improve efficiently soft-starter transients' · pdf fileimprove efficiently soft-starter...

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Improve efficiently soft-starter transients' immunityBy Delcho Penkov

& Alain Côte Schneider Electric

Summary

Abstract ........................................................................................................ 1

Nomenclature ................................................................................................. 1

Introduction .................................................................................................... 2

Overwiew of SCR functionning and electrical transient issues ......................... 4

Case study ..................................................................................................... 5

Modeling of the RVSS and case study power system in EMTP-ATP ............... 6

Analysis of the thyristor turn on current transient ............................................. 7

Overview of the voltage transient during switching .......................................... 9

Estimation of the risk of high current transient ............................................... 11

Development of protection sizing tool ........................................................... 12

Conclusion ................................................................................................... 14

Acknowledgements ...................................................................................... 15

Appendices .................................................................................................. 15

Vita ............................................................................................................... 16

COM-POWER-WP--EN Rev1 | 1

Improve efficiently soft-starter transients' immunity

Abstract

In this paper the authors present results of measurements and

mathematical analysis on MV Silicon Controlled Rectifiers (SCR, Thyristors)

for the purposes of the power electronics components protection during

turn on. Current transient was identified as responsible for damaging

soft-starters in field applications. This work is focused on the identification

of the major parameters playing role in the current transient.

Formulae for calculating the current rate-of-rise and risk identification

procedure are derived. A simple tool for practical SCR transient protection

design is described.

Index Terms — Soft-starter, current transient, EMTPATP

Nomenclature

φ Phase shift between current and voltage, in ms.

α Thyristor turn on delay with respect to voltage zero crossing, in ms.

COM-POWER-WP--EN Rev1 |


Introduction

Silicon Controlled Rectifiers (SCR) or Reduced Voltage Soft-Starters (RVSS) are modern techniques used for smooth motor starting in MV power systems. Thus many papers on SCR design discuss motor or system protection issues like torque pulsations or harmonic reduction, however little is said about protection of SCR itself. Major electrical transient constraints on the semiconductors are overvoltages during switching off, and current rate of rise while turning on. For power systems with rated voltages of 5.5 kV and above the current transient may become important, and depending on the application, damage the semiconductors, some believe because long connecting cables are used. Since it is common to use one RVSS to start several motors sequentially, failure of the RVSS can lead to substantial production losses. Generally a 100 μH series reactor is likely to be sufficient. However, from an economical point of view and lack of space, it would be much better to determine a more rigorous way to size this reactor. Risk assessment is crucial, together with a clear explanation of the current rate of rise phenomenon. In this paper the authors investigate field measurements and mathematical models of the SCR in order to understand the mechanisms the current transient depends on. In a first step it came out that the transients depend on the immediate environment around the SCR including both upstream and downstream installation, and not only on the motor cable length. Further analysis helped us to build a mathematical constant parameters model in EMTP-ATP that reproduces accurately the system behaviour during thyristor turn on. This was important step since it allowed us to go further and by hand analysis establish a formula to calculate the current transient on turn on without performing a simulation. It allowed in-depth understanding of what the current transient was the most depending on. Consideration of the overall behaviour of the switching angle and motor acceleration during start made it possible to put forward recommendations to reduce the potential risk of SCR damage before installation of additional protection. Yet if this is not sufficient, calculation tool gives recommendation on the sizing of the protection reactance together with the expected current transient. The solution is generalised and user may account also for the installed protection capacitors. Unfortunately, due to a very high installation conditions dependency simple rule, just built on voltage/rated power/cable, is difficult to be formulated, given the necessity to make some preliminary calculations. Required input data concerns immediate upstream and downstream installation details, and how the soft-starter is set up. Practical tests confirmed the accuracy of the developed tool. Thus full SCR service continuity in optimized installation conditions will be ensured. The paper is organized as follows: Section II introduces the principles of the soft-starter functioning and the constraints it is exposed to. Section III is a brief overview of a field application where a soft-starter was damaged during motor start. Section IV presents the modelling of the soft-starter and the simplification assumptions that were validated by comparison with field measurements.Sections V and VI will describe the mathematical analyses made for deriving of the current rate-of-rise computation formula and estimation of critical transient voltage values. Section VII defines the critical moments (zones) during the motor start. Section VIII discusses the development of risk assessment tool capable of inductance sizing calculation.



