- Email: [email protected]
Comparative study of high voltage bushing designs suitable for apparatus containing cryogenic helium gas
Cryogenics 57 (2013) 12–17
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Comparative study of high voltage bushing designs suitable for apparatus containing cryogenic helium gas H. Rodrigo a,⇑, L. Graber a, D.S. Kwag b, D.G. Crook a, B. Trociewitz a a b
Center for Advanced Power Systems, Florida State University, 2000 Levy Avenue, Tallahassee, FL 32310, USA Department of Fire Safety, Kyungil University, 33 Buho-ri, Hayang-eup, Gyeongsan-si, 712-701 Korea
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 14 February 2013 Accepted 24 April 2013 Available online 3 May 2013 Keywords: High voltage Bushing Helium gas Cryogenics Partial discharge
a b s t r a c t The high voltage bushing forms a critical part of any termination on cables, transformers and other power system devices. Cryogenic entities such as superconducting cables or fault current limiters add more complexity to the design of the bushing. Even more complex are bushings designed for superconducting devices which are cooled by high pressure helium gas. When looking for a bushing suitable for dielectric cable tests in a helium gas cryostat no appropriate device could be found that fulfilled the criterion regarding partial discharge inception voltage level. Therefore we decided to design and manufacture a bushing in-house. In the present work we describe the dielectric tests and operational experience on three types of bushings: One was a modified commercially available ceramics feed through which we adopted for our special need. The second bushing was made of an epoxy resin, with an embedded copper squirrel cage arrangement at the flange, extending down about 30 cm into the cold end of the bushing. This feature reduced the electric field on the surface of the bushing to a negligible value. The third bushing was based on a hollow body consisting of glass fiber reinforced polymer and stainless steel filled with liquid nitrogen. The measurements showed that the dielectric quality of all three bushings exceeded the requirements for the intended purpose. The partial discharge (PD) data from these studies will be used for the design and fabrication of a cable termination for a specialized application on board a US Navy ship. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Unlike terrestrial power systems where space is not a serious concern high voltage power devices on board ships have severe space restrictions. These concerns require novel solutions such as superconducting cables. Since liquid cryogens were not acceptable for the Navy due to potential asphyxiation hazards, gaseous helium was chosen as the cooling medium for the superconducting cable. Gaseous helium is a very weak dielectric at ambient temperature and thus is not one that is in general usage for power cables. However, as the density is increased due to low temperature and elevated pressure there is very marked improvement in its performance as a suitable dielectric [1,2]. The thermal properties and in particular the cooling performance of gaseous helium is also inferior to liquid nitrogen. Solid dielectric materials form an important integral part of many fixtures used in high voltage systems. The failure of the system is often due to deterioration of the solid dielectric [3]. Other researchers have reported using some novel techniques in designing bushings for high temperature superconducting (HTS) cables. In one case they had transitioned from liquid nitrogen (77 K) to ambient by traversing through two gaseous
⇑ Corresponding author. Tel.: +1 978 250 1507. E-mail address: [email protected] (H. Rodrigo). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.04.002
chambers, one with cold gaseous nitrogen and the next, being SF6 [4]. The other case was similar in construction where the transition was by going through two chambers with cold gaseous nitrogen and finally warm nitrogen [5]. Work that is of interest to us has been reported [6,7] at the Large Hadron Collider (LHC) in CERN and at the ATLAS project in CERN [8]. There are some similarities between the present work which is designed for a cable termination rated at 1 kV, 3 kA (3 MVA). In the cases reported at CERN the current ratings ranged from 600 A to 13 kA for the LHC and 20.5 kA for the ATLAS project. However, unlike the present work the operating voltages were of the order of 5 V. The test voltages for insulation integrity for the LHC case were between 1.5 kV and 3.1 kV for current leads rated at 600 A and 13 kA respectively. For the ATLAS project the maximum test voltage was 2 kV at rated current. In both these cases at CERN the current leads were of the superconducting types where the temperature transitioned from 4.2 K in liquid helium to ambient (293 K) with an intermediate enclosure of gaseous helium at 20 K. The end that is immersed in liquid helium is of the low temperature superconducting (LTS) type and the end close to the warm end is of the HTS type. The pressure level for the LHC case is 0.35 MPa and for the ATLAS project it is 3.0 MPa. However, we were looking for a less complex solution which does not include additional gas spaces. Since no suitable bushing was commercially available, we decided to design a bushing in-house resulting in three different designs.
