Hyun, “Design and Experiments of Novel Hybrid Type Superconducting Fault Current Limiters,” IEEE Trans.. Ok-Bae Hyun, Jungwook Sim, Hye-Rim Kim, Kwon-Bae Park, Seong-Woo Yim, Il-Sung Oh,
Trang 1Superconductivity Application in Power System 69
Fig 27 IEEE 39 bus systems (HTS cable application: red line)
SI calculation of sample system
To consider power system reliability, N-1 contingency criteria was applied Equation (3.1)
and (3.2) shows the severity index (SI, over load index and voltage index) used in ranking
Over-load index
Equation 3.1 represents over-load index
2
max, 1
L i i i
P PI
P
Voltage index
Consumption of reactive power can be known by voltage ranker which represents
increment of reactive power loss by increased load factor of line Equation 3.2 represents
voltage index
2 1
L
i i i
where P i is active power, X ireactance, and P max,ipower ratings of i-line
The results of SI on sample system results are shown in Table 3.4 and Table 3.5 As a result
of calculation, the first two contingency cases of each SI are determined as the object cases of
voltage stability calculation
Trang 2(a) before
(b) after Fig 28 P-V curve (HTS cable application)
Trang 3Superconductivity Application in Power System 71
Ranking
No
Contingency Line
PI[p.u.]
From Bus To Bus
Table 8 Performance index by line overload index
Ranking
No
Contingency Line
PI[p.u.]
From Bus To Bus
1 28 29 10.8884
2 2 3 10.3888
3 16 21 10.2108
4 2 25 9.9931
5 6 7 9.8334 Table 9 Performance index by line voltage index of case I
Table 10 is the summary of the overloaded lines at severe contingency cases HTS cable is applied as the order of severity of overloaded line The replaced system is shown as Fig.29 Considered HTS cable constants are L = 0.10[uH/km], C=0.29[uF/km] respectly
Incremented transfer capacity after HTS cable replacement is 8,880MW in base case and 5720MW in N-1 contingency case Therefore, increased transfer capacity becomes 1820MW
from to contingency rating flow overload(%)
Table 10 Overloaded lines at N-1 contingency
5.2 SFCL
In power system, proper SFCL application places are considered as (a)~(c) points of Fig 29 Point (a) is to limit fault current of distribution feeder SFCL at (b) point reduces fault
Trang 4current impact of adjacent transformer in case of parallel operation and protects bus bar Point (c) is general solution to reduce transformer secondary fault current and extend Circuit Breaker changing time when distribution system experiences high fault current
Fig 29 SFCL application
6 Conclusion
The infrastructure of electric power system is based on conductor With the change of power industry, such as Kyoto protocol and Energy crisis, superconducting technology is very promising one not only to increase efficiency of electricity but also to upgrade security of power system Among various superconducting technology, most applicable ones –HTS cable, Fault current limiters, Dynamic SC are introduced and discussed how to apply Other superconducting facilities, like transformer, generator, SMES, Superconducting Flywheel, are in testing and will be implemented with the changes of power market needs However, the most critical obstacle of power system application is superconductor material and cooling system Present HTS superconductors have to be improved much more than conventional ones, but still have difficulties in general use, such as extreme low temperature operation, hard manufacturing, AC loss and high cost Cooling system is also hard task which have close relation of HTS failure due to quench mechanism In operating point of view, monitoring and control to protect the local hot spot is another task to overcome More advanced superconductors and application methods are expected in power system usage in near future
7 Acknowledgment
Thanks to support all referenced paper authors and researchers in the field of superconductor application in power system, especially Dr OK-Bae Hyun and Si-Dol Hwang in KEPRI
Trang 5Superconductivity Application in Power System 73
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Trang 6D W A Willen et al, “Test results of full-scale HTS cable models and plants for a 36kV,
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HTS cables and fault-current limiters on power systems", IEEE Trans On Applied Superconductivity Vol 13, No 2, pp 1818-1821, 2003
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Trang 74
Current Distribution and Stability of a Hybrid
Superconducting Conductors Made of LTS/HTS
Yinshun Wang
Key Laboratory of HV and EMC Beijing, State Key Laboratory for Alternate Electrical
Power System with Renewable Energy Sources, North China Electric Power University,
Beijing, China
1 Introduction
Although having made great progress in many applications, such as high magnetic field
inserts in magnets at helium temperature and electrical engineering application in low
magnetic fields at nitrogen temperature, the high temperature superconductor (HTS) is less
commercially viable in mid- and large- scale magnets because of its high cost, low
engineering critical current density, mechanical brittleness and low n value compared with
conventional low temperature superconductors (LTS)
The superconductor with a high n value transfers quicker from superconducting state to the
normal conducting state From the standpoint of application, the transient characteristics
strongly affect its stability With a high current, in the low n value area, flux flow voltage
becomes lower than in the high n value area Generally, it is considered that quenching
occurs at a weak point, which is defined as a low Ic and low n value area However, when
such transition is observed, it is predicted that the limit current of quenching will be reached
sooner for the high n value than for the lower n value (Torii et al., 2001, Dutoit et al, 1999)
In general, the traditional superconductor has a higher n value than the Bi2223/Ag tape In
order to improve its stability, a LTS is always connected to a conventional conductor with