The SCR driven motor start is based on the use

of parallel thyristors connected in reverse parallel

configuration to each other that are switched on

by a command signal. The command signal

is intentionally delayed from the voltage zero-

crossing so that a smaller current is provided

to the motor sufficient to start but lower than

the rated starting current.

The overall behavior is described in the Fig. 1.

(V,I)

t(s)Current zero time

Ithyristor

Vupstream

Figure 1 – Principle of controlled current in soft-starter.

tcurrent_zero = α - φ

The current through the thyristor stops naturally

on zero crossing. The moment of zero crossing

depends on the phase shift between the current

and voltage.

This phase shift varies during the motor start as

Overwiew of SCR functionning and electrical transient issues

the motor is equivalent to variable impedance and

decreases with the acceleration of the motor.

The command signal (represented by α ) is also

varied during the motor start.

Globally the behavior shown in Fig. 2 is observed:

(ms)

t(s)

t(s)

t(s)

leffinit = Veffinit.kd

Veffinit

init = f (Veffinit)

leff (% x Ir)

Veff (%)

Ramp time

Current limit

Rated current

Rated voltage

Full wavevoltage

Figure 2 – Basic evolution of main soft-starter depending variables.

Somewhere after passing in full-wave conduction

of the thyristors a parallel by-pass contactor is

closed and RVSS stops.

The user dependent settings are:

1. initial voltage, in % of rated voltage

2. ramp time (time to go to the current limit), in s

3. current limitation, in % of rated motor current.

Thyristor Functioning Principles

Electrical transient switching constraints

These are the surge voltage on switching off and

current rate-of-rise on turn on. The surge voltage

issue is solved by a parallel connected snubber,

sized according to the expected overvoltage and

energy. This is a standard issue, scheduled during

the development of the soft starter. Fig. 3 shows

an example on a 6kV power system.

The overvoltage on the thyristors goes up to

270 % of the peak rated single phase voltage.

Connecting 2 or 3 thyristors in series reduces

the overvoltage applied individually on them.

t(ms)55 59 63 67 71-12

-8

-4

0

4

8

12k(V)

Upstreamvoltage

Voltage acrossthe thyristor

: VP2F-VaP2F

: VaP2F

Figure 3 – Measurement of Voltage transient during a complete fundamental period.

(1)



The current rate-of-rise issue differs because

it depends on the installation conditions of the

soft-starter. Its solving is not generalized but case

dependent. An example of current turn on rate of

rise on 6 kV power system is shown in Fig. 4.

t(ms)55 59 63 67 71-1500

-1000

-500

0

500

1000

1500(A)

Turn on currenttransient

: IP1F

Figure 4 – Measurement of current through the thyristor during a complete fundamental frequency period.

As it can be seen there is a high current transient

on turn on.

If it overpasses the thyristor limit (100-200 μA/s

typically) the thyristor is damaged as well as the

softstarter itself. This is the subject of this paper.

Fig. 5 shows a zoom on the current during this

transient:

t(ms)63 63.05 63.10 63.15 63.20 63.25

(A)

-100

100

200

300

400

500

0

: IP1F

Figure 5 – Current transient on switching on of the thyristor.

In this paper the authors will focus on the main

parameters this transient depends on.

Case study

The problem of high current rate-of-rise emerged

on a 6 kV power system in an oil refinery based in

Spain (Fig. 6).

Grid

Power transformer

6 kV bus

Upstream cable

Soft-starter

Downstream cable

Motor

Figure 6 – Case study power system.

The power system is described hereafter:

1. Rated voltage of 6 kV

2. 30-40 m of 120 mm² cable upstream to

the softstarter, 2 conductors per phase

3. 320 m of 120 mm² cable downstream of

the softstarter, 2 conductors per phase

4. 3.1 MW Motor.

The motor has been started several times before

the soft-starter was damaged. Substantial

measurements on site, shown in Fig. 3 and 5

revealed high current rate-ofrise, over 170 A/μs

whereas the thyristors were only able to withstand

up to 150 A/μs repetitive rate-of-rise.

