**4.2.2 Current regulated delta modulator**

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 grid current most follow the reference current value.

Power Quality Improvement by Using

current signal is a modulated waveform.

detrimental effects[5].

following are the set values

below

**5. Simulation results of grid connected VSI** 

**5.1 Simulation results of hysteresis current controller** 

DC link reference voltage (udc \*) = 2200V DC link capacitance (C) = 600UF Reference Reactive power (Q\*) = 0 VAr Hysteresis band width (H) = 20 Amp Ramp generator frequency = 2 KHz Proportional Gain (Kp) = 0.01 Integral Gain (Ki) = 60

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 123

The generated error signals are compared to a triangular waveform of fixed frequency and amplitude. If the current error signal is positive and larger than the triangular wave, the switches are activated to apply +VB to the load. However, if current error signal is positive and smaller then the triangular wave, the switches are activated to apply –VB to the load. Some hysteresis has been added to the controller, in order to prevent multiple crossings of the error signals with the triangular wave. The ramp-comparator controller is a modulation system. The frequency of the triangular wave is the carrier frequency, while the error

Since this controller uses a fixed-frequency triangular wave, it has the effect of maintaining a constant switching frequency of the inverter. This is the main advantage of this controller. However, it has some disadvantages, as the output current has amplitude and phase errors. This results in a transmission delay in the system. Moreover, a zero vector is applied to the load. This means load is disconnected several instants over the fundamental period of the output voltage. In order to overcome the above difficulties, a new ramp- comparator controller is proposed. Fig.9 shows the schematic diagram of this controller, in which the phase shifters are included. The actual values of the three phase load currents are measured and compared to the three 1200 phase-shifted triangular waveforms having the same fixed frequency and amplitude. The performance of this scheme is considered identical to those for three independent single-phase controllers. It is to be noted that there is no interaction between the operations of the three phases. As a result, the zero voltage vectors will be eliminated for balanced operation. This does not necessarily lead to the possibility of creating the positive and/or negative sequence sets due to the controller alone. The zero voltage vectors eliminate the necessity of neutral connections for some applications and, in such cases; no harmonic neutral current can flow in the load. Thus, there is no problem of incorporating the proposed controller in industrial motor drives. For applications in UPS's and active filters, the higher harmonics lead to more losses. However, the shifting of current harmonics to higher orders does not usually create noticeable problems in motor drives, as the machine inductance filters out the higher harmonics and limits the associated

Simulation of virtual grid flux oriented control of grid connected VSI was done by using MATLAB/Simulink and results for different types of current controllers has shown

For the simulation of virtual grid flux oriented control of grid connected inverter the

Fig. 8. Current regulated delta modulator scheme

The delta modulation offers an opportunity of on-line harmonic minimization of pulse width modulated inverter without conventional optimization processes, like selective harmonic elimination or harmonic weighting techniques.

#### **4.2.3 Modified ramp type current controller**

A conventional ramp-comparator controller is also shown in fig. 9, the phase shifters are bypassed. The actual values of the three-phase load currents are measured and compared to the references currents.

Fig. 9. Modified ramp type current control scheme

The delta modulation offers an opportunity of on-line harmonic minimization of pulse width modulated inverter without conventional optimization processes, like selective

A conventional ramp-comparator controller is also shown in fig. 9, the phase shifters are bypassed. The actual values of the three-phase load currents are measured and compared to

Fig. 8. Current regulated delta modulator scheme

**4.2.3 Modified ramp type current controller** 

Fig. 9. Modified ramp type current control scheme

the references currents.

harmonic elimination or harmonic weighting techniques.

The generated error signals are compared to a triangular waveform of fixed frequency and amplitude. If the current error signal is positive and larger than the triangular wave, the switches are activated to apply +VB to the load. However, if current error signal is positive and smaller then the triangular wave, the switches are activated to apply –VB to the load. Some hysteresis has been added to the controller, in order to prevent multiple crossings of the error signals with the triangular wave. The ramp-comparator controller is a modulation system. The frequency of the triangular wave is the carrier frequency, while the error current signal is a modulated waveform.

Since this controller uses a fixed-frequency triangular wave, it has the effect of maintaining a constant switching frequency of the inverter. This is the main advantage of this controller. However, it has some disadvantages, as the output current has amplitude and phase errors. This results in a transmission delay in the system. Moreover, a zero vector is applied to the load. This means load is disconnected several instants over the fundamental period of the output voltage. In order to overcome the above difficulties, a new ramp- comparator controller is proposed. Fig.9 shows the schematic diagram of this controller, in which the phase shifters are included. The actual values of the three phase load currents are measured and compared to the three 1200 phase-shifted triangular waveforms having the same fixed frequency and amplitude. The performance of this scheme is considered identical to those for three independent single-phase controllers. It is to be noted that there is no interaction between the operations of the three phases. As a result, the zero voltage vectors will be eliminated for balanced operation. This does not necessarily lead to the possibility of creating the positive and/or negative sequence sets due to the controller alone. The zero voltage vectors eliminate the necessity of neutral connections for some applications and, in such cases; no harmonic neutral current can flow in the load. Thus, there is no problem of incorporating the proposed controller in industrial motor drives. For applications in UPS's and active filters, the higher harmonics lead to more losses. However, the shifting of current harmonics to higher orders does not usually create noticeable problems in motor drives, as the machine inductance filters out the higher harmonics and limits the associated detrimental effects[5].
