JPH0875689A - Deterioration detector - Google Patents

Abstract

PURPOSE: To detect a degree of deterioration of electric insulating material and electrical apparatuses quantitatively and quickly by providing a bridge circuit, voltage-vector detecting means and an operating means. CONSTITUTION: A bridge circuit is formed of a standard capacitor 21, a variable impedance 23, a sample to be measured 22 and a fixed resistor 24. The first and second detected signals A and B outputted from the first and second output terminals of this circuit are inputted into vector detecting means 3 and 4. The voltage vector values of the respective detected signals are detected with the detecting means 3 and 4. An operating part 5 adjusts and controls the impedance value of the impedance 23 based on the respective voltage vector values. Thus, the balance adjustment for obtaining the balance condition of the circuit 2 can be performed readily and strictly. Then, the effects of the resistance of a cable housing and the stray current are suppressed extremely, and the degree of the characteristic deterioration of the measured data can be detected highly accurately and quantitatively under the hot-line state. At the same time, this detecting operation can be automated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deterioration detecting device for detecting deterioration of an electrically insulating material, an electric device and the like, and more particularly to a deterioration detecting device for applying an AC voltage to detect the degree of deterioration as a voltage vector value.

[0002]

2. Description of the Related Art Conventionally, as this kind of deterioration detecting device, there is an insulation resistance measuring device for measuring deterioration by measuring the insulation resistance of various cables or electric equipment. This insulation resistance measuring device is used for diagnosing the insulation of a transmission line such as an insulation resistance method, a direct current leakage current method, etc. when a power failure occurs in the transmission line.
In some cases, such as the nδ method, the DC component method, the composite (combination of the hot line Tanδ method and the DC component method) determination method, and the DC superposition method, the insulation of the transmission line is diagnosed in the live state of the transmission line.

FIG. 6 is a schematic block diagram of a conventional insulation resistance measuring device by a direct current superposition method. In the figure, the conventional device based on the DC superposition method is the primary middle point of the grounding transformer GPT or
A DC voltage of 6 to 50 V is superposed between the high voltage path and the ground through a reactor dedicated for measurement, the DC leakage current is detected from the ground wire of the target cable of the measurement sample, and the insulation resistance value is calculated from this DC leakage current. Then, the degree of deterioration of the target cable is determined. Further, in this DC superposition method, since the insulation resistance value is obtained from the DC leakage current, the influence of the stray current is large, and in order to eliminate this influence, the polarity is reversed (positive / negative switching) for measurement.

When the deterioration of the measurement sample is determined by this DC superposition method, the insulation resistance can be automatically measured, and the trend of the data can be analyzed by monitoring the measurement sample of up to 20 lines.

[0005]

Although there are various kinds of conventional insulation resistance measuring devices as deterioration detecting devices, each diagnostic method for power failure is not practical, and each diagnostic method for hot line is practical. Is excellent. Also in each of the above-mentioned diagnostic methods at the time of hot line, for example, in the direct current superposition method, the high voltage charging unit must be contacted, and the influence of the resistance of the cable jacket is large, and the water tree (Water Tree) must be on the verge of penetration. It has a problem that it is difficult to detect. The hot-line Tan δ method and the composite judgment method must be in contact with the high-voltage charging section, and can be detected effectively only when the target cable is uniformly deteriorated, but it is effective when uneven deterioration occurs. It had a problem that it could not be detected.

Further, in the case of the direct current component method, effective detection is difficult unless the water tree is on the verge of penetration, and there is a problem that the resistance of the cable jacket and stray current are large and accurate detection cannot be performed. . Each of the conventional deterioration detecting devices described above can only obtain a qualitative detection result even if the detection operation is properly performed, and cannot detect the degree of deterioration quantitatively.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a deterioration detecting device capable of quantitatively and rapidly detecting the degree of deterioration of an electrically insulating material and an electric device. .

[0008]

FIG. 1 shows the principle of the present invention. In the figure, the deterioration detecting device according to the present invention is
The connection middle point where the standard capacitor and the variable impedance are connected in series is used as the first output terminal, and the connection middle point where the measurement sample and the resistance are connected in series is used as the second output terminal, and each series circuit is an AC power source. A bridge circuit formed in parallel with each other, and a voltage of each voltage of the first detection signal output from the first output terminal and the second detection signal output from the second output terminal. Voltage vector detecting means for detecting a vector value, and a first of the voltage vector values
And an arithmetic processing unit that adjusts and controls the impedance value of the variable impedance based on each of the second detection signals, and controls the connection of the resistor. The characteristic deterioration of the measurement sample is judged from each resistance value.

