Radio Interference Due To High Voltage Insulator String
The local electric breakdown of air or the corona is quite common on the high voltage power transmission line hardware. The operating stress is ideally lower than the corona inception levels, however, due to some manufacturing defects, damages caused during the transportation and installation, deposition of contaminants like dust particles or water droplets etc. the local field can get significantly intensified. As a result, the corona can occur on line conductors, nuts and bolts of the hardware, arcing horns, guard rings, suspension clamps, etc. Also, since the conductors and tie-wires with the tops of the insulator; and the pins with the entire surface of the thread in the pin holes, do not make perfect electrical contacts, corona may occur in the intervening air gaps.
Radio noise in High Voltage transmission line is associated with the pulsating modes of corona discharges developing at the line conductor and hardware, sparking resulting from poor electrical contact and scintillations on the contaminated insulator surfaces. The current pulse associated with individual corona discharges typically possess a rise time measurable in 10s of ns, which is followed by a slow tail measured in 100s of ns. A several discharges are produced in every half cycle of the power frequency voltage and there could be several sites producing corona and the noise generated has considerably wide frequency spectrum. The RI level is high in the broadcast frequency range (0.5-1.6MHz) and then decreases gradually at higher frequencies. Detailed studies of corona current characteristics have shown that positive corona is the main source of radio noise from transmission line.
The Radio Interference (RI) from electric power transmission line hardware, if not controlled, poses serious electromagnetic interference to system in the vicinity Also, in future, if the transmission lines are to be employed for general communications, it becomes imperative to limit the corona generated electromagnetic noise .With regard to the transmission lines, the sources of RI are both line conductors and the line hardware including the insulator strings . The present work mainly concerns with the insulator string along with the associated line hardware.
The existing standards have two tests pertaining to RI and corona. First one involves measurement of conducted RI through suitable circuit configuration and a radio noise meter. The second one involves identification of onset a visual corona, which is relatively subjective.
Hence, governing standards have prefixed upper limits for radio interference levels from different components of high voltage transmission lines. For convenience, the laboratory testing for the RI levels are carried out through the measurement of the conducted radio interference levels.
The RI measurement does not really locate the coronating point, as well as, the modes of corona. At the same time experience shows that it is rather difficult to locate the coronating points by mere inspection. The associated geometry involves both highly localized field intensification points, as well as, relatively extended moderate field intensification points. This in turn leads to both point corona and a diffuse corona to start with, which later transform into Details of experimental investigations Experimental arrangement commonly used test circuits for measuring radio interference are those recommended by IEC and NEMA. For the present work the IEC circuit shown in Figure 1 is employed.
The main components of the circuit are high voltage source (50 Hz, 150 kV, 300 kVA transformer with primary voltage of 230/440 V and with a rated continuous current of 2A), low pass filter which can be tuned to the required frequency, high voltage bus end terminations, coupling capacitor (0.00161 realised by two units of 0.00322 F of GE make connected in series), measuring impedance radio noise meter type SMV 11, VEB Messelektronik Berlin make is used for the measurements. The input voltage to the transformer is 400 V two phase ac. The testing arrangement is so designed to be simpler for operation and all the necessary precautions have been incorporated for proper safety and protection with essential tripping arrangements. The test object consisted of 9-disc insulators(132kV system) and the test voltage was 85kV
The International Standard specifies the procedure for a radio interference (RI) test carried out in a laboratory on clean and dry insulators at a frequency of 0,5 MHz or 1 MHz or, alternatively, at other frequencies between 0.5 MHz and 2 MHz. The frequencies of 0.5 or 1 Mhz are preferred because, usually the level of radio noise at this part of the spectrum and also because 1 MHz lies between the low and medium frequency radio broadcast bands. As per the standard , the measuring apparatus, as per the specification of CISPR 16-1, has been currently used for the RI characteristics of insulators.
Figure: 5.15. RIV measuring circuit as per IEC augmented with ground end current measurement
The voltage is gradually applied in steps, to reach a value of 90 kV (15% above phase voltage), held for at least five minutes, to allow RIV phenomenon to stabilize. Then, voltage is reduced slowly in steps. The radio noise generated by the insulator string is observed. Three such cycles are repeated, and RIV in dB (above 1 mV) at different voltages is recorded for four insulator strings. The experiments were repeated at least five times to check for repeatability.
The corona current in principle is measurable at two ends of the string i.e. from high voltage end and from ground end. Of course, for very accurate measurements, optical link between measuring system and the oscilloscope would be essential. At present, due to the non-availability of such a system at our laboratory, conventional method is only employed. The current is indirectly sensed by measuring the voltage across a 50 Ω resistor connected at the ground end. It applies to both the ground end lead of insulator string, as well as, the input to RIV.meter as indicated in the figure. However, for safety purpose, in the ground end lead of insulator string a high resistance 5 kΩ is also inserted. However, before proceeding further on measurement, the following needs to be discussed.
The corona current pulses are known to have short front durations measured in 10s of ns. As a consequence, their propagation characteristics would be more like waves on antenna rather than classical circuit domain pulses and further, their propagation is not governed by the applied voltages. A very similar situation prevails with the measurement of partial discharge pulses in high voltage power apparatus and cables. Therefore, the quantity measured at any given point on the circuit need not be and will not be the actual corona current pulse generated at the source. Nevertheless, owing to the linearity of the system for such pulses, measured current should be directly related to the corona pulse at the generation point.
Amongst the two possible current measurement points, the investigation is started with measuring the current at the input to RIV meter. The reasons for the same are as follows. Firstly, the reference value as per prevailing standards, the RIV measurement as per the prescribed circuit is the testing method and therefore, the current coupled through the RIV coupling capacitor governs the test result. Therefore, it would be prudent first to consider this current and study whether intended identification of coronating source could be carriedout. Secondly, as mentioned before, the corona pulse will propagate on ground lead in an antenna mode and hence several reflections and attenuation can be expected in the path of the ground lead which has several bends and runs along supporting steel frame. It will therefore be quite involved to correlate the signal strength at the RIV input. Considering these, first the input to the RIV meter itself is considered for its characteristics.
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