Theory and Application of
Superconducting Magnetic Energy Storage
Ju Wen, Jian X. Jin
Center of Applied Superconductivity and
Electrical Engineering,
University of Electronic Science and
Technology of China, Chengdu 610054, China
Email: jxjin@uestc.edu.cn
ABSTRACT
Superconducting magnetic energy storage (SMES)
technology is one of the most active research areas of
applied superconductivity in the recent years especially
after the high temperature superconducting (HTS)
materials were discovered. The theory of SMES and the
structure of its system will be introduced and discussed
in this paper, as well as its power conditioning system
control strategies and prospective applications in the
future.
1.
where L is the inductance of the SMES coil, ISMES is the
current flowing in the SMES coil. Supposing the SMES
coil discharging with constant power P0 within specific
time ts, the energy in the SMES coil E(t) at t<ts is
E (t ) = E SMES − P0 t
(2)
INSTRUCTION
Superconducting magnetic energy storage (SMES) is an
energy storage device that stores energy in the form of
dc electricity that is the source of dc magnetic field. In
the recent 30 years, SMES technology is one of the hot
research areas of superconductor applications, especially
when the high temperature superconducting (HTS)
materials were discovered 20 years ago in 1986.
Research on SMES is promoted and its technology has
achieved significant progress. Compared to other energy
storage devices and methods, SMES has some better
performances. Firstly, the current density of SMES coil
is about 10 to 100 times lager than the common coil, and
has virtually no resistive losses. Consequently, the
energy with a higher density can be stored in a persistent
mode until required. Secondly, the efficiency of SMES
can get up to 95% and energy exchanging speed with
power systems within milliseconds. Furthermore, SMES
can be easily controlled. With the well developed power
electronic technology, SMES can enhance power system
stability and improve the power quality through active
and reactive power compensation.
2.
You G. Guo, Jian G. Zhu
Faculty of Engineering,
University of Technology,
Sydney, PO Box 123, Broadway,
NSW 2007, Australia
Email: youguang@eng.uts.edu.au
THEORY AND CONFIGURATION OF SMES
A SMES system is mainly consisted of a
superconducting coil as magnet, a cryogenic system, a
protection system, a power conditioning system (PCS)
and a controller, as shown in Fig. 1 [1]. The
superconducting coil is obtaining energy while charging
from the power system, then releases the energy stored
through discharging. The energy stored in SMES coil
can be described as
ESMES =
LI
2
SMES
2
Figure 1: Components of a typical SMES system.
When t=ts, the current in SMES coil is
Is =
(3)
where v is the voltage across the coil during discharging.
If the current in the coil drops below the critical value Is,
the system can no longer discharge with constant power
P0. However, discharge may be continued at a reduced
power and this depends on the depth of discharge. The
term depth of discharge λ of the coil is represented as a
ratio of deliverable energy Ed to stored energy ESMES, that
can be described as
Ed
Pt
= 0s
E SMES E SMES
λ=
(4)
The current at any given time t is obtained from (1) and
(3) as
I (t ) =
P0
t
1− λ
ts
v 1− λ
(5)
This current depends on the λ and the coil operating
voltage v. The energy stored is obtained from (2) and (4)
as
E (t ) = E SMES −
P0 t s
(1)
P0
v
E SMES λ
t
ts
t
=
(1 − λ )
ts
λ
(6)
2.1.
SMES COIL TECHNOLOGY
The SMES coil is the main element in a SMES system,
A SMES coil design follows Virial law [2], and the law
is shown as
M st = (1 + 2Qc ) ρ st E / σ st
(7)
where Mst is the mass of the coil; Qc is the compressive
quality factor, its value is within 0 - 1 that depends on
the SMES coil configuration; ρst is the configuration
density; σst is the average design stress; E is the stored
energy in the coil. According to (7), to reduce the coil
mass requires a small Qc value. When Qc = 0, the
minimum of mass can be described as
M st = ρ st E / σ st
(8)
To design a coil having less mass than (8) is impossible.
According to (7), the medium-sized SMES coils have
been designed and the results are shown in Table 1 [2].
Table 1. The results of medium-sized SMES coil
SMES coil
Interior hole
capacity /MWh
/m
1
5
5
8
10
10.5
20
13.5
Height /m
8
12
15
17.5
Operating
current /kA
50
50
75
100
The superconducting coil can be divided into a toroidal
coil and a solenoid coil. The toroidal coil is suitable for
medium-sized and small-sized SMES mainly, the ideal
structure adopts the multistage structure, and the
advantages of this structure are reducing the magnetic
field leakage and the floor space. The solenoid coil is
suitable for large-sized SMES, its advantage is having a
simple structure, but produces leaking magnetic fields.
In a toroidal coil, the electromagnetic force is inward as
shown in Fig. 2(a), and in a solenoid coil it works
outward as shown in Fig. 2(b) [3].