After inserting a reactance of 100 μH in series

with the softstarter the current rate-of-rise was

measured as 40 A/μs during the current limitation

period. The solution was very effective, however

bulky. We focused our analysis on the identification

of the origins of the problem.



EMTP-ATP is a software package dedicated to

modeling of power system transients.

It allows analysis of high frequency phenomena,

i.e. switching transients, and was chosen as most

adequate to the aim of our modeling.

Moreover EMTP-ATP allows in depth modeling

of the power system together with suitable RVSS

model, critical, since the analysis aimed to see

the impact of the surrounding power system

on the semiconductors.

Please note that the modeling was focused on

an accurate simulation of the transients during

switching on. At first, we modeled the complete

power system using frequency-dependent cable

models and RVSS with its command circuit,

capable to represent the power system during

the complete motor start. The comparison with

real test measurements showed the very good

accuracy of the models. However, due to the very

high number of setting parameters, such as

the RVSS settings and motor, cable specifications,

such approach was not applicable for large scale

analysis on the current rate-of-rise, since it is

difficult to establish, monitor and explore

the very high number of various study cases it

would require. Furthermore, indepth analysis

showed that the current rate-of rise varies during

motor start, which requires simulation of

the complete process. With a timestep of 100 ns

for an average of 15 s to be simulated,

the generated output files would have been

difficult to handle.

We proceeded to simplification of the used models:

1. The motor was implemented as series R-L

fixed impedance with phase-to-earth parasitic

capacitance. This assumption is valid since the

transients on the thyristors are much faster than the

speed variation of the motor shaft, responsible

for the variation of it’s equivalent impedance

(this is also stated in IEC 62271-100)

2. Power feed was assumed as source behind an

equivalent short-circuit impedance (comprising

the transformer), with phase-to-earth

equivalent capacitance, including protection

capacitors if any.

3. Cables were modeled in π, however in order to

avoid discharging of the immediate upstream

of the RVSS cable capacitance into the

downstream one, a different approach was

applied: the upstream and downstream cables

have been considered as an equivalent cable

whose capacitances were placed on its ends.

The softstarter is placed along this single cable,

according to the real data. Its precise position

does not have any impact on the current

rate of rise since the cable capacitances are

concentrated at the ends of this equivalent cable.

Thus the resulting power system model was

greatly simplified as shown in Fig. 7.

Cupstream

Upstreamcable

DownstreamcableZcc Motor

Cdownstream

Figure 7 – Simplified power system model.

Cupstream = Csystem + Cprotection capacitors + Cupstream cable

Cdownstream = Cmotor + Cdownstream cable

The results obtained with this simplified model

were still precise (<5 % error) with respect to

the measured values of current rate-of-rise,

oscillation frequency and attenuation.

Example curves are shown in Appendix C.

However this simplified model, built with only linear

elements had another important advantage:

it allowed clear analysis of the switching events.

Modeling of the RVSS and case study power system in EMTP-ATP

(2)



As it was stated before the simplification of

the power system model led to the possibility

of using simple mathematics in order to derive

the behavior of the current in the first instants

after turning on.According to the measurements

(see Fig. 5) the current transient may be

approximated to a second order step response.

Analysis of the thyristor turn on current transient

Thus in electrical terms this means that it may

be summarized as a second order circuit

comprised of an inductor, capacitor and resistor,

the latter providing the attenuation of the signal.

The estimation of their values will be given by

the mathematical analysis.

1. During switching on, due to the very high

frequency of the current transient (50-200 kHz)

the upstream power circuit, except the

connecting cable, is equivalent to a capacitor

connected to earth. In fact, if this was not

the case, the current rate-of-rise will be

quite limited by the short-circuit equivalent

inductance of the upstream circuit

2. During switching, again due to the very high

frequency, the motor is assumed as

a capacitor connected to earth, its equivalent

RL circuit having a very high value.

3. During switching on, the fundamental (50 Hz)

current increase is neglected with respect to

the first quarter of the oscillation of

the transient current

Hypothesis

The following parameters are defined:

C1, C2 equivalent capacitors of the power

system upstream and downstream of

the soft-starter, the value being the sum

of the upstream or motor equivalent

capacitance and that of the upstream or

downstream cable

R, L equivalent parameters of the connecting

cable comprising both the upstream and

downstream part, the resistance being

calculated at the presumed oscillating

frequency of the current

VC1, VC2 phase to earth voltages at the moment

of switching, upstream and downstream

to the soft-starter

The turn on of a thyristor is represented by

the equivalent circuit in Fig. 8.

