SURGE SUPPRESSION USING A FUZZY HYSTERESIS-BAND CONTROLLED RESISTOR ARRAY

Transient over voltage protection has always been an important part of the electrical design of power systems. Metal-oxide varistor (MaY) has been a useful tool to limit the impulse voltage stress in recent years. This device has some disadvantages: 1. Its protection voltage is selected a little higher than the system nominal voltage. The main reason is the variation of the power frequency peak voltage, 2. It is only a two-valued resistor (switches between high and low impedances), 3. May has a relatively large slope resistance in conduction mode, 4. May s protection process depends only on the voltage amplitude and has no obvious dependence on the variation rate of the voltage amplitude, 5. Threshold voltage of a May is a constant value and depends on its physical parameters. This paper presents a method to alleviate the aforementioned disadvantages and introduces a resistor array controlled by a fuzzy hysteresis-band controller to achieve a variable resistor. The first disadvantage is addressed using a power frequency voltage amplitude detector. Appropriate values of resistors and a switching strategy to parallel suitable resistors in the resistor array can lead to diverse values of the needed resistor. In this method load voltage derivative is utilized to help surge suppression and the threshold voltage has an adaptive variance due to power frequency voltage amplitude. It is also shown that the proposed circuit has negligible leakage current.