**1. Introduction**

The use of renewable energy sources either as distributed generators in public AC networks or as isolated generating units supplying is one of the new trends in power-electronic technology. Renewable energy generators equipped with electronic converters can be attractive for several reasons, such as environmental benefits, economic convenience, social development.

The main environmental benefit obtained by using renewable sources instead of traditional sources, is the reduction in carbon emission. Many countries have adopted policies to promote renewable sources in order to respect the limits on carbon emission imposed by international agreements.

Moreover, renewable energies can be economically convenient in comparison with traditional sources, if the economic incentives for grid connected renewable sources are taken into account or in other particular situations to supply stand alone loads. In some cases, it can be more convenient to supply an isolated load with renewable local source instead of extending the public grid to the load or to supply it with diesel electric generators. In this case, in order to evaluate the economic benefits of renewable energy solution, It is necessary to take in account either the cost of the fuel and the cost of its transport to the load, that can be located in remote and hardly reachable areas.

In addition to the economic benefits, the use of distributed renewable generation units contributes to decentralise the electrical energy production, with a positive impact on the development of remote areas. The exploitation of local renewable sources supports local economies and lightens the energy supply dependency from fuels availability and prices fluctuations.

The integration in the electric grid of distributed power generation systems, located close to the loads, reduces the need to transfer energy over long distances through the electric grid. In this way several benefits are achieved, such as the reduction of bottle-neck points created by overcharged lines, the increase of global efficiency and the limitation of thermal stress on grid conductors. Renewable distributed generation units, if properly controlled and designed can improve the power flow management on the grid and reduce the probability of grid faults, so increasing the power quality of the energy supply.

It's important to evaluate also the possible drawbacks of the increasing number of renewable energy sources on the power-supply stability and quality, both in grid connected

Integration of Hybrid Distributed Generation Units in Power Grid 5

In the following sections some guidelines of the design of renewable generation units will be analysed, focusing on the aspects that influences the power quality of the electrical system. Considering that the renewable sources are mainly wind and photovoltaic, the hybrid generation unit that will be considered, it is composed of a wind turbine, a photovoltaic generator and a battery bank. The hydroelectric source will not be considered, because of its atypical features (no need of static power converter, power availability often regulated by means of natural or artificial basins) that makes it similar to a traditional regulated thermic

When it is necessary to supply electrical loads in stand-alone configuration there is no particular standard that define the power quality requirements. Anyway, the mains concepts of power quality regulation in grid connected systems can be used also for standalone systems and as general guidelines, a power quality level close to the one guaranteed

On the following sections, a list of the main power quality concepts is presented, focusing on how this concepts, originally dedicated to grid connected systems, can be adapted to stand-alone configurations, in order to obtain some guidelines to improve the performance

For general purpose and to simplify the analysis, a stand-alone system, including some loads and one renewable hybrid generation unit equipped with a battery bank is

These considerations can be generalised and adapted to a system of several renewable generation units parallel connected in order to supply the loads in stand-alone configuration. The main considerations regarding the battery bank can be adapted also to other energy storage systems, such as flyback wheels, hydraulic reservoirs, fuel cells, etc.

In the considered renewable power units, the interface between power generators and loads is made by one ore more static converters. It is important that one of this static converter is a voltage controlled inverter (interface converter) that generates a voltage waveform corresponding to a certain reference given to its control system. By regulating the voltage reference signal, it's possible to obtain a sinusoidal voltage waveform with desired

In order to maintain the nominal voltage level on the loads, it is important to include in the voltage regulator of the interface converter a compensation term that takes into account the voltage drops on the line impedance, by measuring the current drawn by the loads as it is

• The line connecting the power plants and the loads, in small application is a low voltage line, thus characterised by a mainly resistive impedance. The compensation term in the regulator should take into account the line impedance value and typology (reactive or

**3.1 Requirements on voltage waveform (voltage amplitude in normal operating** 

**3. Hybrid renewable generation units in stand-alone mode** 

frequency and amplitude, within the limits of the power converter.

Some consideration can be made about the Fig.2:

**2. Renewable generation units in stand alone mode** 

for loads fed by the grid can be choose.

generation units.

of these systems.

considered.

**conditions)** 

shown in Fig.2.

resistive).

and stand-alone configurations, in order to prevent possible problems with a proper design and management of this generation units.