C1

RL I

VC1VL VR

C2 VC2

Figure 8 – Equivalent circuit for current transient analysis.

Equivalent circuit of the system during switching on



The following equation is derived from Fig. 8:

From (3) the following expression of the current

in the first moments after thyristor turn on is

obtained:

Obviously from (4) the current transient depends

both on upstream and downstream circuit.

However, as often in practical cases the upstream

cable is quite short, it is often said that the current

transient is only dependent on the downstream

cable. This is valid for the equivalent cable

inductance; however the upstream capacitance,

together with the one downstream will determine

the equivalent capacitance of the circuit and thus

the frequency of the current transient.

The current rate-of-rise is calculated within 1 μs

interval. Since the initial value of the current is zero,

the current rate-of-rise 1 μs after turn on is equal

to the value of current at 1 μs which is given in (5).

Equation (5) allows the current rate-of-rise to be

computed, once the power system is simplified

to the equivalent circuit of Fig. 8. Should any

protection inductance be incorporated into

the circuit, its value is to be added to the cable

inductance.

It should be noted that the current rate-of-rise is

proportional to the power system rated voltage.

The higher the rated voltage, the higher will be

the current rate of rise. For example, in a 6.6 kV

system it will be approximately twice as high as for

a 3.3 kV system. There are two important variables

that do not depend on the power system itself but

on the switching conditions. Those are the values

of the upstream and downstream voltages on

the capacitors. Their estimation requires

consideration of the transients on the voltages

during switching off and on of the thyristors.

Mathematical expression of the current rate-of-rise during switching on

(3)

(4)

(5)

R.I + L. + .∫I.dt + VC1 t=0+ .∫I.dt + VC2 t=0

= 0dIdt

1 C2

1C1

I(t) = .e -t/T .sin(w.t)(VC1 t=0

+ VC2 t=0 )w. L

T = 2LR

LCw = 4.R 2

12L√

C = C1.C2

C1 + C2

dI(t)dt = I(t)

t=1µs=

- VC1 t=0 + VC2 t=0)(w.L .e-10 -6/ T.sin(w.10 -6)



A measurement of the voltage transient during

switching off and on is given in Fig. 9

t(ms)60 61 62 63 64-7000-5250

-3500

-1750

1750

0

3500

5250

7000

(V)

DownstreamVoltage

: VP2F

: VaP2F

Figure 9 – Measured Voltages on the soft-starter outgoing terminals during on-off switching.

The analysis of these measured voltages results in

the use of another equivalent circuit representing

the power system between on-off switching.

Overview of the voltage transient during switching

Frequency of the voltage transient

The calculation of the transient voltage frequency

requires establishing the equivalent circuit of the

power system at the moment of switching off.

Given the measured frequency of the transient

voltage, 3-4 kHz, the following simplifying

assumptions are made:

1. The upstream circuit, including cable is

represented by the short-circuit impedance of

the power system, whose value is quite low

compared to the one of the capacitors to earth.

This assumption is valid when the transformer

neutral earthing is made with small impedance,

which is generally the case.

2. The cable impedance is negligible compared to

that of the motor

3. The short-circuit impedance is negligible

compared to that of the motor, thus

the upstream circuit is considered to be

directly earthed.

According to the above assumptions,

the following equivalent circuit for the voltage

transient may be drawn:

Lm Rm

CdownstreamCsnubber

Figure 10 – Equivalent circuit for the voltage transient during switching off.

The frequency of the voltage transient is therefore:

The calculated frequency is slightly higher than

the real one because of the assumptions made for

the short-circuit impedance of the upstream circuit.

This will be accounted for in the next point.

Fdownstream_voltage

= 1√2.. L

eq.C

eq

Leq

= Lm + Lm = Lm 12

32

Ceq

= Csnubber

+ Cdownstream

(4)



Consideration of the transient voltage value

The transient voltage is the difference between

the upstream and downstream phase-to-earth

voltage. Depending on the turn on instant,

the voltage across the thyristor will be different.

