**4.2 Different current control techniques**

#### **4.2.1 Hysteresis current control technique**

Hysteresis current controller is an instantaneous feedback system which detects the current error and produces driver commands for switches when error exceeds specified band.

Fig. 6. Hysteresis current control scheme

The main task of current controller in PWM VSI is to force the load current according to the reference current trajectory [6] by comparing to command phase currents with measured instantaneous values of phase currents, the current controller generates the switching states for the converter power devices in such a way that error current should be minimized. Hence current control implements two tasks, error compensation (decrease in current error)

1. No phase and amplitude errors (ideal tracking) over a wide output frequency range.

3. Limited or constant switching frequency of converter to ensure safety operation of

Some of the requirements like fast response and low harmonic distortion contradict each

Hysteresis current controller is an instantaneous feedback system which detects the current error and produces driver commands for switches when error exceeds specified band.

5. compensation of effects due to load parameter variations( resistance and reactance).

6. Compensation of semiconductor voltage drop and dead times of converter.

7. Compensation of DC link and AC side voltages.

**4.1.1 Basic scheme of current controlled PWM** 

and modulation (determination of switching states).

**4.1.2 Basic requirement and performance criteria**  Basic requirements of current controller are as fallows [16]

2. To provide high dynamic response of system.

converter semiconductor power devices.

4. Low harmonic content

other.

5. Good dc-link voltage utilization.

**4.2 Different current control techniques 4.2.1 Hysteresis current control technique** 

Fig. 6. Hysteresis current control scheme

The purpose of the current controller is to control the load current by forcing it to follow a reference one. This is achieved by the switching action of the inverter to keep the current within the hysteresis band. Simplified diagram of a typical three-phase hysteresis current controller is shown in fig. 6, The load currents are sensed and compared with the respective command currents using three independent hysteresis comparators having a hysteresis band H. the output signals of the comparators are used to active the inverter power switches. Based on the band, there are two types of current controllers, namely, the fixed band hysteresis current controller and sinusoidal band hysteresis current controller [5].

Fig. 7. Hysteresis current control operation
