**1. Introduction**

The technological advances of the last decades favored a widespread of power electronics converters in the majority of household appliances, industrial equipment connected to the Low Voltage (LV) grid and, more recently, in distributed power generation, near the consumer – microgeneration (μG).

Most of this electronic equipment is a strong producer of current harmonics, polluting the LV network and generating sensitivity to dips, unbalances and harmonics, being also more sensitive to Power Quality issues. In the future, the massive use of renewable and decentralized sources of energy will probably worsen the problem, increasing Total Harmonic Distortion (THD), RMS voltage values, increasing unbalances and decreasing Power Factor in Low Voltage Networks.

In these and in other Power Quality related issues, power electronics became, to a certain extent, the cause of the problem. However, due to the continuous development of power semiconductors characteristics, less demanding drive circuits, integration in dedicated modules, microelectronic control circuits improvement, allowing their operation at higher frequencies and with higher performance modulation and control methods, power electronics converters also have the potential to become the solution for the problem. Still, even the non polluting grid connected converters are not usually exploited to their full capability as, in general, they are not used to mitigate Power Quality problems.

The smart exploitation of μG systems may become very attractive, using power electronics converters and adequate control strategies to allow the local mitigation of some power quality problems, minimizing the LV grid harmonics pollution (near unitary power factor) and guaranteeing their operation as active power filters (APF).

Based on these new challenges, the main aim of this work is to create a virtual LV grid laboratory to evaluate some power quality indicators, including power electronics based models to guarantee a more realistic representation of the most significant loads connected to the LV grid. The simulated microgenerators are represented as Voltage Source Inverters (VSI) and may be controlled to guarantee: a) near unity power factor (conventional μG); b) local compensation of reactive power and harmonics (active μG).

Design of a Virtual Lab to Evaluate and Mitigate

The magnetizing reactance *Xm* is given by (4):

and the short-circuit losses *Pcc*.

assumed to be equal. Then:

**2.2 Distribution cables** 

by the physical construction of the cable.

and length.

Power Quality Problems Introduced by Microgeneration 187

2 *m* 2 *m m n <sup>I</sup> B G U* =− −

1

*m*

*cc*

*n*

2 *n cc <sup>t</sup> <sup>I</sup>*

The resistance and leakage reactance from the primary and secondary windings may be

1 2 2

1 2 2

In this work a 400kVA 30kV/400V distribution transformer (base values *Sb*=400kVA, *Ub*=30kV, /( 3 ) *bb b IS U* = ) is used. From the no-load test a magnetizing current *Im*=2.9% and no-load losses of *P0*=1450W are considered. From the short-circuit test it is assumed

The distribution cables models are based on the π model (Fig. 2) and their section is chosen according to the current nominal values. The series resistance and inductance and the shunt admittance may be obtained from the manufacturers values depending on the cables section

In LV distribution networks four-wire cables are used (three phase conductors and a neutral conductor insulated separately), all enclosed by an outer polyethylene insulation mantle. Usually the conductors are sector shaped. The shunt and series impedance are determined

*m*

*cc*

Then, from (5) and (6) it is possible to determine the leakage reactance *Xt* (7):

*Ucc*=4.5%, with nominal current *In* (1 pu) and short-circuit losses *Pcc*=8.8 kW.

*<sup>U</sup> <sup>Z</sup>*

The magnetizing impedance is much higher than the series branch impedances (Fig. 1). Then, from the short-circuit test, it is possible to obtain the short-circuit impedance *Zcc* (5) and the total resistance *Rt* (6) from the transformer primary and secondary windings, knowing the short-circuit voltage *Ucc*, necessary to guarantee the current nominal value *In*

*X*

(3)

*<sup>B</sup>* <sup>=</sup> (4)

*<sup>I</sup>* <sup>=</sup> (5)

*<sup>P</sup> <sup>R</sup>* <sup>=</sup> (6)

2 2 *X ZR t cc t* = − (7)

*Rt R R*= = (8)

*Xt X X*= = (9)

From the obtained results, active μG have the capability to guarantee an overall Power Quality improvement (voltage THD decrease and Power Factor increase) allowing a voltage THD decrease when compared to voltage THD values obtained with conventional μG.
