**Power Quality Improvement by Using Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter**

Vasanth Reddy Bathula and Chitti Babu B. *MIC College of Technology, NIT Rourkela India* 

### **1. Introduction**

112 Electrical Generation and Distribution Systems and Power Quality Disturbances

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> The conventional energy sources are limited and have pollution to environment as more attention and interest have been paid on the utilization of renewable energy sources such as wind energy, fuel cells and solar energy. Distributed power generation system (DPGS) is alternative source of energy to meet rapidly increase energy consumption. These DPGS are not suitable to be connected directly to the main utility grid. Rapid development of power electronic devices and technology, double sided converters are used to interface between DPGS and utility grid as they match the characteristics of the DPGS and the requirements of the grid connections. Power electronics improves the performance of DGPS and increase the power system control capabilities, power quality issues, system stability [1].

> Rapidly increase in number of DGPS's leads to complexity in control while integration to grid. As a result requirements of grid connected converters become stricter and stricter to meet very high power quality standards like unity power factor, less harmonic distortion, voltage and frequency control, active and reactive power control, fast response during transients and dynamics in the grid. Hence the control strategies applied to DGPS become of high interest and need to further investigated and developed [3].

> In this chapter, a virtual grid flux oriented vector control [2] (outer loop controller) and three different types of current controllers such as hysteresis current controller, current regulated delta modulator, modified ramp type current controller (inner current loop) techniques are proposed, with main focus on DC link voltage control, harmonic distortion, constant switching frequency, unity power factor operation of inverter. Vector control of grid connected inverter is similar to vector control of electric machine. Vector control uses decoupling control of active and reactive power. The control system for the vector control of grid connected converter consists of two control loops. The inner control loop controls the active and reactive grid current components. The outer control loop determines the active current reference by controlling the direct voltage. A cascaded control system, such as vector control is a form of state feedback. One important advantage of state feedback is that the inner control loop can be made very fast. For vector control, current control is the inner control loop. The fast inner current control nearly eliminates the influence from parameter variations, cross coupling, disturbances and minor non-linearity in the control process. Vector control uses PI-controllers in order to improve dynamic response and to reduce the

Power Quality Improvement by Using

inverter switching frequency.

**3. Virtual grid flux oriented control** 

position is given by

**3.1 Action of Phase lock loop (PLL)** 

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 115

of the power flow. The voltage is controlled by the transformer turn ratio and/or the rectifier inverter firing angle in a closed-loop control system. The frequency must be exactly equal to that of the grid, or else the system will not work. To meet the exacting frequency requirement, the only effective means is to use the utility frequency as a reference for the

Earlier, control and stabilization of the electricity system as taken care only by large power system like thermal, nuclear etc. due to large penetration of DPGS the grid operators requires strict interconnection called grid code compliance. Grid interconnection requirements vary from country to country. Countries like India where the wind energy systems increasing rapidly, a wind farm has to be able to contribute to control task on the same level as conventional power plants, constrained only by limitation of existing wind conditions. In general the requirements are intended to ensure that the DPGS have the control and dynamic properties needed for operation of the power system with respect to both short-term and long-term security of supply, voltage quality and power system stability. In this paper most significant requirement is power quality. The power quality

Virtual grid flux vector control of grid connected Pulse Width Modulated (PWM) converter has many similarities with vector control of an electric machine. In fact grid is modeled as a synchronous machine with constant frequency and constant magnetization[2]. A virtual grid flux can be introduced in order to fully acknowledge the similarities between an electric machine and grid. In space vector theory, the virtual grid flux becomes a space vector that defines the rotating grid flux oriented reference frame, see in fig.2. The grid flux vector is aligned along d-axis in the reference frame, and grid voltage vector is aligned with q-axis. Finding the position of grid flux vector is equivalent to finding the position of the grid voltage vector. An accurate field orientation can be expected since the grid flux can be measured. The grid currents are controlled in a rotating two-axis grid flux orientated reference frame. In this reference frame, the real part of the current corresponds to reactive power while the imaginary part of the current corresponds to active power. The reactive and active power can therefore be controlled independently since the current components are orthogonal. Accurate field orientation for a grid connected converter becomes simple since the grid flux position can be derived from the measurable grid voltages. The grid flux

measurement is mainly the harmonic distortion and unity power factor [2].

cos( ) , *<sup>g</sup> g*

θ

*g*

The implementation of the grid voltage orientation requires the accurate and robust acquisition of the phase angle of the grid voltage fundamental wave, considering strong distortions due to converter mains pollution or other harmonic sources. Usually this is accomplished by means of a phase lock loop (PLL). PLL determines the position of the virtual grid flux vector and provides angle (θg) which is used to generate unit vectors cos(θg), sin(θg) for converting stationary two phase quantities in stationary reference frame

 = sin( ) *<sup>g</sup> g*

θ

*g*

= − (1)

*e e* α

*e e* β

cross coupling between active and reactive powers. Hysteresis current controller is used as inner current control loop to provide switching pulses to inverter, it has good dynamic response it is more suitable as inner current controller where we need fast acting inner current loop, but drawback of this hysteresis current controller is variation of switching frequency with parameters of grid voltage, filter inductor and output current is having lower order harmonics. In current regulated delta modulator switching frequency of inverter is limited by using latch circuit, but in this case also switching frequency is not maintaining constant during fundamental period. In Modified ramp type current controller switching frequency is limited, also maintain switching frequency constant. This controller ramp signal is generated at particular frequency to maintain switching frequency constant [4],[5],[6].
