JP2004510290A - Superconducting cable - Google Patents

Abstract

To provide a flexible oxide superconducting cable with low AC loss, a tape-shaped superconducting wire coated with a stabilizing metal is wound around a flexible former. It is preferable that the superconducting wire is arranged on the former such that the bending strain does not exceed 0.2%. When arranging in a former, a large number of tape-shaped superconducting wires are arranged in parallel with a core material to form a first layer. A second layer is formed by arranging a predetermined number of tape-shaped superconducting wires in parallel on the first layer. The former can be made of metal, plastic, reinforced plastic, polymer, or composite, and provides flexibility to the superconducting wires and cables.
[Selection diagram] Fig. 1

Description

高温超電導ケーブル送電システムにおいては、液体窒素の２相流は許可されないものとする。その第１の理由は、２相流の圧力低下は、単相流の場合より高いからである。さらに、誘電体中の気泡がケーブル電気絶縁レベルを低下させうるからである。
【０１０５】
高温超電導送電ケーブルシステムの向流損失および熱分析を行った。これは、ケーブルシステムのエネルギ収支を実行することにより行った。高温超電導ケーブルの１次元エネルギ収支は、次のように書ける。
【０１０６】
【数１】

ここでρは密度、Ｃは熱容量、ｚはケーブル軸に沿った座標方向、ｋは熱伝導率、Ａはケーブル断面積である。
【０１０７】
高温超電導ケーブルエネルギ収支は、対流熱伝導項Ｑ_{ｃｏｎｖ，ｉ}を含む。この項は、フォーマにおけるおよびケーブルと極低温内側パイプとの間の環状領域における液体窒素流への対流を含む。熱伝導率とケーブル断面積との積ｋＡ_ＨＴＳは、本例において定数であり、１ケルビン当たり０．１６ワットメートルに等しい。液体流のために次のエネルギ収支式がさらに必要である。
【０１０８】
【数２】

ここでｉは各液体窒素流（フォーマ流、環状流、および該当する場合は戻り流）である。内側流への対流熱伝導は、高温超電導ケーブルフォーマ内のみに存在する。外側窒素流は、高温超電導ケーブルの外側と２重壁可撓極低温の内側とに対し、対流熱交換を行う。
【０１０９】
対流熱伝達係数は次の式により計算する。
【０１１０】
【数３】

ここでＮ_Ｎｕはヌッセルト数、Ｃ_ｈは熱伝達係数、ｋ_ＬＮ２は液体窒素の熱伝導率、Ｎ_Ｒｅはレイノルズ数、Ｎ_Ｐｒはプラントル数である。
【０１１１】
前記式を差分法に適用し、時間的に数的に積分し、定常状態を得る。温度特性を決定したら、流路全長に渡って次の式を積分することにより、圧力低下を近似する。
【０１１２】
【数４】

ここでＶは流速であり、ｆは摩擦係数である。
【０１１３】
厳密な処理の場合、熱水力解法を結合する必要がある。しかしながら圧縮降下および密度変化は、本例において考慮している条件下での液体窒素については、小さいと考えられる。従って、温度および圧力特性解法を個別に実行した。波形ベローズにおける極低温液体流の摩擦係数は、滑らかなパイプの４倍であると考えられている。本例において、レイノルズ数は、１０^５〜１０^６の範囲にあり、一定摩擦係数ｆ＝０．０７を用いた。端末の圧力低下は、小さいと推定されるので無視する。
【０１１４】
高温超電導ケーブルクライオスタットは、表５に示した寸法を有する可撓２重壁構造とする。シンク温度は、Ｔ_∞＝３００Ｋである。代表的な市販真空断熱可撓クライオスタットは、有効実際設置熱伝導率ｋ_ｅｆｆ＝０．０００８ワット／メートル／ケルビンを有する。単位長当たりの局所熱伝達は、計算可能であり、局所液体窒素温度Ｔ_ｖｉ（ｘ）およびクライオスタット内側管径Ｄ_ｃｉおよび外側管径Ｄ_ｃｏに依存する。この熱伝達項を駆動する温度差は、外側クライオスタットについて代表的に２２０Ｋである。
【０１１５】
【数５】

臨界電流は、５メートルシステムにおける以前の測定から計算した。測定温度の線形近似と７７Ｋにおける３０００Ａの基準値とを用いることにより、温度に対応する臨界電流は次のように求まる。
【０１１６】
【数６】

交流損失Ｐ_ＡＣ（ワット／メートル）は、モノブロックモデルを用いて計算する。
【０１１７】
【数７】

ここでＦ＝Ｉ_ｐ／Ｉ_ｃは交流サイクルにおけるピーク電流と超電導体の臨界電流との比率、ｆは周波数、ｈ＝（Ｄ_ｏ ^２−Ｄ_ｉ ^２）／Ｄ_ｏ ^２である。この研究は、１５００Ａ_ｒｍｓおよび２０００Ａ_ｒｍｓの動作電流に対する結果を示す。
【０１１８】
誘電損失は、ケーブルの設計電圧に依存する。定格値として０．０５ワット／メートルが予測され、この値は初期のテストと一致している。
【０１１９】
このモデルを５メートル高温超電導ケーブルシステムでの測定値と比較した。図２０は、測定動作温度との比較を示す。この測定において、５メートル高温超電導ケーブルは、２１０グラム／秒の流量の液体窒素を７９．２Ｋの温度および５．４気圧の圧力で供給して冷却した。ケーブルには１２５０Ａ_ｒｍｓの電流を印加した。測定値は、定性的に計算値と一致している。温度の相違の主原因は、端末用に簡易熱モデルを使用したことにある。短いケーブルの場合、端末はシステム熱負荷の大部分を発生する。端末は、真空断熱および電気絶縁を有する。各端末は、２本の最適化した電流リードを含み、定格電流１２５０Ａ_ｒｍｓを越える電流を搬送する。端末熱負荷は、各端部に付き約３００ワットである。動作電流のレベルにより端末熱負荷には多少の変動があるものの、その差は小さく、長いケーブルでは特にその差が小さいため、無視する。
【０１２０】
高温超電導送電システムに関し、端末熱負荷は一定であり、送電ケーブルシステムの長さには関係しない。クライオスタットを介する交流損失および熱負荷は、高温超電導送電ケーブルシステムの長さと、冷媒流の構成、供給温度、流量とに依存する。
【０１２１】
液体窒素の条件を一定とした。すなわち圧力１０気圧、温度６７Ｋとした。液体窒素の特性は、ＧＡＳＰＡＫを使用して取得した。前記圧力は、市販可撓クライオスタットの性能内に十分納まり、前記温度は、極低温冷却装置の代表的な温度である。窒素の三重点は約６３．２Ｋであるため、例えば６５Ｋの低温は閉サイクル冷却装置を用いて実現できる。
【０１２２】
図２１および図２２は、２例の長ケーブルについて、１０００グラム／秒／位相の流量における両冷却配置の臨界電流および温度曲線を示す。向流の場合に温度限界の存在がはっきりと分かる。この場合、下温度線はフォーマ流温度であり、中央線は高温超電導ケーブル温度であり、上線は環状戻り流温度である。図２３において、５００メートル長向流の圧力低下は、４．７バールである。ケーブル温度を下げるために流量を上昇させると、圧力低下も大きくなる。圧力はすでに高いため、ケーブル内での沸騰の可能性が出てくる。両冷却構成において、電流が高い場合、より高い温度が発生し、高温超電導ケーブル臨界電流を下げ、交流損失を上昇させる。
【０１２３】
図２３および図２４は、両冷却構成の６７Ｋにおける冷却負荷を長さの関数として示す。これら結果から、ケーブルで搬送する電流がより低いと交流損失が著しく低減するが、クライオスタット損失はケーブル内の電流レベルに全く依存しないことが分かる。
【０１２４】
高温超電導送電ケーブルシステムでは、重要な要素の多くが冷媒流量に依存する。２５０メートル向流の場合および１０００メートル平行流の場合に関し分析を行い、異なる流量における電流２０００Ａ_ｒｍｓでの最大ケーブル温度および圧力低下を求めた。その結果を図２５および図２６に示す。
【０１２５】
両構成において、低流量においてケーブル内温度は高くなる。向流の場合、液体窒素が冷却装置へ戻るシステム温度は、ケーブル最大温度よりも低い。平行流冷却構成の場合、それとは逆になる。流量が低いと、向流冷却ケーブル最大温度は高くなるため、ケーブルの超電導特性が著しく低下する。流量を増やすと、最大温度が下がるが、圧力低下が大きくなる。これら流量に関する事柄は、平行流の場合、２０００メートルの長さでも見られない。
【０１２６】
図１９に示す高温超電導ケーブル構成は、単一クライオスタット向流冷却構成である。高温超電導ケーブルのフォーマは、可撓波形ステンレス鋼管である。高温超電導ケーブルは、低温誘電構成であり、超電導遮蔽層を必要とする。この遮蔽層は、誘電体によって主導体から分離する。前記遮蔽は、主導体と同じ電流を搬送するような設計である。
【０１２７】
同軸設計の特徴は、１）遮蔽層内のイメージ電流は、外部磁界およびクライオスタットおよびダクト内の渦電流を減少させる、２）高温超電導体および誘電体はテープ状態で巻く、３）極低温誘電体は、サイズを小さくして、電流搬送能力を上げる、４）可撓ケーブルはリール巻きを可能にする。
【０１２８】
超電導テープをフォーマに巻き付けるための特別設計機械を用いて、３０メートル高温超電導ケーブルを製造した。主導体、極低温誘電体、および遮蔽を含む第１の２本のケーブルを巻いた後、それらをクライオスタットに配置した。クライオスタット用特別エンドキャップを用意し、大気圧において液体窒素でのケーブル冷却を可能にした。主導体および遮蔽導体の端部に臨時電圧タップを設け、超電導体だけの臨界電流を測定可能にした。図２８は、３０メートル高温超電導ケーブルの第２位相主導体の直流Ｖ−Ｉ曲線を示す。１μＶ／ｃｍ基準において観察した臨界電流Ｉ_ｃは２９８０Ａ、ｎ値は９であった。第１位相主導体の臨界電流は３０００Ａ（電力供給限界）以上であった。同じく図２８に示す通り、５メートルケーブルの１例の場合、臨界電流が１０９０Ａ、ｎ値が３である。５メートルケーブルおよび３０メートルケーブルの設計電流は、１２５０Ａであった。５メートルケーブルおよび３０メートルケーブルには、同一数の層および同一数の超電導テープを使用した。５メートルケーブル用超電導テープを調達した時から３０メートルケーブル用超電導テープを調達するまでの期間に、テープ性能は劇的に向上した。その結果、３０メートルケーブルは定格１２５０Ａに設計されているにもかかわらず、実際には３０００Ａまでの導体となっている。このように大きなマージンを有するため、約１５００Ａ以下の電流で動作する場合、超電導体は抵抗を示さなかった。これは図２８で明らかである。２５００Ａにおいて、電圧降下は０．２５μＶ／ｃｍ、直流抵抗は０．０１μΩ／ｍであった。臨界電流の測定後、臨時電圧タップを取り外した。
【０１２９】
図２９において高温超電導ケーブルの敷設箇所は、点線で囲んだ部分であり、３カ所の製造工場への配電システムの一部である。このプロジェクト用に建設した切換変電所は、超電導ケーブルおよび架空線の一方または両方によって負荷を提供できる。制御棟は、超電導ケーブル用電気制御および保護パネル、極低温制御システム、および会議室を含む。
【０１３０】
２本の１１５ｋＶ送電線により、前記送電変電所へ電力を提供する。送電変電所は、総容量４０ＭＶＡを有し、２台の２０ＭＶＡ整合非調整降圧変圧器を有する。これら変圧器の高圧側は１１５ｋＶ、低圧側は１２．４ｋＶである。
【０１３１】
２本の１２．４ｋＶ給電線が変電所を出る。両給電線は、１０３３．５ＡＣＳＲ（鋼心アルミより線）を使用する。これは代表的タイプの導線である。１２．４ｋＶ保護システムとして真空遮断器を設け、ロックアウト前に３回の再閉路を行う再閉路継電器を用いる。対称３相障害電流は、１２．４ｋＶにおいて約１４，０００Ａである。５メートル超電導ケーブルは、このレベル以上の障害電流におけるテストに合格している。
【０１３２】
高温超電導ケーブルは、極低温液体窒素を３相に循環させて冷却する。５メートルケーブルテストプログラムに基づき、３０メートルケーブル極低温システムの条件を、熱負荷３０００Ｗ、動作温度範囲７０〜８０Ｋ、流量１．３ｌ／ｓ（２１ｇｐｍ）、最大動作圧力１０バールと決定した。２つの極低温冷却システム設計をオプションとして検討した。これらは、開ループ沸騰槽冷却装置と閉サイクル冷却装置である。開ループ装置を選択した理由は、資本設備コストが安いからである。両装置の運転コストは同等である。その理由は、小型冷却装置よりも大型の液体窒素製造プラントの方が高効率だからである。この小型冷却装置は、３０メートル高温超電導ケーブルシステムに使用する予定だったものである。ケーブルシステムの動作寿命を長くするためには、主装置の保守中に使用する開ループ窒素バックアップ装置を含む閉ループ冷却装置がより良い選択であろう。なぜなら閉ループ冷却装置は、大型貯蔵デュワーを頻繁に補給しないでよいからである。
【０１３３】
極低温システム設計は、３つの主要部からなる。すなわち液体窒素貯蔵タンク、極低温スキッド、および真空外装配管である。
【０１３４】
極低温スキッドは、低温ボックスを含む。この低温ボックスは共通断熱真空にあり、過冷却装置、分相器、２個の緩衝容器を含む。低温ボックスと関連配管とは、多層断熱で覆うことにより背景熱負荷を最小にする。極低温スキッドは、さらに３台の真空ポンプを含む。これらは過冷却槽の圧力を下げ、システムの動作温度を下げるために使用する。主およびバックアップ用液体窒素循環ポンプを設ける。液体窒素の流れは、前記循環ポンプからシステムを通過し、過冷却器を通過し、ケーブルへの供給ラインを通過し、戻りラインを通過し、ポンプ入り口の分相器へ戻る。分相器は、システム起動時に使用し、蒸気が循環ポンプに至るのを防止する。過冷却器は、大量貯蔵タンクから補充を受ける。過冷却器の沸騰は、８０Ｋ以下で運転の際、真空ポンプを経由して大気へ放出する。緩衝タンクは交互に使用する。すなわち１つはシステム圧力印加に使用し、別の１つは補充して待機する。
【０１３５】
液体窒素貯蔵タンクは、４０，０００リットル容量である。このタンクは、コンクリート土台に水平設置し高さを抑えてある。タンク液面は、電話回線を利用して遠隔監視し、必要に応じて液体窒素供給装置によって補填する。
【０１３６】
真空外装配管は、スキッドとケーブルとを接続する。３本の配管がある。すなわち入り口配管、戻り配管、冷却配管であり、ケーブル３相をそれぞれ接続する。この構成により、ケーブル３相のどのような組合せでも、稼働、非稼働、冷却処理できる。
【０１３７】
通常運転中は、プログラム可能論理制御システムを使用して極低温システムを動作させる。システム冷却および再始動は手動で行う。これは、これら動作の回数が少ないからであり、プログラミングの費用がかさむからである。
【０１３８】
最初のオフラインテストは、可変交流電圧電源を使用してケーブルに電圧を印加した。その電源から１１〜１２ｋＶの電圧を１度に１相印加し、それを３０分間保持し、ケーブル誘電システムをテストした。位相１および位相２は、定格電圧の１６６％に維持して絶縁破壊を起こさなかった。位相３は、少し異なる形状を有し、定格電圧の２３０％に維持して絶縁破壊を起こさなかった。
【０１３９】
テスト中、充電電流を測定し、それを元にケーブル静電容量を決定し、計算静電容量と比較した。これを表６に示す。また計算ケーブルインダクタンスも示す。ケーブルサージインピーダンスは４Ωであり、これは従来の銅ケーブルより低い。この低いサージインピーダンスにより、交流超電導ケーブルは、従来の銅ケーブルと比較すると、より長い臨界長さ（充電電流が定格電流と等しくなる長さ）を有する。このため超電導ケーブルは、充電電流がケーブルの電流搬送能力の大半を占めるまで、より長い連続長さにおいて敷設できる。
【０１４０】
【表６】

