DEVICE AND METHOD FOR DETECTING A SURGE ARRESTER RESISTIVE LEAKAGE CURRENT TO REDUCE COMPUTATIONAL LOAD

The present invention relates to a device and method for detecting a surge arrester resistive leakage current for reducing the computational load, which may significantly reduce the computational load as the Fourier transform process need not be repeated in the process of searching to match a section to be Fourier transformed to a section in-phase with AC voltage to extract the resistive leakage current component from the Fourier series of surge arrester leakage current and enables real-time detection of the resistive leakage current by shortening the section searching process of one-period leakage current by searching for the section of one-period leakage current, which is to be initially Fourier-transformed, based on the characteristic pattern that it is repeatedly generated by application of an AC voltage. The present invention is implemented by a leakage current detection unit 10 detecting a leakage current in a surge arrester 1, a reference point search unit 20 of selecting a time when the leakage current has a highest left-right symmetry, as a reference point, a Fourier transform unit 30 obtaining a Fourier series for the one-period leakage current starting at the reference point, and a resistive leakage current extraction unit 40 correcting the reference point so that a characteristic pattern of the surge arrester resistive leakage current is shown in the sum the sine terms of the Fourier series.

TECHNICAL FIELD

The present invention relates to a device and method for detecting a surge arrester resistive leakage current for reducing the computational load, which may significantly reduce the computational load as the Fourier transform process need not be repeated in the process of searching to match a section to be Fourier transformed to a section in-phase with AC voltage to extract the resistive leakage current component from the Fourier series of surge arrester leakage current and enables real-time detection of the resistive leakage current by shortening the section searching process of one-period leakage current by searching for the section of one-period leakage current, which is to be initially Fourier-transformed, based on the characteristic pattern that it is repeatedly generated by application of an AC voltage.

BACKGROUND ART

Surge arresters are installed to protect electric devices, such as transformers and circuit breakers, connected to the power system from abnormal voltages due to lightning strikes and switching surges. Recently, metal-oxide surge arresters (MOSA) are widely used.

The metal-oxide surge arrester is modeled as a circuit in which a non-linear resistance is connected in parallel to a capacitance, and the leakage current flowing when a grid voltage is applied may be interpreted as a composite current of the resistive leakage current through the non-linear resistance and the capacitive leakage current through the capacitance.

When the surge arrester deteriorates, the capacitance is almost constant and, thus, the capacitive leakage current hardly fluctuates. With the progress of the deterioration, however, the non-linear resistance gradually decreases and the resistive leakage current increases significantly. Therefore, a most preferable way to accurately determine the deterioration state of the surge arrester is to extract the resistive leakage current from the leakage current flowing through the surge arrester and determine the deterioration state.

According to Korean Patent No. 10-2068028, by the inventors of the present invention, it is possible to extract the resistive leakage current by detecting only the surge arrester leakage current without detecting the voltage, using the fact that the characteristic patterns of the surge arrester leakage current and the resistive leakage current are shown before and after the time when the voltage is 0 V, and the fact that the resistive leakage current component may be obtained by extracting the sine term from the Fourier series of the leakage current of the surge arrester for one period that starts at the time when the voltage is 0 V.

According to Korean Patent No. 10-2068028, since the symmetry of the surge arrester leakage current is highest at the time when the voltage is 0 V, the resistive leakage current may be accurately extracted by selecting the time when the symmetry is the highest as a reference point and then stepwise correcting the reference point until the composite component of the sine term extracted from the Fourier series of the one-period surge arrester leakage current starting at the reference point shows the characteristic pattern of the resistive leakage current.

However, since the technology disclosed in Korean Patent No. 10-2068028 requires a new Fourier series to be obtained each time the reference point is corrected, digital computational processing is burdened. Moreover, even high-order components of the surge arrester leakage current need to be detected and, to accurately extract the resistive leakage current from the surge arrester leakage current, the reference point is required to be accurately corrected sample-by-sample. To this end, the sampling rate needs to be increased, and the number of samples of one period increases accordingly. Further, the memory and computational load for obtaining the Fourier series increases. The increase in memory and computational load renders it difficult to detect the resistive leakage current in real time and diagnose degradation.

