High frequency magnetic and electric field measurements and simulation of a 245kV GIS

The transient high frequency electric and magnetic fields are generated in Gas insulated substations(GIS) systems during switching operations by both radiation and conduction mechanisms. In this paper the radiated transient electric and magnetic fields are measured by using high frequency field sensors at important locations in the 245kV GIS were presented. The measurements are carried out at three important locations including at bus link, cable termination and C.T position respectively. The results of measured values are compared with the calculated values. The results shows that transient electric & magnetic field values are correlate with the measured values at bus link, near current transformer(CT)and at cable termination , the calculated values are slightly deviated from the measured values when moves along with XLPE cable. This may be due to a fast attenuation rate of electromagnetic fields with distance away from the DS3. These results are very useful in order to prepare some precautionary protection measure to avoid malfunctioning of secondary equipment and metering equipment in GIS systems.

I. Introduction

Gas insulated substations (GIS) have been used in power systems for more than 30 years, even up to the system voltages of 745kV. However, the estimation of transient electric and magnetic fields are important for the GIS systems operating more than 132kV. In the present study a 245kV GIS system is considered for the measurements Transient Electric and Magnetic fields. These measurements are important for the insulation diagnosis of gas insulated switchgear [1]. For the insulation diagnosis of such apparatus, the detection of electromagnetic wave caused by DS (Disconnector switch) is a practical technique, and a lot of researches have been performed [1] [3]. There are two types in these researches; one is UHF technique [4] [8] in which the electromagnetic wave propagated in GIS is directly detected by UHF disc sensor in GIS, the other is antenna detection method [5] [7] in which the electromagnetic wave radiated out of GIS is detected by outside antennas. In both cases for the high-sensitive measurement, it is necessary to discriminate EM (Electromagnetic) signal in SF₆ gas from external noises [8]. The frequency characteristics of EM signal may also be affected by the construction of GIS aperture such as spacer configuration, bushing, and besides the polarization and the radiation pattern of electromagnetic wave [10]. As a result, the received electromagnetic wave due to DS operation detecting by sensors finally has the complicated spectrum consisted of total frequency characteristics mentioned above. However, such frequency characteristics have not been elucidated in spite of the necessity from the viewpoint of insulation diagnosis of GIS. Therefore, it is important to clarify how individual frequency characteristic is determined and how these frequency characteristics will be superimposed. In this paper, we measured the radiated electromagnetic waves which was affected by the construction of model GIS, such as aperture like insulation spacer and bushing. We compared the magnitudes of EM fields at various locations in a 245kV GIS system, which is located at Bandra - Kurla complex to meet the power requirement in south Mumbai. The measurements are performed at three important locations during switching operation. Due to the switching operation of circuit breakers and disconnectors in GIS, a very fast transient over voltages and over currents will be generated along the three phases in the network. The generated transients are main reason for the generation of transient electromagnetic fields in a GIS, secondary equipment in HV switchgear is processor oriented, entirely electronic and very sensitive to Electromagnetic disturbances[9]. CIGRE working group 23.02 stated in its report on 2^nd international GIS service experience survey that of the sample of 73024 CB/bay/year, 16% reported problems with mal operation of control, protection or secondary circuits, although in general, improved reliability of GIS in service was confirmed However, due to the switching operation of circuit breakers and disconnectors in GIS a very fast transient over voltages will be generated along the three phases further more it will generate the transient potential rise on the external surface of GIS enclosure and then radiate electromagnetic fields from the enclosure to disturb control and protection devices near by. The switching transient fields have been measured in GIS by a number of investigators. But recent measurements are carried out with advanced field meters with very high sensitivity. The field measurements are carried out at three important locations in a 245kV GIS, in order to determine main field sources, the Electric and magnetic field sensors were placed a at required locations as shown in the Fig. 2. Once the main field sources have been determined, recommendations can be made to reduce the field levels and substation design can be modified to make use of this information. This paper presents the results of measurement and simulation of electric and magnetic fields in a 245kV GIS system at Bandra-Kurla complex to meet the power requirement in south Mumbai. The measurements are taken in the month of October 2009. These studies are very useful for estimation of main field sources in the sub station. For taking measurements the meters Field starl000EF and field starl000MF along with the field sensors are used.