KR20160038580A - Calculating imsulation thickness of high temperature superconducting DC cable and HTS DC cable made by this method - Google Patents

Abstract

The present invention relates to a method for calculating the insulation thickness of a high temperature superconducting direct current (DC) cable. The method includes the following steps of: preparing a mini-model cable including a semi-conductive layer and an insulating layer; obtaining respective values of a DC insulation characteristic, a DC impulse insulating characteristic, and a DC polarity-inverted insulating characteristic by respectively applying a DC voltage, an impulse voltage, and a DC polarity-inverted voltage to the mini-model cable under the pressure of liquid nitrogen; calculating a DC insulation thickness, a DC impulse insulation thickness, and a DC polarity-inverted insulation thickness respectively through the values of insulation characteristics and determining the greatest thickness among the insulation thicknesses as the insulation thickness of the mini-model cable. Thus, the present invention provides an effect in that the insulation thickness value can be obtained by respectively applying the DC voltage, the DC impulse voltage, and the DC polarity-inverted voltage to the mini-model cable, and the obtained value can be applied to the thickness of a core in a laminated insulation system for a high temperature superconducting DC cable.

Description

TECHNICAL FIELD The present invention relates to a method of calculating insulation thickness of a high-temperature superconducting DC cable, and a high-temperature superconducting DC cable produced by the method.

The present invention relates to a method of estimating insulation thickness of a high-temperature superconducting DC cable, and more particularly, to a method of estimating insulation thickness of a high-temperature superconducting DC cable capable of determining insulation thickness of a cable by applying a DC voltage, an impulse voltage and a DC polarity reversal voltage to the mini- And a high-temperature superconducting DC cable produced by the method.

Recently, in a wide area such as a high temperature superconducting cable, a superconducting transformer, a superconducting fault current limiter, a superconducting magnetic energy storage, a superconducting generator, The high-temperature superconducting technology used is attracting great attention from the viewpoint of energy technology development.

In particular, superconducting cables are evaluated as being the most applicable, and research and development is actively being carried out in various countries around the world. Until now, it was mainly aimed at R & D of high-temperature superconducting AC cable. However, in recent years, in the large-capacity power transportation for long distance in the land, or the large capacity power transportation using cable in the sea floor, Cable (High temperature superconducting DC cable) is under research and development. The insulation system of high temperature superconducting DC cable is mainly developed as a complex insulation system of liquid nitrogen refrigerant and polymer. Therefore, in order to develop superconducting DC cable, it is important to establish insulation technology at cryogenic temperature in addition to insulation material technology in DC environment.

Among liquid nitrogen systems, PPLP (PolyPropylene Laminated Paper) laminated insulation systems are mainly developed for AC cables. However, multilayer PPLP insulation systems are also applicable to superconducting DC cables in terms of stability, reliability, and space charge.

However, in laminated insulation systems for high temperature superconducting DC cables, the design voltage is required to determine the core insulation thickness. In addition, unlike AC, the maximum destructive field must be obtained through insulation evaluation tests such as DC, DC polarity reversal and lightning impulse considering space charge.

Korea Patent Office Registration No. 10-0489268

Accordingly, an object of the present invention is to provide a method of calculating insulation thickness of a high-temperature superconducting DC cable capable of determining insulation thickness of a cable by applying a DC voltage, an impulse voltage and a DC polarity reversal voltage to the mini-model cable, .

The above object is achieved by a method of manufacturing a miniature model cable comprising the steps of: preparing a mini model cable including a semiconductive layer and an insulating layer; Applying a DC voltage, an impulse voltage, and a DC polarity reversal voltage to the mini model cable under a liquid nitrogen pressure to obtain a DC insulation characteristic, a DC impulse insulation characteristic, and a DC polarity reverse insulation characteristic, respectively; Calculating a DC insulation thickness, a DC impulse insulation thickness and a DC polarity reversing insulation thickness through the insulation characteristic values and determining an insulation thickness of the mini-model cable as the thickest insulation thickness among the insulation thicknesses Of the insulation thickness of the high-temperature superconducting DC cable.