Fig. 1. Example of a HDGU structure with DC and AC bus

The negative influences on power quality of renewable energies derives mainly from two typical characteristics of renewable sources: their randomly varying availability and the presence of a static converter as output interface of the generating plants (with exception for hydroelectric).

The sudden variation of generated power from renewable sources can lead to frequency fluctuation and to grid instability, in grid connected systems, and to total or partial loss of loads power supply in stand-alone systems.

The output interface converter introduces in the electric system voltage and current harmonics that affect negatively the system power quality. On the other hand, if properly designed and controlled, the interface converter can efficiently support the grid in normal operations (reactive power and voltage control) and during grid faults. To reduce this influence, many actions can be taken, typical of the traditional power converters' design technique, such as: low emission topologies, high switching frequency, optimised control techniques and regulators and output passive filters (typically in LC configuration)

To mitigate the effect of renewable sources randomness on power quality different actions are possible: equipping the renewable generation unit with a storage system, integrating to the renewable source a non-renewable source (e.g. a diesel generation unit), integrating in the renewable source a storage system and a non-renewable programmable source.

What is interesting to talk about, as a specific aspect of renewable generation units, is the electric system that lays beyond the interface converter (renewable source, energy storage, power flow control, ecc…) and that allows to transform the random renewable sources into a regulated, flexible and grid-friendly (in grid connected mode) or load-friendly (in standalone mode) generation units.

First off all it will be analysed a first solution consists of a renewable source equipped with an energy storage system in stand-alone mode. Secondly a grid connected application of diffused hybrid generation units itwill be analysed.

and stand-alone configurations, in order to prevent possible problems with a proper design

ICE EG

CHCP

IM

Hydropower generator

PMSG Wind energy conv. system

Energy storage

The negative influences on power quality of renewable energies derives mainly from two typical characteristics of renewable sources: their randomly varying availability and the presence of a static converter as output interface of the generating plants (with exception for

The sudden variation of generated power from renewable sources can lead to frequency fluctuation and to grid instability, in grid connected systems, and to total or partial loss of

The output interface converter introduces in the electric system voltage and current harmonics that affect negatively the system power quality. On the other hand, if properly designed and controlled, the interface converter can efficiently support the grid in normal operations (reactive power and voltage control) and during grid faults. To reduce this influence, many actions can be taken, typical of the traditional power converters' design technique, such as: low emission topologies, high switching frequency, optimised control

To mitigate the effect of renewable sources randomness on power quality different actions are possible: equipping the renewable generation unit with a storage system, integrating to the renewable source a non-renewable source (e.g. a diesel generation unit), integrating in

What is interesting to talk about, as a specific aspect of renewable generation units, is the electric system that lays beyond the interface converter (renewable source, energy storage, power flow control, ecc…) and that allows to transform the random renewable sources into a regulated, flexible and grid-friendly (in grid connected mode) or load-friendly (in stand-

First off all it will be analysed a first solution consists of a renewable source equipped with an energy storage system in stand-alone mode. Secondly a grid connected application of

techniques and regulators and output passive filters (typically in LC configuration)

the renewable source a storage system and a non-renewable programmable source.

DC/DC Boost converter

AC/DC Converter

AC/DC Converter

Thermal load

AC/DC Converter

> DC/AC Converter

DC/AC Converter

DC/AC Converter

DC/AC Converter

DC/AC Converter

> Costumer load

PUBLIC GRID

AC Bus

and management of this generation units.

AC/DC Converter

DC/DC Boost converter

DC Bus

loads power supply in stand-alone systems.

alone mode) generation units.

diffused hybrid generation units itwill be analysed.

Fig. 1. Example of a HDGU structure with DC and AC bus

AC/DC Converter

AC/DC Converter

Thermal load

PV Array

Energy storage

hydroelectric).

ICE EG CHCP

IM Hydropower generator

PMSG Wind energy conv. system Costumer load

PV Array

DC/AC Converter In the following sections some guidelines of the design of renewable generation units will be analysed, focusing on the aspects that influences the power quality of the electrical system. Considering that the renewable sources are mainly wind and photovoltaic, the hybrid generation unit that will be considered, it is composed of a wind turbine, a photovoltaic generator and a battery bank. The hydroelectric source will not be considered, because of its atypical features (no need of static power converter, power availability often regulated by means of natural or artificial basins) that makes it similar to a traditional regulated thermic generation units.
