Grid Connected Solar PV System: Issues & Challenges
(Paper ID: 412ET3011201707)
Pawar Heena Kaushikrao Prof. Amarpal Kanojiya
Electrical Department Electrical Department Dr.Jivraj Mehta Institue of Technology Dr.Jivraj Mehta Institue of Technology
Mogar, Anand, Gujrat Mogar, Anand, Gujrat
[email protected] [email protected]
Abstract: India is a country which has a rich amount of sunlight i.e. solar energy and it is exploited in large amount. We can generate large amount of electricity from solar energy. Solar energy is renewable energy and it is free from pollution and most important factor is it is free of cost. Solar energy is converted into electricity by photovoltaic cell. Before it is costly to generate electricity from photovoltaic cell. But nowadays cost is reduced of PV system. In this paper we can study that how the Grid connected solar PV system used to feed the National Grid & Residential load as well. We can also study the comparison of Grid connected &
Off Grid system. What are the issues there as Solar PV system included both AC & DC current mode. What are the challenges we face during the solar power generations.
Keywords: PV system, Grid connected PV system, Array.
I. INTRODUCTION
Solar Power is an important renewable resource for energy production all over the world. It is clean, environment friendly, free of cost and available in large amount. India has lots of potential of solar energy. All the year 5000trillilon KWH/year energy is incident over Indian land over 300 sunny days & it receives 4-8KWH/Sqm/day. As such, solar power offers significant potential to meet a large share of the country's energy needs. Solar energy, therefore, has a tremendous potential as a future energy source. It has the advantage of permitting decentralized distribution of energy, large centralized power generation plants, smaller distributed heat and power plants; or scaled down, at the individual consumer level. Thus, it has the leverage of empowering people at grass root level.
As on September, 2017, Power system of India had an installed generating capacity around 329.3GW and thermal power plants powered by coal, gas, naphtha or oil are connected for approximately 66.6% of total power generation capacity. Hydropower share of 13.6%.
Renewable sources of energy account for around 17.7% of the total power generated.[1]
Installed capacity of wind power having a share of around 55.76%. Bio gas shares 14.23%, solar shares 22.49% and last small hydro plants shares 7.52% of total installed capacity of renewable resources. Thus, there is a need for large-scale power generation from both Non-renewable and renewable sources of energy to sustain a targeted GDP growth of 8% to 10% per year[4].
Solar System installation capacity in India of Grid interactive PV system is increased from 2.49 GW in 2012 to 13.12GW IN June 2017[1]. The installed grid connected solar power is 1035MW and the capacity of off-grid solar PV is 85MW as in October 2012 [2].India targeted of 20GW of solar installation till 2022 according to Government of India Mission for solar sector i.e. “Jawaharlal Nehru National Solar Mission (JNNSM).[3]
One feature of the global PV industry is that PV-generated electricity is replacing other forms of electricity at an increasingly high rate. This is most evident in the growth- rate for the grid connected Electricity, which has become the dominant market for PV-generated electricity. Other applications for PV-generated electricity include communication and signaling, special commercial and industrial applications, rural off-grid systems, consumer use and large-sized power plants not connected to the grid.[5]
II. PHOTOVOLTAIC CELL
Fig. 1. Fig 1. Photovoltaic cell
Photovoltaic cells use semiconductor material to convert sunlight into electricity. It is the core equipment of the solar system. The technology for doing so is related to the solid- state technologies which are used to make transistors, diodes, and all of the other semiconductor devices that we use so many of these days. Grid-connected solar Photovoltaic (PV) systems use the direct conversion of
sunlight into electricity which is fed directly into the electricity grid without storing it in batteries. This option, is generally carbon free or carbon neutral and as such does not emit greenhouse gases during its operation, since global warming and climate change are mostly caused by the release of carbon dioxide and other greenhouse gases into the atmosphere. Photovoltaic systems are solar energy supply systems that convert sunlight directly to electricity.
Fig.1 shows basic diagram of a cell [5].
The module of PV cells to be used for a wide range of power applications ranging from a few mill watts in wrist watches and scientific calculators to several Megawatts in central power stations.
