JP2017175795A - Power conversion device and ground fault detection method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power conversion device and a ground fault detection method therefor, having improved ground fault detection accuracy in a low voltage region.SOLUTION: The power conversion device includes: a DC power supply 1 which supplies DC power; an inverter 3 which converts an output voltage of the DC power supply 1 into AC to drive an AC motor 4; high resistance resistors 2A, 2B which high-resistance grounds the output end of the DC power supply 1; grounding detection means 7 which detects currents flowing through the high-resistance resistors 2A, 2B, to determine to be the ground fault of the device when the magnitude of the current is a predetermined threshold or larger; and a main control unit 9 which controls the output of the inverter 3. When the output voltage of the inverter 3 is a predetermined threshold or lower, the main control unit 9 adds a predetermined DC bias to the output voltage reference of each phase of the inverter 3.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power conversion device and a ground fault detection method thereof.

  In order to detect the ground fault of the power converter, normally, for example, the DC link portion is grounded with high resistance, and when the ground current exceeds a predetermined threshold, it is considered as a ground fault. In this case, the level and sensitivity of ground fault detection are important, but when the output frequency of the power converter changes greatly, the impedance of the route through which the zero-phase current flows due to the influence of stray capacitance in the high frequency region, For this reason, since the detection sensitivity is lowered, it has been reported that this should be corrected (for example, see Patent Document 1).

Japanese Utility Model Publication No. 63-182634 (Overall)

  The technique disclosed in Patent Document 1 takes into consideration the influence of zero-phase charging current that occurs regardless of the presence or absence of a ground fault in a high frequency region in order to make the ground fault current detection sensitivity constant in all frequency regions. It tries to detect. However, when trying to detect a low-impedance ground fault, the influence of frequency is not so much, and it is actually difficult to detect a ground fault in a low voltage region.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a power converter and a ground fault detection method thereof that improve the ground fault detection accuracy in a low voltage region.

In order to achieve the above object, a power converter according to the present invention includes a DC power supply for supplying DC power,
An inverter that drives the AC motor by converting the output voltage of the DC power supply to AC, a high resistance that grounds the output terminal of the DC power supply with high resistance, and a current that flows through the high resistance are detected. A ground detecting means for determining a ground fault of the apparatus when the threshold is equal to or greater than a predetermined threshold; and a main controller for controlling the output of the inverter, wherein the main controller has an output voltage of the inverter equal to or lower than a predetermined threshold In this case, a predetermined DC bias is applied to the output voltage reference of each phase of the inverter.

  According to the present invention, it is possible to provide a power conversion device and a ground fault detection method thereof with improved ground fault detection accuracy in a low voltage region.

The circuit block diagram of the power converter device which concerns on one Example of this invention. Operation | movement explanatory drawing of the bias generator of the power converter device of the power converter device which concerns on one Example of this invention. The simulation result (the 1) of the power converter device which concerns on one Example of this invention. The simulation result (the 2) of the power converter device which concerns on one Example of this invention.

  Hereinafter, a power converter according to an embodiment of the present invention will be described with reference to FIGS. 1 to 4.

  FIG. 1 is a circuit configuration diagram of a power converter according to the present invention. DC power is fed from the DC power source 1 to the inverter 3 and converted into a desired AC voltage to drive the AC motor 4. A series circuit of high resistances 2A and 2B is connected in parallel with the DC power source 1, and the middle point of the series circuit is grounded. The ground current, that is, the sum of the currents flowing through the high resistances 2A and 2B is detected by the current detector 6, and the detection signal is given to the ground detection circuit 7. When the RMS value of the ground current detected by the current detector 6 exceeds a predetermined threshold value, the ground detection circuit 7 performs a protection / alarm operation such as stopping the operation of the apparatus or issuing an alarm.

  The rotational position of the AC motor 4 is detected by the rotation angle detector 5, the input current of the AC motor 4 is detected by the current detector 8, and the detected amount is fed back to the main control unit 9 for controlling the inverter 3. Is given as a quantity. Hereinafter, the internal configuration of the main control unit 9 will be described.