Further, since the technology disclosed in Korean Patent No. 10-2068028 selects a reference point according to the symmetry of the leakage current of the surge arrester into which noise has been introduced, a time when the voltage is significantly deviated from the time when the voltage is 0 V may be selected as the reference point, and the number of times of the correction may be thus increased, and more loads may be posed on the computation.

PRIOR TECHNICAL DOCUMENTS

Patent Documents

DETAILED DESCRIPTION OF THE INVENTION

Technical Problem

Thus, the present invention aims to provide a device and method for detecting a surge arrester resistive leakage current, which may significantly reduce the computational load in the digital area by estimating as close to the time when the voltage is 0 V a time as possible upon estimating the time when the voltage is 0 V according to the pattern of the detected surge arrester leakage current and, upon correcting the period which is subject to a Fourier-transform until the characteristic pattern of the resistive leakage current is shown in the sine term of the Fourier series of the surge arrester leakage current, calculating the Fourier series without performing a Fourier-transform.

Technical Solution

To achieve the above objects, according to the present invention, a method for detecting a surge arrester resistive leakage current comprises a leakage current detection step S10of detecting a total leakage current ITflowing through a surge arrester1to which a periodic AC voltage is applied, as a digital data value, a reference point search step S20of selecting a time when the total leakage current IThas a highest left-right symmetry, as a reference point, a Fourier transform step S30of obtaining a Fourier series by Fourier-transforming the total leakage current ITduring one period starting at the reference point, a reference point verification step S40of verifying the reference point depending on whether a characteristic pattern of a resistive leakage current IRaccording to a non-linear resistive characteristic of the surge arrester1is shown in a sum of sine terms of the Fourier series, a reference point correction step S50of reperforming the reference point verification step S40by correcting the reference point, if the characteristic pattern is not shown in the reference point verification step S40, and obtaining a Fourier series of the one-period total leakage current ITvarying according to reference point correction from an equation resultant from reference point time-shifting a Fourier series before the reference point correction, and a resistive leakage current extracting step S60of setting the sum of the sine terms of the Fourier series as the resistive leakage current IRif the characteristic pattern is shown in the reference point verification step S40.

According to an embodiment, the reference point correction step S50sets a sine term coefficient of the Fourier series according to the reference point time shift as

and a cosine term coefficient as

wherein m is an order of the Fourier coefficient, amis an mth-order cosine term Fourier coefficient of the Fourier series before time shifting, bmis an m-order sine term Fourier coefficient of the Fourier series before time shifting, N is a number of samples in one period, and Δn is a sample interval resultant from time-shifting the reference point.

To achieve the above objects, according to the present invention, a device for detecting a surge arrester resistive leakage current comprises a leakage current detection unit10sampling a total leakage current ITflowing through a surge arrester1to which a periodic AC voltage is applied, and detecting a digital data value, a reference point search unit20selecting a time when the total leakage current IThas a highest left-right symmetry, as a reference point, a Fourier transform unit30obtaining a Fourier series by Fourier-transforming the total leakage current ITduring one period starting at the reference point, and a resistive leakage current extraction unit40correcting the reference point until a characteristic pattern of a resistive leakage current IRaccording to a non-linear resistive characteristic of the surge arrester1is shown in a sum of sine terms of the Fourier series, obtaining the Fourier series of the one-period total leakage current ITvarying according to the reference point correction from a Fourier series before the reference point correction, and setting the sum of the sine terms of the Fourier series, where the characteristic pattern of the resistive leakage current IRis shown, as the resistive leakage current IR.

Advantageous Effects

As configured as above, the present invention may perform a Fourier-transform only once during the course of modifying the one-period surge arrester leakage current period where development as a Fourier series is to be performed to obtain the resistive leakage current using the Fourier series of the surge arrester leakage current and obtain it from the equation resultant from time-shifting the Fourier series according to the period modification. Thus, as compared with the method of repeating a Fourier-transform according to the period modification, the present invention may significantly reduce computational load and may thus enable real-time detection of the resistive leakage current and diagnosis of the deterioration state of the surge arrester.

According to an embodiment of the present invention, even upon detecting the period of the one-period surge arrester leakage current to be Fourier-transformed, a result of obtaining at a plurality of times based on periodicity may be reflected. Thus, the period modification process may be reduced, and the computational load may be further decreased.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention are described with reference to the accompanying drawings to be easily practiced by one of ordinary skill in the art.

