JP2002116235A - Method for detecting partial discharge of gas-blast load- break apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly sensitively detect a partial discharge signal which occurs in an insulated gas-blast load-break apparatus and to distinguish the detected partial discharge signal from external noise to reduce error determination. SOLUTION: In the device for detecting from outside an insulating spacer partial discharge of an insulated gas-blast load-break apparatus of a system in which the insulating spacer and a metal flange are fixed by metal bolts, by detecting frequency bands equal to the fundamental resonance frequency of standing waves or more excited between the metal bolts with the metal bolts as nodes between the metal bolts when the electromagnetic waves of the partial discharge which occurs due to insulation failure inside the insulated gas-blast load-break apparatus are transmitted between the bolts of the metal bolts of the insulating spacer and leaked to outside the insulated gas-blast load-break apparatus, electromagnetic signals from inside the insulated gas-blast load-break apparatus are efficiently detected to improve sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partial discharge detecting method for detecting a partial discharge of a gas switchgear such as a gas insulated switchgear in which an insulating spacer and a metal flange are fixed with metal bolts. The present invention relates to a partial discharge detection method capable of improving the detection sensitivity and enabling discrimination with external noise to prevent erroneous determination.

[0002]

2. Description of the Related Art FIG.
FIG. 7 is a configuration diagram showing a partial discharge detection device of this type of conventional gas insulated switchgear described in Japanese Patent No. 731. In this figure, reference numeral 1 denotes an antenna for detecting electromagnetic waves due to partial discharge leaking from an insulating spacer of a gas insulated switchgear. The output of the antenna 1 is input to a diagnostic unit through a bandpass filter 2 and an amplifier 3. The diagnostic unit includes a comparator 4, a signal generator 5, and a buzzer 6,
The comparator 4 outputs a signal when the level of the input signal from the band-pass filter 2 exceeds the reference level Vref, activates the signal generator 5, and sounds the buzzer 6.

Next, the operation of the conventional device will be described. When partial discharge occurs inside the gas insulated switchgear,
An electromagnetic wave leaking from the insulating spacer disposed between the metal flanges to the outside is detected by the antenna 1, the electromagnetic wave signal is amplified by the amplifier 3 through the band-pass filter 2 and input to the comparator 4, and the level of the input signal is changed to the reference level Vref. Buzzer 6 sounds. The band-pass filter 2 is set so as to avoid (filter) a frequency band of radio or the like that is known in advance.

[0004]

Since the conventional partial discharge detecting device is configured as described above, the detected signal is an electromagnetic wave signal of the partial discharge leaked from inside the gas switch,
It is difficult to determine whether the signal is external electromagnetic wave noise that has entered from the outside of the gas switchgear, and there has been a problem that a noise signal from the outside is erroneously determined to be a signal from the inside. Further, since the frequency characteristics of the electromagnetic wave passing between the metal bolts for fixing the insulating spacer between the metal flanges are not taken into consideration, the detection sensitivity is inefficient.

The present invention has been made to solve the above problems, and detects a partial discharge signal generated in a gas insulated switchgear with high sensitivity, and detects the detected partial discharge signal and external noise. It is an object of the present invention to provide a method for detecting partial discharge of a gas insulated switchgear, which makes it possible to reduce the erroneous judgment by making it possible to judge the discharge.

[0006]