The DC insulation thickness is obtained by the following equation (1)

<Formula 1>

Preferably, each of the DC impulse insulation thickness and the DC polarity reversal insulation thickness is obtained by the following equation (2).

<Formula 2>

Here, it is preferable that the insulating layer is an insulating layer in which a plurality of polypropylene laminated paper (PPLP) are laminated, and the PPLP insulating layer further includes a butt-gap which is a space formed between PPLPs.

Preferably, the applying of the DC polarity reversal voltage repeats the application of the positive DC voltage to the mini model cable, the positive DC voltage discharge, the application of the negative DC voltage, and the negative DC voltage discharge, The pressure is preferably 0.2 to 0.6 MPa.

It is still another object of the present invention to provide a method of calculating a DC insulation thickness, a DC impulse insulation thickness and a DC polarity reverse insulation thickness of a 250 kV class high temperature superconducting direct current cable including an insulation layer, Insulation superconducting cable of the present invention.

Here, the insulating layer is preferably an insulating layer in which a plurality of polypropylene laminated paper (PPLP) are laminated.

According to the structure of the present invention described above, a DC voltage, an impulse voltage and a DC polarity reversal voltage are respectively applied to a mini model cable to obtain an insulation thickness value, which is applied to insulation thickness of a core in a laminated insulation system for a HTS cable Provides a possible effect.

FIG. 1 is a flowchart of a method of calculating an insulation thickness of a high-temperature superconducting DC cable according to an embodiment of the present invention,
2 is a cross-sectional view of a mini-model cable,
3 is a graph showing the dependence of the DC breakdown voltage on the interval between the butt-gaps,
4 is a graph showing a method of applying a DC polarity reversal voltage,
5 is a graph comparing values of DC (+), Imp (+) and DC polarity reversal dielectric breakdown fields of the mini model cable,
6 is a graph showing the polarity effect of the DC dielectric breakdown field and the maximum breakdown field value of the bimodal probability distribution,
7 is a graph showing the polarity effect of the impulse dielectric breakdown field and the maximum breakdown field value of the bimodal probability distribution,
8 is a graph showing the characteristics of the number of times of DC applied voltage-polarity inversion.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for calculating insulation thickness of a high-temperature superconducting DC cable according to the present invention will be described in detail with reference to the drawings.

Mini-model cables are prepared as shown in Fig. 1 (S1).

2, a mini model cable 10 is prepared in the form of a pipe 11 made of stainless steel having a diameter of 28 mm and the inside and the outside of the pipe 11 are connected to a semi- . Here, the semiconductive layer 13 is preferably made of carbon paper.

An insulating layer 15 in which a plurality of PPLPs (PolyPropylene Laminated Paper) are stacked is added between the semiconductive layer 13 disposed inside and outside the pipe 11 as described above. Here, PPLP is a semi-synthetic paper produced by squeezing kraft paper and polypropylene, and the thickness of the sample is 110 to 120 탆.

The PPLP insulating layer 15 preferably stacks three PPLPs, and may further include a butt-gap as the case may be. Here, Butt-gap refers to the minute space between the upper layer and the lower layer, which overlaps the PPLP when the cable is made. This Butt-gqp affects the insulation strength of the cable.

The effective length for the insulation breakdown test is 50 mm, and the ends of the mini model cable 10 are reinforced and insulated for the field relaxation and creepage distance. The total length of the mini model cable 10 thus completed is 400 mm.

3 is a graph showing the dependence of the DC (+) dielectric breakdown voltage of the PPLP sheet on the gap (L) between the butt-gaps under a liquid nitrogen pressure of 0.4 MPa. As can be seen from the graph, the tendency to saturate at 8 mm or more after the breakdown voltage rises as L increases. Also, when L was 8 mm or more, insulation breakdown occurred vertically immediately through the butt-gap of the upper layer without causing surface discharge in the butt-gap of the upper and lower layers. In this experiment, the mini model DC cable has an overlap ratio of 30% according to the formula (L / w) × 100 (%) with respect to the width 25mm of the PPLP insulation paper, Respectively.