When determined to make the subject matter of the present invention unnecessarily unclear, a detailed description of the known functions or known configuration as disclosed in Korean Patent No. 10-2068028 may be skipped.

Referring to the block diagram shown inFIG.1and the flowchart shown inFIG.2, a device for detecting a surge arrester resistive leakage current, according to an embodiment of the present invention, includes a leakage current detection unit10for performing a leakage current detection step S10, a reference point search unit20for performing a reference point search step (S20), a Fourier transform unit30for performing a Fourier transform step (S30), and a resistive leakage current extraction unit40for performing a reference point verification step (S40), a reference point correction step (S50), and a resistive leakage current extracting step (S60).

The leakage current detection unit10detects the current flowing through a surge arrester1connected between a grid bus2of a power system and a ground line3as a digital data value and performs the leakage current detection step (S10). To this end, the leakage current detection unit10may include a current transformer11that transforms the current flowing through the surge arrester1to the ground line3as the grid voltage u, which is a periodic alternating current (AC) voltage, is applied through the grid bus2, into a current within a specific range and outputs the transformed current and an A/D converter12that converts the analog current signal output from the current transformer11into digital data by sampling the analog current signal at a predetermined sample period.

The surge arrester1may be equivalent to a circuit in which a capacitance C and a non-linear resistance R are connected in parallel, and may be, e.g., a metal-oxide surge arrester (MOSA). Accordingly, the current flowing through the surge arrester1and detectable by the leakage current detection unit10may be regarded as a composite current of a capacitive leakage current ICflowing through the capacitance C and a resistive leakage current IRflowing through the non-linear resistance R, based on a modeling of the surge arrester1. In the following description, the composite current of the capacitive leakage current ICand the resistive leakage current IRis referred to as a total leakage current IT.

The grid voltage u is a periodic AC voltage having a grid frequency of 60 Hz in the case of Korea, and when applied to the surge arrester1, the leakage current illustrated inFIG.3may flow.

FIG.3is a graph illustrating a waveform of the one-period total leakage current ITdetected from the surge arrester1, a waveform of the capacitive leakage current ICincluded in the one-period total leakage current IT, and the resistive leakage current IRincluded in the one-period total leakage current IT. As can be seen inFIG.3, the resistive leakage current IRis a current in phase with the grid voltage u, and the capacitive leakage current ICis a leading current whose phase is 90 degrees ahead of the grid voltage u.

The resistive leakage current IRis less than a current ΔIth, which is tiny enough to be neglectable, in the period where the grid voltage u is a predetermined value or less, due to the characteristics of the non-linear resistance R and, when the grid voltage u exceeds the predetermined value, the resistive leakage current IRrapidly increases according to the magnitude of the grid voltage u. Accordingly, the total leakage current ITshows a characteristic pattern in which symmetry is the highest at the times (t0 and t1) when the grid voltage u is 0 V, and the resistive leakage current IRshows a characteristic pattern in which it becomes below the tiny current ΔIthin the periods before and after the times (t0 and t1) when the grid voltage u is 0 V.

The surge arrester1has a low limiting voltage and excellent discharge characteristics due to the characteristics of the non-linear resistance R. However, as the surge arrester1deteriorates, the resistive leakage current gradually increases and the period, in which the tiny current ΔIthor less, flows narrows, it may lose its performance. However, as the characteristics of the capacitive load C do not change significantly, it is necessary to diagnose the deterioration by detecting the resistive leakage current IR.

To that end the reference point search unit20, the Fourier transform unit30, and the resistive leakage current extraction unit40search the time when the grid voltage u is 0 V using the characteristic pattern of the resistive leakage current IRand the characteristic pattern of the total leakage current ITshown in the periods before and after the time when the voltage is 0 V like disclosed in Korean Patent No. 10-2068028, sets a time when the grid voltage u is 0 V as a start reference point of the period of a one-period total leakage current ITto be developed as Fourier series, and extracts the resistive leakage current IRfrom the Fourier series of the total leakage current IT.

However, according to the present invention, the excessive computational load, which is a problem with Korean Patent No. 10-2068028, is greatly reduced. To that end, the computational load is significantly reduced by raising the accuracy of searching for a reference point according to the characteristic pattern of the total leakage current ITto thereby correct the reference point, reducing the number of numbers in which the process of obtaining the Fourier series is performed, and calculating the coefficients of the Fourier series that fluctuate according to the correction of the reference point.

