JP3794092B2

JP3794092B2 - Power converter

Info

Publication number: JP3794092B2
Application number: JP05625697A
Authority: JP
Inventors: 伸三玉井
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1997-03-11
Filing date: 1997-03-11
Publication date: 2006-07-05
Anticipated expiration: 2017-03-11
Also published as: JPH10257676A

Description

【０００１】
【発明の属する技術分野】
この発明は電力変換装置、特に電力系統に接続される超電導エネルギー貯蔵設備における交流直流変換装置に関するものである。
【０００２】
【従来の技術】
超電導エネルギー貯蔵設備は、電力系統から超電導コイルに直流電流を流してエネルギーをコイルの磁気エネルギーとして貯蔵しておき、電力需要家の消費電力が増大し、系統の電力が不足する場合にコイルエネルギーを交流電力に戻して電力需要家に供給し電力不足を補おうとするものである。このような用途には、交流を直流に、また直流を交流に変換する電力変換器が用いられる。また、これら設備は容量が数十、数百ＭＷに達する非常に大きいものがあり、変換器はコイルに流す電圧、電流に応じて、直列または並列に複数台接続される。
【０００３】
図１８は典型的なこのような従来の電力変換装置の構成例を示した構成図である。図１８において、１０１はコイル、１はコイル１０１を含む直流回路、２０１、２０２、２０３は他励式変換器、２は交流を直流に変換する電力変換器群、３は電力系統、４は進相コンデンサ、５は進み無効電力を可変制御できるＳＶＣ（Static Var Compensator）、６は電力変換器群から発生する高調波を抑制する高調波フィルタである。
【０００４】
次に動作について説明する。電力変換器群２のそれぞれの他励式変換器２０１、２０２、２０３は、電力系統３からの交流電力を直流に変換し、コイル１０１に直流電流を流す。電力変換器群２はサイリスタを用いた他励式変換器で構成され、位相制御される。進相コンデンサ４は電力変換器群２の交流側に接続され、その進み無効電力で他励式変換器で構成される電力変換器群２の発生する遅れ無効電力をキャンセルする。進相コンデンサ４は数バンクに分けて構成され、他励式変換器の無効電力量に応じて、適当な量だけ切り換えて選択し、交流側に接続できるようになっている。
【０００５】
しかし、進相コンデンサのバンク数をむやみに多くすることは、スイッチの数も増え、経済的にも不利であり、装置も大きくなるため好ましくない。このため、進相コンデンサのバンク数には限度があり、通常は他励式変換器の無効電力と一致した進相コンデンサ量を選ぶことができず、無効電力に多少のアンバランスが生じる。そこで、電力系統によっては、更にきめ細かく無効電力を調整できるよう、無効電力を無段階調節できるＳＶＣ５を設置し、進相コンデンサ４の進み無効電力と他励式変換器の遅れ無効電力とのアンバランス分を補償する。この時、進相コンデンサ４とＳＶＣ５を合わせた最大の進み無効電力が、電力変換器群２の発生する最大無効電力量に見合うように進相コンデンサ４とＳＶＣ５の容量を設定しなければならない。
【０００６】
電力変換器群２を構成する他励式変換器の代表例であるサイリスタ変換器を備えた従来の電力変換装置の動作を図１９を用いて説明する。図において、直流側に接続されたコイル１００の直流電流Ｉ_dを計測装置で測定し、コイル電流指令との差を０に近くするように制御器２５１がコイル電圧指令Ｖ_dREFを出力する。直流電圧Ｖ_dと制御角αの関係は、交流相電圧の振幅をＶとすると、
【０００７】
【数２】

【０００８】
であり、これを制御パルス位相計算回路２５１で計算して、制御角αとし、他励式変換器を構成するそれぞれの素子のＯＮ、ＯＦＦ指令を作成する。サイリスタ変換器の動作を簡単に説明するため、図２０に単相サイリスタ変換器を、図２１にその電圧電流波形例を示す。図２１に示すように、単相交流電圧の０点から位相αだけ遅れた角度で電流Ｉ_adが流れる。次のサイリスタがスイッチングするのは、電圧が同じ位相関係となる１８０度後になるので、電流は１８０度の方形波となる。この電流が流れる期間は交流電圧で決まり、単相では１８０度、３相では１２０度で固定されてしまう。そのため、有効電力を変更しようとして制御角を変えると、電圧と同相の電流成分は勿論変化するが、それと同時に電圧と９０度位相のずれた無効分電流も変化してしまう。これを式で説明すると次のようになる。
【０００９】
まず、交流相電圧ｖ=Ｖsinθに対して交流電流の基本波は、
【００１０】
【数３】

【００１１】
となる。ここでは問題に関係の小さい、転流時の電流変化時間を無視し、変換器に付属する変圧器の巻き数比は１と仮定して考える。
さて、式（２）は、
【００１２】
【数４】

【００１３】
と変形できるから、交流電圧に対して９０度位相のずれた成分、即ち無効分電流Ｉ_Qは、
【００１４】
【数５】

【００１５】
となる。
従って、無効電力Ｑは回路を３相回路として、
【００１６】
【数６】

【００１７】
となる。また、他励式変換器の直流電圧Ｖ_dは、
【００１８】
【数７】

【００１９】
であるから、式（６）のαをＶ_dで置き換えると、
【００２０】
【数８】

【００２１】
となる。
【００２２】
無効電力は、式（５）または式（７）で表されるため、他励式変換器の制御角αと直流電圧Ｖ_d、または直流電圧Ｖ_dと直流電流Ｉ_d に依存して変化する量となる。従って、コイル電流を変化させようとすると、直流電圧Ｖ_dを変化させてコイルに電圧をかけて制御するため、本構成の電力変換装置は運転状態によって必ず無効電力が変化する。このような無効電力の変化は、進相コンデンサの切り換え、あるいはＳＶＣによる調節により補償する。従って、他励式変換器群の発生する無効電力の最大値に合わせた進相コンデンサ、ＳＶＣが必要となる。
【００２３】
さらに、他励式変換器は電流をオン、オフするため、高調波を発生する。その高調波は、基本波交流に対して、１１次、１３次などの低次の高調波が多く、その抑制のために高調波フィルタ６を設置しなければならない。
【００２４】
また、図２２は典型的な従来の電力変換装置の他の構成例を示した構成図である。図１８と同一符号は同一または相当部分を示すので説明を省略する。
図２２において、１０２、１０３、１０４、１０５はコイル、２０４、２０５は他励式変換器である。
【００２５】
次に動作について説明する。この構成例は、図１８の構成例に対して、コイルが複数あった場合であり、電力変換器群２の中で、電力変換器２０１、２０２はコイル１０１の電流を制御し、電力変換器２０３はコイル１０２の電流を制御する。電力変換器２０４、２０５はコイル１０３の電流を電流バランス用コイル１０４、１０５を通じて制御する。
このようにコイルが１台だけでなく、複数あった場合には、それぞれのコイルに流す電流、電圧によって変換器を直列構成にしたり、並列構成にしたり、単体で用いたりする。
【００２６】
この場合も図１８の構成例と同様に、コイル電流、電圧の制御動作によって、交流側の無効電力が変化することは言うまでもない。従って、これら他励式変換器群の無効電力を補償するために、図１８と同様に、他励式変換器群の発生無効電力の最大値に合わせて進相コンデンサ４、ＳＶＣ５が必要となる。
さらに、この場合も他励式変換器の低次の高調波が多く、その抑制のために高調波フィルタ６を設置しなければならない。
【００２７】
また、図２３は典型的な従来の電力変換装置の他の構成例を示した構成図である。図１８、図２２と同一符号は同一または相当部分を示すので説明を省略する。
【００２８】
次に動作について説明する。この構成例は、図１８の実施例に対して、コイルに流す電流を増やすために電力変換器２０１、２０２、２０３の直流側を並列に接続した場合である。それぞれの電力変換器の直流電流をバランスさせるために電流バランス用コイル１０４、１０５、１０６を通じて並列接続している。電力変換器群２の中で、それぞれの電力変換器２０１、２０２、２０３は、それぞれコイル１０４、１０５、１０６の電流を制御し、その合計としてコイル１０１の電流を制御する。
【００２９】
この場合も図１８、図２２の構成例と同様に、コイル電流、電圧の制御動作によって、交流側の無効電力が変化することは言うまでもない。従って、これら他励式変換器群の無効電力を補償するために、図１８、図２２と同様に、他励式変換器群が発生する無効電力の最大値に合わせて進相コンデンサ４、ＳＶＣ５が必要となる。
さらに、この場合も他励式変換器の低次の高調波が多く、その抑制のために高調波フィルタ６を設置しなければならない。
【００３０】
【発明が解決しようとする課題】
従来の電力変換装置は以上のように構成されているので、電力変換器群の発生する遅れ無効電力が一定ではなく、変換器の直流電流および直流電圧により変化するため、無効電力補償のための進相コンデンサ４、ＳＶＣ５を他励式変換器の動作範囲における最大無効電力に従って容量を決め、設置しなければならなかった。また、他励式変換器の無効電力の変化に合わせてＳＶＣ５を制御したり、進相コンデンサ４のいくつかをオンオフしなければならず、進相コンデンサの切り換えスイッチが必要となり、装置の構成が複雑となった。さらに、他励式変換器の高調波を抑制するための高調波フィルタが必要であった。これら交流側の付加設備は広い面積をとるため、設備全体の敷地面積が大きくなるという問題点があった。
【００３１】
この発明は上記のような問題点を解消するためになされたもので、ＳＶＣを設置する必要がなく、また、進相コンデンサもいくつかに分けてオンオフすることがなく、一定の状態でよく、かつ容量も少なく、または無くしてしまうことのできる電力変換装置を得ることを目的とする。
また、高調波フィルタの容量も低減、または無くしてしまうことのできる電力変換装置を得ることを目的とする。
【００３２】
【課題を解決するための手段】
この発明の電力変換装置は、少なくとも１台の他励式変換器と、少なくとも１台の自励式変換器の双方からなり、電力系統からの交流を直流に変換する複数台の変換器と、上記自励式変換器と上記他励式変換器とに接続され、変換された直流を通流させるエネルギー貯蔵用のインダクタンス要素とを備えた電力変換装置であって、自励式変換器の発生する無効電力により、他励式変換器の発生する無効電力の変化をキャンセルして、上記複数台の変換器の発生する無効電力が直流電流および直流電圧に関わらず一定になるように上記自励式変換器を制御したものである。
【００３３】
この発明の電力変換装置は、上記他励式変換器における制御角αと直流電流Ｉ_d とから、次式
【００３４】
【数９】

