SCR Series Operation Tutorial
Series Operation of SCRs
Necessity of SCR series connection:
For some industrial applications, the demand for voltage and current ratings is so high that a single SCR cannot meet such requirements. In such cases, SCRs are connected in series in order to meet the high voltage demand and in parallel for meeting the high current demand.
- Series connection of power devices are often required to increase the overall voltage rating.
- For example we have to use SCR as a power switch in the power electronic circuit having voltage rating of 1000 volts.
- But we have a couple of power thyristors having voltage rating of 600 volts only.
- Then by connecting two thyristors in series we can implement the circuit.
Problems in SCR series operation:
- When the thyristors are connected in series, they have small differences in their ratings. We know that in the world no two devices are having identical characteristics.
- Consider that two thyristors with same ratings are connected in series.
- The thyristor having highest internal resistance will have minimum leakage current.
- So high voltage will appear across it in off state.
- This creates voltage imbalance in the series connection.
- Hence equalization is necessary in the series connection.
String Efficiency & Derating Factor
For SCR series operation, it should be ensured that each SCR rating is fully utilized and the system operation is satisfactory.
String efficiency is a term that is used for measuring the degree of utilization of SCRs in a string.
The string efficiency of SCRs connected in series/parallel is defined as
String Efficiency = [Actual voltage/current rating of the whole string]/[Individual voltage/current rating of one SCR]*[Number of SCRs in the string]
- In practice, this ratio is less than one.
- To get highest possible string efficiency, the SCRs connected in series string must have identical V-I characteristics.
- As a consequence, string efficiency can never be equal to one.
- However, unequal voltage/current sharing by the SCRs in a string can be minimized to a great extend by using external equalizing circuits.
- The measure of the reliability of string is given by a factor called derating factor DRF defined as
DRF = 1 - String Efficiency
- A uniform voltage distribution in steady state can be achieved by connecting a suitable resistance across each SCR such that each parallel combination has the same resistance.
- This shunt resistance R is called as static equalizing circuit.
- The series connected SCRs suffer from unequal voltage distribution across them during their turn-on and turn-off processes and also during their high frequency operation which means more frequent turning on and turning off of the devices.
- Thus a simple resistor used for static voltage equalization cannot maintain equal voltage distribution under transient condition.
- During the turn-off process, due to the difference in junction capacitance, there is the differences in stored charge for the series connected SCRs.
- It will cause unequal reverse voltage sharing among the thyristors. This problem is solved by connecting capacitor across each thyristor.
- The value of capacitors should be large enough to swap the junction capacitance.
- A small resistance in series with this capacitance will limit the discharge current through the thyristor during turn-on process.
- The R2-C network will also act as a snubber network to limit the rate of rise of voltage across the thyristor at switch-on. The circuit arrangement for dynamic voltage equalization is shown in Fig
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