Current Source Inverter

CSI

1. CURRENT SOURCE INVERTERS (CSI) In the CSIs, the input current is constant. The amplitude of output current from the CSI is independent of the load, but the magnitude of output voltage and its waveform is dependent upon the nature of load. A CSI converts the input DC current to an AC current, and the frequency of the AC current depends upon the frequency of Switching Devices (rate of triggering the SCRs). The amplitude of the AC output current can be adjusted by controlling the magnitude of the DC input current.

2. Since CSI is a constant current system, it is used typically to supply high power factor loads where impedance will be remain constant or decreases at harmonic frequencies in order to prevent problems either on switching or with harmonics voltage. An VSI requires feedback diode whereas a CSI does not require any feedback diode. The commutation circuit of CSI is very simple as it contains only capacitors. Because power semiconductors in a CSI have to withstand reverse voltage, devices like power transistors, power MOSFETS, and power BJTs cannot be used in CSIs. THYRISTORS are the Best Power Semiconductor Switches used in CSI Drives

3. Applications of CSI Speed control of AC motors Lagging VAR compensation Solar photovoltaic utility systems Synchronous motor starting

4. Concept of Current Source

5. Single-Phase Full-Bridge Inverter (CSI) with Pure C Load I T 1 T 2 T 3 T 4 V  + LOAD V in Current source  + V

6. Principle of Operation with Waveforms

7. T /2 Current input to CSI I t T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 i I - I T t t t 2T T T/2 Output current Output voltage Input voltage 3T/2 f = 1/T f = 2/T   in

8. Voltage Shape and its Value at t = T/2 T/4 ≤ t ≤ T/2

9. T/2 ≤ t ≤ 3T/4

10. 3T/4 ≤ t ≤ T

11. T ≤ t ≤ 5T/4

12. Practical Circuit C 1  + LOAD I S  + C 2 T 2 D 2 T 3 D 3 T 4 D 4 T 1 D 1 V C 2 V C 1 i

13. A basic thyristor-based single-phase CSI is shown. Capacitors are used here as a commutating device. Working of this inverter can be explained using following points: Initially, T1 and T2 are conducting, and the capacitors are charged with the polarities, as shown. The positive load current is flowing through T1, D1, load, D2, and T2. To turn off the conducting thyristors T1 and T2, the other two thyristors T3 and T4, are triggered. When these thyristors (T3 and T4) start conducting, the capacitor voltage VC1 and VC2 appears as reverse voltage across the thyristors T1 and T2, respectively. So, thyristors T1 and T2 get turned off.

14. Now, the load current flows through T3, C1, D1, load, D2, C2, and T4. So, the polarity of the capacitors reverses due to the charging current in the reverse direction, that is, the right plate of capacitors C1 and C2 become positive, and the left plate becomes negative. When the capacitor voltage becomes equal to the load voltage, the load current becomes zero, and the diodes D1 and D2 get turned off. The load current now flows in the reverse direction through T3, D3, load, D4, and T4. Similarly, when the thyristors T1 and T2 are triggered, the capacitor voltage VC1, VC2 turn off T2 and T4 in a similar fashion. In this way, the cycle repeats and output AC voltage of the desired frequency is obtained.

15. Conclusions The slope of Current is ( I S /C ) The Peak Value of Voltage from Zero axis as reference is V P = (I S T/4C) The Peak-to-Peak value of the Voltage in the circuit is V P-P = 2V P = (I S T/2C) For Pure Capacitive Load in this Full-Bridge Inverter (CSI) each pair of Thyristors will equally conduct for a period of T/2

16. In CSI Output Current shape is Square, But output VOLTAGE shape depends upon the nature of load (a) R Load - Square Shape (b) Pure C Load - Triangular Shape (c) R-C Load - Exponential (Rise and Fall) Shape

17. Three - Phase CSI A basic three-phase CSI, feeding a star-connected resistive load, is shown. Diodes are used in series with each switch to handle reverse voltage, which should be avoided to appear across the power semiconductor-based switches. The switches are turned on in the sequence of their numbers, with an interval of 60°, and each switch conducts for 120° duration.

18. I S S 1 S 3 S 5 S 4 S 6 S 2 D 1 D 3 D 5 D 4 D 6 D 2 3-phase star connected load

19. At a time two switches conduct, that is, one from the upper group (T1, T3, T5) and other from the lower group (T4, T6, T2). The load current of a particular phase is +I S when upper switch of the corresponding arm conducts, and it is -I S when the lower switch conducts.

20. T 1 T 2 I i c i b I s i a II III IV V V I I II III T 1 ,T 6 T 3 , T 2 T 4 ,T 3 T 4 ,T 5 T 6 ,T 5 T 1 ,T 6 T 1 ,T 2 T 3 ,T 2  60  120  180  240  300  360  420  480  54   t  t  t I s +I s I s I s I s

21. Advantages of CSI It has inherent short-circuit fault protection because the input is a controlled-current source, so the current does not rise to a dangerously high level during the fault, so, it is more reliable. It requires a simple commutation circuit, that is, only capacitors. It does not require any feedback diode. It may use a converter-grade SCR, which is cheaper than inverter-grade SCR. It has inherent four-quadrant operation capability due to converter–inverter combination.

22. Disadvantages of CSI It suffers from stability problems at light loads. It suffers from operating problems at high frequency. Its response is sluggish.