Solar cells are usually made of semiconductor materials such as silicon, gallium arsenide, cadmium telluride or copper indium dieseline. Solar cells are available in three major forms, which is based on the nature of the material used in their production [7].
The two main forms are 1.Crystalline solar cells, 2. Thin film solar cells. Mono crystalline and Polycrystalline cell are main types of thin film cells & 3.Concentrating PV.
(A) Crystalline silicon PV:
sliced from ingots or castings or grown from ribbons
It is the first generation solar technology and have been around a long time, providing evidence of their durability and longevity. PV panels made from crystalline solar cells are able to convert the highest amount of solar energy into electricity of any type of flat solar panel.
Crystalline solar panel is not hazards to environment.[7]
(B) Thin film solar cell:
photo-sensitive materials deposited in thin layers on a low cost backing, e.g. Glass, stainless steel (produces lower efficiency cell than crystalline silicon). Typical photo-sensitive materials include amorphous silicon (a-Si), copper indium diselenide (CIS,CIGS) and cadmium telluride (CdTe).
Better performance in weak sunlight environment
Silicon thin film PV with high absorption coefficient also brings the benefit of module installation against any direction of sunlight.
Better performance at high ambient temperature.
Silicon thin film PV with low temperature coefficient has much better capability in hot environment.
More resistant against Shading.
Silicon thin film PV outputs power robustly even in shading condition. While, crystalline silicon PV doesn’t work well due to some cells without output power.
High energy yield.[7]
(C) Concentrated Photovoltaic (CPV):
Commercially available over 16 billion kWh of operational experience;
operating temperature potential up to 500°C (400°C commercially proven)
Commercially proven annual net plant efficiency of 14% (solar radiation to net electric output)
Commercially proven investment and operating costs
Modularity & Good land-use factor
Lowest materials demand[7]
III. PVMODULES,PANELS &ARRAY.
Fig. 2. PV cell, PV module & arrays
The major component of a PV system is the solar panel which is formed by putting together several PV cells.
Several PV cells forms a PV module, several modules form arrays and several arrays form panels. The modules are constructed like a sandwich which has a cover of low iron glass and a back sheet to protect them from the harsh environment. Iron glass protects the front surface of material to maintain the high transmissivity. The structure frame is used to protect the glass. The power generated from the panels goes into the inverters.[6]
A typical crystalline cell might be 100 x 100mm. Cells are combined to form modules. The size of PV array is assumed by the available area, amount of the solar radiation and the load required. In PV system the panel can be selected by the manufacturer, technology and power.
PV CELL
PV
MODULE ARRARY
Table 1. Mechanical data for240Wp Solar PV Module
Sr.
No
Parameters Value
1 Cell 156mm2 high power Polycrystalline solar cell 2 Connection 60 cells in series
3 Dimension 1644.6x982.2x40mm (1.62 sqm)
4 Weight 18.60Kg
5 Connection type
Cable with plug connector (compatible with MC4 connectors)
6 Bypass diode
3(three) 15A bypass diode
Table 2. Electrical data for240Wp Solar PV Module
Sr.N o.
Parameters Values
1 Maximum Power 240Wp
2 Open Circuit Voltage 37.26V 3 Short Circuit Current 8.39A 4 Voltage at point of
maximum power
30.85V 5 Current at point of
maximum power
7.80A
6 Max System Voltage 600/1000V(UL/IE C)
7 Tolerance at Pmax 0% TO 3%
8 Temperature Coefficient- Power
-0.45%/OC
IV. ENERGY PATHWAY IN RENEWABLE POWER
Fig. 3. Energy pathways in a renewable power mini-grid connected with the utility grid.
In fig 3. Shows that how renewable energy is generated by wind, water, bio-mass & solar, which is available locally.
These all are the resources, that is Primary Energy. Wind energy converted by wind turbine into kinetic energy then
by Induction generator into AC power. Same as water’s hydro energy converted into the kinetic energy and then into the AC. Biomass is converted into gases which run the grassfire engine i.e. kinetic energy, which is converted into electricity. Last solar energy is converted by PV cell into DC power then by help of inverter it converted into AC.