low resistivity and high thermal conductivity, such as copper and aluminum, which then
reduces its engineering critical current
To enhance the performance of conventional composite NbTi superconductors with large
current capacity (several tens of kA) utilized in large helical devices (LHD), a new LTS/HTS
hybrid in which HTS is used as a part stabilizer in place of low-resistivity metals, was
proposed (Wang et al, 2004; Gourab et al, 2006; Nagato et al, 2007) Thus its cryogenic stability
against thermal disturbance, steady-state cold-end recovery currents and the minimum
propagation currents (MPC) can be greatly improved because the HTS has low resistance and
current diffusion which is faster than that in a pure conventional conductor matrix
n c c
J
E E J
=
Based on the power-law model (1) fitted in range of 0.1μV/cm ≤E≤1μV/cm, LTS has a
higher n value (≥25) than HTS with a relative lower n value (<18) due to its intrinsic and
Trang 80.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Normalized current density j=J/Jc
n=1
Fig 1 Schematic E vs J plots of superconductors with nH and nL(nL>nH), normal metal with n=1
granular properties (Yasahiko et al., 1995; Rimikis et al., 2000) According to different n values between LTS and HTS shown as Fig 1, n=1 refers to the normal conductor according
to the Ohm law We firstly suggested a type of LTS/HTS hybrid composite conductor in
2004 in order to improve the stability of mid- and large scale superconducting magnets, in particular the cryo-cooled conduction superconducting magnet application
Due to the different n values between LTS and HTS, the transport current flows initially through the LTS in the hybrid conductor If there is a normal-transition in the LTS with some disturbance, the transport current will immediately transfer to the HTS, then the heat generation can be suppressed and full quench may be avoided On the other hand, since the thermal capacity of HTS is two orders of magnitude higher than that of LTS, temperature rise can be smaller in the hybrid conductor than in the LTS Therefore, the hybrid conductor can endure larger disturbances and maintain a higher transport temperature margin In this chapter, we report on the current distribution and stability of a LTS/HTS hybrid conductor
by simulation and experiment near in the range of 4.2K
2 Numerical models of current distribution and stability
2.1 Current distribution
This kind of LTS/HTS hybrid conductor consists of soldering LTS wire and HTS tape together or by directly winding several LTS wires and HTS tapes together in parallel mode The LTS/HTS superconductor is combination of LTS wire and HTS tapes shown in Fig.2
Fig 2 Schematic view of LTS/HTS hybrid conductor with combination of LTS and HTS conductors
Trang 9Current Distribution and Stability of a Hybrid Superconducting Conductors Made of LTS/HTS 77
Fig 3 Equivalent parallel circuit consisting of LTS/HTS hybrid conductor
According to its processing technology, the LTS/HTS hybrid conductor can be
approximately considered to be equivalent parallel circuit consisting of LTS, HTS and metal
matrix, shown in Fig 3
Let UH, UL, UM be the voltages of the pure HTS, LTS conductors and the normal metal
matrix including metal sheath, solder, etc, respectively; and JH, JL, JM the corresponding
branch current densities Those parameters satisfy the following equations
H n H
cH
J
J
L n L
cL
J
J
where Uc=EcL0, Ec is critical electric field (Ec =E(Ic)),and is usually equal to 1.0 μV/cm, L0 is
the length of the hybrid superconductor, nH and nL are the n indices of HTS and LTS,
respectively; JcH and JcL are their critical current densities RM, the resistance of the matrices,
is approximately given by
0
M avg
M
L R
S
ρ
where ρavg and SM are the effective resistivity and cross-sections of the matrices, estimation
of ρavg is given , shown as Fig 5
n i avg i 1 i
where fi and ρi are volumetric ratio and resistivity of i-th components in matrices except for
the LTS and HTS Since the resistivity of superconductors is more at least one order than the
metal conductor, it is reasonable to neglect the resistances of superconductors in this
chapter
Based on Eq (2) through Eq (6), following relations are found for unit length of the hybrid
conductor
Trang 104 10
H
n H
avg M cH
I
−
(8)
where IT is total transport current of hybrid conductor, and IH, IL and IM the transport
currents through HTS, LTS and matrices, respectively The temperature dependence of
critical currents of LTS and HTS in the hybrid superconductor can be approximately
expressed as polynomial expressions with constant coefficients Then the current
distribution can be simulated according to Eq (8)
2.2 Thermal stability
In order to conveniently analyze the thermal stability of the hybrid superconductor under
the adiabatic condition, the heat source, made of heater, is located at the center of conductor
with 200 mm length, and the length of heaters along the conductor is 10 mm, as
schematically shown in Fig 4
Fig 4 Schematic view of heating on the hybrid conductor
The length of any segments is much larger than their cross-section and then the physical
properties are assumed to be homogeneous over the cross-section The numerical simulation
may be simplified by choosing following one-dimensional, nonlinear, transient, heat balance
equation (Wilson, 1983; Iwasa, 1994)
0
where (γC)avg is average heat capacity (J·m-3·K-1), kavg the average thermal conductivity
(W·m-1·K-1), Q the joule heat (W) generated in hybrid conductor, G the initial heat
disturbance (W) applied by heater, V the total volume of the hybrid conductor and V0 the
volume of hybrid conductor surrounded by heater
Both of average heat capacity and thermal conductivity are estimated according to Fig.5
Assuming that a composite conductor consists of n kinds of material, the heat capacity of
each material is (γiCi) in which γi and Ci are mass density and heat specific, respectively, ki
and ρi its thermal conductivity and resistivity, the volumetric ratio of each component to