POWER QUALITY MEASUREMENT AND ANALYSIS WITH CLUSTERS OF PHOTOVOLTAIC INVERTERS

By

ITUMELENG ISAAC SELEKE

A mini-dissertation submitted for the partial fulfilment of the requirements for the degree

BACCALAUREUS INGENERIAE

In

ELECTRICAL AND ELECTRONIC ENGINEERING SCIENCE

At the

UNIVERSITY OF JOHANNESBURG

STUDY LEADER: PROF TWALA

Contents2REQUIREMENTS ANALYSIS32.1INTRODUCTION32.1IDENTIFIED ISSUES AND CONSTRAINTS32.1.1TECHNICAL42.1.2FINANCIAL AND ECONOMIC42.1.3LEGAL42.2REQUIREMENTS SPECIFICATIONS43LITERATURE REVIEW63.1INTRODUCTION63.2DESCRIPTION OF GRID-CONNECTED PHOTOVOLTAIC SYSTEM AND ELECTRICITY DISTRIBUTION SYSTEM73.2.1CENTRAL STATION PRODUCTION93.2.2DISTRIBUTED PRODUCTION103.3POWER QUALITY ISSUES RELATED TO GRID-CONNECTED PHOTOVOLTAIC SYSTEMS113.3.1HARMONICS113.3.2VOLTAGE RISE/OVERVOLTAGES143.3.3GRID IMPEDANCE VARIATION153.4STANDARDS AND LEGAL CONSTRAINTS OF GRID-CONNECTED PHOTOVOLTAIC SYSTEMS163.5TOOLS163.6CONCLUSION173.7REFERENCES18

Chapter 2 : REQUIREMENTS ANALYSIS Requirements Analysis As with any project that needs to carried out there are certain requirements that should be taken into account by the person undertaking the project in order for the projects objectives to be accomplished accounting for the ultimate success of the project. These requirements are compiled in the foundation stages of the project, this section of the project documentation provides a list and does analysis of these requirements. It will include tasks that go into formulating the needs or conditions to fulfil the project goal.Introduction In Chapter1, the Problem Statement was introduced where with countries all over the world grappling with power crisis due to increase in the cost of electricity production, the environmental considerations that need to understood in the production of electricity by conventional means and high sanctions that are placed on power utilities should they falter on these restrictions. A partial solution to these problems has been the integration of Renewable Energy electric power production looking specifically at a Photovoltaic system, into national grids. The objective of the project will be then to Power Quality analysis on the power generated from such a system, this basically just a measure of the how the power produced from a Photovoltaic system deviates from ideal. In Section1.3 we listed the scope of the project which included locating a LV network which had a PV system coupled onto it and measuring of the power exported into grid by the system at PCC. Analyse the measured data, compare the data to regulatory standards, and make suggestions on how PQ can be improved. Section15 also covered the methodology of the project, setting a path of how the work in accomplishing project objective will be carried out. In Chapter2, the analysis will be on the constraints and other issues which are linked to technicality, economy, impact on society and environment, ethics, usability, exclusions and assumptions are presented.

2.1 Identified issues and Constraints In this section a thorough examination is done in identifying the issues and constraints that surround carrying out power quality measurements and analysis of grid-connected PV with clustered inverters to determine their impact on the grid. These issues are considered important and are likely to be faced during the different stages to the completion of project. These issues will be addressed in order to meet project objective. Technical The power quality issue that is a direct cause of inverters is Harmonics thus the student we only be limited to doing measurements and analysis on the harmonic content present in the grid due to inverters. Power quality parameters such as reactive power, power factor and flicker will not be taken into consideration. Initial projections of the project show that because its power systems measurements and analysis will be simulation based. Here the data to be gathered using simulations The simulations will be done using DIgSILENT PowerFactory as this software package is readily accessible to student as opposed to other packages and it is understood to be the popular power system software in industry. Financial and Economic The cost related to the fulfilling project objective should not exceed R1500, 00 as this amount is the maximum allowed amount for funding final year engineering projects by Faculty of Engineering and Built Environment at the University of Johannesburg. Every cent used in the project and receipt should be accounted for. Legal The project carries some legal attributes that should be adhered to. If any source in the form of literature, images, video, data etc. is used by student in completion of project, such a source should be referenced using IEEE referencing style. Failure to do so it will assumed that the student is claiming work that is not theirs and that will result in punishment as plagiarism is seen as information theft and is not tolerated by University of Johannesburg. The punishment for such an offence is failure of the course.

