> **NSTX ELECTRICAL Power Systems**
>
> S. Ramakrishnan, C. Neumeyer, E. Baker, R. Hatcher, A. Ilic, R.
> Marsala**,** D. O'Neill, A. von Halle
>
> Princeton Plasma 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 to address the need for eleven
power loops. Power converters had to 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/    Volts   Peak Amps ESW@ Peak
                                     Parallel   (kV)    (kA)      Amps (sec)
                                     PSS                          

  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.

![](media/image1.wmf){width="3.4784722222222224in"
height="2.8201388888888888in"}

[Figure 1 - AC One Line]{.underline}

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.

![](media/image2.wmf){width="3.0416666666666665in"
height="1.6527777777777777in"}

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.

![](media/image3.wmf){width="3.4791666666666665in"
height="1.4097222222222223in"}

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.

![](media/image4.wmf){width="3.3333333333333335in"
height="1.4722222222222223in"}

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.

![](media/image5.wmf){width="3.4784722222222224in" height="1.71875in"}

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 that the current can be ramped up to 50 kA in 26
milliseconds if 1 kV is applied. The rise time improves to 14
milliseconds by applying 2 kV. In order to inject a current through the
plasma, the vacuum vessel is divided into two parts a) inner vessel and
b) outer vessel, insulated from one another. The two electrodes to which
the potential is applied are the inner vessel and the outer vessel. The
inner vessel is part of the center stack of the machine. The center
stack houses also the OH coil , inner part of TF coils, and
miscellaneous diagnostics. The outer vessel has several ports and
diagnostics equipment. Special design efforts were undertaken to insure
that the two vessel parts are insulated one another and from the other
mounted components. Two redundant suppressor banks are installed for
CHI.

*Special Cost Saving Measures*

Special cost measures have been undertaken while designing, and adapting
the TFTR power supplies for NSTX use. Among them were the changes within
the Safety Disconnect Switches (SDS). TFTR PF coils were divided into
three power loops with groups of coils. Each group of coils was
connected in a series sandwich configuration in order to reduce the
circuit voltage to ground. The outgoing cables from one SDS (from both
legs) did not land in one cable tray. There are multiple cables
connected to each outgoing leg to handle the current. NSTX required
eleven independent coil circuits and each circuit had to be provided
with an SDS. The cables were reconnected such that the positive and
negative return currents were in the same cable tray. Similar
interchanges of cables were made at the receiving end also.

To reduce the cost of additional cabling from the location where TFTR
cabling ended to the NSTX, six circuits were configured on a 3-wire
basis. This resulted in the reduction of extensions required from 22 to
19. The extended cables were terminated in a corner of the NSTX Test
Cell (NTC). Air-cooled buses were then provided to within about 10 feet
of the center of the machine.

All the existing DC current transducers were repositioned in the
circuits. Each circuit was provided with redundant current measurement.
For circuits with parallel branches, each parallel branch was provided
with redundant current sensors wherever possible. A total of 31 DC
Current Transducers were available from TFTR, requiring an additional
five current sensors for NSTX use. Shunts were purchased and installed
with fiber optic transmitters and receivers to complete the measurement
requirements.

*Dummy Load*

A toroidal coil of an old dismantled machine in PPPL is used as the
Dummy Load for testing the power supplies.

**C.** **Protection**

Each power supply is provided with a fault detector. The fault detector
trips the power supply in case of a fault including an overcurrent, a
pulse overtime condition and other miscellaneous fault conditions.
Basically the faults are grouped into three categories. The first
category produces an alarm only. The second category suppresses the
rectifier and fires the bypass module. For more serious faults, such as
failure to suppress, the AC breaker of the power supply is tripped.
Thus, electronic measures (suppression and bypass) are undertaken before
activation of a breaker trip thereby reducing the wear and tear on the
mechanical breaker. An important feature includes an advance signal to
the power supplies to suppress and bypass when any trip command is given
to the AC breaker trip coil.

The existing Hardwired Control System (HCS) of TFTR was retained, except
for the feature that closed the ground switches on a bypass failure.
NSTX coils being of comparatively very low inductance did not require
this feature. There are four existing HCS control boards and are
assigned to a) TF b) OH c) PF1 and d) PF2, 3, 4, CHI. However the
controls were combined for items (b), (c) and, (d). Thus a fault in any
of the PF, OH and CHI systems results in the suppression of all the
associated power supplies. The faults were grouped based on the magnetic
and electrical independence of the TF system from the PF/OH systems.
PPPL proposes to upgrade the HCS boards that are using electromechanical
relays with a Programmable Logic Controller (PLC) system in future.

Five existing analog modular units in the Electrical Fault Detection
System (EFDS) are used to protect five of the coils. The five circuits
are TF, OH, PF2 Lower, PF3 Upper, and PF3 Lower coils. These coils were
chosen based on their current carrying capability as compared to the
levels at which they are routinely operated. The protection primarily
includes overcurrent, and projected heating as computed by ∫ I\^2\*t.
There is also a feature to compare the signals from the two redundant DC
current transducers and if the deviation is more than a preset value the
system is tripped by suppressing the associated rectifiers. In addition
a permissive interlock is provided to the CHI circuit to prevent CHI
power supplies from firing unless there is a preset minimum value of TF
current.

