JPS5828812B2 - Leakage current suppression device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a type of alternating current power supply having a safety grounding conductor in addition to the power transmission line, and more particularly to a circuit which minimizes the current flow in the grounding conductor. be.

In recent years, there has been increasing concern about excessive leakage currents flowing in the ground conductors of electrical equipment.

Both utilities and customers have complained that the large reactive currents generated by line filter capacitors associated with calculators and other electrical equipment often mask the presence of actual leakage currents. There is.

In such cases, leakage current detectors installed in the consumer's power equipment may operate incorrectly or become inoperable in the event of an accident requiring maintenance or repair. be.

Strict power regulations limit leakage currents in the ground conductors of electrical devices to about 1 milliampere.

However, based on the consensus among engineers that such a low limit is not practical in the computer field, the guidelines state that the leakage current of the ground wire (green wire) in computer systems other than small computers is 3.5. It is stipulated that it is less than 1000mIJ amperes.

Until now, research has been carried out with the goal of reducing the leakage current of the grounding wire to 1 milliampere or less, but no technology has been developed to achieve this goal within economically acceptable limits. is the current situation.

The basic concept of the present invention is based on the fact that leakage currents in ground wires in computers and other electrical devices are approximately constant and predictable, and are based on the fact that leakage currents in ground wires in computers and other electrical devices are approximately constant and predictable. By adding a circuit that causes the ground wire to generate a current having

Prior art that may be helpful in understanding the present invention is US Pat. No. 1,485,361, US Pat.
No. 728, No. 3473091, No. 3946738, etc. (disclosed here).

U.S. Pat. No. 1,485,361 shows an AC power transformer circuit that suppresses line fault currents without overloading the power line during normal operation.

U.S. Pat. No. 1,537,371 teaches inserting an inductor in the neutral ground wire of an AC power supply to provide a lagging current to compensate to some extent for leading currents due to large capacitances between the line and earth. It shows.

U.S. Pat. No. 2,791,728 shows a simple construction safety grounding arrangement for medical appliances having an autotransformer-resistor capacitor network to suppress power line hum from recording equipment. ing.

US Pat. No. 3,473,091 shows a basic ground fault detector suitable for use in conjunction with a circuit according to the present invention.

An autotransformer is used, the secondary part of which is connected to the ground wire and the primary part connected to the energized wire.

With such a configuration, even if the ground wire is grounded at both ends, a fault occurring on one side of the autotransformer will unbalance the differential transformer and cause the protective relay to open.

U.S. Pat. No. 3,946,738 shows a surgical appliance that applies high frequency current to a patient, with a small leakage current to ground via stray capacitance.

A compensation capacitor and a high frequency power source are connected to prevent this leakage current.

As noted above, these U.S. patents show different circuits for different purposes, but none of them are related to ground fault detection systems, so it is not surprising that such systems do not use AC power supplies. It does not provide a solution to the problem of excessive current flowing through the grounding wire, causing malfunctions.

The present invention provides a multiwire AC system in which an independent grounding wire is provided between the machine frame and the earth, and a considerable current flows in the grounding wire due to the capacitive reactance between the ungrounded wire and the grounded wire. In a power supply circuit, the purpose is to create a current that counteracts an excessive current during normal operation by providing capacitors with approximately the same capacitance and momentarily applying voltages in opposite directions.

The desired polarity reversal operation is performed by a transformer, in particular an autotransformer.

The principles of the invention are also applicable to cases where excessive currents occur due to inductive reactance.

In this case, an inductor is used instead of a capacitor.

Embodiments of the present invention will be described in detail with reference to these drawings, but before doing so, some general points will be made.

Generally, electronic devices such as computers are equipped with electromagnetic and radio frequency interference (EMI/RFI) filters on power lines to suppress propagating nozzles in both directions.

Such filters can be as simple as just a feedthrough capacitor, or as complex as a multistage inductance-capacitance (LC) network, which connects the power line to the grounded machine frame. They have the common feature that a capacitor is connected between them.

If a feedthrough capacitor is used alone, a capacitance on the order of 0.1 to 0.5 microfarads is typically required to provide sufficient attenuation.

As a result, a relatively large reactive current flows.

By the way, many low pass filters are designed to allow reactive currents of less than 3.5 microIJ amperes to flow through the ground wire.

These limitations limit the usable line-to-ground capacitance in single phase 120 volt circuits to approximately 0.05 microfarads.

Conventional common core inductor filters require large line capacitors to eliminate differential mode noise.

