a Physics Laboratory, P.O. Box 451
Princeton, New Jersey 08543
ABSTRACT
The National Spherical Torus Experiment (NSTX) has been
designed and installed in the existing facilities at Princeton
Plasma Physic Laboratory (PPPL). Most of the hardware, plant
facilities, auxiliary sub-systems, and power systems originally
used for the Tokamak Fusion Test Reactor (TFTR) have been used
with suitable modifications to reflect NSTX needs. The design of
the NSTX electrical power system was tailored to suit the available
infrastructure and electrical equipment on site. Components were
analyzed to verify their suitability for use in NSTX. The total
number of circuits and the location of the NSTX device drove the
major changes in the Power system hardware. The NSTX has
eleven (11) circuits to be fed as compared to the basic three power
loops for TFTR. This required changes in cabling to insure that
each cable tray system has the positive and negative leg of cables
in the same tray. Also additional power cabling had to be
installed to the new location. The hardware had to be modified t o
address the need for eleven power loops. Power converters had t o
be reconnected and controlled in anti-parallel mode for the Ohmic
heating and two of the Poloidal Field circuits. The circuit for the
Coaxial Helicity Injection (CHI) System had to be carefully
developed to meet this special application. Additional Protection
devices were designed and installed for the magnet coils and the
CHI. The thrust was to making the changes in the most cost-
effective manner without compromising technical requirements.
This paper describes the changes and addition to the Electrical
Power System components for the NSTX magnet systems.


INTRODUCTION
The existing power system, originally used for TFTR
magnets, has been reconfigured and modified for feeding the
NSTX coil systems. The AC power to the facility remains
unchanged and is comprised of the substation fed at 138 kV by
Public Service Gas & Electric Co. (PSE&G). Two 138 kV/
13.8 kV transformers are fed from the transmission line and
feed power to two 13.8 kV 60 Hz switchgears, located in D-
Site. One of these switchgears supplies the motors of two
Motor Generator (MG) units and associated auxiliaries. One of
the two MG units each rated at 475 MVA, 60 to 90 Hz., 13.8
kV, is used to supply the NSTX coil system via two variable
frequency buses.
Some of the TFTR thyristor power supplies are used for the
NSTX coil system. The main power supply systems for
NSTX are for the Toroidal Field (TF), Ohmic Heating Field
(OH), and Poloidal Field (PF, eight individual circuits), and
the Coaxial Helicity Injection (CHI) circuits. Table 1 gives
the details of the NSTX coil system circuits along with their
pulse rating.
POWER SYSTEM DESCRIPTION
A. AC System
The existing AC system at D-Site (See Figure 1) is used
without any change for NSTX. The electrical utility service
to the PPPL site is a 138 kV transmission line thermally rated
at 600 Amps steady state (equivalent to about 140MVA).
Table 1
NSTX Circuit. Types, Poles, Power Supply Sections, & Ratings

(Type I - 2 wire unidirectional; Type II - 3 wire unidirectional;
Type III - 2 wire bi-directional; Type IV - 3 wire bi-directional).
Circuit
Type
DC
Poles
Series/
Parallel
PSS
Volts
(kV)
Peak
Amps
(kA)
ESW@
Peak Amps
(sec)
TF
I
2
1/4
1
72
0.6
OH
III
2
6/2
6
+/-24
0.525
PF1a Upper
II
3
2/1
2
15
5
PF1a Lower
2/1
2
15
5
PF1b
I
2
2/1
2
20
1
PF2 Upper
II
3
2/1
2
20
5
PF2 Lower
2/1
2
20
5
PF3 Upper
IV
3
2/1
2
20
5
PF3 Lower
2/1
2
20
5
PF4/5
I
2
3/1
3
20
5
CHI
I
2
2/2
2
50
0.8
Present agreements with the local utility, (PSE&G), permit
PPPL to take a 120 MVA dynamic load without phase
correction. Power from the local grid is received at 138 kV
and two transformers of 30/40/50 MVA each, step down the
power to 13.8 kV and feeds the two fixed frequency buses S1
& S2 located at D-Site. The bus S1 feeds power to each of the
two motors and generator exciters, of the MG units. Each
generator is rated 475 MVA, 13.8 kV, 60 to 90 Hz. The
Generator output is fed to the coil system, via the Variable
Frequency buses SV1 & SV2.
Utility Feed
stepped down
138/13.8 kV
BUS S2
13.8kV BUS SV1 3000A (60-90 Hz.)
ACCLR
G1-
CLR
3.5 %
2500
MVA
475 MVA
13.8 kV
Power
Convertesrs
(Typical)
Other 11
breakers
Other 11
breakers
SV2
13.8kV BUS S1 , 60 Hz.
G1
G2
NSTX
Loads.
Typical
for
house
loads