H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
13
The bushings described in this article have been specifically built to be used in the high pressure cryostat as described in Ref. [9]. The pressure cryostat (Fig. 1) was used for testing 1 m long sample cables in helium gas. Liquid helium (3) was injected in order to establish a 40–70 K helium environment inside the pressure cryostat. Gaseous helium was used to increase the pressure inside the cryostat. A heat exchanger (2) was used in order to pre-cool the helium gas. The pressure level was adjusted throughout the experiments, typically between 0.45 MPa and 2.17 MPa. A liquid nitrogen jacket (4) reduced heat influx by radiation. The cryostat featured a 76 mm wide neck (5). The helium gas in the neck section is substantially warmer than in the main compartment of the cryostat due to heat influx through the main flange, port flanges and the bushing itself. The reduced density leads to reduced dielectric strength. Therefore, the neck area is the most critical zone for the bushing. The sample cables to be tested in the cryostat were expected to exhibit PD inception voltages (PDIV) of up to 10 kV (RMS) at the intended pressure and temperature conditions. Therefore, any bushing to be used for these experiments required a PDIV in excess of 10 kV (and preferably higher for future applications). 2. Description of the bushings 2.1. Ceramics feed through (‘‘Bushing A’’) Initially we purchased a ceramics feed through manufactured by SST of New York (Model FA20855CA, rated at 100 kV, 10 A, 2.17 MPa). When we tested this feed through in the cryostat under helium pressure of 2.17 MPa for partial discharge (PD) its performance was not suitable for the application it was intended, which was to study PD behavior of model cables. We then set out to modify this feed through. Using Finite Element Analysis (COMSOL Multiphysics 4.1) we determined the electric field distribution along the length of this feed through and set about modifying it. The modified feed though is shown in Fig. 2. The ceramics part of this feed through is hollow (1), exposing the conductor to helium gas at room temperature. The low density of the gas around the conductor led to corona activity at voltages above a few kilovolts. The solution was to fill that volume with epoxy resin of type Stycast 2850FT. The feed through was permanently mounted onto a larger flange (DN100) which featured a smooth transition of the opening to a larger diameter (3). The cast was extended 400 mm in order to provide more than adequate creepage distance on the cast [10]. This was also long enough to clear the critical neck section of the cryostat. The 25 mm diameter of the cast allowed for
Fig. 2. Modified ceramic feed through (‘‘Bushing A’’). The original ceramic part (1), flange DN100 (2), copper ring and truncated epoxy cone (3), aluminum conductor (4), terminating sphere (5).
more than adequate clearance from the earthed body of the cryostat. This distance could have been shortened by incorporating sheds along some portion of its length, however, it was deemed unnecessary as the present length did not interfere with the planned experiments. The result was that we were able to use it for our intended purpose. It is of utmost importance that there are no voids in the epoxy cast. Therefore the epoxy was degassed in vacuum before pouring it into the feed through. Immediately afterwards the filled feed through was put in a vacuum chamber for about five minutes. This procedure reduced air in the cast even further. For the curing process, the chamber was pressurized with 2 MPa nitrogen gas. Any present micro voids would shrink even more and hold higher internal pressure, increasing PD inception voltage level. This elaborate procedure guaranteed best possible quality of the epoxy cast. 2.2. Large epoxy bushing (‘‘Bushing B’’)
Fig. 1. Bushing (1), high pressure gaseous helium inlet via heat exchanger (2), liquid helium inlet (3), liquid nitrogen inlet (4), and neck section of cryostat (5).
In order to allow for experiments at higher voltages in the pressure cryostat, a new bushing was designed. The goal of this new bushing was to guarantee zero electric field in the critical neck section of the cryostat. It had to be substantially larger in most dimensions. A ceramic feed through (as in A) has a small diameter conductor which increases the electric field, severely restricting the flexibility of reducing the electric field to an optimal value. Therefore we decided to design our own bushing without a ceramic feed through. This required a full mold to accommodate all parts of the bushing. The mold was CNC machined of a rigid plastic foam (FR4500, supplied by General Plastics, Tacoma, Washington). As shown in Fig. 3 the material was machined to a pattern in two halves to take a central conductor mounted on a DN 100 stainless steel flange with a squirrel cage (1) made of copper. The two halves of the mold were coated appropriately, bolted together and placed
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 3. Steps taken in the fabrication of Bushing B showing the prepared mold (1), the mold in a bag during the cast process (2), the vacuum pump (3), and the supply of epoxy resin (4).
inside a thick plastic bag with tubes (2). A vacuum pump (3) was connected and the air was evacuated from the bag until a vacuum gauge read about 0.1 Pa. The prepared epoxy resin of type Stycast 2850FT (4) was now sucked into the mold until the whole mold was filled with the epoxy resin. It was allowed to stand for two days and the mold removed. The finished bushing is shown in Fig. 4. Fig. 4 shows finished Bushing B with epoxy cast (2), a central conductor made of aluminum, terminated in a brass cylinder (1). Due to a leak in our mold the cast did not extend all the way up to the threads but stopped approximately 10 cm below (3). To even out the high electric field stress at the point at which the bushing bolted onto the main pressure cryostat we incorporated a squirrel cage arrangement made of copper (5). This part of the bushing was inside the cryostat, which experienced the high gaseous helium pressure and high temperature gradient from ambient temperature at the flange to 40–70 K at the lower end. The bushing was set up as was the case with Bushing A, by inserting it into the pressure cryostat and securely fixing it with the DN100 flange (4). A 50 mm diameter sphere (6) terminated the bushing.
this termination the conductor carrying current transitions from 50 K at a pressure of 2.17 MPa in gaseous helium to ambient temperature and pressure. In order to have a gradual gradation of temperature the bushing goes through an enclosure of liquid nitrogen. The liquid nitrogen acts as a heat intercept and therefore keeps the surface of the insulator body cold. Bushing C comprised a ceramic feed through (11) terminated in a brass cylinder (12). The feed through was welded onto one end of a stainless steel barrel (8), which was inside the cryostat and bolted onto it by a DN100 flange (7). Liquid nitrogen was carried into the barrel by the inlet tube (6). The second tube (5) was capped. The flange (4) DN40 carried a G10 tube (2), which had two holes drilled in it (3) for the liquid and gaseous nitrogen to escape. The central conductor was terminated in a brass cylinder (1). At the other end of the conductor inside the barrel was a plug (9) for assembling and disassembling the bushing. Between the plug and the end of the feed through was a spring (10) which allowed for expansion and contraction of the conductor. Three E type thermocouples were fixed onto the outer surface of the stainless steel barrel (8).