【００３５】
で演算した遅れ無効分電流Ｉ_Q を各他励式変換器毎に求め、これら遅れ無効分電流の和と、電力系統に接続された進相コンデンサの進み無効分電流との差を求め、これを自励式変換器の無効分電流の制御指令値とし、有効分電流指令値と共に上記自励式変換器を制御したものである。
【００３６】
この発明の電力変換装置は、上記他励式変換器群が同じ交流母線に接続され、交流母線の他励式変換器群の接続点よりも電力系統側、かつ自励式変換器よりも他励式変換器群側の交流母線の電圧、電流から他励式変換器群の遅れ無効分電流を計測し、上記遅れ無効分電流と、電力系統に接続された進相コンデンサの進み無効分電流との差を求め、これを自励式変換器の無効分電流の制御指令値とし、有効分電流指令値と共に上記自励式変換器を制御したものである。
【００３７】
この発明の電力変換装置は、他励式変換器群が同じ交流母線に接続され、交流母線の他励式変換器群の接続点よりも電力系統側、かつ自励式変換器よりも他励式変換器群側の交流母線の電圧、電流から他励式変換器群の遅れ無効電力を計測し、上記遅れ無効電力と、電力系統に接続された進相コンデンサの進み無効電力に相当する値との差を求め、これを自励式変換器の無効電力の制御指令値とし、上記制御指令値を系統電圧で除した無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したものである。
【００３８】
この発明の電力変換装置は、交流母線の全ての変換器群の接続点よりも電力系統側、かつ電力系統に接続された進相コンデンサよりも変換器群側の交流母線の電圧、電流から全ての変換器群の無効分電流または無効電力を計測し、上記無効分電流または無効電力と、上記進相コンデンサの進み無効分電流または進み無効電力に相当する値との差を求め、これを自励式変換器の無効分電流または無効電力の制御指令値とし、上記制御指令値が０またはそれに近い値となるように上記自励式変換器の無効分電流指令値を作成する制御器を備え、上記無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したものである。
【００４０】
この発明の電力変換装置は、交流母線の全ての変換器群および進相コンデンサの接続点よりも電力系統側の交流母線の電圧、電流から無効分電流または無効電力を計測し、上記無効分電流または無効電力が、０またはそれに近い値となるように上記自励式変換器の無効分電流指令値を作成する制御器を備え、上記無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したものである。
【００４１】
この発明の電力変換装置は、上記各電力変換装置において、自励式変換器を電流形コンバータで構成したものである。
【００４２】
この発明の電力変換装置は、上記各電力変換装置において、自励式変換器を、直流側にコンデンサを接続した、交流直流変換のための電圧形コンバータと、上記電圧形コンバータのコンデンサとインダクタンス要素との間に接続され、直流電圧を可変に制御して上記インダクタンス要素に流れる電流を制御するチョッパとで構成したものである。
【００４３】
この発明の電力変換装置は、上記電力変換装置において、電力系統に接続された進相コンデンサの進み無効電力を、システムの運転状態において他励式変換器が発生する最大遅れ無効電力と最小遅れ無効電力の和の１／２としたものである。
【００４４】
この発明の電力変換装置は、上記各電力変換装置において、他励式変換器の高調波電流を計測し、自励式変換器の無効分電流指令値及び有効分電流指令値より、上記他励式変換器の高調波電流を減算して他励式変換器の高調波電流をキャンセルするようにしたものである。
【００４５】
【発明の実施の形態】
実施の形態１．
以下、この発明の実施の形態を図について説明する。図１はこの発明の実施の形態１による電力変換装置を示す構成図である。図において、従来のものと同一符号は同一または相当部分を示すので説明を省略する。図において、２１０は交流を直流に変換する自励式変換器、２２１、２２２は他励式変換器２０１、２０２の直流電流、制御角から無効分電流を演算する演算回路、２２３は演算された各無効分電流の総和をとる総和演算器、２２４は加減算器、２２５は進相コンデンサ４の無効分電流値を設定する設定回路、２２６は自励式変換器２１０の有効分電流指令を設定する設定回路、２２７は自励式変換器２１０の交流電流制御器である。
【００４６】
次に動作について説明する。まず、従来のものと同様に、変換器２０１、２０２は直流電流を制御するために制御角αを調整する。演算回路２２１、２２２は、他励式変換器２０１、２０２の直流電流Ｉ_d 、制御角αから、各他励式変換器の無効分電流Ｉ_Q を
【００４７】
【数１０】