ケーブルの直流電圧と電流の関係（Ｖ−Ｉ曲線）を測定するため、３０００Ａ直流電力を供給した。主ブッシングおよび遮蔽ブッシングの外部に電圧タップを臨時に設けた。Ｖ−Ｉ曲線は１度に２相測定した。この測定は、一端において２相に直流電源を接続し、他端において相間に短いジャンパ線を接続した。位相２および位相３を測定し、次に位相１および位相２を測定した。従って位相２は２回測定した。主導体のＶ−Ｉ曲線を測定した。次に遮蔽導体を測定した。これはケーブルが、同軸導体を有する低温誘電設計だからである。直流臨界電流は、高温超電導テープ性能およびケーブル設計から予測されたものだった。図３０は、位相２および位相３のＶ−Ｉ曲線を示す。電流範囲のほとんどにおいて線形挙動が見られるが、これは外部電圧タップの位置による。外部電圧タップは、超電導ケーブル各端部の先に延びる銅母線およびコネクタを含む。
【０１４１】
直流電圧電流テストは、６ヶ月間の運転および４〜６回の冷却・暖機サイクル運転の後、２０００年６月に可変負荷条件下で再度実施し、ケーブル性能を求めた。
【０１４２】
超電導ケーブルの平均負荷、定格負荷、および緊急負荷をシミュレートするため、直流負荷電流テストを行った。図３１に示す通りの広範な負荷電流テストを、８００Ａ，１２００Ａ，１４００Ａにおいて、各８時間、１度に２本の主導体に対して直流電源を用いて行った。この第１回負荷テストにおいて、ケーブル冷却システム温度に変化は見られなかった。
【０１４３】
次のテストは、広範な開回路定格電圧テストであり、変電所から電力を供給して行った。一端のケーブル遮断器を閉じ、他端を開放に維持し、ケーブルに電流を流さないようにした。各相の位相電圧をいくつかの順序で１２時間まで維持した。ケーブル誘電性能は設計通りであった。
【０１４４】
図３２は、位相１における液体窒素帰路温度変化を示す（Ｙ軸値は示さず）。ケーブル温度変化は、約１Ｋである。
【０１４５】
図３３は、位相１における液体窒素帰路圧力を示す（Ｙ軸値は示さず）。ケーブル圧力変化は、約０．２８バール（４ｐｓｉ）である。
【０１４６】
極低温熱負荷テストを数度行い、ケーブルシステムの総熱損失を測定した。３０メートルケーブルおよび端末の全３相に対する６００Ａでの熱損失は、１４９０Ｗであった。様々な成分を直接あるいは別々に測定し、熱損失の内訳を推定した。各ケーブル端末について熱損失は２３０Ｗであり、６個の端末がある。ケーブルクライオスタットは、各々７．６２ｃｍｘ１２．７ｃｍ（３”ｘ５”）の真空外装パイプであり、１Ｗ／ｍ／相である。６００Ａにおいて導体および遮蔽は、０．２Ｗ／ｍ／相に相当する。
【０１４７】
本発明を詳細に説明し図示したが、これらは例示的なものであり、本発明を制限するものではない。本発明の精神および範囲は、請求の範囲によってのみ限定されるものである。
【図面の簡単な説明】
【図１】
本発明に基づく多層構造を示す斜視図である。
【図２】
本発明の一実施例を示す断面図である。
【図３】
本発明の他の実施例を示す断面図である。
【図４】
本発明に使用するエンボス加工パターンを示す図である。[0001]
Technical field
The present invention relates to a superconducting cable using a flexible oxide superconductor. In particular, it relates to the formation of a superconducting cable.
[0002]
Background art
A superconducting substance is a substance whose electric resistance approaches zero (1 uv / cm) below a critical temperature. This critical temperature depends on the substance. Superconductivity is defined within the critical surface. That is, it is defined in a graph or diagram centered on temperature, current, and magnetic field. A critical current curve is defined for a given service temperature. This critical current is a function of the magnetic field generated by or applied to the superconductor.
[0003]
The best known superconducting materials are NbTi and Nb₃Sn. However, the working temperature of these substances is only 4.2K. This is the boiling temperature of liquid helium. This is a major limitation and these superconducting materials cannot be applied on a large scale. Therefore, these superconductors are used almost exclusively for magnet windings. Wire (NbTi and Nb₃Sn) or tape (Nb)₃Sn) and high critical current density (3500 A / mm for NbTi)²Small magnets using windings that provide 5 Tesla) create large, high magnetic fields (up to 18 Tesla).
[0004]
Such a superconducting magnet is used for forming a medical image by nuclear magnetic resonance (MRI) and for analyzing a substance by the same principle (NMR). For example, there are an ore separation magnet, a high magnetic field research magnet, and the like. Also used for large particle accelerators (SSC, HERA, KEK, etc.).
[0005]
An oxide superconductor with a higher critical temperature was discovered in 1986. These are intermetallic compounds, include metal oxides and rare earths, and have a perovskite (mica) crystal structure. The critical temperatures of these materials vary from 30K to values near room temperature, and their critical magnetic fields are above 60 Tesla. Therefore, these materials are promising, and NbTi and Nb₃It is considered to replace Sn and is expected to be applied to fields such as power transmission, which cannot be realized with liquid helium. Such materials have not previously been available as wires, cables, films, tapes, and sheets.
[0006]
The oxide superconductor enters a superconducting state at the temperature of liquid nitrogen, and thus is considered to be advantageous for application to a superconducting cable having a liquid nitrogen cooling medium. In such a field, the oxide superconductor can simultaneously simplify the heat insulation system and reduce the cooling cost as compared with a superconducting cable requiring liquid helium.
[0007]
Superconducting cables must be able to transmit high currents in small conductors with low energy losses. Generally, power transmission is performed by alternating current. A superconductor used for AC cannot avoid energy loss generally called AC loss. The AC loss is a hysteresis loss, a coupling loss, an eddy current loss, and the like, and depends on a critical current density of the superconductor, a filament size, a conductor structure, and the like.
[0008]
Various types of superconducting cables have been prototyped using metal superconductors, and structures for reducing AC loss have been studied. For example, a superconductor in which a composite multifilament superconductor is spirally wound around the outer periphery of a normal conductor has been prototyped. It is formed by wrapping layers of a composite multifilament superconductor in a clockwise and counterclockwise direction and overlapping them alternately. The winding direction of the conductor is made different for each layer to reduce the magnetic field generated in the conductor, reduce the impedance, and increase the current carrying capacity. This conductor has a high resistance layer or an insulating layer between each layer.
[0009]
When making a cable conductor using an oxide superconductor, the technology of a metal superconductor cannot be used. Oxide superconductors, or ceramic superconductors, are more susceptible to mechanical strain and weaker than metal superconductors. For example, according to the disclosure of one conventional example, a superconductor is spirally wound around a normal conductor, and the winding pitch is made equal to the diameter of each superconductor. However, if a superconducting wire made of an oxide superconductor covered with a silver sheath is wound at such a short pitch, the oxide superconductor is likely to be broken, and the current is likely to be interrupted. If the oxide superconducting wire is bent extremely, the critical current becomes extremely low. Cable conductors must be somewhat flexible to facilitate handling. It is difficult to manufacture flexible cable conductors from hard and brittle oxide superconductors.
[0010]
In the United States, high-voltage underground power cables using approximately 3,500 miles of copper conductors provide power to metropolitan areas. These cables are aging and many will be replaced in the next 20 years. Urban areas also continue to grow and require new cables. Power companies are seeking new cable technologies that increase power density, reduce losses and costs, and maintain the same high reliability as conventional cables. Superconducting cables have the potential to meet these difficult requirements.
[0011]
The cost of laying new cables in urban areas is high. Approximately 30% to 50% of that cost is cable cost, and most of the rest is installation cost. Superconducting cables can replace existing copper cables and increase power density by a factor of 3-8. Most of the existing cable designs are high pressure oil-filled pipe type cables that contain three copper cables and oil in a 4 to 8 inch diameter steel pipe. Removing these old copper cables and oils and replacing them with superconducting cables would significantly increase current capacity and save the cost of installing new cable systems.
[0012]
Superconducting cables propose new applications for cables in urban areas. Existing high-voltage copper cables carry electricity from the city's periphery to the center. Then, the voltage is reduced at the transmission substation, and the power is distributed to the customer by the distribution system. The new superconducting technology's low-voltage, high-current superconducting cables can transmit power to urban centers, allowing utilities to move high-voltage transmission substations out of urban centers. These substations are extremely expensive to install and maintain in the city center. Urban land prices range from $ 100 to $ 600 per square foot.
[0013]
The present invention enables superconducting cables with distribution voltages and high currents to be applied to utilities. Applicable utilities include transmission in 1) substation to customer, 2) substation to substation, 3) substation bus extension, 4) substation high speed feeder, 5) generator to boost transformer. .
[0014]
Disclosure of the invention
An object of the present invention is to provide a superconducting cable comprising an oxide superconductor having flexibility and excellent superconductivity, and particularly exhibiting a high critical current and a high critical current density.
[0015]
Another object of the present invention is to provide a superconducting cable with low AC loss.
[0016]
The superconducting cable according to the invention uses an oxide superconductor. The cable includes a flexible core and a plurality of tape-shaped oxide superconducting wires wound on the core. No electric insulating layer is interposed between the superconducting wires or between the core material and the superconducting wires. In the present invention, each of the oxide superconducting wires is basically formed of an oxide superconductor and a stabilizing metal covering the same. The plurality of tape-shaped superconducting wires arranged on the core material form a plurality of layers. Each layer is formed by arranging a plurality of the tape-shaped superconducting wires in parallel. These layers are successively stacked on the core. The core material imparts flexibility to the superconducting cable of the present invention. The superconducting cable according to the present invention maintains a superconducting state at liquid nitrogen temperature.
[0017]
Further, the conductor according to the present invention provides an AC conductor with low AC loss.
[0018]
The above and other objects, features, aspects and advantages of the present invention will be described in detail below with reference to the accompanying drawings.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a high-temperature superconducting cable used in a shielded or unshielded structure. The applications of shielded and unshielded cables are wide.
[0020]
In a modification of the present embodiment, the high-temperature superconducting tape of the cable is insulated with a dielectric, and another high-temperature superconducting layer is provided on the dielectric. Then put the whole cable in the cryostat (cryostat) or construct the cryostat so as to cover the cable. Such a coaxial structure confine the magnetic field between the inner layer and the outer layer of the HTS tape. Therefore, there is substantially no magnetic field outside the high-temperature superconducting tape, and no eddy current flows through the metal sheath. With this structure, a large-capacity current can be carried according to the number of tapes in the cable. A limitation in this cable design is that the dielectric must be kept at cryogenic temperatures so that materials that do not physically and mechanically degrade at cryogenic temperatures must be used. Polymerized dielectrics according to one embodiment of the present invention have good physical and mechanical properties below liquid nitrogen temperature. This material has a high dielectric strength and a high breakdown voltage. The cable of the present invention preferably uses a flexible stainless steel corrugated tube. The corrugated tube may be covered with a braided wire or a mesh wire. Preferably, a hole is formed in the corrugated tube. The size and pattern of the holes is such that liquid nitrogen can flow through it, so that the liquid nitrogen enters the butt gap of the HTS tape and soaks the dielectric. The high-temperature superconducting tape is arranged in a special way and constitutes a two-layer structure to maximize the current flowing through the cable.
[0021]
Preferably, the dielectric comprises a semiconductive tape, an aluminum vapor deposition shielding tape, and a polymerized dielectric tape. FIG. 3 shows a typical structure of a shielded cable. An unshielded cable can be formed by omitting the outer layer of the HTS tape. Such a cable structure is shown in FIG. The present invention includes both shielded HTS cables and unshielded HTS cables. The unshielded cable structure of the present invention differs in design from known cable structures when extruding dielectric over an insulated cryostat. Regarding the shielded high-temperature superconducting cable, there is no conventional example that discloses a method of forming the same.
[0022]
FIG. 1 shows a superconducting cable 10 according to the present invention. The superconducting cable 10 has a flexible vacuum double-walled outer tube 11. The liquid nitrogen 12 flows through the outer tube 11 to the cooler. The ground potential superconducting shielding material 17 surrounds the dielectric shielding layer 16. The shielding layer 16 surrounds the current carrying superconductor 15. The superconductor 15 surrounds the inner tube 13 of the flexible porous wall. The inner tube 13 is a central tubular portion and conveys liquid nitrogen from the cooler. In one embodiment, inner tube 13 has a braided surface that contacts superconductor 15.
[0023]
FIG. 2 shows one embodiment of an unshielded cable. The semiconductive base tape 22 is arranged around the former 21. The superconducting tape 23 is disposed thereon. Another semiconductive base tape 24 is wound on the superconducting tape 23. The shielding layer 25 is wound on the base tape 24. The dielectric layer 26 is wound on the shielding layer 25. The shielding layer 27 is wound on the dielectric layer 26. A semiconductive underlayer 28 is wound on the shielding layer 27. An adhesive tape 29 is wound on the underlayer 28. The alignment ring 30 is wound on the fixing tape 29. The covering cryostat 31 is wound on the centering ring 30.
[0024]
FIG. 3 shows an embodiment of the shielded cable. The outer cryostat 53 surrounds the alignment ring 52. The alignment ring 52 surrounds the fixing tape 51. The fixing tape 51 surrounds the semiconductive tape 50. The semiconductive tape 50 surrounds the superconducting tape 49. Superconducting tape 49 surrounds semiconductive underlying tape 48. The base tape 48 surrounds the shielding layer 47. The dielectric 46 surrounds the shielding layer 45. The shielding layer 45 surrounds the semiconductive tape 44. Superconducting tape 43 surrounds semiconductive underlying tape 42. The base tape 42 surrounds the former 41.
[0025]
