Download PAPER Title Superconducting Fault Current Limiters

PAPER
1/6
Superconducting Fault Current Limiters
as New Devices for Distribution Installations
Title
Registration Nº: (Abstract)
Summary
Superconducting fault current limiters (SFCLs) as versatile devices are excellent means to
overcome the existing problems resulting from high short circuit currents. Due to the strong
increase in resistance of a SFCL initiated by a fault the technology is well suited to protect
today’s electricity grids. The very low reactance under all operating conditions is a further
advantage of these superconducting devices. Nexans SuperConductors has for the first time
transferred such devices from the R&D phase into commercial production. Several SFCLs have
been designed, built, tested, and commissioned and are operated by DNOs or power plant
operators. One of the systems built has been especially designed for multiple applications at
different installation locations. A recent SFCL system is installed together with a HTS cable to
supply up to 40 MVA on 10 kV medium voltage in the inner city of Essen, Germany, an
installation which has become known as “AmpaCity”. The SFCL is protecting the HTS cable and
the grid lying behind and is at the same time enabling a very compact cross section for the
concentric HTS cable. Further systems under construction will be installed in the UK grid to
enable improve network short-circuit strength and enable increased power generation with low
CO2 footprint. The latest development achievements as well as test and operation results are
presented.
Authors of the paper
Name
Joachim Bock*
Country
Germany
e-mail
[email protected]
Achim Hobl
Germany
[email protected]
Judith Schramm
Germany
[email protected]
Christian Jänke
Germany
[email protected]
Simon Krämer
Germany
[email protected]
*corresponding author
Name of company and address
Nexans SuperConductors GmbH
Chemiepark Knapsack, 50351 Huerth, Germany
Tel.: +49 2233 48 6597 FAX: +49 2233 48 6847
Key words
Growing electricity demand, distributed generation, short circuit power increase,
Superconducting Fault Current Limiter (SFCL)
PAPER
2/6
Superconducting Fault Current Limiters
as New Devices for Distribution Installations
1.
INTRODUCTION
While on the one hand the demand for electricity and
system reliability continues to increase, on the other hand
utilities around the world are adding more and more
distributed generation and are building new interconnections resulting in evermore tightly meshed
networks. These tendencies result in higher fault current
levels challenging the ability of substation equipment to
withstand mechanical and thermal loads associated with
these high fault levels. Utilities must react by adapting
their equipment to meet the challenges of the higher level
of fault currents.
Electric utilities around the world have long employed a
variety of fault current mitigation techniques such as fault
current limiting reactors like air cores, selective tripping
schemes, and so forth. Each of these measures has distinct
drawbacks. An ideal solution would be a re-usable,
automatic device that does not affect the operation of the
power system during normal operation, but yet would
limit fault currents already during the first cycle peak.
Utilities in Europe have recently begun to employ
Superconductor Fault Current Limiters (SFCLs).
Resisitive type stand-alone fault current limiters based on
high temperature superconductors (HTS) technology have
been tested in the grid since a full decade now.
Nexans SuperConductors has for the first time transfered
such a device from the R&D phase into commercial
production. Characteristics and typical applications of this
type of SFCL are presented along with the operational
advantages and their potential role in the deployment of
future electrical networks such as the so called “Smart
Grids”.
2.
FUNCTIONALITY AND APPLICATIONS
CASES
Working Principle of resistive SFCL
SFCLs have the unique characteristics of almost zero
impedance under normal operating conditions and high
impedance at fault conditions. An excellent overview
about all aspects of development and application of
SFCLs is given in [1]. Several principles have been
developed, inductive types, “saturated iron core” or
“shielded iron core”, which typically require a device like
a substation transformer with similar footprint. Compact
and efficient SFCLs based upon the resistive type have
been demonstrated in several real grid installations.
The working principle is based on the nonlinear voltagecurrent-characteristic in the superconducting material of
the form U ~
( I I c )n , where Ic is the critical current of
the superconductor, defined as the current, at which 1 µV
voltage drop per cm superconductor length is reached.
The exponent n is usually between 5 and 30 for typical
high temperature superconductors (HTS) and differs with
the material type.
