Vol. No: I, Issue: 1, 2005
SUPERCONDUCTING MAGNETIC ENERGY STORAGE
Dr. S.C. Tripathy
Department of ECE and EI
Abstract
Operating Principle
Research and development in the field of
superconducting magnets has been going on for
many years leading to consideration of very large
electromagnets with superconducting windings as a
medium of energy storage in power system.
However, because of the high cost of extremely low
temperature (4.5 Kelvin ) superconducting material
and difficulty in converting energy between electrical
and magnetic forms, application to utility system was
not possible till l981. In l982 a 30-MJ
superconducting magnetic energy storage device was
built and field tested by the Bonneville Power
Administration in the U. S. A. In this paper the
design and test results are presented.
The elements of the complete electrical assembly are
depicted in Figure . Electrical energy is stored in the
magnetic field of superconducting inductor coil,
which is connected to the ac power system through
the inverter/converter unit. Each converter is of
conventional 6-pulse type. Two such converters are
connected as shown provide a 12-pulse arrangement
outside the dewar with only the inductor L as a load
on the dc side.
Introduction
The major components of a SMES unit are its
superconducting coil, the non-magnetic vacuum
vessel, the cryogenic system with liquid helium
refrigerator the ac/dc thyristor converter and the local
control system. In this paper we discuss only the low
temperature superconducting device. The coil made
of NbTi is immersed in a superfluid helium bath
supplied from helium refrigeration system and is
contained in a helium vessel maintained at a low
temperature of 1.8 Kelvin (Critical temperature of
the material). The helium vessel is called a cryostat.
The helium vessel is surrounded by and supported
from a vacuum vessel in nitrogen shroud surrounding
the helium vessel. The vacuum vessel assembly is
known as Dewar.
Basic ckt. elements for SMES unit for
power system applications
The interfacing of I-C set between the ac system and
SMES inductor magnet consists of conventional
thyristor bridge(12-pulse type). However, voltage
source inverters consisting of IGBT devices can also
be used in SMES application . The control of power
from ac system to the SMES unit and vice versa can
be achieved by varying the commutation angle a of
the thyristor controlled inductor-converter set. To
have a control over the operation of SMES unit,
delay angle of commutation a is varied which
The superconducting coil once charged with dc
current from ac/dc converter supports a magnetic
field of approximately 1.2 Tesla without any losses.
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Vol. No: I, Issue: 1, 2005
Cryogenic System
produces variation in bridge voltage Ed throughout a
wide range of plus and minus values.
A SMES system is based upon the nondissipative
feature of superconducting material carrying dc
current. For the 30-MJ coil, the superconducting
material is the ductile alloy NbTi embedded as fine
filaments in a copper matrix. Small eddy current and
hysteresis losses do occur in superconducting cables
as a result of the magnetic field variation when the
SMES coil is charged and discharged. Other heat
loads to the 4.5 Kelvin liquid helium bath are from
radiation and conduction through the containing
dewar and heat conduction down and I2R or Joule
heating in the helium vapour cooled leads that
deliver the power from the ambient condition to the
coil at 4.5 Kelvin. These heat loads are compensated
by a CTI Model 2800 helium refrigerator. The
refrigerator also is to cool the storage coil from
ambient temperature to 4.5 K and to liquefy the
helium for the coil immersion bath. Nevertheless the
problem of heat loss necessitated the design of a
nonconducting dewar made of fiberglass-reinforced
epoxy to hold the SMES coil.
Neglecting the losses, then Ed may be described as
below in accordance with converter theory.
Ed =Ed’ Cos a - Id Xc /2
Where,
Ed ‘ =no load per unit bridge voltage
Id = per unit dc current of the bridge
Xc = per unit commutating reactance
The dc voltage Ed across the inductor can be
expressed as L dId/dt, where t is thetime in second.
The response of dc current is
Id = Id0 + (1/L) ò Ed dt
Where Id0 is the initial value of current in the SMES
coil having an inductance L . For charging the coil at
maximum rate Ed should be held at its maximum
value which corresponds to the rectifier operation of
the converter with delay angle of the thyristor firing a
equal to zero. Current Id then builds up as a linear
function of time t until the rated current value Id = 1.0
per unit is reached. Then the firing angle
is
increased slowly so that the dc current is kept
constant.