The present invention relates to a cable using an oxide superconductor. This cable has a flexible core and 2kgf / mm²A plurality of tape-shaped oxide superconducting wires disposed on said core with a tensile force not exceeding 0.2% and a bending strain not exceeding about 0.2%. Each of the tape-shaped superconducting wires basically consists of an oxide superconductor and a stabilizing metal covering the same. A layer is formed by arranging the plurality of tape-shaped superconducting wires in parallel, and a plurality of such layers are formed. The plurality of layers are continuously stacked on the core without inserting an insulating layer between the plurality of layers and the core. The core imparts flexibility to the superconducting cable. The superconducting cable can maintain a superconducting state at liquid nitrogen temperature. The wire has a substantially uniform superconducting phase in the longitudinal direction of the wire. The c-axis of the superconducting phase is substantially parallel to the thickness direction of the line. The superconducting wire is formed by grains aligned in parallel and extending in the longitudinal direction of the wire. The crystal grains are stacked in the thickness direction of the wire.
[0026]
Preferably, the superconducting cable has flexibility such that it does not substantially degrade superconductivity in bending up to about 50 times the cable diameter. The core is preferably selected from the group consisting essentially of metals, plastics, reinforced plastics, polymers and composites. In one embodiment, the core material of the superconducting cable is a tube having a surface selected from a spiral groove surface, a mesh surface, a braided surface, and a mat surface on the outer surface of the tape-shaped superconducting wire. The superconducting cable of the present invention has no insulating layer between a plurality of layers of the tape-shaped superconducting wire. In the tape-shaped superconducting wire, it is preferable that the plurality of tape-shaped superconducting wire layers are sequentially arranged on the surface of each layer on the core material. In another embodiment, the wires are twisted within the tape-like stabilizing metallization. Within the superconducting cable, the tape-shaped superconducting wires are arranged at an angle of about 90 degrees, preferably at an angle of about 10 degrees to about 60 degrees, more preferably at an angle of about 20 degrees to about 40 degrees. One embodiment of the present invention provides a superconducting cable comprising at least two separate tape-like superconducting wire layer groups. It is preferable that the arrangement angle of the tape-shaped superconducting wire layer is set such that the arrangement direction or the pitch changes between successive layers. Each of the continuous tape-shaped superconducting wire layers is preferably composed of at least two tape-shaped superconducting wires. Preferably, the at least two independent tape-like superconducting wire layer groups are separated by a dielectric layer. It is preferable to separate the tape-shaped superconducting wire layer closest to the core material from the core material by a dielectric layer. Preferably, said dielectric is selected from the group consisting of polypropylene, polyethylene and polybutylene. In one embodiment of the invention, the at least two separate tape-like superconducting wire layer groups carry approximately equal amounts of current in the cable. Preferably, the taped superconducting wire layer group furthest from the core provides shielding of current flowing through other layers to reduce magnetic fields or eddy currents in the cable. The stabilizing metal is preferably selected from the group consisting of silver, silver alloy, nickel, and nickel alloy, and may be provided with a buffer layer.
[0027]
In one embodiment of the present invention, each tape-shaped multifilament oxide superconducting wire has a structure composed of a large number of filaments. These are basically structures in which an oxide superconductor is contained in a stabilizing metal of silver, a silver alloy, nickel, and a nickel alloy. The oxide superconductor can be made of an oxide such as bismuth, strontium, calcium or the like and a copper oxide.
[0028]
Each of the plurality of layers preferably includes at least two tape-shaped silver-coated superconducting wires. Preferably, each of the plurality of layers includes at least four tape-shaped lines. In one embodiment of the present invention, it is preferable that an insulating layer is interposed between the second and third layers. When the plurality of layers is four or more, it is preferable to interpose an insulating layer between the second layer and the third layer.
[0029]
In the cable of the present invention, the core material is generally called a former, and holds the tape-shaped superconducting wire at a predetermined range of bending strain. The former has the required length of the superconducting cable conductor and is located at the center of the superconducting cable conductor. The former is substantially cylindrical or helical, has a tape-like line disposed thereon, and has a substantially constant diameter over its entire length. The former can be made of at least one material selected from the group consisting essentially of metals such as stainless steel, copper and aluminum, plastics, reinforced plastics and ceramics.
[0030]
In the present invention, the former is preferably a flexible tubular member. A pipe having a spiral groove (hereinafter referred to as a spiral pipe) can also be used as a former having sufficient strength and flexibility. A bellows tube having a bellows may be used as a former. The former may be formed from a spirally wound member such as a spiral steel strip. Each of these shapes is employed to provide sufficient flexibility to the former. The flexible former imparts flexibility to the conductor of the present invention. The flexible conductor of the present invention can be wound around a drum.
[0031]
In the practice of the present invention, it is possible to arrange or wind several tape-like multifilament superconducting wires on a former. The tape-shaped wires may be arranged in two or more layers, and the surface thereof may be directed to the former. Each layer can be formed with any number of tape-like lines. It is also possible to arrange several tape-like lines parallel to each other on the former, cover the former surface with tape-like lines, and wind another tape-like line thereon. A sufficient number of tape-like lines are wound as a second layer on the first layer of tape-like lines, and a third layer of tape-like lines is wound thereon. No insulating layer is disposed between adjacent layers.
[0032]
In the method of the present invention, each tape-shaped multifilament oxide superconducting wire is arranged or wound around a former having a predetermined diameter in a predetermined range of bending strain or curvature and a predetermined range of pitch. A relatively gentle bend is applied to this tape-like line along its length. The tape-shaped wire wrapped around the former bends at a bending strain not exceeding 0.4%, preferably not exceeding 0.3%. In the case of bending in such a range of bending strain, the superconductivity of the tape-shaped wire hardly decreases as compared with the linear state.
[0033]
In the present invention, it is preferable to adjust the pitch and diameter of the former so that the bending strain of the superconducting wire does not exceed 0.2%. When each tape-shaped multifilament oxide superconductor is wound around a former, the tensile force is 2 kgf / mm.^-20.5 to 2 kgf / mm^-2It is preferable to set it in the range.
[0034]
The core can be made of an electrically insulating material or an electrical conductor. In consideration of reducing the AC loss, an electrically insulating material is preferable. In consideration of strength, a metal that is a conductor is preferable. When a metal pipe or a metal bellows pipe having a spiral groove is used as a core material, it is possible to impart flexibility to the conductor and maintain a constant strength. The use of a metal core is safe if an abnormal current is accidentally generated. In this case, the optimum resistance of the core can be set in consideration of the AC loss of the conductor and the abnormal current to the core.
[0035]
When using a metal pipe or a metal bellows pipe which can have a spiral groove as a core material, a metal tape conductor is arranged or wound on the core material. Then, a dielectric tape is arranged on the outer surface of the metal tape. The superconducting tape does not buckle because the metal tape can form a smooth surface over all the grooves of the core material. It is possible to cover the grooves with metal tape while maintaining the flexibility of the core.
[0036]
The present invention may use a tape-shaped multifilament wire having twisted filaments. The filaments forming the superconducting multifilament tape are twisted at a predetermined pitch. By twisting the filaments, the induced current flowing between the stabilizing metal and the filament separates into small loops at each twisting pitch, limiting the current value. This suppresses the generation of Joule heat in the stabilizing metal and reduces the AC loss as compared with a superconducting wire having a non-twisted filament.
[0037]
Since the superconducting cable conductor according to the present invention has flexibility, the superconductivity does not deteriorate even if it is bent up to 50 times the cable diameter. This conductor can be wrapped around a drum and stored and / or transported.
[0038]
The present invention can provide a long oxide superconducting cable conductor having flexibility and excellent superconductivity. In one embodiment of the invention, eddy currents and coupling currents flowing or traversing between the superconducting tapes are suppressed by the second or subsequent layers of the tubular superconducting wire layer. The present invention provides a practical AC superconducting cable conductor.
[0039]
The superconducting material is obtained by inserting ceramic crystal grains into a silver tube and stretching it to about 1 to 2 mm. A number of the drawing tubes according to the final required capacity of the cable are inserted into a silver tube and drawn to a required use size. Alternatively, the tubes may be cut first into small sections, added to a second silver tube and stretched. The silver flat tape material thus formed is about 80-60% silver and about 20-40% ceramic by weight. Preferably, about 65 percent is silver and about 35 percent is ceramic.
[0040]
The invention further relates to a novel process or method for producing a polymerized tape suitable for cryogenically operating superconducting power cables, and to a tape so produced. This process biaxially stretches a polyethylene, polypropylene, or polybutylene film having a maximum dielectric constant of about 3.0 and embosses a random pattern into the film. This polymeric material as a low dielectric constant biaxially oriented embossed film overcomes the problems of brittleness, cracking, and extreme shrinkage. Polymeric materials produced by known processing methods have these problems and cannot be used in cryogenically operated power cable systems. Embossing the film allows the dielectric fluid to flow relatively freely through the cable.
[0041]
The polyolefin sheet material is biaxially stretched before use in the cable of the present invention. This involves stretching the sheet lengthwise with a stretch ratio of about 5: 1 to about 10: 1, while simultaneously stretching in the transverse direction.
[0042]
Sheets or tapes made from sheets thus treated from polyolefin materials at suitable stretch rates have many excellent qualities in cable manufacturing. Further processing is required to prevent tape microfibrillation and to prevent a single rift from spreading the full length. This processing includes biaxial stretching in the sheet transverse direction. This stretches the sheet transversely at a rate of up to about 50% to form a tape. This tape is sufficiently biaxially stretched to sufficiently suppress microfibrillation.
[0043]
The polyethylene, polypropylene, and polybutylene tapes produced by the above process are embossed with a specific pattern under predetermined conditions to ensure proper impregnation and heat transfer. The embossing pattern consists of random or irregular grooves and points in the lateral direction. The stretched sheet is cut or taped and used in the form of a single layer, a plurality of layers, or a laminate.
[0044]
The pattern allows for the flow of impregnation in the longitudinal and lateral directions, as well as flow across the tape and between the butt gaps. Such a flow aids the layer-to-layer impregnation and promotes convective heat transfer. The cable itself is comprised of multiple layers of tape made of polyolefin, polyethylene, polybutylene, or polypropylene. To make the cable easier to bend, different layers of polyolefin tape may be used in each layer. This tape width may increase as the distance from the cable conductor increases.
[0045]
A polyethylene, polypropylene, or polybutylene film according to the present invention has a dielectric constant no greater than about 3.0. This preferred maximum is about 2.3. The first processing step is biaxial stretching or drawing, preferably in a ratio of about 5: 1 to about 6: 1 in the longitudinal direction and up to about 2: 1 in the transverse direction. The stretched tape is then embossed at a temperature of about 80 ° C. to about 140 ° C. to form an irregular or random groove pattern in the tape, which is basically oriented laterally.
[0046]
Polymerized tapes that do not undergo the novel processing steps described above cannot be used in cryogenically operating superconducting power cable systems due to some inherent problems. For example, in a liquid nitrogen environment at 77 ° K, most polymerized tapes harden to glass. For this reason, the tape undergoes tensile fracture due to thermal contraction exceeding the intrinsic elongation and simple collapse. Another problem is cracking in liquid nitrogen. Liquid nitrogen has a boiling point of 77 ° K and is known to be a strong cracking agent for polymers. Cracks cause stress rupture and eventually fracture the tape. The biaxial stretching process overcomes these brittleness, extreme shrinkage, and cracking problems.
[0047]
Many polymers exhibit two prominent yield modes. One of the yield modes is due to the applied shear stress. However, the yield phenomenon itself is due to the normal stress component acting on the yield surface. Another yield mode is due to maximum principal stress. This type of yield is a crack or normal stress yield. Cracking can occur due to stress or a combination of stress and dissolution. Cracks exhibit similar characteristics in all polymers that produce them. Cracks are fine reticulated cracks that almost always travel in the direction perpendicular to the maximum principal stress. Generally, cracks occur at local stress concentration points on the surface. In static testing, cracking occurs when stress or strain reaches a certain critical value. However, cracking can occur at relatively low stress levels for long-term loading.
[0048]
Detailed electron microscopic examination of the cracked area shows that molecular chain orientation has occurred in the cracked area, with oriented microfibers traversing the cracked surface.
[0049]
