Power Converter Characteristics Design & Operation Responsibles Power Converter Architecture Power Part Control Part

Magnet Protection Power Converter Components Magnet Types Machine Installation Production Contract & Contact History

Power Converter Characteristics

Design & Operation Responsibles

1st Intervention	Piquet SY-EPC LHC
Responsibles:	Raul BIANCHI
	Yves THUREL
	CERN SY-EPC-LPC

Power Converter Architecture

This Power Converter is used in LHC Machine to power superconductive magnets. It is located in the LHC underground installation, close to the loads to limit cable losses in the underground installation.

Different parts were designed and produced separately, Power Converter being finally integrated in a housing rack, with 3 main parts:

• High Precision Current sensors: DCCTs, able to measure DC current at the required precision.
• Power Part: Power Rack and its removable Power Module
• A Digital Controller (FGC) using WorldFip bus in charge of:
  • The high level control from and to the Cern Control Room
  • The high precision digital current loop
  • Collecting and reporting all status, faults, and measurements from all the different parts to the remote services, for diagnostic and operation purposes.

Power Converter simplified Architecture .ppt

Power Part

Voltage Source is based on a variable switching frequency (50kHz..200kHz) half bridge resonnant topology (dual thyristor) followed by a 4 quadrant linear stage to allow the 4 quadrant operation. This topology gives a very low output EMC noise, at a cost of some control complexity given by the different stages.

A 19'' rack provides connectivity from AC network and to the load, housing up to 2 Voltage Source Power Modules.

Power Part simplified Architecture / Topology .vsd

3 different types exist A, B, C corresponding to.

• A-Type: Power Module is integrated in rack providing a DC Contactor between Power Module and Rack Crowbar.
• B-Type: Power Module is integrated in rack without any DC Contactor between Power Module and Rack Crowbar.
• C-Type: Power Module is integrated in rack providing a DC Contactor between Power Module and Rack Crowbar, and a external Crowbar placed in a Power Module slot being modified is additionnaly connected (Inner Triplet Case).

Typical Curves

Control Part

Control & regulation principles are summarized in a detailled schematics representating only the part involved in the output current regulation scheme.

Regulation Control simplified schematic .vsd

High precision current control loop is managed by the digital controller called FGC (Function Generator Controller). This unit includes a high precision Sigma Delta Analog to Digital Converter which digitalize the analog current measurement coming from 2 DCCTs (DC current Transducer). Precision is then directly relying on sensor precision: DCCT, the ADCs, and the algorithm being used for the regulation loop. Voltage source is then used as a power amplifier, powering the load through a high bandwidth voltage loop (>500Hz).

Magnet Protection

Power Converter is part of magnet protection scheme, even if not directly fully responsible of the monitoring and diagnostic of the superconductive magnet status. Dedicated systems QPS (Quench Protection System) + PIC (Power Interlock Controller) can interlock Power Converter if magnet safety requires it.

Power Converter is expected to:

• Always ensure that external protection system can stop the Power Converter.
  Power Converter provide a safe incoming signal called Fast Abort. This redundant signal uses 2 paths to interlock and stop the converter and its redundancy is checked each time it acts. It directly acts on AC and DC Contactor bobbin, ensuring their opening as required.
• Stop powering the load always providing a safe path for magnet current.
  Magnet current path is ensured through a dedicated system called crowbar when combined with an output DC contactor (Power Module in case its output stage dies in short would collect all magnet current without the DC Contactor). Crowbar active system is located in the rack and provides a safe resistive discharge path for magnet current, with a capability to dissipate, most of the time, part of magnet energy.
• Monitor Earth current of the total circuit and take the right action if threshold reached.
  Total circuit = converter + load (magnet and its DC cables).
• Monitor the voltage across the 2 current leads, and take the right action if threshold reached .

• Crowbar

  The system is based on a 50 mOhms Power Resistance series back-to-back thyristors being fired at a given output voltage (±13V), and then providing a safe path for magnet current, providing energy to handle is less than 108 kJ by design. Additional DC-Contactor (Converter type A) ensure that no potential short-circuits at the level of the Power Module can prevent the magnet energy to be actually dissipated in the Crowbar resistance. By the way these additional DC-contators prevent the converter while being in OFF-state to actually deviate some current during magnet discharge (converter output stage is not fully opened in OFF state). One should never forget a Power Module dying with its output stage in short would prevent the crowbar from dissipating magnet energy in the case no DC-Contactor is present.

