Partial discharge measurement device

Abstract

To provide a partial discharge measurement device that can measure a partial discharge without an affection of noise caused by an inverter even when the fundamental frequency is a high frequency.SOLUTION: One end of a capacitor VC is connected to a measurement target. The capacitor VC has a capacitance of 40 pF or less, and a cutoff frequency of 80 MHz or more. One end of a detection impedance Z is connected to the other end of the capacitor VC, and the other end of the impedance is connected to a ground. One end of a low-pass filter LPF is connected to a connection point between the capacitor VC and the detection impedance Z. Each one end of high-pass filters HPF1 and HPF2 is connected to the connection point between the capacitor VC and the detection impedance Z. A measurement unit (oscilloscope 4; PC5 for waveform processing) is connected to the other end of the low-pass filter LPF and the other ends of the high-pass filters HPF1 and HPF2 to measure a partial discharge.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a partial discharge measuring device that is used for diagnosing insulation degradation and detects partial discharges generated from high-voltage equipment (for example, rotating machines, inverters, static equipment).

Background Art

[0002] Conventional partial discharge measuring devices have mainly been used for diagnosing the insulation degradation of high-voltage equipment to which an alternating voltage of commercial frequency (50 Hz or 60 Hz) is applied. However, in recent years, due to high-efficiency operation, there has been an increasing number of device configurations that use inverters to adjust the frequency and voltage.

[0003] For example, a method has been conceived in which a high voltage of commercial frequency of 3 kV or more is converted to a frequency exceeding the commercial frequency (60 Hz to several hundred Hz) via an inverter to operate an electric motor. At this time, noise is generated due to the switching of the power semiconductors inside the inverter and their peripheral circuits, making it difficult to capture partial discharges.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] (Patent Document 1) In order to capture partial discharge generated from an inverter-driven rotating machine, it has a structure combining a capacitor and a detection resistor, and the detected PD (partial discharge) signal is measured by a measuring instrument via a high-pass filter to measure PD.

[0007] This Patent Document 1 has the following problems. ·As far as FIG. 5 is concerned, it is not clear that the fundamental frequency is 36 Hz and it can be measured at frequencies higher than the commercial frequencies (50 Hz, 60 Hz). ·The frequency band of the capacitor for capturing the PD signal is not clear, and the effect of noise removal is not clear. ·A high-pass filter is provided, but its frequency band is not shown and the effect of noise removal is not clear. ·As specified in

[0040] , since noise removal uses a high-frequency antenna, the number of components as a measuring device and the calculation become complicated.

[0008] (Patent Document 2) It has a structure that selects only the partial discharge signal generated by the partial discharge in the motor by removing the switching noise from the inverter device by a high-pass filter connected to the power supply cable connected to the power supply terminal of the motor for each phase.

[0009] This Patent Document 2 has the following problems. ·As a high-pass filter, it is specified that it is preferable to use one with a low-cutoff frequency in the range of 100 MHz to 500 MHz, but the frequency band of the capacitor for capturing the PD signal is not specified, and it is unclear whether the PD signal can really be captured. · The fundamental frequency of the inverter is not specified, and it is not clear that it can be measured at frequencies higher than the commercial power frequencies (50 Hz, 60 Hz).

[0010] [Problem 1: Frequency Band of Capacitor] As a general interpretation, the frequency band of the capacitor is as shown in Table 1. Cp in Table 1 represents capacitance (pF), Z represents impedance (MΩ), L represents inductance (kH), and R1 represents resistance (MΩ).