(a) FBC
(b) TTC
Figure 3: The toroidal coil.
The structure of solenoid coil is simpler than toroidal
coil; design of solenoid coil focuses on the optimum the
geometry parameters, so that the solenoid coil can store
greater energy with less superconducting material and
less volume. The ordinary cross sectional shape of
solenoid coil is rectangular. However, a new cross
sectional shape of step shape has been proposed [7]. By
using the proposed shape, the winding volume and the
loss of the SMES coils can be reduced effectively. It is
possible to design the larger SMES for power system
stabilization with higher energy storage by using the
proposed shape.
2.2.
POWER CONDITIONING SYSTEM
Power conditioning system (PCS) is the interface
between SMES coil and power system, generally there
are two types: current source converter (CSC) PCS and
voltage source converter (VSC) PCS. The configurations
of CSC-PCS and VSC-PCS are shown in Fig. 4 and Fig.
5 respectively [8]. VSC-PCS usually uses in a largesized SMES system. VSC-PCS is a voltage source
converter in series with a DC/DC chopper, the charge
and discharge of a SMES coil is controlled through
invert circuit and the chopper. The four-quadrant voltage
source converter is utilized to accomplish the power
transformation between the three-phase AC power
system and the DC bus.
(a) Toroidal coil
(b) Solenoid coil
Figure 2: Electromagnetic force on toroidal and solenoid
coils.
A Force-Balanced-Coil (FBC) [3] has been proposed,
and the conceptual structure of a FBC is shown as Fig.
3(a) [4], the dark hatch indicates one complete helical
winding, and a FBC coil using 340m of Ag sheathed Bi2223 HTS tapes has been designed, the coil operates
current 12A and is cooled by liquid nitrogen. The test
result shows, the tensile stresses and the bending stresses
cause the critical current decrease of the coil [5]. To
reduce the stress problem, a kind of new coil structure Tilted-Toroidal-Coils (TTC) have been proposed by
changing two tilted angles on the basis of toroidal field
coil (TFC) structure, and as shown in Fig. 3(b) [6].
Figure 4: Voltage source converter of PCS.
The configuration of CSC-PCS is simple compared to
VSC-PCS, and also its control is easier. The most
important feature is that the response of power exchange
with a CSC-PCS is much faster than with a VSC-PCS,
because a SMES stores energy in the form of dc current.
CSC-PCS technique is suitable for medium- and minisized SMES, several modules of CSC-PCS with parallel
connection are able to be used in the case of high or ultra
high power.
as shown in Fig. 7 [11]. This new method mainly has
four advantages: (1) Increasing the current capacity to
the level required by the superconducting magnet. (2)
Reducing conduction losses. (3) Reducing harmonic
waves by harmonic cancellation without the need of
expensive filter. (4) Achieving a high frequency
bandwidth.
2.3.2.
NON-LINEAR ROBUST CONTROL
CONTROL
AND
FUZZY
Figure 5: Current source converter of PCS.
The control strategies of PCS are very important in a
SMES system, the strategies should be chosen according
to the capacity of the SMES, stability of power system,
AC harmonic wave and etc.
2.3.
CONTROL STRATEGIES
A SMES device performance is also determined by its
controller, which is a main technique to be considered in
a SMES system design. Fig. 6 gives the block diagram
of a SMES device control [9], where power system
variables are the deviations of voltage, frequency, and
current; Pd and Qd are the demanded active and reactive
power; α1 and α2 are the firing angles of power
converters; and Ps and Qs are the SMES active and
reactive power outputs.
The stability of the power system is a typical non-linear
problem, any practical power systems will be affected by
uncertain factors, for instance the parameter uncertainty
and interruption from outside. Therefore it is difficult to
use the accurate mathematic models to describe the
dynamic behaviour of the system. Switch frequency has
a close relationship with the load parameters and the
input voltage, and the uncertainty of the changes of these
parameters make the accurate design limited; Using
robust control and fuzzy control can overcome the
limitation of accurate modelling. In order to verify the
feasibility, the traditional PI control and fuzzy control
are compared in transient stability enhancement of
power system with three-phase-to ground (3LG) and
single-line-to ground (1LG) faults near the generator, as
shown in Fig. 8 and Fig. 9 respectively [12].
Figure 6: Block diagram of control with a SMES
device.
The demanded active and reactive power, needed to
support power system performance as measured by
voltage, stabilizing power oscillation and improving
power transfer, is determined by the external control
based on deviations in voltage, frequency, and current.
These input variables are in turn dependent on the
system state. The internal control determines the firing
angles of the PCS, which are the input signals to the
SMES device. The internal control controls the firing
angles according to the signals of demanded reactive and
active powers. The detailed control strategies are
discussed as follows.