2.3. Liquid nitrogen bushing (‘‘Bushing C’’)
3. Experimental characterization of the bushings
The third bushing, C, is shown in Fig. 5. The basic idea was to use a relatively small ceramics feed through whose surface is at a temperature of 77 K. This guarantees that helium gas in contact with its surface has very high density and therefore high dielectric strength [1,2]. This can be achieved by an internal volume of liquid nitrogen. The secondary aim of this particular bushing – besides the use in the pressure cryostat – has been to utilize the experience and knowledge to design and fabricate terminations for a 1 kV, 3 MVA power cable. A patent for this principle is pending [11]. In
A full and detailed description of the equipment used and the methodology followed has been given in Ref. [9]. In the present work we substituted the cable and feed through used in [9] with each of the bushings shown in Figs. 2, 4 and 5. When a bushing was put in place inside the high pressure cryostat the output of the partial discharge (PD) measurement system was connected to it. The input voltage to the transformer was switched on and the voltage was gradually increased in steps of about 500 V. At each voltage level the peak level of apparent charge of the PD was
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 4. Photograph and drawing of Bushing B showing the aluminum conductor with brass cylinder (1), the epoxy body with sheds (2), the shortened top part due to vacuum problem (3), flange to pressure cryostat (4), field shaping insert (‘‘squirrel cage’’) (5), and the bottom sphere (6).
Fig. 5. Photograph and drawing of Bushing C showing brass cylinder (1), G10 top part (2), vent holes (3), flange to connect to bottom part (4), unused/blocked vent line (5), fill line (6), flange to pressure cryostat (7), stainless steel barrel (8), conductor connection (9), spiral (10), hollow ceramics feed through (11), and bottom brass cylinder (12).
100
(3)
Bushing B Bushing C
90
(4)
(2)
80
Apparent charge [pC]
measured. This procedure was common for all three bushings. For tests on Bushing A the pressure cryostat was prepared in a manner identical to that given in [9] for the cable tests, except that in this case there was no cable in place. The bushing was terminated in a 50 mm diameter aluminum sphere. Thermocouples were fixed onto a stainless steel tube inside the cryostat which was the supply line for gaseous helium. The thermocouples were spaced 50 cm apart in line with the top, middle and bottom of the cable had it been in place. This allowed to set the required helium temperature surrounding the lower end of the bushing by alternately admitting liquid helium and high pressure helium gas into the cryostat. For Bushing C the procedure was somewhat different from that given above. It was installed in the cryostat with a brass cylinder (11) terminating the feed though. The preparation was as described for Bushings A and B. However, in this case we did not use liquid helium to cool the cryostat and the thermocouples were affixed onto the outside of the stainless steel barrel (8) which was at earth potential. A Dewar of liquid nitrogen was connected to the inlet tube (6) of the bushing and liquid nitrogen was admitted into the barrel until the liquid squirted out of the two vent holes (3) in the hollow G10 cylinder (2). The temperature readings on all three thermocouples were 77 K indicating that the barrel was always full of liquid nitrogen. The cryostat was pressurized with gaseous helium up to 1.48 MPa. The power supply was connected and the experiment was conducted as was the case before. However, we were only able to do four different pressure levels. At 1.12 MPa and above the external part of the bushing broke down due a surface flashover before it could reach a higher level of PD within the bushing. Fig. 6 shows the level of apparent charge as a function of applied voltage (RMS) for Bushings B and C at each pressure level. It is
(5)
70
(1)
60 50
(2)
(3)
(1)
(4)
40 30
(6)
20 10 0
0
5
10
15
20
25
30
35
40
Voltage [kV] Fig. 6. The variation of apparent charge as a function of voltage at different levels of pressure. Labels (1) through (6) correspond to pressure levels of 0.45 MPa through 2.17 MPa in increments of 0.34 MPa.
clearly visible that Bushing B performs slightly better than Bushing C. The higher background noise level of approximately 5 pC with Bushing B could be attributed to external sources. A more suitable method to compare PD data is to look at the voltage at which PD initiates. We define partial discharge inception voltage (PDIV) in Fig. 7, where the initiation voltage is taken as the value of voltage when the PD exceeds 5 pC above the background value.
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 7. Definition of PDIV as used in the present work. The threshold value QThres was defined as 5 pC above background PD activity QBG. The value of PDIV UPDIV was interpolated between the last value below QThres (n) and the first value above QThres (n+1).