Further, according to the present invention, there is provided an interrupting means connected in parallel with a fixed resistor of the bridge circuit, if necessary, and the arithmetic processing section outputs the first output terminal with the interrupting means turned on. The voltage level value of the first detection signal is input from the voltage vector detection means, the disconnection means is opened, and the voltage of the difference between the first and second detection signals output from the first and second output terminals. A vector value is input from the voltage vector detection signal, the deviation vector value of each voltage vector value is calculated, and the voltage vector value of the second detection signal is calculated from the deviation voltage vector value and the voltage vector value of the first detection signal. Is calculated, and the impedance value of the variable impedance is adjusted and controlled so that the voltage vector value of the first detection signal matches the calculated voltage vector value of the second detection signal. Than it is.

Further, according to the present invention, the capacitance to ground is connected in parallel to the connection midpoint of each series circuit of the bridge circuit, if necessary. Further, according to the present invention, the variable impedance of the bridge circuit is such that a variable resistance and a variable capacitance are connected in parallel, if necessary. Further, according to the present invention, the first and second detection signals output from the respective output terminals are connected between the first and second output terminals of the bridge circuit and the voltage vector detection means, if necessary. Is provided with a differential amplifier for calculating the deviation.

[0011]

In the present invention, a bridge circuit is formed from a standard capacitor and variable impedance, a sample to be measured and a resistance, and the first and second detections output from the first and second output terminals of the bridge circuit. The signal is input to the voltage vector detecting means, the voltage vector detecting means detects the voltage vector value of each detection signal, and the arithmetic processing means adjusts and controls the impedance value of the variable impedance based on each voltage vector value. Therefore, it is possible to easily and rigorously perform the balance adjustment for obtaining the equilibrium condition of the bridge circuit in the live state when the AC voltage is applied, and suppress the influence of the resistance of the cable jacket and the stray current as much as possible. In this state, the degree of characteristic deterioration of the measurement sample can be detected with high accuracy and quantitatively, and the detection operation can be automated.

Further, according to the present invention, the arithmetic processing unit controls the connection / disconnection means to turn on / off the bridge circuit so that the first
Detection signal and the signal of the difference between the first and second detection signals are respectively output, and the voltage vector value of each output signal is calculated as the deviation voltage vector value of the voltage vector value, The voltage vector value of the second detection signal is calculated from the voltage vector value of the detection signal and the deviation voltage vector value, and the voltage vector value of the first detection signal matches the voltage vector value of the second detection signal. Since the impedance value of the variable impedance is adjusted and controlled as described above, the balance condition of the bridge circuit can be adjusted more accurately, and the degree of characteristic deterioration of the measurement sample can be detected with higher accuracy.

Further, in the present invention, the balance condition of the bridge circuit can be adjusted in consideration of the capacitance to ground at the connection midpoint of each series circuit, and the degree of deterioration can be detected more precisely. Further, in the present invention, since the variable impedance is composed of the parallel circuit of the variable resistance and the variable capacitance, it is possible to adapt it to the circuit characteristics of the measurement sample, and it is possible to detect the degree of deterioration more precisely.

Further, in the present invention, the first and second detection signals output from the first and second output terminals of the bridge circuit are operated by a differential amplifier to calculate the deviation of each detection signal. Therefore, it is possible to input each signal under the same condition with a single differential amplifier and cancel the noise component contained in each signal to adjust the balance condition of the bridge circuit more precisely. The degree of can be detected with higher accuracy.

[0015]

【Example】

(One Embodiment of the Invention) One embodiment of the present invention will be described below with reference to FIGS. 2 is an overall circuit configuration diagram of the deterioration detection apparatus according to the present embodiment, FIG. 3 is an operation flowchart in the embodiment apparatus shown in FIG. 2, and FIG. 4 is a description of balance adjustment in the embodiment apparatus shown in FIG. FIG.