Since the existing EFDS coil overcurrent protection was available only
for five of the coils, a separate Analog Coil Protection (ACP) unit was
designed and installed. The ACP was designed to trip the power supplies
(suppress and bypass) if a) an overcurrent is detected or b) the
presence of a current at a low level is detected for more than an
adjustable time duration that is adjustable in one second increments
from 1 to 15 seconds. This corresponds to the pulse duration required
for any circuit. The trip signal is sent to the HCS system which invokes
a trip (suppress and bypass) of the affected group of power supplies
i.e. TF and/ or PF/OH/CHI. The ACP also trips the power supplies if more
than one pulse attempted during a preset interval of time adjustable
between 256 and 383 seconds. Destructive forces are generated if the
polarity sensitive product of OH and PF1b coil currents exceed a limit.
The ACP is designed and set to trip the PF/OH/CHI power supplies if the
product of OH and PF1b coil current exceed a set value.

A Pulse Duration and Period (PDP) electronic timer was also designed and
installed to give permissive to the power supplies only for a preset
pulse duration and a preset period. This prevents pulses more than the
permissible duration to be applied as well as prevents imposition of
more than one pulse during an adjustable time period. The PDP is an
additional back up protection.

*Ground Fault Protection*

The TFTR ground fault detection (GFD) scheme was modified and applied to
the coil systems. There were four GFD circuits for TFTR. These detectors
are reused and connected to a) TF, PF1a, PF1b, b) OH, c) CHI and, d)
PF2,3 & 4 coils circuits. A unique method of interconnecting independent
electrical power loops via high resistances was adopted for ground
detection as in the case of item (a) and (d) above. Thus no additional
ground detection relays had to be purchased. Typically these are set to
trip at about 5 mA.

![](media/image6.wmf){width="3.388888888888889in"
height="1.7361111111111112in"}

Figure 6

Ground Fault detection - combining circuits

**D. Controls**

The design of the power supply controls requires that several types of
analysis be performed, in order to operate within the overall envelope
of the circuit voltages, impedances, and component ratings.

1\. DC Voltage drop vs. current vs. supply frequency;

2\. AC & DC Short circuit;

3\. Control of voltage and current plus transition modes:

\- Rectifier anti-parallel mode zero crossing

\- Rectifier transition into/out of anti-parallel mode

Based on the circuit analysis, a Power Supply Real Time Controller
(PSRTC) computer code was designed and provided to control the coil
currents.

Voltage and current requirements are driven by:

1\. The basic plasma scenario;

2\. Plasma shape and position control.

The PSRTC acts on the power supply equipment and provides control of the
application of voltage to the NSTX coils either in direct response to
commands from the NSTX Plasma Real Time Controller (PRTC), or as
required to produce a programmable current through the coils via
feedback control. The PRTC includes the plasma controller as well as
associated fault sensing features which serve to integrate the operation
of the entire NSTX plant including the Gas Injection, Heating etc. In
addition to controlling the interaction of the power supply equipment
with the coils, the PSRTC provides control of the application of voltage
and delivery of current to the CHI. The same basic function is applied
to all of the circuits. The output of the control function consists of
the firing angle (α) and convert bit commands which are delivered to the
Power Supply Control Words. Since the firing angle is related to the
voltage, these commands are essentially voltage commands. The voltage
commands are derived either from closed loop proportional - Integral
(PI) current control against a programmable reference, or from a direct
voltage request. The direct voltage request is obtained from either a
programmable reference, or a signal supplied in real time by PRTC. When
in the voltage control mode, a current clamp feature is applied if the
current approaches a programmable limit. Additional features include
rate limiting, and control of antiparallel rectifiers, including the
requisite α limiting.

**CONCLUSIONS**

By locating NSTX in D-Site at PPPL, it has been possible to utilize the
site credits for the maximum advantage of NSTX. TF coil turns were
optimized. Other capabilities were increased. Future upgrades are
accomplished easily, utilizing existing power system. All equipment had
been earlier used effectively for TFTR and had been well maintained.
TFTR had an impeccable Safety Record. That tradition will continue for
NSTX with all the well-proven safety interlock measures. Cost has been
reduced to the maximum extent by judiciously choosing equipment
configuration with changes to accomplish desired design goals.

**ACKNOWLEDGMENTS**

The work was supported by USDOE Contract \# DE-AC02-CHO 3073. The
authors wish to express their appreciation to Martin Peng, Masa Ono and
Michael Williams, who provided encouragement and assistance in the
various stages of design and implementation of the NSTX Power System.
Our thanks are also due to Roger Raman of University of Washington who
provided input to design the CHI power system.

**REFERENCES**

\[1\] C.Neumeyer, et. al.,\"NSTX Real Time Control Systems\", 11th
IEEE/NPSS Real Time Conference, Santa Fe, NM, June 1999

\[2 C. Neumeyer, et. al., \" Engineering Overview of National Spherical
Torus Experiment \", 17th IEEE/NPSS Fusion Symposium, IEEE No.
0-7803-4226-7/98

\[3\] Systems Requirements Document - NSTX Power Systems, PPPL Document
\# NSTX-SRD-5X-019

\[4\] Pwr. System Real Time Controller Software Requirement
Specification - PPPL Document \# NSTX-SPEC-5X-049