[0011] Assuming the input impedance on the measuring instrument side is 50 Ω, the capacitance of the capacitor (Comparative Example 1) often used in the insulation diagnosis of high-voltage equipment is 1210 pF, and the cut-off frequency is about 3 MHz. The capacitance of the capacitor (Comparative Example 2) often used for insulation diagnosis in large generators, etc., is 80 pF, and the cut-off frequency is about 40 MHz. The PD signal of the frequency components above the cut-off frequency will be greatly attenuated. Here, the following capacitors were used as comparison targets. Comparative Example 1: Coupling capacitor (Model PDC12-1000) from Sparks Instruments Comparative Example 2: Epoxy-mica capacitor from Iris Power

[0012]

Table 1

[0013] On the other hand, the frequency band of the noise caused by the switching of a general inverter is assumed to be 40 MHz or less (Non-Patent Document 1). Since the capacitors of Comparative Example 1 and Comparative Example 2 overlap with the frequency band of the noise, it becomes difficult to detect signals other than very large PD signals.

[0014] Fig. 1 shows an example of PD measurement observed when a high voltage (3.81 kV) was applied to the rotating machine coil using the capacitor of Comparative Example 2. The measuring instrument used is the PORTABLE PARTIAL DISCHARGE ANALYZER (Model TMS-6141) manufactured by SPARKS. Fig. 1(a) shows the case without noise from the inverter, and Fig. 1(b) shows the case with noise from the inverter. The horizontal axis represents one cycle of the voltage replaced by the angle (°), and the vertical axis represents the PD signal intensity replaced by the discharge charge amount (pC or nC). As shown in Fig. 1(b), PD signals of about 2 nC or less are completely buried in the noise, and it is impossible to distinguish between PD and noise.

[0015] [Problem 2: Impedance of the capacitor] As shown in Table 1, the impedances of the capacitors when a voltage is applied at a frequency of 1 kHz are Comparative Example 1: 0.13 MΩ and Comparative Example 2: 2 MΩ. In particular, in Comparative Example 1, the impedance of the capacitor is too small to be used for high-voltage equipment applications with a frequency of 1 kHz and a voltage of 3 kV or more.

[0016] In the field of rotating machines, in addition to the commercial frequencies of 50 Hz and 60 Hz, there are rotating machines driven by inverters at 400 Hz. Furthermore, by adjusting the number of poles of the rotor, assuming that the frequency can be adjusted to 400×2 = 800 Hz and a frequency variation of 20% is allowed, it may also operate at frequencies up to 960 Hz.

[0017] From the above, it becomes an issue to provide a partial discharge measuring device that is not affected by the noise generated from the inverter and can measure partial discharge even when the fundamental frequency is high.

Means for solving the problem

[0018] The present invention has been devised in view of the above-described conventional problems. One aspect thereof is a partial discharge measuring device that is connected to a measurement target and measures partial discharge, including a capacitor having a capacitance of 40 pF or less and a cut-off frequency of 80 MHz or more, one end of which is connected to the measurement target; a detection impedance having one end connected to the other end of the capacitor and the other end grounded; a low-pass filter having one end connected to the connection point of the capacitor and the detection impedance; a high-pass filter having one end connected to the connection point of the capacitor and the detection impedance; and a measurement unit connected to the other end of the low-pass filter and the other end of the high-pass filter for measuring the partial discharge.

[0019] Also, as one aspect, the capacitor is a vacuum capacitor.

[0020] Also, as one aspect, the high-pass filter includes two high-pass filters, a first high-pass filter and a second high-pass filter. The cut-off frequency of the first high-pass filter is 9 kHz or more, and the cut-off frequency of the second high-pass filter is 50 MHz or more.

[0021] Also, as one aspect, the cut-off frequency of the low-pass filter is 100 kHz or less.

Effects of the Invention

[0022] According to the present invention, it is possible to provide a partial discharge measuring device that can measure partial discharge without being affected by noise generated from an inverter, even when the fundamental frequency is high.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0024] Hereinafter, Examples 1 to 3 of the partial discharge measurement device in the present invention will be described in detail with reference to FIGS. 2 to 4.

[0025] The configuration of the partial discharge measurement device that solves [Problem 1: Frequency band of the capacitor] and [Problem 2: Impedance of the capacitor] is shown in FIG. 2.