It is preferred to add a dye to the very transparent polyolefin insulation tape to assist in cable laying. This technique can produce very useful cables. The operator must distinguish each spiral layer of the tape. Using common, very transparent polyethylene, polybutylene, or polypropylene tapes, the operator cannot distinguish the butt gap for each of the eight or ten tape layers. By adding a predetermined amount of the selected dye to the tape, the operator can easily distinguish the edges and butting gaps of each tape layer by layer. This is because the darkness of the colors increases significantly from layer to layer. The dye is chosen to minimize the increase in the loss factor of the raw material.
[0050]
The tape width can also be varied. The width should be narrow near the conductor and wider outside. The direction of each layer may be reversed at a predetermined radial thickness. The thickness may be determined by the design of the taping machine.
[0051]
The dielectric tape is wrapped around a helical layer in a superposed manner, and each butt gap between the spirals of the same layer is shifted from the butt gap of the layer below it. This structure is facilitated by the dye-containing insulating tape.
[0052]
Polyolefin tapes such as polyethylene, polybutylene, polypropylene, etc. are transparent when highly oriented as required by the present invention. This transparency is disadvantageous if the butt gap of many layers is clearly visible through the cable surface. In a state where the butting gap is shifted in each layer, it becomes difficult for the operator to distinguish the butting gap in each layer.
[0053]
Thus, the tape of the present invention contains a dye. Therefore, the lower layer looks darker in the cable. Organic dyes are preferred as dyes. Organic dyes have less adverse effects on loss factor and dielectric constant of tapes than inorganic metal salts.
[0054]
In order to balance the required color with the effect on the electrical properties, the organic dye is added in parts per million at a ratio of 100 to 1000.
[0055]
This results in reducing the light transmittance of the tape by 10 to 50 percent. Use of this tape in a cable reduces visibility to one to four layers. On the other hand, when the dye is not added, the butt gap having a depth of 8 to 10 layers can be seen in the insulating region.
[0056]
Longitudinal stretching achieves a 5: 1 to 10: 1 thickness reduction by stretching or tentering the sheet.
[0057]
This thickness reduction rate is actually a measurement of the linear stretching of the sheet, and serves as an index of the change in the tensile properties of the polymer. This treatment is preferably performed at a temperature of about 80C to 140C.
[0058]
The sheet is further stretched in the transverse direction at a reduction rate of up to 50%. Without this treatment, the polymer will be microfibrillated, the fibers will separate in the width direction and the tape will tear in the longitudinal direction.
[0059]
In contrast, the treated polyolefin tape has a tensile modulus in the longitudinal direction of at least 250,000 psi and satisfies all cable manufacturing requirements.
[0060]
This tensile strength of the tape obtained through the above process provides resistance to degradation and is a requirement for use in cable taping machines. The tape subjected to the above treatment can be wound around the cable sufficiently firmly in a conventional cable manufacturing machine.
[0061]
Prior to final construction as a cable, the polyolefin tape is embossed to create gaps between the tape layers to provide a relatively free flow of impregnation within the cable and promote heat transfer.
[0062]
This is achieved by a specific embossing technique. The embossing of the tape is preferably performed by a roller. FIG. 4 is a top view showing a typical pattern of embossing, and shows a part of the tape 60. In the figure, valleys 61 appear as dark lines in the pattern.
[0063]
The feature of the embossing pattern is irregular and is to promote the flow of the impregnant laterally against the longitudinal direction of the tape. As shown in FIG. 4, the irregular valley pattern runs essentially across the width of the tape and satisfies the requirement. This pattern does not engage adjacent tape layers, unlike regular groove and channel patterns. Thus, the non-uniform and irregular pattern allows each tape layer to move relative to each other for a short distance, providing the flexibility required for cable manufacture and installation.
[0064]
The pattern that provides the lateral flow imparts heat transfer and permeability to the cable. Although the polymer is impervious, the infiltration and heat transfer structures provided by the cable of the present invention do not depend on the permeability of the material itself.
[0065]
The embossing pattern increases the effective tape thickness. That is, the peak-to-peak thickness is twice the original tape thickness. The tape is compressed during winding. Embossing is performed by a roller. These rollers indent one side of the tape and project the other side. As the tape is wrapped around the cable, the irregular surface separates the tape layers. However, since the pattern encourages lateral flow, the impregnant can proceed to the next gap between the tapes if it flows up to one-half the tape width from the butt gap. Thereby, the path from the outside of the cable to the conductor is relatively short.
[0066]
The two representative embossing patterns are as follows. One is a coarse pattern, which typically has an intermediate height of valleys of 0.1 mm and a distance between adjacent peaks of 0.2 mm. The other is a fine pattern, which typically has an intermediate height of valleys of 0.025 mm and a distance between adjacent peaks of 0.05 mm.
[0067]
The availability of embossed patterns ranging from coarse to fine patterns allows designers to find a compromise between heat transfer and operating stress. The coarse pattern provides the best heat transfer but slightly reduces operating voltage stress. The fine pattern has the opposite effect.
[0068]
The present invention further includes cables, terminals, and test procedures. The cable test facility includes a 5 meter HTS cable between two terminals and an associated support system. Related support systems include AC and DC power supplies, cryogenic coolers, and data acquisition systems. Cryogenic systems use sub-coolers to provide up to 1 kW of cooling for cables and terminals. The boiling of liquid nitrogen in the subcooler corresponds to the total equipment heat load. The outer shell side of the sub-cooling device is maintained at a liquid nitrogen temperature of 70 to 77K by a vacuum pump. However, most of the test programs did not use sub-cooling, and were performed in an average cable temperature range of 79 to 81 k. The system cycle is open, and exhaust from the outer shell side of the sub-cooling device is finally discharged to the atmosphere. Cooling fluid (10 bar or less of nitrogen) passes through the tube side of the sub-cooler and goes to the cable supply manifold. Nitrogen from the cable or terminal flows to the inlet of the liquid nitrogen circulation pump. This pump provides a pressure head for the closed loop flow of pressurized nitrogen.
[0069]
Electrical data and cryogenic data are acquired by a dedicated sensor. These sensors scan with three 10-channel multimeters. The multimeter connects to a personal computer (PC) based data acquisition system. This system uses the LabVIEW program. Thereby, the measurement of the DC VI characteristics and the AC loss of the cable, the insulation integrity through the partial discharge measurement, the cryogenic performance at the rated voltage (7.2 kV AC effective value) and the rated current (1250 A) can be diagnosed. The cable is tested with a high voltage power supply (1A, 18kV rating) at a peak voltage level 2.5 times the operating voltage. The dedicated impulse power supply is increased from 100 kV to 200 kV, and a 25 kA pulse power supply is arranged to simulate a cable overcurrent due to a system failure.
[0070]
Two terminal concepts have been developed. Most of the cable test programs were run with the vacuum terminal concept. It uses vacuum for both electrical and thermal terminals. Each vacuum terminal has two sets of feedthroughs. One set is for phase conductors and the other set is for high temperature superconducting neutral conductors. Each set has a warmed ceramic bushing and transitions from outside air (295K, 1 atm) to vacuum (295K). Each set also has a second ceramic bushing to make the transition from vacuum to liquid nitrogen (72-81K) up to 10 bar. These bushings are rated for maximum cable current and voltage. The two terminals are evacuated by a common vacuum header with a mechanical turbomolecular pump station. A typical vacuum here is 10^-5-10^-4Tor range. This combination of vacuum and multi-layer super insulation provides sufficient insulation. There were concerns about reliability during cooling and long-term operation. Small leaks in the low temperature bushing due to thermal or electromechanical stress could degrade the vacuum at the end. Even with a small amount of liquid nitrogen leakage, the vacuum will be at the Paschen minimum of nitrogen gas and will break down at voltages up to 250V. This voltage is much lower than the operating voltage. Accordingly, an alternative terminal embodiment has been developed that operates on pressurized fluid and gaseous nitrogen. This is shown in FIG.
[0071]
As with the vacuum version, the system of FIG. 5 is set at 18 kV AC withstand voltage and 110 kV basic impulse insulation level. This is smaller than the vacuum terminal of FIG. The basic concept is that there is no cold phase or shielding bushing. Each terminal has two conventional thermal bushings and has no significant thermal stress. A copper tube is concentrically arranged between the heat retaining bushing and the end of the high-temperature superconducting cable. These copper tubes are designed to minimize axial heat transfer at operating cable currents. A non-metallic cylinder is used to provide electrical insulation between the phase copper tubing and the neutral copper tubing. The dynamic vacuum system is omitted, a conventional static vacuum system is used, and radiative heat losses are reduced using multi-layer super insulation. The entire terminal is at cryogenic system pressure and there is a natural heat transfer from liquid to gas along the copper conduit. If there is a leak, the internal pressure will gradually decrease to atmospheric nitrogen pressure (can withstand 30 kV / cm) and the system is designed to prevent breakdown in such cases. This terminal design is more efficient in vacuum leak tests and can eliminate pump failures.
[0072]
The following tests were successfully completed in the long-term test program at the pressurized terminal.
[0073]
Long term operation at design current (1250 A AC rms) and voltage (7.2 kV phase-ground).
[0074]
-AC withstand voltage test to withstand 18kV for 30 minutes without breaking.
[0075]
Impulse test to 90 kV (see below for cable test).
[0076]
-Measurement of terminal heat load.
[0077]
As shown in FIG. 7, the heat load gradually decreased through a series of design improvements. The best performance was seen at the 30m cable end.
[0078]
Six of the pressurized terminals in the three-phase 30-m high-temperature superconducting cable were tested. The thermal load of the pressurized terminals installed on the 30 m cable was about 230 W per terminal at an alternating current of 600 A. Each end has two conductive cooling copper leads (pipes). The theoretical minimum heat load for each lead is 44 W / kA.
[0079]
The single-phase 5m cable tested had an internal cable conductor with four layers of Bi-2223 / Ag tape helically wrapped around the conductor. The four layers were selected to provide sufficient performance for a design current of 1250A. These tapes were mechanically wound at an angle of 30 ° on a 38 mm outer diameter stainless steel former. A Creoflex® dielectric tape was wrapped between the inner and outer high temperature superconductors. The outer high-temperature superconducting cable conductor is similar to the inner conductor, providing shielding of the current flowing through the inner conductor and eliminating magnetic fields or eddy currents in the outer structure. The outer high temperature superconductor is at ground potential.
[0080]
The 5m cable test results include a successful overcurrent test up to 12.8 kA with a 2 second pulse length (see FIG. 8). This exceeds 10 times the design current and simulates a short on the load side.
[0081]
In addition, the standard impulse insulation level (BIL) of the cable was tested to simulate lightning and other surges. The requirement for a 15 kV class distribution cable is a reference impulse insulation level of 110 kV, and the pulse rises to this maximum in 1.2 μs. High-temperature superconducting cables and terminations withstand a reference impulse insulation level of about 90 kV (see FIG. 9) and appear to break above this value. When the test voltage was reduced below 90 kV, the system again withstood the test pulse.
[0082]
The 5 m cable was removed from the test facility and bent over a wooden frame of the same diameter (2.44 m) as the cable shipping spool. The cable was bent in one direction and the opposite direction, and this was repeated four times. In the table test before and after bending, no failure was found in the dielectric system. That is, the breakdown voltage and the impulse insulation did not change from the previous test. After bending, the cable critical current decreased by about 15% (see FIG. 10).
[0083]
A low temperature dielectric triaxial cable was developed and tested. A cable up to 1.5 m long was manufactured. This structure is smaller than three individual single-phase cables and requires very little high-temperature superconducting tape. This is because a balanced load does not generate a net induced current outside the three homocentric layers. FIG. 11 shows a cross-sectional view of the cable, and compares the sizes of the single-phase cable and the triaxial cable. The cross-sectional area through which liquid nitrogen flows is almost the same, but the dielectric of the triaxial cable is slightly thicker. This is because the phase-phase voltage between the high-temperature superconductors of the triaxial cable is higher than the phase-ground voltage of the coaxial single-phase cable.
[0084]
AC superconducting cables provide low loss transmission capability. Although there are known superconducting cables for this application, their construction is complex and extremely expensive to manufacture. The phase conductor is made of a metallic superconducting material and requires separate cooling of each phase. For this purpose, the space in the phase conductor is used as a passage for the cooling substance, and the cooling system uses closed-loop liquid nitrogen or liquid helium.
[0085]
In one embodiment of the invention, the superconducting cable is smaller, requires less material, and the cooling mechanism is smaller than known superconducting cables.