In each of the drawings, the deterioration detecting apparatus according to this embodiment has a standard capacitor 21, a measurement sample 22, a variable impedance 23 and a fixed resistor 24 connected in a bridge shape, and a switch 25 is connected to the fixed resistor 24 in parallel. AC voltage vector E (which is connected to the bridge circuit 2 connected to each other, the connection midpoint c of the standard capacitor 21 and the measurement sample 22 and the connection midpoint d of the variable impedance 23 and the fixed resistor 24 of the bridge circuit 2 Hereinafter, the connection point b (hereinafter referred to as the first output terminal b) of the power supply unit 1 for applying E (~), the standard capacitor 21 and the variable impedance 23 of the bridge circuit 2, the measurement sample 22, and the fixed resistor 2
A differential amplifier 3 which receives and operates each of the first and second detection signals A and B from the connection middle point a (hereinafter, second output terminal a) 4 of FIG. The lock-in amplifier 4 for calculating the voltage vector value for each of the detection signals A and B, and the impedance value of the variable impedance 23 is adjusted and controlled based on each voltage vector value, and the opening / closing of the switch 25 is controlled. This is a configuration including the arithmetic processing unit 5.

The standard capacitor 21 is, for example, a low voltage side electrode formed of a single wire conductor as a center conductor of a coaxial cable, a polyethylene resin insulator covering the outer periphery of the low voltage side electrode, and an outer periphery of the polyethylene resin insulator. A high-voltage electrode formed of an intermediate conductor in close contact with, a nylon tape of an outer conductor that covers the outer circumference of the high-voltage side electrode, and a polyethylene resin insulator, a protective conductor, and a nylon tape that sequentially cover the outer circumference of the nylon tape. It can also be configured.

The measurement sample 22 is, for example, a coaxial type power cable, and can be represented as a parallel circuit of a resistor Rx and a capacitor Cx as an equivalent circuit.
The variable impedance 23 is composed of a parallel circuit of a correction variable resistor Rv and a correction variable capacitor Cv, and the resistance value of the correction variable resistor Rv and the capacitance value of the correction variable capacitor Cv are controlled by the arithmetic processing unit 5. It is a configuration that changes. The fixed resistor 24 has a configuration in which a detection resistor Rd and a ground capacitance Csh are connected in parallel.

When the voltage vector values of the first and second detection signals A and B coincide with each other under the control of the variable impedance 23 and the switch 25, the arithmetic processing unit 5 determines from the respective input values in the case of coincidence. In this configuration, the insulation resistance value of the measurement sample 22 is output. Next, the operation of the apparatus of this embodiment based on each of the above configurations will be described. First, the switch 25 is turned on under the control of the arithmetic processing unit 5, and the bridge circuit 2
In, the detection resistor Rd is short-circuited (step 1). Since the detection resistor Rd is in a short-circuited state, the second
The second detection signal A is output as "zero" from the output terminal a, and the first detection signal B is output at a predetermined value only from the first output terminal b (step 2).

The first detection signal B is input to the lock-in amplifier 4 via the differential amplifier 3, and the lock-in amplifier 4 outputs the voltage value B and the phase value θB of the absolute value of the first detection signal B. The voltage vector value B of the first detection signal calculated
(Hereinafter, shown as the first detection signal B (-)) (step 3). This first detection signal B is shown in FIG. From the respective values of the first detection signal B, the standard capacitance Cs of the standard capacitor 21, and the correction variable resistance Rv and the correction variable capacitor Cv set when the first detection signal B is output. Vector value AC voltage E (hereinafter E
(Shown as ()), the absolute voltage value E is calculated according to the following equation (step 4).

[0021]

[Equation 1]

In this equation (1), Ws = 2πf · Cs
・ Rv = ω ・ Cs ・ Rv, Wv = 2πf ・ Cv ・ Rv =
ω · Cv · Rv.

Further, from the respective values of the standard capacitance Cs, the correction variable resistor Rv and the correction variable capacitor Cv, the first value is obtained.
Of the detection signal B (-) of
B is calculated by the arithmetic processing unit 5, and the phase Δθ of the AC voltage E (∼) with respect to the reference phase of the device itself is obtained from this phase φB and the phase value θB calculated in step 3 to correct the phase of the device itself. The value is Δθ (step 5). The calculation of the phase correction value Δθ is as follows when the phase display range of the apparatus itself is 0 to 2π radians (0 to 360 degrees) (step 6).

