JPS60174956A - Ac phase voltage detection circuit - Google Patents

Abstract

PURPOSE:To enable the detection of a phase voltage free from effect of any zero phase current by connecting dummy resistors star-connected to a three- phase AC circuit. CONSTITUTION:Dummy resistors 21-23 star-connected are connected to a three- phase AC circuit 3 while a variable resistor 24 connected to a control power source of an inverter is connected to the neutral point P of the dummy resistors. In addition, the phase voltage detection resistors 31-33 are star-connected and then, connected to the circuit 3. As a higher harmonic is generated with the operation of the inverter connected to the circuit 3, a zero phase current flows into the control power source via the resistors 21-23 and 24 and the potential at the point P varies with respect to the zero volt as reference potential of the control power source. But the potential at the neutral point P of the resistors 31-33 will not be affected by the zero phase current. Therefore, when a voltage divided properly is taken out from the resistors 31-33, phase voltages in respective phases can be obtained as potential from the zero volt as reference potential. The voltage thus divided is utilized as a phase voltage signal for the control of the inverter.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] This invention relates to a three-phase AC circuit whose secondary winding is delta-connected and which is connected to a three-phase AC circuit fed from the secondary side of a three-phase transformer. The present invention relates to an AC phase voltage detection circuit that obtains a phase voltage that is not affected by zero-sequence current in order to control an inverter.

[Prior art and its problems]

A power supply side converter made of switching semiconductor elements is connected to a three-phase AC power supply to convert AC power into DC power, and this DC power is converted to AC power of a desired voltage and frequency by a motor side converter also made of switching semiconductor elements. The so-called vVV, which can be converted to variable speed operation of an induction motor,
Inverters are often used, but when an inverter is used to slow down or stop an induction motor, it converts the kinetic energy held by the motor into electrical energy and returns it to the AC power source. You can also let them drive.

To operate the inverter as described above, it is necessary to detect the phase voltage of the AC circuit in order to detect the synchronous phase, but when detecting this phase voltage, it is necessary to ensure that it is not affected by the zero-sequence current. Must be.

FIG. 1 is a circuit diagram showing a conventional example of a phase voltage detection circuit.

In FIG. 4, a three-phase transformer 2 whose primary and secondary sides are connected in delta is connected to an AC power supply 1 consisting of a 3-phase, an S-phase, and a T-phase. e17) Transformer 2 Then, the voltage is transformed to the required voltage, and a three-phase AC circuit 3 consisting of U phase, ■ phase, and W phase is obtained. Then, phase voltage detection resistors 11, 12, and 13 are connected to each of the U phase, V phase, and W phase, and the other ends of each phase voltage detection resistor 11, 12, and 13 are connected in common to form a star connection. , this neutral point is connected to a variable resistor 14 connected to an arbitrary power source for control. If a voltage is extracted from each of the phase voltage detection resistors 11 , 12 , and 13 by appropriate voltage division, this is the phase voltage of the three-phase AC circuit 3 .

However, in the circuit shown in FIG. 1, if a zero-sequence current flows through a wire, for example due to harmonics generated with the operation of an inverter, the star-connected phase voltage detection resistor 11
, 12, and 13 are affected by point Y, which is the neutral point. When returning power from the DC side to the AC power supply, the power supply side converter controls the on/off of each switching element based on the magnitude relationship of the phase voltage of each phase of the three-phase AC power supply. In order to be able to continue the conversion operation without any problems even if the phase voltage fluctuates due to the influence of the zero-sequence current, various countermeasures are required, so the disadvantage is that the control device circuit becomes expensive with multiple lines. There is.

[Purpose of the invention]

This invention provides an AC system that can detect AC phase voltage for controlling an inverter whose power source is a three-phase transformer whose secondary winding is delta-connected, without being affected by zero-phase current. The purpose is to provide a phase voltage detection circuit.

[Key points of the invention]

In this invention, a star-connected dummy resistor is connected to a three-phase AC circuit whose power source is a three-phase transformer whose secondary winding is delta-connected, and is connected to the control power source of an inverter. While connecting the variable resistor to the neutral point of this dummy resistor, connect the phase voltage detection resistor, which is also star-wired, to the three-phase AC circuit, and obtain the appropriately divided phase voltage from this phase voltage detection resistor. By taking it out, it is intended to cut off the influence of zero-sequence current.

