Download Improve efficiently soft-starter transients` immunity

Improve efficiently soft-starter
transients' immunity
ByDelcho 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
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 | Improve efficiently soft-starter
transients' immunity
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.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Improve efficiently soft-starter
transients' immunity
Overwiew of SCR functionning
and electrical transient issues
Thyristor Functioning Principles
The SCR driven motor start is based on the use
the motor is equivalent to variable impedance and
of parallel thyristors connected in reverse parallel
decreases with the acceleration of the motor.
configuration to each other that are switched on
The command signal (represented by α ) is also
by a command signal. The command signal
varied during the motor start.
is intentionally delayed from the voltage zero-
Globally the behavior shown in Fig. 2 is observed:
crossing so that a smaller current is provided
(ms)
to the motor sufficient to start but lower than
init = f (Veffinit)
the rated starting current.
The overall behavior is described in the Fig. 1.
leff (% x Ir)
(V,I)
Vupstream
leffinit = Veffinit.kd
Rated current
Veff (%)
Rated voltage
t(s)
t(s)
Ramp time
Ithyristor
Figure 2 – B
asic evolution of main soft-starter depending
variables.
Figure 1 – Principle of controlled current in soft-starter.
tcurrent_zero = α - φ
t(s)
Full wave
voltage
Veffinit
Current zero time
t(s)
Current limit
(1)
Somewhere after passing in full-wave conduction
of the thyristors a parallel by-pass contactor is
The current through the thyristor stops naturally
closed and RVSS stops.
on zero crossing. The moment of zero crossing
The user dependent settings are:
depends on the phase shift between the current
1. initial voltage, in % of rated voltage
and voltage.
2. ramp time (time to go to the current limit), in s
This phase shift varies during the motor start as
3. current limitation, in % of rated motor current.
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
k(V)
12
energy. This is a standard issue, scheduled during
4
the development of the soft starter. Fig. 3 shows
0
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.
Upstream
voltage
8
-4
Voltage across
the thyristor
-8
-12
55
59
63
67
71
t(ms)
: VP2F-VaP2F
: VaP2F
Figure 3 – M
easurement of Voltage transient during
a complete fundamental period.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
The current rate-of-rise issue differs because
If it overpasses the thyristor limit (100-200 μA/s
it depends on the installation conditions of the
typically) the thyristor is damaged as well as the
soft-starter. Its solving is not generalized but case
softstarter itself. This is the subject of this paper.
dependent. An example of current turn on rate of
Fig. 5 shows a zoom on the current during this
rise on 6 kV power system is shown in Fig. 4.
transient:
(A)
(A)
1500
500
1000
400
500
300
0
200
Turn on current
transient
-500
-1000
-1500
55
100
0
59
63
67
71
t(ms)
: IP1F
-100
63
63.05
63.10
63.15
63.20
63.25
t(ms)
: IP1F
Figure 4 – Measurement of current through the thyristor
during a complete fundamental frequency period.
Figure 5 – C
urrent transient on switching on of the thyristor.
As it can be seen there is a high current transient
In this paper the authors will focus on the main
on turn on.
parameters this transient depends on.
Case study
The problem of high current rate-of-rise emerged
The power system is described hereafter:
on a 6 kV power system in an oil refinery based in
1. Rated voltage of 6 kV
Spain (Fig. 6).
2.30-40 m of 120 mm² cable upstream to
Grid
Power transformer
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.
6 kV bus
The motor has been started several times before
the soft-starter was damaged. Substantial
Upstream cable
Soft-starter
Downstream cable
Motor
Figure 6 – C
ase study power system.
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.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Modeling of the RVSS and case study
power system in EMTP-ATP
EMTP-ATP is a software package dedicated to
3.Cables were modeled in π, however in order to
modeling of power system transients.
avoid discharging of the immediate upstream
It allows analysis of high frequency phenomena,
of the RVSS cable capacitance into the
i.e. switching transients, and was chosen as most
downstream one, a different approach was
adequate to the aim of our modeling.
applied: the upstream and downstream cables
Moreover EMTP-ATP allows in depth modeling
have been considered as an equivalent cable
of the power system together with suitable RVSS
whose capacitances were placed on its ends.
model, critical, since the analysis aimed to see
The softstarter is placed along this single cable,
the impact of the surrounding power system
according to the real data. Its precise position
on the semiconductors.
does not have any impact on the current
Please note that the modeling was focused on
rate of rise since the cable capacitances are
an accurate simulation of the transients during
concentrated at the ends of this equivalent cable.
switching on. At first, we modeled the complete
power system using frequency-dependent cable
Thus the resulting power system model was
models and RVSS with its command circuit,
greatly simplified as shown in Fig. 7.
capable to represent the power system during
the complete motor start. The comparison with
real test measurements showed the very good
Zcc
Upstream
cable
Downstream
cable
Motor
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
Cupstream
Cdownstream
Figure 7 – Simplified power system model.