Figure 1 - AC One Line
Disconnect switches are provided to parallel the buses and to
provide alternate feed from the 60 Hz utility system via bus S1. The power converters are fed from the buses SV1 & SV2.
Only one MG is required for operating NSTX. Each MG has
a stored deliverable energy of 2250 MJ. However, only 170
MJ is required for a typical NSTX Pulse. The MG speed is
initially kept at 68 Hz, and droops down only by about two
hertz to deliver the required energy. Figure 1 shows the one
line of the Power system. The 13.8 kV Bus S2 provides house
power.
B. Thyristor Power Supplies
PPPL has 39 modular power supplies (Figure 2) with
conversion and bypass modules at D-Site. A power supply has
two sections, each of which provides an equivalent rating of 1
kV, 24 kA - 6 seconds equivalent square wave (ESW) every
300 seconds. Each power supply is fed by one three winding
transformer with a polygon primary and delta/wye secondary.
The polygon is arranged to produce + 7.5
°
or -7.5
°
phase shift
depending on the phase sequence of the 13.8 kV input to the
polygon.
R
B
ACB
AC CLR
R
B
SV Bus
SV Bus - 13.8 kV Variable Freq.
ACCLR = AC Feeder Reactor
RT - Conversion Transformer
ACB = AC Breaker
R = 6-pulse rectifier
B = bypass thyristor array
RT
RT
Figure 2
Typical Power Supply with two sections
Toroidal Field (TF) Power Supply
The NSTX TF coil is rated at 1 kV, 72 kA pulsed current of
0.6 seconds duration every 300 seconds (See Figure 3). The
existing TFTR TF1 power converter is used to feed the NSTX
TF coil. Four power supply sections are connected in parallel
with a combined rating of 1 kV, 86.6 kA for 6 seconds
equivalent square wave (ESW) every 300 seconds, with an
allowance of 10% of unbalanced current sharing between the 4
parallel paths. Each parallel path has a DC current limiting
reactor.
To Grd. fault
detection ckt.
TF
DC CLR
R
B
Figure 3
TF Power Supply circuit (Type 1)
The remaining unused TF1 power supplies are put on
permanent
bypass. One power supply in each parallel path
has been commissioned to be available as installed spare. The
existing cabling was extended to the NSTX Test Cell (NTC).
The surge suppressors were changed to reflect the new
requirements. Simulations indicate that the maximum pulse
current of 72 kA can be ramped up in the coil in 0.6 seconds.
Ohmic Heating Coil Power Supplies
The NSTX OH coil requires a 4 kV (reversible) supply with a
rating of +/- 24 kA for 0.5 seconds every 600 seconds with a
provision to increase the voltage to 6 kV in both directions
(See Figure 4). The RMS coil current is 710 Amps. TFTR
TF2 power supplies are used. These have four parallel strings
with six power supply sections of 1 kV in each string. Two
strings are connected in antiparallel configuration to provide
four-quadrant control (the type III circuit of Table 1). Each
string is rated 6 kV, 24 kA - 6 seconds ESW every 300
seconds. Analysis of the circuit with the system parameters
indicate that I^2*t of the coil is not exceeded for the pulse
requirements. However the current derivative is 333 kA/ sec
compared to the desired value of 410 kA/ sec. To achieve the
higher derivative, if required, 6 kV can be readily introduced in
each string. The surge suppressors were retained as is.
OH
DC CLR
R
To Grd. fault
detection ckt.
6, 1 kV pwr.
supplies in each
branch
Figure 4
OH Power supply circuit (Type III )
Poloidal Field (PF) Power Supplies
The PF system consists of 8 circuits, with each with a
dedicated power supply system as shown in Table 1. PF1a,
PF2 and PF3 coil systems have been provided with individual
feed to the upper and lower coils using a three wire feed
system to reduce the number of leads from 4 to 3 for a cost
effective installation. All the circuits use the existing TFTR
power supplies depicted in Figure 2. The rating for each circuit
is given in Table 1. The three wire unipolar configuration is
shown in Figure 5.
2 , 1 kV PSS in each string
Lower Coil
Upper Coil
Grd. fault
detection
Grd. fault
detection
Figure 5
Unidirectional three wire Circuit (Type II )
for PF1aU, L & PF2U , L The PF3 Upper and Lower coils circuits are four quadrant
systems with rectifiers connected in antiparallel using a three
wire configuration (Type IV circuit. - same as figure 5 except
that the rectifiers are in antiparallel) to feed the upper and
lower coil systems. The surge suppressors were changed for all
circuits to meet the new requirements.
Coaxial Helicity Injection
The coaxial helicity injection (CHI) requires a total of 50 kA.
This is a Type I circuit as shown in Figure 3 except that a)
only two parallel paths are provided and b) only a common
line switch isolation is provided for the two branches together.
In this system the plasma is the load, the impedance of which
is projected to be 80 uH and 7 milliohms. Simulation
indicates t