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 full bridge ZVS-ZCS switching mode topology (f.switching = 25 kHz) followed by a 4 quadrant linear stage to allow the 4 quadrant operation. This soft switching topology gives a very low output EMC noise, with also a high reduction of low frequency voltage output ripple (rejected by two cascades loops) at a cost of some control complexity given by controlling the two different stages.

A 19'' rack provides connectivity from AC network and to the load, housing one Voltage Source (2x Power Sub-Modules).

Power Part simplified Architecture / Topology .vsd

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.

LHC600A-40V Power Converter is also responsible of the current leads protection. These current leads are the 2 (minus and plus polarities) electrical conductors located at the transition between cold and warm environment.

Power Converter is then 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.
• Stop powering the load always providing a safe path for magnet current.
  Magnet current path is ensured through a dedicated system called crowbar or through Power Module in case its output stage dies in short. 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 (±53V), and then providing a safe path for magnet current. 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.

  

  Crowbar System simplified schematic .ppt

• 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=to=[0ms]    → Fast Abort Request from Machine Interlock
  2) t=[1ms]         → Power Converter Output Stage opens and becomes not conductive
  3) t=[1ms..xms] → Load energy is transfered to the crowbar = a Capacitor up to V_crowbar = ±53V
                                 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=[xms..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

  Detection system is an active / passive system, since relying on a converter state-controlled 100mA current source. Active system is disabled when Power Converter is set ON, S1 being ON and short-circuiting the 100 Ohms resistor, with a 10 Ohms resistance series a fuse only being connected between negative polarity and earth.

  Active system is enabled when Power Converter is set OFF. The active 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 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).

  Active system is disabled when Power Converter is set ON, S1 being ON and short-circuiting the 100 Ohms resistor. Detection system relies on a 10 Ohms monitored resistance series a fuse only being connected between negative polarity and earth. Passive system detection condition is provided through the level of current, the cable resistance value, and the location of the earth fault.

  Overcurrent protection is achieved , in both active / passive cases, through a 1A-100V fast fuse in series in the path provided for the earthing fault current.

  

  Earthing System simplified schematic .vsd

• Current Leads Protection

  These current leads are protected monitoring the voltage across them. Per design these current leads must not developp more than 150mV at full current. Such a case would mean their thermal stabilization is not correct.

  Design is simply facing some common mode voltage coming from Earthing system added from the differential voltage, when trying to measure a small voltage of some mV across long distances.

  

  Current Leads Protection 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
  • Sub-Module 1 (Input + Inverter + Pwr Elec. Ctrl)
  • Sub-Module 2 (Transfo + Rect. +4QLS)
  • FGC2 Fan tray unit
  • 2x Current sensors: DCCTs
    • DCCT Electronic
    • DCCT Head
  • Electronic Chassis (Ref: HCRFEDA - Type 3) .edms
    • Digital Controller: FGC2
    • AC/DC Power Module
    • PSU FGC Tri-volt
    • PSU DCCT Bi-volt

Magnet Types

Machine Installation .xlsm

Production Contract & Contact History

Developped	Transtechnik
	2017: 04 units | LIU Booster
	2016: 06 units | SFRS & HL-LHC
	2004: 50 units | LHC
Manufactured	Germany
	Transtechnik
	PowerTech
Production	2004-2017: 57 racks + 61 Power Modules Kits
CERN Contact	Yves THUREL

Converter Circuit Names