JP5275069B2 - DC / AC circuit ground fault detection method for non-grounded AC circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC/AC circuit ground fault detecting method for a non-grounding AC circuit, for easy reduction of cost in the entire ground fault accident protection system of an AC circuit and a DC circuit in an AC/DC power distribution system. <P>SOLUTION: The ground fault accident of a DC circuit 30 connected, through a rectifier 20, to a non-grounding AC circuit 10 on the secondary side of a main transformer 1, is detected through a change in a DC potential of a grounding resistor 50 for limiting a DC ground fault current which is arranged at an AC ground fault sensor 40 on the AC circuit 10 side. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a DC / AC circuit ground fault detection method in a non-grounded AC circuit in which a DC circuit is connected to a non-grounded AC circuit on the secondary side of a main transformer such as a factory transformer via a rectifier.

  The AC / DC distribution system that transforms the three-phase AC of the primary distribution line with the main transformer of the power consumer, rectifies it with the three-phase rectifier, and supplies the DC load with the ungrounded AC circuit on the secondary side of the main transformer. Prepare. The non-grounded AC circuit is a non-grounded three-phase AC circuit and a high-resistance grounded three-phase AC circuit according to the non-grounded three-phase AC circuit. A DC load is installed in a DC circuit connected to the non-grounded AC circuit via a three-phase rectifier. In such an AC / DC power distribution system, the AC circuit side is set to a non-grounded system, so that the fault current of the AC side ground fault is suppressed to be small. In addition, an AC ground fault sensor for detecting an AC ground fault is normally installed on the AC circuit side, and this AC ground fault sensor is grounded in a state where a high resistance is equivalently inserted. Further, a DC ground fault sensor (for example, see Patent Document 1) that detects a DC ground fault is disposed on the DC circuit side as necessary.

  In the AC / DC distribution system shown in FIG. 5, an AC ground fault sensor 40 is installed in the non-grounded AC circuit 10 on the secondary side of the main transformer 1, and a DC ground fault sensor 70 is installed in the DC circuit 30. The circuit breaker 11 is inserted in each of the three-phase three-wires of the AC circuit 10, and each of the three-phase three-wires has a ground capacitance Co. A DC circuit 30 is connected to the AC circuit 10 via a diode bridge three-phase rectifier 20, and a DC load 33 is connected to a positive terminal 31 and a negative terminal 32 of the DC circuit 30.

  The AC ground fault sensor 40 generally has a mode using a zero phase transformer (EVT) shown in FIG. 5 and a mode using a zero phase voltage divider (ZPD) (not shown). The AC ground fault sensor 40 of FIG. 5 includes a zero-phase transformer (EVT) 41 connected to the secondary-side three-phase three-wire of the main transformer 1 and a zero-phase flowing through the neutral point ground line 42 of the zero-phase transformer 41. Although a zero-phase current transformer (ZCT) 43 for detecting current is provided, either one may be provided. The primary side of the zero-phase transformer 41 is connected to the secondary side of the main transformer via the EVT protection fuse F. An AC ground fault relay 46 is connected to the tertiary side of the zero-phase transformer 41 via an AC current limiting resistor 45. When a ground fault occurs at any one terminal of the AC circuit 10, a zero-phase current Io flows through the neutral ground line 42 of the zero-phase transformer 41, and this is connected to the zero-phase current transformer 43. The relay 44 detects. When detecting the zero-phase current Io, the ground fault relay 44 displays an AC ground fault alarm or the like, and opens the circuit breaker 11 to protect the AC circuit 10 as necessary. Alternatively, the ground fault relay 46 detects the zero-phase voltage Vo and protects the AC circuit 10. The alternating current limiting resistor 45 of the ground fault relay 46 suppresses the zero-phase current Io flowing through the zero-phase transformer 41 and prevents the zero-phase transformer 41 from being overloaded.

  The DC circuit 30 includes a smoothing reactor 34 and a smoothing capacitor 35 that smooth the DC voltage rectified by the three-phase rectifier 20. A DC ground fault sensor 70 is installed in parallel with the smoothing capacitor 35. The DC ground fault sensor 70 has a ground fault detector for detecting a DC ground fault current Idg grounded on the positive side or the negative side of the DC circuit 30, and an alarm based on a ground fault current detection signal of the ground fault detector. A drive controller for generating and controlling the DC circuit to an open circuit as necessary is provided. When the DC ground fault sensor 70 detects the DC ground fault current Idg and controls the DC circuit 30 to be an open circuit, the DC ground fault current disappears, the DC circuit 30 is protected from its own ground fault, and the AC circuit Ripple to 10 is prevented.