(2)
Cupstream = Csystem + Cprotection capacitors + Cupstream cable
Cdownstream = Cmotor + Cdownstream cable
showed that the current rate-of rise varies during
The results obtained with this simplified model
motor start, which requires simulation of
were still precise (<5 % error) with respect to
the complete process. With a timestep of 100 ns
the measured values of current rate-of-rise,
for an average of 15 s to be simulated,
oscillation frequency and attenuation.
the generated output files would have been
Example curves are shown in Appendix C.
difficult to handle.
However this simplified model, built with only linear
elements had another important advantage:
We proceeded to simplification of the used models:
it allowed clear analysis of the switching events.
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.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Analysis of the thyristor turn
on current transient
As it was stated before the simplification of
Thus in electrical terms this means that it may
the power system model led to the possibility
be summarized as a second order circuit
of using simple mathematics in order to derive
comprised of an inductor, capacitor and resistor,
the behavior of the current in the first instants
the latter providing the attenuation of the signal.
after turning on.According to the measurements
The estimation of their values will be given by
(see Fig. 5) the current transient may be
the mathematical analysis.
approximated to a second order step response.
Hypothesis
1.During switching on, due to the very high
2.During switching, again due to the very high
frequency of the current transient (50-200 kHz)
frequency, the motor is assumed as
the upstream power circuit, except the
a capacitor connected to earth, its equivalent
connecting cable, is equivalent to a capacitor
connected to earth. In fact, if this was not
RL circuit having a very high value.
3.During switching on, the fundamental (50 Hz)
the case, the current rate-of-rise will be
current increase is neglected with respect to
quite limited by the short-circuit equivalent
the first quarter of the oscillation of
inductance of the upstream circuit
the transient current
Equivalent circuit of the system during switching on
The following parameters are defined:
VC1, VC2phase to earth voltages at the moment
of switching, upstream and downstream
C1, C2equivalent capacitors of the power
system upstream and downstream of
to the soft-starter
the soft-starter, the value being the sum
of the upstream or motor equivalent
The turn on of a thyristor is represented by
capacitance and that of the upstream or
the equivalent circuit in Fig. 8.
downstream cable
L
R
R, Lequivalent parameters of the connecting
cable comprising both the upstream and
downstream part, the resistance being
VL
C1
VC1
I
VR
C2
VC2
calculated at the presumed oscillating
frequency of the current
Figure 8 – Equivalent circuit for current transient analysis.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Mathematical expression of the current rate-of-rise
during switching on
The following equation is derived from Fig. 8:
R.I + L.
The current rate-of-rise is calculated within 1 μs
interval. Since the initial value of the current is zero,
dI
1
1
+
.∫I.dt + VC1 + C2 .∫I.dt + VC2 = 0 (3)
dt C1
t=0
t=0
the current rate-of-rise 1 μs after turn on is equal
From (3) the following expression of the current
to the value of current at 1 μs which is given in (5).
in the first moments after thyristor turn on is
- ( VC1 t=0 + VC2 t=0) -10 -6/ T
dI(t)
= I(t)
=
.e
.sin(w.10 -6) (5)
w.L
dt
t=1µs
obtained:
I(t) =
T=
(VC1 t=0+ VC2 t=0 )
w. L
Equation (5) allows the current rate-of-rise to be
.e -t/T .sin(w.t)
2L
R
computed, once the power system is simplified
(4)
to the equivalent circuit of Fig. 8. Should any
protection inductance be incorporated into
1
L
w=
R 2 4.
2L√
C
the circuit, its value is to be added to the cable
C1.C2
C=
C1 + C2
It should be noted that the current rate-of-rise is
Obviously from (4) the current transient depends
The higher the rated voltage, the higher will be
both on upstream and downstream circuit.
the current rate of rise. For example, in a 6.6 kV
However, as often in practical cases the upstream
system it will be approximately twice as high as for
cable is quite short, it is often said that the current
a 3.3 kV system. There are two important variables
transient is only dependent on the downstream
that do not depend on the power system itself but
cable. This is valid for the equivalent cable
on the switching conditions. Those are the values
inductance; however the upstream capacitance,
of the upstream and downstream voltages on
together with the one downstream will determine
the capacitors. Their estimation requires
the equivalent capacitance of the circuit and thus
consideration of the transients on the voltages
the frequency of the current transient.
during switching off and on of the thyristors.
inductance.
proportional to the power system rated voltage.