[0024]

[Equation 2]

When the phase display range is −π to + π radians (−180 degrees to +180 degrees), the following equation is obtained.

[0026]

(Equation 3)

AC voltage E obtained in step 5
(-), The standard capacitance Cs, the correction variable resistor Rv, and the correction variable capacitor Cv, xy based on the AC voltage E.
X component Bx in the first detection signal B (-) in the coordinate system
And the first y component By is calculated by the following equations.
The detection signal B '(-) is calculated (step 7). Since the calculated first detection signal B '(-) is a value calculated from the value of each element, a noise component like the first detection signal B (-) detected in step 2 is generated. Not included.

[0028]

[Equation 4]

[0029]

(Equation 5)

In the equations (4) and (5), the AC voltage E (-) is used to calculate the x and y components Bx and By, respectively. 1
It is also possible to perform calculation as in the following equation using the detection signal B itself.

[0031]

(Equation 6)

[0032]

(Equation 7)

Next, the arithmetic processing unit 5 opens the switch 25 and connects the fixed resistor 24 to the bridge circuit 2 (step 8). In this state, the first and second detection signals A and B simultaneously output from the first and second output terminals a and b are input to the differential amplifier 3 to obtain the deviation voltage A-B. In this way, the differential amplifier 3 operates based on the first and second detection signals A and B that are simultaneously output, and the noise components included in the respective signals become extremely equal values, and the noise components are canceled in the differential amplifier. Because I am doing
This deviation voltage A-B does not include a noise component (or is suppressed to an extremely low value). That is, since the voltage vectors A and B are simultaneously output, they have the same noise component n, and can be set as A = a + n and B = b + n. The difference (A−B) between the voltage vectors A and B is {a−b
+ (N−n)}, and when this is calculated, (ab) is obtained. The deviation voltage (A-
When the deviation between B) and the first detection signal B is obtained, the noise component contained in the voltage vector A is removed and the original detection signal value a is obtained. Based on the deviation voltage AB, the lock-in amplifier 4 measures the deviation voltage vector AB (-) as the absolute value AB and the phase θA-B (step 9).

Further, using the phase correction value Δθ obtained in step 5, the deviation voltage vector AB (∼) is phase φA-B in the xy coordinate system with the AC voltage E as a reference.
Is calculated as follows.

[0035]

[Equation 8]

The x and y components of the deviation voltage vector A-B (-) thus calculated and the x and y components of the first detection signal B '(-) calculated in the step 7 The components Ax and Ay of x and y of the second detection signal A (-) are calculated from the components Bx and By as shown in the following equation. A noise component is also included in the second detection signal A (-) calculated from the fact that each deviation vector AB (-) and the first detection signal B '(-) do not include a noise component. Absent.

[0037]

[Equation 9]

[0038]

[Equation 10]

Here, the arithmetic processing unit 5 determines whether or not the difference value between the first and second detection signals A (-) and B (-), which are simultaneously input, is less than or equal to a preset reference accuracy value Q. It is determined (step 10). If it is determined that the value is equal to or less than the reference accuracy value Q, the arithmetic processing unit 5 determines that the standard capacitance Cs, the correction variable resistance Rv, the correction variable capacitance Cv, the detection resistance Rd, and the ground electrostatic capacitance Csh at the time of the matching. Based on the above, the insulation resistance value of the power cable forming the measurement sample 22 is calculated and output (step 11).

The insulation resistance value can be calculated based on each of the above values by a general equilibrium condition expression of a bridge circuit. A specific example of this calculation is as follows. Here, the calculation of the insulation resistance in an ideal circuit in which the electrostatic capacitance to ground can be ignored is set to E = 1 to simplify the calculation, and the electrostatic capacitance Csh of the fixed resistor 24 is set to Csh = 0. First, under the above conditions

[0041]

[Equation 11]

[0042]

[Equation 12] Here, Ws = ωCsRv and Wv = ωRvCv.