[0086]
The superconducting cable of the present invention has a configuration in which only one neutral conductor is required for three-phase conductors (R, S, T). Furthermore, the phase conductor, the neutral conductor, and the cooling passage are arranged concentrically around each other to realize a very compact structure. Cooling of the cable is performed with liquid nitrogen. The electrical insulation is made of polyethylene or polypropylene and is placed between the phase conductors (R, S, T) and between the neutral conductors. The cable limits heat loss to the outside by vacuum and / or insulation. The refrigerant circulates through the cable core and the annular passage. This annular passage connects directly to the vacuum / insulation.
[0087]
FIG. 27 shows a cross section of a superconducting cable (1) according to the present invention. The cable core is formed around the passage (2), has a diameter of about 50-200 mm, and allows the coolant to pass through. This diameter can be other dimensions. The passage (2) defines a boundary of the first phase conductor (R). The outer shell of the passage (2) is formed by a superconductor. The phase conductor (R) is preferably formed by a superconducting tape. This superconducting tape has a silver sleeve filled with a high-temperature superconducting ceramic material. These sleeves are rolled into a surface tape. Then, these tapes are wound around a mandrel to form a conductor (R). The thickness of the conductor (R) is preferably about 0.1 to 10 mm. An electrical insulation (3) is arranged on the first phase conductor. This insulation is preferably made of polyethylene or polypropylene. These insulations are wrapped around the phase conductor tape (R) to a desired thickness. The thickness of the insulation is preferably about 10 to 50 mm. A second phase conductor (S) is wound around the insulation (3). This conductor is also made of a superconductor tape and wound around the insulation (3). The phase conductor (S) has the same capacity as the phase conductor (R). The insulation (4) is wound around the phase conductor (S). The method of making the insulation and the capacity are the same as those of the insulation (3). A third phase conductor (T) is formed in the same manner and capacity as the phase conductors (R) and (S) and wound on the insulation (4). Further, an insulation (5) is formed in the same manner as the insulations (3) and (4) and wound around the phase conductor (T). The thickness of the insulation (5) may be smaller than the thickness of the insulation (3) or (4). The boundary of the neutral conductor (6) is outside the insulation (5). This neutral conductor (6) carries only small currents under symmetric loads and may be made of a common conductor, preferably copper. And the thickness may be several millimeters. This return conductor also serves as the boundary of the closed ring cooling passage (7). This cooling passage circulates liquid nitrogen. The diameter of the cooling passage (7) is preferably about 150-500 mm. The outside of the cooling passage (7) is vacuum or super adiabatic and defines a boundary inner surface (8I) and a boundary outer surface (8A). An annular space is formed between the two boundary surfaces, and this space is evacuated or filled with super insulation.
[0088]
The inventive cable was cooled to about 81K with liquid nitrogen. The liquid nitrogen was at about 4.7 atmospheres and at a flow rate of about 0.19 l / s (3 gpm). A short current pulse was applied to the cable, much larger than the critical current Ic of the cable (approximately 910 A) to simulate the fault current in a short circuit during operation. During the pulse period and the post-pulse period, the voltage between the phase conductor and the joint, the current and voltage of the shield conductor, and the temperature and pressure of the refrigerant were observed. The pulse application started with a one second pulse and gradually increased the current from 4.8 to 12.8 kA. Next, the pulse length was increased to 2 seconds, and a current pulse up to 12.8 kA was applied again. The pulse length was reduced to 0.5 seconds, and a current of 15.3 kA was applied. Finally, the pulse length was extended to 5 seconds, and a current of 6.8 kA was applied.
[0089]
FIG. 12 shows the current and voltage course of the cable during a typical pulse application. A fault current of about 12.8 kA was programmed to be applied to the cable for 2 seconds. As soon as the current reached 12.8 kA, a voltage of about 3.2 V (cable voltage) developed across the cable. This voltage and the voltage drop along the terminal and the external power cable clearly exceeded the power supply limit (12V) and caused a current drop. By the end of the 2 second pulse, the current had dropped to about 6.9 kA. However, the cable voltage continued to rise until it exceeded 5V, indicating heat generation in the conductor. On the other hand, the voltage between the cable and the connector joint (joint voltage) dropped from about 0.3V to 0.17V. This is the same rate as the current drop.
[0090]
A rise in cable voltage indicates that a rise in temperature has occurred. The measured voltage was divided by the corresponding current to obtain the resistance response of the cable. The result is shown in FIG. At the start of the 12.8 kA pulse, the cable resistance was 0.25 mΩ (compared to 0.54 μΩ at the critical current) and rose to 0.72 mΩ at the end of the 2 second pulse. (The discontinuity at the beginning and end of the current pulse is due to the cable voltage divided by the current near zero). Based on the change in silver resistance as a function of temperature, the change in cable resistance indicates that the high temperature superconductor had heated to about 170K by the end of the pulse. According to FIG. 12, the cable voltage almost disappears after the current pulse, but the cable resistance of FIG. 13 decreases relatively slowly, and reaches about 0.1 mΩ after 7 seconds.
[0091]
In the cable structure, the high-temperature superconducting tape is separated from the refrigerant by the base tape at the inner diameter portion, and is separated from the refrigerant by the cryogenic dielectric tape at the outer diameter portion. Therefore, cooling of the high-temperature superconducting tape is basically performed only by heat conduction. The heating of the HTS tape during the fault current pulse can be considered adiabatic. When the product (power) of the current and the voltage in FIG. 12 is integrated, the total heat generation energy of the conductor in this pulse application is about 80 kJ. Using the silver specific heat integration, the conductor is estimated to heat up to about 175K adiabatically. FIG. 13 shows that the joint resistance during pulse application remains at 24 μΩ. There is no noticeable temperature rise at the joint. This is because the cooling condition of the joint is good (direct contact with liquid nitrogen).
[0092]
The fact that the cable resistance becomes much higher than the value at Ic, depending on the overcurrent, means that the HTS cable has an inherent current limiting function. Determining the cable resistance increase range during the overcurrent period indicates the current limit range of the cable. For 15 overcurrent pulses, the cable resistance was determined based on the peak current arrival time of the pulse current. The result is shown in FIG. 14 as a function of the current. The resistance of the 5 m cable jumped from 0.54 μΩ at Ic to 0.31 mΩ at 15.3 kA. That is, it has risen almost 600 times.
[0093]
According to the DC VI curve (see FIG. 18), the cable of the present invention increases in proportion to I to the 3.8th power (n value) above Ic. For this reason, the resistance of the superconductor at the overcurrent I increases from the value of 0.54 μΩ at Ic to (I / Ic).^2.8It is obtained by the coefficient of The high temperature superconducting tape of the present invention contains 70% silver in addition to the superconductor. By using a resistance of 0.3 mΩ, the resistance of the silver substrate in the cable is estimated to be about 0.25 mΩ at liquid nitrogen temperature. For this reason, the cable resistance in the overcurrent situation was calculated by comparing the obtained high-temperature superconductor resistance with the silver resistance. The resulting calculated curve is shown in FIG. This calculated curve is in good agreement with the measured data. Therefore, it is appropriate to obtain the high-temperature superconductor resistance equal to or higher than Ic by the power law.
[0094]
This means that the high-temperature superconductor in the cable of the present invention shared a fault current at 8.1 kA (about 9 times the critical current) as well as the silver base material. When the value is less than this value, the current almost flows through the superconductor. When the value is more than this value, the current flowing through the silver base material gradually increases. At 15 kA, the superconductor can carry only 15% of the fault current. Above 10 kA, the measured data exceeds the calculated curve. This indicates that the tape heating reached the peak value before the fault current.
[0095]
Since high voltage drops across the cable occur during the fault overcurrent period, longer pulse lengths result in higher power and significant total energy loss. When this energy is discarded by the refrigerant, it causes an increase in temperature and pressure, and impairs the cooling system. The pulse application of FIG. 12 produced the largest energy loss among all pulse applications. FIG. 15 shows the response of the temperature sensor in the pulse application. The sensor “outside T” was arranged close to the refrigerant outlet of the cable inside the terminal. The sensor “T far” was placed on the cable side of the far end terminal. These sensors are present in the flowing refrigerant, but do not show any increase in temperature during and after the current pulse. Only the sensor "T bus" located on the bus side of the terminal at the far end showed a temperature rise of about 5K 3 seconds after the pulse. This sensor was cooled by the resident gas and was at a higher temperature of about 96K.
[0096]
The total energy generated in the cable by this pulse application was about 80 kJ. If half of this energy was immediately dumped into liquid nitrogen in the inner pipe (former), the temperature rise would have been about 5K. However, no such temperature increase was observed. The phase conductor is not directly cooled by the coolant. It takes several tens of seconds for the conductor to cool (release heat to the refrigerant). A liquid nitrogen flow of about 0.2 m / s in the pipe is sufficient to prevent a measurable increase in refrigerant temperature.
[0097]
FIG. 16 shows the corresponding pressure changes under the same pulse application. Contrary to the temperature response, both the inlet pressure and the outlet pressure begin to rise one second after the start of the pulse and reach a peak value of about 0.34 atmosphere (5 psi) one second after the pulse. Both pressure measurement points were several meters away from the refrigerant inlet and outlet of the cable. The pressure wave reached there for a fraction of a second (at the speed of sound in liquid nitrogen). Since no temperature increase in the refrigerant was observed, the pressure increase was due to transient heat generation in the terminal.
[0098]
In the simulation over one hour by applying 15 fault current pulses, the cumulative temperature rise of the refrigerant at the cable outlet was about 1K. There were no significant system pressure changes. The fault current was repeated every few minutes, and no damage was found in the high-temperature superconducting cable and the cryogenic system of the present invention.
[0099]
Also of concern are the voltages and currents that fault currents in the phase conductors cause in the shielding loop. In the experiment, the two ends of the superconducting shield were connected with a copper cable and a shunt, and the induced current was observed. FIG. 17A shows the induced current of the shielding loop when a 12.8 kA fault current is applied for 2 seconds. Only transient currents of about 350 A and 120 A were induced in the shield at the rise and fall of the phase conductor overcurrent. These low values are due in part to the relatively long rise and fall times (about 300 ms) of overcurrent from the present power supply. If the fault current rises faster, the induced transient in the shield will be higher. There was no measurable induced current in the shield during the 2 second slow drop in overcurrent.
[0100]
FIG. 17B shows that the maximum voltage generated at the shielding conductor was 0.35 mV or less. Since this voltage was below the critical current voltage for the cable of 0.5 mV and the induced transient was below the critical current, the shield conductor remained superconductive during the fault current pulse.
[0101]
To determine whether the simulation with the fault current pulse reduced the cable quality, the DC VI curve of the cable was measured after the fault current test. FIG. 18 shows the VI curve of the cable compared to the measurement one year ago. No difference is seen between the two VI curves. Between these two measurements of DC VI, a high voltage resistance test of 18 kV, an impulse test of 90 kV, a long time test (72 hours) at design current and voltage, and cooling and warming up several tens of times were performed. In these tests, the cable did not show any degradation in DC characteristics. On a 1 μV / cm basis, the critical current of the cable remained at about 910A.
[0102]
HTS cables have been proposed to replace existing underground cables. In general underground cable laying, three individual phases are laid in separate ducts. It seems that the cooling device for supplying the cryogenic liquid nitrogen may be provided at one end of the cable. FIG. 19 shows a single cryogenic countercurrent cooling arrangement for a high temperature superconducting cable. The HTS cable former and cryogenic wall are typically flexible corrugated stainless steel tubing. The high temperature superconducting cable of the present invention is of a low temperature dielectric configuration, requires a superconducting shielding layer, and isolates it from the main conductor by a dielectric. This shielding layer carries the same current as the main conductor.
[0103]
In a counter-current cooling arrangement, the liquid nitrogen passes through the high-temperature superconducting cable former, cools the end and the cable, and returns in the annulus between the cable outer and the inner cryogenic wall. In a parallel flow arrangement, liquid nitrogen flows through the cable former and the annulus in the same direction. Liquid nitrogen is returned through another vacuum jacket duct. This single return cryogenic device is identical to the cable cryogenic device. Table 5 shows the dimensions used in the study of these two cases.
[0104]
[Table 5]