JP 2005-137095 A

  Each of the AC ground fault sensor on the AC circuit side and the DC ground fault sensor on the DC circuit side installed in the AC / DC power distribution system as shown in FIG. 5 has an excellent function of protecting the corresponding AC circuit and DC circuit from a ground fault. . Each of these ground fault sensors is large and expensive. In particular, the DC ground fault sensor is expensive, making the entire ground fault protection system of the AC / DC power distribution system expensive. In a distribution system in which a plurality of DC circuits are connected to a common AC circuit, it is difficult to install an expensive DC ground fault sensor in each of the plurality of DC circuits.

  In addition, the DC ground fault sensor of the DC circuit functions so that only an alarm is issued when a DC ground fault is detected, power supply from the AC circuit to the DC circuit is continued, and a power failure of the DC circuit is avoided. There is. Further, among a plurality of DC circuits connected to a common AC circuit, there is a DC circuit in which no DC ground fault sensor is provided. When a DC ground fault occurs in a DC circuit having a DC ground fault sensor like the former or a DC circuit not equipped with a DC ground fault sensor like the latter, a DC ground fault current flows to the AC circuit side, and May cause trouble.

  For example, as shown in the DC circuit 30 of the distribution system in FIG. 5, when a DC ground fault occurs on the negative terminal 32 side, the terminal voltage of the smoothing capacitor 35 becomes the battery voltage, and the three-phase three-wire distribution system of the AC circuit 10 A DC ground fault current Idg flows in the direction of the broken line arrow in FIG. 5 via the neutral point ground of the zero-phase transformer 41. Further, when a DC ground fault occurs on the positive electrode terminal 31 side, a DC ground fault current Idg flows in the direction opposite to the dashed arrow in FIG. When such a DC ground fault current Idg flows through the zero-phase transformer 41, the zero-phase transformer 41 is DC-biased and the excitation current becomes excessive, causing the protective fuse F to blow. If no fuse is installed, the continuous flow of excessive excitation current and DC ground fault current Idg may lead to zero-phase transformer burning. For this reason, it is necessary to take measures for preventing the zero-phase transformer from being burned out due to the DC ground fault current on the AC ground fault sensor side, the AC ground fault sensor becomes more expensive, and the entire ground fault accident protection system becomes even more expensive. There was a bug.

  An object of the present invention is to provide a DC / AC circuit ground fault detection method in an ungrounded AC circuit that facilitates cost reduction of the entire ground fault protection system.

  The method of the present invention to achieve the above object is to provide a DC ground fault current arranged in an AC ground fault sensor on the AC circuit side in the event of a ground fault in a DC circuit connected to a non-grounded AC circuit on the secondary side of the main transformer. It is detected by a change in DC potential of the limiting grounding resistor.

  Here, the ungrounded AC circuit includes a non-grounded three-phase AC circuit having an AC ground fault sensor having a zero-phase transformer (EVT) and a zero-phase current transformer (ZCT), and a zero-phase voltage divider. The present invention can also be applied to a high resistance grounding circuit having a (ZPD) AC ground fault sensor. The grounding resistor for limiting the DC ground fault current disposed in the AC circuit has a resistance value that does not break the non-grounding system for AC in the AC circuit, and one end is connected to the AC circuit side and the other end is grounded. When a ground fault occurs on the DC circuit side, the DC ground fault current Idg flows to the DC ground fault current limiting ground resistor through the ground, and the terminal voltage of the ground resistor is displaced. By detecting and monitoring this voltage displacement, the occurrence of a ground fault on the DC circuit side can be detected, and a protective measure against the occurrence of a DC ground fault can be quickly implemented. By detecting and monitoring the DC ground fault on the DC circuit side on the AC circuit side in this way, it is possible to spread a DC ground fault accident to the AC circuit side without installing an expensive DC ground fault sensor on the DC circuit side. It is easy to reduce the cost of both AC and DC ground fault protection systems.

In the method of the present invention, the grounding resistor for limiting the DC ground fault current is neutralized by being inserted into the primary side neutral point ground line of the zero-phase transformer for AC ground fault detection arranged as an AC ground fault sensor in the AC circuit. characterized by a point resistance.