[0026] An alternating current power supply (high-frequency high-voltage power supply) 1 is connected to a partial discharge measurement device 3 via a stator coil 2. The stator coil 2 is a component of the rotating machine and is the measurement object of the partial discharge measurement device 3. In FIG. 2, the stator coil 2 of the rotating machine is shown as the measurement object, but it can be used for all high-voltage devices in general. Other examples include, for example, inverters and static devices.

[0027] The partial discharge measurement device 3 includes a capacitor (for example, a vacuum capacitor: hereinafter referred to as a vacuum capacitor) VC, a detection impedance Z, a low-pass filter LPC, a first high-pass filter HPF1, a second high-pass filter HPF2, an oscilloscope 4, and a waveform processing PC (electronic computer) 5. The oscilloscope 4 and the PC 5 form a measurement unit.

[0028] One end of the vacuum capacitor VC is connected to the stator coil 2. One end of the detection impedance Z is connected to the other end of the vacuum capacitor VC. The other end of the detection impedance Z is grounded.

[0029] At the connection point of the vacuum capacitor VC and the detection impedance Z, one end of the low-pass filter LPF, one end of the first high-pass filter HPF1, and one end of the second high-pass filter HPF2 are connected. The other end of the low-pass filter LPF, the other end of the first high-pass filter HPF1, and the other end of the second high-pass filter HPF2 are connected to the oscilloscope 4. A PC5 is connected to the oscilloscope 4. Hereinafter, each device and each component will be described.

[0030] (1) Vacuum capacitor VC Generally, the frequency band of the PD signal generated from high-voltage equipment is several MHz to several GHz. Among these PD signals, in order to capture the components with frequencies above the noise frequency band (about 40 MHz or less), a vacuum capacitor with a capacitance (about 40 pF or less) is selected such that the cut-off frequency is twice or more the noise frequency band, specifically 80 MHz or more. This is because the definition of the cut-off frequency is the frequency at which the signal magnitude attenuates by 3 dB (1 / 2 as the power quantity), and the frequency components of the cut-off frequency pass through slightly.

[0031] The vacuum capacitor VC has characteristics such as a vacuum structure inside, high insulation, and extremely small dielectric loss. By reducing the capacitance of the vacuum capacitor VC, as shown in the electrical characteristics of Example 1, Example 2, and Example 3 in Table 1, the impedance of the vacuum capacitor VC can be increased. The capacitance and cut-off frequency of the vacuum capacitor are as follows in Examples 1 to 3.

[0032] Example 1: Capacitance 24.06 pF, cut-off frequency 132 MHz Example 2: Capacitance 20.37 pF, cut-off frequency 156 MHz Example 3: Capacitance 10.51 pF, cut-off frequency 303 MHz.

[0033] By raising the cut-off frequency to 80 MHz or more, PD signals above the noise frequency are captured without attenuation.

[0034] (2) Detection impedance Z As described in Patent Document 3 or Patent Document 4, select the resistance value so that the ratio R1 / R2 of the resistance component (R1) of the vacuum capacitor VC to the resistance component (R2) of the detection impedance Z is in the range of 999 to 4999 when the fundamental frequency is 1 kHz. Specifically, it was as shown in Table 2.

[0035]

Table 2

[0036] An overvoltage protection diode or a varistor is provided in the detection impedance Z. In Examples 1, 2, and 3, the overvoltage protection diode is a diode that does not exceed the voltage (25V) described in the "Low-Voltage Circuit Grounding Protection Guidelines" of the Japan Electrical Association, and when it exceeds 6.4V, current starts to flow and does not exceed 11.3V even when flowing up to the allowable current value (53A) (Name: TVS Diode Manufacturer: ON Semiconductor Model: P6KE7V5CA).

[0037] (3) Low-pass filter (LPF) To pass an AC signal with a fundamental frequency of 50 Hz to 1 kHz, a low-pass filter having a cut-off frequency of 100 kHz or less is used. In Examples 1, 2, and 3, the manufacturer is Atosense Co., Ltd., the model number is BN-LF050K, and the cut-off frequency is 50 kHz.

[0038] (4) First and second high-pass filters HPF1 and HPF2 Two high-pass filters, a first high-pass filter HPF1 and a second high-pass filter HPF2, are installed.