The following description focuses primarily on technical features distinct from those disclosed in Korean Patent No. 10-2068028, with the known art in Korean Patent No. 10-2068028 excluded from the description.

The reference point search unit20selects a reference point n0according to the pattern of the total leakage current ITdetected as digital data via the leakage current detection unit10, as shown in the waveform diagram ofFIG.4and performs the reference point search step S20.

The waveform diagram ofFIG.4is a waveform of the total leakage current ITdetected using 3,600 samples (the number N of samples is 3,600) during one period.

The reference point n0is a time when the grid voltage u is estimated to be 0 V and is selected by searching for a time when the total leakage current IThas the highest left-right symmetry and, as described below, the reference point n0is applied as a start point of the one-period total leakage current ITwhich is to be developed as a Fourier series.

The reference point n0may be searched and selected by one of the following three methods.

In a first method, a time when the one-period total leakage current IThas the highest left-right symmetry in a half-period period having a positive (+) value may be searched and selected as the reference point n0. The left-right symmetry may be evaluated according to the degree of symmetry calculated for the data of a predetermined prior-subsequent period width (Δn) as described in Korean Patent No. 10-2068028. Given the width of the period in which a current not more than the tiny current ΔIthbefore and after the time when the grid voltage u is 0 V and the possibility of fluctuation of the period width due to deterioration, the prior-subsequent period width (Δn) may be predetermined as a proper value to prevent it from being erroneously detected as the time corresponding to the peak value of the resistive leakage current IR.

Of course, a time when the highest left-right symmetry is shown in a half-period period having a negative (−) value may be searched and selected as the reference point n1.

In a second method, any one reference point may be corrected and selected according to the size of the period between the reference point n0searched in the positive (+) period of the one-period total leakage current ITand the reference point n1searched in the negative (−) period.

As shown inFIG.4, there is supposed to be a half-period (n) difference between the positive (+) period reference point n0and the negative (−) period reference point n1, but may not be due to influence by noise introduced upon detection, detection errors, and A/D conversion resolution. Thus, the reference point is corrected depending on the difference between the half period (n) and the interval between the two reference points n0and n1. The search time may be reduced by searching for one of the two reference points n0and n1and then searching the periods before and after the time when the half-period difference is made, for the other reference point.

A specific example for selecting a reference point may be to time-shift, by half period (n), one of the two reference points n0and n1towards the other reference point and then select the average value as a reference point.

In a third method, the reference point selected by the above-described first or second method may be corrected according to the reference point of the prior one-period total leakage current IT.

To that end, the reference point search unit20may include a correction unit21that memorizes the reference point applied for the prior one-period total leakage current ITand searches and selects a reference point for the current one-period total leakage current IT.

For example, the reference point applied when the resistive leakage current IRis extracted by the resistive leakage current extraction unit40is stored as described below.

The reference point search unit20searches for the reference point nzfor the current period (S21), identifies whether the reference point n0for the prior period is stored (S22) and, if the reference point n0for the prior period is stored, activates the correction unit21to correct the reference point n2(S23).

The correction unit21corrects the reference point n2for the current period depending on the difference between the searched reference point nzfor the current period and the reference point n0for the prior period, minus one period (2n). As an example correction method, the reference point n0for the prior period is time-shifted by one period (2n) towards the current period, and then, the average of the time-shifted reference point and the reference point n2searched in the current period may be selected.

Meanwhile, upon searching for the reference point nzin the total leakage current ITfor the current period, a predetermined period from a time which is one period (2n) after the reference point n0applied and stored in the prior period is set as a search period, and the reference point n2is searched in the search period.

The one-period total leakage current ITwhich starts from the reference point is varied by the reference point modification or correction. However, as described in Korean Patent No. 10-2068028, an rearranging method may be applied which attaches the prior period to the subsequent period, or the subsequent period to the prior period, for the detected one-period total leakage current IT.

The Fourier transform unit30performs the Fourier transform step S30to Fourier-transform the one-period total leakage current IT, which starts at the reference point selected by the reference point search unit20, to thereby develop it as a Fourier series as shown in Equation 1 below.