In order to achieve the above object, a partial discharge detection method according to the present invention is applied to a gas switchgear in which an insulating spacer and a metal flange are fixed by metal bolts. In an apparatus for detecting partial discharge from the outside, when an electromagnetic wave of a partial discharge generated due to insulation failure inside the gas switchgear penetrates between the bolts of the metal bolts of the insulating spacer and leaks outside the gas switchgear. Preferably, a frequency band equal to or higher than the fundamental resonance frequency of the standing wave excited between the metal bolts is detected using the metal bolts as nodes. Further, the partial discharge detection method according to the present invention,
One or a plurality of resonance frequencies excited between volts are determined, and the resonance frequency is detected through a narrow band filter having the center frequency as a center frequency. Further, in the partial discharge detection method according to the present invention, the antenna for detecting the electromagnetic wave of the partial discharge is moved along the space between the bolts, and the frequency to be detected is made coincident with the one or more resonance frequencies. When the spectrum intensity distribution between the bolts matches the intensity distribution of the standing wave that resonates, the occurrence of partial discharge is determined. Still further, in the partial discharge detection method according to the present invention, instead of moving the antenna, a plurality of antennas are arranged between the bolts and signals are taken in at the same time. Further, in the partial discharge detection method according to the present invention, the antenna is configured by a magnetic field detection antenna that detects a magnetic field, and the magnetic field detection antenna is configured such that a magnetic field detection direction at which a maximum gain is obtained is perpendicular to the metal flange. Are arranged. Further, in the partial discharge detection method according to the present invention, the antenna is configured by an electric field detection antenna that detects an electric field, and the electric field detection antenna is configured so that an electric field detection direction at which a maximum gain is obtained is parallel to the metal flange. Are arranged. Further, the partial discharge detection method according to the present invention is characterized in that, when an electromagnetic wave of a partial discharge generated due to insulation failure inside the gas switchgear propagates through the gas switchgear, the coaxial waveguide structure of the gas switchgear The distance between the metal bolts is calculated such that the resonance frequency of the TE wave or TM wave of the standing wave excited according to the resonance frequency matches the resonance frequency excited between the metal bolts of the insulating spacer. The detection is performed by arranging the metal bolts. Furthermore, in the partial discharge detection method according to the present invention, instead of adjusting the distance between the metal bolts, the insulating spacer is made of a material having a dielectric constant that matches a resonance frequency. Further, the partial discharge detection method according to the present invention may be configured such that the gas insulated switchgear is used as the resonance frequency inside the gas switchgear to be matched with the resonance frequency of the metal bolt portion instead of the resonance frequency of the TE wave or TM wave. Characterized by using the resonance frequency of the standing wave excited between the impedance discontinuities. Furthermore, in the partial discharge detection method according to the present invention, a metal shield that shields electromagnetic waves leaking from the inside of the gas switchgear is disposed so as to cover the outer periphery of the insulating spacer, and a part of the metal shield leaks from the inside of the gas switchgear. Opening a detection window through which electromagnetic waves can be transmitted, making the width of the detection window the same as the thickness of the spacer, making the vertical width shorter than the distance between bolts and larger than the horizontal width of the detection window, Arrange the position of the detection window in the window frame so that the metal bolts do not overlap,
The frequency band for detecting partial discharge may be a resonance frequency determined by a vertical width of the detection window, instead of a resonance frequency determined between the metal bolts.

[0007]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a configuration diagram showing a partial discharge detection device for performing a partial discharge detection method for a gas switchgear according to Embodiment 1 of the present invention. Hereinafter, a case where the partial discharge detection method of the present invention is applied to a gas insulated switchgear as an example of a gas switchgear will be described. In FIG. 1, a gas insulated switchgear as a gas switchgear includes a metal container 20 containing a conductor 21 therein together with an insulating gas SF6, and an insulating member inserted between connecting portions of the metal containers 20, ie, between the flanges 22a and 22b. Spacer 22 and bushing 14 for connecting the entire device to the outside
And Also, as shown in FIG.
2 are sandwiched between the metal flanges 22a and 22b, and a plurality of metal bolts 26a, 26b,.
(Only one is shown).

The partial discharge detecting device according to the present invention comprises a metal container 2
0, which receives electromagnetic waves generated inside and leaking into the air from the insulating spacer 22;
16 for amplifying the output from the amplifier 16 and a signal processing device 17 for performing signal processing based on the output from the amplifier 16
And a partial discharge determination device 18 that determines the presence or absence of a partial discharge based on the output of the signal processing device 17.

FIG. 2 shows a plurality of metal bolts 26a, 26b,... Attached to metal flanges 22a, 22b.
The figure shows a state in which an electromagnetic wave transmitted between the metal bolts resonates between metal bolts to form a standing wave. As shown in the graph of FIG. 2, the electromagnetic wave radiated from the inside of the gas insulated switchgear to the outside of the gas insulated switchgear through the insulating spacer 22 when passing between the metal bolts 26a, 26b,. Using the metal bolts 26a, 26b,... As nodes of waves, resonance occurs between the metal bolts 26a, 26b,. There are a plurality of resonance frequencies fc, and the frequency is represented by the following equation (1). fc = C × n / (2 × L × √ε) (1) where C is the speed of the electromagnetic wave in vacuum, L is the distance between bolts, and ε is the relative permittivity of the spacer.