Since A = B is obtained by the balance adjustment,

[0044]

[Equation 13]

Here, when (X / Y = D) is set, the equation (1)
From 1),

[0046]

[Equation 14]

[0047]

(Equation 15)

Here, since R is a high insulation resistance, R
(1 / ωC), and the capacitance C of the sample 22 is

[0049]

[Equation 16]

It becomes Also, from equations (12) and (13)

[0051]

[Equation 17]

It becomes Furthermore, substituting equations (16) and (17) into equation (15),

[0053]

(Equation 18)

By rearranging this equation (18), the resistance Rx of the sample 22 is

[0055]

[Formula 19]

Here, Ws = ωCsRv, Wv = ωCx
Rx.

As another calculation method, a low frequency voltage (for example, 1 to several tens Hz, 50 V or less) is superimposed between the line of the AC voltage applied from the power source section 1 and the ground, and the power to be the measurement sample 22 is obtained. The superposed low-frequency current is taken out from the ground wire of the cable, the low-frequency voltage is micro-changed and superposed in the same manner as described above, and in this case, the micro-changed low-frequency current superposed from the ground wire of the measurement sample 22. Take out. It is also possible to calculate the insulation resistance from the low-frequency currents extracted respectively.

When it is judged in step 10 that the reference accuracy value is equal to or higher than Q, the arithmetic processing section 5 determines the second detection signal from the first detection signal B (-) and the deviation voltage vector AB (-). A (-) is calculated (step 12). The arithmetic processing unit 5 sets the condition (value of the correction variable resistor Rv and the correction variable capacitor Cv so that the vector value of the first detection signal B (-) becomes equal to the vector value of the second detection signal A (-). ) Is calculated according to the following equation (step 13).

[0059]

[Equation 20]

[0060]

[Equation 21]

In the equations (11) and (12), the calculation is performed using the AC voltage E, but the first detection signal B
It is also possible to calculate using the following equation.

[0062]

[Equation 22]

[0063]

[Equation 23]

Each value of the variable impedance 23 is set so as to satisfy the conditions of the correction variable resistance Rv and the correction variable capacitance Cv obtained in the above step 13 (step 13).

After the set condition value is set, the process returns to step 1 and the above operation is repeated. In this way, since the first and second detection signals A (-) and B (-) are detected as voltage vector values and compared,
The balance adjustment can be performed quickly and precisely, and the adjustment operation can be automated. When the values of the contact resistance r and the stray capacitance Cp of the variable impedance 23 in the above-described embodiment are not negligible, if the variable resistance Rv ′ and the variable capacitance Cv ′ are set, the correction variable resistance Rv and the correction variable resistance Rv are corrected. The variable capacitance Cv for each is represented by the following equation.

[0066]

[Equation 24]

[0067]

(Equation 25)

(Other Embodiments of the Present Invention) In each of the above-mentioned embodiments, the measurement sample has been described by taking a power cable as an example. However, various kinds of deterioration such as deterioration of various electric devices, deterioration of water pipes due to electrolytic corrosion, etc. The degree can be detected quantitatively.
Further, in each of the above-described embodiments, the insulation resistance value is obtained to detect various types of deterioration, but the dielectric loss tangent test (Tangent
When δ, hereinafter Tan δ) is obtained from the equilibrium state of the Schering bridge circuit, this equilibrium condition may be used for adjustment.

[0069]

As described above, in the present invention, a bridge circuit is formed from the standard capacitor and the variable impedance, the measurement sample and the fixed resistor, and the first and second output terminals of the bridge circuit output. The first and second detection signals are input to the voltage vector detection means, the voltage vector detection means detects the voltage vector value of each detection signal, and the arithmetic processing means calculates the variable impedance based on each voltage vector value. Since the impedance value is adjusted and controlled, it becomes possible to easily and rigorously perform the balance adjustment for obtaining the equilibrium condition of the bridge circuit in the live state when the AC voltage is applied, and the influence of the resistance of the cable jacket and the stray current. It is possible to detect with high accuracy and quantitatively the degree of characteristic deterioration of the measured sample while suppressing the There is an effect that can automate the work. Further, in the present invention, the arithmetic processing unit outputs the first detection signal and the sum signal of each of the first and second detection signals from the bridge circuit by controlling the connection / disconnection of the connection / disconnection means. The voltage vector value of each output signal is calculated as the deviation voltage vector value of the voltage vector value, and the first voltage vector value is calculated.
The voltage vector value of the second detection signal is calculated from the voltage vector value and the deviation voltage vector value of the detection signal of
Since the impedance value of the variable impedance is adjusted and controlled so that the voltage vector value of the first detection signal matches the voltage vector value of the detection signal of, the balance condition of the bridge circuit can be adjusted more precisely, and the measurement This has the effect that the degree of deterioration of the characteristics of the sample can be detected with higher accuracy. Further, in the present invention, the balance condition of the bridge circuit can be adjusted in consideration of the capacitance to ground at the connection midpoint of each series circuit, and there is an effect that a more accurate deterioration degree can be detected. Further, in the present invention, since the variable impedance is configured by the parallel circuit of the variable resistance and the variable capacitance, it is possible to adapt it to the circuit characteristics of the measurement sample, and it is possible to detect a more precise deterioration degree. . Furthermore, in the present invention, the first and second detection signals output from the first and second output terminals of the bridge circuit are operated by the operational amplifier to calculate the deviation of each detection signal. One differential amplifier can be used to input each signal under the same conditions, cancel noise components contained in each signal, and adjust the balance condition of the bridge circuit more precisely. It has an effect that it can be detected with high accuracy.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of a deterioration detection device according to the present invention.