40 Bushing A Bushing B Bushing C
35
PDIV [kV]
30 25
this critical part of the bushing. We have been helped significantly in the final design of the cable termination. In the termination the lower part of the bushing is in liquid nitrogen and uppermost part close to the main flange containing the squirrel cage is in gaseous nitrogen [11]. The dielectric strength of liquid nitrogen is very much stronger than that of gaseous nitrogen, thus we have gained markedly in reducing the chances of the bushing failing under operating conditions of the cable assembly. Between about 0.4 MPa and 1.2 MPa there was linear increase in the PDIV curve at approximately 15 kV (MPa) 1. After about 1.5 MPa it leveled off reaching just above 35 kV at the highest pressure of 2.17 MPa. Bushing C was one that we made and tested not because it would be used in the conventional practical sense, but rather as a means of studying an entity which will be used under special circumstances in the terminations of a superconducting cable [11]. Essentially, our goal was to determine the partial discharge characteristics of a solid insulator surface exposed to a high pressure gaseous helium environment at a temperature of the order of 77 K. The ceramic feed through welded onto a stainless steel barrel (Fig. 5) has given us very good results, as shown in Fig. 8. Had it not been for the fact that there was external breakdown at the higher gas pressures it might have surpassed the performance of the epoxy Bushing B.
20
5. Conclusions 15 10 5 0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Pressure [MPa] Fig. 8. PDIV of all three bushings in comparison.
4. Results and discussion Fig. 8 shows PDIV of all three bushings as a function of helium pressure level. The values are taken from the same measurements shown in Fig. 6 but processed according to our PDIV definition as in Fig. 7. The modified ceramic feed through (Bushing A) was adequate for the for the purpose of testing model cables within the cryostat, and its performance characteristic shows that there is steady but modest improvement up to a pressure of about 1.5 MPa. After that it levels off and reaches about 12 kV at the highest pressure of 2.17 MPa thus satisfying the criterion of PDIV higher than 10 kV. The design value of the electric field at the surface of this bushing with the corona ring, epoxy filling and truncated epoxy cone was 0.5 kV mm 1, when the voltage was set at 9 kV. This calculation was done for conditions in helium gas at room temperature (293 K). However, the surface conditions prevailing under the experimental conditions with higher density of helium was considerably better. The temperature on average at the top section of Bushing A, in the neck part of the cryostat, was about 230 K, so the density of helium at 2.17 MPa is 4.44 kg m 3 whereas the density at 100 kPa and 293 K it is only 0.166 kg m 3[12]. It has been shown that the dielectric characteristics of gaseous helium improves significantly with increase in density [1,13]. The large epoxy bushing (Bushing B) gave the best performance as shown in Fig. 8; here we had the advantage of making a bushing from scratch which was suitable for the cryostat that has some limitations as indicated in Section 2.1. We incorporated the ring and squirrel cage arrangement which gave no surface electric field in
We have shown three different high voltage bushings designed for operation in our high pressure helium cryostat. They all fulfill the requirements regarding dielectric strength and in particular showed very low partial discharge activity in the intended voltage range. The results showed that minimizing the electric field at the surface of a bushing, in particular one where there is limited clearance, improves the dielectric performance significantly. Helium gas density is a critical parameter with regard to surface discharges along epoxy surfaces in such an environment. Keeping the surface of the bushing at very low temperature helps to reduce the required creepage distance. Ceramics feed throughs designed for operation with liquid nitrogen are not suitable for helium gas. However, marked improvements can be achieved by reducing the electric field stress at critical points in a feed through and filing the hollow ceramic section with a cryogenically suitable epoxy. The dielectric and cryogenic properties of liquid nitrogen can be used effectively to transition from cold compressed gaseous helium to finally ambient, as would be the case, in terminations of cryogenic high pressure power cables cooled by gaseous helium.
Acknowledgements The authors would like to thank Steve L. Ranner for excellent welding and brazing work on our bushings. This Project is sponsored in part by the US Office for Naval Research (ONR) under Grant Number N00014-02-1-0623.
References [1] Meats RJ. Pressurized helium breakdown at very low temperatures. Proc IEE 1972;119(6):760–6. [2] Fallou B et al. Dielectric breakdown of gaseous helium at very low temperatures. Cryogenics 1970;10:142–8. [3] Kosaki M et al. Solid insulation and its deterioration. Cryogenics 1998;38(11):1095–104. [4] Shimonosono T et al. Development of a termination for the 77 kV class high TC superconducting power cable. IEEE Trans Power Del 1997;12(1):33–8. [5] Kwag DS et al. The electrical insulation characteristics for a HTS cable termination. IEEE Trans Appl Supercond 2006;16(2):1618–21.
H. Rodrigo et al. / Cryogenics 57 (2013) 12–17 [6] Ballarino A. High temperature superconducting current leads for the Large Hadron Collider. LHC project report 337 – presented at 4th EUCAS 1999, September 14–17, Barcelona, Spain. [7] Turtu S et al. Cryogenic test of high temperature superconducting current leads at ENEA, LHC project report 865 – presented at the CEC-ICMC’05 Conference 29 August–2 September 2005, Keystone, Colorado, USA. [8] Dudarev AV. 20.5 kA current leads for ATLAS barrel toroid superconducting magnets. IEEE Trans Appl Supercond 2002;12(1):1289–92. [9] Rodrigo H et al. Electrical and thermal characterization of a novel high pressure gas cooled DC power cable. Cryogenics 2012;52:310–4.