When the electromagnetic wave transmitted between metal bolts has a fundamental resonance frequency fc = C / (2 × L × √ε) or less, the length of the wavelength becomes larger than the length L between metal bolts.
Transmission between metal bolts becomes difficult, and transmission efficiency decreases. Therefore, if a high-pass filter in which the transmission frequency band of the filter 25 shown in FIG. 1 is equal to or higher than the above-mentioned fundamental frequency is used, transmitted electromagnetic waves can be detected efficiently. The detection sensitivity can be improved.

Embodiment 2 Embodiment 2 of the present invention
In the first embodiment, the detection frequency is a single frequency. In the second embodiment, the filter 2 shown in FIG.
5 uses a band-pass filter whose transmission frequency is adjusted to the resonance frequency fc obtained by the above equation (1).

Since the detection sensitivity is improved by narrowing the frequency band to be detected, according to the second embodiment, the sensitivity can be further improved as compared with the first embodiment.

Embodiment 3 FIG. 3 shows a main part of a third embodiment of the present invention. The configuration of the gas insulated switchgear itself is the same as that of FIG. 1, but the configuration of the partial discharge detection device is partially different. That is, in the third embodiment, as shown in FIG. 3, instead of the antenna 15 of FIG. 1, detection is performed by moving between adjacent metal bolts from one metal bolt 26 a to the other metal bolt 26 b. A magnetic field probe 27 is provided. Further, in FIG. 3, the partial discharge detection device
A filter 25, an amplifier 16, a signal processing device 17, and a partial discharge determination device 18 are provided. The filter 25 is the band-pass filter that matches the resonance frequency used in the second embodiment. The processing device 17 and the partial discharge determination device 18 are the same as those in the first embodiment shown in FIG.

Next, a method of determining a signal detected by the magnetic field probe 27 will be described. When the filter 25 of FIG. 3 is set to the basic resonance frequency (resonance frequency when n = 1 in the above equation (1)) and detected, the vertical axis indicates the distance between metal bolts and the horizontal axis indicates the distance based on the detection result. In terms of the detected intensity of the signal detected by the magnetic field probe 27, if an electromagnetic wave is generated inside the gas insulated switchgear, the signal intensity is maximum at the center between the metal bolts as shown in the graph of FIG. , And a minimum bow-shaped curve is obtained near the metal bolt. Further, the filter 25 is connected to the second resonance frequency (n =
If 2), a curve as shown in FIG.

On the other hand, when the magnetic field probe 27 detects electromagnetic noise entering from the outside of the gas insulated switchgear, the electromagnetic noise is radiated uniformly regardless of the position of the metal bolt. As described above, a characteristic that does not depend on the distance between metal bolts and has no change in signal strength can be obtained.

On the other hand, it is possible to accurately determine whether or not partial discharge has occurred inside the gas insulated switchgear from the resonance frequency set in the filter 25 and the corresponding distance between metal bolts-detection strength characteristic. it can.

Embodiment 4 FIG. 5 shows a configuration of the fourth embodiment of the present invention. In the fourth embodiment, in the configuration of FIG. 3, the number of magnetic field probes 27 is increased to a plurality, and the connection between the magnetic field probe 27 and the filter 25 is selectively switched by the switch 28 so that measurement can be performed. It was made. With such a configuration, in addition to the effect of the third embodiment, the trouble of moving the magnetic field probe 27 can be omitted, and the measurement time can be reduced.

Embodiment 5 FIG. 6 shows the directions of an electric field component and a magnetic field component of an electromagnetic wave transmitted between metal bolts. When the electromagnetic waves of the partial discharge generated inside the gas insulated switchgear penetrate the portion sandwiched between the metal flanges 22a and 22b, the electric field component of the electromagnetic waves is perpendicular to the end face (cross section) of the metal flange. Have. Further, since the magnetic field component is orthogonal to the electric field component, the magnetic field component is oriented parallel to the end faces of the metal flanges 22a, 22b and perpendicular to the axes of the metal bolts 26a, 26b,.

