JPH07194132A - Inverter facility - Google Patents
Inverter facilityInfo
- Publication number
- JPH07194132A JPH07194132A JP5335629A JP33562993A JPH07194132A JP H07194132 A JPH07194132 A JP H07194132A JP 5335629 A JP5335629 A JP 5335629A JP 33562993 A JP33562993 A JP 33562993A JP H07194132 A JPH07194132 A JP H07194132A
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- Prior art keywords
- voltage
- inverter
- zero
- phase
- ground
- Prior art date
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- Supply And Distribution Of Alternating Current (AREA)
- Rectifiers (AREA)
- Control Of Ac Motors In General (AREA)
- Control Of Multiple Motors (AREA)
Abstract
Description
【０００１】[0001]
【産業上の利用分野】本発明は、複数台のＰＷＭ形イン
バータ装置を低圧母線に接続したインバータ設備に係
り、特に高次零相高調波対策を施したインバータ設備に
関する。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter facility in which a plurality of PWM type inverter devices are connected to a low voltage bus bar, and more particularly to an inverter facility in which measures against high-order zero-phase harmonics are taken.
【０００２】[0002]
【従来の技術】ＰＷＭインバータは、低次高調波を抑制
した交流出力によって誘導電動機等の負荷に供給できる
ため、交流電動機の電源にして騒音低減や高速応答制御
等に効果的となる。2. Description of the Related Art Since a PWM inverter can supply a load such as an induction motor with an AC output in which low-order harmonics are suppressed, it can be used as a power source of the AC motor and effective for noise reduction and high-speed response control.
【０００３】図４は、誘導電動機群を負荷とするインバ
ータ設備の配電系統を示す。配電用変圧器１で降圧され
た低圧母線２には多数のインバータ装置３1〜３Nがフィ
ーダ結線され、各インバータ装置３1〜３Nによって負荷
となる誘導電動機４1〜４Nに可変電圧可変周波（ＶＶＶ
Ｆ）の交流電力を供給して可変速運転を行う。FIG. 4 shows a power distribution system of inverter equipment having an induction motor group as a load. A large number of inverter devices 3 1 to 3 N are feeder-connected to the low-voltage bus 2 stepped down by the distribution transformer 1, and a variable voltage is applied to the induction motors 4 1 to 4 N, which are loads by the inverter devices 3 1 to 3 N. Variable frequency (VVV
The AC power of F) is supplied to perform variable speed operation.
【０００４】また、インバータ装置を介することなく、
低圧母線２から直接に誘導電動機５を運転する負荷構成
も設けられる。Further, without passing through an inverter device,
A load arrangement is also provided to drive the induction motor 5 directly from the low voltage bus 2.
【０００５】インバータ装置３1〜３Nは、インバータ装
置３1に代表して示すように、遮断器ＣＢと交流リアク
トルＡＣＬ及びフィルタＦＩＬを介してＰＷＭ形のＶＶ
ＶＦインバータＩＮＶに引き込み、インバータＩＮＶに
誘導電動機４1の運転出力を得る。The inverter devices 3 1 to 3 N , as represented by the inverter device 3 1 , represent a PWM type VV via a circuit breaker CB, an AC reactor ACL and a filter FIL.
It is drawn into the VF inverter INV and the operation output of the induction motor 4 1 is obtained in the inverter INV.
【０００６】このような配電系統の保護手段として、イ
ンバータＩＮＶのフィーダ側に地絡過電流リレー（５１
Ｇ）ＯＣＧが設けられる。また、低圧母線２には、三相
計器用変圧器ＧＰＴを介して地絡過電圧リレー（６４）
ＯＶＧ及び零相電圧リレー（Ｖ0）ＶＺＲが設けられ
る。As a protection means for such a distribution system, a ground fault overcurrent relay (51
G) OCG is provided. Further, the low voltage bus 2 is connected to the ground fault overvoltage relay (64) via the transformer GPT for three-phase meters.
An OVG and zero phase voltage relay (V 0 ) VZR are provided.