[0039] First high-pass filter HPF1: To eliminate the attenuation of the output pulse signal of the calibrator that determines the relationship between the magnitude of the PD signal and the discharge charge amount, a high-pass filter with a cut-off frequency in the range of several kHz to several tens of kHz is used. Generally, alternating current contains harmonics that affect PD signal processing, and it is necessary to remove at least the third harmonic (3 kHz when the frequency is 1 kHz) and the fifth harmonic (5 kHz when the frequency is 1 kHz). Therefore, the cut-off frequency of the first high-pass filter HPF1 is preferably 9 kHz or higher. In Examples 1, 2, and 3, a high-pass filter with a manufacturer: THORLABS, model: EF121, and a cut-off frequency of 9 kHz was used.

[0040] Second high-pass filter HPF2: To remove noise, a high-pass filter with a cut-off frequency of 50 MHz or higher is used. In Examples 1, 2, and 3, a high-pass filter with a manufacturer: CRYSTEK, model: CHPFL-0050, and a cut-off frequency of 55 MHz was used.

[0041] (5) Oscilloscope 4 For the oscilloscope 4, it is advisable to use one with the following specifications, for example.

[0042] 1. Sampling rate: 2.5 GS / s (giga-samples per second) = 2.5×10 9 samples / second (data interval: 0.4 nanoseconds).

[0043] 2. Frequency band Generally, the required frequency band of the oscilloscope should be at least 5 times the highest frequency component contained in the measurement signal. Specifically, to capture a signal with a frequency band of 80 MHz, an oscilloscope with a frequency band of 450 MHz or higher per channel is used.

[0044] 3. Resolution With an 8-bit resolution, when the range is 1 V, it is 1 / 2 8 = 3.9 mV. Assuming the calibration signal is 1 V (1000 mV) = 1000 pC, the resolution of the measured charge amount is 3.9 pC.

[0045] 4. Input Impedance Low-pass filter LPF: Since the low-frequency voltage cannot be captured if it is smaller than the resistance component of the detection impedance Z, the input impedance shall be 1 MΩ.

[0046] First and second high-pass filters HPF1, HPF2: To allow high-frequency components to pass through, the input impedance shall be 50 Ω.

[0047] (6) PC5 for waveform processing PC5 for waveform processing performs the following processing as an example.

[0048] (i) Correct the phase shift of the low-frequency voltage waveform measured through the low-pass filter LPF. Since the phase of the voltage signal output through the power supply 1 and the low-pass filter LPF is shifted due to the capacitance of the vacuum capacitor VC and the phase delay of the low-pass filter LPF, this shift amount is corrected.

[0049] (ii) Integrate the PD signals measured through the first and second high-pass filters HPF1, HPF2. In order to obtain the PD pattern, the maximum discharge charge amount, and the frequency, integration is performed every unit time, and it is graphed and digitized for display. Specifically, the processing shown in FIG. 3 of Patent Document 5 is performed.

[0050] [Function and Operation] When the vacuum capacitor VC and the detection impedance Z are connected as shown in FIG. 2 and an AC voltage of 1 kHz and 6900 Vrms is applied, Table 2 shows the result of calculating the power consumed by the vacuum capacitor VC.

[0051] The power consumption of the vacuum capacitor VC (Example 1, Example 2, Example 3) is very small, being 1 / 3 or less compared to the capacitor of Comparative Example 2. The power consumption of the capacitor of Comparative Example 1 is as large as 362 W and it cannot be used under the condition of a frequency of 1 kHz.

[0052] Furthermore, a low-pass filter LPF with a cut-off frequency of 50 kHz, a first high-pass filter HPF1 with a cut-off frequency of 9 kHz, a second high-pass filter HPF2 with a cut-off frequency of 55 MHz, and an oscilloscope 4 (manufacturer: Tektronix, model: DPO7254) were connected, and FIG. 3 shows a comparison of partial discharge measurement results when 6900 V was applied to the high-voltage rotating machine stator coil. FIG. 3 shows the measurement results in a state where there is no noise generated from the inverter. In Examples 1, 2, and 3 and Comparative Example 2, PDs are captured from the outputs of the first and second high-pass filters HPF1 and HPF2.