Requirements SpecificationsHere a description of specific tasks or objectives that should be accomplished with this project. Identify the local and/or systemwide impacts of grid-connected PV systems on the power distribution grid Quantify steady-state impacts Quantify dynamic impacts Provide utility customers with guidelines regarding the expected impacts as a function of the penetration level of grid-connected PV systems Assess potential mitigation measures for any problem discovered during the study. Develop best-practice interconnection guidelines for grid-connected PV systems readiness studies [1].

Chapter 3 : LITERATURE REVIEWLiterature ReviewIntroductionThe use of renewable energy to produce bulk electric power to supplement the already existing conventional methods has grown quite bit over the years. Current trends show how renewable energy production, wind and solar energy (PV) has grown over the past two decades, with PV system capacity enjoying a per annum increase of 55% from 2004 to 2010 [see Figure 1]. At 40GW installed capacity worldwide by the end of 2010. With the technological advances made in semiconductors used in solar cells and more efficient power electronic devices make PV systems a more attractive option for future exploration. Future projections by European Photovoltaic Industry Association (EPIA) show that the global installed PV technology could be 800GW absorbing a large share of the global electricity demand [2].

Figure 1: Comparison of installed capacity of wind and PV systems [2] The project is aimed at determining the power quality of electric power produced by Photovoltaic system with is connected to the grid [see Figure 2]. The objective will be to conduct an impact study on the integration of PV power generation in our grid. In the previous chapters, chapter 1 and 2 the problem statement of the project was introduced and following that the project objective was defined by compiling the requirements specifications, a list of constraints and issues related to such project were documented.

Figure 2: Grid connected PV power system [3]The following will provide literature related to PV systems, giving a description of such a system and its electricity distribution system. This chapter will also provide literature on PQ issues related to PV systems. A look at the standards that govern PV systems that are grid connected will also be looked at. From this chapter the student will equipped with the knowledge to determine how PQ measurement and analysis of a PV system with clustered inverters can be carried out.Description of Grid-connected Photovoltaic system and electricity distribution system A human beings definition of Renewable energy is that an inexhaustible energy source, which is continuously flowing. Solar energy can be thought of as such a source for the Earth, providing an infinite source of energy. Energy delivered from the sun to the earth is thought to be which is said to be 10000 times more than what is currently consumed by the electricity users [4]. The goal over time has always been to harness this energy and use it supplement the current need of electric power. One method used to harness this energy is called Photovoltaic effect, which is the direct conversion of the energy of light from the sun into electric energy. When light rays from the sun strikes the solar cells which is a semi conductive device, the energy of these light rays passes on this energy which is enough to energize negatively charged particles. This action frees these electrons, allowing the potential barrier in the cell to act on them (electrons),separating electrons and holes, each going to opposite sides of the cell to produce a potential difference or voltage which can be used to drive current (direct current) through a circuit [see Figure 3] [5].

Figure 3: A potential barrier in a solar cell separates light-generated charge carriers, creating voltage [5] Because one single solar cell can only provide a limited amount power, depending on the application they must be electrically connected together (in series or parallel) so that it able to provide enough power for whatever chosen application thus the physical size of the connected solar cells depends entirely on its application. The equivalent circuit of PV cell is shown in Figure 4.

Figure 4: PV cell equivalent circuit

(1)Where Ip = Photocurrent [A]Vpv = Terminal voltage of cell [V]ID = Diode current [A]Io = Saturation current [A]Ish = Shunt current [A]N = Ideality factor (value is between 1 and 2)q = Electron charge [C]k = Boltzmanns constant T = Junction Temperature [K]Rs = Series resistance []Rsh = Shunt resistance []

There are currently number of applications that photovoltaic systems are used for namely:Table 1: Applications of PV systems [4]PV ApplicationExample

Consumer ProductsWatches, Cellphone chargers

Off-grid i.e. StandaloneResidential power systems for individual households

Off-grid IndustrialWater management, Lighting and telecommunication systems

Grid connectedIntegrated into the grid, in roofs, outer walls, on huge plots of land (Solar Parks)

