Name: transformer sequence networks in per unit
Uploaded: 2024-11-21T14:54:31.000Z
Duration: 1 min 26 s
Channel: JoVE

中性インピーダンスを介して接地された理想的なY-Yトランスは、バランスの取れた正または負のシーケンス電流が流れると、単相トランスと同様の単位当たりシーケンスネットワークを持ち、中性電流と電圧降下を打ち消します。
3つの相すべてからのゼロシーケンス電流が結合して中性電流を形成し、中性インピーダンスの両端で電圧降下を引き起こし、低電圧巻線電圧に影響を与えます。
実用的なY-Yトランスは、単位あたりのシーケンスネットワークに外部インピーダンスを含んでいます。各位相は、磁化インダクタンスと並列のコア損失抵抗を表します。
これらのトランスフォーマーは、ユニットあたりの正と負のシーケンスインピーダンスが同一ですが、ゼロシーケンスネットワークはニュートラルインピーダンスに依存します。
デルタデルタトランスは、同一の正と負のシーケンスネットワークを持ち、ユニットあたりのインピーダンスは巻線接続に依存しません。
三相のユニットあたりのシーケンスをモデル化するために、3 つの同等の単相トランスを共通の S ベースと比例電圧ベースを使用して接続することで、3 つの三巻線トランスを作成します。
ゼロシーケンスネットワークでは、高電圧接続は高電圧巻線の構成に依存します。

単位あたりのシーケンスモデル

Per-Unit Sequence Models

理想的な Y-Y 変圧器は、中性インピーダンスを介して接地され、平衡した正または負のシーケンス電流を受けると、単相の理想的な変圧器の単位あたりのシーケンスネットワークに似たネットワークを表示します。これらの電流は中性電流を生成せず、それに伴う電圧降下も生成しません。 
ゼロシーケンス電流は大きさと位相が同一であり、中性電流を生成し、中性インピーダンスと低電圧巻線で電圧降下を引き起こします。変圧器の中性線が接地されていない場合、ゼロシーケンス電流は地面に流れることができず、巻線内を循環することができます。 
実際の Y-Y 変圧器における単位あたりのシーケンスネットワークには、外部インピーダンスが組み込まれています。並列分岐は平衡 Y インピーダンス負荷を表し、各位相は磁化インダクタンスと並列のコア損失抵抗器に対応します。これらの変圧器は、単位ごとに同一の正相および逆相インピーダンスを持ちますが、中性インピーダンスはゼロシーケンスネットワークに影響します。 
デルタデルタ変圧器は、同一の正相および逆相インピーダンスを持つ単位あたりのシーケンス ネットワークを備えています。ただし、実際の変圧器では、異なる巻線接続がこれらのインピーダンスに影響を与える可能性があります。3 相 3 巻線変圧器の単位あたりのシーケンスモデルは、端子用の共通の S ベースと比例電圧ベースを使用して、3 つの同一の単相変圧器を接続することによって構築できます。 
一般的なゼロシーケンスネットワークでは、高電圧巻線の構成によって高電圧接続が決まります。 単位あたりの 負シーケンスネットワークインピーダンスは、常に正シーケンスネットワークインピーダンスと一致します。

中性インピーダンスを介して接地された理想的なY-Yトランスは、平衡正または単相トランスと同様のユニットあたりのシーケンスネットワークを備えています。

Education

JoVE Core

Electrical Engineering

電気工学

Unit 1

ユニット1

Symmetrical and Unsymmetrical Faults

対称故障および非対称故障

KEY TERMS AND CONCEPTS

主要用語と概念

transformer sequence networks in per unit

Transformer sequence networks in per unit show how three-phase transformers respond to positive-, negative-, and zero-sequence currents. For an ideal Y-Y transformer grounded through neutral impedances, the per-unit sequence networks act much like those of a single-phase ideal transformer when the current is balanced. Balanced positive-sequence and negative-sequence currents do not create neutral current, so they do not cause voltage drops across the neutral path.