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[10] Tuncer E et al. Electrical insulation characteristics of glass fiber reinforced resins. IEEE Trans Appl Supercond 2009;19(3):2359–62. [11] Graber L et al. Cable termination for high voltage power cables cooled by a gaseous cryogen. 10 May 2012, Prov. Pat No: 61/645,304. [12] Mann DB. The thermodynamic properties of helium from 3 to 300 K between 0.5 and 100 atmospheres. Nat Bureau of standards Technical Note 154, 1962 [13] Gerhold J. Dielectric breakdown of helium at low temperatures. Cryogenics 1972:370–6.
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Comparative study of high voltage bushing designs suitable for apparatus containing cryogenic helium gas H. Rodrigo a,⇑, L. Graber a, D.S. Kwag b, D.G. Crook a, B. Trociewitz a a b
Center for Advanced Power Systems, Florida State University, 2000 Levy Avenue, Tallahassee, FL 32310, USA Department of Fire Safety, Kyungil University, 33 Buho-ri, Hayang-eup, Gyeongsan-si, 712-701 Korea
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 14 February 2013 Accepted 24 April 2013 Available online 3 May 2013 Keywords: High voltage Bushing Helium gas Cryogenics Partial discharge
a b s t r a c t The high voltage bushing forms a critical part of any termination on cables, transformers and other power system devices. Cryogenic entities such as superconducting cables or fault current limiters add more complexity to the design of the bushing. Even more complex are bushings designed for superconducting devices which are cooled by high pressure helium gas. When looking for a bushing suitable for dielectric cable tests in a helium gas cryostat no appropriate device could be found that fulfilled the criterion regarding partial discharge inception voltage level. Therefore we decided to design and manufacture a bushing in-house. In the present work we describe the dielectric tests and operational experience on three types of bushings: One was a modified commercially available ceramics feed through which we adopted for our special need. The second bushing was made of an epoxy resin, with an embedded copper squirrel cage arrangement at the flange, extending down about 30 cm into the cold end of the bushing. This feature reduced the electric field on the surface of the bushing to a negligible value. The third bushing was based on a hollow body consisting of glass fiber reinforced polymer and stainless steel filled with liquid nitrogen. The measurements showed that the dielectric quality of all three bushings exceeded the requirements for the intended purpose. The partial discharge (PD) data from these studies will be used for the design and fabrication of a cable termination for a specialized application on board a US Navy ship. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Unlike terrestrial power systems where space is not a serious concern high voltage power devices on board ships have severe space restrictions. These concerns require novel solutions such as superconducting cables. Since liquid cryogens were not acceptable for the Navy due to potential asphyxiation hazards, gaseous helium was chosen as the cooling medium for the superconducting cable. Gaseous helium is a very weak dielectric at ambient temperature and thus is not one that is in general usage for power cables. However, as the density is increased due to low temperature and elevated pressure there is very marked improvement in its performance as a suitable dielectric [1,2]. The thermal properties and in particular the cooling performance of gaseous helium is also inferior to liquid nitrogen. Solid dielectric materials form an important integral part of many fixtures used in high voltage systems. The failure of the system is often due to deterioration of the solid dielectric [3]. Other researchers have reported using some novel techniques in designing bushings for high temperature superconducting (HTS) cables. In one case they had transitioned from liquid nitrogen (77 K) to ambient by traversing through two gaseous
⇑ Corresponding author. Tel.: +1 978 250 1507. E-mail address: [email protected] (H. Rodrigo). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.04.002
chambers, one with cold gaseous nitrogen and the next, being SF6 [4]. The other case was similar in construction where the transition was by going through two chambers with cold gaseous nitrogen and finally warm nitrogen [5]. Work that is of interest to us has been reported [6,7] at the Large Hadron Collider (LHC) in CERN and at the ATLAS project in CERN [8]. There are some similarities between the present work which is designed for a cable termination rated at 1 kV, 3 kA (3 MVA). In the cases reported at CERN the current ratings ranged from 600 A to 13 kA for the LHC and 20.5 kA for the ATLAS project. However, unlike the present work the operating voltages were of the order of 5 V. The test voltages for insulation integrity for the LHC case were between 1.5 kV and 3.1 kV for current leads rated at 600 A and 13 kA respectively. For the ATLAS project the maximum test voltage was 2 kV at rated current. In both these cases at CERN the current leads were of the superconducting types where the temperature transitioned from 4.2 K in liquid helium to ambient (293 K) with an intermediate enclosure of gaseous helium at 20 K. The end that is immersed in liquid helium is of the low temperature superconducting (LTS) type and the end close to the warm end is of the HTS type. The pressure level for the LHC case is 0.35 MPa and for the ATLAS project it is 3.0 MPa. However, we were looking for a less complex solution which does not include additional gas spaces. Since no suitable bushing was commercially available, we decided to design a bushing in-house resulting in three different designs.