This is given to the main grid with common coupling and the onsite AC loads. [8]
V. STRUCTURE OF GRID CONNECTED PVSYSTEM
The topologies of the grid connected PV system occur in three forms as shown in Fig. 4(A)Central Inverter topologies, (B)String topology and (C) Module topology.
[9],[10]
Fig. 4. Existing topologies of the grid connected Photovoltaic Systems: (A) Central topologies. (B) String topology (C) Module topology
In Central Inverter topology the integrated arrangement of the PV module is connected to the centralized inverter and then to the grid. It’s centralized inverter cost is low, it is robust and easy in maintenance, which increased its efficiency. The efficiency of Central topology system is about 97.5%[11]. But it has one disadvantage is high
mismatch loss, inverter sensitivity to the voltage on DC side. It is used in residential application.
In String topology PV panels connected in strings, and these strings are connected to an inverter, which finally connected to the grid. The string has an advantage is that each string can be oriented in directions of maximum power. It is normally used in large power plant applications.
In Modular topology each PV modules has an inverter integrated to it. It is advantageous that the introduction of inverter separately reduces the inverter functionality. Each panel can be optimally tracked. It is not used mostly, but typical in 200W power application.
The selection of the above topology is made with the priority of power handling capability. For low power module topology is used, and the string topology and central topology is in large scale power systems. For more optimal power and large power capacity string topology is recommended. So there are many factors such as cost, maximum power control, efficiency, power handling capacity etc [11],[12].
VI. BLOCK DIAGRAM OF GRID CONNECTED PVSYSTEM
Fig. 5. Block Diagram of the grid connected PV system
As shown in block diagram there are 6 main components of the grid connected PV system. We can discuss one by one as below.
(1) PV array: As discuss above PV modules are connected together and from that PV array is constructed. These arrays are used in the grid. For sizing a grid-connected PV array there are a number of important points to keep in mind:
• Energy demand:
This is the most important factors for the installation of array size. The current price difference between purchased electricity and sold electricity, the energy using on site makes more financial sense.
• Contribution to the overall load:
Size of array depends on the basis of a contribution to the overall load generated for the particular area rather than to meet a particular load.
• Contribution to the annual load:
Usually, sizing is to determine the contribution to the total annual load but one can also consider the contribution to the annual load during daylight hours.
• Available area:
The available facade area may restrict the array size, particularly in smaller installations.
• Budget:
Often the available budget is the dominant constraint.
(2) PV array circuit combiner box(junction box): In the Junction boxes, individual module strings are connected together and form a bundle. Then it is safely routed to the inverter. It is a combination of well-organized string monitoring system and a safety concept adapted to the PV technology. The junction boxes will have suitable cable entry points for both incoming and outgoing cables fitted with cable glands of appropriate sizes.
They monitor the output of solar PV arrays. If difference between string outputs is too large, the operator is informed though monitoring system. Active disconnection allows string voltages to be measured separately. These junction boxes are enclosed in an IP 65 rated stainless steel housing, making it ideal for long term use in PV systems. In addition, the direct connection between the strings and the spring clam connectors ensures a durable and safe installation.
(3) Ground fault protector: The array structure of the PV yard will be grounded properly using adequate number of earthing kits. All metal casing/shielding of the plant shall be thoroughly grounded to ensure safety of the power plant.
(4) DC/AC inverter: The inverter is used to convert DC power generated by PV module into the AC power and then it is given to the grid. The inverter automatically does the start up and the shut down function. To minimize the power loss, during the conversion process, the inverter technology uses IGBT(Insulated Gate Bipolar Transistor) technology and through the use of iron powder chocks and high-quality transformers with the losses less than 1%. For large power installation multiply inverters are parallel. The string/central inverters operates on MPPT (Maximum Power Point Tracking) mode to ensure maximum output from the solar generators at different condition. String inverter use higher system voltage to reach high plant efficiency.
Factures of inverter: The conversion efficiency is 98%
peak. Enhance availability, compact and easy to maintain, anti islanding, high accuracy MPPT algorithm over a wide voltage range. Communication and PC tools for local or remote configuration and monitoring.[3],[7].