This mini model cable is attached to the bottom of the high voltage bushing installed in the cryostat, which is a device for maintaining cryogenic temperatures.

A DC voltage, an impulse voltage and a DC polarity reversal voltage are respectively applied to the mini model cable (S2).

After introducing commercial liquid nitrogen with a mini model cable, DC (direct current), impulse voltage and DC reversal voltage of polarity are respectively applied between mini model cable electrodes under a pressure of 0.4 MPa .

For high voltage application, the DC voltage was increased at a rate of 2kV / sec using a power supply with a maximum voltage of 100kV, and a standard impulse power supply with a impulse of 15kJ capacity and a 1.2 × ㎲ waveform of a maximum voltage of 400kV was used.

The DC polarity reversal voltage applying method uses a method as shown in FIG. As shown in Fig. 4, firstly positive voltage (+) DC voltage is applied for 60% of insulation breakdown for 2 minutes to confirm the breakdown voltage. After that, the high-voltage electrode is discharged for 2 minutes in the grounded state, and then the polarity is reversed again and the negative (-) voltage is applied for 2 minutes. At this time, the voltage rise was 5 kV, and the constant voltage application time was maintained for 2 minutes. When the voltage is continuously increased in the period of + or - polarity inversion of the constant voltage, the insulation breakdown value when the insulation breakdown occurs is taken as the DC polarity reverse breakdown voltage. Also, in the DC VN characteristic test, the number of polarity inversions until the insulation breakdown occurs is measured by applying a constant DC voltage to the + and - polarities periodically for 2 minutes. The dielectric breakdown test is repeatedly tested using about 5 to 10 samples under given conditions.

DC insulation property, DC impulse insulation property, and DC polarity reversal insulation property (S3).

The maximum destructive field ( _Emax ) value can be obtained from the DC insulation property, the impulse insulation property, and the DC polarity reversal insulation property measured in step S2 by using the following equations, The minimum value of the cable insulation thickness can be obtained. Since it is necessary to obtain an insulation thickness that can accommodate all three insulation characteristics, the thickness of the insulation of the mini-model cable is determined by the thickest of the three insulation thickness obtained from the three insulation characteristics.

Fig. 5 shows the results of comparing the _Emax values of the DC (+), Imp (+) and DC polarity reversal dielectric breakdown fields of the mini model cable under a pressure of 0.4 MPa. Table 1 shows the average dielectric breakdown field and standard deviation of the mini model cable.

Voltage Average dielectric breakdown field
(kV / mm) Maximum dielectric breakdown field
(kV / mm) DC (+) 131 112 Imp (+) 130 105 DC polarity reversal 117.4

Here, it can be seen that the voltage is higher in the order of DC (+), Imp (+) and DC polarity reverse breakdown electric field. The DC (+), Imp (+) and DC polarity reversal average dielectric breakdown fields are 131, 130 and 117.4 kV / mm, respectively. Particularly, the lower DC polarity reverse breakdown voltage means that the electric field is distorted by the electric field of the space charge accumulated in the PPLP due to the DC polarity reversal, thereby causing the insulation breakdown at a low voltage. Therefore, this should be considered in the insulation design of high temperature superconducting DC cables. In addition, the standard deviation (?) Of all the dielectric breakdown fields is 3.1 to 4.5%.

A. Determine mini-model cable insulation thickness through DC insulation properties

6 shows the polarity effect of the DC dielectric breakdown field of the mini model cable under the liquid nitrogen pressure of 0.4 MPa and the maximum breakdown field (E _max ) value of 1% and the Weibull probability distribution. The liquid nitrogen pressure dependence of the DC breakdown voltage was measured under a liquid nitrogen pressure of 0.4 MPa because it saturates at 0.4 MPa or higher, similar to the case of AC. As can be seen from the graph, the maximum breakdown field values of DC (+) and DC (-) are 112 kV / mm and 115 kV / mm, respectively.

