JPH0226747B2 - - Google Patents
Info
- Publication number
- JPH0226747B2 JPH0226747B2 JP56120740A JP12074081A JPH0226747B2 JP H0226747 B2 JPH0226747 B2 JP H0226747B2 JP 56120740 A JP56120740 A JP 56120740A JP 12074081 A JP12074081 A JP 12074081A JP H0226747 B2 JPH0226747 B2 JP H0226747B2
- Authority
- JP
- Japan
- Prior art keywords
- current
- iron core
- residual current
- excitation
- residual
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical group [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 20
- 230000004907 flux Effects 0.000 claims description 10
- 238000001514 detection method Methods 0.000 claims description 8
- 238000000034 method Methods 0.000 claims description 7
- 230000035945 sensitivity Effects 0.000 claims description 5
- 238000004804 winding Methods 0.000 claims description 5
- 230000005284 excitation Effects 0.000 description 23
- 230000001052 transient effect Effects 0.000 description 17
- 238000010586 diagram Methods 0.000 description 8
- 230000035699 permeability Effects 0.000 description 4
- 239000000696 magnetic material Substances 0.000 description 3
- 230000005389 magnetism Effects 0.000 description 2
- 230000007257 malfunction Effects 0.000 description 2
- 230000000149 penetrating effect Effects 0.000 description 2
- 230000007423 decrease Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 229910000889 permalloy Inorganic materials 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/175—Indicating the instants of passage of current or voltage through a given value, e.g. passage through zero
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Testing Of Short-Circuits, Discontinuities, Leakage, Or Incorrect Line Connections (AREA)
- Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)
Description
【発明の詳細な説明】
三相電路を零相電流を検出する方法として、三
相電路の各用に変流器をそれぞれ設置して検出す
る方法があるが、この方法においては、各CTに
バラツキがあるため、残留電流特性が悪いという
欠点がある。[Detailed Description of the Invention] As a method for detecting zero-phase current in a three-phase circuit, there is a method of installing a current transformer for each of the three-phase circuits and detecting the zero-phase current. Due to variations, there is a drawback that residual current characteristics are poor.
この欠点を解決する発明として、各変流器の負
荷抵抗（負担抵抗）と巻線抵抗の値を、各変流器
の定格電流を流して二次側を開放したときの一次
側の励磁電流のベクトル和が定格電流での感度電
流以下になるような値にし、かつ各変流器の鉄芯
の磁束密度を1000ガウス以内にすることにより残
留電流を減少させる方法の発明がある。 As an invention to solve this drawback, we calculated the value of the load resistance (burden resistance) and winding resistance of each current transformer by the excitation current on the primary side when the rated current of each current transformer flows and the secondary side is opened. A method has been invented to reduce the residual current by setting the vector sum to a value that is less than the sensitivity current at the rated current and by setting the magnetic flux density of the iron core of each current transformer to within 1000 Gauss.
この発明の方法では、従来の検出方法に比べ残
留電流特性は良いが、一次電流が急激に変化した
時、残留電流が大巾に上昇し、低下し、安定する
までに長時間を要し、そのため組合せた継電器が
誤動作するという問題点がある。 Although the method of this invention has better residual current characteristics than conventional detection methods, when the primary current changes rapidly, the residual current rises and falls significantly, and it takes a long time for it to stabilize. Therefore, there is a problem that the combined relay may malfunction.
第１図には、三相電路が各変流器（以下CTと
いう）を設置して、零相電流を検出する場合の回
路が示してあり、同図においてCT−１，CT−
２，CT−３は各変流器、R1，R2，R3は各変流器
の負担抵抗と巻線抵抗の和、R0は零相電流検出
用抵抗を示す。この回路において、一次電流が
I〓a、I〓b、I〓cの時、残留電流は、基本波分は、
残留電流(1)≒I〓a・R1／ωL1＋I〓b・R2／ωL2＋I〓cR
3／ωL3
第三高調波分は、
残留電流(2)≒｛｜I〓a・R1／ωL1｜＋｜I〓b・R2／ωL
2｜
＋｜I〓c・R3／ωL3｜｝×（３〜５％）
（磁束密度1000G以下の時）
となり、ほぼ基本波と第三高調波の和が残留電流
となる。 Figure 1 shows a circuit in which a three-phase circuit is equipped with current transformers (hereinafter referred to as CT) to detect zero-phase current.