Table 3. Electrical data of Inverter.[3]
Sr.No. Parameter Values
1 AC Current 1800A
2 DC Current 2100A
3 Inverter Power at 50OC 1060KWp
4 Max THD 2.28%
5 Euro Efficiency 98%
6 MPPT voltage range 400-800V
7 Maximum no. of internal DC landings points
18
VII. CONVENTIONAL OFF GRID AND GRID TIED SYSTEM
Off gird system: PV systems which are installed without being connected to the conventional grid line are called “Off grid system” and the most of these systems uses the large capacity battery as a storage device. These PV systems are not connected to the grid of public electricity. They require a storage system which stores energy. During day solar energy is available but at night it is not there, so storages system required to store energy which used at night. They are mostly used in areas where main utility grid is not possible to install or where this is not cost effective. They are recommended in developing countries in remote areas, where supply of the main grid is not possible. These are used in industrial applications such as telecommunications and traffic signs, remote habitations such as solar home systems and water pumping applications. [6]
The Grid connected PV system: PV systems connected to a large independent grid and feed the power to the grid directly are called the “Grid connected PV system.” These are used in both the centralized &decentralized grid connected PV applications.[13] Decentralized grid connected PV system include rooftops PV generators, where PVS mounted on the rooftops of the buildings & its integrated systems in which the PV systems are incorporated into the buildings. These are connected to the public grid via house or building grid. Their capacities are in the lower range of kilowatts. The central grid connected to the public grid directly. The central grid connected PV system have capacities are in the range of higher kilowatts to megawatts. They are simple to install, reliable and flexible and it has high efficiency.[6]
VIII. ISSUES AND CHALLENGES OF PVSYSTEM
There are random changes in the nature of the renewable sources, its integration into the grid causes technical challenges which are targeted and solved. The technical challenges cover the reduction in power quality, power fluctuation causing unreliability, storage & protection
issues, optimal positioning of Distributed Generator (DG) and anti islanding [13], [14].
(1) Problems Concerned with Power Quality
As the renewable Distributed Generators are integrated through a power electronic converter to the grid, they usually inject harmonics into the system. Due to switching mechanism of the power electronic switches, Harmonics are generated in the inverter, which produce poor quality of power to be supplied to the consumers. Hence, to overcome the harmonics the soft switching control schemes of the inverter are used. Also active or passive filters are to be used for overcoming the harmonics.[17]
Due to varying nature of the DG set, the operating frequency and the operating voltage can also changes, for that the power flow is affected.[13] Sometimes it required disconnecting and reconnect the renewable energy sources to the grid as the load demand changes which causes voltage flicker. Better tap settings for the transformer connected to the grid through the feeder should be made, which is more useful when two or more feeders are supplied by the same transformer, but the DG is concentrated on only one of the above feeder. [15] To solve the problem, digital voltage control algorithms are used. [14] When there is interconnection of 1 phase source with 3 phase source is done unbalanced voltage profile arises or vice-versa. This is leads to unbalanced current which deduces the quality of the system [16].
(2) Reverse Power Flow
Conventional power systems are unidirectional power flow.
But as a renewable energy source is integrated to the conventional power system the reversal of power flow takes place which affects the operation of protection circuits [13]
(3) Storage
Due to the interconnection of renewable sources or PV sources into the grid power path flow, the grid’s standard comes down. According to generation of power from the DG set, the grid may act as a source or sink of power. The battery can be used to store the excess of energy, if the PV power generation is surplus or in case of a weak grid.[13].