PHASE-SHIFTING SPWM TECHNOLOGY
Figure 7: Current source converter with tri-logic
PWM control.
With the development of Gate Turn-off (GTO) Thyristor,
Sinusoidal Pulse Width Modulation (SPWM) technology
has more and more extensive applications in the electric
and electronic devices. However, because the maximum
of switch frequency is very low, it produces a large
number of harmonic waves, and the bandwidth of PCS is
narrow, and high switch frequency can produce heavy
switch loss, this makes a contradiction of higher switch
frequency with less switch losses. In order to solve this
problem, a new phase-shifting method and technology
that combines the SPWM technology and multiple
modules technology together has been proposed [10],
using dynamic SPWM tri-logic as the operating strategy,
Figure 8: 3LG fault.
2.3.1.
low temperature liquefier; vacuum device; relief valve
when the pressure is too high; helium reserve pot; and
the cooling case.
Figure 9: 1LG fault.
2.4.
SUPERCONDUCTIVITY MEASUREMENT
PROTECTION SYSTEM
AND
The SMES coil operates dynamically in power system
for this application, sometimes it is required to absorb
and release great power within some milliseconds, so the
SMES coil must be more stable than other common coils.
A SMES coil operates stably only in certain conditions,
or it may result in damage. Overheating and high voltage
arcing of the SMES coil may make it lose
superconductivity, then the energy stored in the SMES
coil exhausts in the form of heat if no action has been
taken, that may destroy the SMES coil seriously, so the
protection system is very important in a SMES system.
The protection system should satisfy the requirement as
follows: Firstly, energy exhaustion should be reduced
and overheating of the wire should be prevented.
Secondly, in order to prevent the evaporation of the
cryogenic liquid in cryogenic system, the energy
exhausting in the cryogenic container should be reduced.
There
are
several
methods
that
measure
superconductivity of SMES coil, such as temperature
detection [13], pressure detection [13], ultrasonic
detection [14], flow velocity detection [15] and voltage
detection [16]. Temperature detection detects the change
of temperature of the conductor; pressure detection
measures the change of pressure in the cryogenic
container; ultrasonic detection detects the change of
transfer function through measuring the ultrasonic signal
from input to output; flow velocity detection detects the
change of flow velocity of the liquid; voltage detection
detects the change of resistive voltage. However, the
voltage detection is more practical than another method.
The protection of SMES magnet requires that the
detection of superconductivity must be fast enough and
exhausting the energy without destroying the SMES coil.
Some methods that prevent SMES coils lose
superconductivity should be considered during the
design, manufacture and operation of the SMES coil.
2.5.
CRYOGENIC SYSTEM
The cryogenic system mainly consists of stainless steel
refrigeration device, the distribution system with low
temperature liquid, a pair of automatic helium liquefiers,
etc. The main compositions of the distribution system
are electric connection on the refrigeration device top;
low-temperature control valve case in which the helium
flows; refrigeration between the device, valve case and
There are two kinds of traditional SMES coil cooling
methods: one is to let the SMES coil dip in the liquid
helium, the other is the forced pressure cooling to flow
the ultra critical helium through the conductor. The first
method is good in stability, but not for AC losses and
voltage-proof; the second method has good
performances in machine intensity, AC losses and over
voltage, but improving the stability is needed. These two
methods all consist of the complex system of cryogenic
liquid, and the compensation of liquid is needed if the
SMES system keep on operating for a long time. In order
to solve this problem, it has been proposed not using the
liquid helium but using cryocooler to cool the magnet
[17]; it is much safer and is no need to make high
pressure to liquefaction. It is not only more suitable for
operation but also high effective. Because of the
limitation of the HTS material, the cryogenic system of a
HTS SMES system mostly uses this method. Fig. 10 is
the conduction cooling system with a GM cryocooler
without coolant such as liquid helium. This equipment
was designed for testing the transient thermal
characteristics of cryocooler-cooled HTS coil [18],
where the cryocooler-cooled LTS coils were used to
generate a background magnetic field.
Figure 10: The conduction cooling system with
GM cryocooler.
A two-stage GM cryocooler to cool 1MJ SMES [19] has
been adopted, the operation result shows that the power
of cryocooler is 60W at 73K and 20W in 20K. However,
the cooling method using cryocooler has broken the
limitation of the traditional cooling method, even its
stability is not high enough. GM cryocooler can not offer
power of 50-100W in 20-40K, therefore the routine GM
cryocooler products do not meet the need of SMES
magnets using HTS materials. It is very essential to
develop the one stage GM cryocooler with high
refrigeration capacity for cooling a HTS-SMES.
3.
APPLICATION OF SMES AND THE STATUS
OF DEVELOPMENT
3.1.