【００４８】
で演算する。それらの総和を総和演算器２２３でとると、全ての他励式変換器が発生する遅れ無効分電流が計算できる。
【００４９】
そこで、交流系統に接続されている進相コンデンサ容量とほぼ同じ無効分電流設定値Ｉ_CQを設定回路２２５で設定し、この設定値から上記遅れ無効分電流の値を加減算器２２４で減算すると、他励式変換器の遅れ無効分電流と進相コンデンサの進み無効分電流の差の無効分電流（Ｉ_CQ −Ｉ_Q）が演算できる。それを自励式変換器２１０の無効分電流指令として交流電流制御器２２７に出力する。
一方、変換器２１０は直流電流を制御するために設定回路２２６で必要とする自励式変換器２１０の有効分電流の値を設定し、この設定値を自励式変換器２１０の有効分電流指令として交流電流制御器２２７に出力する。
交流電流制御器２２７は上記無効分電流指令と上記有効分電流指令を用いて自励式変換器２１０の交流電流を制御する。
【００５０】
なお、設定回路２２６および交流電流制御器２２７は、自励式変換器２１０の構成によって異なる。図２は電流型変換器を自励式変換器として用いた例である。コイル電流指令と測定コイル電流の差を０に近くなるように制御器が有効分電流指令を制御する。有効分電流指令と無効分電流指令は、互いに９０度位相がずれているので図３に示すように、ベクトル的に合成して和を取り、交流電流指令を作る。その交流電流指令を三角波発生器と比較してパルス幅変調をかけ、制御パルス発生回路を通してスイッチング素子をＯＮ、ＯＦＦする。図４、図５は単相自励式変換器の場合の動作説明図である。交流電圧に対して、任意の位相で電流を交流側へ流すようＳＷ１、ＳＷ４をＯＮしたり、ＳＷ２、ＳＷ３をＯＮしたりできる。電流０の期間は、直列に接続されたスイッチをＯＮして直流側を短絡し、交流には電流を流さない。
【００５１】
このようにして自励式変換器２１０において交流電流を制御することにより、他励式変換器２０１、２０２、自励式変換器２１０、進相コンデンサ４の全体で流れる無効分電流の和は、
Ｉ_Q＋（Ｉ_CQ −Ｉ_Q）−Ｉ_CQ＝０・・・（９）
となり、キャンセルされて０になる。
【００５２】
図６は本実施の形態による電力変換装置の無効分電流の大きさの変化を示すものであり、図に示すように、他励式変換器の無効分電流Ｉ_Q が時間とともに変化しても、それに応じて自励式変換器の無効分電流が制御される、例えば、他励式変換器の遅れ無効分電流の大きさが進相コンデンサの無効分電流の大きさよりも大きかった場合、自励式変換器は進みの無効分電流を流して、遅れの無効分電流を補償する。また、他励式変換器の遅れ無効分電流の大きさが進相コンデンサの無効分電流の大きさよりも小さかった場合、自励式変換器は遅れの無効分電流を流す。このようにして、全体として無効分電流を一定に制御するため、交流側の進相コンデンサをオンオフする必要はなく、一定のままでよい。また、別にＳＶＣを置いて無効電力補償量を変化させなくとも、電力変換器群２を構成する自励式変換器自体が無効電力の変化分を吸収し、常に無効電力を一定にするように制御されているため、ＳＶＣも必要ない。
【００５３】
また、進相コンデンサの容量は、全ての変換器が他励式の従来例の場合、他励式変換器の最大無効電力に合わせて進相コンデンサの容量を決めなければならなかったが、本実施の形態においては、他励式変換器の遅れ無効電力の最大値から、自励式変換器が発生する進み無効電力分だけ差し引いた小さな容量で良く、進相コンデンサの容量も大幅に少なくできる。
【００５４】
なお、上記他励式変換器は電流形の変換器が通常用いられており、超電導エネルギー貯蔵においては直流コイルの電源であるので電流形変換器が都合がよい。本実施の形態１においては自励式変換器２１０も電流形変換器を用いたので、コイル電源としてそのまま利用でき、構成が簡単になる。
【００５５】
実施の形態２．
上記実施の形態１では、他励式変換器の直流電流Ｉ_d 、制御角αから、他励式変換器の無効分電流Ｉ_Q を演算で求めたが、図７に示すように、他励式変換器群と自励式変換器の間の交流母線の電圧、電流から交流母線電圧より９０度位相の遅れた無効分電流を無効分電流測定回路２２８で測定して、他励式変換器群の遅れ無効分電流Ｉ_Q を求め、以下同様に、設定回路２２５で設定された設定値から上記遅れ無効分電流の値を加減算器２２４で減算して自励式変換器２１０を制御してもよく、上記実施の形態１と同様の効果が得られる。
【００５６】
実施の形態３．
上記実施の形態１では、他励式変換器の直流電流Ｉ_d 、制御角αから、他励式変換器の無効分電流Ｉ_Q を演算で求め、自励式変換器２１０を制御したが、図８に示すように、他励式変換器群と自励式変換器の間の交流母線の電圧、電流から遅れ無効電力Ｑを無効電力測定回路２３０で測定し、測定された遅れ無効電力Ｑを用いて自励式変換器２１０を制御してもよい。この場合は、交流系統に接続されている進相コンデンサ容量とほぼ同じ無効電力設定値Ｑ_C を設定回路２３２で設定し、この設定値Ｑ_C から他励式変換器群の上記遅れ無効電力Ｑを加減算器２３１で減算し、この値を自励式変換器２１０の無効電力の制御指令値として割算器２２９に出力する。割算器２２９では上記制御指令値を系統電圧で除した値を無効分電流指令値とし、交流電流制御器２２７に出力する。以下、実施の形態１と同様、設定回路２２６で必要とする自励式変換器２１０の有効分電流の値を設定し、この設定値を自励式変換器２１０の有効分電流指令として交流電流制御器２２７に出力する。交流電流制御器２２７は上記無効分電流指令と上記有効分電流指令を用いて自励式変換器２１０の交流電流を制御する。
【００５７】
実施の形態４．
上記実施の形態１では、他励式変換器の直流電流Ｉ_d 、制御角αから、他励式変換器の無効分電流Ｉ_Q を演算で求め、自励式変換器２１０を制御したが、図９に示すように、全ての他励式、自励式変換器群の合流する交流母線の電圧、電流から無効電力Ｑを無効電力測定回路２３０で測定し、測定された全変換器の遅れ無効電力Ｑを用いて自励式変換器２１０を制御してもよい。この場合は、交流系統に接続されている進相コンデンサ容量とほぼ同じ無効電力設定値Ｑ_C を設定回路２３２で設定し、この設定値から全変換器の上記遅れ無効電力を加減算器２３１で減算すると、全変換器の遅れ無効電力と進相コンデンサの進み無効電力の差の無効電力（Ｑ_C −Ｑ）が演算できる。それが０または０に近くなるように、制御器２３３において自励式変換器の無効分電流指令を制御し、この指令値を交流電流制御器２２７に出力する。以下、実施の形態１と同様、設定回路２２６で必要とする自励式変換器２１０の有効分電流の値を設定し、この設定値を自励式変換器２１０の有効分電流指令として交流電流制御器２２７に出力する。交流電流制御器２２７は上記無効分電流指令と上記有効分電流指令を用いて自励式変換器２１０の交流電流を制御する。
【００５８】
なお、本実施の形態では全ての他励式、自励式変換器群の合流する交流母線の電圧、電流から無効電力を測定して自励式変換器２１０を制御したが、実施の形態２と同様、無効分電流を測定して制御してもよい。
【００５９】
実施の形態５．
上記実施の形態１では、他励式変換器の直流電流Ｉ_d 、制御角αから、無効分電流Ｉ_Q を演算で求め、自励式変換器２１０を制御したが、図１０に示すように、全ての他励式、自励式変換器群、進相コンデンサの合流する交流母線の電圧、電流から無効電力を無効電力測定回路２２８で測定し、測定された無効電力が０または０に近くなるように制御器２３３において自励式変換器の無効分電流指令値を制御し、この指令値を交流電流制御器２２７に出力する。以下、実施の形態１と同様、設定回路２２６で必要とする自励式変換器２１０の有効分電流の値を設定し、この設定値を自励式変換器２１０の有効分電流指令として交流電流制御器２２７に出力する。交流電流制御器２２７は上記無効分電流指令と上記有効分電流指令を用いて自励式変換器２１０の交流電流を制御する。
【００６０】
なお、本実施の形態では全ての他励式、自励式変換器群、進相コンデンサの合流する交流母線の電圧、電流から無効電力を測定して自励式変換器２１０を制御したが、実施の形態２と同様、無効分電流を測定して制御してもよい。
【００６１】
実施の形態６．
次に、上記実施の形態１ないし５において、自励式変換器２１０として電圧型自励式変換器を用いた場合を説明する。
電流形自励式変換器はコイルに流れる直流電流を直流回路で環流（直流短絡）させたり、交流側に流したりしながら交流側に流れる電流を調節する。従って制御できる交流電流は、コイル電流に依存し、コイル電流以上の電流を流すことはできない。このことは、コイル電流が小さい電流である運転状態の場合、自励式変換器の流せる無効分電流が小さくなる。直流電流が小さい場合、他励式変換器の無効電力も小さくなるが、交流側には進相コンデンサが接続されており、自励式変換器は遅れの無効電力を積極的に流して進相コンデンサの進み無効電力をキャンセルする必要がある。自励式変換器が電流形の場合、無効分電流が不足する場合がある。
これに対して、電圧形自励式変換器は電流形と逆に、直流側コンデンサの電圧、または０電圧（交流短絡）を交流側に交互に印可して交流側電流を流すため、交流側電流は、直流電流に依存しない。
【００６２】
図１１は実施の形態６による電力変換装置の主要部を示す構成図である。２１０は電圧形自励式変換器であり、電圧形コンバータ２１１は直流コンデンサ２１２の電圧を制御する。直流コイル電流を自由に制御するためには、直流電圧は０から可変にできなくてはならないが、自励式変換器の直流コンデンサ電圧は０にはできないため直流母線をそのまま直流コイルに接続できない。そこで、直流電圧を自由に制御できるＤＣチョッパ２１３を通して直流コイルに接続する。
本構成では、電圧形自励式変換器２１０はＤＣチョッパ２１３を通じてコイル１の電流を増減させるための有効電力を制御する他、交流側の無効電力を制御する。また、電圧形自励式変換器２１０は、上記電流形自励式変換器のような、直流電流が小さい場合に無効分電流を流せない不都合はないので、装置の最大電流容量まで、自由に電流を制御することができる。
【００６３】
次に動作について説明する。
図１２は電圧形自励式変換器を自励式変換器２１０として用いた場合の電力変換装置の動作を説明する図である。直流電圧Ｖ_d は一定であるので、コイル電流指令と測定コイル電流の差を０にするように、制御回路２１４がＤＣチョッパ２１３の制御パルスを制御し、コイルにかかる電圧を調整する。ＤＣチョッパ２１３からコイル１にエネルギーが移動すると、コンデンサ電圧Ｖ_d が変化する。コンデンサ電圧Ｖ_d を一定にするように、設定回路２２６において直流電圧指令から測定直流電圧Ｖ_d の差を取り、それが０近くなるように制御器で有効分電流指令を制御する。交流電流制御器２２７で有効分電流指令と無効分電流指令をベクトル的に合成して和を取り、交流電流指令を作る。その交流電流指令と、測定された交流電流の差を取り、それが０に近くなるように制御器で電圧指令を制御する。交流電流は変換器出力電圧と系統電圧の差電圧によって流れるため、交流電圧測定値を加えて交流電圧指令とし、その交流電圧指令を三角波発生器と比較してパルス幅変調をかけ、スイッチング素子をＯＮ、ＯＦＦする。図１３、図１４は単相変換器の場合の動作説明図である。交流電圧に対して、任意の位相で電圧をかけることができ、電圧０の期間はＳＷ１、ＳＷ２またはＳＷ３、ＳＷ４をＯＮして交流側を短絡する。
【００６４】
なお、上記電圧形自励式変換器２１０の容量をなるべく小さくするには、予想される他励式変換器の運転状態において、最大の無効電力Ｑ_max と最小の無効電力Ｑ_min の間を、自励式変換器が補償できればよい。従って、進相コンデンサの進み無効電力Ｑ_c は、
【００６５】
【数１１】