In the sixth embodiment of the present invention, the above properties are used, and as shown in FIG. 7, in the third and fifth embodiments, the loop antenna 29 is used as a magnetic field detecting antenna.
And the magnetic field detection direction of the loop antenna 29 is arranged so as to be parallel to the end faces of the metal flanges 22a and 22b. By arranging the loop antenna 29 in this way, the magnetic field detection sensitivity can be improved, and therefore, partial discharge can be detected with high sensitivity.

Embodiment 6 Embodiment 6 of the present invention
In the fifth embodiment, instead of an antenna for detecting a magnetic field, an antenna for detecting an electromagnetic wave of partial discharge is used,
The antenna for electric field detection is used, and the direction of the antenna for electric field detection (electrolysis detection direction) is perpendicular to the end face of the metal flange. With such a configuration, the electric field detection sensitivity of the electric field detection antenna can be improved, and therefore, partial discharge can be detected with high sensitivity.

Embodiment 7 When a partial discharge occurs in a gas insulated switchgear, the electromagnetic wave of the partial discharge propagates through the bus of the gas insulated switchgear, and the TE wave (Transverse Electron) corresponds to the coaxial structure of the gas switchgear.
ic Wave) and TM wave (Transverse)
(Magnetic Wave) is formed.

Further, when the electromagnetic wave of the partial discharge is radiated from the insulating spacer to the outside, the resonance frequency characteristic formed between the metal bolts represented by the above formula (1) is further superimposed. Therefore, the frequency characteristic of the electromagnetic wave of the partial discharge detected by the insulating spacer is a result of multiplying the frequency characteristic of the TE wave and the TM wave by the resonance frequency generated between the metal bolts.

Therefore, in the seventh embodiment of the present invention, the frequency of the TE wave or the TM wave is made to coincide with the resonance frequency formed between the metal bolts, and the signal intensity of the electromagnetic wave of the partial discharge is increased. is there. For this reason, in formula (1), the distance between the metal bolts is calculated such that the resonance frequency fc matches the TE wave or the TM wave, and the distance between the metal bolts is arranged in accordance with the calculated distance. Can be detected.

Embodiment 8 FIG. Embodiment 8 of the present invention
In the seventh embodiment, the partial discharge can be detected with high sensitivity by calculating the dielectric constant ε so that the resonance frequency matches the equation (1) and arranging the spacer having the dielectric constant according to the dielectric constant ε. Can be.

Embodiment 9 Embodiment 9 of the present invention
In the seventh or eighth embodiment, as the resonance frequency inside the gas-insulated switchgear to be matched with the resonance frequency between the metal bolts, instead of TE or TM, it is excited between impedance discontinuities of the gas-insulated switchgear. The resonance frequency of the standing wave is used.

FIG. 8 shows an example of a specific detection method according to the ninth embodiment. At both ends of the bus A of the gas insulated switchgear,
A bus B and a bus C having different shapes from the bus A are connected to each other, and an apparatus for detecting a partial discharge is arranged near the insulating spacer 22 of the bus A. The insulating spacer 22 supports the high-voltage conductor 21 of the bus A. In the ninth embodiment, the configuration of the partial discharge detection device (15 to 18, 25) for detecting the partial discharge is the same as that of the first embodiment. When partial discharge occurs inside bus A,
Although the electromagnetic wave of the partial discharge propagates toward the bus portion B and the bus C, the buses B and C have different bus shapes from the bus A, and a part of the electromagnetic wave is reflected due to impedance mismatch at the junction. While repeating multiple reflections, a standing wave depending on the length La of the bus A occurs inside the bus A, and f =
A resonance frequency expressed by C × n / (2La) is excited. Metal bolts may be arranged as described in the seventh embodiment so that the resonance frequency excited by the bus A and the resonance frequency represented by the above equation (1) match.
By determining the dielectric constant of the insulating spacer 22 as in the eighth embodiment, it is possible to detect a partial discharge with high sensitivity.

