**4. Current control approach to VSI**

#### **4.1 Objectives**

118 Electrical Generation and Distribution Systems and Power Quality Disturbances

The following derivation of direct voltage controller assumes instantaneous impressed grid currents and perfect grid flux orientation. The instantaneous power flowing into grid can be

<sup>3</sup> ( )

3 2

*DC DC DC DC du P ui uC*

Assuming the converter losses are neglected, the power balance in the direct voltage link is

*DC s g s g gq du u C P P P ei*

Where Ps is the distributed energy system power is assumed to be independent of the DC voltage. A transfer function of between direct voltage and active grid current Ig is obtained

> 3 2

The transfer function is non-linear. It is acceptable to substitute the direct voltage with the reference set value since the objective is to maintain a constant direct voltage. The

*g DC gq DC*

> \* 3 2

*DC*

3 *dc g*

\*

*g DC gq*

*e u i*

<sup>1</sup> 2

α

*Cu F G p e*

From eq.8, a P-controller is obtained for regulating the direct voltage. The P- controller is optimal for an integrator process in the sense that the P- controller eliminates the remaining error for steps in the reference value. However, there will be a remaining error for steps in

*e u i*

When neglecting capacitor leakage, the direct voltage link power is given by

*DC*

Applying internal model control gives the direct voltage link controller as

α

3 3 \* ( ) 2 2 *S P jQ e i e i j e i g g g gg g q g* =+ = = + *<sup>d</sup>* (2)

<sup>2</sup> *S e i je i g gq g* = + *<sup>d</sup>* (3)

*P ei g g gq* = (4)

*dt* = = (5)

*dt* =− − =− − (6)

*pCu* ≈ − (7)

*pCu* ≈ − (8)

<sup>−</sup> = =− (9)

3 2

**3.2.1 DC voltage controller (outer loop)** 

The active power is real part of equation.3.

assumption gives linearized transfer function.

written as

given by

as

The current control methods play an important role in power electronic systems, mainly in current controlled PWM voltage source inverters which are widely used in ac motor drives, active filters, and high power factor, uninterruptable power supply (UPS) systems, and continuous ac power supplies [5]. The performance of converter system is largely dependence on type of current control strategy. Therefore current controlled PWM voltage source inverters are one of the main subjects in modern power electronics. Compared to conventional open loop PWM voltage source inverter, the current controlled PWM voltage source inverters have fallowing advantages:


Power Quality Improvement by Using

Hysteresis tolerance band

+vdc


Fig. 7. Hysteresis current control operation

**4.2.2 Current regulated delta modulator** 

grid current most follow the reference current value.

[5].

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 121

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

Actual current

Reference current

Current regulated delta modulator operation is same as that of hysteresis current controller. Main advantage of delta modulator is switching frequency variation is very less and we can limit switching frequency to desirable frequency. Circuit diagram of current regulated delta modulator is shown in fig. 8. This consists of comparators and latch to limit switching frequency [7]. Actual grid currents compared with reference currents provides error currents, this error currents are flowing into comparator. These comparators are acting like hysteresis limiters which limit error current between two bands and generate pulses. These pulses are given to latching circuit as binary values 0 and 1. Latching circuit is operated when clock signal is enabling the latch. Switching frequency of inverter is decided clock frequency of latch. Operation of latch is that it holds the input until the clock signal enables the latch. When clock signal applied latch will enable and it will give output to gate drive circuit, which drives the inverter in such a way that error current should be minimized and