On the other hand, in the case of AC, the n value of polyethylene, kraft paper, etc. in liquid nitrogen is reported to be in the range of 14 to 100. Therefore, the K ₁ = 1.8 ask the aged deterioration coefficient K1 by the following equation 1, if the driving time of 40 years.

<Expression 1-1>

Since the dielectric breakdown voltage of DC (-) is higher than that of DC (+), insulation design of high temperature superconducting DC cable adopts DC (+) insulation breakdown voltage. The calculation of the insulation thickness of the DC cable is based on the modified cylindrical equation as shown in Equation 2 for temperature dependence by resistivity (ρ) and electric field dependence.

<Formula 1>

Here, E is the design electric field (kV / mm), V _DC is the DC target voltage (kV), r ₁ is the inner diameter (mm) of the insulating layer, and r ₂ is the outer diameter (mm) of the insulating layer.

The DC target voltage VDC in Equation 1 is expressed by the following Equation 1-2.

<Expression 1-2>

Here, U ₀ denotes an operating voltage (250 kV), K ₁ denotes an aged deterioration coefficient (1.8) obtained from Equation 1-1, K ₂ denotes a temperature coefficient (1.0), and K ₃ denotes an unfavorable factor do.

The DC target voltage based on Equation 1-2 is calculated as 495 kV. The DC design electric field is expressed by the following equations 1-3.

<Equation 1-3>

Here, E _max is the maximum breakdown field value (112 kV / mm), and K ₄ is the design acceptance value (1.2). The DC design field based on Equation 1-3 was calculated to be 99.2 kV. Therefore, for r ₁ = 14 mm, the DC insulation thickness of the 250 kV class high-temperature superconducting cable is 5.4 mm using Equation 1.

B. Determine mini model cable insulation thickness through DC impulse insulation properties

7 shows the polarity effect of the impulse breakdown field and the maximum breakdown field (E _max ) of the 1% Weibull probability distribution. As shown in the graph, the dielectric breakdown voltage of Imp (-) is higher than that of Imp (+) and the E _max of Imp (-) and Imp (+) are 105 kV / mm and 108 kV / mm, respectively.

<Formula 2>

DC superposition Polarity Impulse insulation thickness calculation is based on the cylindrical equation as shown in Equation 2 based on the dielectric constant (ε). Imp (+) dielectric breakdown field is applied because the dielectric breakdown field of Imp (-) is higher than that of Imp (+).

The impulse target voltage V _imp is expressed by the following Equation 2-1.

<Formula 2-1>

Where U ₀ is the operating voltage (250kV), K ₅ is the overvoltage factor (2.5), and K ₆ is the safety factor for the uncertainty factor (1.1). The Imp target voltage (V _imp ) based on Equation 2-1 was calculated to be 688 kV.

The Imp design field is expressed by the following Equation 2-2.

<Formula 2-2>

Here, E _max is the maximum Imp breakdown field (105 kV), and K ₄ is the tolerance (1.2). Therefore, the DC design electric field based on Equation 2-2 is calculated as 94.8 kV.

For r ₁ = 14, the DC overlap of a 250 kV class high-temperature superconducting cable. The reverse insulation polarity Imp insulation thickness is 10.5 mm using Equation 2.

C. Determine mini-model cable insulation thickness through DC polarity reversal insulation characteristics

The applied voltage-polarity reversal frequency (V-N) characteristic is generally Vn · N = const. 8 shows the DC application voltage-polarity reversal frequency characteristics of the mini model cable at 0.4 MPa atmospheric pressure. In this experiment, the relation between the applied voltage and the number of polarity inversion is expressed by Equation 3-1.