Here, N is the number of samples of the one-period total leakage current ITand, according to the waveform diagram illustrated inFIG.4, N is set to 3600. n is the sample number of the total leakage current IT, and m is the order. In general, since the harmonics mixed with the grid voltage u are odd-numbered ones and only up to the ninth harmonic may be considered, as the order expressed by m, only the 1st, 3rd, 5th, 7th, and 9th order including the 1st order of the fundamental wave may be considered.

amis the Fourier coefficient of the mth-order cosine term, and bmis the Fourier coefficient of the mth-order sine term, and are obtained by Equation 2 below.

The resistive leakage current extraction unit40includes a verification unit41that verifies the accuracy of the reference point according to the pattern of the signal composed of the sum of the sine terms in the Fourier series of Equation 1 to thereby perform the reference point verification step S40and a correction unit42that, when the reference point is determined to be incorrect as a result of the verification, corrects the reference point and then allows it to be verified again to thereby perform the reference point correction step S50. The resistive leakage current extraction unit40performs the resistive leakage current extracting step S60that definitely determines that the signal resultant from extracting only sine terms from the Fourier series of the one-period total leakage current IT, which starts at the finally corrected reference point only when the reference point is determined to be accurate as a result of the verification, and summating the extracted sine terms, is the resistive leakage current IR.

The reference point verification step S40by the verification unit41includes extracting the signal composed of the sine terms from the Fourier series of Equation 1 using Equation 3 below (S41) and identifying whether the characteristic pattern of the resistive leakage current IRis shown (S42).

However, as described in Korean Patent No. 10-2068028, the sum of the sine terms expressed by Equation 3 in the Fourier series differs according to the selection position of the reference point as shown inFIG.5.

InFIG.5, IR,ais the waveform of the sum of the sine terms obtained when the reference point is selected as the time when the applied voltage u is 0 V and, in the initial period (0 to nth) starting at the reference point, such a characteristic pattern is shown where a current below a neglectable tiny current ΔIthflows. IR,bis the waveform when a time later than the time when the applied voltage u is 0 V is selected as the reference point, and a section when the current is less than 0 A occurs in the initial period 0 to nth. IR,cis the waveform when a time earlier than the time when the applied voltage u is 0 V is selected as the reference point, and a section when the current is more than the neglectable ΔIthoccurs in the initial period 0 to nth.

The accuracy of the selected reference point is verified depending on whether the characteristic pattern of the resistive leakage current IR, which has a value not more than the tiny current ΔIthin the initial period 0 to nth is shown in the sum of the sine terms of the Fourier series.

A proper length of the initial period 0 to nth may be determined considering the length of the period when the resistive leakage current IRof the surge arrester1, which has not be deteriorated, flows in the magnitude not more than the tiny current ΔIth, as described above in connection withFIG.3, and may then be used upon verification. For example, the proper length of the initial period 0 to nth may be determined to be a range from 0 to n/6.

Meanwhile, as detailed in Korean Patent No. 10-2068028 and shown inFIG.5, a plurality of times in the initial period 0 to nth are set and, for verification, the value at the corresponding time may be compared with the tiny current ΔIth.

The reference point correction step S50by the correction unit42is performed when the characteristic pattern of the resistive leakage current IRis not shown in the sum of the sine terms of the Fourier series as a result of the verification, and corrects the reference point (S51), obtains the Fourier series of the one-period total leakage current ITwhich varies according to the correction of the reference point (S52), and allows the reference point verification step S40by the verification unit41to be performed again. In other words, the reference point is repeatedly corrected until the characteristic pattern of the resistive leakage current IRis shown in the sum of the sine terms of the Fourier series, and the Fourier coefficients of the Fourier series are modified accordingly.

Here, the method described in Korean Patent No. 10-2068028 may be used to correct the reference point. That is, if a reference point is selected in the positive (+) period, when the current is less than 0 A at, at least any one, of a plurality of times in the initial period 0 to nth, the reference point is stepwise time-shifted to be brought forward until the current is 0 A or more at all of the plurality of times in the initial period 0 to nth and, when the current exceeds the tiny current ΔIthat all of the plurality of times in the initial period 0 to nth, the reference point is stepwise time-shifted to be put back until the current is 0 A or less at, at least any one, of the plurality of times in the initial period 0 to nth.