Figure 1: current – voltage characteristics of a
superconductor compared to a normal conductor
With the short circuit event the current increases and the
voltage follows this power law. In that way the resistance
of the material rises about several orders of magnitude
until the material completely leaves the superconductor
state and behaves like a normal resistor with a
proportional current-voltage-law (Fig. 1). Within a few
ms the associated heating brings the superconductor
above its critical temperature, whereby the transition is
reached even faster. After the first half wave a further
heating of the material during the short circuit also in the
normal conducting state leads to further increasing
resistance and a decrease in the limited current (Fig. 2).
This is associated with a phase change of the material and
the transition from the superconducting to the normal
conducting regime is called a “quench”. SFCLs make use
2/6
PAPER
3/6
of this transition, when the normal amplitude of the rated
current is below the critical current Ic and the quench is
triggered by the high current occurring during a short
circuit event.
Behaviour in the Grid
The SFCL technology provides the unique possibility to
design new innovative grid structures: The grid can have
very low impedance, e.g. through grid coupling or due to
low impedance equipment, but without the risks
associated to high short circuit power.
With a resistive SFCL in the line, the limiter also
minimises the phase shift between current and voltage
during a short circuit. This effect strongly reduces the
stress and thus the requirements on the circuit breakers in
line, because the current and voltage are almost in phase,
which also means zero crossing is occurring
simultaneously. In any case, all circuit breakers, busses
and cables downstream of a limiter can have much lower
ratings and significant equipment cost can be saved.
Especially high savings are expected for power plant
installations in the case of building new blocks or
expanding existing equipment.
An interesting aspect of using SFCLs is also that
equipment can be operated closer to its limits. With this
an SFCL is providing more headroom at the substation
e.g. for further integration of distributed generation. All in
all investment can be avoided or shifted by temporarily
installing a SFCL.
nearly pure resistive behavior of the limiter. This follow
current can be adapted in amplitude by selection of
appropriate materials.
Grid Type Applications
SFCL as versatile devices for fault current management
can be applied at different positions within a typical grid,
as shown in Fig. 3.
Figure 3: grid installations of SFCLs
Feeder Application
Depending on the protective function, the SFCL can be
used either in incoming feeders, e.g. as transformer
feeder, or in the outgoing feeders. This in-line application
protects all elements downstream of the point of
installation.
Busbar Coupling
The SFCL is especially advantageous for busbar
couplings, as fully redundant feed-in is possible without
a normally associated increase in short-circuit currents. In
case of a fault, the limiter ensures that the short-circuit
contribution from the un-faulted bus is strongly reduced.
Even more, the un-faulted side can maintain almost stable
voltage and operation. An additional advantage for busbar
coupling is that under normal operating conditions perfect
load levelling can be reached resulting in lower
operational losses and less stress for the equipment.
Figure 2: Prospective and limited current during a shortcircuit event
The red line in Fig. 2 marks the prospective current
flowing without limiter. The green line denotes the
limited current and shows effective and reliable limitation
already in the first half wave. The extensively
symmetrical curve progression in the following half
waves, the so called follow current, comes out of the
FIRST INSTALLATIONS
Following successful SFCL field tests in a distribution
level substation in 2005 [2], NSC installed the world’s
first HTS-based Fault Current Limiter in a power station
in 2009 [5]. NSC has since implemented several SFCLs
in distribution level networks.
3/6
PAPER
4/6
First installations have been realized in the distribution
network of different DNOs in UK. The SFCLs are based
upon BSCCO-2212 bulk superconductor modules which
are manufactured by NSC by a melt casting process [4].
Using mono- or bifilar components – the latter ones for
reduced AC losses – NSC has built and integrated several
complete SFCL systems based on this material type.
Bamber Bridge 33/11 kV Substation (ENW, UK)
The existing switchgear at the substation was rated at 11
kV with a short circuit capability of 150 MVA equating
19.7 kA making capacity and 7.87 kA breaking capacity.
Upgrading two 33/11 kV transformers increased fault
current contributions of each unit to 11 kApeak, 4.2 kA at
the substation. A Nexans SFCL deployed in a bus section
configuration limits the fault contribution from healthy to
faulted busbar in case of a fault on an outgoing circuit.
The total of fault contribution and contribution from the
transformer directly feeding the busbar are now limited to
95% of the switchgear’s rating. SFCL implementation
saves upgrading the substation with a new 11 kV
switchboard of primary distribution circuit-breakers [3].
