Power Converter Characteristics

Project Team

Xavier Bonnin - Emilien Coulot - Christophe Mutin - Stephane Reignier

Power Converter Topology

For the BOREAL brick, a 18kV/50 Hz transformer (galvanic isolation – voltage adaptation – phase shift), a diode rectifier, and a DC-Link filter are used for the AC/DC conversion. An IGBT bridge (4 quadrant operation) with an output filter are used for DC/DC conversion.

The energy of the magnet is:

• Recovered in a capacitors bank on the DC-Link during cycling mode of operation,
• Dissipated in a brake chopper in case of too small energy storage on the DC-Link,
• Dissipated In a bipolar crowbar in case of fault,

One module BOREAL:

                                                         BOREAL topology .vsd
 

Two modules BOREAL are assembled in parallel to increase the current capability (BOREAL_2P):

                                                     BOREAL_2P topology .vsd
 

Two modules BOREAL are assembled in series to increase the voltage capability (BOREAL_2S):

                                                    BOREAL_2S topology .vsd
 

Four modules BOREAL are assembled in parallel with two other BOREAL modules assembled in series (BOREAL_2P_2S):

                                                  BOREAL_2P_2S topology .vsd
 

Four modules BOREAL are assembled in parallel to increase the voltage capability (BOREAL_4P):

                                                BOREAL_4P topology .vsd
 

Magnet Protection

Power converter is part of magnet protection scheme, even if not directly fully responsible of the monitoring and diagnostic of the magnet status. PIC/WIC (Power/Warm Interlock Controller) can interlock power converter if magnet safety requires it.

Power converter is then expected to:

• Always ensure that external protection system can stop the power converter through a safe signal called Fast Abort.
• Stop powering the load in safe way (handling the magnet energy even when stopping, through dedicated system called Crowbar). This active system provides a safe resistive discharge path for magnet current (energy).
• Monitor earth current of the total circuit: converter + load (magnet and its DC cables), and take the right action if threshold reached.

► Active Crowbar

The system is based on axx Ω power resistor connected to two thyristors which are triggered when there is a faulty condition on the converter or when the voltage Vmeas goes beyond a predefined threshold (BOD protection).

 BOREAL Crowbar system simplified schematic.vsd

Earth fault detection ►

The detection system is a passive solution which is independent of the configuration of the converter (serial or parallel). It’s composed of a voltage divider connected between positive polarity, the negative polarity and the earth:

• The voltage threshold V_EARTH is set to 0.5 V hence limiting the fault current to 5 mA (R_EARTH = 0.1kΩ),
• R₁ is chosen with respect to the requested sensitivity of the system:

V_NOM:   Nominal value of the voltage,
k: voltage unbalance factor (0.05…0.1).

                      BOREAL Earth System simplified schematic .vsd