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Overview of the voltage transient
during switching
A measurement of the voltage transient during
The analysis of these measured voltages results in
switching off and on is given in Fig. 9
the use of another equivalent circuit representing
the power system between on-off switching.
(V)
7000
5250
Downstream
Voltage
3500
1750
0
-1750
-3500
-5250
-7000
60
61
62
63
64
t(ms)
: VP2F
: VaP2F
Figure 9 – Measured Voltages on the soft-starter outgoing
terminals during on-off switching.
Frequency of the voltage transient
The calculation of the transient voltage frequency
Lm
requires establishing the equivalent circuit of the
Rm
power system at the moment of switching off.
Given the measured frequency of the transient
voltage, 3-4 kHz, the following simplifying
Csnubber
assumptions are made:
Cdownstream
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,
Figure 10 – E
quivalent circuit for the voltage transient
during switching off.
The frequency of the voltage transient is therefore:
Fdownstream_voltage=
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,
Leq= Lm +
1
2.�.√ Leq.Ceq
3
1
Lm = Lm
2
2
(4)
Ceq= Csnubber + Cdownstream
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.
the following equivalent circuit for the voltage
transient may be drawn:
COM-POWER-WP--EN Rev1 | Improve efficiently soft-starter
transients' immunity
Consideration of the transient voltage value
The transient voltage is the difference between
In other terms, and with respect to the commutation
the upstream and downstream phase-to-earth
angle and the time interval between switching off
voltage. Depending on the turn on instant,
and on, the voltage is approximated as:
the voltage across the thyristor will be different.
Each side of the soft-starter should be considered
individually. The upstream voltage value will be
Vdownstream
(pu of V(t))
2
determined by the moment of switching on, i.e.
t - period of downstream
voltage oscillation
1
the delay given by the control circuit, (α).
This value is easily estimated. The downstream
0.5
voltage is defined both from the commutation
< t/2
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.
Current zero time
Figure 12 – A
pproximation 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:
Vth_turn_on
This data is unavailable for many reasons.
> 2.t
t=α
= (1+Vdownstream(α _ φ)).Vupstream
t=α
(6)
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)
Vreal
t(s)
-0.5
-1.0
-2.0
Vapproximated
Figure 11 – Approximation of the downstream voltage
evolution after turn off of the thyristor.
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transients' immunity
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)
init = f (Veffinit)
init
Rated
T(s)
First
danger zone
Second
danger zone
End of
ramp time
Closing of
bypass contactor
Full wave
voltage
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:
Vrmsramp_end =
Imax
Kd
(7)
Where:
Imaxrequested current limitation, pu of rated
current
kdmotor 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 %.
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Development of protection sizing tool
The above equations and considerations were
Its’ application will ensure optimized installation
implemented in a tool capable of calculating
conditions for the SCR application as well as its
the current rate of rise for the two designated
service continuity.
danger zones.
Required Input Data
Input data for the protection sizing tool.
Designation
Rated voltage
Rated frequency
Upstream capacitance
Upstream cable material resistivity
Upstream cable length
Upstream cable relative insulation dielectric constant
Number of cables per phase
Downstream cable material
Downstream cable length
Downstream cable insulation dielectric constant
Number of cables per phase
Motor rated power
Motor rated power factor
Motor Starting power factor
Motor Efficiency
Motor Starting current
RVSS Snubber capacitance
Current limitation setting
Thyristor current rate-of-rise limit
Units
kV
Hz
nF
Ωm
m
Ωm
m
kW
%
x In
nF
% of In
A/μs
Results
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
Current rate of rise (A/µjjs)
300
because of propitious installation/setting/load
250
conditions
200
zone 2 is accounted, being more constraining
than danger zone 1
Zone 1
Zone 2
350
accounted, danger zone 2 will not take place
- Zone 2, (usual case), when only the danger
Current rate of rise as function
of installed protection inductance
400
Current limit set to: 380 % In
Thyristor limit
150
100
50
0
0
10
20
30
40
50
60 70 80 90
Protection inductance (µjjH)
Figure 14 – R
esults of calculation of current rate-of-rise.