【０００７】[0007]
【発明が解決しようとする課題】低圧母線１から多数の
インバータ負荷及びその他の一般用負荷に配電する系統
において、ＰＷＭ形インバータＩＮＶの負荷側には高次
零相高調波成分の電流・電圧が発生する。In a system in which the low voltage bus bar 1 distributes a large number of inverter loads and other general loads, a current / voltage of a high-order zero-phase harmonic component is present on the load side of the PWM inverter INV. Occur.
【０００８】この零相高調波成分は、ＰＷＭ形インバー
タＩＮＶのキャリア（搬送波）周波数が高くなるほど零
相電流や零相電圧の増加として現れ、インバータの出力
段に設けられるフィルタでは除去しにくい。特に、最近
ではインバータを低騒音化するためにキャリア周波数を
高くする傾向にあり、零相高調波成分がますます高くな
ってきている。This zero-phase harmonic component appears as an increase in zero-phase current and zero-phase voltage as the carrier frequency of the PWM inverter INV increases, and is difficult to remove with a filter provided at the output stage of the inverter. In particular, recently, there is a tendency to increase the carrier frequency in order to reduce the noise of the inverter, and the zero-phase harmonic component is becoming higher and higher.
【０００９】上記の零相高調波電流・電圧は、配電母線
２側にも回り込みを起こし、地絡過電圧リレーＯＶＧや
零相電圧リレーＶＺＲ及び地絡過電流リレーＯＣＧの誤
動作を招く問題があった。これを以下に詳細に説明す
る。The above-mentioned zero-phase harmonic current / voltage also spills over to the distribution bus 2 side, which causes a malfunction of the ground fault overvoltage relay OVG, the zero-phase voltage relay VZR, and the ground fault overcurrent relay OCG. This will be described in detail below.
【００１０】インバータＩＮＶの負荷側に発生する高調
波零相電圧は、負荷側の接続ケーブルのキャパシタンス
及び誘導電動機ＩＭ内部の対地キャパシタンスを介して
大地を通り、配電系統に回り込む。The harmonic zero-phase voltage generated on the load side of the inverter INV passes through the ground via the capacitance of the connection cable on the load side and the ground capacitance inside the induction motor IM, and then flows into the power distribution system.
【００１１】この回り込みは、（１）抵抗接地系では接
地抵抗ＮＧＲを通って、（２）直接接地系では変圧器１
の二次中性点を通って、（３）非接地系ではインバータ
ＩＮＶの一次側漂遊キャパシタンスを通って起きる。This sneak is (1) through the ground resistance NGR in the resistance ground system, and (2) in the transformer 1 in the direct ground system.
(3) occurs in the ungrounded system through the primary side stray capacitance of the inverter INV.
【００１２】この回り込みは、高調波電流・電圧がイン
バータＩＮＶに戻るという閉ループを構成して零相電流
を流し、この零相電流が接地抵抗ＮＧＲや漂遊キャパシ
タンスを通って電圧降下を起こし、これが地絡過電圧リ
レーＯＶＧや零相電圧リレーＶＺＲを動作させると共
に、インバータＩＮＶのフィーダの地絡過電流リレーＯ
ＣＧを動作させてしまうことがある。This sneak constitutes a closed loop in which the harmonic current / voltage returns to the inverter INV to flow a zero-phase current, and this zero-phase current causes a voltage drop through the ground resistance NGR and stray capacitance, which causes the grounding. In addition to operating the overvoltage relay OVG and the zero-phase voltage relay VZR, the ground fault overcurrent relay O of the feeder of the inverter INV is operated.
The CG may be operated.
【００１３】これらの動作は、地絡が起きていないにも
拘わらず生ずるもので、誤動作になる。These operations occur even though no ground fault has occurred, resulting in malfunctions.
【００１４】本発明の目的は、高次零相高調波成分の電
流・電圧による保護装置の誤動作を防止したインバータ
設備を提供することにある。It is an object of the present invention to provide inverter equipment which prevents malfunction of a protection device due to current / voltage of high-order zero-phase harmonic components.