  The existing equipment similar to the zero-phase transformer 41 shown in FIG. 5 can be applied to the zero-phase transformer for AC ground fault detection here. When this existing zero-phase transformer 41 is applied, a neutral point resistor is connected in series as a DC ground fault current limiting ground resistor to the existing neutral point ground line 42. Such a neutral point resistor can be small and inexpensive, and the cost of both the AC and DC ground fault protection systems can be further reduced. Further, the neutral point resistance suppresses the bias of the zero-phase transformer due to the DC ground fault current Idg, and prevents the fuse blown on the zero-phase transformer side or the transformer body from burning. Further, the neutral point resistance can be used also as an AC ground fault sensor for detecting an AC ground fault current on the AC circuit side, thereby simplifying the entire AC ground fault sensor on the AC circuit side.

In the present invention, the DC blocking capacitor connected in series to the neutral point resistor, the series connection state of the neutral point resistor and the capacitor, and the changeover switch for disconnecting the capacitor from the neutral point resistor and releasing the series connection state a structure having, characterized in that switching the neutral point resistor and capacitor in series connection state changeover switch based on ground fault detection signal of the direct current circuit from the DC potential change of the neutral point resistor.

  The change-over switch here is an automatic transition switch that closes the neutral point resistance side until a ground fault occurs on the DC circuit side. The neutral point resistance is connected to the neutral point ground line on the zero-phase transformer side. Only the series connection is maintained, and the function of detecting the DC ground fault by the neutral point resistance is maintained. When the occurrence of a ground fault in the DC circuit is detected due to the voltage displacement of the neutral point resistance, the changeover switch is automatically switched by this detection signal, and the state where the neutral point resistor and the capacitor are connected in series is maintained. At this time, the DC ground fault current Idg is blocked by the capacitor and does not flow into the zero-phase transformer, and the zero-phase transformer is protected from the demagnetization due to the DC ground fault current Idg. And since an alternating current supplies with electricity, a capacitor | condenser is continued without impairing the AC ground fault detection function by the side of a zero phase transformer.

  In the present invention, the magnitude of the neutral point resistance can be set to a resistance value that does not cause saturation of the zero-phase transformer of the AC ground fault sensor by the DC ground fault current Idg. By doing so, it is possible to suppress excessive excitation current of the zero-phase transformer at the time of occurrence of a DC ground fault, and it is possible to prevent troubles such as blown fuses and transformer burnout on the zero-phase transformer side.

  Furthermore, in the present invention, the DC ground fault current limiting grounding resistor is connected to one terminal on the secondary side of the main transformer in which an AC ground fault detecting zero-phase voltage divider is provided as an AC ground fault sensor in the AC circuit. The connected high resistance for DC detection can be obtained. The high resistance for DC detection here is such that the AC circuit is a high resistance grounding circuit, prevents excessive biasing of the main transformer, and does not disturb the impedance balance of the zero-phase voltage divider (for example, about 1 MΩ) ) And is connected to one terminal of the secondary side three-phase three-wire of the main transformer. As the zero-phase voltage divider, an existing one that detects an AC ground fault occurring on the AC circuit side based on the three-phase zero-phase voltage can be applied.

  In the present invention, the occurrence of a ground fault in the DC circuit is detected by monitoring the change in the DC potential of the ground resistor for limiting the DC ground fault current and the sign of the change direction. It is possible to determine a ground fault pole as to whether a DC ground fault has occurred on the positive side or the negative side of the circuit.

  According to the method of the present invention, since the occurrence of the DC ground fault on the DC circuit side is detected by the AC ground fault sensor on the AC circuit side, the direct current can be detected even if an expensive DC ground fault sensor is not provided on the DC circuit side. The occurrence of a ground fault can be detected, and the excellent effect of facilitating cost reduction of both AC and DC ground fault protection systems can be achieved. In addition, by inserting a capacitor in the zero-phase transformer neutral point grounding wire in the event of a DC ground fault, the DC ground fault can be prevented from spreading to the AC circuit side, and even in the event of some DC circuit faults, the AC circuit The operation of the AC ground fault sensor on the side can be continued, and the supply reliability of the AC and DC systems is improved.

It is a circuit diagram of a power distribution system showing an embodiment of the method of the present invention. It is a circuit diagram which shows other embodiment. It is a circuit diagram which shows other embodiment. It is a circuit diagram which shows other embodiment. It is a circuit diagram of a power distribution system for demonstrating the conventional DC / AC circuit ground fault detection method.