【００６６】
とし、自励式変換器の制御可能無効電力Ｑ_Iは、
【００６７】
【数１２】

【００６８】
であれば、他励式変換器と自励式変換器の無効電力の和を
【００６９】
【数１３】

【００７０】
に一定に制御できるので、進相コンデンサの進み無効電力とキャンセルすることができる。この結果、自励式変換器の容量を少なくすることができる。
【００７１】
なお、上記実施の形態１ないし６では、進相コンデンサ４を一定量入れて、全体でキャンセルするようにしたが、図１５に示すように自励式変換器の容量を増やせば、進相コンデンサを無くすことができ、装置をより簡単にできる。
【００７２】
実施の形態７．
上記実施の形態１ないし６では、無効電力を補償したが、さらに、図１６に示すように、他励式変換器の電流を検出し、その検出電流から高調波電流検出回路２４０で基本波分を取り除いて他励式変換器の高調波電流を検出し、他励式変換器の高調波電流を自励式変換器の高調波電流によりキャンセルするよう自励式変換器２１０の電流を制御する構成を取ることができる。２４１、２４２は各々、自励式変換器の無効分電流指令値及び有効分電流指令値とから、それぞれ他励式変換器の高調波電流を減算する加減算器である。この場合は、高調波フィルタ６が不要になるか、自励式変換器で補償できない高い周波数の高調波のみをフィルタリングする小さな高調波フィルタで済む効果がある。
【００７３】
なお、上記実施の形態７では、無効分電流検出回路を他励式変換器群と自励式変換器の間に挿入し、自励式変換器の無効分電流指令値を制御したが、実施の形態１や実施の形態３ないし６のように自励式変換器の無効分電流指令値を制御してもよい。
【００７４】
また、上記実施の形態７では、進相コンデンサを一定量入れて、全体でキャンセルするようにしたが、図１７に示すように自励式変換器の容量を増やせば、進相コンデンサを無くすことができ、装置をより簡単にできる。
【００７５】
【発明の効果】
以上のように、この発明によれば、少なくとも１台の他励式変換器と、少なくとも１台の自励式変換器の双方からなり、電力系統からの交流を直流に変換する複数台の変換器と、上記自励式変換器と上記他励式変換器とに接続され、変換された直流を通流させるエネルギー貯蔵用のインダクタンス要素とを備えた電力変換装置であって、自励式変換器の発生する無効電力により、他励式変換器の発生する無効電力の変化をキャンセルして、上記複数台の変換器の発生する無効電力が直流電流および直流電圧に関わらず一定になるように上記自励式変換器を制御したので、交流側の進相コンデンサを一定かつ少ない容量、または、無くしてしまってもよく、装置の構成が簡単になる効果がある。
【００７６】
この発明によれば、各他励式変換器の遅れ無効分電流を直流電流と制御角から計算して求め、これら遅れ無効分電流の和と、電力系統に接続された進相コンデンサの進み無効分電流との差を求め、これを自励式変換器の無効分電流の制御指令値とし、有効分電流指令値と共に自励式変換器を制御したので、無効電力や無効分電流を検出する回路を交流母線上に設ける必要が無く、構成が簡単になる効果がある。
【００７７】
この発明によれば、他励式変換器群の遅れ無効分電流の和を無効分電流検出回路で検出し、上記遅れ無効分電流と、電力系統に接続された進相コンデンサの進み無効分電流との差を求め、これを自励式変換器の無効分電流の制御指令値とし、有効分電流指令値と共に自励式変換器を制御したので、他励式変換器からの無効分電流計算が必要なく、制御装置が簡単になる効果がある。
【００７８】
この発明によれば、他励式変換器群の遅れ無効電力を計測し、上記遅れ無効電力と、電力系統に接続された進相コンデンサの進み無効電力に相当する値との差を求め、これを自励式変換器の無効電力の制御指令値とし、上記制御指令値を系統電圧で除した無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したので、他励式変換器からの無効分電流計算が必要なく、制御装置が簡単になる効果がある。
【００７９】
この発明によれば、他励式変換器と自励式変換器の無効分電流の和または無効電力の和を検出し、上記無効分電流または無効電力と、電力系統に接続された進相コンデンサの進み無効分電流または進み無効電力に相当する値との差を求め、これを自励式変換器の無効分電流または無効電力の制御指令値とし、上記制御指令値が０またはそれに近い値となるように上記自励式変換器の無効分電流指令値を作成する制御器を備え、上記無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したので、他励式変換器からの無効分電流計算が必要なく、制御装置が簡単になる効果がある。
【００８１】
この発明によれば、他励式変換器と自励式変換器と進相コンデンサの無効分電流の和または無効電力の和を検出し、上記無効分電流または無効電力が、０またはそれに近い値となるように自励式変換器の無効分電流指令値を作成する制御器を備え、上記無効分電流指令値と、有効分電流指令値とにより上記自励式変換器を制御したので、他励式変換器からの無効分電流計算が必要なく、制御装置が簡単になる効果がある。
【００８２】
この発明によれば、上記各電力変換装置において、自励式変換器を電流形コンバータで構成したので、自励式変換器が他励式変換器との直流母線に直接接続され、直流回路が簡単になる効果がある。
【００８３】
この発明によれば、上記各電力変換装置において、自励式変換器を、直流側にコンデンサを接続した、交流直流変換のための電圧形コンバータと、上記電圧形コンバータのコンデンサとインダクタンス要素との間に接続され、直流電圧を可変に制御して上記インダクタンス要素に流れる電流を制御するチョッパとで構成したので、交流無効分電流を直流電流の大きさに関わらず制御できるため、無効電力制御機能が直流回路の運転状態に影響されにくい効果がある。
【００８４】
この発明によれば、上記電力変換装置において、電力系統に接続された進相コンデンサの進み無効電力を、システムの運転状態において他励式変換器が発生する最大遅れ無効電力と最小遅れ無効電力の和の１／２としたので、自励式変換器の容量を少なくすることができる。
【００８５】
この発明によれば、上記各電力変換装置において、他励式変換器の高調波電流を計測し、自励式変換器の無効分電流指令値及び有効分電流指令値より、上記他励式変換器の高調波電流を減算して他励式変換器の高調波電流をキャンセルするようにしたので、高調波フィルタを不要にするか、自励式変換器で補償できない高い周波数の高調波のみをフィルタリングする小さな高調波フィルタで済ませることができ、装置構成が簡単になる効果がある。
【図面の簡単な説明】
【図１】この発明の実施の形態１による電力変換装置を示す構成図である。
【図２】この発明の実施の形態１による電力変換装置の動作を説明する説明図である。
【図３】この発明の実施の形態１による電力変換装置の動作を説明する説明図である。
【図４】この発明の実施の形態１による電力変換装置の動作を説明する説明図である。
【図５】この発明の実施の形態１による電力変換装置の動作を説明する説明図である。
【図６】この発明の実施の形態１による電力変換装置の無効分電流の大きさの変化を示す図である。
【図７】この発明の実施の形態２による電力変換装置を示す構成図である。
【図８】この発明の実施の形態３による電力変換装置を示す構成図である。
【図９】この発明の実施の形態４による電力変換装置を示す構成図である。
【図１０】この発明の実施の形態５による電力変換装置を示す構成図である。
【図１１】この発明の実施の形態６による電力変換装置の主要部を示す構成図である。
【図１２】この発明の実施の形態６による電力変換装置の動作を説明する説明図である。
【図１３】この発明の実施の形態６による電力変換装置の動作を説明する説明図である。
【図１４】この発明の実施の形態６による電力変換装置の動作を説明する説明図である。
【図１５】この発明の実施の形態６による電力変換装置の主要部を示す構成図である。
【図１６】この発明の実施の形態７による電力変換装置を示す構成図である。
【図１７】この発明の実施の形態７による電力変換装置を示す構成図である。
【図１８】従来の電力変換装置を示す構成図である。
【図１９】従来の電力変換装置の動作を説明する説明図である。
【図２０】従来の電力変換装置の動作を説明する説明図である。
【図２１】従来の電力変換装置の動作を説明する説明図である。
【図２２】従来の他の電力変換装置を示す回路構成図である。
【図２３】従来の他の電力変換装置を示す回路構成図である。
【符号の説明】
１直流回路、１０１，１０２，１０３，１０４，１０５，１０６コイル、２電力変換器群、２０１，２０２，２０３，２０４，２０５他励式変換器、２１０自励式変換器、２１１電圧形自励式変換器、２１２直流コンデンサ、２１３ＤＣチョッパ、２１４制御回路、２２１，２２２演算回路、２２３総和演算器、２２４，２３１，２４１，２４２加減算器、２２５，２２６設定回路、２２７交流電流制御器、２２８無効分電流測定回路、２２９割算器、２３０無効電力測定回路、２３２設定回路、２３３制御器、２４０高調波電流検出器、３電力系統、４進相コンデンサ、５ＳＶＣ、６高調波フィルタ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a power conversion device, and more particularly to an AC / DC conversion device in a superconducting energy storage facility connected to a power system.
[0002]
[Prior art]
The superconducting energy storage facility stores direct current from the power system to the superconducting coil to store the energy as magnetic energy in the coil, which increases the power consumption of the power consumer and causes the coil energy to be stored when the system power is insufficient. It is intended to make up for the power shortage by returning it to AC power and supplying it to power consumers. For such applications, power converters that convert alternating current to direct current and direct current to alternating current are used. Some of these facilities have a very large capacity of several tens or several hundreds MW, and a plurality of converters are connected in series or in parallel according to the voltage and current flowing through the coil.
[0003]
FIG. 18 is a block diagram showing a typical configuration example of such a conventional power converter. In FIG. 18, 101 is a coil, 1 is a DC circuit including the

coil

101, 201, 202, and 203 are separately excited converters, 2 is a power converter group that converts AC to DC, 3 is a power system, and 4 is a phase advance. A capacitor 5 is an SVC (Static Var Compensator) capable of variably controlling the advance reactive power, and 6 is a harmonic filter that suppresses harmonics generated from the power converter group.
[0004]
Next, the operation will be described. Each separately-