H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
13
The bushings described in this article have been specifically built to be used in the high pressure cryostat as described in Ref. [9]. The pressure cryostat (Fig. 1) was used for testing 1 m long sample cables in helium gas. Liquid helium (3) was injected in order to establish a 40–70 K helium environment inside the pressure cryostat. Gaseous helium was used to increase the pressure inside the cryostat. A heat exchanger (2) was used in order to pre-cool the helium gas. The pressure level was adjusted throughout the experiments, typically between 0.45 MPa and 2.17 MPa. A liquid nitrogen jacket (4) reduced heat influx by radiation. The cryostat featured a 76 mm wide neck (5). The helium gas in the neck section is substantially warmer than in the main compartment of the cryostat due to heat influx through the main flange, port flanges and the bushing itself. The reduced density leads to reduced dielectric strength. Therefore, the neck area is the most critical zone for the bushing. The sample cables to be tested in the cryostat were expected to exhibit PD inception voltages (PDIV) of up to 10 kV (RMS) at the intended pressure and temperature conditions. Therefore, any bushing to be used for these experiments required a PDIV in excess of 10 kV (and preferably higher for future applications). 2. Description of the bushings 2.1. Ceramics feed through (‘‘Bushing A’’) Initially we purchased a ceramics feed through manufactured by SST of New York (Model FA20855CA, rated at 100 kV, 10 A, 2.17 MPa). When we tested this feed through in the cryostat under helium pressure of 2.17 MPa for partial discharge (PD) its performance was not suitable for the application it was intended, which was to study PD behavior of model cables. We then set out to modify this feed through. Using Finite Element Analysis (COMSOL Multiphysics 4.1) we determined the electric field distribution along the length of this feed through and set about modifying it. The modified feed though is shown in Fig. 2. The ceramics part of this feed through is hollow (1), exposing the conductor to helium gas at room temperature. The low density of the gas around the conductor led to corona activity at voltages above a few kilovolts. The solution was to fill that volume with epoxy resin of type Stycast 2850FT. The feed through was permanently mounted onto a larger flange (DN100) which featured a smooth transition of the opening to a larger diameter (3). The cast was extended 400 mm in order to provide more than adequate creepage distance on the cast [10]. This was also long enough to clear the critical neck section of the cryostat. The 25 mm diameter of the cast allowed for
Fig. 2. Modified ceramic feed through (‘‘Bushing A’’). The original ceramic part (1), flange DN100 (2), copper ring and truncated epoxy cone (3), aluminum conductor (4), terminating sphere (5).
more than adequate clearance from the earthed body of the cryostat. This distance could have been shortened by incorporating sheds along some portion of its length, however, it was deemed unnecessary as the present length did not interfere with the planned experiments. The result was that we were able to use it for our intended purpose. It is of utmost importance that there are no voids in the epoxy cast. Therefore the epoxy was degassed in vacuum before pouring it into the feed through. Immediately afterwards the filled feed through was put in a vacuum chamber for about five minutes. This procedure reduced air in the cast even further. For the curing process, the chamber was pressurized with 2 MPa nitrogen gas. Any present micro voids would shrink even more and hold higher internal pressure, increasing PD inception voltage level. This elaborate procedure guaranteed best possible quality of the epoxy cast. 2.2. Large epoxy bushing (‘‘Bushing B’’)
Fig. 1. Bushing (1), high pressure gaseous helium inlet via heat exchanger (2), liquid helium inlet (3), liquid nitrogen inlet (4), and neck section of cryostat (5).
In order to allow for experiments at higher voltages in the pressure cryostat, a new bushing was designed. The goal of this new bushing was to guarantee zero electric field in the critical neck section of the cryostat. It had to be substantially larger in most dimensions. A ceramic feed through (as in A) has a small diameter conductor which increases the electric field, severely restricting the flexibility of reducing the electric field to an optimal value. Therefore we decided to design our own bushing without a ceramic feed through. This required a full mold to accommodate all parts of the bushing. The mold was CNC machined of a rigid plastic foam (FR4500, supplied by General Plastics, Tacoma, Washington). As shown in Fig. 3 the material was machined to a pattern in two halves to take a central conductor mounted on a DN 100 stainless steel flange with a squirrel cage (1) made of copper. The two halves of the mold were coated appropriately, bolted together and placed
14
H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 3. Steps taken in the fabrication of Bushing B showing the prepared mold (1), the mold in a bag during the cast process (2), the vacuum pump (3), and the supply of epoxy resin (4).
inside a thick plastic bag with tubes (2). A vacuum pump (3) was connected and the air was evacuated from the bag until a vacuum gauge read about 0.1 Pa. The prepared epoxy resin of type Stycast 2850FT (4) was now sucked into the mold until the whole mold was filled with the epoxy resin. It was allowed to stand for two days and the mold removed. The finished bushing is shown in Fig. 4. Fig. 4 shows finished Bushing B with epoxy cast (2), a central conductor made of aluminum, terminated in a brass cylinder (1). Due to a leak in our mold the cast did not extend all the way up to the threads but stopped approximately 10 cm below (3). To even out the high electric field stress at the point at which the bushing bolted onto the main pressure cryostat we incorporated a squirrel cage arrangement made of copper (5). This part of the bushing was inside the cryostat, which experienced the high gaseous helium pressure and high temperature gradient from ambient temperature at the flange to 40–70 K at the lower end. The bushing was set up as was the case with Bushing A, by inserting it into the pressure cryostat and securely fixing it with the DN100 flange (4). A 50 mm diameter sphere (6) terminated the bushing.
this termination the conductor carrying current transitions from 50 K at a pressure of 2.17 MPa in gaseous helium to ambient temperature and pressure. In order to have a gradual gradation of temperature the bushing goes through an enclosure of liquid nitrogen. The liquid nitrogen acts as a heat intercept and therefore keeps the surface of the insulator body cold. Bushing C comprised a ceramic feed through (11) terminated in a brass cylinder (12). The feed through was welded onto one end of a stainless steel barrel (8), which was inside the cryostat and bolted onto it by a DN100 flange (7). Liquid nitrogen was carried into the barrel by the inlet tube (6). The second tube (5) was capped. The flange (4) DN40 carried a G10 tube (2), which had two holes drilled in it (3) for the liquid and gaseous nitrogen to escape. The central conductor was terminated in a brass cylinder (1). At the other end of the conductor inside the barrel was a plug (9) for assembling and disassembling the bushing. Between the plug and the end of the feed through was a spring (10) which allowed for expansion and contraction of the conductor. Three E type thermocouples were fixed onto the outer surface of the stainless steel barrel (8).