The present invention will be described below with reference to the embodiments shown in FIGS.
[Embodiment 1]

  FIG. 1 is applied to the AC / DC power distribution system of FIG. 5, and the same or corresponding parts as in FIG. The difference between FIG. 1 and FIG. 5 is that the DC ground fault sensor is omitted from the DC circuit 30 and the neutral point ground line of the zero-phase transformer 41 provided as the AC ground fault sensor 40 in the non-grounded AC circuit 10. 42, a grounding resistor 50 for limiting a DC ground fault current is additionally inserted. A grounding resistor 50 shown in FIG. 1 is a neutral point resistor 51 that grounds the neutral point of the zero-phase transformer 41. In FIG. 1 and FIG. 2 to be described later, the DC ground fault current limiting grounding resistor 50 is referred to as a neutral point resistor 51.

  In the method of the present invention, the DC ground fault occurrence state on the DC circuit 30 side is detected based on the displacement of the neutral point DC voltage detected by the neutral point DC potential detector 52 connected in parallel to the neutral point resistor 51. Monitor. This monitoring is specifically performed as follows.

In the state of FIG. 1, when a DC ground fault occurs on the negative electrode side of the DC circuit 30, a DC ground fault current Idg flows through the ground as in the case of FIG. This DC ground fault current Idg flows through the neutral point resistor 51 in the negative direction from the ground side, and the neutral point DC potential detector 52 detects the neutral point DC potential Vng. By determining whether the neutral point DC potential Vng is positive or negative, it can be determined that a DC ground fault has occurred on either the positive side or the negative side of the DC circuit 30. Further, when the ground fault resistance when the DC ground fault current Idg flows is Rdg, the resistance value of the neutral point resistor 51 is Rng, and the rectifier DC voltage of the DC circuit 30 is Vdc, the ground fault resistance Rdg is expressed by the following equation (1). I want.
Rdg = Rng [(Vdc / 2) −Vng] / Vng (1)

  From the detection results over many items such as detection of a DC ground fault by detecting the DC ground fault current Idg, determination of a DC ground fault occurrence accident pole, and measurement of the ground fault resistance Rdg, a multi-function on the DC circuit side is obtained. A ground fault protection system can be constructed, and an expensive dedicated DC ground fault sensor can be omitted from the DC circuit 30 side.

The neutral point resistor 51 acts as a DC limiting resistor that suppresses the DC ground fault current Idg flowing into the zero-phase transformer 41 side. As a result, the DC bias of the iron core on the zero-phase transformer 41 side is suppressed, the excessive excitation current is suppressed, and the occurrence of blowout of the protective fuse F and the occurrence of burnout of the zero-phase transformer 41 can be prevented.
[Embodiment 2]

  Next, the embodiment of FIG. 2 will be described. FIG. 2 shows a modification of a part of the circuit shown in FIG. 1. A direct current blocking capacitor 53 is connected in series to a neutral point resistor 51, and the neutral point resistor 51 and the capacitor 53 are connected in series and the neutral point resistor 51. A changeover switch 54 is added to release the series connection state by disconnecting the capacitor 53. The changeover switch 54 indicated by the solid line in FIG. 2 maintains a state where the neutral point resistor 51 is connected in series to the neutral point ground line 42 and the capacitor 53 is disconnected from the neutral point resistor 51. In this state, the DC ground fault monitoring operation on the DC circuit 30 side described above is performed. In this DC ground fault monitoring operation, when the occurrence of a ground fault in the DC circuit 30 is detected by the voltage displacement of the neutral point resistor 51, the changeover switch 54 is connected to the capacitor 53 side as shown by the chain line in FIG. The neutral point resistor 51 and the capacitor 53 are connected in series.

When the changeover switch 54 is automatically switched to the capacitor 53 side based on the detection signal of the occurrence of the ground fault, the DC ground fault current Idg is blocked by the capacitor 53 and does not flow into the zero phase transformer 41, and the zero phase transformer. The 41 side is protected from the demagnetization due to the DC ground fault current Idg. Further, since the capacitor 53 blocks direct current but energizes alternating current, the AC ground fault detection function on the zero-phase transformer 41 side as the AC ground fault sensor on the AC circuit 10 side continues.
[Embodiment 3]

  1 and 2 show an embodiment in which a zero-phase transformer 41 is used as the AC ground fault sensor 40 on the AC circuit 10 side. FIG. 3 shows an embodiment in which a zero-phase voltage divider (ZPD) 61 is used as the AC ground fault sensor 40. In the case of FIG. 3, an AC ground fault detection zero-phase voltage divider 61 is arranged in the AC circuit 10 as the AC ground fault sensor 40, and a DC ground fault current limiting ground is connected to one terminal on the secondary side of the main transformer 1. A DC detection high resistance 55 is inserted as the resistance 50. By monitoring the change or positive / negative of the DC voltage detected by the DC potential detector 56 connected in parallel to the DC detection high resistance 55, the occurrence of a DC ground fault in the DC circuit 30 is detected as in FIG. A ground fault resistance Rdg is calculated.