excited converter

201, 202, 203 in the power converter group 2 converts AC power from the power system 3 into DC and causes DC current to flow through the coil 101. The power converter group 2 includes a separately-excited converter using a thyristor and is phase-controlled. The phase advance capacitor 4 is connected to the AC side of the power converter group 2 and cancels the delayed reactive power generated by the power converter group 2 constituted by a separately excited converter with the advanced reactive power. The phase-advancing capacitor 4 is divided into several banks, and can be selected by switching an appropriate amount according to the reactive power amount of the separately excited converter and connected to the AC side.
[0005]
However, it is not preferable to increase the number of the phase-advancing capacitor banks unnecessarily because the number of switches increases, which is economically disadvantageous and the device becomes large. For this reason, there is a limit to the number of banks of the phase advance capacitor, and usually the amount of phase advance capacitor that matches the reactive power of the separately-excited converter cannot be selected, and some unbalance occurs in the reactive power. Therefore, depending on the power system, an SVC 5 that can adjust the reactive power steplessly is installed so that the reactive power can be adjusted more finely, and the amount of imbalance between the advanced reactive power of the phase advance capacitor 4 and the delayed reactive power of the separately excited converter To compensate. At this time, the capacities of the phase advance capacitor 4 and the SVC 5 must be set so that the maximum advance reactive power combining the phase advance capacitor 4 and the SVC 5 matches the maximum reactive power generated by the power converter group 2.
[0006]
The operation of a conventional power conversion device including a thyristor converter, which is a typical example of a separately excited converter constituting the power converter group 2, will be described with reference to FIG. In the figure, the DC current I of the coil 100 connected to the DC side is shown. _d Is measured by a measuring device, and the controller 251 controls the coil voltage command V so that the difference from the coil current command is close to zero. _dREF Is output. DC voltage V _d And the control angle α, the amplitude of the AC phase voltage is V,
[0007]
[Expression 2]

[0008]
This is calculated by the control pulse phase calculation circuit 251 to obtain the control angle α, and an ON / OFF command for each element constituting the separately excited converter is created. In order to briefly explain the operation of the thyristor converter, FIG. 20 shows a single-phase thyristor converter, and FIG. As shown in FIG. 21, the current I is at an angle delayed by a phase α from the zero point of the single-phase AC voltage. _ad Flows. Since the next thyristor switches 180 degrees after the voltage has the same phase relationship, the current becomes a square wave of 180 degrees. The period during which this current flows is determined by the AC voltage, and is fixed at 180 degrees for the single phase and 120 degrees for the three phases. Therefore, if the control angle is changed in order to change the active power, the current component in phase with the voltage naturally changes, but at the same time, the reactive current that is 90 degrees out of phase with the voltage also changes. This can be explained by the following formula.
[0009]
First, the fundamental wave of the alternating current with respect to the alternating phase voltage v = Vsinθ is
[0010]
[Equation 3]

[0011]
It becomes. Here, the current change time at the time of commutation, which is not related to the problem, is ignored, and the turn ratio of the transformer attached to the converter is assumed to be 1.
Now, equation (2) is
[0012]
[Expression 4]

[0013]
Component that is 90 degrees out of phase with respect to the AC voltage, that is, the reactive current I _Q Is
[0014]
[Equation 5]

[0015]
It becomes.
Therefore, the reactive power Q is a three-phase circuit,
[0016]
[Formula 6]

[0017]
It becomes. Also, the DC voltage V of the separately excited converter _d Is
[0018]
[Expression 7]

[0019]
Therefore, α in equation (6) is set to V _d Replace with
[0020]
[Equation 8]

[0021]
It becomes.
[0022]
Since the reactive power is expressed by Expression (5) or Expression (7), the control angle α of the separately excited converter and the DC voltage V _d Or DC voltage V _d And DC current I _d The amount varies depending on. Therefore, if the coil current is changed, the DC voltage V _d Therefore, the reactive power of the power conversion device of this configuration always changes depending on the operation state. Such a change in reactive power is compensated by switching the phase advance capacitor or adjusting by SVC. Therefore, a phase advance capacitor, SVC, that matches the maximum reactive power generated by the separately excited converter group is required.
[0023]
Further, the separately excited converter turns on and off the current, and generates harmonics. The harmonics include many lower-order harmonics such as the 11th order and the 13th order with respect to the fundamental wave alternating current, and the harmonic filter 6 must be installed for the suppression.
[0024]
FIG. 22 is a configuration diagram showing another configuration example of a typical conventional power conversion device. The same reference numerals as those in FIG. 18 denote the same or corresponding parts, and thus description thereof is omitted.
In FIG. 22,

reference numerals

102, 103, 104, and 105 denote coils, and 204 and 205 denote separately-excited converters.
[0025]
Next, the operation will be described. This configuration example is a case where there are a plurality of coils as compared to the configuration example of FIG. 18. In the power converter group 2, the

power converters

201 and 202 control the current of the coil 101. 203 controls the current of the coil 102. The

power converters

204 and 205 control the current of the coil 103 through the current balance coils 104 and 105.
Thus, when there are not only one coil but a plurality of coils, the converters are configured in series, in parallel, or used alone depending on the current and voltage flowing through each coil.
[0026]
In this case as well, it is needless to say that the reactive power on the AC side is changed by the control operation of the coil current and voltage as in the configuration example of FIG. Accordingly, in order to compensate for the reactive power of these separately excited converter groups, the phase advance capacitor 4 and the SVC 5 are required in accordance with the maximum value of the reactive power generated by the separately excited converter groups as in FIG.
Furthermore, in this case as well, there are many low-order harmonics of the separately-excited converter, and the harmonic filter 6 must be installed for the suppression.
[0027]
FIG. 23 is a configuration diagram showing another configuration example of a typical conventional power conversion device. The same reference numerals as those in FIGS. 18 and 22 denote the same or corresponding parts, and thus description thereof is omitted.
[0028]
Next, the operation will be described. This configuration example is a case where the DC sides of the

power converters

201, 202, and 203 are connected in parallel with the embodiment of FIG. In order to balance the direct currents of the respective power converters, they are connected in parallel through current balancing coils 104, 105 and 106. In the power converter group 2, each

power converter

201, 202, 203 controls the current of the

coils

104, 105, 106, and controls the current of the coil 101 as a sum thereof.
[0029]
In this case as well, it is needless to say that the reactive power on the AC side is changed by the control operation of the coil current and voltage as in the configuration examples of FIGS. Therefore, in order to compensate the reactive power of these separately excited converter groups, the phase advance capacitor 4 and the SVC 5 are required in accordance with the maximum value of the reactive power generated by the separately excited converter groups as in FIGS. It becomes.
Furthermore, in this case as well, there are many low-order harmonics of the separately-excited converter, and the harmonic filter 6 must be installed for the suppression.
[0030]
[Problems to be solved by the invention]
Since the conventional power converter is configured as described above, the delay reactive power generated by the power converter group is not constant, and varies depending on the DC current and DC voltage of the converter. The capacities of the phase-advancing capacitors 4 and SVC 5 must be determined according to the maximum reactive power in the operating range of the separately excited converter. In addition, the SVC 5 must be controlled in accordance with the change in reactive power of the separately excited converter, or some of the phase advance capacitors 4 must be turned on and off, and a phase advance capacitor changeover switch is required, resulting in a complicated apparatus configuration. It became. Furthermore, a harmonic filter for suppressing harmonics of the separately excited converter is necessary. Since these additional facilities on the AC side take a large area, there is a problem that the site area of the entire facility becomes large.
[0031]
The present invention was made to solve the above-described problems, and it is not necessary to install an SVC, and the phase advance capacitor is not divided into several parts, and may be in a certain state. An object of the present invention is to obtain a power conversion device that has a small capacity or can be eliminated.
It is another object of the present invention to provide a power converter that can reduce or eliminate the capacity of the harmonic filter.
[0032]
[Means for Solving the Problems]
This invention Power of The force transducer is Consisting of at least one separately-excited transducer and at least one self-excited transducer, Multiple converters that convert alternating current from the power system into direct current And a power conversion device comprising an energy storage inductance element connected to the self-excited converter and the separately-excited converter and allowing the converted direct current to flow therethrough, By the reactive power generated by the self-excited converter ,other The self-excited converter is controlled so that the reactive power generated by the converters is canceled and the reactive power generated by the plurality of converters is constant regardless of the DC current and DC voltage. is there.
[0033]
This invention Power of The force transducer is the above Control angle α and DC current I in separately excited converter _d And the following formula
[0034]
[Equation 9]

