NuMI Horn Production Power Supply Safety Analysis Report

Ken Bourkland

July 18, 2001

**Purpose:** A power supply system has been built for the purpose of
pulsing the NuMI focusing horns. This system will first be used in MI-8
for the continued testing of the prototype NuMI horns and, later, for
operation in the NUMI beamline . Various field, temperature, vibration,
and life tests will be carried out on the horns using this equipment.
The charging source and capacitor bank, working together, can produce
the pulse train as shown in Figure 1. It is capable of producing pulse
widths of 2.6 ms or 5.2 ms and designed to operate continuously at a
rep-rate of 1.9 seconds.

In the MI-8 test area, a dummy load is included in series with the horn
to represent the electrical equivalent of the additional resistance and
inductance of the full length stripline and second horn that are a part
of the beamline installation.

![](media/image1.wmf)

Figure 1. Output waveform from capacitor bank

**Equipment:** The circuit contains a capacitor bank that consists of
120 units of 7.5 mF, each rated at 670 working-volts. For safety, the
capacitor bank is divided into twelve independent sections. The
capacitors within each section can be connected in parallel for 75 mF,
or in series-parallel for18.8 mF, to achieve the two different output
pulse widths. Each of the twelve capacitor sections is isolated from the
other sections and the charging source by diodes, and from each other
and the load by SCRs. Three sections of capacitors, working together
with their respective SCRs, drive one of the conductor pair of the
output stripline. The other three stripline conductor pairs are driven
in the same manner by their respective quadrants of the capacitor bank.

The capacitor bank output is connected to the dummy load and horn via an
eight layer, four conductor pair, stripline. The dummy load is connected
into the stripline, electrically in series between the capacitor bank
and horn, to provide the equivalent resistance and inductance the system
will encounter in beamline operation in order to achieve the proper
pulse width and expected system loses. The capacitor bank is charged by
two 240kW PEI power supplies connected in series to provide up to 1,200
volts. The PEIs are standard units but have been up-dated to include
later model regulation electronics, new water hoses, and provisions for
a lock on the rear doors. A schematic of the equipment is shown in
Figure 2.

The capacitor bank is constructed within a heavy duty enclosure equipped
with multiple doors, all of which are secured with latches and key
locks. The equipment is installed in the south end of the MI-8 service
building. A floor plan of the equipment configuration is shown in Figure
3.

**Hazards:** Certain hazards inherent with high voltage, high current,
and capacitive energy storage exist within this circuit. A list of each
hazard and the mitigation steps for that hazard is as follows.

[Voltage:]{.underline} The PEI power supplies used to charge the
capacitor bank are connected in series, one configured for 800 volts
output and the second for 400 volts output. This configuration can
provide from 0-1,200 volts at 300 Amps and will be operated to
approximately 950 volts during horn testing. The PEI enclosures are
fitted with door interlock switches on their respective rear doors. This
interlock will turn off the supplies if either set of doors is opened.
The rear doors will be locked during operation.

The PEI front doors are locked by the front panel disconnect switch
mechanism when the disconnect is in the ON position. All internal bus
associated with either 480 Vac or the DC output in each PEI is covered
with protective insulated shields to prevent personnel exposure to
energy that could originate from the second series connected PEI power
supply. There will be no need to enter either PEI during start-up or
shut-done of horn operations in MI-8.

The PEIs are isolated from the capacitor banks by diodes to prevent any
energy stored in the capacitors from back-feeding into the PEI should
there be a fault internal to the PEI.

The isolation diodes are rated at 2,500V~PIV~.

All of the doors of the capacitor bank enclosure are fitted with
interlock switches. These switches are connected into the PEI interlock
chain to turn off the charging supply upon entry. All of the capacitor
bank doors (24) are secured by Best® cylinder locks, all keyed alike and
operated by a single key. This same key also operates the gates of the
fenced-in area around the horn load and the locks securing the back
doors of the PEIs. The key will be captured by the MI-8 control system
and must be in place to permit operation.

The dummy load, mounted above the capacitor bank, rises to the capacitor
bank voltage when the horn is pulsed. It is enclosed by fencing to
prevent personnel access.

[Current:]{.underline} Each capacitor bank section is designed to
provide up to 17 kA~PK~ to its respective SCR. The capacitor row buses
have a tab for each respective capacitor bushing. The tabs are
constructed of 1/16" x 1-1/2" copper, 0.094 in^2^ cross-sectional

![](media/image2.wmf)

![](media/image3.wmf)

Figure 3. Layout of equipment in the MI-8 service building.

area, and operate up to a maximum of 65 A~RMS~. The main bus material in
each capacitor row is 1/4\" x 4" copper, 1 in^2^, and will operate up to
a maximum of 605 A~RMS~. The combined output of the three SCRs on each
quadrant switch panel sums three capacitor sections to produce 51
kA~PK~, 1,815 A~RMS~. The bus material in each SCR switch panel, and the
center bay stripline of the enclosure, is 1/4\" x 12" copper, providing
3 in^2^ cross-sectional area.