In some types of power configurations, such as grounded delta three-phase power systems, several line-to-line components function between the line and the grounded machine frame.

As a result, filters that are strictly designed to carry small leakage currents may have more than acceptable levels of electrode flow.

FIG. 1 shows the reactive current flow for a typical single-phase power circuit and its line-to-ground (frame) capacitor that is grounded on the -power side.

The secondary winding 12 of the distribution transformer 10 has a tap connected to a ground point 14 by the electric utility.

The frame 16 of the customer's machine is connected to another ground point 1 by a safety ground wire 20, commonly referred to as a green wire.
8 is connected.

Such a connection is made on the power transmission line side of a main circuit breaker provided at the entrance to the consumer's power equipment.

Load 22 is resistive.

As shown, all the reactive current Ia related to the capacitor 24 flows to the ground line 20.

Note that it is assumed that the reactive current flows clockwise at this point.

FIG. 2 shows the power circuit of FIG. 1 in which an autotransformer 26 and a capacitor 28 are provided in place of the capacitor 24.

Due to the action of the autotransformer 26, the charging current Ib for the capacitor 28 flows counterclockwise through the grounding wire 20.

The current Ib, which has a phase difference of 180 degrees from the current Ia, is connected to the grounding wire 20.
is determined so as to cancel out the current Ia at .

FIG. 3 shows a combination of the configuration of FIG. 1 and the configuration of FIG. 2 in accordance with the present invention.

A composite (difference) current Ic of currents Ia and Ib flows through the grounding line 20.

As can be seen from the figure, the ground wire of the distribution transformer 10 and the ground wire 20 serve as a common path for the currents Ia and Ib.

FIG. 4 shows the resulting flow of reactive current Id in the configuration of FIG.

This current Id is a vector sum of the original reactive current and a compensation reactive current that is generated by the action of the autotransformer 26 with a phase difference of 181 degrees from the original reactive current.

If the magnitude of the original reactive current and the compensation reactive current are equal, the combined ground line current Ic becomes zero.

In fact, the reactive current path common to the two types of currents Ia and Ib is shifted from the safety ground line 20 to the ground power line.

The components required to achieve the objective according to the invention are an autotransformer with a 1:1 transformation ratio for instantaneous polarity reversal and an ungrounded side of the load for determining the instantaneous value of the current. There are only two capacitors with capacitances equal to the overall line-to-ground capacitance of .

Note that instead of such a capacitor, a non-matching capacitor may be used and the voltage may be adjusted by adjusting the transformation ratio.

The principles of the invention described in connection with a single-phase circuit can also be applied to a three-phase constant force circuit by simply adding an autotransformer and a compensation capacitor.

This will be explained below.

Note that the diagrams we will refer to from now on also contain many arrows indicating the flow of current, but these arrows do not indicate the direction of current at the same point in time, as in the case of a single-phase circuit. Please be careful.

FIG. 5 shows a widely used neutral-grounded Y-type three-phase circuit.

The reactive current Ih flowing through the safety grounding wire is the current Ie, I
It is the vector sum of f and Ig.

However, since there is a mutual phase difference of 120 degrees as shown in Figure 6a, the vector sum of three equal currents becomes zero as shown in Figure 6b.

When there is excessive leakage current (actually neutral current) in a neutral grounded Y-type three-phase circuit, the currents in the three phases become unbalanced.

In order to suppress the reactive current in the ground line, a line-to-ground capacitor may be added to one or two phases to equalize the currents in the three phases.

Next, in a grounded delta (or corner grounded) type circuit, only two of the three line-to-ground capacitors are included in the AC circuit, as is commonly observed.

FIG. 7 shows the reactive current in a delta type circuit.

As in the Y-type circuit, the ground conductor current I7 is the vector sum of the reactive currents Ij and Ik.

However, unlike the Y-type circuit, only the reactive currents of two phases are involved.

Therefore, the vector sum does not become zero as shown in Figure 8b.

Note that FIG. 8a shows the phase difference between the two phase currents.

As can be seen by following the current path, in the positive half cycle the current Ik flows into the ground wire in the opposite direction to the arrow of the current Ih.

In such a state, the direction of Ik in FIG. 8b is reversed.

In order to generate a compensation current having the same magnitude Ij+Ik as the current Il and having a different phase, a compensation circuit consisting of a transformer 72 and a capacitor 74, and a compensation circuit consisting of a transformer 76 and a capacitor 78 are required, as shown in FIG. It is necessary to connect the compensation circuit consisting of the following to the two wires on the non-grounded side.

