and reached

the motor. Therefore, the peak voltage can be found by

determining the total voltage due to reflections at the

terminals of the motor, from eqns. (4-7), after three transitions

of the cable, and adding this to the incident wave voltage

magnitude, Edc, as shown in eqns. (8-9).

ELL,p=dc

r

mdcc E+tv

El

3 for tt< tr/3 ----(8)

ELL,p= Edc* m+ Edc for tt tr/3---------(9)

The normalized peak motor terminal voltage for longer

pulse rise times, i.e. tr/3 > tt, can be written as a function of


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rise time as:

dc

P,LL

E

E = 13

+

r

mc

tv

l ---------- (10)

Therefore

r

mc

tv

l

31, pulse width is no longer a sinusoidal function of the

angular position of the pulse.

For low values of frequency modulation index (MF= fc/f)

to eliminate the even harmonics, a synchronized PWM should

be used and MF should be an odd integer. More ever MF

should be a multiple of 3 to cancel out the most dominant

harmonics in the line to line voltage.

B. High Frequency Model of the Power Cable

An adequate estimation of the power cable parameters

is needed in order to have an accurate computation of the over

voltage. Long cable lengths contribute to a damped high

frequency ringing at the motor terminals due to the distributed

nature of the power cable leakage inductance and coupling

capacitance which results in over voltages and further stress

the motor insulation. In addition, voltage reflection is a

function of inverter output pulse rise time and the length of

the motor cables, which behave as a transmission line for theinverter output pulses. So the cable representation resembles

like lumped parameter model of the transmission line.

Fig. 6. High Frequency Model of the Power Cable

The fig.6shown above is the per phase representation

of one lumped segment of the power cable used in the

MATLAB simulation .The model conjugates low and high

frequency representations of the power cable.R1,R2,C3areresponsible to represent the low frequency and remaining for

the high frequency phenomena.


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C. High Frequency Model of the Induction Motor

Another key factor for an accurate over-voltage analysis is

the high frequency representation of the ac motor input

impedance, which must be valid over a broad range of

frequency. It is not necessary to verify how voltage will

distribute inside the AC machine winding in order to calculate

the over voltage at the terminals. It is important, rather, to

know the value of the ac motor input impedance and how it

varies as a function of frequency. The fig.7shows the

schematic of the proposed model of an induction motor is

implemented in the simulation program to evaluate the over-

voltage analysis. The model conjugates low and high

frequency representations of the motor. The suggested model

is a lumped-parameter representation of the motor input

impedance. The low frequency equivalent model parameters,

r1, r2, l1, l2and lm is partly responsible for capturing the low

frequency transients, while the remaining R-L-C network is

responsible to represent the high frequency phenomena.

Winding-to-ground capacitance and winding turn-to-turn

capacitance play the major role in the high frequency

phenomena. Their relation with the leakage inductance formsthe dominant poles in the frequency response. The parameter

Cg represents the winding-to-ground capacitance. The

parameter Rg is added in the circuit to represent the

dissipative effects that are present in the motor frame

resistance. The circuit formed by the parameters Rt, Lt, and Ctis the part of the network responsible to capture the second

resonance in the frequency response, which is related to the

winding turn-to-turn capacitance. The parameter Re is

responsible to account for the losses introduced by eddy

current inside the magnetic core [7].

Fig. 7(a). High Frequency Model of the Induction motor

D. Frequency Response of Induction Motor

High Frequency response of an Induction motor as shown

in fig 7(b) is useful identify the surge impedance of an

induction motor. Knowing the surge impedance of the

induction motor, we can identify whether the cable impedance

is match with the motor impedance. If the two impedances are

matches, reflection will not occur and surge voltage will not

appear at the motor terminal.

Fig. 7(b). Frequency response of the Induction motor

V.SIMULATION RESULTS AND EXPERIMENTAL VERIFICATION

Using model of PWM Inverter and by using the high

frequency models [8] [9] of cable and Induction motor

(shown in fig7), a simulation program has been developed in

MATLAB to calculate the over voltages. The voltage pulse

rise times (350ns-1.2 s,) driving an induction motor of power

ratings 3hp and 15hp are considered for simulation.

Experimental results for 3HP, 10mt and 14mt cable lengths

are shown in fig.9.

A. Cable Length Vs Peak Voltage

MATLAB simulation results for the 3HP motor rating and4mt, 10mt and 14mt cable lengths are shown below in fig 8

Fig. 8. Simulation results for 3HP, 4mt, 10mt and 14mt cable respectively.


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reduces.

D. Effect of Rise Time (dv/dt)

From equation (13),critical cable length can be found by

m

r

3

2.0t

v=l c -----------(14)

i.e. for a 440V ac system with 594V dc bus, the allowable

peak voltage would be 1.2*594V = 712.8V. From the cableparameters v= 160m/s is obtained and the critical rise time

(tr) for 10m of cable and m= 0.9 would be 2.566s.

Therefore, a rise time of less than 2.566s (higher dv/dt) will

result in an over-voltage at the terminals of the motor greater

than 20%.

Table III shows the minimum cable length and rise time

after which virtual voltage doubling occurs at the terminals of

the motor. The cable measurement carried by using a LCR

meterTABLE III

MINIMUM CABLE LENGTH FOR VOLTAGE DOUBLING

Rise time ( s) Cablelength(mt)

0.2566 3.6

0.5 18

2.566 36

5.132 54

2.566 92.32

3.0 106.44

3.5 126

4.0 144

5.0 180

From the above table & waveforms, it is observed that

1. As the cable length increases the over voltage reduces.

2. For large HP ratings over voltage is less.3. As the rise time increases the dv/dt reduces.

E. Effect of High dv/dt

High switching speeds and zero switching loss schemes

drastically improve the performance of the PWM inverter, but

high rate of voltage rise (dv/dt) of 0 to 400v in less than 0.1s

has adverse effects on the motor insulation and bearings and

deteriorates waveform quality. This leads to overvoltage

problems and hence reduces the motor life [10] [11].

Fig 13 shows the dv/dt for supply voltage of 440v, 10mt

cable length and different frequencies. Usually PWM

Inverters operates at higher switching frequencies and for

these values of frequencies the dv/dt is more as shown in

fig13.Hence shorter rise time will have more dv/dt.

Fig. 13. dv/dt for supply voltage of 440v, 10mt cable length and different

frequencies

Fig. 14. Voltage and time curve at inverter output and at motor terminal with

filter and without filter.

Over voltage can reduce by using filters at motor

terminal or at output of inverter [5] [6]. One of the filter

design method to reduce the overvoltage is RC filter at motor

terminal. In this method cable is terminated by a first order

filter consisting of a capacitor in series with the resistor tomatch with the cable and provide the proper level of damping

to control the voltage overshoot.

By observing fig15 over voltages are more at motor

terminal without filter and less over voltages with filter.

Hence dv/dt can be reduced by adopting filter at motor

terminal or at output of the inverter terminal.

VI.COMPARISON OF EXPERIMENTAL AND SIMULATION

RESULTS

Experimental and simulation results are compared for 3HP

induction motor connected by different length of cable from

PWM Inverter as shown in fig. 16

VI. CONCLUSION

Fig. 16. Experimental and simulation results for 3HP induction motor with 10mt

cable length.

Observing fig 16, simulation and experimental wave forms

are verified, which shows simulation results are validated by

experimental results.


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Modelling, Simulation and Experimental Transient Over Volta