The stripline leading from the center bay of the enclosure, and external
to the enclosure, is constructed of 3/8\" x 12\" 6101-T61 aluminum
electrical bus alloy. Both materials can operate at 7250 A~RMS~
continuously.

Any access to high current connections requires LOTO procedures to be
followed so that no hazard exists.

[Stored energy:]{.underline} The energy stored in each capacitor bank
section is 11.4 kJ. Each diode providing capacitor section isolation
from the PEIs is rated for 2.5 kV~PIV~ and 410 A~IF(AV)~. The Eupec SCRs
switching the output are rated for 2 kV V~DRM~ and V~RRM~. The NWL
capacitors are rated for a Hi-pot voltage of 1,340 volts
terminal-to-terminal, 2 kV terminal-to-case, and a working voltage of
670 volts. The case construction withstand specification was for 25 kJ
containment. Case material is 14 Ga. (0.075") steel.

Two high voltage, high current, Ross® shorting relays, with independent
discharge resistors, are series connected across each of the capacitor
sections for discharging the capacitors upon turn-off of the charging
supplies. The relays close the resistors across the capacitor banks
while in the de-energized state. The dual shorting scheme provides
redundancy for additional electrical safety. The shorting relays are
mounted such that gravity assist is accommodated for fail-safe
operation, eliminating dependence upon springs for contact closure.
Auxiliary contacts on the shorting relays are connected into the control
interlock chain to withhold the "Enable" signal to the PEIs unless both
shorting relays are energized.

Two resistive ground sticks are provided to discharge the twelve
capacitor groups, one mounted on each side of the enclosure. Two
hard-ground sticks are also provided, one on each side of the enclosure.
Anchored grounding clip leads are provided to maintain capacitor
grounding as needed.

[Capacitor over pressure:]{.underline} The NWL capacitors are of the
segmented metalized foil design. The typical failure mode for this type
construction is voltage punch through followed by self clearing of the
metalization. This process, should it continue within a capacitor, will
result in gradual loss of capacitance and build-up of pressure within
the capacitor case. All of the capacitors are fitted with normally
closed pressure switches. These switches, all connected in series, are a
part of the controls interlock chain and will stop power supply
operation should an over pressure condition occur.

[Induced voltage on Equipment During Faults to Ground]{.underline}:

The low side of the capacitor bank (+) and the enclosure are bonded
together to form a single point ground within the enclosure. This common
point is in turn connected to the external steel stripline support
structure via two 900 mcm cables to provide a current return path to the
capacitor bank in the event a fault should occur either at the horn or
stripline. Ground fault current detection is provided by a LEM current
monitor on these cables.

The PEI enclosures, the controls rack, and the stripline support
structure steel are all bonded to the MI-8 building ground system grid
via 500 mcm bare copper conductor.

[Acoustic:]{.underline} It is not anticipated that the sound levels will
be high enough to be hazardous to building occupants but sound levels
will be measured if so indicated.

In the past, horn operations have been limited to nights and weekends to
spare personnel the annoyance of day shift operations.

[Hazardous Materials:]{.underline} All of the NWL energy storage
capacitors are of identical construction and are impregnated with
Calchem® C-102, manufactured by Calgene Chemical, Inc. A MSDS sheet is
on hand for this product. The product description states this material
is rape seed oil (Canola oil) and does not contain any PCBs. The
capacitors, while mounted on their sides to facilitate removal and
replacement, are mounted with their pressure switch toward the top of
the case to minimize leakage volume should a leak occur at the switch
location. The enclosure design in the capacitor bank bays is such that
it has an oil containment capacity of \~14 gallons. Each capacitor
contains \~2.5 gallons of oil.

The SCR snubber capacitors are manufactured by GE and are marked "Non
PCB." The impregnant, presumably oil, is unknown.

No other materials known to be hazardous exist within the equipment.

**LOTO:** Due to there being two sources of energy for the PEIs, and the
potential for stored energy in the capacitors, a written LOTO procedure
will be necessary. The LOTO procedure describes the steps to safely lock
out energy sources and discharge stored energy.

480 V~AC~ power for the PEIs is supplied via a dedicated circuit breaker
in power panel

DHP-MI8-1, breaker #17. It is located in the southeast corner of MI-8.

The circuit breaker for the 120 V~AC~ control power in the PEIs is
located in power panel PP-MI8-1A-5-A1, breaker #13.

The circuit breaker for the 120 V~AC~ power for the controls rack is
located in power panel PP-MI8-1A-5-A1, Circuit Breaker #15. All 120
V~AC~ power in the rack is installed according to NEC.

The circuit breaker for the 120 V~AC~ power for the capacitor bank
enclosure is located in power panel PP-MI8-1A-5-A1, Circuit Breaker #17.
All 120 V~AC~ power in the capacitor bank enclosure is guarded by
utility boxes and guards over terminal strips.

All breakers are fitted with a locking device for LOTO locks.

**Warning Device:** A flashing red warning light is installed atop the
capacitor bank enclosure, on the end nearest the horn load. It will
operate at any and all times either of the PEI charging supplies is in
the \"DC ON\" mode.