Power Converter Characteristics
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
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 VEARTH is set to 0.5 V hence limiting the fault current to 5 mA (REARTH = 0.1kΩ),
- R1 is chosen with respect to the requested sensitivity of the system:
VNOM: Nominal value of the voltage,
k: voltage unbalance factor (0.05…0.1).
BOREAL Earth System simplified schematic .vsd