[0035]
Delay reactive current I calculated by _Q For each separately-excited converter, and the sum of these delayed reactive currents And the difference between the lead reactive current of the phase advance capacitor connected to the power grid This is the control command value of the reactive current of the self-excited converter, and the self-excited converter is controlled together with the effective current command value.
[0036]
This invention Power of The force transducer is the above Separately-excited converter group is connected to the same AC bus, the voltage and current of the AC bus on the power system side from the connection point of the separately-excited converter group of the AC bus and the separately-excited converter group side of the self-excited converter Measure the delayed reactive current of the separately excited converter group from And the difference between the lead reactive current of the phase advance capacitor connected to the power grid This is the control command value of the reactive current of the self-excited converter, and the self-excited converter is controlled together with the effective current command value.
[0037]
This invention Power of In the force conversion device, the separately excited converter group is connected to the same AC bus, the AC line is connected to the AC system on the power system side than the connection point of the separately excited converter group, and the separately excited converter group side from the self-excited converter side. The delayed reactive power of the separately excited converter group is measured from the bus voltage and current, and the above delayed reactive power is measured. And the value corresponding to the lead reactive power of the phase advance capacitor connected to the power grid The self-excited converter is controlled by the reactive current command value obtained by dividing the control command value by the system voltage and the effective current command value. is there.
[0038]
This invention Power of The power converter is on the power system side from the connection points of all converter groups of the AC bus, and Connected to the power grid Measure the reactive current or reactive power of all converter groups from the voltage and current of the AC bus on the converter group side of the phase-advancing capacitor. And the value corresponding to the lead reactive current or lead reactive power of the above phase advance capacitor A controller for generating a reactive current command value for the self-excited converter so that the control command value becomes 0 or a value close to the control command value for the reactive current or reactive power of the self-excited converter. The self-excited converter is controlled by the reactive current command value and the effective current command value.
[0040]
This invention Power of The force converter measures the reactive current or reactive power from the voltage and current of the AC bus on the side of the power system from the connection points of all converter groups of the AC bus and the phase advance capacitor. Includes a controller that creates a reactive current command value of the self-excited converter so that the value becomes 0 or a value close to 0, and the self-excited converter according to the reactive current command value and the effective current command value. Is controlled.
[0041]
This invention Power of In the power converter, the self-excited converter is a current source converter in each of the power converters described above.
[0042]
This invention Power of In each of the power converters described above, the power converter includes a self-excited converter, a voltage source converter for AC / DC conversion in which a capacitor is connected on the DC side, and a capacitor between the capacitor of the voltage source converter and an inductance element. A chopper that is connected and variably controls the direct current voltage to control the current flowing through the inductance element.
[0043]
The power converter of this invention is the above power converter, Connected to the power grid The advance reactive power of the phase advance capacitor is ½ of the sum of the maximum delay reactive power and the minimum delay reactive power generated by the separately excited converter in the operating state of the system.
[0044]
This invention Power of The power converter measures the harmonic current of the separately excited converter in each of the power converters described above, and determines the harmonics of the separately excited converter from the reactive current command value and the active current command value of the self-excited converter. The harmonic current of the separately excited converter is canceled by subtracting the current.
[0045]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 FIG.
Embodiments of the present invention will be described below with reference to the drawings. 1 is a block diagram showing a power conversion apparatus according to Embodiment 1 of the present invention. In the figure, the same reference numerals as those in the prior art indicate the same or corresponding parts, and the description thereof will be omitted. In the figure, 210 is a self-excited converter that converts AC to DC, 221, 222 are DC currents of separately-

excited converters

201, 202, an arithmetic circuit that calculates an ineffective current from the control angle, and 223 is each ineffective calculated A sum calculator for taking the sum of the divided currents, 224 is an adder / subtracter, 225 is a setting circuit for setting an invalid divided current value of the

phase advance capacitor

4, and 226 is a setting circuit for setting an effective current command of the self-excited converter 210, Reference numeral 227 denotes an alternating current controller of the self-excited converter 210.
[0046]
Next, the operation will be described. First, like the conventional ones, the

converters

201 and 202 adjust the control angle α to control the direct current. The

arithmetic circuits

221 and 222 are connected to the DC current I of the separately

excited converters

201 and 202, respectively. _d From the control angle α, the reactive current I of each separately excited converter _Q The
[0047]
[Expression 10]

[0048]
Calculate with. When these sums are taken by the sum calculator 223, the delay reactive current generated by all the separately excited converters can be calculated.
[0049]
Therefore, the reactive current set value I that is almost the same as the capacity of the phase advance capacitor connected to the AC system. _CQ Is set by the setting circuit 225, and the value of the delayed reactive current is subtracted from the set value by the adder / subtracter 224, the reactive component of the difference between the delayed reactive current of the separately-excited converter and the advanced reactive current of the phase advance capacitor. Current (I _CQ -I _Q ) Can be calculated. This is output to the AC current controller 227 as a reactive current command for the self-excited converter 210.
On the other hand, converter 210 sets the effective current value of self-excited converter 210 required by setting circuit 226 to control the direct current, and this set value is used as the effective current command of self-excited converter 210. Output to the AC current controller 227.
The alternating current controller 227 controls the alternating current of the self-excited converter 210 using the reactive current command and the effective current command.
[0050]
The setting circuit 226 and the alternating current controller 227 differ depending on the configuration of the self-excited converter 210. FIG. 2 shows an example in which a current type converter is used as a self-excited converter. The controller controls the effective current command so that the difference between the coil current command and the measurement coil current is close to zero. Since the effective current command and the reactive current command are 90 degrees out of phase with each other, as shown in FIG. The AC current command is compared with a triangular wave generator and subjected to pulse width modulation, and the switching element is turned on and off through a control pulse generation circuit. 4 and 5 are operation explanatory diagrams in the case of a single-phase self-excited converter. SW1 and SW4 can be turned on, and SW2 and SW3 can be turned on so that a current flows to the AC side at an arbitrary phase with respect to the AC voltage. During the current 0 period, the switches connected in series are turned ON to short-circuit the DC side, and no current flows through the AC.
[0051]
In this way, by controlling the alternating current in the self-excited converter 210, the sum of the reactive currents flowing through the separately