Embodiment 10 FIG. FIG. 9 shows a configuration of a tenth embodiment of the present invention, in which (a) is a side view of a gas insulated switchgear, and (b) is a front view thereof. In FIG.
Reference numeral 31 denotes a metal shield that blocks electromagnetic waves, and the shield 31 covers the entire insulating spacer 22 except for a measurement window 32 for measuring the partial discharge electromagnetic wave on the outer peripheral portion of the insulating spacer 22. Metal flange 22a, 2
2b and is electrically grounded through the metal flanges 22a and 22b. The width of the measurement window 32 is the same as the width of the insulating spacer 22, and the length thereof is smaller than the distance between the metal bolts. The position of the measurement window 32 does not overlap with the metal bolts 26a, 26b,. Are located in

In the methods of the first to ninth embodiments,
When the frequency band needs to be changed suddenly, such as when the noise level of the frequency band to be measured is large, the distance between the metal bolts cannot be easily changed structurally because it is determined by the distance between the metal bolts. There is. On the other hand, by arranging the shield 31 having the measurement window 32 corresponding to the frequency band to be measured by the above equation (1),
Since the frequency band can be set arbitrarily in a frequency band higher than the fundamental frequency determined by the distance between metal bolts while avoiding a frequency band having a large noise level, partial discharge can be detected with high sensitivity.

Also, due to the shield effect of the shield 31, electromagnetic noise radiated from the outside is reduced by the insulating spacer 2.
2 and penetrates the insulating spacer 22 again to prevent entry into the partial discharge detection device, and partial discharge can be detected with high sensitivity.

[0030]

As described above, according to the first and second aspects of the present invention, the partial discharge electromagnetic wave generated inside the gas switchgear is detected outside the insulating spacer, and the electromagnetic wave passes between the metal bolts. By setting the filter in accordance with the resonance frequency of the standing wave generated when the signal is generated, it is possible to determine whether or not the signal is from the inside of the gas switchgear, thereby improving the determination accuracy.

According to the third and fourth aspects of the present invention, the filter is adjusted to the resonance frequency of the standing wave generated when the electromagnetic waves of the partial discharge pass between the metal bolts, and the standing wave between the metal bolts is further adjusted. By measuring the intensity distribution, it is possible to determine whether or not the signal is from the inside of the gas switchgear, thereby improving the determination accuracy.

According to the fifth and sixth aspects of the present invention, the detection sensitivity can be improved by directing the detection direction of the antenna in such a direction as to obtain the maximum gain.

According to the seventh to ninth aspects of the present invention, when a partial discharge electromagnetic wave generated inside the gas insulated switch propagates through the gas switch, the standing wave excited by the structure of the gas switch is provided. And, by arranging the metal bolts and determining the dielectric constant of the insulating spacer so that the resonance frequency of the standing wave excited when transmitting between the metal bolts of the insulating spacer is matched, the intensity of the electromagnetic wave is reduced. It is possible to increase the detection sensitivity and improve the detection sensitivity.

According to the tenth aspect of the present invention, the outer periphery of the insulating spacer is covered with a metal foil, and an electromagnetic wave signal is detected by an antenna from a detection window provided on the foil. As a result, electromagnetic noise entering the inside of the gas-insulated switch can be reduced, and the detection sensitivity can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a partial discharge detection method according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a resonance phenomenon between metal bolts in the first embodiment.

FIG. 3 is an explanatory diagram of a partial discharge detection method according to a third embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a method for determining the presence or absence of a partial discharge in a third embodiment.

FIG. 5 is an explanatory diagram of a partial discharge detection method according to a fourth embodiment of the present invention.

FIG. 6 is an electromagnetic field distribution of an electromagnetic wave leaking from an insulating spacer in the partial discharge detection method according to the fifth embodiment of the present invention.

FIG. 7 is an explanatory diagram of a partial discharge detection method according to a fifth embodiment of the present invention.

FIG. 8 is an explanatory diagram of a partial discharge detection method according to a ninth embodiment of the present invention.

FIG. 9 is an explanatory diagram of a partial discharge detection method according to a tenth embodiment of the present invention.

FIG. 10 is an explanatory diagram of a conventional partial discharge detection device.