<Formula 3-1>

The number N of times of DC polarity inversion increases as the applied voltage decreases, and the DC breakdown field E1000 = 104 kV / mm at N = 1000 times. However, n is a repetitive deterioration coefficient of n = 62.5. In this study, the repeatability deterioration index n = 50 in consideration of safety. Therefore, when the polarity reversal is carried out for 6 times for 8 days, the repetitive deterioration coefficient is expressed by the following Equation 3-2.

<Formula 3-2>

However, considering CIGRE recommendation, K ₇ = 1.45 × 1.03 = 1.49. The DC polarity inversion target voltage (V _RDC ) based on Equation 3-2 was calculated as 411 kV, and the DC polarity reversal design electric field based on Equation 3-3 was calculated as 86.6 kV / mm. Therefore, in case of r1 = 14mm, DC polarity reversal dnk of 250kV high-temperature superconducting cable. Since the polarity changes continuously, it uses general cylindrical equation like impulse in general, so insulation thickness is 5.6mm .

The comprehensive results of the target voltage and design field of the PPLP insulated high-temperature superconducting DC cable are shown in Table 2.

Voltage Target voltage
(kV) Design field
(kV / mm) Insulation thickness
(mm) DC 495 93 5.8 Impulse 688 88 10.5 DC polarity reversal 411 87 5.6

The insulation thickness of the 20 kV high-temperature superconducting cable core obtained from the above DC, Imp and DC polarity reversal experiments is 10.5 mm, which is the insulation thickness obtained from the Imp test obtained as the result of the thickest value.

(+) And DC (-) of a mini model cable are 112 and 115 kV / mm, respectively, and the Imp (+) and Imp (- Were measured at 105 and 108 kV / mm, respectively. The number N of times of DC polarity inversion is equal to the DC breakdown field E1000 = 104 kV / mm at N = 1000 times. Through DC, Imp, and DC polarity reversal experiments, insulation design was studied for each design variable. The insulation thickness of a 250kV class high temperature superconducting DC cable core insulated with PPLP was estimated to be 10.5mm, the thickest of the three.

10: Mini model cable
11: pipe
13:
15: Insulating layer

Claims (9)

A method of calculating an insulation thickness of a high-temperature superconducting DC cable,
Preparing a mini model cable including a semiconductive layer and an insulating layer;
Applying a DC voltage, an impulse voltage, and a DC polarity reversal voltage to the mini model cable under a liquid nitrogen pressure to obtain a DC insulation characteristic, a DC impulse insulation characteristic, and a DC polarity reverse insulation characteristic, respectively;
Calculating a DC insulation thickness, a DC impulse insulation thickness and a DC polarity reversing insulation thickness through the insulation characteristic values and determining an insulation thickness of the mini-model cable as the thickest insulation thickness among the insulation thicknesses Method of calculating insulation thickness of high temperature superconducting DC cable. The method according to claim 1,
Wherein the DC insulation thickness is obtained by the following equation (1).
<Formula 1>

The method according to claim 1,
Wherein each of the DC impulse insulation thickness and the DC polarity reversal insulation thickness is obtained by the following equation (2).
<Formula 2>

The method according to claim 1,
Wherein the insulation layer is an insulation layer in which a plurality of polypropylene laminated paper (PPLP) are laminated. 5. The method of claim 4,
Wherein the PPLP insulation layer further comprises a butt-gap which is a space between the PPLPs. The method according to claim 1,
The application of the DC polarity reversal voltage,
Applying a positive DC voltage to the mini model cable, repeating the positive DC voltage discharge, the negative DC voltage, and the negative DC voltage discharge. The method according to claim 1,
Wherein the liquid nitrogen pressure is 0.2 to 0.6 MPa. In a high temperature superconducting DC cable,
The DC insulation thickness, the DC impulse insulation thickness and the DC polarity reverse insulation thickness of the 250 kV class high temperature superconducting DC cable including the insulation layer are calculated and the insulation thickness of the cable is applied as the insulation thickness of the cable High temperature superconducting DC cable. 9. The method of claim 8,
Wherein the insulation layer is an insulation layer in which a plurality of polypropylene laminated paper (PPLP) are laminated.

Images