Ainsworth Lane 33/11 kV Substation (Scottish Power,
UK)
The installation of a new transformer increased the fault
level at the substation to 270 MVA. This would have
exceeded UK DNO 250 MVA design fault level in a
region where 33 kV and 11 kV networks are meshconnected as opposed to the radial connecting prevalent
in the UK. The SFCL is installed in the substation’s bus
section and reduces fault currents passing through it from
a prospective ~7.5 kA to less than 3.2 kA, thus limiting
the busbar fault level to around 200 MVA, equalling 80%
of the capability of the switchgear in place.
Protecting Power Plant Auxiliary System
The resistive SFCLs are attractive devices for short circuit
protection of the auxiliary supply in power stations.
Special need is seen for coal power stations due to high
power drives for coal crushing and induced draught fans.
Two unique features of the resistive SFCL were most
relevant for the customer to have a device installed: The
fail safe behaviour and the fact that the grid is not dark
after the short circuit event, the device was designed to fit
into the existing grid protection scheme.
Boxberg Power Plant (I), (Vattenfall, D)
In 2009, a Nexans SFCL based upon BSCCO-2212 bulk
superconductor modules was deployed at the power plant
for short circuit protection of the auxiliary supply
including lignite crushers [5]. The device in a 12 kV
feeder bar limits a 63 kApeak prospective short circuit
current to less than 30 kApeak immediately and to about
7 kA after 10 milliseconds. With a nominal current rating
of 800 A, its design allows inrush currents of up to 4.1 kA
during coal crusher start up and of 1.8 kA for subsequent
15 seconds without tripping the limiting function. Field
testing of the device was successfully completed.
Boxberg Power Plant (II), (Vattenfall, D)
In 2011, a Nexans SFCL based upon YBCO coated
conductor modules replaced the first device and was
commissioned after successful testing [6]. The YBCO
based SFCL combines further improved energy efficiency
with a faster response to fault currents and stronger initial
limitation: 63 kApeak prospective short circuit current are
limited to 16 kApeak. Essentially, SFCL deployment
permits internal MV supply switchgear to be downsized
to standard component level and thus affords a
substantially lower investment requirement.
TOWARDS A STANDARD SOLUTION
The superconducting part of Nexans SFCLs consists of
series- and parallel-connected modules for each current
phase.
Figure 4: Installation of SFCL at Boxberg
“Stacking” these units provides the system with the
specific fault current limiting capacities, durations, and
reaction rates required in individual grid environments.
Stacked modules are immersed in liquid nitrogen within a
cryogenic vessel equipped with a commercial cooling
4/6
PAPER
5/6
system. The modules operate maintenance-free at
temperatures of -200 °C.
Palma de Mallorca 16 kV Substation (Endesa, E)
Košice 110/24 kV Substation (VSE, SK)
The EU-funded ECCOFLOW project aimed at
developing a multi-purpose SFCL providing flexibility
for a range of power grid applications. Nexans acted as
coordinator and device manufacturer in the ECCOFLOW
consortium which involved five European power utilities
and eight scientific and industrial partners [7]. The
ECCOFLOW design is based upon YBCO coated
conductor modules. The SFCL’s power rating is 1 kA at
24 kV. Combined with inductive or resistive shunts, the
SFCL limits fault currents for extended intervals of up to
1,000 ms.
Figure 6: SFCL 12-2400 for the AmpaCity Project
A superconductor cable can be laid in existing ducts
which affords substantially lower construction costs. In
total the superconductor system facilitates the
refurbishment of inner city power infrastructures by
offering high performance and savings of space and cost.
AmpaCity at the RWE grid is an international lighthouse
project and the HTS system is now constantly supplying
the inner city of Essen since April 2014.
Figure 5: Installed SFCL 24-1000 system
ENABLING NEW MV METROPOLITAN CABLES
AmpaCity 10 kV Superconducting Cable (RWE, D)
End of 2013, a superconductor power transmission
system by Nexans has been integrated in the inner city
power grid of Essen, Germany [8]. It comprises a 1 km
long 10 kV three-phase concentric HTS cable overloadprotected by a SFCL. The superconductor cable has a
rating of 40 MVA and will replace an entire conventional
110 kV installation of equal capacity by simultaneously
providing low-loss transmission. The AmpaCity project
thus provides a blueprint for removing high-voltage
installations and a good part of today’s substations from
inner cities in the course of rebuilding metropolitan grids
for future requirements. Overload protection by a Nexans
SFCL enables a compact cable design, as shunts in
parallel to the superconductors become expendable [9].