In a balanced state with this configuration, the combined reactive current does not flow through the grounding wire 20, but instead flows through the grounded phase wire.

When performing partial compensation, a portion of the reactive current flows through the ground wire 20 as shown in FIG.

When using a transformer with a fixed transformation ratio,
In order to adjust the overall compensation capacitance to minimize the ground wire current, it is necessary to connect several small capacitors in parallel.

An effective technique for making such capacitance adjustments is to use a capacitor with a capacitance approximately equal to the line-to-ground capacitance of the circuit, and to adjust the applied voltage by selecting the appropriate winding taps of the transformer. It is something.

A circuit configuration according to such a technique is shown in FIG.

For various applications, a second winding to the center tap to produce a voltage of 480 volts and a fourth winding of 480 volts.
2, and a second winding for producing a voltage of 8 volts.

By connecting the first winding and the second winding in series, the compensation voltage can be adjusted in 2% increments up to ±14%.

However, 12% is excluded. To make an adjustment of 12 fO, it is necessary to add another tap to the second winding to make an adjustment of 2 fO.

Such a configuration allows voltage adjustment to accommodate most capacitance tolerances.

When adjusting the compensation current, the current may reach an apparent minimum value due to overcompensation in one phase and undercompensation in the other phase.

This apparent minimum value is, of course, different from the true minimum value obtained with proper compensation.

A true minimum value is obtained only when the amplitudes of the out-of-phase compensation current components in the two phases are equal to the amplitudes of their corresponding in-mode reactive current components.

A current minimum is achieved by repeating the process of first increasing and then decreasing the compensation capacitance associated with the second phase while leaving the first phase slightly undercompensated. Ru.

It may be convenient to use a pair of variable capacitors for quick adjustment and then replace them with a fixed capacitor of corresponding capacitance.

The ground current that exists after the compensating reactive current is properly adjusted is considered to be the true leakage current, ie, the common mode current in the resistive leakage current path in the circuit.

If it is desired to compensate for such common mode currents, it is necessary to add a potentiometer.

However, in order to perform permanent compensation, it is necessary to fully consider the stability of the leakage current mechanism.

FIG. 10 shows a configuration in which a potentiometer is added to a single phase power circuit for complete compensation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a single-phase power circuit and its line-to-top reactive current path, FIG. 2 is a diagram showing a configuration in which a leakage current compensation circuit is added to the single-phase power circuit, and FIG. 3 is a diagram showing a configuration in which a leakage current compensation circuit is added to the single-phase power circuit. A diagram showing a combination of the configuration in FIG. 1 and the configuration in FIG. 2 together with a current path, FIG. 4 is a diagram showing a reactive current path obtained as a result of the configuration in FIG. 3, and FIG. Diagram showing the flow of reactive current in a grounded Y-type three-phase power circuit, No. 6a
Figures 6 and 6b are diagrams showing the mutual relationship of reactive current vectors in the Y-type three-phase power circuit of Figure 5, Figure 7 is a diagram showing the flow of reactive current in the grounded delta type three-phase power circuit, and Figure 8a is a diagram showing the flow of reactive current in the grounded delta type three-phase power circuit. and FIG. 8b shows the interrelationship of reactive current vectors in the delta three-phase power circuit of FIG. 7, and FIG. 9 shows the flow of reactive current in a grounded delta three-phase power system fully compensated according to the invention. FIG. 10 is a diagram showing a configuration in which a circuit for compensating for both reactive current components and common-mode leakage current components is added to a single-phase circuit according to the present invention. 10...Distribution transformer, 20...Grounding wire, 24...Capacitor, 26...Auto transformer, 28... capacitor.

Claims (1)

[Scope of Claims] 1. A power source connected to an electrical load by a plurality of conductors, and a given flux actance connected between a first conductor of the plurality of conductors and a machine frame. an electrical element, another conducting wire connected between the second conducting wire of the plurality of conducting wires and the grounding point, and a grounding wire connected between the machine frame and the grounding point. a power supply circuit having a reactance substantially equal to a given reactance of the electrical element, and having first and second terminals, the first terminal being connected to the machine frame. a reactor; a phase inversion circuit having a terminal connected to the first conducting wire, a terminal connected to the second conducting wire, and a terminal connected to the second terminal of the reactor; , a leakage current suppression device that suppresses leakage current flowing through the grounding wire.