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Power Quality Analysis of a System Based on Renewable Energy Supplying a Local Distribution

Network Cristian Drago Dumitru #1, Adrian Gligor#2

# Electrical Engineering Departament, Petru Maior University of Tg. Mure Nicolae Iorga St., No. 1, Tg. Mure, Romania

1 cdumitru@engineering.upm.ro

2agligor@upm.ro

AbstractThis paper deals with the quality analysis of an

electrical local network supplied with energy provided by renewable resources. The presence on a large scale of renewable energy resources brought some issues concerning generally the electromagnetic compatibility and particularly the quality of energy regarding the harmonic pollution. Modelling and analysis of the electric energy quality in the proposed system is conducted by programs developed in Matlab/Simulink environment.

I. INTRODUCTION Nowadays, renewable energy represents an alternative to

consider for solving of environmental friendly energy demand and consumption. Renewable sources of energy have very different characteristics. They are much smaller in scale and benefit from economies of mass production rather than economies of scale. For wind, water and solar power, the fuel is free but intermittent. Solar energy is available at the point of use but wind and water resources are generally further from load centres [1].

With the introduction of renewable energy sources the distribution systems becomes an active system with both energy generation and consumption. This introduces some major problems concerning power quality. As this type of sources imply the intensive using of electronic power devices one relevant issue is the appearance of power quality distortion phenomena. These problems are often difficult to solve and may allow different solutions, solutions that require higher investments, so the choice is not always simple for engineers, professionals and managers.

This paper presents an application that constitutes a useful tool for emphasise and understand the most relevant phenomena concerning renewable energy systems power quality.

II. POWER QUALITY AND INDICES Power quality has become a real problem over the last

years due to the use of power electronics, as mentioned above, but also due to sensitive load equipments. Inverter-connected generators, commonly used in case of renewable energy, may inject non-sinusoidal currents into the distribution network and cause additional power quality distortion phenomena.

Power quality distortion phenomena can be subdivided into:

- harmonic distortion standards such as IEEE 519-1992 impose limits for different order harmonics and the total harmonic distortion (THD). The limits for system voltage distortion are 5% for THD and 3% for any individual harmonics [2];

- transients IEC Publication 60071-1 defines transient overvoltage as a short-duration overvoltage of a few milliseconds or less, oscillatory or non-oscillatory, usually highly damped [3];

- voltage dips a voltage dip, according to IEC, is a sudden reduction of the voltage at a particular point of an electricity supply system below a specified dip threshold ( within a time period no shorter than 10 ms) followed by its recovery after a brief interval. ;

- voltage swells - temporary increase in RMS value of AC voltage with the magnitude of the retained voltage between 110% and 180% of the rated voltage;

- flicker impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time [4];

- voltage asymmetry (unbalance) the loads cause non equal phase currents to flow in the power system and unsymmetrical voltage drop on the system components. As a consequence, voltage balance in the node is lost;

- frequency variations represent a change in frequency from the normally stable utility frequency. This may be caused by erratic operation of emergency generators or unstable frequency power sources.

Power quality system analysis requires an accurate knowledge of the spectral components of non-stationary current and voltage waveforms. One of the main applications of spectral components in the field of power quality refers to the waveform distortion indices. Several indices are in common use for the characterization of waveform distortions and they generally refer to periodic signals, which allow a definition of harmonic components. Those indices are:

- total harmonic distortion (THD) general purpose, standards;

- power factor (PF) potentially in revenue metering ; - telephone influence factor audio circuit interference; - C message index communications interference; - IT product - audio circuit interference, shunt

capacitor stress;

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- VT product voltage distortion index; - K factor transformer derating; - crest factor dielectric stress; - unbalance factor three phase circuit balance; - flicker factor - incandescent lamp operation; bus

voltage regulation; sufficiency of short circuit capacity;

III. MATLAB SIMULINK MODEL DEVELOPMENT FOR PARAMETER QUALITY ANALYSIS

The proposed wind-solar hybrid system for power generation is used as an instructional resource for teaching renewable energy and power quality concepts. Each technology has its own individual instrumentation requirements to measure and control system variables. Although the wind-solar power system based on renewable energy have helped in teaching real-world applications of the renewable energy, the lack of computer aided instrumentation forced us to develop and simulate in Matlab Simulink environment such tools and to implement new blocks.[5]

The analysed system is based on specific mathematical models of the components: photovoltaic and wind system.

A. Matlab/Simulink model development of photovoltaic ideal and real system for parameter quality analysis

Fig. 1 shows the Matlab/Simulink model implementation of photovoltaic system consisting from a solar model, PV and inverter model.

Fig. 1. Matlab/Simulink model implementation of photovoltaic system

B. Matlab/Simulink model development of wind ideal and real system for parameter quality analysis

Fig. 2 shows the Matlab/Simulink model implementation of wind system consisting from a wind model, wind turbine and inverter model.

Fig. 2. Matlab/Simulink model implementation of wind system

C. Matlab/Simulink model development of hybrid solar-wind real system for parameter quality analysis

Fig. 3. Matlab/Simulink model implementation of hybrid system

IV. EXPERIMENTAL RESULTS

a)

b)

c)

Fig. 4. Harmonic analysis of the voltage waveform corresponding to a) phase A, b) phase B and c) phase C in case of system supplied from power system

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a) b) c)

Fig. 5. Harmonic analysis of the voltage waveform corresponding to a) phase A, b) phase B and c) phase C in case of system supplied from PV

a) b) c)

Fig. 6. Harmonic analysis of the voltage waveform corresponding to a) phase A, b) phase B and c) phase C in case of power supply from wind generator

a) b) c)

Fig. 7. Harmonic analysis of the voltage waveform corresponding to a) phase A, b) phase B and c) phase C in case of power supply from hybrid PV and wind generators

V. CONCLUSION Depending on the methods or technologies, rational use of

energy can have a positive or a negative impact on power quality. The cause of power quality problems is mainly the improper use of local loads and equipments. But, even if efficient loads are connected to a grid by a power electronics interface, the power quality might deteriorate. There are also devices with malfunction when the power quality becomes too poor. Connecting the hybrid solar-wind generation system to a local AC distribution network implies the solving of some difficult issues concerning:

- network overloads in normal and short-circuit regime; - influence on power quality; - voltage profile changing and reactive power flow

control; - contribution of renewable sources to power system

services;

- renewable sources adaptability to normal disturbances of power system;

- influence on power system stability; - influence on power system protections; - insulation issues and insulation coordination; - power system reliability and availability;

REFERENCES [1] J.W.Plastow., Energy services for an electricity industry based on

renewable energy, Power Engineering Journal -- October 2001 -- Volume 15, Issue 5, p. 239-247

[2] Baggini, Angelo: Handbook of power quality. West Sussex, John Wiley and Sons, 2008

[3] IEC 60071-1,Insulation co-ordination Part 1: Definitions, principles and rules, Eight Edition,2006;

[4] IEC 60050 (161), International Electrotechnical Vocabulary (IEV) Chapter: Electromagnetic Compatibility

[5] Dr.Recayi Pecen, Dr. MD. Salim, Mr. Ayhan Zora. A LabView Based Instrumentation System for a Wind-Solar Hybrid Power Station, Journal of Industrial Technology, Volume 20, Number 3, June 2004 through August 2004 www.nait.org