FIG. 2 is an overall circuit configuration diagram of a deterioration detection device according to an embodiment of the present invention.

FIG. 3 is an operation flowchart of the deterioration detecting device according to the embodiment of the present invention.

FIG. 4 is a vector diagram for explaining balance adjustment of the deterioration detecting device according to the embodiment of the present invention.

FIG. 5 is another circuit configuration diagram of the variable impedance of the deterioration detecting device according to the embodiment of the present invention.

FIG. 6 is a schematic circuit configuration diagram of an insulation resistance measuring device as a conventional deterioration detecting device.

[Explanation of symbols]

 1 power supply section 2 bridge circuit 3 operational amplifier 4, 40 lock-in amplifier 5, 50 arithmetic processing section 21 standard capacitor 22 measurement sample 23 variable impedance 24 fixed resistance 25 switch Rv correction variable resistance Cv correction variable capacitance Rd detection resistance Csh ground Capacitance

Claims (5)

[Claims] 1. A connection middle point in which a standard capacitor and a variable impedance are connected in series is used as a first output terminal, and a connection middle point in which a measurement sample and a resistor are connected in series is used as a second output terminal. A bridge circuit formed by connecting a series circuit in parallel to an AC power source, a first detection signal output from the first output terminal, and a second detection signal output from the second output terminal. Voltage vector detection means for detecting the vector value of each voltage of the signal, and adjusting and controlling the impedance value of the variable impedance based on the first and second detection signals of the voltage vector value, and connecting the resistor. Determining the characteristic deterioration of the measurement sample from each value of the standard capacitor, the variable impedance and the resistance of the bridge circuit which is adjusted and controlled, and which has an arithmetic processing unit for controlling. Deterioration detecting apparatus according to claim. 2. The deterioration detecting device according to claim 1, further comprising: an intermittent means connected in parallel with a fixed resistor of the bridge circuit, wherein the arithmetic processing section puts the intermittent means into a closed state to provide a first output terminal. The voltage level value of the first detection signal output from the voltage vector detection means is input from the voltage vector detection means, and the connection / disconnection means is opened to output the first and second output terminals.
And the voltage vector value of the difference between the second detection signals is input from the voltage vector detection signal to calculate the deviation vector value of each voltage vector value, and the deviation voltage vector value and the voltage vector value of the first detection signal. And a voltage vector value of the second detection signal is calculated from the impedance value of the variable impedance so that the calculated voltage vector value of the second detection signal matches the voltage vector value of the first detection signal. A deterioration detecting device characterized by adjusting and controlling. 3. The deterioration detecting device according to claim 1, wherein a capacitance to ground is connected in parallel to a connection midpoint of each series circuit of the bridge circuit. Detection device. 4. The deterioration detecting device according to claim 1, wherein the variable impedance of the bridge circuit is formed by connecting a variable resistor and a variable capacitance in parallel. . 5. The deterioration detecting device according to claim 1, wherein the deterioration detecting device is connected between the first and second output terminals of the bridge circuit and the voltage vector detecting means, and the output terminals are connected to each other. A deterioration detecting apparatus comprising a differential amplifier for calculating a deviation between each of the first and second detection signals output from the deterioration detecting apparatus.