2, CT-3 is each current transformer, R 1 , R 2 , R 3 is the sum of the burden resistance and winding resistance of each current transformer, and R 0 is the zero-sequence current detection resistance. In this circuit, the primary current is
When I〓a, I〓b, I〓c, the residual current, the fundamental wave component, is: Residual current (1)≒I〓a・R 1 /ωL 1 +I〓b・R 2 /ωL 2 +I〓cR
3 /ωL 3rd harmonic component is: Residual current (2)≒｜I〓a・R 1 /ωL 1 ｜＋｜I〓b・R 2 /ωL
2 | + | I〓c・R 3 /ωL 3 |}×(3 to 5%) (when the magnetic flux density is 1000G or less), and the residual current is approximately the sum of the fundamental wave and the third harmonic.
なお、上記残留電流(1)、(2)の式において、
L1はI〓aの時のCT−１の二次側から見たインダク
タンス
L2はI〓b 〃 〃
L3はI〓c 〃 〃
ここで各Ｒは小さくし、ωLを大きくすること
により残留電流は小さくなり、磁束密度Ｂが
1000G以下ならば高感度になるのであるが、この
時、Ia＝√２Iasinωtがωt＝−π／２でONになり、
CTの残留磁気が０だとすると、定常状態時の残
留電流と過渡状態時の残留電流との値の違いは、
第２図、第３図の通りである。第３図において磁
束変化及び励磁電流の時間軸を表わすと、第４図
及び第５図のようになる。鉄心内の磁束密度の変
化は、振巾の大きさはほぼ一定であるが、過渡時
の前に鉄心内に残留磁気があると初期時加算され
る。そのため励磁電流は最初大きく、徐々に減少
し、安定域に入る。 In addition, in the equations of residual currents (1) and (2) above, L 1 is the inductance seen from the secondary side of CT-1 when I〓a, L 2 is I〓b 〃 〃 L 3 is I〓c 〃 〃 Here, by making each R smaller and increasing ωL, the residual current becomes smaller and the magnetic flux density B becomes
High sensitivity is obtained below 1000G, but at this time, Ia = √2 Iasinωt turns on at ωt = -π/2, and if the residual magnetism of the CT is 0, then the residual current in the steady state and the residual current in the transient state are The difference in value from the residual current is
As shown in Figures 2 and 3. In FIG. 3, the time axes of magnetic flux changes and excitation current are shown in FIGS. 4 and 5. The amplitude of the change in the magnetic flux density within the iron core is approximately constant, but if there is residual magnetism within the iron core before the transition, it will be added at the initial stage. Therefore, the excitation current is large at first, gradually decreases, and enters a stable region.
このため、高感度にすると、過渡現象の起きた
CTは励磁電流が大きく、安定域に入るまで長時
間を要する。 For this reason, when the sensitivity is set high, transient phenomena occur.
CT has a large excitation current, and it takes a long time to reach a stable range.
高感度3CTの場合、トランスや通常のCTと同
様に過渡現象を起しているCTは励磁電流が非常
に大きい。よつて３個の励磁電流のベクトル和が
残留電流であるから、１個のCTのみ励磁電流が
大きいと、残留電流は大きく、安定するまで数10
秒以上かかるので、負荷変動のある所では使用で
きない。 In the case of a high-sensitivity 3CT, the excitation current of the CT, which causes transient phenomena like a transformer or normal CT, is extremely large. Therefore, since the vector sum of the three excitation currents is the residual current, if the excitation current of only one CT is large, the residual current will be large and will take several tens of seconds until it stabilizes.
Since it takes more than a second, it cannot be used in places where the load fluctuates.