  The phase angle detected by the rotation angle detector 5 is converted into the rotation speed ωr by the differentiator 91, and compared with the given speed reference ωr * and the electric motor controller 92 which is the main controller, and is amplified by the current reference. It becomes. This current reference, together with a separately provided magnetic flux reference, is amplified and compared with the two-axis converted output of the current detector 8 inside the motor controller 92, respectively, and d- and q-axis voltage references ED_R and EQ_R. Get. The phase reference θ1 based on the phase angle detected by the rotation angle detector 5 is used for these two-axis conversions. The d-axis voltage reference ED_R and the q-axis voltage reference EQ_R are converted by the voltage reference converter 93 into the three-phase basic voltage references EU_R, EV_R, and EW_R, which are AC quantities, using the phase reference θ1 again. The output of the bias generator 95 is added to each of the three-phase basic voltage references EU_R, EV_R, and EW_R by an adder 94, resulting in output voltage references VU_REF, VV_REF, and VW_REF. These three-phase voltage references VU_REF, VV_REF, and VW_REF are compared with a predetermined PWM carrier by the PWM controller 96, and a gate signal for each switching element is generated.

  FIG. 2 is an explanatory diagram showing the operation of the bias generator 95. The horizontal axis represents the rotational speed of the AC motor 4, and the vertical axis represents the magnitude of the voltage. In FIG. 2, a straight line going up to the right through the zero point represents the voltage level of the original voltage reference, that is, the three-phase basic voltage references EU_R, EV_R, and EW_R. Here, the magnitude of the voltage may be assumed to be, for example, an RMS value based on an AC three-phase basic voltage for one phase. A straight line (broken line) in which the voltage VT is 0 at the rotation speed 0 and the voltage 0 is the rotation speed ωT is the zero-phase bias Vbias that is the output of the bias generator 95. As will be described later, this Vbias may be a positive or negative DC voltage or an AC voltage, but here it is a positive DC voltage.

  In this way, the sizes of the three-phase voltage references VU_REF, VV_REF, and VW_REF are as shown by the thick solid lines in FIG. However, it should be noted that the constant value from the speed 0 to the speed ωT simply indicates a value obtained by adding the zero-phase bias Vbias to the voltage magnitude based on the three-phase basic voltage reference.

  3 and 4 show simulation results of the effect of improving the ground fault detection accuracy in the low voltage region in the above configuration. The simulation condition is that when the output voltage of the DC power supply 1 is 600 V and the motor speed ωr is 5% of the rated value, one phase of the three-phase output buses of the inverter 3 is grounded at an impedance of 1Ω at time 0. The detection current I_GR_FLT (mA) of the ground detection circuit 7 when the zero-phase bias Vbias is 0 (pu) as shown in the upper part of FIG. 3A is shown in the lower part of FIG. Further, as shown in the upper part of FIG. 3B, the detection current I_GR_FLT (mA) of the ground detection circuit 7 when the zero-phase bias Vbias is 0.7 (pu) is shown in the lower part of FIG. In the conventional method without the zero-phase bias in FIG. 3A, I_GR_FLT has a detection value of about 0.8 mA and the detection amount is very small, but when the positive DC bias in FIG. 3B is applied. Since a zero-phase voltage is applied, the detection value is about 13 mA, which is 15 times or more, and it can be seen that the sensitivity of ground fault detection increases.

  Note that VT in FIG. 2 is preferably set to several tens of percent of the rated voltage. If this is 40%, VT will be about 240V. Further, when the rotation speed or the output voltage is increased in the state of no bias shown in FIG. 3A, the detection current I_GR_FLT increases substantially in proportion to the increase. This is considered because the zero-phase component included in the output of the inverter 3 is proportional to the output voltage. If VT in FIG. 2 is 40% of the rated voltage, the detected current I_GR_FLT at the rotational speed ωT (output voltage VT) is about 8 mA, which is about 8 times that in FIG. 3A, and can be sufficiently detected without erroneous detection.

  FIG. 4 shows simulation results when the zero-phase bias Vbias is alternating current. FIG. 4A shows I_GR_FLT when the zero-phase bias Vbias is a sine wave having a peak value of 0.7 (pu) and a frequency of 10 Hz as shown in the upper part. Similarly, FIG. 4B shows I_GR_FLT in the lower stage when the zero-phase bias Vbias is a rectangular wave having a peak value of 0.7 (pu) and a frequency of 10 Hz as shown in the upper stage. In either case, the detection current includes a zero-phase bias ripple, but about 7 mA for a sine wave and about 11 mA for a rectangular wave, a sufficiently detectable value is obtained. 3 and 4, a delay in the detected current is observed at time 0. This delay corresponds to a delay in the calculation time of the RMS calculator inside the ground detection circuit 7.

  As shown in FIG. 2, according to this embodiment, since a bias is applied only at a low voltage or low speed, the apparatus can be operated more safely without applying unnecessary stress to the apparatus. It is. On the other hand, if the bias is applied even in the high speed range, for example, the shaft current flowing through the bearing of the AC motor 4 may increase, which may affect the life of the bearing.