The issues of sizing of battery introduced to the grid connected PV system. Also, it generates the issues of battery current and voltage control.[17]
(5) Protection Issues
Traditional power systems are protected by over current, over voltage relays and circuit breakers. But as energy conversion systems i.e. solar energy system is introduced the protection of the network becomes more complex. The
issues of changes in the short circuit level, reverse power flow and lack of sustained fault current persists.[17]
(5) Short Circuit Level Change
In the design of protective devices such as circuit breakers and relays, the short circuit level is an important design parameter. Short circuit level change usually characterized by the equivalent system impedance at the fault point [13]
and it indicates the amount of fault current for the relay to react upon the fault. The equivalent impedance doesn’t vary with the grid power system, but varies with the Distribution Generator network systems as the input changes to it changes instantaneously. The Short Circuit Change varies the forecast of the fault current magnitude changes which cannot be withstood by the designed circuit breaker rating right through the operation.[17]
(6) Lack of Sustained Fault Current
In traditional power system we used switch gears and circuit breakers for the protection of the system from the fault current which differentiates the fault current from the normal current. This is done because the fault current is greater than the normal current. It is the toughest task for the circuit breaker to identify the magnitude of the fault current form the normal current, if the magnitude of the fault current varies from the DG set. Solar systems mainly used the power electronic switches which do not supply sustained fault currents [13].
(7) Islanding
In the grid connected PV systems islanding is a unique problem. Islanding occurs on grid failure. It is the condition in which a distributed generator continues to power a location even though power from the electric utility grid is no longer present. Islanding is dangerous to the utility workers, who may not realize that a circuit is still powered.
Even though there is no power form the electric grid. For that DG must detect islanding and immediately stop producing power, this is refers as anti islanding. At the point of common coupling of the grid and the renewable generator, an auto reclosure relay is used. It is kept open for the separation of the utility network with the grid. Otherwise the voltage builds up on power generation systems without the energy absorption by the grid causing huge voltage unbalance resultingly in system deterioration. Thus the anti islanding control technique came into picture [13], [16]. The standard anti islanding control techniques include over frequency and under frequency relays, over voltage and under voltage relays. In addition to the standard schemes, active and passive schemes are introduced for reducing the probability of islanding. Passive schemes include voltage
harmonic monitoring, slide mode frequency shift, phase jump detection and branch out. Whereas the active scheme includes impedance measurement and active frequency drift.[17].
IX. ADVANTAGES &DISADVANTAGES OF GRID CONNECTED
PVSYSTEM
There are lots of advantages of the grid connected PV system.
(1) The grid connected PV systems are simple to install as they do not require a battery system.
(2) They have a high efficiency.
(3) They are reliable and flexible and safer as well.
(4) Free from periodic maintenance as no moving parts are there.
(5) The cost of generation is reduced once it is installed. As solar energy is free, the power bill will reduce. It is possible to sell surplus electricity produced to the local electricity supplier.
(6) They have effective utilization of generated power because there are no storage losses involved.
(7) The photovoltaic power system is carbon negative over its lifespan, as any energy produced over and above that to build the panel initially offsets the need for burning fossil fuels. Even though the sun doesn’t shine always, any installation gives a reasonable predictable average reduction in carbon consumption.
(8) They are noise free.
The disadvantages of the grid connected PV system.
(1) Grid connected PV system can causes issues with voltage regulation. The traditional grid operates under the assumption of one way or radial flow.
But electricity injected to the grid increases voltage, and can drive levels outside the acceptable bandwidth of +5 %.
(2) They are compromise power quality. Their intermittent nature causes rapid changes in voltage.
This is not only wears out voltage regulators due to frequent adjusting, but also can result in voltage flicker.
(3) Connecting to the grid poses many protections related challenges. In addition to islanding as mentions above, too high levels of the grid connected results in problems like relay desensitization, nuisance tripping, and interference with automatic reclosers.
X. LIMITATIONS OF GRID CONNECTED PVSYSTEM. The grid connected PV system can’t supply power during night and during rainy days because sunlight is not
sufficient. As there are no storage devices so they cannot supply power. The renewable energy sources are intermittent like wind, water, biomass, and solar. Due to these renewable sources there are fluctuations which affect the power grid frequency, voltage, component performance, and instability in the power generation system. And due to all these interrupted service to the customers.
XI. CONCLUSION
By reviewing all research papers, finally we can concluded that in future to fulfill the energy demand we have to use renewable energy broadly, like solar, wind, hydro, etc.
among them solar energy generated from PV cell is most elegant as there is no moving parts are there. No emission of any gases which are harmful to our environment. So it is environment friendly. Because of the solar energy, country does not depend on the others for fossil fuels, nuclear fuels, etc.
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