その対策として、通常鉄芯が飽和しない範囲で
Ｒを大きくするか、ωLを小さくするかで、時定
数を短くすれば、問題ないのであるが、3CTの場
合、残留電流が大きくなり、意味がなくなつてし
まう。 As a countermeasure, it is usually okay to shorten the time constant by increasing R or decreasing ωL within a range that does not saturate the iron core, but in the case of 3CT, the residual current becomes large and it is meaningless. It will disappear.
本発明は、3CTの鉄芯のみ分割とし、エアギヤ
ツプをとり、巻線、シールドは非分割とし、過渡
現象時の問題点を解消し、高感度の零相電流検出
を可能にしようとするものである。 The present invention aims to solve problems during transient phenomena and enable highly sensitive zero-sequence current detection by dividing only the iron core of the 3CT, removing the air gap, and leaving the windings and shield undivided. be.
以下本発明を詳細に説明すると、磁性材料、例
えばパーマロイは初期透磁率以下でも磁化力Ｈに
対する磁束密度Ｂは第６図のＢ−Ｈ曲線で示すよ
うに非直線である。そして透磁率μは第７図の透
磁率曲線で示すように変化する。 The present invention will be described in detail below. Even if the magnetic material, such as permalloy, has an initial magnetic permeability or less, the magnetic flux density B with respect to the magnetizing force H is non-linear as shown by the B-H curve in FIG. 6. The magnetic permeability μ changes as shown by the magnetic permeability curve in FIG.
そのため、交流電圧が同じでもＢの動作点が第
６図のＯとＰの場合では、磁界の交流分は異な
る。これが上記のように残留電流を大きくする影
響を及ぼす。 Therefore, even if the AC voltage is the same, the AC component of the magnetic field is different when the operating points of B are O and P in FIG. This has the effect of increasing the residual current as described above.
これに対し第８図示のように鉄芯１に空隙Δl1
を入れると、磁気抵抗は
Ｒ＝ｌ／μA＝l1／μ0μsA＋Δl1／μ0A
＝１／μ0A（l1／μs＋Δl1）
ここで第１項は空隙のない磁の磁気抵抗値でμs
が磁界Ｈに対し変化するため、Ｂの変化巾が一定
でも動作点によつて励磁電流は変る。 On the other hand, as shown in Figure 8, there is a void Δl 1 in the iron core 1.
Then, the magnetic resistance is R=l/μA=l 1 /μ 0 μsA+Δl 1 /μ 0 A = 1/μ 0 A (l 1 /μs+Δl 1 ) Here, the first term is the magnetic reluctance of a magnet with no air gap. value in μs
Since B changes with respect to the magnetic field H, the excitation current changes depending on the operating point even if the range of change in B is constant.
例えば、Δl1＝（５〜10）・l1／μs程度にΔl1を選
ぶ
ことにより、磁気抵抗Ｒが５〜10倍上昇し、Ｒ／ωL
は、Ｌが５〜10分の１になるため、Ｂ≦1000Gで
はほとんど直線になり、過渡現象が起きても、励
磁電流の交流分は変化せず、残留電流は一定であ
り、過渡現象のみなくなる。すなわち交流分の過
渡現象が少なくなるが、直流分は変らない。 For example, by choosing Δl 1 to be approximately Δl 1 = (5 to 10) l 1 /μs, the magnetic resistance R increases by 5 to 10 times, and R/ωL becomes 1/5 to 10 times L. Therefore, when B≦1000G, it becomes almost a straight line, and even if a transient phenomenon occurs, the alternating current component of the excitation current does not change, the residual current remains constant, and only the transient phenomenon disappears. In other words, transient phenomena in the AC component are reduced, but the DC component remains unchanged.
その理由を示すと次のとおりである。 The reasons for this are as follows.