In the HTS cable transmission system, two-phase flow of liquid nitrogen shall not be permitted. The first reason is that the pressure drop in a two-phase flow is higher than in a single-phase flow. In addition, bubbles in the dielectric can reduce the level of cable electrical insulation.
[0105]
Countercurrent loss and thermal analysis of the HTS transmission cable system were performed. This was done by performing the energy balance of the cable system. The one-dimensional energy balance of a HTS cable can be written as:
[0106]
(Equation 1)

Where ρ is the density, C is the heat capacity, z is the coordinate direction along the cable axis, k is the thermal conductivity, and A is the cable cross-sectional area.
[0107]
The energy balance of the HTS cable is calculated by the convective heat conduction term Q_{conv, i}including. This term includes convection to the liquid nitrogen stream in the former and in the annular region between the cable and the cryogenic inner pipe. Product kA of thermal conductivity and cable cross section_HTSIs a constant in this example and is equal to 0.16 watt-meters per kelvin. The following energy balance equation is further required for the liquid flow.
[0108]
(Equation 2)

Where i is each liquid nitrogen stream (former stream, annular stream, and if applicable, return stream). Convective heat transfer to the inside flow exists only in the HTS cable former. The outer nitrogen flow provides convective heat exchange between the outside of the high temperature superconducting cable and the inside of the double walled cryogenic.
[0109]
The convective heat transfer coefficient is calculated by the following equation.
[0110]
(Equation 3)