excited converters

201 and 202, the self-excited converter 210, and the phase advance capacitor 4 is
I _Q + (I _CQ -I _Q ) -I _CQ = 0 (9)
And canceled to zero.
[0052]
FIG. 6 shows a change in the magnitude of the reactive current of the power conversion device according to this embodiment. As shown in the figure, the reactive current I of the separately excited converter is shown. _Q Even if changes with time, the reactive current of the self-excited converter is controlled accordingly.For example, the delayed reactive current of the separately excited converter is larger than the reactive current of the phase advance capacitor. If it is larger, the self-excited converter supplies a leading reactive current to compensate for the delayed reactive current. If the magnitude of the delayed reactive current of the separately excited converter is smaller than the magnitude of the reactive current of the phase advance capacitor, the self-excited converter passes a delayed reactive current. In this way, since the reactive current is controlled to be constant as a whole, it is not necessary to turn on and off the AC phase advance capacitor, and it may remain constant. In addition, the self-excited converters constituting the power converter group 2 absorb the change in the reactive power and control the reactive power to be always constant without separately changing the reactive power compensation amount by placing the SVC. SVC is not necessary.
[0053]
In addition, when all converters are separately-excited type, the capacity of the phase-advancing capacitor had to be determined according to the maximum reactive power of the separately-excited converter. In the embodiment, a small capacity obtained by subtracting the amount of the advanced reactive power generated by the self-excited converter from the maximum value of the delayed reactive power of the separately excited converter is sufficient, and the capacity of the phase advance capacitor can be greatly reduced.
[0054]
Note that a current source converter is usually used as the separately excited converter, and a current source converter is convenient because it is a DC coil power supply in superconducting energy storage. In the first embodiment, since the self-excited converter 210 also uses a current source converter, it can be used as a coil power source as it is, and the configuration becomes simple.
[0055]
Embodiment 2. FIG.
In the first embodiment, the direct current I of the separately excited converter _d From the control angle α, the reactive current I of the separately excited converter I _Q As shown in FIG. 7, the reactive current that is 90 degrees behind the AC bus voltage from the AC bus voltage and current between the separately-excited converter group and the self-excited converter is invalidated. Measured by the current measuring circuit 228, the delayed reactive current I of the separately excited converter group I _Q Similarly, the self-excited converter 210 may be controlled by subtracting the value of the delayed reactive current from the set value set by the setting circuit 225 by the adder / subtractor 224, as in the first embodiment. Similar effects can be obtained.
[0056]
Embodiment 3 FIG.
In the first embodiment, the direct current I of the separately excited converter _d From the control angle α, the reactive current I of the separately excited converter I _Q Was calculated and the self-excited converter 210 was controlled. As shown in FIG. 8, the reactive power Q delayed from the voltage and current of the AC bus between the separately-excited converter group and the self-excited converter was measured. The self-excited converter 210 may be controlled using the delayed reactive power Q measured by the circuit 230. In this case, the reactive power setting value Q is almost the same as the capacity of the phase advance capacitor connected to the AC system. _C Is set by the setting circuit 232, and this set value Q _C Then, the delay reactive power Q of the separately excited converter group is subtracted by the adder / subtractor 231, and this value is output to the divider 229 as the reactive power control command value of the self-excited converter 210. In the divider 229, a value obtained by dividing the control command value by the system voltage is set as a reactive current command value, and is output to the AC current controller 227. Hereinafter, as in the first embodiment, the effective current value of the self-excited converter 210 required by the setting circuit 226 is set, and this setting value is used as the effective current command of the self-excited converter 210 as an AC current controller. Output to H.227. The alternating current controller 227 controls the alternating current of the self-excited converter 210 using the reactive current command and the effective current command.
[0057]
Embodiment 4 FIG.
In the first embodiment, the direct current I of the separately excited converter _d From the control angle α, the reactive current I of the separately excited converter I _Q , And the self-excited converter 210 was controlled. As shown in FIG. 9, the reactive power measurement circuit calculates the reactive power Q from the voltages and currents of the AC buses of all the separately excited and self-excited converter groups. The self-excited converter 210 may be controlled using the delayed reactive power Q of all converters measured at 230. In this case, the reactive power setting value Q is almost the same as the capacity of the phase advance capacitor connected to the AC system. _C Is set by the setting circuit 232, and the delay reactive power of all the converters is subtracted by the adder / subtracter 231 from this set value, the reactive power (Q of the difference between the delayed reactive power of all the converters and the advanced reactive power of the phase advance capacitor) _C -Q) can be calculated. The controller 233 controls the reactive current command of the self-excited converter so that it becomes 0 or close to 0, and outputs this command value to the AC current controller 227. Hereinafter, as in the first embodiment, the effective current value of the self-excited converter 210 required by the setting circuit 226 is set, and this setting value is used as the effective current command of the self-excited converter 210 as an AC current controller. Output to H.227. The alternating current controller 227 controls the alternating current of the self-excited converter 210 using the reactive current command and the effective current command.
[0058]
In this embodiment, the reactive power is measured from the voltage and current of the AC bus that joins all separately-excited and self-excited converter groups, and the self-excited converter 210 is controlled. As in the second embodiment, The reactive current may be measured and controlled.
[0059]
Embodiment 5. FIG.
In the first embodiment, the direct current I of the separately excited converter _d , Reactive current I from control angle α _Q Was calculated and the self-excited converter 210 was controlled. However, as shown in FIG. 10, the reactive power is invalidated from the voltages and currents of all the separately-excited, self-excited converters, and AC buses where the phase-advancing capacitors merge The controller 233 controls the reactive current command value of the self-excited converter so that the measured reactive power is 0 or close to 0, and the command value is sent to the AC current controller 227. Output. Hereinafter, as in the first embodiment, the effective current value of the self-excited converter 210 required by the setting circuit 226 is set, and this setting value is used as the effective current command of the self-excited converter 210 as an AC current controller. Output to H.227. The alternating current controller 227 controls the alternating current of the self-excited converter 210 using the reactive current command and the effective current command.
[0060]
In this embodiment, the self-excited converter 210 is controlled by measuring the reactive power from the voltage and current of the AC buses joined by all separately-excited and self-excited converter groups and the phase advance capacitors. Similarly to 2, the reactive current may be measured and controlled.
[0061]
Embodiment 6 FIG.
Next, a case where a voltage type self-excited converter is used as the self-excited converter 210 in the first to fifth embodiments will be described.
The current source self-excited converter adjusts the current flowing in the AC side while circulating the DC current flowing in the coil in a DC circuit (DC short circuit) or flowing it in the AC side. Therefore, the AC current that can be controlled depends on the coil current, and it is not possible to pass a current higher than the coil current. This means that in the operating state where the coil current is small, the reactive current that can flow through the self-excited converter becomes small. When the direct current is small, the reactive power of the separately-excited converter is also small, but a phase-advancing capacitor is connected to the AC side, and the self-excited converter actively sends delayed reactive power to the phase-advancing capacitor. It is necessary to cancel the reactive power. When the self-excited converter is a current type, the reactive current may be insufficient.
On the other hand, the voltage-type self-excited converter, contrary to the current type, causes the AC side current to flow by alternately applying the voltage of the DC side capacitor or 0 voltage (AC short-circuit) to the AC side. Is independent of direct current.
[0062]
FIG. 11 is a configuration diagram illustrating a main part of the power conversion device according to the sixth embodiment. Reference numeral 210 denotes a voltage source self-excited converter, and the voltage source converter 211 controls the voltage of the DC capacitor 212. In order to freely control the DC coil current, the DC voltage must be variable from 0. However, since the DC capacitor voltage of the self-excited converter cannot be set to 0, the DC bus cannot be directly connected to the DC coil. Therefore, the DC voltage is connected to the DC coil through a DC chopper 213 that can freely control the DC voltage.
In this configuration, the voltage type self-excited converter 210 controls the reactive power on the AC side in addition to controlling the active power for increasing and decreasing the current of the coil 1 through the DC chopper 213. Further, the voltage source self-excited converter 210 has no inconvenience that a reactive current cannot flow when the direct current is small, unlike the current source self-excited converter 210. Can be controlled.
[0063]
Next, the operation will be described.
FIG. 12 is a diagram for explaining the operation of the power converter when a voltage source self-excited converter is used as the self-excited converter 210. DC voltage V _d Is constant, the control circuit 214 controls the control pulse of the DC chopper 213 to adjust the voltage applied to the coil so that the difference between the coil current command and the measurement coil current is zero. When energy is transferred from the DC chopper 213 to the coil 1, the capacitor voltage V _d Changes. Capacitor voltage V _d In the setting circuit 226 so that the measured DC voltage V _d The effective current command is controlled by the controller so that it becomes close to zero. An alternating current controller 227 vectorally combines the effective current command and the reactive current command and takes the sum to create an alternating current command. The difference between the alternating current command and the measured alternating current is taken, and the voltage command is controlled by the controller so that it is close to zero. Since the alternating current flows due to the voltage difference between the converter output voltage and the system voltage, the alternating voltage measurement value is added to form an alternating voltage command, the alternating voltage command is compared with a triangular wave generator, pulse width modulation is applied, and the switching element is turned on. Turns on and off. FIG. 13 and FIG. 14 are operation explanatory diagrams in the case of a single phase converter. A voltage can be applied to the AC voltage at an arbitrary phase, and during the period of voltage 0, SW1, SW2 or SW3, SW4 are turned ON to short-circuit the AC side.
[0064]
In order to reduce the capacity of the voltage source self-excited converter 210 as much as possible, the maximum reactive power Q is expected in the expected operating state of the separately excited converter. _max And minimum reactive power Q _min It is only necessary that the self-excited converter can compensate between the two. Therefore, the lead reactive power Q of the phase advance capacitor _c Is
[0065]
[Expression 11]

[0066]
Controllable reactive power Q of self-excited converter _I Is
[0067]
[Expression 12]

[0068]
The sum of the reactive power of the separately excited converter and the self-excited converter.
[0069]
[Formula 13]