2.3. Liquid nitrogen bushing (‘‘Bushing C’’)
3. Experimental characterization of the bushings
The third bushing, C, is shown in Fig. 5. The basic idea was to use a relatively small ceramics feed through whose surface is at a temperature of 77 K. This guarantees that helium gas in contact with its surface has very high density and therefore high dielectric strength [1,2]. This can be achieved by an internal volume of liquid nitrogen. The secondary aim of this particular bushing – besides the use in the pressure cryostat – has been to utilize the experience and knowledge to design and fabricate terminations for a 1 kV, 3 MVA power cable. A patent for this principle is pending [11]. In
A full and detailed description of the equipment used and the methodology followed has been given in Ref. [9]. In the present work we substituted the cable and feed through used in [9] with each of the bushings shown in Figs. 2, 4 and 5. When a bushing was put in place inside the high pressure cryostat the output of the partial discharge (PD) measurement system was connected to it. The input voltage to the transformer was switched on and the voltage was gradually increased in steps of about 500 V. At each voltage level the peak level of apparent charge of the PD was
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 4. Photograph and drawing of Bushing B showing the aluminum conductor with brass cylinder (1), the epoxy body with sheds (2), the shortened top part due to vacuum problem (3), flange to pressure cryostat (4), field shaping insert (‘‘squirrel cage’’) (5), and the bottom sphere (6).
Fig. 5. Photograph and drawing of Bushing C showing brass cylinder (1), G10 top part (2), vent holes (3), flange to connect to bottom part (4), unused/blocked vent line (5), fill line (6), flange to pressure cryostat (7), stainless steel barrel (8), conductor connection (9), spiral (10), hollow ceramics feed through (11), and bottom brass cylinder (12).
100
(3)
Bushing B Bushing C
90
(4)
(2)
80
Apparent charge [pC]
measured. This procedure was common for all three bushings. For tests on Bushing A the pressure cryostat was prepared in a manner identical to that given in [9] for the cable tests, except that in this case there was no cable in place. The bushing was terminated in a 50 mm diameter aluminum sphere. Thermocouples were fixed onto a stainless steel tube inside the cryostat which was the supply line for gaseous helium. The thermocouples were spaced 50 cm apart in line with the top, middle and bottom of the cable had it been in place. This allowed to set the required helium temperature surrounding the lower end of the bushing by alternately admitting liquid helium and high pressure helium gas into the cryostat. For Bushing C the procedure was somewhat different from that given above. It was installed in the cryostat with a brass cylinder (11) terminating the feed though. The preparation was as described for Bushings A and B. However, in this case we did not use liquid helium to cool the cryostat and the thermocouples were affixed onto the outside of the stainless steel barrel (8) which was at earth potential. A Dewar of liquid nitrogen was connected to the inlet tube (6) of the bushing and liquid nitrogen was admitted into the barrel until the liquid squirted out of the two vent holes (3) in the hollow G10 cylinder (2). The temperature readings on all three thermocouples were 77 K indicating that the barrel was always full of liquid nitrogen. The cryostat was pressurized with gaseous helium up to 1.48 MPa. The power supply was connected and the experiment was conducted as was the case before. However, we were only able to do four different pressure levels. At 1.12 MPa and above the external part of the bushing broke down due a surface flashover before it could reach a higher level of PD within the bushing. Fig. 6 shows the level of apparent charge as a function of applied voltage (RMS) for Bushings B and C at each pressure level. It is
(5)
70
(1)
60 50
(2)
(3)
(1)
(4)
40 30
(6)
20 10 0
0
5
10
15
20
25
30
35
40
Voltage [kV] Fig. 6. The variation of apparent charge as a function of voltage at different levels of pressure. Labels (1) through (6) correspond to pressure levels of 0.45 MPa through 2.17 MPa in increments of 0.34 MPa.
clearly visible that Bushing B performs slightly better than Bushing C. The higher background noise level of approximately 5 pC with Bushing B could be attributed to external sources. A more suitable method to compare PD data is to look at the voltage at which PD initiates. We define partial discharge inception voltage (PDIV) in Fig. 7, where the initiation voltage is taken as the value of voltage when the PD exceeds 5 pC above the background value.
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17
Fig. 7. Definition of PDIV as used in the present work. The threshold value QThres was defined as 5 pC above background PD activity QBG. The value of PDIV UPDIV was interpolated between the last value below QThres (n) and the first value above QThres (n+1).