NI＝Ｒ・φより
過渡状態においても第９図示のCTの等価回路
における励磁電圧υは一定でＩのみが変るだけと
考えることができる。例えば、３個のCTの励磁
特性が第１０図示のとおりとすると、残留電流は
第１１図のベクトル図より50ｍＡとなる。 From NI=R·φ, it can be considered that even in a transient state, the excitation voltage υ in the equivalent circuit of CT shown in Figure 9 is constant and only I changes. For example, if the excitation characteristics of the three CTs are as shown in Figure 10, the residual current will be 50 mA from the vector diagram in Figure 11.
この時I1＝Φ／ＮＲ＝Φ／Ｎ・１／μ0A・l1／μs＝K
1l1／μ1s
＝0.25A
I2＝I3＝K1l1／μ2s＝0.3A
となる。 At this time, I 1 =Φ/NR=Φ/N・1/μ 0 A・l 1 /μs=K
1 l 1 / μ 1 s = 0.25A I 2 = I 3 = K 1 l 1 / μ 2 s = 0.3A.
ここで鉄芯１を分割して約10倍励磁電流を大き
くしたとすると、
I′1＝K1（l1／μ1s＋Δl1）＝0.25＋K1Δl1
ここにl′1＝l1−Δl1 Δl1は微少だからl′1とl1は
計
算上はほぼ等しい。 If we divide the iron core 1 and increase the excitation current by about 10 times, then I' 1 = K 1 (l 1 /μ 1s + Δl 1 ) = 0.25 + K 1 Δl 1 where l' 1 = l 1 − Since Δl 1 Δl 1 is very small, l′ 1 and l 1 are almost equal in calculation.
K1Δl≒10・Ｋl1′／μ1sとすると I′1≒0.25＋2.5＝2.75A I′2＝I′3＝K1l′1／μ2s＋K1Δl1≒0.3＋2.5 ＝2.80A となり、残留電流はおよそ2.8−2.75＝0.05A ＝50ｍＡ となり、分割前とほぼ同じである。 If K 1 Δl≒10・Kl 1 ′/μ 1s , then I′ 1 ≒0.25+2.5=2.75A I′ 2 =I′ 3 =K 1 l′ 1 /μ 2s +K 1 Δl 1 ≒0.3+2.5 = 2.80A, and the residual current is approximately 2.8-2.75 = 0.05A = 50mA, which is almost the same as before division.
そのため、鉄芯１を分割したことにより、コイ
ルを多く巻く必要はなく貫通形の設計で良く、外
部磁界のみ（内部も含む）を気をつければ良い。 Therefore, by dividing the iron core 1, there is no need to wind many coils, and a through-type design is sufficient, and only the external magnetic field (including the internal one) needs to be taken care of.
鉄芯１の分割は、分割面を１か所にしない方が
外部磁界の影響を受けにくい。例えば鉄芯１を４
枚重ねにする場合、第１２図示のように分割面を
45度ずつずらせて重ねると、外部磁界の影響を受
けにくい。 The division of the iron core 1 is less susceptible to external magnetic fields if the division plane is not in one place. For example, iron core 1 is 4
When stacking sheets, divide the dividing plane as shown in Figure 12.
By stacking them at a 45-degree angle, they are less susceptible to external magnetic fields.
なお、過度現象は、定常状態では励磁電圧、励
磁電流が第１３図示のようにＢ−Ｈ曲線のＯ点で
振巾するが、過渡状態では第１４図のように励磁
電圧のピーク値がυ2＝2υ1近くまで上昇し、励磁
電流のピーク管の値は定常状態における√２
Isinωtの２倍、すなわち√２Isinωtよりもはるか
に大となり、他の相２つの過渡状態が少ないた
め、大きな残留電流が残る。 In addition, in a transient phenomenon, in a steady state, the excitation voltage and excitation current swing at the O point of the B-H curve as shown in Figure 13, but in a transient state, the peak value of the excitation voltage is υ as shown in Figure 14. 2 = 2υ 1 , and the peak value of the excitation current is √2 in the steady state.
It is twice Isinωt, that is, much larger than √2Isinωt, and since there are few transient states in the other two phases, a large residual current remains.