  The high resistance 55 for direct current detection uses the alternating current circuit 10 as a high resistance ground circuit, suppresses excessive biasing of the main transformer 1 and does not disturb the impedance balance of the zero-phase voltage divider 61, for example, about 1 MΩ. It is a resistor having a high resistance value. The DC detection high resistance 55 may be one in which a DC ground fault current detection voltage dividing terminal is arranged in the DC detection high resistance in addition to a single high resistance value. As the zero-phase voltage divider 61 shown in FIG. 3, an existing one that detects an AC ground fault occurring on the AC circuit 10 side based on the three-phase zero-phase voltage can be applied.

  For example, the zero-phase voltage divider 61 includes capacitors C1 to C3 having one end connected to each of the three phases of the AC circuit 10, and a common zero-phase current detecting capacitor C4 connected to the other ends of the capacitors C1 to C3. An insulation transformer 62 connected in parallel to the capacitor C4 is provided. The zero-phase voltage detection capacitor C4 is grounded. When no AC ground fault has occurred in the AC circuit 10, the output voltage of the isolation transformer 62 is zero. When an AC ground fault occurs at any one of the three-phase three-wire terminals of the AC circuit 10, a voltage is supplied from any of the capacitors C1 to C3 to the zero-phase current detection capacitor C4, and the secondary side of the insulation transformer 62 The voltage rises and the occurrence of an AC ground fault is detected.

In the case of the AC / DC power distribution system of FIG. 3, when a ground fault occurs on the DC circuit 30 side, the DC ground fault current Idg is largely current-limited by the DC detection high resistance 55 and flows. The occurrence of a DC ground fault is detected by the potential change of the resistor 55. Even after the occurrence of the DC ground fault is detected, the DC ground fault current Idg flows while the current is greatly limited by the DC detection high resistance 55. This limited current does not cause biasing on the main transformer side.
[Embodiment 4]

  In order to reliably prevent the DC ground fault current Idg from flowing into the main transformer 1, for example, an open / close switch 57 is provided in series with the DC detection high resistance 55 as shown in FIG. The open / close switch 57 is opened. By doing so, the function of the AC ground fault sensor 40 can be continued without using the DC blocking capacitor as shown in FIG.

  In addition, the form which implements the method of this invention is not restricted to FIGS. 1-4, Of course, a various change can be added in the range which does not deviate from the summary of this invention.

DESCRIPTION OF SYMBOLS 1 Main transformer 10 AC circuit 20 Three-phase rectifier 30 DC circuit 33 DC load 40 AC ground fault sensor 41 Zero phase transformer for AC ground fault detection 42 Neutral point ground wire 43 Zero phase current transformer 51 Neutral point resistance 52 Neutral point DC potential detector 53 DC blocking capacitor 54 Changeover switch 55 High resistance for DC detection 56 DC potential detector 57 Open / close switch 61 Zero phase voltage divider for AC ground fault detection
Idg DC ground fault current

Claims (1)

A ground fault in a DC circuit connected to a non-grounded AC circuit on the secondary side of the main transformer is caused by a change in the DC potential of a ground resistor for limiting the DC ground fault current disposed in the AC ground fault sensor on the AC circuit side. detected,
The grounding resistance is a neutral point resistance inserted in a primary side neutral point grounding wire for an AC ground fault detection zero-phase transformer arranged as an AC ground fault sensor in the AC circuit,
A DC blocking capacitor connected in series to the neutral point resistor, a series connection state of the neutral point resistor and the capacitor, and a changeover switch that disconnects the capacitor from the neutral point resistor and releases the series connection state. ,
A non-grounded AC circuit , wherein the neutral point resistor and the capacitor are switched to a series connection state by the changeover switch based on a ground fault detection signal of the DC circuit due to a DC potential change of the neutral point resistor. DC / AC circuit ground fault detection method.

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