Each side of the soft-starter should be considered

individually. The upstream voltage value will be

determined by the moment of switching on, i.e.

the delay given by the control circuit, (α).

This value is easily estimated. The downstream

voltage is defined both from the commutation

moment and the transient evolution.

The instantaneous fundamental frequency value

of the downstream voltage is half the one on

the other side, as shown in appendix A.

Precise estimation of the voltage requires having

the complete power system parameters, especially

for the attenuation constant calculation.

This data is unavailable for many reasons.

Thus it is necessary to formulate a simplified

method for the downstream voltage estimation.

The next figure presents how its value may be

approximated, of course in excess of the real one:

(V,pu)

-0.5

-1.0

-2.0

t(s)

Vapproximated

Vreal

Figure 11 – Approximation of the downstream voltage evolution after turn off of the thyristor.

In other terms, and with respect to the commutation

angle and the time interval between switching off

and on, the voltage is approximated as:

t - period of downstreamvoltage oscillation

Current zero time

Vdownstream(pu of V(t))

0.51

2

< t/2 > 2.t

Figure 12 – Approximation of the downstream voltage value as function of current zero time.

The value of the upstream voltage being always

1pu of itself, the voltage across the thyristor is

therefore:

(6)Vth_turn_on t=α

= (1+Vdownstream

(α _ φ)).Vupstream t=α



Estimation of the risk of high current transient

The above equations and considerations allow the computation of the current rate-of-rise at each thyristor turn on. Yet this requires selection of particular moments when this calculation is critical for the estimation of the need of adding a protection reactor. First let’s have a look on the evolution of the turn on delay (α) and the current zero delay (φ) with respect to the voltage zero crossing:

(ms)

T(s)

init = f (Veffinit)

init

Seconddanger zone

Firstdanger zone

Full wavevoltage

End oframp time

Closing ofbypass contactor

Rated

Figure 13 – Time variation of the control delay and current phase shift during the motor start.

The critical instants during motor start are those where the current zero time is the smallest, in this case the voltage across the breaker would be at its potentially highest value. As it can be seen, there can be designated two critical zones:1. First danger zone – it is the moment of from

start to ramp end, where the control delay (α) will make a first brake, according to the requested current limitation, and slow its decrease, the current phase shift (φ) will continue decreasing with the acceleration of the motor

2. Second danger zone – the period after ramp end until going into full wave conduction. Current limitation means to maintain the current to the desired value. With the speed increase the current is getting smaller and the control angle has to be decreased. This increases in turn the current, much like a proportional control. The control angle decrease may lead to experience very high di/dt. Earlier it happens after the ramp end and higher is the current rate of rise.

Between these two danger zones, the second one is potentially more critical.

This is because of the certainty that during this period the current zero time will decrease, to values lower than half of the oscillation period (Fig. 12) and the voltage across the thyristor must be taken as 3 times the upstream voltage, (6). If this happens during the current limitation phase the control angle will be still relatively high, i.e. the voltage will be close to its’ maximum on turn on. Generally, in order to consider the worst case, the current rate of rise is to be calculated immediately after the ramp end.The estimation of the voltage constraint on the thyristor requires a preliminary estimation of the control delay and the current phase shift. The control delay can be derived from the required current limitation. First it is necessary to estimate the voltage rms value on the motor at ramp end:

Where:Imax requested current limitation, pu of rated

currentkd motor starting current, in pu of the rated

current

The estimation of the control delay for a certain rms voltage at ramp end is made by dedicated algorithm, explained in Appendix B. The exact value of the phase shift can not be estimated with sufficient precision. This would require a close look on the motor equivalent parameters and speed evolution during start, which in turn is very load dependent. That is why it was preferred to use the stalled rotor phase shift. Parametric analysis on the phase shift impact showed that a higher power factor leads to a slightly higher di/dt (5-6 % over). Generally this should not play a role since selected reactor values will always be higher than the exactly needed, because choice is generally made among fixed by manufacturer values. Taking the immediate greater value will normally add additional security margin, far beyond 5-6 %. Of course, depending on the available reactor values, it may be sometimes important to increase the di/dt values by 5-6 % before going to selection.With the above assumptions, the ratio between the current rates of rise Zone2/Zone1 will be either 150 % or 200 %.