3. CONCLUSIONS
Two different superconducting material options are
currently available for manufacturing resistive SFCL
systems. For the BSCCO-2212 bulk material, NSC has
proven the viability of SFCLs from the basic
superconducting material to the complete system. SFCLs
using YBCO tape with stronger limitation also have been
produced, type- and field tested.
Both technologies provide the exceptional possibility to
design new innovative grid structures: The grid can have
very low impedance, e.g. by grid coupling or low
impedance equipment, but without the risk associated to
high short circuit power.
With a resistive SFCL in the line, the limiter minimises
the phase shift between current and voltage during a short
circuit, an effect which strongly reduces the stress and
requirements on the circuit breakers in line, because the
current and voltage are zero almost simultaneously. All
circuit breakers, busses and cables downstream of a
limiter can have much lower ratings and significant
equipment cost can be saved. For e.g. the power plant
installation this would be attractive in the case of building
new blocks or extending the existing ones. An interesting
aspect of using SFCLs is also that in case of equipment
loaded close to its operating limits, investment can be
shifted by temporarily installing an SFCL.
5/6
PAPER
6/6
The present movement in the power generation towards
distributed and renewable energies leads to a fundamental
downsizing of average power generation units. Where the
integration of distributed generation meets existing grids
and leads to excess short circuit overloading, SFCLs can
be a very promising solution: Most generation units are
able to supply directly into the medium voltage grid, but
that would need a higher short-circuit capability than in
many cases available. The typical solution is the
connection to the high voltage grid with the need for a
separate and expensive transformer with considerable
additional cost for breakers, cables, and civil works. An
alternative solution would be the integration via SFCL, as
shown in Figure 7.
All installations realized today are on the medium voltage
level. However, due to the performance of SFCLs, they
will also be large interest to integrate such devices into
high voltage grids.
[5] J. Bock, M. Bludau, R. Dommerque, A. Hobl, S.
Kraemer, O.M. Rikel; S. Elschner “HTS Fault
Current Limiters – First Commercial Devices for
Distribution Level Grids in Europe” IEEE Trans.
Appl. Supercond., vol. 21, No. 2, pp.1202-1205, 2011
[6] S. Elschner, A. Kudymow, S. Fink, W. Goldacker, F.
Grilli, C. Schacherer, A. Hobl, J. Bock, M. Noe,
“ENSYSTROB - Resistive Fault Current Limiter
based on Coated Conductors for Medium Voltage
Application”, IEEE Trans. Appl. Supercond., (Proc.
ASC 2010), in press
[7] J. Bock, A. Hobl, S. Krämer, M. Bludau, J.
Schramm, C. Jänke, M. Rikel, S. Elschner „Nexans
Superconducting Fault Current Limiters for Medium
Voltage Applications Status and prospects” CIRED
2011, Frankfurt
[8] A. Hobl, S. Elschner, J. Bock, S. Krämer, C. Jänke,
J. Schramm, “Superconducting fault current limiter
– a new tool for the ‘grid of the future’”, CIRED
2012, Lisbon
[9] J. Bock, A. Hobl, J. Schramm, “Superconducting
Fault Current Limiters – a New Device for Future
Smart Grids –”, CICED 2012, Shanghai
Figure 7: grid connection of DG with a SFCL
BIBLIOGRAPHY
[1] M.
Noe,
M.
Steurer,
“High-temperature
superconductor fault current limiters: concepts,
applications, and development status” Supercond.
Sci.Tech , 20 R15, 2007
[2] J. Bock, F. Breuer, H. Walter, S. Elschner, M.
Kleimaier, R. Kreutz, M. Noe, IEEE Transact. Appl.
Supercond., Vol. 15, pp. 1955-1958, 2005
[3] D. Klaus, A. Wilson, R. Dommerque, J. Bock, D.
Jones, J. McWilliam, A. Creighton „Fault Limiting
technology trials in distribution networks” CIRED
2009, Prague
[4] J. Bock, S. Elschner, P.F. Herrmann, “Melt-cast
processed MCP-BSCCO 2212 tubes for power
applications up to 10 kA”, IEEE Trans. Appl.
Supercond., vol. 5, No. 2, pp.1409-1413, 1995
6/6