For the purpose of this research project the focus will be on grid connected PV systems. Photovoltaic source produces DC voltage which for some applications is appropriate but for some applications which are powered by AC voltage the voltage needs to be converted from DC to AC. The action of converting the DC-AC voltage is done by a device called an inverter. The inverter uses power electronic equipment for the conversion process to supply power to ordinary electric equipment. It essentially is a switching device that regularly flips polarities at the output terminals [6]. Such a conversion process is used particularly for grid-connected PV systems where AC power flows. Here clustered PV systems are connected and interfaced to medium to low voltage networks. Below is a typical diagram of a grid connected PV system. It consist of a PV array which is made out of a number of PV cells which are electrically connected, cells are usually connected in series to form a solar module which are then connected in series to form a string. Theses strings are then connected in parallel to form an array. There are number of configurations that these PV arrays can be arranged namely; Centralized Configuration: Here one inverter is connected to the to the PV array String Configuration: With this configuration each string in the PV array is connected to a one inverter. This configuration increases the reliability of the system. Multi-string Configuration: Here each string is connected to DC-DC converter for voltage amplification and an implementation of maximum power point tracking (MPPT). AC Module Configuration: This is the most recent configuration where an inverter is embedded into each module. an inverter for DC-AC conversion which was discussed, a filter which is used to get rid of any harmonic content which is different to the fundamental electrical frequency. The system also consist of a transformer which is used to step-up the voltage from the inverter to a nominal voltage which is at the same voltage and frequency value as that of the grid [see Figure 4]. There are other components which form part of the PV system one of which is the diode which is used to block any reverse current flow to the PV array causing potential damage to it. Figure 5: Schematic diagram of grid-connected PV system [6]

Grid connected PV systems can be broken down into two categories, distributed production and central station production. Central Station ProductionA central station grid-connected PV system is Megawatt (MW) size plant at utility scale. Here utilities use the electricity generated for either base load or peaking load. The system is coupled into grid by distribution substations through feeders. Grid-connected PV systems of MW-size used for central station applications make use of large number of power electronic inverters modules which are connected in parallel [1]. A system like this makes use of a number of interconnection transformers [see Figure 5]. An electricity generation schemes like this are often touted as a future competitor against turbine-generated electric power generation (using fossil and nuclear fuels).

Figure 6: Utility sized PV-system connected to distribution substation

Distributed Production Distributed grid-connected PV systems are seen as possibly being the solution to peak loading issues, they are small to medium scale systems. The distributed system has capacities that range from 10 kW to 1000 kW, here installations are on residential establishments, economic complexes, industrial sites and other buildings. Here clustered PV systems are connected close to the loads. The scale of the PV system is comparable to the load, they can either be three phase or single phase. The PV arrays are located on rooftops or places of high solar irradiance near the load. There can be a number of PV-systems connected to a single feeder [see Figure 6].

Figure 7: Distributed grid-connected PV system

The advantages of implementing such PV systems being that it is clean energy source with no emissions, no use of fuels and water. It requires little maintenance as it has no moving parts. The integration of such systems into the grid is also able to create new forms of employment and decrease cost of electricity to consumers. More importantly the efficient management of grid-connected PV systems is also able to provide electric supply security. There are still studies that being carrying on the impact of the integration of PV systems in distribution systems, at present there a positive and negative issues related to PV systems integration in power systems related to the operating characteristics of the network itself and the characteristics of PV system.

Power Quality issues related to Grid-connected Photovoltaic systems Though the implementation of PV systems is seen as a solution to help with over loading on utility transmission and distribution lines, provide peak load shaving, give overall grid support and curb environmental issues related to power generation yet there are impacts that need to be studied on the high penetration of clustered PV systems into networks. There are however potential adverse effects this system posers to the network due to the use of certain devices. This speaks to the Power Quality (PQ) electrical power generated from such a renewable source. A high density of grid-connected PV systems means there will be a number of inverters connected to the grid. Here are some of the issues seen as potential challenges in the integration such as Harmonics, Voltage rise, grid impedance variation and voltage imbalance, all these issues are serious enough to warrant studies [7].HarmonicsAlthough the topic of power quality covers harmonics, unbalance, flicker, sag, swell and other concepts power electronic devices are seen as the major cause of harmonics. This has become the main concept that power quality engineers working on mitigating. The function of the inverter is to supply current that is synchronized with the grid, this device can also change the characteristics of the grid. The inverter is seen as nonlinear device, this means the electrical device which does not have linear relationship with between current and voltage because of high frequency switching of semiconductor devices with pulse width modulation harmonic distortion can be generated. Harmonics refers to the sinusoidal current or voltage generated from the inverter having frequencies that are integer of the frequency at which the distribution network system is designed to operate, this includes the electrical equipment that connected to the network [8]. For example the supply frequency component in South Africa is 50 Hz which is the fundamental, because of the inverters higher frequency components will always be multiples of the fundamental frequency which are then called harmonics. Harmonic order is then used to describe the ratio of the harmonic frequency to the supply which are 3, 5, 7, and 11, and so on [see Figure 7]. These higher order frequencies will distort the sinewave causing it deviate from ideal, every cycle of the waveform is then distorted equally.