  

  

  

  

  No Fault No fault    -    converter → OFF converter → OFF    -    Crowbar Fully Active Crowbar Fully Active

  A-Type (no REE installed in the circuit). Crowbar System simplified schematic .vsd

  

  

  

  

  No Fault No fault    -    converter → OFF converter → OFF    -    Crowbar Fully Active Crowbar Fully Active    -    REE Active (if required) REE Active (if required)

  B-Type (REE installed in the circuit). Crowbar System simplified schematic .vsd

• Fast Abort Interface

  Machine Interlock system can request a Fast Abort to the converter, in case a quench is detected. Converter is then assumed to react as soon and as quick as possible, stopping providing energy to the load. Delay time between a Fast Abort request and actual opening of the 4-quadrant output power stage is less than 1mS, but 20ms, AC Mains Contactor delay time, should be considered as a worst case (internal control malfunction case). A typical sequence could be described as follow:
  

  1. t=[=0ms] → Fast Abort Request from Machine Interlock
  2. t=[=1ms] → Power Converter Output Stage opens and becomes not conductive
  3. t=[>xms] → Load energy is transfered to the crowbar = a Capacitor up to V.crowbar = ±13V.
     Capacitor charge depends on initial load current (I=C.dV/dt). C=C.Crowbar//C.Power-Module
  4. t=[=xms] → Crowbar Thyrsitor is fired, absorbing Crowbar capacitor + Load energy.
     An initial over-current generally happens at initial thyristor start-up.
  5. t=[>end] → Load energy is dissipated in the crowbar (R series Thyristor ON voltage) and in the load resistance.

  This signal being part of the magnet safety scheme, it is acting redundantely at the level of Converter AC Mains Contactor. 2 paths are used and monitored to stop the contactor.
  A possible schematic is described below:

  

  Earthing System simplified schematic .vsd

• Earth System

  The circuit injects a 100 mA DC current on a grounded resistive branch, resulting in a common mode voltage at the output circuit easing the earthing fault detection. Output circuit Common mode voltage is, without any earth fault, around 10V (=100 mA x 100 Ohms), and is not relying on load operation, making possible to detect an earth fault even with converter being OFF. (OFF, not condamned).

  If an earth fault occurs on the output circuit, a faulty current will be deviated from the initial path back to earth by using the shunt 10 Ohms resistor path (monitored for detecting this fault).

  Overcurrent protection is achieved through a 1A-100V fast fuse in series in the path provided for the earthing fault current.

  Earth fault current is filtered at the level of the Power Module, and a strong earth fault will make the converter trip 20ms after it happens. See this real measurement curves .png.

  

  Earthing System simplified schematic .vsd

Power Converter Components .vsd

A power converter is actually a sum of different equipments under several different sections in the SY-EPC group. The modularity is a key factor for easier maintenance with regards to LHC tunnel access conditions.

Power Converter Rack can accept up to 2 Power Converters. Electronic Chassis, fan tray unit and AC-DC Power Module are shared in such a case.

• Power Converter Rack (up to 2 converters per rack)
  • Power Module
  • Rack items
    • 2x DC Contactors in // and their control card [AE40]
    • Current Sensor LEM [...]
    • Rack Earthing Card [AA5F]
    • Rack Crowbar and its control cards [AA5G + AA5E]
    • Rack Center Panel (Display + I.Limits) (back) with its card [AA5D]
    • Rack Flowmeter
  • FGC2 Fan tray unit
  • 2x Current sensors: DCCTs
    • DCCT Electronic
    • DCCT Head
  • Electronic Chassis (Ref: HCRFECA - Type 2) .edms
    • Digital Controller: FGC2
    • AC/DC Power Module
    • PSU FGC Tri-volt
    • PSU DCCT Bi-volt

Magnet Types

Machine Installation

Production Contract & Contact History

Developped	EEI-CIRTEM
	2001-2005
Manufactured	France-Italy
	EEI-CIRTEM
Production	444 Pc
CERN Contact	Yves THUREL

Converter Circuit Names