Where N_NuIs the Nusselt number, C_hIs the heat transfer coefficient, k_LN2Is the thermal conductivity of liquid nitrogen, N_ReIs the Reynolds number, N_PrIs the Prandtl number.
[0111]
Applying the above equation to the difference method, numerically integrating over time to obtain a steady state. After determining the temperature characteristics, the pressure drop is approximated by integrating the following equation over the entire length of the flow path.
[0112]
(Equation 4)

Where V is the flow velocity and f is the coefficient of friction.
[0113]
For rigorous processing, it is necessary to combine thermo-hydraulic solutions. However, the compression drop and density change are considered to be small for liquid nitrogen under the conditions considered in this example. Therefore, temperature and pressure characteristic solutions were performed individually. It is believed that the coefficient of friction of the cryogenic liquid flow in the corrugated bellows is four times that of a smooth pipe. In this example, the Reynolds number is 10⁵-10⁶And a constant coefficient of friction f = 0.07 was used. The terminal pressure drop is ignored because it is presumed to be small.
[0114]
The high-temperature superconducting cable cryostat has a flexible double-wall structure having the dimensions shown in Table 5. The sink temperature is T_∞= 300K. A typical commercially available vacuum insulated flexible cryostat has an effective actual installation thermal conductivity k_eff= 0.0008 watts / meter / kelvin. The local heat transfer per unit length can be calculated and the local liquid nitrogen temperature T_vi(X) and cryostat inner pipe diameter D_ciAnd outer pipe diameter D_coDepends on. The temperature difference driving this heat transfer term is typically 220K for the outer cryostat.
[0115]
(Equation 5)

The critical current was calculated from previous measurements on a 5 meter system. Using a linear approximation of the measured temperature and a reference value of 3000 A at 77 K, the critical current corresponding to the temperature is determined as follows.
[0116]
(Equation 6)

AC loss P_AC(Watt / meter) is calculated using a monoblock model.
[0117]
(Equation 7)

Where F = I_p/ I_cIs the ratio of the peak current in the AC cycle to the critical current of the superconductor, f is the frequency, h = (D_o ²-D_i ²) / D_o ²It is. This study is 1500A_rmsAnd 2000A_rmsThe results for the operating currents of FIG.
[0118]
The dielectric loss depends on the design voltage of the cable. A nominal value of 0.05 watts / meter is expected, which is consistent with earlier tests.
[0119]
This model was compared with measurements on a 5 meter high temperature superconducting cable system. FIG. 20 shows a comparison with the measurement operation temperature. In this measurement, a 5 meter high temperature superconducting cable was cooled by supplying 210 grams / second of liquid nitrogen at a temperature of 79.2K and a pressure of 5.4 atmospheres. 1250A for the cable_rmsWas applied. The measured values are qualitatively consistent with the calculated values. The main cause of the temperature difference is the use of a simplified thermal model for the terminal. For short cables, the terminals generate most of the system heat load. The terminal has vacuum insulation and electrical insulation. Each terminal includes two optimized current leads, rated current 1250A_rmsCarry more than The terminal heat load is about 300 watts at each end. Although the terminal thermal load varies slightly depending on the level of the operating current, the difference is small and is ignored for long cables, especially since the difference is small.
[0120]
For HTS power transmission systems, the terminal heat load is constant and independent of the length of the transmission cable system. AC losses and heat loads through the cryostat depend on the length of the HTS transmission cable system and the configuration, supply temperature, and flow rate of the refrigerant flow.
[0121]
The liquid nitrogen conditions were kept constant. That is, the pressure was 10 atm and the temperature was 67K. Liquid nitrogen properties were obtained using GASPAK. The pressure is well within the performance of commercial flexible cryostats, and the temperature is typical of cryogenic cooling systems. Since the triple point of nitrogen is about 63.2K, a low temperature of, for example, 65K can be achieved using a closed cycle cooler.
[0122]
Figures 21 and 22 show the critical current and temperature curves for both cooling arrangements at a flow rate of 1000 grams / second / phase for two examples of long cable. The presence of a temperature limit is clearly visible in the case of countercurrent. In this case, the lower temperature line is the former flow temperature, the center line is the high temperature superconducting cable temperature, and the upper line is the annular return flow temperature. In FIG. 23, the pressure drop for a 500 meter long countercurrent is 4.7 bar. Increasing the flow to lower the cable temperature also increases the pressure drop. Since the pressure is already high, there is a possibility of boiling in the cable. In both cooling configurations, when the current is high, a higher temperature is generated, lowering the critical current of the HTS cable and increasing the AC loss.
[0123]
Figures 23 and 24 show the cooling load at 67K for both cooling configurations as a function of length. From these results, it can be seen that the lower current carried in the cable significantly reduces the AC losses, but the cryostat loss is completely independent of the current level in the cable.
[0124]
In high-temperature superconducting power transmission cable systems, many important factors depend on the refrigerant flow rate. Analyzes were performed for a 250 meter countercurrent case and a 1000 meter parallel flow case, and a current of 2000 A at different flow rates._rmsThe maximum cable temperature and pressure drop at were determined. The results are shown in FIGS.
[0125]
In both configurations, the temperature inside the cable increases at low flow rates. In the case of countercurrent, the system temperature at which liquid nitrogen returns to the cooling device is lower than the cable maximum temperature. The reverse is true for a parallel flow cooling configuration. If the flow rate is low, the maximum temperature of the countercurrent cooling cable will be high, and the superconductivity of the cable will be significantly reduced. Increasing the flow reduces the maximum temperature but increases the pressure drop. These flow issues are not seen at a length of 2000 meters for parallel flow.
[0126]
The high-temperature superconducting cable configuration shown in FIG. 19 is a single cryostat countercurrent cooling configuration. The former of the HTS cable is a flexible corrugated stainless steel tube. High temperature superconducting cables have a low temperature dielectric configuration and require a superconducting shielding layer. This shielding layer is separated from the main conductor by a dielectric. The shield is designed to carry the same current as the main conductor.
[0127]
Features of the coaxial design are: 1) image current in the shielding layer reduces external magnetic fields and eddy currents in the cryostat and duct; 2) high temperature superconductors and dielectrics are wound in tape; 3) cryogenic dielectrics Reduces the size and increases the current carrying capacity. 4) The flexible cable enables reel winding.
[0128]
A 30 meter high temperature superconducting cable was manufactured using a specially designed machine for winding superconducting tape around a former. After winding the first two cables including the main conductor, the cryogenic dielectric, and the shield, they were placed on the cryostat. A special end cap for cryostat was prepared to allow cable cooling with liquid nitrogen at atmospheric pressure. Temporary voltage taps are provided at the ends of the main conductor and the shield conductor, so that the critical current of only the superconductor can be measured. FIG. 28 shows a DC VI curve of the second phase main conductor of the 30-meter high-temperature superconducting cable. Critical current I observed on the basis of 1 μV / cm_cWas 2980A and n value was 9. The critical current of the first phase main conductor was 3000 A (power supply limit) or more. As also shown in FIG. 28, in the case of one example of a 5-meter cable, the critical current is 1090 A and the n value is 3. The design current for the 5 meter and 30 meter cables was 1250A. The same number of layers and the same number of superconducting tapes were used for the 5 meter and 30 meter cables. From the time of procurement of the superconducting tape for 5 meter cable to the time of procuring the superconducting tape for 30 meter cable, the tape performance improved dramatically. As a result, a 30 meter cable is actually a conductor up to 3000A, despite being designed for a rating of 1250A. Due to such a large margin, the superconductor did not exhibit resistance when operated at a current of about 1500 A or less. This is evident in FIG. At 2500 A, the voltage drop was 0.25 μV / cm and the DC resistance was 0.01 μΩ / m. After measuring the critical current, the temporary voltage tap was removed.
[0129]
In FIG. 29, the place where the high-temperature superconducting cable is laid is a portion surrounded by a dotted line, and is a part of a power distribution system to three manufacturing factories. The switching substation constructed for this project can provide the load with one or both of superconducting cables and overhead lines. The control building includes electrical control and protection panels for superconducting cables, cryogenic control systems, and conference rooms.
[0130]
Power is provided to the transmission substation by two 115 kV transmission lines. The transmission substation has a total capacity of 40 MVA and has two 20 MVA matched unregulated buck transformers. The high voltage side of these transformers is 115 kV and the low voltage side is 12.4 kV.
[0131]
Two 12.4 kV feeders exit the substation. Both feeders use 1033.5 ACSR (steel core aluminum stranded wire). This is a typical type of conductor. A vacuum circuit breaker is provided as a 12.4 kV protection system, and a reclosing relay that performs reclosing three times before lockout is used. The symmetric three-phase fault current is about 14,000 A at 12.4 kV. The 5 meter superconducting cable has passed tests at fault currents above this level.
[0132]
The high-temperature superconducting cable is cooled by circulating cryogenic liquid nitrogen in three phases. Based on the 5 meter cable test program, the conditions for the 30 meter cable cryogenic system were determined to be 3000 W thermal load, operating temperature range 70-80 K, flow rate 1.3 l / s (21 gpm), and maximum operating pressure 10 bar. Two cryogenic cooling system designs were considered as options. These are an open loop boiling bath cooler and a closed cycle cooler. The reason for choosing an open loop device is that capital equipment costs are low. The operating costs of both devices are comparable. The reason is that a large liquid nitrogen production plant has higher efficiency than a small cooling device. This compact cooling device was to be used in a 30 meter high temperature superconducting cable system. To extend the operating life of the cable system, a closed loop cooling system including an open loop nitrogen backup device used during main unit maintenance would be a better choice. This is because the closed loop refrigeration system does not need to replenish large storage dewars frequently.
[0133]
Cryogenic system design consists of three main parts. That is, a liquid nitrogen storage tank, a cryogenic skid, and a vacuum outer pipe.
[0134]
The cryogenic skid includes a cryogenic box. The cryobox is in a common adiabatic vacuum and includes a subcooler, a phase splitter, and two buffer vessels. The cryobox and associated piping is covered with multi-layer insulation to minimize background heat load. The cryogenic skid further includes three vacuum pumps. These are used to lower the pressure in the subcooling bath and lower the operating temperature of the system. Provide main and backup liquid nitrogen circulation pumps. The liquid nitrogen stream passes from the circulation pump through the system, through a subcooler, through a supply line to the cable, through a return line, and back to the phase splitter at the pump inlet. The phase splitter is used at system startup to prevent steam from reaching the circulation pump. The subcooler receives replenishment from the bulk storage tank. Boiling of the subcooler releases to the atmosphere via a vacuum pump when operating below 80K. Buffer tanks are used alternately. That is, one is used for system pressure application and the other is refilled and on standby.
[0135]
The liquid nitrogen storage tank has a capacity of 40,000 liters. This tank is installed horizontally on a concrete base to reduce its height. The tank liquid level is monitored remotely using a telephone line and supplemented with a liquid nitrogen supply device as needed.
[0136]
The vacuum exterior piping connects the skid and the cable. There are three pipes. That is, an inlet pipe, a return pipe, and a cooling pipe, and three phases of cables are connected to each other. With this configuration, any combination of the three phases of the cable can be activated, deactivated, and cooled.
[0137]
During normal operation, the cryogenic system is operated using a programmable logic control system. System cooling and restarting is performed manually. This is because the number of these operations is small, and the programming cost increases.
[0138]
The first off-line test applied voltage to the cable using a variable AC voltage power supply. A voltage of 11-12 kV was applied from the power supply one phase at a time and held for 30 minutes to test the cable dielectric system. Phase 1 and phase 2 were maintained at 166% of the rated voltage and did not cause dielectric breakdown. Phase 3 had a slightly different shape and was maintained at 230% of rated voltage with no breakdown.
[0139]
During the test, the charging current was measured, based on which the cable capacitance was determined and compared with the calculated capacitance. This is shown in Table 6. The calculated cable inductance is also shown. The cable surge impedance is 4Ω, which is lower than conventional copper cables. Due to this low surge impedance, the AC superconducting cable has a longer critical length (the length at which the charging current becomes equal to the rated current) as compared with the conventional copper cable. Thus, superconducting cables can be laid in longer continuous lengths until the charging current accounts for most of the current carrying capacity of the cable.
[0140]
[Table 6]