In contrast, if a reference point is selected in the negative (−) period, when the current is 0 A or more at, at least any one, of a plurality of times in the initial period 0 to nth, the reference point is stepwise time-shifted to be brought forward until the current is 0 A or less at all of the plurality of times in the initial period 0 to nth and, when the current is less than the tiny current ΔIthat all of the plurality of times in the initial period 0 to nth, the reference point is stepwise time-shifted to be put back until the current is 0 A or more at, at least any one, of the plurality of times in the initial period 0 to nth. Of course, it is preferable to time-shift at each sample interval.

According to Korean Patent No. 10-2068028, Fourier coefficients are calculated by again Fourier-transforming the one-period total leakage current ITthat varies according to the correction of the reference point.

However, according to the present invention, the Fourier coefficients that fluctuate according to the correction of the reference point are calculated by applying the addition theorem of the trigonometric function in an equation resultant from reference point time-shifting the Fourier series before the correction of the reference point correction.

The Fourier coefficients according to the reference point time-shifting may be calculated as follows.

The Fourier series resultant from time-shifting the reference point by Δn may be expressed as in Equation 4 below by time-shifting Equation 1, which is expressed as the Fourier coefficient before time-shifting the reference point.

As mentioned above, n is the sample number in order, N is the total number of the samples in one period, and m is the order. However, n is the sample number assigned from the time-shifted reference point.

Referring to Equation 4, the cosine term and the sine term on the right side each are time-shifted and are thus expressed as the terms whose phase has been varied by

and may thus be arranged according to the addition theorem of trigonometric functions as shown in Equation 5 below.

Referring to Equation 5, the cosine term and sine term including the phase angle

according to the time-shifting may be expressed as two trigonometric functions using the trigonometric function of the phase angle as a coefficient value according to the trigonometric addition theorem.

The terms may be arranged in such a way as to sum up the terms having the same trigonometric function and may thus be expressed as the Fourier series equation as shown in Equation 6 below.

In Equation 6, amis the Fourier coefficient of the mth-order cosine term of the Fourier series before time shifting the reference point, and bmis the Fourier coefficient of the mth-order sine term of the Fourier series before time shifting the reference point. Amis the Fourier coefficient of the mth-order cosine term of the Fourier series after time shifting the reference point, and Bmis the Fourier coefficient of the mth-order sine term of the Fourier series after time shifting the reference point.

Referring to Equation 6, the number N of the samples for one period is a value determined by the leakage current detection unit10, if only time shift Δn is set to a predetermined value,

in the equation of calculating the Fourier coefficients Am and Bm after the reference point is time-shifted, except for the Fourier coefficients amand bmbefore the reference point is time-shifted become fixed values for each order. Thus, the values of

calculated with the absolute value of Δn set to ‘1’ may be previously stored and, upon calculating the Fourier series according to reference point correction, be used.

In other words, the Fourier transform unit30performs the Fourier transform process only once, and the Fourier coefficient obtained by the Fourier transform is updated by Equation 6 according to the reference point correction, obtaining the Fourier coefficient according to the correction of the reference point. Thus, computational load may be significantly reduced.

As such, if the characteristic pattern of the resistive leakage current IRis shown in the sum of the sine terms of the Fourier series while performing the reference point verification step S40according to the Fourier coefficient updated as the reference point is corrected, the sum of the sine terms of the Fourier series finally updated is determined to be the resistive leakage current IR(S60).

Of course, to obtain the resistive leakage current IRin the total leakage current ITin the next period, the process is restarted from the leakage current detection step S10, so that the resistive leakage current IRmay be continuously obtained per period. Further, since the resistive leakage current IRcontinuously obtained may be expressed as the Fourier coefficient of the sine term, only the Fourier coefficients of the sine terms are stored, and they may be continuously obtained. To obtain information on the capacitive leakage current IC, the Fourier coefficients of the cosine terms may be stored together.

Meanwhile, the reference point finally corrected for each period is transmitted to the reference point search unit20so that the reference point to be stored and used in the correction unit21is updated. Accordingly, as described above, the correction unit21may correct the reference point searched for in the current one-period total leakage current ITaccording to the corrected reference point for the previous one period.

While the inventive concept has been shown and described with reference to exemplary embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes in form and detail may be made thereto without departing from the spirit and scope of the inventive concept as defined by the following claims. Therefore, such modifications should be regarded as belonging to the scope of the present invention, and the scope of the present invention should be determined by the claims to be described later.

DESCRIPTION OF SYMBOLS