Evaluation of Results
Results need to be evaluated in order to obtain an indication of site severity or to estimate flashover risk. Level of possible data analysis depends on how many parameters are captured. For example, multivariate analysis techniques can be used to better understand the relationship between environmental parameters and leakage current when time-stamped leakage current and weather parameters are logged. However, there are limitations in such an analysis since level of leakage current is not only a function of the environment at that time instant but also of weather conditions preceding the event. For example, contamination accumulating over a long dry period before a wetting event will result in higher levels of leakage current than when the time of measuring is preceded by a rainy period. Nevertheless, leakage current as a fundamental parameter to characterize contamination performance of insulators can still provide useful information for:
1. Pollution site severity measurement
Information gathered enables selection of insulator designs and dimensions with respect to contamination performance. For this purpose, leakage current activity on the insulator should be correlated with that during artificial contamination tests under known pollution severity.
2. Initiator for insulator maintenance
Increased leakage current activity on insulators can be used to trigger insulator maintenance before critical conditions arise.
3. Insulator characterization
Leakage current can be used to compare performance of different types of insulators installed at the same testing site and determine the most appropriate design and dimensions for those conditions.
4. Condition of RTV coatings & ageing of polymeric insulators
Measuring Site Pollution Severity
Determining site severity through leakage current measurement is described in the latest version of IEC 60815-1 (illustrated in Fig. 6) and is comprised of the following steps:
1. The relationship of leakage current to contamination severity is determined through laboratory tests on the insulator of interest. This has traditionally been done with the salt-fog test, considered representative of coastal conditions across Europe;
2. On site, leakage current is monitored across the insulator type for which this leakage current relationship has been determined;
3. Monitoring of leakage current continues for a considerable time period, i.e. at least a year;
4. Maximum current measured over the monitoring period is converted to equivalent site severity with the relationship determined in the first step.
Monitoring leakage current can also be an early warning system to initiate maintenance on ceramic or glass insulators, e.g. cleaning. Leakage current activity across an insulator increases as it becomes more contaminated and higher contamination levels mean greater flashover risk. As such, increasing leakage current activity indicates higher risk of flashover, as illustrated in Fig. 7.
The curve showing probability of flashover for any given leakage current can also be derived from laboratory test results. In this EPRI implementation, both peak leakage current and also more complex algorithms based on duration and magnitude of the event are used to generate alarms.
Leakage current measurements are often used as an objective way to quantify and compare discharge activity on different types of insulators exposed to the same contaminated conditions. Comparing absolute levels of leakage current peaks allows an impression of the relative flashover performance of insulators whereas other current parameters, e.g. cumulative charge, are often associated with erosion stress a polymeric material is subjected to (i.e. ageing performance).
EPRI has developed a robust, low cost sensor for monitoring leakage currents across insulators. These self-contained units report measured data and status via radio frequency link to a local base station where data is integrated into an asset health system. The sensor has the following features:
• Current measurement is done using a ferrous current transformer surrounding a straight current conductor as primary winding. The saturation characteristics of the magnetic circuit and low impedance of the primary winding protects the internal electronics of the sensor from overvoltages;
• The device stores the statistical parameters of leakage current peaks that occur. Leakage current is monitored continuously and the peak value over a 1-minute period is binned and stored. Amplitude of the highest peak current occurring during the last measuring cycle is also reported. The device has 6 bins for which leakage current threshold can be changed wirelessly at any time;
• It keeps track of the time since the last reset, thereby limiting the data message sent by the device to a bare minimum. It also limits internal circuit complexity and power consumption for the device.
• The communication system of the sensors is based on the IEEE 805.15.4 architecture but with a customized communication protocol. The communication protocol allows two-way communications enabling the user to reset the bins or change bin levels remotely;
• The electronics are embedded in potting compound and housed in a metallic enclosure in order to protect internal components from environmental as well as electromagnetic influences;
• Power is supplied by on-board, long life batteries. Based on battery characteristic and low power consumption of the unit, battery life is estimated at over 12 years.
EPRI has two standard settings for leakage current threshold for the bins. These bin ranges are programmable and can be adjusted (within limits) remotely while the sensor is in service