[Explanation of symbols]

Reference Signs List 14 bushing, 15 partial discharge detection discharge detection antenna, 16 amplifier, 17 signal processing device, 18 partial discharge determination device, 20 metal container, 21 conductor, 22 insulating spacer, 28 changeover switch, 29 loop antenna, 33 metal bolt, 31 Electromagnetic wave shield, 32
Measurement window for partial discharge detection.

Continuing on the front page (72) Inventor Ichigaki Ichigaki 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Corporation (72) Inventor Masanori Osumi 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsui Electric F term in the company (reference) 2G015 AA10 BA02 CA01 5G017 EE02 5G365 DH11 DN04

Claims (10)

[Claims] In a device for detecting partial discharge from outside of an insulating spacer for a gas switching device in which an insulating spacer and a metal flange are fixed by metal bolts, the device is generated due to insulation failure inside the gas switching device. When the electromagnetic wave of the partial discharge penetrates between the bolts of the metal bolts of the insulating spacer and leaks to the outside of the gas switchgear, the electromagnetic bolts are excited between the metal bolts with the metal bolts as nodes. A partial discharge detection method comprising detecting a frequency band equal to or higher than a fundamental resonance frequency of a standing wave. 2. The partial discharge detection method according to claim 1, wherein one or a plurality of resonance frequencies excited between volts are determined, and the resonance frequency is detected through a narrow-band filter having the frequency as a center frequency. Characteristic partial discharge detection method. 3. The partial discharge detection method according to claim 2, wherein an antenna for detecting an electromagnetic wave of the partial discharge is moved along the space between the bolts, and a frequency to be detected is made coincident with the one or more resonance frequencies. Detecting the occurrence of partial discharge when the spectrum intensity distribution between the volts at that frequency matches the intensity distribution of the standing wave that resonates. 4. The partial discharge detecting method according to claim 3, wherein a plurality of antennas are arranged side by side between said bolts and signals are taken in simultaneously instead of moving the antenna. 5. The partial discharge detection method according to claim 3, wherein the antenna is constituted by a magnetic field detection antenna for detecting a magnetic field, and a magnetic field detection direction in which a maximum gain is obtained is perpendicular to the metal flange. A partial discharge detection method, wherein the magnetic field detection antenna is arranged such that 6. The partial discharge detection method according to claim 3, wherein the antenna is constituted by an electric field detection antenna for detecting an electric field, and an electric field detection direction in which a maximum gain is obtained is parallel to the metal flange. A partial discharge detection method, wherein the electric field detection antenna is arranged such that 7. The partial discharge detection method according to claim 1, wherein an electromagnetic wave of a partial discharge generated due to an insulation failure inside the gas switchgear propagates through the gas switchgear. The resonance frequency of the standing wave TE wave or TM wave excited according to the coaxial waveguide structure of the gas switchgear is matched with the resonance frequency excited between the metal bolts of the insulating spacer. A method for detecting a partial discharge, wherein a distance between metal bolts is calculated, and the metal bolts are arranged in accordance with the calculated distance to perform detection. 8. The partial discharge detecting method according to claim 7, wherein the insulating spacer is made of a material having a dielectric constant matching a resonance frequency, instead of adjusting the distance between the metal bolts. Detection method. 9. The partial discharge detection method according to claim 7, wherein the resonance frequency of the TE wave or the TM wave is set as the resonance frequency inside the gas switchgear to be matched with the resonance frequency of the metal bolt portion. Alternatively, a partial discharge detection method using a resonance frequency of a standing wave excited between impedance discontinuities of the gas insulated switchgear. 10. The partial discharge detection method according to claim 1, wherein a metal shield for shielding electromagnetic waves leaking from inside the gas switchgear is provided so as to cover an outer periphery of the insulating spacer. Open the detection window through which electromagnetic waves leaking from inside the gas switchgear can be transmitted, and make the width of the detection window the same as the thickness of the spacer, and set the vertical width to be smaller than the distance between bolts. Shorter and larger than the width of the detection window, the position of the detection window is arranged in a window frame so that the metal bolts do not overlap, and the frequency band for partial discharge detection is determined between the metal bolts. A method for detecting a partial discharge, wherein a resonance frequency determined by a vertical width of the detection window is used instead of the resonance frequency.