Design Data
Table - I
Stored Energy = 30 MJ =8.4 kWh
Peak Power = 10 MW
Firing angle settings beyond 140 degrees are
possible, but there is a risk of commutation failure. In
the buck-boost mode of control each α can be
controlled independently and apparent complex
power equation is
S=1.35 EId(1/2)[(cos 1+cos 2)+ j(sin 1+ sin 2)]
SMES coil current = 4.9 kA
Operating frequency = 0.35 Hz
(charge and discharge rate of SMES)
Where E is the input ac voltage to the thyristor
bridge. The firing angles of the two 6-pulse thyristor
bridges are controlled independently. At any time
during the charging period, the stored energy in per
unit system is given by
Coil material : Niobium Titanium NbTi
Embedded in copper strands Cu : NbTi
Coil maximum operating temperature= 4.5 Kelvin
Inductance of superconducting coil L= 2.5 Henry
Maximum magnetic field = 2.85 Tesla
Maximum coil terminal voltage = 2.1 kV
Refrigeration is done by liquid Helium
Table –II
SMES Coil Ampere Turns = 4.51x 106
Number of layers in the coil = 40
WL = (1/2) L Id2
It should be noted that L is a inductance of the coil in
Henry which is a measure of the size of the SMES.
Number of turns per layer = 23
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Ido = Intial dc current flowing in the coil
Coil Configuration = 20 Double Pancake Coils
Average Coil Diameter = 2.7 m (inside)
= 3.38 m (outside)
Coil height = 1.21 m
NbTi Filament dimension = 6.5 micro-meter dia.
Zo = Rating constant of the SMES device
Tdc1 = Time constant of the converter
S = Laplace transform complex variable
Commissioning Test
Copper strand dimension = 0.511 mm dia.
Number of NbTi filaments = 1464
Modulation of the SMES unit addresses control
objectives similar to those of HVDC modulation at
the BPA Celilo converter station, but its capabilities
and operating environment are substantially different.
The SMES unit is sized for small signal use, and
SMES modulation signal is a random one with time
variable statistics, so management of stored control
energy reserves and effect of sustained operation
near the device limits must be considered in overall
controller design. A key element is the relative
sensitivity of power system dynamic response to real
and reactive components of complex power
modulation at the substation site where the SMES is
installed.
Protective System
1. In case of fault in the transformer or converter,
the circuit breaker on the ac side should be
opened.
2. Bypass thyristors must be connected immediately
after the 12-pulse converter for circulating the
coil current.
3. One bypass thyristor valve must be provided for
each 6-pulse thyristor. One 6-pulse thyristor
converter unit is supplied from Y-connected
transformer and the other 6-pulse thyristor
converter unit is supplied from Delta-connected
transformer unit.
Each of the two SMES converter bridges presents a
variable complex power load to the ac system. With
parallel control of the two converter bridges with
same firing angle, a reactive power signal is
produced that varies nonlinearly with the real power.
This mode of operation is not likely to raise serious
difficulties during normal operation because the
expected level of real power modulation and
associated reactive power signal are small. The
reactive power signals will act mainly as an
unwanted noise peaking at twice the ac intertie swing
frequency near 0.7 Hz. High signal levels are
required during system response tests because
ambient noise on the ac intertie can approach 10 MW
rms and the gain presented to test inputs is quite low
at some frequencies.
4. In case of internal trouble like break in the SMES
coil conductor, energy should be dumped in a
separate resistor.
5. For prolonged disconnection of SMES in case of
permanent fault, a superconducting switch must
be provided inside dewar. Coil current will
circulate through the closed switch.
6. To protect against overvoltage transients like
lightning, surge arrestors must be connected at
the coil terminals outside dewar.
Transfer Function
The transfer function of the coil when it is connected
to the frequency controller of power system is given
by the following equation:
dPd =df (Ido Zo)/(Tdc1S + 1)
Model studies indicate that operation of the SMES
unit according to the buck – boost logic with
different phase and control signals to each of the
converter bridges could hold the reactive power
where,
dPd = Power deviation due to change in load
df = frequency deviation of the utility due to change
in load
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Vol. No: I, Issue: 1, 2005
constant at + or - 8 MW for a possible preferred
mode of operation.
storage hydro scheme has found general acceptance
as an important component for long range planning
and operation.
Conclusions
References
This paper presents the basic concepts, circuit
requirements, and terminal operating characteristics
of energy storage inductor-converter units for power
systems. Such units, properly controlled, would
provide significant power system benefits. Because
of the inherent quick response characteristics of the
inductor-converter units, damping capabilities are
excellent. Improvements in transient stability may
also be realized.
1. H. A. Peterson, N. Mohan and R. W. Boom,
“Superconductive Energy Storage Inductor-Converter
Units for Power System”, IEEE Transactions on Power
Apparatus and Systems”, Vol. PAS-94, No. 4, July/August
1975, pp. 1337-1346
2. J. D. Rogers, R. L. Schermer, B. L. Miller, andJ. F. Hauer,
“30-MJ Superconducting Magnetic Energy Storage
System for Electric Utility Transmission Stabilization”,
Proceedings of IEEE, Vol. 71, No. 9, September 1983,
pp. 1099-1107.
‰
While other kinds of energy storage have received
some attention at various times, only the pumped
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