Zero-sequence current behaves differently because all three phase currents have the same magnitude and phase. This creates a neutral current and produces voltage drops across both the neutral impedance and the low-voltage winding. If the neutral is not grounded, zero-sequence current cannot flow to ground, but it can still circulate inside the windings.

Practical Y-Y transformers add external impedances to the per-unit sequence network. Their shunt branches represent a balanced-Y impedance load, with each phase modeled by a core loss resistor in parallel with the magnetizing inductance. In these transformers, the per-unit positive-sequence and negative-sequence impedances are the same, while the neutral impedances affect the zero-sequence network.

Delta-delta transformers also have identical per-unit positive-sequence and negative-sequence impedances. In practical transformers, however, the winding connection can change these impedances. Three-phase, three-winding transformer models can be built by connecting three identical single-phase transformers with a common S-base for the terminals and proportional voltage bases. In the general zero-sequence network, the high-voltage winding configuration sets the high-voltage connection, and the negative-sequence network impedance always matches the positive-sequence network impedance.

Transformer Sequence Networks in Per Unit

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Fault Current in a Series R-L Circuit

A series resistor-inductor, or R-L, circuit can model a three-phase short circuit when the switch closes at the start of the time period. In this fault condition, all three phases of an unloaded synchronous machine are short-circuited. With no fault impedance and no initial current, the starting voltage depends on the source voltage phase angle. Kirchhoff's Voltage Law, or KVL, is used to analyze the circuit and find the total asymmetrical fault current. This current has two parts. The AC...

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Synchronous Machine Fault Current Decay

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Subtransient fault current analysis in a power system uses simplified models for the main equipment. Transformers are represented by their leakage reactances. Transmission lines are modeled with equivalent series reactances, and synchronous machines are treated as constant voltage sources behind their subtransient reactances. Several elements are left out of the calculation because their effect on the immediate fault current is small or they make the model harder without changing the result...

bus impedance matrix

Three-Phase Fault Current Analysis

Three-phase fault current analysis in an N-bus power system uses the positive-sequence network and the bus impedance matrix. It helps estimate subtransient fault currents when a short circuit occurs at one bus. The method uses superposition, which means the problem is split into two circuits that are studied separately. In the first circuit, all machine voltage sources are short-circuited. Only the prefault voltage source at the faulted bus remains. The positive-sequence bus impedance matrix...

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Protective Devices in Power Systems

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Generator Sequence Currents and Impedances

Generator sequence currents and impedances explain how a Y-connected synchronous generator behaves under balanced and unbalanced conditions. When the generator is grounded through a neutral impedance, it is set up to produce balanced internal phase voltages with only positive-sequence components. The source voltage therefore appears only in the positive-sequence network, and the line-to-ground voltages at the generator terminals show the sequence components of this setup. Zero-sequence current...

per-unit sequence models

Transformer sequence networks in per unit show how three-phase transformers respond to positive-, negative-, and zero-sequence currents. For an ideal Y-Y transformer grounded through neutral impedances, the per-unit sequence networks act much like those of a single-phase ideal transformer when the current is balanced. Balanced positive-sequence and negative-sequence currents do not create neutral current, so they do not cause voltage drops across the neutral path. Zero-sequence current behaves...

fault types

Power System Faults in Sequence Networks

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key terms and concepts

Learn how Y-Y, delta-delta, and three-winding transformers use per-unit sequence networks to model positive, negative, and zero-sequence currents.

Transformer Sequence Networks in Per Unit | JoVE K12

JoVE Core 電気工学 は、電気回路の基本概念と回路解析テクニックを網羅したアニメーションビデオシリーズです。オペアンプと回路変換および 1 次・ 2 次回路の概念を網羅しています。実験を撮影した実写動画では、設計例や現代社会での応用が含まれており、複雑な概念も分かりやすく学習いただけます。

electrical engineering

Transformer Sequence Networks in Per Unit

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