APPLICATION OF SMES
SMES having fast response characteristic makes it to be
able to apply to the power system extensively. Its
function has two main aspects: Enhancing the power
system stability and improving power supply quality.
Improving the stability demonstrates that SMES can
damp system oscillations, thus improve the transmitting
capacity of the power system. In addition, SMES can
prevent voltage drops, caused from the generator fault or
the large load inserted in the system, through the active
and reactive power compensation. Improvement of
power supply quality has several considerations: (1)
Improving flexible AC transmission system (FACTS); (2)
Compensation of fluctuating loads; (3) Spinning reserve;
(4) Improving power system symmetry; (5) Protection of
critical loads; (6) Compensation of dynamic voltage
(VAR compensation) in dynamic voltage support; (7)
Backup of power supply. The potential applications of
SMES are shown on Fig. 11 [20], the shaded areas
represent that the applications have reached the
commercialized application level.
Figure 11: Energy-power
potential SMES applications.
3.2.
characteristics
of
STATUS OF DEVELOPMENT
A 30MJ SMES system was installed in BPA (Bonneville
Power Administration) Tacoma transformer substation in
1982-1983, its system has steadily worked for 1200
hours, and its results indicate that the system meets its
design requirement [21]. In 1988-1989, a 20MWh SMES
have been designed by University of Wisconsin,
Westinghouse, Teledyne and et al. [22]. 1MJ and 3MJ
SMES units can also be supplied by Superconductivity
Inc. of USA. At present in USA a SMES system of
100MJ/50MW is under designing, it is the biggest SMES
system so far, the goal of designing this system is to
damp the low frequency oscillation in the transmission
power system. The SMES magnet has been built in 2003;
this magnet has been tested in the Center for Advanced
Power System [23].
The Superconductive Energy Storage Research
Association was established in Japan in 1986, its task is
to promote the practical application using SMES. In
1991, Kyushu Electric Corporation designed a 30kJ
SMES to improve the stability of a 60kW hydroelectric
generator; the test results indicate the suitable
performance of SMES achieved [24]. In order to decide
the suitable SMES capacity having the best ratio of
quality to cost, 1kWh/1MW and 100kWh/20MW SMES
systems have been designed. The validity of the SMES
of 1kWh/1MW has been under investigation by linking
its prototype to a 6kV power system and a 66kV power
system. Experiments were conducted to compensate load
fluctuation in a 6kV and a 66kV power system by
varying the degree of controlling energy output of the
SMES, according to energy capacity variations stored in
the SMES and the load at the demand side [25]. In 2004,
researchers in Japan carried on the conceptual design of
a 15MWh SMES in order to improve the stability of the
wind-force 100MW electricity generation system [26].
In Russia, the T-15 superconducting magnet fabricated
in 1988, its capacity reaches 370-760MJ [27]. Since the
1990s, researchers of Russia have carried on the design
of the energy storage magnet of 100MJ/20MW [28]. In
Korea a 1MJ/300kVA SMES system has been
fabricated using as an uninterruptible power supply
(UPS), it is shown that a 3s power interruption is
successfully compensated, and high on-line efficiency
(96%), low output voltage (2.5%) and input current (3%)
are achieved [29]. In 2005, Korea Electrotechnology
Research Institute designed a 3MJ/750kVA SMES
system for improving the power supply quality [30],
with 1000A operation current. DGA (Délégation
Générale pour L’Armement) strongly supports applied
superconductivity research in France. After a successful
100kJ SMES built using Bi-2212 tapes with cooling in
liquid helium, the DGA required to develop this
technology at a higher energy storage level with
operation temperature at 20K. In 2004, DGA had a
project aimed to build a 800kJ HTS SMES operating at
20K, engineering current densities exceed 300MA/m2
(20K, 5T) [31]. In Germany a few companies have
joined and designed a 150kJ/20kVA SMES system used
for UPS [19].
At present, the study on SMES system concentrates on
the following four aspects: (1) Study of PCS, it
concentrates on the study of the topological structure of
the circuit and its control strategies; (2) Application of
SMES, it mainly expends the field of application of
SMES; (3) SMES magnet design, it concentrates on the
study of HTS materials and optimuim of the structure of
the magnet; (4) Protection system, it focuses on the
methods to measure the loss of superconductivity and to
ensure the SMES system in a security operation.
4.
CONCLUSIONS
With the fast development of the HTS technology,
SMES devices with high efficiency, rapid response with
active and reactive power compensation, will play an
irreplaceable role in the power systems. According to the
estimation made by various investigators, the new
developing superconductive industry's market will
exceed 25 billion dollars by 2010, among them it is 30%
in the power system; and will reach 70 billion dollars by
2020. Nearly all kinds of superconducting power
products can reach the commercialized use in about
2015, among the electricity application, the SMES
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technology is more
development prospects.
applicable
and
captivating
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