(7)Vrmsramp_end

= ImaxK

d



The above equations and considerations were

implemented in a tool capable of calculating

the current rate of rise for the two designated

danger zones.

Its’ application will ensure optimized installation

conditions for the SCR application as well as its

service continuity.

Development of protection sizing tool

Required Input Data

The case study data was entered for the

estimation of the risk of current rate-of-rise and it’s

evolution with the protection reactance. The next

figure shows the results of calculation, sorted as:

- Zone 1, when only danger zone 1 is

accounted, danger zone 2 will not take place

because of propitious installation/setting/load

conditions

- Zone 2, (usual case), when only the danger

zone 2 is accounted, being more constraining

than danger zone 1

400

350

300

250

200

50

100

150

00 10 20 30 40 50 60 70 80 90

Zone 1

Current limit set to: 380 % In

Current rate of rise as functionof installed protection inductance

Thyristor limit

Zone 2

Protection inductance (µjjjH)

Current rate of rise (A/µjjjs)

Figure 14 – Results of calculation of current rate-of-rise.

Designation UnitsRated voltage kVRated frequency HzUpstream capacitance nFUpstream cable material resistivity ΩmUpstream cable length mUpstream cable relative insulation dielectric constant -Number of cables per phase -Downstream cable material ΩmDownstream cable length mDownstream cable insulation dielectric constant -Number of cables per phase -Motor rated power kWMotor rated power factor -Motor Starting power factor -Motor Efficiency %Motor Starting current x InRVSS Snubber capacitance nFCurrent limitation setting % of InThyristor current rate-of-rise limit A/μs

Results

Input data for the protection sizing tool.



The obtained results show that insertion of 100 μH

inductance leads to 50 A/μs current rate-of-rise for

danger zone 1, whereas the measured value was

about 40 A/μs. Comparing the required inductance

sizing for Zone 1 and on Zone 2, it may be seen

that the required inductance is more than 5 times

higher. This higher value will cost more because

installation requirements will differ.

Sometimes, in order to get rid of Zone 2 constraint,

an earlier by-pass closing may be convenient.

But this solution is not sure to be efficient.

Also earlier by-pass closing is not really a solution;

one may ask why we need a soft-starter if it is to

not use it completely?

An elegant solution in order to reduce current

rate of rise would be to control α in a way that

the current zero time remains higher than twice

the voltage transient period.

This will keep the current rate of rise at the smallest

possible value. This will require signal processing

of the upstream or downstream voltage oscillation

immediately after the current interruption in

the thyristor. The drawback of this solution is that

it will not be efficient when the current limitation

setting is very close to the minimum acceptable

value, under which the motor will simply not start.

In fact, reducing the current rate of rise requires

increasing of the current zero time intervals that

decrease the current rms value.

Of course, in some existing installations over sizing

the inductance may be the simplest and the most

convenient solution.

Discussion of the results



Conclusion

In this paper we presented in-depth analysis of

the mechanisms and estimation of current rate-of-

rise in MV soft-starter semiconductors.

Based on field measurements and reasonable

simplification assumptions a mathematical

definition of the current transient immediately

after the thyristors turn on was established.

It showed that the current transient issues

increases with the application rated voltage.

Furthermore, soft-starter control analysis allowed

defining two zones of major importance for correct

sizing of protection solutions: one immediately after

ramp end (beginning of current limitation phase)

and another starting shortly after, until closing

of the by-pass contactor.

The second one requires greater value of

the protection inductance. Also there were

proposed partial solutions for decreasing

the current rate of rise, to close earlier the by-pass

contactor or to control the turn on delay (α).

The most effective solution is sizing of

the protection inductance by considering

the second danger zone. All the considerations

and mathematical formulae were implemented

in a tool which was also presented and validated

by comparison with field test measurements.

This advancement will ensure that soft-starter

installation conditions are optimized and safe for

full SCR service continuity.



Acknowledgements

Authors would like to thank Mr. C. Durand for his

work on the simulation models, Mr. P.A. Claudel

and Mr. R. Catalan-Herrero for the measurements

they performed on site and Mr. R. Henri for his

benefic contribution to the final version of this paper.