[0053] The partial discharge measurement devices of Examples 1 to 3 exhibit the most effectiveness in an environment where the inverter is operating and a large amount of noise is generated due to switching.

[0054] FIG. 4 shows the results of measuring PDs in the configuration of the partial discharge measurement device of FIG. 1 using the capacitors of Examples 1, 2, and 3 and Comparative Example 2 in a state where noise is generated from the inverter as shown in FIG. 1(b).

[0055] In Example 1 and Comparative Example 2, it is captured that PDs occur at the output from the first high-pass filter HPF1. However, in Examples 2 and 3, they are completely buried in noise and it is impossible to determine whether PDs are occurring. On the other hand, at the output from the second high-pass filter HPF2 with a cut-off frequency of 55 MHz, PDs are captured without being affected by noise in all of Examples 1, 2, and 3. In Comparative Example 2, PDs are also captured, but the output signal intensity from the second high-pass filter HPF2 is slightly small.

[0056] As described above, the effects of the partial discharge measurement devices of Examples 1 to 3 are as follows compared to the devices using the capacitors of the conventional Comparative Examples 1 and 2.

[0057] By reducing the capacitance of the vacuum capacitor VC to be small (40 pF or less), the impedance increases, the leakage current when a high voltage is applied decreases, and the power consumption decreases. Therefore, partial discharge can be measured even when the fundamental frequency is high (for example, up to 1 kHz).

[0058] By using a vacuum capacitor VC with a frequency band of 80 MHz or higher (capacitance 40 pF or less), PD in a frequency band with little influence of noise (frequency band 40 MHz or less) can be captured.

[0059] Also, by using a second high-pass filter HPF2 with a cut-off frequency of 50 MHz or higher, PD in a frequency band with little influence of noise (frequency band 40 MHz or less) can be measured.

[0060] Note that when the noise from the inverter is very small or non-existent, the PD output from the first high-pass filter HPF1 with a cut-off frequency of several kHz to several tens of kHz may be measured.

[0061] As described above, in the present invention, although only the specific examples described have been explained in detail, it is obvious to those skilled in the art that various modifications and corrections are possible within the scope of the technical idea of the present invention, and it is natural that such modifications and corrections belong to the scope of the claims.

Explanation of symbols

[0062] VC... Vacuum capacitor (capacitor) Z... Detection impedance LPF... Low-pass filter HPF1, HPF2... First high-pass filter, second high-pass filter 1... Power supply (high-frequency high-voltage power supply) 2... Stator coil (measurement target) 3... Partial discharge measurement device 4... Oscilloscope 5... PC (electronic computer)

Claims

1. A partial discharge measuring device connected to a measurement target and measuring partial discharge, comprising: a capacitor having a capacitance of 40 pF or less and a cut-off frequency of 80 MHz or more, one end of which is connected to the measurement target; a detection impedance having one end connected to the other end of the capacitor and the other end grounded; a low-pass filter having one end connected to the connection point of the capacitor and the detection impedance; a high-pass filter having one end connected to the connection point of the capacitor and the detection impedance; a measurement unit connected to the other end of the low-pass filter and the other end of the high-pass filter, for measuring the partial discharge; and a partial discharge measuring device characterized by comprising the above.

2. The partial discharge measuring device according to claim 1, wherein the capacitor is a vacuum capacitor.

3. The high-pass filter includes two high-pass filters, a first high-pass filter and a second high-pass filter, wherein the cut-off frequency of the first high-pass filter is 9 kHz or more, and the cut-off frequency of the second high-pass filter is 50 MHz or more. The partial discharge measuring device according to claim 1.

4. The partial discharge measuring device according to claim 1, wherein the cut-off frequency of the low-pass filter is 100 kHz or less.

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