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The invention relates to a digital device for measurement of resistive leakage current of Zinc-oxide (ZnO) arresters extensively used in electric power network of almost all voltage classes to protect equipment against over voltages. When gapless ZnO arrester is in operation, there is steady state current in the ZnO element column.This steady state current comprises of two parts,namely capacitive current and resistive current. The capacitive current influences the voltage distribution along the multi-unit arrester column under normal system voltage applied to it. The resistive current which is nonlinear, depends upon the much important V - I characteristics, and its magnitude is governed by the applied voltage and the temperature of the ZnO element. Indirectly, it is a measure of the power dissipation in the element and its temperature. Thus, the measurement of resistive current is essential in characterising the ZnO element with respect to voltage and temperature in the laboratory or at the manufacturers premises. It is also important to measure the resistive current of an arrester in service in a substation to determine its healthyness, as the deterioration in the V-I characteristics of ZnO arrester is associated with change in resistive current and steady state power loss.
Presently there are various methods available to measure the resistive leakage current component of ZnO arrester. Two important and widely used methods are (1) differential amplifier, and (2) attenuator compensation technique.
Differential amplifier technique employs an additional lossless capacitor of suitable voltage rating in parallel with the arrester. The difference between the current in the arrester circuit and the parallel capacitor circuit obtained by a differential amplifier will give resistive current. This method is claimed to be most accurate.
There are disadvantages associated with the differential amplifier method. With the increasing voltage rating for the arrester, there are corresponding increase in the voltage rating
of the lossless capacitor. Even though such a capacitor c be employed in the laboratory, it would be almost impossible in the field. Thus, it is not possible to use this method for monitoring the arrester in service.
The attenuator compensation method involves connection of a parallel combination of a resistance and a capacitance in the measuring circuit. The resistance is a variable one and it is required to vary this resistance to get an appropriate voltage across it to determine the resistive current.
There are disadvantages associated with this method of the attenuator compensation. It is difficult to have such a variable shunt resistor in series with a arrester in service, in particular, for EHV arresters. Thus, not suitable for field usage.
In order to overcome these problems some manufacturers market field-grade leakage current analysers for ZnO arresters. These employ high frequency, low voltage, bar primary current transformers for sensing leakage current at the ground end of the arresters and extract only the third harmonic component of the total leakage current.
There are disadvantages also with the field-grade leakage current analysers for ZnO arresters. Though, it may be possible to assess the health of the arrester in service, it will not be effective in determining the peak value and shape of the resistive current for characterising the arrester and to investigate increased power loss problem accurately.
Therefore the main object of the present invention is to provide a simple and effective device for measurement of resistive leakage current which does not employ any compensation network for the realization of resistive current.
According to the present invention there is provided a digital device for measurement of resistive leakage current of zinc-oxide arresters comprising a surge arrester connected
to the test voltage, the said surge arrester is connected at the other end with a current transducer along with a strobe pulse generating capacitive probe and then to an analog to digital converter through an isolation amplifier and interactively connected to a personal computer and a printer.
The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to a non-limiting exemplary embodiments of the invention represented in the accompanying drawings.
Fig.l - Schematic diagram of the device :
Fig.2 - Flow chart of the sequence of operation of the
device as processed in the personal computer : Fig.3 - Measured resistive current waveforms of ZnO
Arrester element stack at 27.0 kV voltage across
it : Fig.4 - Reconstructed resistive current waveforms of ZnO
arrester element stack at 27.0 kV voltage across
it.
In accordance with the present invention the device uses Fourier series expansion for the determination of the in-phase and quadrature harmonic components in the ZnO arrester total internal leakage current. In-phase fundamental and harmonic components of the current are due to the element resistance and quadrature components are due to the element capacitance. These are nothing but the coefficients of the sine and cosine functions in the Fourier series expansion of complex waveform of the total current. The expressions used for the determination of the coefficients are :
Where,
m = number of data points in one cycle of the total current
n=order of the harmonic (1 to 50), k = 1 m; the first data
point I, of the total current always correspond to the positive zero crossover of the voltage applied to the arrester.
The resistive current (i ) waveform only is reconstructed using the coefficients of the sine function (B) with the help of the following expression:
Figure 1 shows the device for implementing the new technique for measuring the total leakage current of a zinc oxide arrester and extraction of its resistive component. The specification of the components of the measurement circuit are as follows:
The surge arrester (SA) input is connected to a test voltage (V) source and output connected to the current transducer (C).
The current transducer (C) alongwith a strobe-pulse generating capacitive probe (CP) including the isolation amplifier (I) have single shot analog bandwidth of notless than 2.5 kHz. The specification of analog to digital (A/D) converter card is 12 bit, 250 kS/s multiple channel sampling rate and has 8 differential channels which can be interfaced with a personal computer (PC) having PCMCIA slot. The analog to dgital (a/D) card is controlled by the personal computer (PC) which is a Pentium processor based note book computer with a printer
(p).
The new measurement techniques thus involves the following steps:
The voltage drop across the current transducer (in series
iwith the arrester as shown in Figure 1) due to the total internal current of the arrester is digitized at a rate of not less than 500 samples in 20 ms (in one power frequency voltage cycle), the first data point being the positive going zero crossover of the voltage applied to the arrester.
Fourier series expansion is performed on this current and resistive component is reconstructed as described earlier. The flow chart of the computer program of the process of measurement of the resistive leakage current using this new technique called "RES-CUR" is given in Figure 2.
Figure 3 shows the resistive current waveforms obtained by measurement using conventional compensation technique as well as the waveforms obtained by the new technique. The peak magnitudes are in agreement showing the validity of the new technique.
Fig.3 indicates total current (1), compensation current (2) and the resistive current (3).
Fig.4 shows the re-constructed current with the present device. The peak value is 6.7 mA.
The device provides the manufacturer of ZnO arresters and testing laboratories in accurately measuring the resistive current to determine their V - 1 characteristics and power loss. Secondly and most importantly,accurate measurement of true resistive current ZnO arrester in service becomes a definite possibility enabling the power generation and transmission utilities to monitor the health of ZnO arresters in service effectively for diagnostic purposes.
The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims.
We CLAIMS
1. A digital device for measurement of resistive leakage current in a zinc oxide surge arrester (SA) comprising:
~ a source for test voltage (V) connected to an input of the surge arrester (SA);
a current transducer (C) with a capacitor prove (CP) tor generating strobe pubises; and
- an analog to digital converter (A/D) controlled by a processor (PC) for digitizing the received signal corresponding to the voltage drop across current transducer (C) due to the total internal current of said surge arrestor (SA) , and for deriving therefrom the resistive component of the leakage current.
2. The device as claimed in claim 1, wherein the current transducer (C) is provided with an isolation amplifier (1) have single short analog bandwidth of not less than 2.5 kHz.
3. The device as claimed in claim 1 wherein the analog-to-digital converter
4. The device as claimed in claim 1 wherein the analog-to-
digital converter (A/D) is capable of digitizing at a rate of not
less than 500 samples in 20 ms.
5. A digital device for measurement of resistive leakage
current of Zinc-Oxide arrester substantially as herein described
and illustrated.
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