[Embodiments of the invention]

FIG. 2 is a circuit diagram showing an embodiment of the present invention.

In FIG. 2, a three-phase transformer 2 whose primary winding and secondary winding are both delta-connected is connected to an AC power supply 1 consisting of R-8-T phases. This three-phase transformer 2 of 2
On the next side, there is a three-phase AC circuit 3 having a tr-v-w phase, which supplies AC power to an inverter (not shown) made of switching semiconductor elements. For example, an electric motor is connected to this inverter, and by performing regenerative braking operation on this electric motor, the kinetic energy held by the electric motor is converted into electric energy and returned to this three-phase AC circuit 3. If so, each switching semiconductor element constituting the inverter must repeatedly turn on and off as appropriate in response to the magnitude relationship of each phase voltage of the three-phase AC circuit 3 Three-phase AC circuit 3 based on the reference potential of Sudame's control power supply
It is necessary to obtain the phase voltage signal of

In the present invention, star-connected dummy resistors 21 , 22 , 23 are connected to the three-phase AC circuit 3 , while a variable resistor 24 connected to the control power supply of the inverter is connected to the neutral point of the dummy resistors 21 , 22 , 23 . Connect to a certain P point.

Furthermore, phase voltage detection resistors 31, 32, and 33 are star-connected and similarly connected to the three-phase variable wave circuit 3. The neutral point of the phase voltage detection resistors 31, 32, and 33 at this time is defined as point Q.

When harmonics are generated due to the operation of the inverter connected to the three-phase AC circuit, dummy resistors 21, 22 . , 23 and the variable resistor 24, the zero-phase current flows into the control power supply, so the potential at point P, which is the neutral point, fluctuates with respect to zero-phase K, which is the reference potential of the control power supply. However, phase voltage detection resistors 31, 32, 3
The potential at point Q, which is the neutral point of No. 3, is not affected by the zero-sequence current. Therefore, the phase voltage detection resistors 31, 32,
33, the phase voltage of each phase is obtained as a potential from zero volts, which is the reference potential mentioned above, so this is used as a phase voltage signal to control the inverter. can do.

〔Effect of the invention〕

According to this invention, when a three-phase transformer whose secondary winding is delta-connected is used as an AC power source for a thyristor,
Connect a star-wired dummy resistor to the phase alternating current circuit, and connect the neutral point of the dummy resistor to the variable resistor connected to the control power supply, while connecting the star-wired phase voltage detection resistor to the same By connecting to a three-phase AC circuit and outputting an appropriately divided voltage, a phase voltage signal that can be used to control an inverter without being affected by zero-sequence current can be obtained. In this way, the influence of zero-sequence current can be removed only by the combination of resistors, so the circuit can be simplified and the increase in cost can be kept very small.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional example of phase voltage detection.Figure 2 is a circuit diagram showing a conventional example of phase voltage detection.
The figure is a circuit diagram showing an embodiment of the present invention.Furt...AC power supply, 2...3-phase transformer, 3...3-phase AC circuit, 11
, 12, 13 - phase voltage detection resistor, 14... variable resistor, 21, 22, 23 dummy resistor, 24...
Variable resistance, 31. 32, 33・Phase voltage detection resistor. v w tusk I same υ VW T 2 fig.

Claims (1)

[Claims] In a circuit in which an inverter consisting of a switching element is connected to a three-phase AC circuit whose power source is the secondary side of a three-phase transformer whose secondary winding is delta-connected, and the phase voltage for controlling the inverter is detected. , a variable resistor having a star-connected dummy resistor connected to the three-phase AC circuit, both ends of which are connected to the control power source of the inverter, and an intermediate terminal of which is connected to the neutral point of the dummy resistor; An AC phase voltage detection circuit comprising: a phase voltage detection resistor connected to an AC circuit in a star connection and further having an intermediate terminal for appropriately dividing each phase voltage of the three-phase AC circuit.