【００１５】[0015]
【課題を解決するための手段】本発明は、前記課題の解
決を図るため、配電用変圧器を電源とする低圧母線を一
次側とし、二次側出力を誘導電動機等の負荷に接続ケー
ブルを介して供給するＰＷＭ形インバータ装置を複数台
備えたインバータ設備において、前記各ＰＷＭ形インバ
ータ装置は、それぞれ二次側接続ケーブルの各相のシー
スを互いに接続して接地抵抗を介して接地し、該接地抵
抗は前記二次側出力に含まれる零相高調波電流・電圧が
前記一次側の保護リレーの誤動作電流・電圧として回り
込むのを抑制する抵抗値にしたことを特徴とする。SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention uses a low-voltage bus that uses a distribution transformer as a power source as a primary side and a secondary side output as a connection cable to a load such as an induction motor. In the inverter equipment provided with a plurality of PWM type inverter devices to be supplied via each of the PWM type inverter devices, each of the PWM type inverter devices connects the sheaths of the respective phases of the secondary side connecting cables to each other and is grounded via a grounding resistance. The grounding resistance is set to a resistance value that suppresses the zero-phase harmonic current / voltage included in the secondary side output from flowing around as a malfunction current / voltage of the protection relay on the primary side.
【００１６】また、本発明は、前記各ＰＷＭ形インバー
タ別にかつ各高調波次数別に求める定電圧源から平均的
な電圧を算出し、各接続ケーブルの長さと単位容量から
平均的な容量を求め、これら平均的な電圧及び容量から
零相電流を求めて必要な前記接地抵抗の抵抗値を求める
ことを特徴とする。According to the present invention, an average voltage is calculated from a constant voltage source obtained for each PWM inverter and each harmonic order, and an average capacitance is obtained from the length of each connecting cable and a unit capacity. It is characterized in that a zero-phase current is obtained from these average voltages and capacitances to obtain a necessary resistance value of the ground resistance.
【００１７】[0017]
【作用】インバータ装置の二次側に現れる零相高調波電
流・電圧が接地系を介して一次側に回り込むのを接地抵
抗によって抑制し、一次側の保護リレーの誤動作を防止
する。The ground resistance prevents the zero-phase harmonic current / voltage appearing on the secondary side of the inverter device from sneaking into the primary side through the ground system, and prevents malfunction of the protective relay on the primary side.
【００１８】低圧母線に複数台のＰＷＭ形インバータ装
置が接続され、接地抵抗を設けたときの設備において、
インバータ装置の二次側から発生する高調波についての
等価回路は、図１に示すようになる。In equipment in which a plurality of PWM type inverter devices are connected to the low voltage busbar and a ground resistance is provided,
An equivalent circuit for harmonics generated from the secondary side of the inverter device is as shown in FIG.
【００１９】定電圧源Ｅ21〜Ｅ2MはＭ台のインバータの
二次側に現れるｎ次高調波の電圧源を示し、コンデンサ
Ｃ21〜Ｃ2Mは各インバータと負荷を接続するケーブルが
持つ静電容量を示し、コンデンサＣ11〜Ｃ1Lは各インバ
ータの二次側の静電容量以外の静電容量を示し、抵抗Ｃ
ＶＲ1〜ＣＶＲMはケーブルシースと接地間に介挿する接
地抵抗を示す。The constant voltage sources E 21 to E 2M represent voltage sources of nth harmonics appearing on the secondary side of the M inverters, and the capacitors C 21 to C 2M are static capacitors provided in a cable connecting each inverter to the load. Capacitors C 11 to C 1L represent capacitances other than the capacitance on the secondary side of each inverter, and the resistance C
VR 1 to CVR M represent ground resistances inserted between the cable sheath and the ground.
【００２０】この等価回路から各高調波次数別に零相電
流が求められ、それぞれを重畳の理に基づき加算するこ
とにより零相電流が求められる。また、ｎ次高調波電圧
が各インバータで異なるときは、二次側静電容量の重み
も加味して次のようにして平均的な電圧ＥnAVRを決定
し、このｎ次高調波電圧が全てのインバータに発生して
いることとして以下の式から算出する。From this equivalent circuit, zero-phase currents are obtained for each harmonic order, and the zero-phase currents are obtained by adding them based on the principle of superposition. Further, when the nth harmonic voltage is different in each inverter, the average voltage EnAVR is determined in the following manner by taking the weight of the secondary side capacitance into consideration. It is calculated from the following formula that it is occurring in the inverter.