このような貫通型の鉄芯を分割すると、その空
隙によりＢ−Ｈ曲線が直線に近くなるために、前
記のように残留電流は変らないが、等価磁気抵抗
が５〜10倍近くなるため、等価μsは５〜10分の１
になり、過渡現象の交流分が少なくなるとともに
時定数Ｌ／Ｒが５〜10分の１と短くなる。 When such a penetrating iron core is divided, the B-H curve becomes close to a straight line due to the air gap, so the residual current does not change as mentioned above, but the equivalent magnetic resistance increases by 5 to 10 times. Equivalent μs is 1/5 to 10
As a result, the alternating current component of the transient phenomenon is reduced and the time constant L/R is shortened to 1/5 to 1/10.
なお、貫通形の鉄芯において残留電流が分割形
の鉄芯と同じ値で、Ｌも同じ、Ｒのみ10倍にすれ
ば、時定数τ＝Ｌ／Ｒは10分の１になり、分割しな
くても同じ特性が得られるように思われるが、そ
のようにすると、励磁電圧が10倍になり、高調波
が大巾に増加し、過渡現象が全く異なり、残留電
流が大巾に増加（時定数は同じ）するので、同じ
特性は得られない。 In addition, if the residual current in the through-type iron core is the same as the split-type iron core, L is the same, and only R is multiplied by 10, the time constant τ = L / R becomes 1/10, and the split-type iron core has the same residual current. It seems that the same characteristics can be obtained without it, but if you do so, the excitation voltage increases ten times, the harmonics increase greatly, the transient phenomena are completely different, and the residual current increases greatly ( (time constant is the same), so the same characteristics cannot be obtained.
残留電流、時定数同一時の貫通形と分割形の残
留電流をクラブで比較すると、基本波は第１５
図、高調波は第１６図のようになり、磁束の変化
の比は第１７図のとおり同じである。 Comparing the residual current of the through-type and split-type clubs when the time constant is the same, the fundamental wave is the 15th.
The harmonics are as shown in Fig. 16, and the ratio of changes in magnetic flux is the same as shown in Fig. 17.
本発明は、叙上のように3CTの鉄芯のみ分割と
し、エアギヤツプをとり、巻線、シールドは非分
割としたから、過渡現象をなくし、組合せた継電
器が誤動作するようなことはなく、高感度の零相
電流検出を可能にする。 As mentioned above, in the present invention, only the iron core of the 3CT is divided, the air gap is removed, and the winding and shield are not divided, so transient phenomena are eliminated, the combined relay does not malfunction, and high Enables sensitive zero-sequence current detection.
第１図は3CT方式による零相電流検出回路図、
第２図は第１図の回路における定常状態時の残留
電流値を示すグラフ、第３図は第１図の回路にお
ける過渡状態時の残留電流値を示すグラフ、第４
図は第３図における磁束変化を時間軸で示すグラ
フ、第５図は第３図における励磁電流を時間軸で
示すグラフ、第６図は磁性材料のＢ−Ｈ曲線を示
す図、第７図は磁性材料の透磁率曲線を示す図、
第８図は本発明に係る変流器の鉄芯を示す図、第
９図は変流器の等価回路図、第１０図は各変流器
の励磁特性の一例を示す図、第１１図は同特性に
おける各変流器の残留電流のベクトル図、第１２
図は本発明に係る変流器の鉄芯の分割面をずらす
例を示す図、第１３図は定常状態の励磁電圧に対
する励磁電流の振巾を示す図、第１４図は過渡状
態の励磁電圧に対する励磁電流の振巾を示す図、
第１５図は貫通形と分割形の残留電流における基
本波分の比較を示すグラフ、第１６図は同残留電
流における高調波分の比較を示すグラフ、第１７
図は貫通形と分割形の磁束の変化の比較を示すグ
ラフである。
１……鉄芯。
Figure 1 is a zero-phase current detection circuit diagram using the 3CT method.
Fig. 2 is a graph showing the residual current value in the steady state in the circuit of Fig. 1, Fig. 3 is a graph showing the residual current value in the transient state in the circuit of Fig. 1, and Fig. 4 is a graph showing the residual current value in the transient state in the circuit of Fig. 1.
