Simulation results for powering serial connected magnets Daniel Siemaszko, Serge Pittet 02.03.2010 OUTLINE : Serial configuration of full rated converters

Simulation results for powering serial connected magnets

Daniel Siemaszko, Serge Pittet02.03.2010

OUTLINE : Serial configuration of full rated converters against the use of active trimmers, principle of neighbourhood assistance in case of failure. Several investigated scenarios in the reconfiguration of the converters.

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Serial converters solution

• Serial connection of converters approach, IDEAL voltage sources with regulated output currents. (Converters ratings : 100A, 20-40V) (Static conditions).

• Advantage : Serious reduction in number of cables. Possibility of using smaller cable section if the difference current between two consecutive magnets is small.

• Questions : Validity of neighbourhood assistance of converters in case of failure?

• Simulation of the failure of fourth converter. Previous converter takes the lead, several scenarios are investigated. (Inductors ratings : 20mH, 150m)

02/03/2010 Daniel Siemaszko, Serge Pittet - EDMS 1060161 2

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No action (short circuit)

• When converter 4 fails, no action is taken, the converters is in short-circuit.• Neighbourhood assistance is impossible since half of the current flows in the cables.

Only half of the nominal current is reachable (depending on cable resistances!).• In case of the need for bypassing a magnet (zero current), a negative voltage must be

applied and the full nominal current will flow through the intermediate cables.


=~0.6s

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Removal of the cable

• When converter 4 fails, the connection to cable 4 is removed. Converter 3 takes the lead and feeds two consecutive magnets with same mean current. In this case, converter 5 could also give the needed assistance.

• Peak current reached in cables is around 1 A, thanks to regulators action. In order to achieve such dynamics, the converters must be twice rated in case of assistance.

• Question of mean current in two consecutive magnets.


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Adding a passive trimmer

• When converter fails, a tuneable resistive load (around 200) is connected in series to the cable 4.

• With accurate tuning the needed current can be achieved with the assistance of converter 3.

• Still, if converter 1 fails this solution is not able to produce the needed assistance.


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Using permanent resistances

• All cables have an added permanent resistance, unbalance in voltage ratings.• When converter 4 fails (short-circuit), current trimming in magnet 4 inadequate.• Value of resistance limited by voltage drop. Instabilities reached when it exceeds the

magnet voltage drop. Slow current recovery and bad voltage sharing.


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Power limitation in converters

• Serial connected converters solution needs the use of double rated converters in case of assistance.

• When the converter reaches its limitation in power, the assistance is not anymore possible and magnet current cannot reach its nominal value (ex. with 2.5kW limit).

• Same behaviour with and without adding a passive shunt.

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Bypassing a magnet sequence

• When a sequence of converters is shorted, the corresponding magnets are bypassed. Non zero current because of cable impedance.

• In order to achieve zero current in a sequence of magnets, the converter corresponding to the first bypassed magnet must compensate with negative voltage.

• In order to achieve zero current in one isolated magnet, the full negative voltage must be applied.

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Daniel Siemaszko, Serge Pittet - EDMS 1060161

Parallel active trimmers

• One big converter with parallel connection of active trimmers approach (Converters ratings : 100A/300V, trimmers ratings 0.1A/15-275V, assuming 16 magnets).

• Advantage : Serious reduction in number of converters. Common ground (higher precision in current measurement), same current ratings for the trimmers.

• Questions : Control, behaviour of the magnet currents in case of failure?

• Simulation of the failure of Converter 4. Open circuit scenario is investigated. (Inductors ratings : 20mH, 150m)


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Removal of the cable

• When converter 4 fails, the connection to cable 4 is removed. As in the neighbour assistance approach, the fourth magnet takes the same current as the previous one. Possible mean value to be reached.

• Oscillating effect due the disconnection of the inductive cable. Can be dumped through a capacitor.

• Again question of mean current in two consecutive magnets.


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Serial active trimmers

• One big converter with serial connection of active trimmers approach (Converter ratings : 100A/300V, trimmers ratings 0.1-1.6A/15-25V, assuming 16 magnets).

• Advantage : Same rated voltage in almost all trimmers.• Questions : Control, stability, behaviour of the magnet currents in case of failure?

• Simulation of the failure of Converter 4. Several scenarios are investigated. (Inductors ratings : 20mH, 150m)


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Serial trimmer, short case

• When converter 4 fails, no action is taken, the converters is in short-circuit.• Same as before, the cables see half of the nominal current.• Need for a high impedance in case of short-circuiting the converter.


=~0.6s

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Serial trimmer, open case

• When converter 4 fails it is brought to an open circuit. The magnet sees the nominal main converter nominal current.

• Peak current reached in cables depends on the position and might reach 1A.• Question of acceptable current in magnet in case of failure.


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Serial trimmer with diodes

• Trimming cables are implemented with serial diodes.• Current profile in the magnet sequence can only present a negative slope.• When converter fails in short-circuit, the diodes do not allow the current magnet to

fall lower than the following one. Open circuit case affects all previous magnets.• Few seconds are needed for stabilising the currents in the remaining magnets.


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Comment on main converters

• Reliability of solutions using trimmers highly depends on the reliability of the main converter

• (N+1) redundancy is achieved with serial connected voltage controlled converters or parallel connected current controlled converters.

• When one module fails, the remaining converters can go on operating.

• Number of module depends on current need for the magnets (drive beam case 100-10A).

• Failed converter must be bypassed in serial approach.• Failed converter must be opened in parallel approach.

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Comment on precision

Solution for powering 16 magnets main converter trimmer

Serial connected converters 100ppm (10mA) -

Parallel connected active trimmers 90ppm (9mA) 500ppm (1/16 mA)

Serial connected active trimmers 90ppm (9mA) 1% (1 mA)

• In serial connected converters, precision of current in magnets depends only on each converter precision. Precision of possible active/passive trimmer influences only the magnet corresponding to the faulty magnet.

• In serial connected active trimmers, precision of current in magnets depends on the big converter precision plus its corresponding trimmer. Use of 1% trimmers possible.

• In parallel connected active trimmers, precision of current in magnets depends on the big converter precision and the number of magnets in the sequence, since each trimmer takes a proportion of the main current.

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Comment on MTBF

• Assuming very reliable converters with MTBF of 3·105, the high number of converters in the main Linac (~45’000), there will be statistically one individual failure every 7 hours.

• With a very optimistic repair time of 4 hours, the whole complex will be down most of the time.

• With the principle of redundancy for main converters in powering groups, it is possible to consider hot swap of failed modules during operation or to stop the whole complex at some specified maintenance days once a month. With one failure every 5 years in one group of 6 modules, the down time of the complex would be theoretically zero.

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Conclusions

• Efficiency of the presented solutions still depends on beam physics requirements. • Reliability of additional passive/active shunts must be taken into account in overall

MTBF considerations.• No serious dynamic issues have been observed assuming static conditions.• The use of active trimmers instead of full rated converters is preferred for price and

volume aspects.• Price of the converters significantly depends on operation type, namely one/two

directions of voltage/current and required dynamics (di/dt in magnets operation).


Solution for powering 16 magnets main converter trimmer

Serial connected converters 16 x (100A/40V) Passive/active?

Parallel connected active trimmers 1 x (100A/300V) 15 x (0.1A/15-300V)

Serial connected active trimmers 1 x (100A/300V) 15 x (0.1-1.6A/20V)

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Documents

Simulation results for powering serial connected magnets Daniel Siemaszko, Serge Pittet 02.03.2010 OUTLINE : Serial configuration of full rated converters