【００２１】[0021]
【数１】 [Equation 1]
【００２２】接地抵抗ＣＶＲ1〜ＣＶＲMは、接続ケーブ
ルの単位長当たりに必要とする抵抗値ＣＶＲ0を求めて
おき、接続ケーブルの長さに応じて各インバータの接地
抵抗を比例演算で求めことができる。例えば、ケーブル
が２倍の長さであれば１／２の抵抗値にする。このとき
の等価回路は図２に示す回路に集約される。For the ground resistances CVR 1 to CVR M , the resistance value CVR 0 required per unit length of the connection cable is obtained in advance, and the ground resistance of each inverter is proportionally calculated according to the length of the connection cable. You can For example, if the cable is twice as long, the resistance value is halved. The equivalent circuit at this time is summarized in the circuit shown in FIG.
【００２３】[0023]
【実施例】図３は、本発明の一実施例を示す要部構成図
であり、インバータ装置３1と誘導電動機４1のケーブル
接続部を示す。DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a schematic view of a main part of an embodiment of the present invention, showing a cable connecting part between an inverter device 3 1 and an induction motor 4 1 .
【００２４】インバータ装置３1のＲ，Ｓ，Ｔ相の出力
端と誘導電動機４1の電源端は、３本の接続ケーブル１
１R，１１S，１１Tによって接続される。各ケーブル１
１R，１１S，１１TのメタルシースＭＳには編組線を芯
線とする絶縁ケーブル１２R，１２S，１２Tの一端が接
続され、この絶縁ケーブルの他端は互いに接続されて接
地抵抗１３を介して接地される。The R, S, and T phase output terminals of the inverter device 3 1 and the power source terminal of the induction motor 4 1 have three connection cables 1
Connected by 1 R , 11 S and 11 T. Each cable 1
One end of an insulated cable 12 R , 12 S , 12 T having a braided wire as a core wire is connected to the metal sheath MS of 1 R , 11 S , 11 T , and the other ends of the insulated cables are connected to each other to form a ground resistance 13 Grounded through.
【００２５】この接続構成によれば、インバータ装置３
1の出力に含まれる高調波電流・電圧は接続ケーブル１
１R，１１S，１１Tの芯線とメタルシースＭＳとの間に
存在するキャパシタンス及び接地抵抗１３を介して接地
側に流れる。According to this connection configuration, the inverter device 3
Harmonic current and voltage contained in one of the output connection cable 1
The current flows to the ground side through the capacitance existing between the core wires of 1 R , 11 S and 11 T and the metal sheath MS and the ground resistance 13.
【００２６】上記の接続構成は、他のインバータ装置３
2〜３Nと誘導電動機４2〜４Nとのケーブル接続にも同様
に施される。The above-mentioned connection configuration is applied to another inverter device 3
The same applies to the cable connection between the 2 to 3 N and the induction motor 42 to 4 N.
【００２７】接地抵抗の抵抗値は、前述のことから、ケ
ーブルの三相分静電容量の単位容量当たりの直列抵抗値
を決め、そのケーブルの静電容量が２倍になるときは単
位当たりの抵抗を２個並列に接続する。From the above, the resistance value of the ground resistance determines the series resistance value per unit capacity of the three-phase electrostatic capacity of the cable, and when the electrostatic capacity of the cable doubles, Connect two resistors in parallel.
【００２８】上記の構成により、配電系統が抵抗接地系
では、接地抵抗ＮＧＲの電圧分担を二次側接地抵抗で軽
減し、また零相電流自体も軽減することによって地絡過
電圧リレーＯＶＧと地絡過電流リレーＯＣＧの誤動作を
防止することができる。また、零相電圧計ＶＺＲの振れ
を防止することができる。With the above configuration, in the case where the power distribution system is the resistance ground system, the voltage sharing of the ground resistance NGR is reduced by the secondary side ground resistance, and the zero-phase current itself is also reduced to reduce the ground fault overvoltage relay OVG and the ground fault overcurrent. It is possible to prevent malfunction of the current relay OCG. Further, the swing of the zero-phase voltmeter VZR can be prevented.