[0070]
Therefore, it is possible to cancel the advance reactive power of the phase advance capacitor. As a result, the capacity of the self-excited converter can be reduced.
[0071]
In the first to sixth embodiments, a fixed amount of the phase-advancing capacitor 4 is added and canceled as a whole. However, if the capacity of the self-excited converter is increased as shown in FIG. Can be eliminated, making the device easier.
[0072]
Embodiment 7 FIG.
In the first to sixth embodiments, the reactive power is compensated. Further, as shown in FIG. 16, the current of the separately-excited converter is detected, and the harmonic current detection circuit 240 determines the fundamental wave component from the detected current. The configuration may be such that the harmonic current of the separately excited converter is detected and the harmonic current of the separately excited converter is detected, and the current of the self excited converter 210 is controlled so as to cancel the harmonic current of the separately excited converter by the harmonic current of the self excited converter. it can. Reference numerals 241 and 242 denote adder / subtracters for subtracting the harmonic current of the separately excited converter from the reactive current command value and the effective current command value of the self-excited converter, respectively. In this case, there is an effect that the harmonic filter 6 becomes unnecessary or a small harmonic filter that filters only high-frequency harmonics that cannot be compensated by the self-excited converter is sufficient.
[0073]
In the seventh embodiment, the reactive current detection circuit is inserted between the separately excited converter group and the self-excited converter, and the reactive current command value of the self-excited converter is controlled. Alternatively, the reactive current command value of the self-excited converter may be controlled as in the third to sixth embodiments.
[0074]
In the seventh embodiment, a fixed amount of a phase advance capacitor is added and canceled as a whole. However, if the capacity of the self-excited converter is increased as shown in FIG. 17, the phase advance capacitor may be eliminated. And make the device simpler.
[0075]
【The invention's effect】
As mentioned above, this departure Clearly According to A plurality of converters each including at least one separately-excited converter and at least one self-excited converter that convert alternating current from a power system into direct current; the self-excited converter and the separately-excited converter A power conversion device having an energy storage inductance element connected to the converter and allowing the converted direct current to flow therethrough, and the reactive power generated by the self-excited converter causes the invalidity generated by the separately excited converter The self-excited converter so that the reactive power generated by the plurality of converters is constant regardless of the DC current and the DC voltage by canceling the power change Therefore, the AC phase-advancing capacitor may have a constant and small capacity or may be eliminated, and the configuration of the apparatus is simplified.
[0076]
This departure Clearly Therefore, the delay reactive current of each separately-excited converter is calculated from the DC current and the control angle, and the sum of these delay reactive currents is calculated. And the difference between the lead reactive current of the phase advance capacitor connected to the power grid Because this is the control command value of the reactive current of the self-excited converter and the self-excited converter is controlled together with the active current command value, it is necessary to provide a circuit for detecting reactive power and reactive current on the AC bus. There is an effect that the configuration becomes simple.
[0077]
This departure Clearly According to the above, the sum of the delayed reactive currents of the separately excited converters is detected by the reactive current detection circuit, and the above delayed reactive current is detected. And the difference between the lead reactive current of the phase advance capacitor connected to the power grid Since this is used as the control command value for the reactive current of the self-excited converter and the self-excited converter is controlled together with the effective current command value, there is no need to calculate the reactive current from the separately excited converter, and the controller is simple There is an effect to become.
[0078]
This departure Clearly According to the above, the delayed reactive power of the separately excited converter group is measured, and the delayed reactive power is And the value corresponding to the lead reactive power of the phase advance capacitor connected to the power grid Since this is the control command value of the reactive power of the self-excited converter, and the self-excited converter is controlled by the reactive current command value obtained by dividing the control command value by the system voltage and the effective current command value, There is no need to calculate the reactive current from the separately excited converter, and the control device is simplified.
[0079]
This departure Clearly According to this, the reactive current sum or reactive power sum of the separately excited converter and the self-excited converter is detected, and the reactive current or reactive power is detected. And the value corresponding to the lead reactive current or lead reactive power of the phase advance capacitor connected to the power grid A controller for generating a reactive current command value for the self-excited converter so that the control command value becomes 0 or a value close to the control command value for the reactive current or reactive power of the self-excited converter. Since the self-excited converter is controlled by the reactive current command value and the active current command value, there is no need to calculate the reactive current from the separately excited converter, and the control device is simplified. is there.
[0081]
This departure Clearly Therefore, the sum of the reactive currents or the reactive powers of the separately excited converter, the self-excited converter, and the phase advance capacitor is detected, and the reactive current or reactive power is automatically set to 0 or a value close thereto. A controller for creating a reactive current command value for the excitation converter is provided, and the self-excitation converter is controlled by the reactive current command value and the effective current command value. No current calculation is required, and the control device is simplified.
[0082]
This departure Clearly Accordingly, in each of the above power converters, since the self-excited converter is configured by a current source converter, the self-excited converter is directly connected to the DC bus with the other-excited converter, and the DC circuit is simplified. .
[0083]
This departure Clearly Accordingly, in each of the above power converters, a self-excited converter is connected between a voltage source converter for AC / DC conversion in which a capacitor is connected on the DC side, and between the capacitor and the inductance element of the voltage source converter. Because the AC reactive current is controlled regardless of the magnitude of the DC current, the reactive power control function can be controlled by the DC circuit. It has the effect of being hardly affected by driving conditions.
[0084]
According to the present invention, in the power converter, Connected to the power grid The advance reactive power of the phase advance capacitor is ½ of the sum of the maximum delay reactive power and the minimum delay reactive power generated by the separately excited converter in the operating state of the system, so that the capacity of the self-excited converter is reduced. Can do.
[0085]
This departure Clearly Accordingly, in each of the power conversion devices, the harmonic current of the separately excited converter is measured, and the harmonic current of the separately excited converter is calculated from the reactive current command value and the effective current command value of the self-excited converter. Since the harmonic current of the separately excited converter is canceled by subtraction, a harmonic filter is not necessary or a small harmonic filter that filters only high-frequency harmonics that cannot be compensated by the self-excited converter can be used. Therefore, the apparatus configuration can be simplified.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a power conversion apparatus according to Embodiment 1 of the present invention.
FIG. 2 is an explanatory diagram for explaining the operation of the power conversion apparatus according to embodiment 1 of the present invention.
FIG. 3 is an explanatory diagram for explaining the operation of the power conversion apparatus according to embodiment 1 of the present invention.
FIG. 4 is an explanatory diagram for explaining the operation of the power conversion apparatus according to embodiment 1 of the present invention.
FIG. 5 is an explanatory diagram illustrating the operation of the power conversion device according to embodiment 1 of the present invention.
FIG. 6 is a diagram showing a change in the magnitude of a reactive current component of the power conversion device according to embodiment 1 of the present invention.
FIG. 7 is a configuration diagram showing a power conversion device according to a second embodiment of the present invention.
FIG. 8 is a configuration diagram showing a power conversion device according to Embodiment 3 of the present invention.
FIG. 9 is a configuration diagram showing a power conversion device according to Embodiment 4 of the present invention.
FIG. 10 is a configuration diagram showing a power conversion device according to Embodiment 5 of the present invention.
FIG. 11 is a configuration diagram showing a main part of a power conversion device according to Embodiment 6 of the present invention.
FIG. 12 is an explanatory diagram illustrating the operation of a power conversion device according to Embodiment 6 of the present invention.
FIG. 13 is an explanatory diagram for explaining the operation of a power conversion device according to Embodiment 6 of the present invention.
FIG. 14 is an explanatory diagram explaining the operation of the power conversion device according to the sixth embodiment of the present invention.
FIG. 15 is a configuration diagram showing a main part of a power conversion device according to Embodiment 6 of the present invention;
FIG. 16 is a configuration diagram showing a power conversion device according to a seventh embodiment of the present invention.
FIG. 17 is a configuration diagram showing a power conversion device according to a seventh embodiment of the present invention.
FIG. 18 is a configuration diagram showing a conventional power converter.
FIG. 19 is an explanatory diagram for explaining the operation of a conventional power converter.
FIG. 20 is an explanatory diagram for explaining the operation of a conventional power converter.
FIG. 21 is an explanatory diagram for explaining the operation of a conventional power converter.
FIG. 22 is a circuit configuration diagram showing another conventional power converter.
FIG. 23 is a circuit configuration diagram showing another conventional power converter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 DC circuit, 101, 102, 103, 104, 105, 106 coil, 2 Power converter group, 201, 202, 203, 204, 205 Separately excited converter, 210 Self-excited converter, 211 Voltage type self-excited converter , 212 DC capacitor, 213 DC chopper, 214 control circuit, 221, 222 arithmetic circuit, 223 total arithmetic unit, 224, 231, 241, 242 adder / subtractor, 225, 226 setting circuit, 227 AC current controller, 228 reactive current Measurement circuit, 229 divider, 230 reactive power measurement circuit, 232 setting circuit, 233 controller, 240 harmonic current detector, 3 power system, quaternary phase capacitor, 5 SVC, 6 harmonic filter.

Claims

A plurality of converters each including at least one separately-excited converter and at least one self-excited converter that convert alternating current from a power system into direct current ; the self-excited converter and the separately-excited converter A power conversion device having an energy storage inductance element connected to the converter and allowing the converted direct current to flow therethrough, and the reactive power generated by the self-excited converter causes the invalidity generated by the separately excited converter A power conversion apparatus, wherein the self-excited converter is controlled such that a change in power is canceled and the reactive power generated by the plurality of converters is constant regardless of a DC current and a DC voltage.

The power conversion device according to claim 1, wherein a control angle α and a direct current I _d in a separately excited converter are

For each separately-excited converter, the delay reactive current I _Q calculated in step 1 is obtained, and the difference between the sum of these delay reactive currents and the advanced reactive current of the phase advance capacitor connected to the power system is obtained . A power conversion apparatus characterized in that the self-excited converter has a control command value for an ineffective current, and the self-excited converter is controlled together with an effective current command value.

2. The power conversion device according to claim 1, wherein the separately excited converter group is connected to the same AC bus, the power system side of the connection point of the separately excited converter group of the AC bus, and the separately excited conversion rather than the self-excited converter. Measure the delayed reactive current of the separately-excited converter group from the voltage and current of the AC bus on the side of the generator group, and calculate the difference between the delayed reactive current and the advanced reactive current of the phase advance capacitor connected to the power system. calculated, which was the control command value of reactive current of self-commutated converter, power converter, characterized in that the controlling the self-commutated converter with active current command value.

2. The power conversion device according to claim 1, wherein the separately excited converter group is connected to the same AC bus, the power system side of the connection point of the separately excited converter group of the AC bus, and the separately excited conversion rather than the self-excited converter. Measure the delayed reactive power of the separately-excited converter group from the voltage and current of the AC bus on the group side, and the difference between the delayed reactive power and the value corresponding to the advanced reactive power of the phase advance capacitor connected to the power system The self-excited converter is controlled by the reactive current command value obtained by dividing the control command value by the system voltage and the effective current command value. The power converter characterized by the above-mentioned.

2. The power converter according to claim 1, wherein the voltage and current of the AC bus on the power system side from the connection points of all the converter groups of the AC bus and on the converter group side of the phase advance capacitor connected to the power system. The reactive current or reactive power of all converter groups is measured from the above, and the difference between the reactive current or reactive power and the value corresponding to the advanced reactive current or advanced reactive power of the phase advance capacitor is obtained . Is a control command value for the reactive current or reactive power of the self-excited converter, and a controller for generating the reactive current command value of the self-excited converter so that the control command value becomes 0 or a value close thereto. , power conversion equipment, characterized in that the controlling the self-commutated converter above a reactive current command value, by the active current command value.

The power converter according to 請 Motomeko 1, all transducers group and phase advance ac busbars voltage of the electric power system side of the connection point of the capacitor of the AC buses, the reactive current or reactive power from the current measured, A controller that creates a reactive current command value for the self-excited converter so that the reactive current or reactive power is 0 or a value close thereto, the reactive current command value and the active current command value; The self-excited converter is controlled by the above.

The power converter according to any one of claims 1 to 6, self-commutated converter, power converter, characterized in that is constituted by the current-source converter.

The power converter according to any one of claims 1 to 6, self-commutated converter, and a capacitor to the DC side, and a voltage-source converter for AC-DC converter, a capacitor and an inductance component of the voltage-source converter And a chopper for controlling the current flowing through the inductance element by variably controlling the DC voltage.

9. The power conversion device according to claim 8, wherein the leading reactive power of the phase advance capacitor connected to the power system is the sum of the maximum delay reactive power and the minimum delay reactive power generated by the separately excited converter in the operating state of the system. The power converter characterized by having set it as 1/2.

The power converter according to any one of claims 1 to 9, the harmonic current of the separately excited converter is measured and from reactive current command value and active current command value of the self-commutated converters, the separately excited converter A power conversion device, wherein the harmonic current of the separately excited converter is canceled by subtracting the harmonic current of the converter.