Current leakage

When considering a ceramic capacitor with parallel electrodes, the leakage current, denoted in A, through the dielectric insulator can be expressed as  

Expressing the leakeage electric current according to Ohm s law, it is possible to identify another physical quantity known as insulation resistance and denoted R.. For any given capacitor, the insulation resistance is largely dependent on the resistivity of the dielectric, which is a property of the material, in turn dependent on the formulation and temperature of measurement. 

The measured insulation resistance is inversely proportional to the capacitance value of the unit being tested. Insulation resistance is a function of capacitance, and hence minimum standards for insulation resistance in the industry have been established as the product of the resistance and the capacitance, RC. For instance, the minimum RC product for ceramic capacitors must be 1 k 2F when measured at 25 C, and it must be 0.1 k 2F at 125 C. 

In addition to thermionic emission, thermionic field emission, and field emission (tunneling) currents, other currents such as defect-assisted turmeling, which may have a quasiohmic nature, can be lumped into what is called the leakage current of more or less unknown origin that can be expressed as 

J t is considered to be a fitting parameter that represents defects and inhomogeneities at the metal-semiconductor interface and is the semiconductor resistance introduced in Equation 8.9. In semiconductors with less than ideal interfaces, a tunneling barrier Eg may not be predicted, in which case it should be considered as another fitting parameter. In practice, the terms iteo and hfeo are also considered to be fitting parameters that represent the magnitude of the contributions to the current from thermionic emission and thermionic field emission, respectively. 

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The MOSEET has three regions of operation. The cutoff region occurs for V g < Up. In this region, the drain-to-source current is the reverse saturation current of the back-to-back source and drain junctions. This leakage current is small but nonzero and allows charge to leak off capacitors which are isolated by cutoff MOSFETs. Because this is how bits are stored in dynamic memory (DRAM) ceUs, DRAMs must be regularly refreshed to retain their memory. 

Mobile ions, such as sodium or potassium, tend to migrate to thep-n junction of the IC device where they acquire an electron, and deposit as the corresponding metal on the p-n junction this consequendy destroys the device. Furthermore, mobile ions also support leakage currents between biased device features, which degrade device performance and ultimately destroy the devices by electrochemical processes such as metal conductor dissolution. 

Irrespective of the class of an insulation system and its quality, it will have some leakage current through its dielectric circuit on application of a high voltage. For all practical piiiposes. therefore, we can consider an insulation system as an imperfect capacitor. 

Figure 9.6 Representation of leakage current in an HT insulation system...

Verification of insulation resistance or measurement of the leakage current 14/435... 

Insulation resistance or measurement of leakage current, both before and after the dielectric test... 

The leakage current is measured as in lEC 60298. This method is generally applicable to an HT system by applying the full rated voltage between the insulating surface, say, between a phase and the ground. The leakage current thus measured should not exceed 0.5 mA. 

Referring to Table 14.7, the recommended insulation value will ensure the following leakage current for an LT system ... 

The values thus shown in Table 14.7 take cognisance of the recommendations of I EC 60298 to maintain the leakage current at less than 0.5 mA for all HT systems. 

In this case also verification of insulation resistance or the measurement of leakage current will be carried out before and after the HV test. 

The design of the C-R combination maintains a negligible level of leakage current through the suppressor in healthy conditions (to contain resistance loss). It is easily achieved, as C provides a near-open circuit in these conditions and permits only a very small leakage current to flow through it. 

A ZnO element basically represents a capacitive leakage circuit. In its leakage current range it may be electrically represented as shown in Figure 18.5, where I q is the leakage current, capacitive in nature, and and I, its capacitive and loss components, respectively. The success of an element will depend upon its low loss-component, which would mean a lower loss during continuous... 

The design and configuration of ZnO disks is such that this resistive current is more than the capacitive leakage current it draws in the MCOV region. 

Reference current I.OriiA Leakage current at MCOV,... 

IZnO,h = maximum leakage current in normal conditions. 

IZnO(2 = actual leakage current measured at site. 

The greatest effect of ageing is reflected by the variation in its resistive current, which is rich in the third harmonic. Variation in is used in assessing the condition of an arrester. By conducting laboratory tests to determine the characteristics of an arrester, we can establish a ratio between the total leakage current, IZnO and the content of If, to assess the condition of the arrester. If we can monitor this current, we can monitor the condition of the arrester. Below we discuss briefly one such method by which this component can be separated out. 

Leakage current monitor - this can be connected perma-nently for continuous reading or periodic monitoring. The normal practice is to measure only periodically for a short period to take average measurements on an hourly, daily, monthly or yearly basis. When not connected permanently, the instrument can also be used as a portable kit to monitor the condition of other arresters installed in the vicinity. Field probe - to compensate the third harmonic of the system voltage to make the IZnO free from the third harmonic of the system voltage. This method of /r measurement therefore provides more accurate and closer monitoring of the arrester. 

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