The figure is a graph showing the magnetic flux change in Fig. 3 on the time axis, Fig. 5 is a graph showing the excitation current in Fig. 3 on the time axis, Fig. 6 is a graph showing the B-H curve of the magnetic material, and Fig. 7 is a diagram showing the permeability curve of a magnetic material,
Fig. 8 is a diagram showing the iron core of the current transformer according to the present invention, Fig. 9 is an equivalent circuit diagram of the current transformer, Fig. 10 is a diagram showing an example of the excitation characteristics of each current transformer, and Fig. 11 is a vector diagram of the residual current of each current transformer with the same characteristics, the 12th
The figure shows an example of shifting the dividing plane of the iron core of the current transformer according to the present invention, Fig. 13 shows the amplitude of the excitation current with respect to the excitation voltage in a steady state, and Fig. 14 shows the excitation voltage in a transient state. A diagram showing the amplitude of the excitation current for
Figure 15 is a graph showing a comparison of the fundamental wave component in the residual current of the through type and split type, Figure 16 is a graph showing a comparison of the harmonic component in the same residual current, and Figure 17 is a graph showing a comparison of the harmonic component in the residual current of the through type and split type.
The figure is a graph showing a comparison of changes in magnetic flux between the penetrating type and the split type. 1... Iron core.
Claims (1)
鉄芯のみを分割とし、巻線、シールドは非分割と
することを特徴とする3CT方式による高感度零相
電流検出法。 ２ 第１項の検出法において各鉄芯の磁束密度を
1000ガウス以下とし、各変流器の感度電流≦I〓a・
R1／ωL1＋I〓bR2／ωL2＋I〓cR3／ωL3とすることを特
徴とする 3CT方式による高感度零相電流検出法。[Claims] 1. High sensitivity zero-sequence current using the 3CT method, characterized in that in each current transformer installed in each phase of a three-phase electric circuit, only the iron core is divided, and the windings and shields are not divided. Detection method. 2 In the detection method described in Section 1, the magnetic flux density of each iron core is
1000 Gauss or less, and the sensitivity current of each current transformer ≦I〓a・
A highly sensitive zero-sequence current detection method using a 3CT method characterized by R 1 /ωL 1 +I〓bR 2 /ωL 2 +I〓cR 3 /ωL 3 .
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP56120740A JPS5822970A (en) | 1981-08-03 | 1981-08-03 | High sensitivity zero-phase current detecting method by 3 current transformer system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP56120740A JPS5822970A (en) | 1981-08-03 | 1981-08-03 | High sensitivity zero-phase current detecting method by 3 current transformer system |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPS5822970A JPS5822970A (en) | 1983-02-10 |
| JPH0226747B2 true JPH0226747B2 (en) | 1990-06-12 |
Family
ID=14793805
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP56120740A Granted JPS5822970A (en) | 1981-08-03 | 1981-08-03 | High sensitivity zero-phase current detecting method by 3 current transformer system |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPS5822970A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008124512A (en) * | 2003-06-10 | 2008-05-29 | Ngk Insulators Ltd | Zero-phase current transformer |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS63311177A (en) * | 1987-06-13 | 1988-12-19 | Sumitomo Electric Ind Ltd | Zero-phase current detecting method utilizing optical magnetic field sensor |
| JP2562774Y2 (en) * | 1989-09-26 | 1998-02-16 | 松下電工株式会社 | Inverter device |
| JP6571290B2 (en) * | 2016-09-20 | 2019-09-04 | 株式会社東芝 | Three-phase through current transformer |
-
1981
- 1981-08-03 JP JP56120740A patent/JPS5822970A/en active Granted
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008124512A (en) * | 2003-06-10 | 2008-05-29 | Ngk Insulators Ltd | Zero-phase current transformer |
| JP4648954B2 (en) * | 2003-06-10 | 2011-03-09 | ミドリ安全株式会社 | Zero phase current transformer |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5822970A (en) | 1983-02-10 |
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