渦電流損失を低減するために、シリコン鋼コアは通常 0.23 mm ～ 0.35 mm の厚さの薄いストリップに積層されます。各積層体は薄い絶縁層 (通常はリン酸塩処理または有機ワニス) でコーティングされており、層間で電流が流れるのを防ぎます。より緊密な積層とより薄いストリップにより、持続的な過負荷状態における損失と熱の蓄積が軽減されます。
の key engineering challenge in split-core design is minimizing the air gap at the joint. Even a microscopic gap in the magnetic circuit dramatically increases the magnetizing current required and introduces errors, particularly at low primary current levels near 5% to 20% of the rated value. Manufacturers address this through:
の toroidal current transformer is by far the most common construction used in modern panel-mount and inline current measurement. Its defining feature is the doughnut-shaped core, which creates a closed magnetic path with no deliberate air gap (in solid-core versions) and no exposed pole faces.
の majority of low-voltage and medium-voltage current transformers used in metering panels and switchgear are manufactured by casting or vacuum-injecting epoxy resin around the wound core assembly. This process, often called molded-case construction, creates a solid, void-free insulating body that:
の secondary burden — the total impedance of connected instruments, wiring resistance, and relay coils — directly affects the accuracy class performance. Most accuracy class specifications (IEC 61869, IEEE C57.13) define performance at a rated burden and at 25% of rated burden. A CT specified as Class 0.5 at a 5 VA burden will perform within 0.5% ratio error only when the connected burden does not exceed 5 VA. Exceeding the rated burden increases secondary terminal voltage, drives the core toward saturation, and degrades accuracy.
高調波性能
可変速ドライブ設備、アーク炉、およびデータセンターの電力システムでは、一次電流に重要な高調波成分 (多くの場合、5 次、7 次、11 次、および 13 次の高調波) が含まれています。標準的なシリコン鋼鉄コアは、高調波周波数で損失と位相角歪みが増加します。ナノ結晶コアとアモルファスコアは、数キロヘルツまで平坦な透磁率曲線を持ち、歪んだ波形に対して大幅に優れた精度を提供し、クラス A の電力品質アナライザに推奨されます。
外側ハウジングと取り付け構成
の enclosure of a current transformer protects the active components from environmental exposure and provides the mechanical interface for installation. Housing design varies significantly between application categories.
の IEEE C57.13 standard uses a letter-number combination: the letter (C, K, T, or X) indicates the performance basis, and the number indicates the secondary terminal voltage at rated burden and 20 times rated current. A C200 CT can deliver its rated secondary current into a burden that develops 200 V at 20 times rated primary current without exceeding 10% ratio error. This classification system is prevalent in North American utility and industrial protection practice.
の core material determines the magnetizing current the transformer requires to function. High-permeability materials (nickel-iron, nanocrystalline) need very little magnetizing current, which directly reduces both ratio error and phase displacement. Silicon steel is adequate for Class 0.5 and Class 1 applications. For Class 0.1 and 0.2 revenue metering, nickel-iron alloy cores are typically required. For harmonic-rich environments, nanocrystalline materials offer the best combination of accuracy and wide-frequency performance.
の accuracy class number represents the maximum allowable ratio error (in percent) at rated burden and at 100% of rated primary current for standard classes, or down to 1% of rated current for "S" class designations. For example, a Class 0.5 CT must not exceed a 0.5% ratio error, while a Class 0.5S CT must also hold within 0.5% ratio error at primary currents as low as 1% of the rated value. Protection-class CTs use different designations (such as 5P20) that express composite error and accuracy limit factor rather than ratio error alone.
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