  Although the embodiments of the present invention have been described above, they are presented as examples and are not intended to limit the scope of the invention. The novel embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  For example, the DC power source 1 in FIG. 1 is normally a rectifier or converter that uses a commercial AC power source as an input, but may be any device that supplies DC power.

  In FIG. 1, the inverter 3 is a two-level inverter, but it may be a multi-level inverter having three or more levels.

  1 performs two-axis control or vector control. However, control other than vector control, for example, V / f constant control may be used.

  Further, the rotation detector 5 in FIG. 1 may be replaced with a speed detector, and these detectors may be omitted. In this case, the rotational speed is indirectly estimated by so-called sensorless control.

  Further, although the ground detection circuit 7 has been described as a detection circuit that performs RMS calculation, it may be a circuit that detects the magnitude of current, for example, an absolute value detection circuit.

  In FIG. 2, the zero-phase bias Vbias is shown as a straight line that monotonously decreases with an increase in speed. However, any bias that increases the sensitivity of ground fault detection in a low speed range may be used. A constant bias may be applied up to the speed ωT.

DESCRIPTION OF SYMBOLS 1 DC power supply 2A, 2B High resistance 3 Inverter 4 AC motor 5 Rotation detector 6 Current detector 7 Ground detection circuit 8 Current detector 9 Main control part 91 Differentiator 92 Motor controller 93 Voltage reference converter 94 Adder 95 Bias Generator 96 PWM controller

Claims (8)

A DC power supply for supplying DC power;
An inverter that drives the AC motor by converting the output voltage of the DC power source into AC;
A high resistance for grounding the output end of the DC power supply with a high resistance;
A ground detection means for detecting a current flowing through the high resistance and determining a ground fault of the device when the magnitude of the current is equal to or greater than a predetermined threshold;
A main control unit for controlling the output of the inverter;
The main control unit
When the output voltage of the said inverter is below a predetermined threshold value, the predetermined DC bias is added to the output voltage reference | standard of each phase of the said inverter, The power converter device characterized by the above-mentioned. A DC power supply for supplying DC power;
An inverter that drives the AC motor by converting the output voltage of the DC power source into AC;
A high resistance for grounding the output end of the DC power supply with a high resistance;
A ground detection means for detecting a current flowing through the high resistance and determining a ground fault of the device when the magnitude of the current is equal to or greater than a predetermined threshold;
Speed detecting means for directly or indirectly detecting the rotational speed of the AC motor;
A main control unit for controlling the output of the inverter;
The main control unit
A power conversion apparatus, wherein a predetermined DC bias is applied to an output voltage reference for each phase of the inverter when the rotational speed detected by the speed detection means is equal to or less than a predetermined threshold.   The predetermined DC bias is a maximum value when the output voltage of the inverter is 0, decreases as the output voltage increases, and becomes 0 when the output voltage reaches a predetermined threshold value. The power conversion device according to claim 1.   The predetermined DC bias has a maximum value when the rotation speed of the inverter is 0, decreases as the rotation speed increases, and becomes 0 when the rotation speed reaches a predetermined threshold value. The power conversion device according to claim 2.   The power converter according to any one of claims 1 to 4, wherein an AC bias is added instead of the DC bias. A DC power supply for supplying DC power;
An inverter that drives the AC motor by converting the output voltage of the DC power source into AC;
A high resistance for grounding the output end of the DC power supply with a high resistance;
A current detector for detecting the current flowing through the high resistance;
A main control unit for controlling an output of the inverter, wherein when the output voltage of the inverter is equal to or lower than a predetermined threshold, a predetermined DC bias is applied to an output voltage reference of each phase of the inverter. In the power conversion device composed of
A ground fault detection method for a power converter, wherein the ground fault of the device is determined when the magnitude of the detected current of the current detector is equal to or greater than a predetermined threshold. A DC power supply for supplying DC power;
An inverter that drives the AC motor by converting the output voltage of the DC power source into AC;
A high resistance for grounding the output end of the DC power supply with a high resistance;
A current detector for detecting the current flowing through the high resistance;
Speed detecting means for directly or indirectly detecting the rotational speed of the AC motor;
A main control unit for controlling an output of the inverter, wherein a predetermined DC bias is applied to an output voltage reference of each phase of the inverter when the rotational speed detected by the speed detection means is equal to or less than a predetermined threshold value; In the power conversion device configured with the main control unit,
A ground fault detection method for a power converter, wherein the ground fault of the device is determined when the magnitude of the detected current of the current detector is equal to or greater than a predetermined threshold.   The ground fault detection method for a power converter according to claim 6 or 7, wherein an AC bias is applied instead of the DC bias.