Figure 8: Illustration of resultant current affected by Harmonics An indicator that used to determine the extent of the distortion of the waveform is called Total Harmonic Distortion (THD). The THD is the ratio of the r.m.s value of all the harmonic components of the sinusoidal waveform to the fundamental (I1 or V1), it is expressed as percentage (%) [9]: For current (I), THDI If the total r.m.s value of the current (IRMS) is known then the following relation to obtain THDI is:

The distorted power generated flows through the distribution network having some effect on the apparatus that make the grid. One of the effects of harmonic is overcurrents flowing through the grid which causes heating of conductors and iron circuits of rotating machines, transformers that are connected. Here because of the harmonic current there is an increase in heating effect of current over the that of the fundamental current, for example if:

So if the harmonic current has 100% I1 which is the fundamental, I3 which is 40% of I1, I5 which is 25% of I1 and I7 is 15% of I1. This will give a total r.m.s current (Irms) or thermal current of 111% I1 which will have heating effect of 1112/1002 100% = 123% meaning the current generated from harmonic inducing source will create a heating effect of 123% over that of the rated or fundamental current. This would deteriorate the conductors of electrical equipment and deteriorate the insulation leading to failure of the apparatus costing power utilities and industrial companies lots of money to replace [see Figure 8]. This would also result in the incorrect operation of overcurrent protection equipment [10].

Figure 9: Equipment burnt due to Harmonic currents [11] [12]Another impact of harmonic currents on the grid is the rise of harmonic voltages. Harmonic currents generate significant overvoltages when they are flowing against impedances. The result of this being the failure apparatus. This issue could be further exaggerated by the presence of resonance in the grid which could cause the harmonic current to generate excessively large harmonic voltage in the grid. Voltage Rise/Overvoltages When the demand due to the increase in the load in the network is very high (Nominal Load) then the operating voltage in the distribution system tends to drop below tolerable range. This disturbance will result in a long duration disturbance called an Under-voltage, with the voltage dropping the further the load is from the source [see Figure 9] [12]. This phenomenon causes the equipment that are connected to grid not to operate as the voltage is below the rated voltage.

Figure 10: Voltage profile of Feeder without (left) and with (right) grid-connected PV systems at Nominal LoadThe integration of grid-connected PV technology to distribution feeders is seen as a solution to the voltage drop dilemma. Particularly with distributed PV systems, they will improve the voltage profile of the network by reducing the voltage drop as the power flow through the feeder is reduced. But this solution also has its limitations because if theres a minimum load or light load condition and the power generated by PV system is greater than demand of the load then the excess power flows back into the network this is called a reverse power flow [see Figure 10] [13]. This phenomenon will cause the voltage to rise in the feeder resulting in misoperation over voltage protection equipment and damage equipment as transformers and lines in the network exceed their thermal limitations.

Figure 11: Voltage profile of Feeder without (left) and with (right) grid-connected PV systems during Light Load (minimum load)Grid impedance variationIn order to combat harmonic frequencies going to the grid and causing problems to all the equipment connected, grid-connected PV power generation systems which make use of inverters are fitted with filters. These output filters are used to attenuate high frequencies so that the PV system can be coupled to the grid, but these filters can introduce stability problems. These filters are made L-C circuits which add some impedance to the grid, varying the inherit impedance in the grid, from transformers and the cables that are connected. This could cause the grid to become resonant [3]. The capacitor is largely responsible for setting up a resonance circuit together with the transformer reactance and cable reactance in the grid. If theres series resonance then the grid impedance is at its lowest which will cause a small harmonic voltage to result in the high harmonic current could harm apparatus connected. If parallel resonance occurs then the grid impedance is at its highest and a small harmonic currents could cause high voltage distortion to occur [14].

Standards and Legal Constraints of Grid-connected Photovoltaic systemsThere are number standards that serve as guidelines for the integration PV systems into the grid. The work of writing up these standards is undertaken by reputable independent organisations, whose work is internationally recognized. Here is look at the most common standards and what they say about level of harmonic distortion permitted in the network. EN 50160: This the European standard which species the recommended levels of different power quality parameters. This code is adopted by most European Grid Codes. It states that the limit for the total harmonic distortion should not exceed 8% up to and including the 40th harmonic in its Voltage characteristics of electricity supplied by public distribution system IEC 61727: Photovoltaic (PV) systems: Characteristics of utility interface this standard wishes to addresses the interface requirements between PV systems and the utility. It imposes a limit for the total current harmonic distortion equal to 5% [15]. IEEE 519-1992: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems standard by the Institute of Electrical and Electronic Engineers specifies that party responsible for generating electricity be limited a total harmonic distortion (THD) for current and voltage of 5% that it can supply to the customer. It also specifies that the limit for maximum individual harmonic components must be 3% for voltages lower than 69kV [16]. These standards or limits apply at point of common coupling (PCC) of utility and consumer. Tools With this project being a power systems project the limitation is the availability and cost of equipment mentioned and safety measures required to perform analysis on a scale that resembles the real life situation. To counter this constraint the student will use a software program to simulate and do analysis of a power system that has PV technology integrated into its grid, the chosen program is DIgSILENT PowerFactory. DIgSILENT is a program that provides tools for modelling of power systems i.e. generation, transmission, distribution and the loads connected for analysis of how they interact [17]. It is used to identify errors and issues that could lead to system instability.

