Improve efficiently soft-starter transients' immunity - D

Improve efficiently soft-starter transients' immunity By 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

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.

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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.

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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.

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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.

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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 ( 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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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: Imax requested current limitation, pu of rated current kd 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 %.

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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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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.

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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.

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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.

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