【００２９】直接接地系では、地絡過電圧リレーＯＶＧ
の誤動作の問題は無いが、次数の高い周波数成分におい
ては零相電圧が小さくともインピーダンスが小ささによ
っては零相電流が増加し、地絡過電流リレーＯＣＧのフ
イルタ容量に問題が発生するが、接地抵抗の介挿によっ
て誤動作を防止できる。In the direct grounding system, the ground fault overvoltage relay OVG
There is no problem of malfunction of the, but in the frequency component of high order, the zero-phase current increases due to the small impedance even if the zero-phase voltage is small, and there is a problem in the filter capacity of the ground fault overcurrent relay OCG, but it is grounded. A malfunction can be prevented by inserting a resistor.
【００３０】非接地系では、インバータの一次側のケー
ブルの静電容量を経由して零相電流が流れると地絡過電
圧リレーＯＶＧの誤動作の恐れがあるが、接地抵抗の介
挿によって誤動作を防止できる。地絡過電流リレーＯＣ
Ｇについては零相電流が小さいため、あまり問題はな
い。In a non-grounded system, if a zero-phase current flows through the electrostatic capacity of the cable on the primary side of the inverter, there is a risk of malfunction of the ground fault overvoltage relay OVG, but the malfunction is prevented by inserting a ground resistance. it can. Ground fault overcurrent relay OC
Regarding G, since the zero-phase current is small, there is not much problem.
【００３１】[0031]
【発明の効果】以上のとおり、本発明によれば、各ＰＷ
Ｍ形インバータ装置は、それぞれ二次側接続ケーブルの
各相のシースを互いに接続して接地抵抗を介して接地
し、接地抵抗は二次側出力に含まれる零相高調波電流・
電圧が一次側の保護リレー側に回り込むのを抑制するよ
うにしたため、ＰＷＭ形インバータのキャリアによる高
次零相高調波によって低圧母線側になるインバータ装置
の一次側の各種リレーが誤動作するのを防止できる効果
がある。As described above, according to the present invention, each PW
The M-type inverter device connects the sheaths of the respective phases of the secondary side connecting cables to each other and grounds them via the grounding resistance, which is the zero phase harmonic current contained in the secondary side output.
Since the voltage has been suppressed from sneaking into the protection relay side on the primary side, it prevents malfunction of various relays on the primary side of the inverter device that are on the low voltage bus side due to high-order zero-phase harmonics generated by the PWM inverter carrier. There is an effect that can be done.
【００３２】また、接地抵抗は、各ＰＷＭ形インバータ
別にかつ各高調波次数別に求める定電圧源から平均的な
電圧を算出し、各接続ケーブルの長さと単位容量から平
均的な容量を求め、これら平均的な電圧及び容量から零
相電流を求めて必要な接地抵抗値を求めるため、ケーブ
ル長変更による接地抵抗の変更等を確実容易にする効果
がある。The ground resistance is calculated by calculating the average voltage from the constant voltage source obtained for each PWM inverter and for each harmonic order, and the average capacitance is obtained from the length of each connecting cable and the unit capacitance. Since the zero-phase current is obtained from the average voltage and the capacitance to obtain the required ground resistance value, there is an effect that the ground resistance can be changed easily by changing the cable length.
【図１】複数台インバータにおける高調波電流・電圧に
関係する等価回路。FIG. 1 is an equivalent circuit related to harmonic current and voltage in multiple inverters.
【図２】図１を集約した等価回路。FIG. 2 is an equivalent circuit in which FIG. 1 is integrated.
【図３】本発明の一実施例を示す要部構成図。FIG. 3 is a configuration diagram of main parts showing an embodiment of the present invention.
【図４】インバータ設備の配電系統図。FIG. 4 is a distribution system diagram of the inverter equipment.