40 Bushing A Bushing B Bushing C
35
PDIV [kV]
30 25
this critical part of the bushing. We have been helped significantly in the final design of the cable termination. In the termination the lower part of the bushing is in liquid nitrogen and uppermost part close to the main flange containing the squirrel cage is in gaseous nitrogen [11]. The dielectric strength of liquid nitrogen is very much stronger than that of gaseous nitrogen, thus we have gained markedly in reducing the chances of the bushing failing under operating conditions of the cable assembly. Between about 0.4 MPa and 1.2 MPa there was linear increase in the PDIV curve at approximately 15 kV (MPa) 1. After about 1.5 MPa it leveled off reaching just above 35 kV at the highest pressure of 2.17 MPa. Bushing C was one that we made and tested not because it would be used in the conventional practical sense, but rather as a means of studying an entity which will be used under special circumstances in the terminations of a superconducting cable [11]. Essentially, our goal was to determine the partial discharge characteristics of a solid insulator surface exposed to a high pressure gaseous helium environment at a temperature of the order of 77 K. The ceramic feed through welded onto a stainless steel barrel (Fig. 5) has given us very good results, as shown in Fig. 8. Had it not been for the fact that there was external breakdown at the higher gas pressures it might have surpassed the performance of the epoxy Bushing B.
20
5. Conclusions 15 10 5 0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Pressure [MPa] Fig. 8. PDIV of all three bushings in comparison.
4. Results and discussion Fig. 8 shows PDIV of all three bushings as a function of helium pressure level. The values are taken from the same measurements shown in Fig. 6 but processed according to our PDIV definition as in Fig. 7. The modified ceramic feed through (Bushing A) was adequate for the for the purpose of testing model cables within the cryostat, and its performance characteristic shows that there is steady but modest improvement up to a pressure of about 1.5 MPa. After that it levels off and reaches about 12 kV at the highest pressure of 2.17 MPa thus satisfying the criterion of PDIV higher than 10 kV. The design value of the electric field at the surface of this bushing with the corona ring, epoxy filling and truncated epoxy cone was 0.5 kV mm 1, when the voltage was set at 9 kV. This calculation was done for conditions in helium gas at room temperature (293 K). However, the surface conditions prevailing under the experimental conditions with higher density of helium was considerably better. The temperature on average at the top section of Bushing A, in the neck part of the cryostat, was about 230 K, so the density of helium at 2.17 MPa is 4.44 kg m 3 whereas the density at 100 kPa and 293 K it is only 0.166 kg m 3[12]. It has been shown that the dielectric characteristics of gaseous helium improves significantly with increase in density [1,13]. The large epoxy bushing (Bushing B) gave the best performance as shown in Fig. 8; here we had the advantage of making a bushing from scratch which was suitable for the cryostat that has some limitations as indicated in Section 2.1. We incorporated the ring and squirrel cage arrangement which gave no surface electric field in
We have shown three different high voltage bushings designed for operation in our high pressure helium cryostat. They all fulfill the requirements regarding dielectric strength and in particular showed very low partial discharge activity in the intended voltage range. The results showed that minimizing the electric field at the surface of a bushing, in particular one where there is limited clearance, improves the dielectric performance significantly. Helium gas density is a critical parameter with regard to surface discharges along epoxy surfaces in such an environment. Keeping the surface of the bushing at very low temperature helps to reduce the required creepage distance. Ceramics feed throughs designed for operation with liquid nitrogen are not suitable for helium gas. However, marked improvements can be achieved by reducing the electric field stress at critical points in a feed through and filing the hollow ceramic section with a cryogenically suitable epoxy. The dielectric and cryogenic properties of liquid nitrogen can be used effectively to transition from cold compressed gaseous helium to finally ambient, as would be the case, in terminations of cryogenic high pressure power cables cooled by gaseous helium.
Acknowledgements The authors would like to thank Steve L. Ranner for excellent welding and brazing work on our bushings. This Project is sponsored in part by the US Office for Naval Research (ONR) under Grant Number N00014-02-1-0623.
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H. Rodrigo et al. / Cryogenics 57 (2013) 12–17 [6] Ballarino A. High temperature superconducting current leads for the Large Hadron Collider. LHC project report 337 – presented at 4th EUCAS 1999, September 14–17, Barcelona, Spain. [7] Turtu S et al. Cryogenic test of high temperature superconducting current leads at ENEA, LHC project report 865 – presented at the CEC-ICMC’05 Conference 29 August–2 September 2005, Keystone, Colorado, USA. [8] Dudarev AV. 20.5 kA current leads for ATLAS barrel toroid superconducting magnets. IEEE Trans Appl Supercond 2002;12(1):1289–92. [9] Rodrigo H et al. Electrical and thermal characterization of a novel high pressure gas cooled DC power cable. Cryogenics 2012;52:310–4.
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[10] Tuncer E et al. Electrical insulation characteristics of glass fiber reinforced resins. IEEE Trans Appl Supercond 2009;19(3):2359–62. [11] Graber L et al. Cable termination for high voltage power cables cooled by a gaseous cryogen. 10 May 2012, Prov. Pat No: 61/645,304. [12] Mann DB. The thermodynamic properties of helium from 3 to 300 K between 0.5 and 100 atmospheres. Nat Bureau of standards Technical Note 154, 1962 [13] Gerhold J. Dielectric breakdown of helium at low temperatures. Cryogenics 1972:370–6.