3000 A DC power was supplied to measure the relationship between the DC voltage and current (VI curve) of the cable. Temporary voltage taps were provided outside the main and shielding bushings. The VI curve was measured two phases at a time. In this measurement, a DC power supply was connected to two phases at one end, and a short jumper wire was connected between the phases at the other end. Phase 2 and phase 3 were measured, and then phase 1 and phase 2 were measured. Therefore, phase 2 was measured twice. The VI curve of the main conductor was measured. Next, the shield conductor was measured. This is because the cable is a low temperature dielectric design with coaxial conductors. The DC critical current was predicted from HTS tape performance and cable design. FIG. 30 shows the VI curves for phase 2 and phase 3. Linear behavior is seen over most of the current range, depending on the location of the external voltage tap. The external voltage tap includes a copper bus bar and a connector extending beyond each end of the superconducting cable.
[0141]
The DC voltage / current test was performed again in June 2000 under variable load conditions after six months of operation and four to six cooling / warming cycles to determine cable performance.
[0142]
A DC load current test was performed to simulate the average load, rated load, and emergency load of the superconducting cable. A wide range of load current tests as shown in FIG. 31 were performed at 800 A, 1200 A, and 1400 A for 8 hours each, using a DC power supply for two main conductors at a time. In this first load test, no change was seen in the cable cooling system temperature.
[0143]
The next test was an extensive open circuit rated voltage test, powered from a substation. One end of the cable breaker was closed and the other end was kept open to prevent current from flowing through the cable. The phase voltage of each phase was maintained in several orders for up to 12 hours. Cable dielectric performance was as designed.
[0144]
FIG. 32 shows the liquid nitrogen return temperature change in phase 1 (Y-axis values are not shown). The cable temperature change is about 1K.
[0145]
FIG. 33 shows the liquid nitrogen return pressure in phase 1 (Y-axis values not shown). The cable pressure change is about 0.28 bar (4 psi).
[0146]
A cryogenic heat load test was performed several times to measure the total heat loss of the cable system. The heat loss at 600A for the 30 meter cable and all three phases of the terminal was 1490W. Various components were measured directly or separately, and the breakdown of heat loss was estimated. The heat loss is 230 W for each cable terminal and there are 6 terminals. The cable cryostats are 3 "x5" vacuum armored pipes each measuring 7.62 cm x 12.7 cm (1 W / m / phase). At 600 A, the conductor and shielding correspond to 0.2 W / m / phase.
[0147]
Although the present invention has been described and illustrated in detail, these are exemplary and not restrictive of the present invention. The spirit and scope of the present invention is limited only by the appended claims.
[Brief description of the drawings]
FIG.
1 is a perspective view showing a multilayer structure according to the present invention.
FIG. 2
FIG. 2 is a cross-sectional view showing one embodiment of the present invention.
FIG. 3
FIG. 6 is a cross-sectional view illustrating another embodiment of the present invention.
FIG. 4
It is a figure showing an embossing pattern used for the present invention.

Claims (47)

A flexible core,
A plurality of tape-shaped oxide superconducting wires each formed basically from an oxide superconductor and a stabilizing metal covering the same and arranged on the core material by a tensile force not exceeding 2 kgf / mm ² ,
Forming a layer by arranging the plurality of tape-shaped superconducting wires in parallel, arranging a plurality of such layers,
The plurality of layers are continuously stacked on the core without inserting an insulating layer between the plurality of layers and the core,
The core material imparts flexibility to the superconducting cable,
The superconducting cable maintains a superconducting state at liquid nitrogen temperature,
The wire has a substantially uniform superconducting phase in the longitudinal direction of the wire;
A c-axis of the superconducting phase is substantially parallel to a thickness direction of the wire;
The superconducting wire is formed by grains aligned in parallel and extending in the longitudinal direction of the wire,
A cable using an oxide superconductor, wherein the crystal grains are stacked in the thickness direction of the wire. The superconducting cable according to claim 1, wherein the cable has flexibility such that the superconductivity of the cable does not substantially degrade in bending up to about 50 times the diameter of the cable. The superconducting cable of claim 1, wherein the core material is selected from a group consisting essentially of metal, plastic, reinforced plastic, polymer, and composite. The superconducting cable according to claim 1, wherein the core material is a tube having a surface selected from a spiral groove surface, a mesh surface, a braided surface, and a mat surface on the outer surface of the tape-shaped superconducting wire. The superconducting cable according to claim 1, wherein no insulating layer exists between the plurality of layers. The superconducting cable according to claim 5, wherein after arranging the first tape-shaped superconducting wire layer on the core material, the plurality of tape-shaped superconducting wire layers are arranged on the surface of each layer. The superconducting cable according to claim 1, wherein the wires are twisted within the tape-shaped stabilizing metal coating. The superconducting cable according to claim 1, wherein the tape-shaped superconducting wires are arranged at an angle of up to about 90 degrees. The superconducting cable of claim 8, wherein the tape-like superconducting wires are arranged at an angle of about 10 degrees to about 60 degrees. The superconducting cable of claim 9, wherein the tape-like superconducting wires are arranged at an angle of about 20 degrees to about 40 degrees. The superconducting cable of claim 1, further comprising at least two separate tape-like superconducting wire layer groups. The superconducting cable according to claim 11, wherein the arrangement angle or the pitch of the tape-shaped superconducting wire layers alternates between successive layers. 13. The superconducting cable according to claim 12, wherein each of the continuous tape-shaped superconducting wire layers is composed of at least two tape-shaped superconducting wires and forms four or more layers. The superconducting cable according to claim 11, wherein the at least two independent tape-shaped superconducting wire layer groups are separated by a dielectric layer. The superconducting cable according to claim 11, wherein a tape-like superconducting wire layer closest to the core is separated from the core by a dielectric layer. The superconducting cable according to claim 11, wherein the dielectric is selected from the group consisting of polypropylene, polyethylene, and polybutylene. The superconducting cable of claim 11, wherein the at least two separate tape-like superconducting wire layer groups carry approximately equal amounts of current in the cable. The superconducting cable of claim 11, wherein a first one of the at least two separate tape-like superconducting wire layer groups carries at least 50 percent of the current flowing through the cable. The superconducting cable of claim 11, wherein a second one of the at least two separate tape-like superconducting wire layer groups carries at least 50 percent of the current flowing through the cable. 18. The superconducting cable of claim 17, wherein the tape-like superconducting wire layer group furthest from the core provides shielding of current flowing through other layers to reduce magnetic fields or eddy currents in the cable. The superconducting cable according to claim 1, wherein the stabilizing metal is selected from the group consisting of silver, a silver alloy, nickel, and a nickel alloy. The superconducting cable according to claim 1, wherein each of the plurality of layers includes at least two tape-shaped wires. The superconducting cable according to claim 1, wherein each of the plurality of layers includes at least four tape-shaped wires. The superconducting cable according to claim 23, further comprising an insulating layer between a second layer and a third layer among the plurality of layers. 24. The superconducting cable according to claim 23, further comprising an insulating layer between the second layer and the third layer among the plurality of layers. 15. The superconducting cable of claim 14, wherein the dielectric has a maximum permittivity of about 3.0. 27. The superconducting cable of claim 26, wherein the dielectric has a maximum permittivity of about 2.3. The superconducting cable of claim 14, wherein the dielectric is biaxially stretched in a longitudinal direction at a ratio of about 5: 1 to about 10: 1. 29. The superconducting cable of claim 28, wherein the dielectric is biaxially stretched in a longitudinal direction at a ratio of about 5: 1 to about 6: 1. 29. The superconducting cable of claim 28, wherein the dielectric is further biaxially stretched in a lateral direction to a ratio of about 2: 1. 29. The superconducting cable of claim 28, wherein the biaxially stretched dielectric is embossed to form irregular and / or random grooves. 32. The superconducting cable of claim 31, wherein the depth of the groove created by embossing the dielectric is about 0.5 to about 2 mils (ml). 32. The superconducting cable of claim 31, wherein the embossing is performed by a roller at a temperature of about 80C to about 140C. 31. The superconducting cable according to claim 30, wherein the pattern formed on the dielectric tape by the embossing allows the impregnation material to flow across the dielectric tape. 32. The superconducting cable according to claim 31, wherein the pattern formed on the dielectric tape by the embossing is irregular peaks and valleys oriented in a lateral direction of the dielectric tape. 15. The superconducting cable of claim 14, wherein the dielectric tape is made from a material containing 100 to 1000 parts per million of organic dye. 32. The superconducting cable according to claim 31, wherein the pattern formed on the dielectric tape by the embossing increases an effective tape thickness. 32. The superconducting cable according to claim 31, wherein the pattern formed on the dielectric tape by the embossing has a distance between adjacent peaks of up to about 0.2 mm. 39. The superconducting cable according to claim 38, wherein a pattern formed on the dielectric tape by the embossing has a peak-to-peak distance of up to about 0.05 mm. The superconducting cable according to claim 14, wherein the tensile coefficient of the dielectric tape is at least 250,000 psi. A superconducting cable for alternating current, comprising: a conduit for cooling a phase conductor and a neutral conductor; and outer peripheral insulation, providing a common neutral point for all three-phase conductors, and wherein the cooling conduits are concentrically arranged. The first phase conductor of the cooling conduit borders a conductive cable core, between the first phase conductor and the second phase conductor, between the second phase conductor and the third phase conductor, and An insulating layer having a predetermined thickness is provided between the third phase conductor and the neutral conductor, an annular cooling conduit is provided between the neutral conductor and the outer peripheral insulation, and each of the phase conductors is made of a superconducting material. The superconducting cable according to claim 41, manufactured by the following. 42. The superconducting cable according to claim 41, wherein each of the phase conductors is made of a superconducting tape made of a rolled flat sleeve in which a ceramic superconducting material is packed in an oxygen porous metal. 42. The superconducting cable according to claim 41, wherein each of the phase conductors is made of a tape formed by packing a ceramic superconducting material in a silver sleeve. 42. The superconducting cable according to claim 41, wherein liquid nitrogen flows through the conduit to cool the superconducting phase. The superconducting cable according to claim 41, wherein the neutral conductor is made of copper. 42. The superconducting cable according to claim 41, wherein the insulating layer between the phase conductors is made of polyethylene or polypropylene.

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