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transients' immunity
Discussion of the results
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.
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Conclusion
In this paper we presented in-depth analysis of
the mechanisms and estimation of current rate-ofrise 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.
COM-POWER-WP--EN Rev1 | 14
Improve efficiently soft-starter
transients' immunity
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
Calculation of downstream voltage
Vn = Va _ Ia.Zm = Vb _ Ib.Zm
Zm
Va
Vn
Vb
Vc
Figure A-1 – Equivalent circuit after current interruption
in one phase.
Ia = _Ib
_
Ia = Va Vb
2.Zm
(A-1)
_
Vn = Va + Vb = Vc
2
2
Calculation of control delay as function of motor rms voltage
Calculation of the control delay requires set up of
Mathematically the rms value calculation is
the limits of control angle variation.
expressed as:
Since the motor is isolated from earth, only control
ϕ−
delay < 2/3 of the fundamental half period is
possible, so that there always at least two
t
V rms =
2
t
thyristors conducting. With this assumption
0
α
ϕ−
ϕ+
t
6
t
6
sin( wt ) +
T
ϕ
α
1
2
. sin( w.t − π )
2
3
6
α+
+ ∫ 0 + ∫ sin 2 ( wt ) + ∫
the voltage variation on one phase of the motor,
for 6kV power system is given on the next figure:
α−
6
2
∫ sin ( wt ) + ∫
ϕ+
2
ϕ
+ ∫ sin 2 ( wt ) +
α−
6
t
sin( wt ) +
(A-2)
T
6
t
1
2
. sin( w.t + π )
2
3
6
t
2
2
+ ∫ sin 2 ( wt )
α+
t
6
Where:
t: period of the fundamental signal.
(V)
6000
A dedicated calculation algorithm iterates on α
Phase to
neutral
voltage
4000
2000
until the required value of the rms voltage is
achieved.
0
-2000
-4000
-6000
0
10
20
30
40
50 t(ms)
: X0001A
: MOTA-N_MOT
Fig. A-2 T
ime variation of the phase to neutral motor voltage
compared to the fundamental phase to earth voltage
upstream of the soft-starter.
COM-POWER-WP--EN Rev1 | 15
Improve efficiently soft-starter
transients' immunity
Comparison of measured and simulated waveforms
(A)
(V)
1500
Measured
1000
500
0
-500
-1000
-1500
0.05
Simulated
0.06
0.07
0.08
0.09
0.10 t(s)
: simu_bpoil_v5. pl4
Fig. A-3 Comparison of line currents, simulated and measured.
(A)
500
200
Measured
100
0
-100
-200
61
Simulated
61.5
62
Simulated
Measured
0.06
0.07
0.08
0.09
t(s)
: simu_bpoil_v5.pl4
: RVSSA-RVSS_A
: RVSSA -RVSSA_A
400
300
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
0.05
62.5
63
63.5
t(ms)
Fig. A-6 Comparison of phase voltages, simulated and
measured.
(V)
10
7.5 Measured
5
2.5
0
-2.5
-5 Simulated
-7.5
-10
59
60
61
62
63
64
65
66
t(ms)
: simu_bpoil_v5.pl4
: RVSSA-RVSS_A
: simu_bpoil_v5. pl4
: RVSSA -RVSSA_A
Fig. A-7 Zoom to Fig. A-6, during current zero in the thyristor.
Fig. A-4 Zoom to Fig. A-3, thyristor turn on.
(A)
1140
1120
1100
1080
1060
1040
1020
1000
980
64
Measured
Simulated
65
66
67
68
69
70
t(ms)
: simu_bpoil_v5.pl4
: RVSSA-RVSS_A
Fig. A-5 Zoom to Fig. A-3, current crest values.
Vita
Delcho Penkov was born in Haskovo, Bulgaria.
Alain Côte received his Electrical Engineering
Hegraduated from Technical University of Sofia in
degree from the National Polytechnic Institute
2002(MSC). In 2006 he received his PhD degree
of Grenoble in 1983. He is currently working
in ElectricalEngineering from the Institut National
on electrical network analyses such as stability,
Polytechnique deGrenoble (INPG).
harmonic and over voltage studies.
He is currently working for Schneider Electric as
He has been personally involved in several
Power Systems Engineer. Member of IEEE.
instances of expertise on equipment failure or
malfunctioning in different fields of industrial plants,
particularly about transient phenomena.
COM-POWER-WP--EN Rev1 | 16
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