❖ Presently, SCRs with voltage and current ratings of 10 kV and 3 kA are available.
❖ However, for some industrial applications, the demand for voltage and current ratings is so high that a single SCR cannot fulfil such requirements.
❖ In such cases, SCRs are connected in series in order to meet the high voltage demand and in parallel for fulfilling the high current demand.
❖ String efficiency is a term that is used for measuring the degree of utilization of SCRs in a string.
❖ String efficiency of SCRs connected in series/parallel is defined as
❖ In practice this ratio is less than one.
❖ For obtaining highest possible string efficiency, the SCRs connected in series/parallel string must have identicalV-I characteristics.
❖ As SCRs of the same ratings and specifications do not have identical characteristics, unequal V/I sharing is bound to occur for all SCRs in a string. Therefore, string efficiency can never be equal to one.
❖ However, unequal V/I sharing by the SCRs in a string can be minimized to a great extent by using external equalizing circuits.
❖ Even in a string provided with external equalizing circuits, the string efficiency is less than unity.
❖ In case one extra unit is added to the series/parallel string, the V/I shared by each device would become lower than its normal rating.
❖ The use of this extra unit will certainly improve the reliability of the string though at an increased cost.
❖ A measure of the reliability of string is given by a factor called derating factor (DRF) defined as DRF=1-string efficiency.
❖ If the value of recommended DRF is more, the number of devices used in series/parallel string will be more.
❖ This will certainly improve the reliability of the string for a given rating of the string.
Password: Snubber
pdf made by Dr M Balasubbareddy