１…配電用変圧器 ２…低圧母線 ３1、３N…ＰＷＭ形インバータ装置 ４1、４N…誘導電動機 １１R、１１S、１１T…接続ケーブル １２R、１２S、１２T…絶縁ケーブル １３…接地抵抗 ＭＳ…メタルシース ＯＣＧ…地絡過電流リレー ＯＶＧ…地絡過電圧リレー ＶＺＲ…零相電圧計1 ... distribution transformer 2 ... low bus 3 1, 3 N ... PWM-type inverter 4 1, 4 N ... induction motor 11 R, 11 S, 11 T ... connection cable 12 R, 12 S, 12 T ... insulated cable 13 ... Ground resistance MS ... Metal sheath OCG ... Ground fault overcurrent relay OVG ... Ground fault overvoltage relay VZR ... Zero-phase voltmeter
フロントページの続き (51)Int.Cl.6 識別記号 庁内整理番号 ＦＩ 技術表示箇所 Ｈ０２Ｐ 7/74 Ｇ Continuation of front page (51) Int.Cl. 6 Identification number Office reference number FI technical display area H02P 7/74 G
Claims (2)
次側とし、二次側出力を誘導電動機等の負荷に接続ケー
ブルを介して供給するＰＷＭ形インバータ装置を複数台
備えたインバータ設備において、前記各ＰＷＭ形インバ
ータ装置は、それぞれ二次側接続ケーブルの各相のシー
スを互いに接続して接地抵抗を介して接地し、該接地抵
抗は前記二次側出力に含まれる零相高調波電流・電圧が
前記一次側の保護リレーの誤動作電流・電圧として回り
込むのを抑制する抵抗値にしたことを特徴とするインバ
ータ設備。1. Inverter equipment comprising a plurality of PWM type inverter devices, each of which has a low-voltage bus, which uses a distribution transformer as a power source, as a primary side, and which supplies a secondary side output to a load such as an induction motor via a connecting cable. The PWM type inverter devices connect the sheaths of the respective phases of the secondary side connecting cables to each other and ground them via a grounding resistance, and the grounding resistance is a zero-phase harmonic current included in the secondary side output. Inverter equipment characterized in that the voltage is set to a resistance value that prevents the voltage from flowing around as a malfunctioning current / voltage of the protection relay on the primary side.
調波次数別に求める定電圧源から平均的な電圧を算出
し、各接続ケーブルの長さと単位容量から平均的な容量
を求め、これら平均的な電圧及び容量から零相電流を求
めて必要な前記接地抵抗の抵抗値を求めることを特徴と
するインバータ設備。2. An average voltage is calculated from a constant voltage source obtained for each PWM inverter and for each harmonic order, and an average capacitance is obtained from the length of each connecting cable and a unit capacitance. Inverter equipment, wherein a zero-phase current is obtained from a voltage and a capacitance to obtain a necessary resistance value of the ground resistance.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP5335629A JPH07194132A (en) | 1993-12-28 | 1993-12-28 | Inverter facility |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP5335629A JPH07194132A (en) | 1993-12-28 | 1993-12-28 | Inverter facility |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JPH07194132A true JPH07194132A (en) | 1995-07-28 |
Family
ID=18290735
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP5335629A Pending JPH07194132A (en) | 1993-12-28 | 1993-12-28 | Inverter facility |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPH07194132A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2006353075A (en) * | 2005-05-16 | 2006-12-28 | Honda Motor Co Ltd | Control circuit device |
| JP2010130704A (en) * | 2008-11-25 | 2010-06-10 | Mitsubishi Electric Corp | Startup device for generator |
| JP2012108080A (en) * | 2010-11-19 | 2012-06-07 | Hasegawa Denki Kogyo Kk | Ground fault detector for ungrounded ac circuit |
-
1993
- 1993-12-28 JP JP5335629A patent/JPH07194132A/en active Pending
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2006353075A (en) * | 2005-05-16 | 2006-12-28 | Honda Motor Co Ltd | Control circuit device |
| JP4680042B2 (en) * | 2005-05-16 | 2011-05-11 | 本田技研工業株式会社 | Control circuit device |
| JP2010130704A (en) * | 2008-11-25 | 2010-06-10 | Mitsubishi Electric Corp | Startup device for generator |
| JP2012108080A (en) * | 2010-11-19 | 2012-06-07 | Hasegawa Denki Kogyo Kk | Ground fault detector for ungrounded ac circuit |
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