Similar Work ConclusionThe Literature Review was used to provide an outline of the project topic, giving it a bit of context as area of renewable energy being investigated. In section 3.1 a brief indication of the current and projected future trend of PV technology was given. In section 3.2 a description of grid-connected PV-systems is given and the different apparatus used in this integration of PV-technology and grid. In section 3.3 power quality issues related to grid-connected PV systems are given such as Harmonics, Overvoltages and Grid impedance variation. Section 3.5 and 3.5 will serve as guidelines for the measurements and analysis that are to be done in the chapters that follow. Using all thats covered in Chapter3 methods that can used for measurement of Power Quality will be determined.

References

[1] F. Katiraei and J. R. Aguero, Solar PV Integration Challenges, IEEE Power & Energy magazine, p. 62, 21 April 2011. [2] R. Shah, N. .Mithulananthan, Bansal, R.C. and V. Ramachandaramurthy, A review of key power system stability challenges for large-scale PV integration, Renewable and Sustainable Energy Reviews, no. 41, pp. 1423-1436, 2015. [3] M. Eltawil and Z. Zhao, Grid-connected photovoltaic power systems: Technical and potential problemsA review, Renewable and Sustainable Energy Reviews, no. 14, pp. 112-129, 2010. [4] M. Zeman, INTRODUCTION TO PHOTOVOLTAIC SOLAR ENERGY, in SOLAR CELLS. [5] Solar Energy Research Institute, Basic Photovoltaic Principles and Methods, Colorado: Technical Information Centre, 1982. [6] A. Fazliana, A. Kadir, T. Khatib and W. Elmenreich, Integrating Photovoltaic Systems in Power System: Power Quality Impacts and Optimal Planning Challenges, International Journal of Photoenergy, 2014. [7] O. Ozgonenela, T. Yalcina, I. Guneyb and U. Kurt, A new classification for power quality events in distribution systems, Electric Power Systems Research, pp. 192-199, 2013. [8] V. Gosbell, Harmonic Distortion in the electric supply system, Technical Note No.3, March 2000. [9] W. A. Maslowski, Harmonics in Power Systems, Mequon: ALLEN-BRADLEY CO., INC.. [10] Solar Edge, Problems and Disadvantages in Current Residential & Commercial On-grid PV Systems, 2012.[11] D. E. Stewart, J. MacPherson and S. Vasilic, Analysis of High-Penetration Levels of Photovoltaics into the Distribution Grid on Oahu, Hawaii, National Renewable Energy Laboratory , Oahu, 2013.[12] E. Demirok, D. Sera, R. Teodorescu and P. Rodriguez, Clustered PV Inverters in LV Networks: An Overview of Impacts and Comparison of Voltage Control Strategies, 2008. [13] A. Stavrou and e. al, Towards the establishment of maximum PV generation limits due to power quality constraints, Electrical Power and Energy Systems, no. 42, pp. 285-298, 2012. [14] M. Liserre, R. Teodoresen and F. Blaabberg, Stability of Grid-Connected PV Inverters with large Grid Impedance Variation, in 2004 34th Annual IEEE Electronics Specialist Conference , Aachen, Germany , 2004. [15] K. Kontogiannis, G. Vokas, S. Nanou and S. Papathanassiou, Power Quality Field Measurements on PV Inverters, International Journal of Advanced Research in Electrical,Electronics and Instrumentation Engineering, vol. 2, no. 11, 2013. [16] J. Muoz, G. Nofuentes, J. Aguilera, M. Fuentes and P. Vidal, Procedure to carry out quality checks in photovoltaic grid-connected systems: Six cases of study, Applied Energy, no. 88, pp. 2863-2870, 2011. [17] J. Lei and N. C. Nirmal-Kumar, Power quality analysis for building integrated PV and micro wind turbine in New Zealand, Energy and Buildings, no. 58, pp. 302-309, 2013.

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