Appendices

Zm

VnVa

Vb

Vc

Figure A-1 – Equivalent circuit after current interruption in one phase.

Calculation of downstream voltage

Vn = Va _ Ia.Zm = Vb _ Ib.Zm

Ia = _Ib

Ia =Va _ Vb 2.Zm

Vn = = Va + Vb 2

Vc 2

_

(A-1)

Calculation of control delay as function of motor rms voltage

Calculation of the control delay requires set up of

the limits of control angle variation.

Since the motor is isolated from earth, only control

delay < 2/3 of the fundamental half period is

possible, so that there always at least two

thyristors conducting. With this assumption

the voltage variation on one phase of the motor,

for 6kV power system is given on the next figure:

-6000

-4000

-2000

0

2000

4000

6000

: X0001A

: MOTA-N_MOT

(V)

t(ms)0 10 20 30 40

Phase toneutralvoltage

50

Fig. A-2 Time variation of the phase to neutral motor voltage compared to the fundamental phase to earth voltage upstream of the soft-starter.

Mathematically the rms value calculation is

expressed as:

++++++

++−++

=

∫∫∫∫

∫∫∫

+

+

+

+

−

−

−

−

2

6

26

6

262

6

26

6

26

0

2

32

21

0

32

21

2t

tt

TT

tt

tt

rms

)wt(sin)t.wsin(.)wtsin()wt(sin

)wt(sin)t.wsin(.)wtsin()wt(sin

tV

α

α

ϕ

ϕ

α

α

ϕ

ϕ

α

α

ϕ

ϕ

π

π

Where:

t: period of the fundamental signal.

A dedicated calculation algorithm iterates on α

until the required value of the rms voltage is

achieved.

(A-2)



(A)

-1500

1500

-1000

1000

-500

5000

0.05

: simu_bpoil_v5. pl4

: RVSSA -RVSSA_A

0.06 0.07 0.08 0.09 0.10 t(s)

Measured

Simulated

Fig. A-3 Comparison of line currents, simulated and measured.

500(A)

400300

200100

0-100-200

61

: simu_bpoil_v5. pl4

: RVSSA -RVSSA_A

61.5 62 62.5 63 63.5 t(ms)

Measured

Simulated

Fig. A-4 Zoom to Fig. A-3, thyristor turn on.

1060104010201000

1140112011001080

98064 65 66 67 68 69 70

(A)

t(ms)

Measured

Simulated

: simu_bpoil_v5.pl4

: RVSSA-RVSS_A

Fig. A-5 Zoom to Fig. A-3, current crest values.

Comparison of measured and simulated waveforms

-10-7.5

-5-2.5

57.510

02.5

(V)

t(s)0.05 0.06 0.07 0.08 0.09

Measured

: simu_bpoil_v5.pl4

: RVSSA-RVSS_A

Simulated

Fig. A-6 Comparison of phase voltages, simulated and measured.

(V)

t(ms)

Measured

-10-7.5

-5-2.5

57.510

02.5

59 60 61 62 63 64 65 66

: simu_bpoil_v5.pl4

: RVSSA-RVSS_A

Simulated

Fig. A-7 Zoom to Fig. A-6, during current zero in the thyristor.

Vita

Delcho Penkov was born in Haskovo, Bulgaria.

Hegraduated from Technical University of Sofia in

2002(MSC). In 2006 he received his PhD degree

in ElectricalEngineering from the Institut National

Polytechnique deGrenoble (INPG).

He is currently working for Schneider Electric as

Power Systems Engineer. Member of IEEE.

Alain Côte received his Electrical Engineering

degree from the National Polytechnic Institute

of Grenoble in 1983. He is currently working

on electrical network analyses such as stability,

harmonic and over voltage studies.

He has been personally involved in several

instances of expertise on equipment failure or

malfunctioning in different fields of industrial plants,

particularly about transient phenomena.

Schneider Electric Industries SAS35, rue Joseph MonierCS 30323F- 92506 Rueil Malmaison Cedex

RCS Nanterre 954 503 439Capital social 896 313 776www.schneider-electric.com

05-2010AMTED110112EN

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