Active and Reactive Power Control in Grid-Connected Solar Photovoltaic Systems - Coursework Example

Active and Reactive Power Control in Grid-Connected Solar Photovoltaic Systems Abstract This project aims to develop an approach of controlling the active and reactive power in grid-connected PV systems in order to maintain voltage stability of the system as a solution to the problems of voltage stability. Despite their numerous potential benefits, the integration of solar photovoltaic systems to the public utility grids is currently faced with a wide range of challenges some of which include voltage instability due to the intermittency of the sunlight intensity as well as the dynamics of interfacing inverters and photovoltaic cells. The proposed methodology to be used in the project will include carrying out a literature review as well as conducting a series of simulations using MATLAB program and Pulse Width Bandwidth to investigate the feasibility of active and reactive power control in grid-connected solar photovoltaic systems. Introduction Grid connected Solar Photovoltaic Systems are electricity systems powered by photovoltaic panels which are integrated into the mains electricity grid. The integration of renewable sources of energy such as solar photovoltaic systems into mains power grids is increasingly gaining popularity as one of the best alternatives and supplements to the traditional fossil fuel generation. According to Darul and Marko (2007, p.58), this is particularly attributed to the rapidly increasing worldwide electricity demand and the rising fuel costs coupled with the growing environmental concerns. Within the last decade, the integration of solar photovoltaic (PV) energy into utility grids has significantly gained popularity due to its immense potential advantages including its noiseless operation, relatively small size, feed in tariff as well as the absence of battery cost. For example, unlike the conventional stand alone solar photovoltaic power systems, grid-connected solar photovoltaic systems seldom require batteries and often supply their surplus power beyond what is required for consumption to the utility grid. In this regard, apart from the no battery cost, -connected solar photovoltaic systems also allow consumers to sell their surplus electricity to local utility companies thereby avoiding the potential storage losses of power. Additionally, solar photovoltaic systems are largely carbon negative over their lifespan and their connection to the grid provides a reasonable reduction in carbon emission (World Energy Council, 2004, p.48). However, despite their numerous potential benefits, the integration of solar photovoltaic systems to the public utility grids is currently faced with a wide range of challenges some of which include voltage instability due to the intermittency of the sunlight intensity as well as the dynamics of interfacing inverters and photovoltaic cells. This project seeks to control the active and reactive power in grid connected PV systems as a solution to the problem of their voltage instability caused by fluctuations in solar radiation. Fig 1: Grid Connected PV system Diagram Statement of the Problem Voltage stability and power quality issues remain some of the major critical challenges that are currently facing the integration of into mains public utility grids. In most cases, the problems of voltage stability are particularly attributed to the intermittency of sunlight as well as the dynamics of interfacing inverters and PV cells. This may result in significant changes in the power supply to the utility grid. For example, due to intermittent sunlight radiation, the input of grid connected solar modules sometimes plunges by up to 50% or more within a few minutes thereby causing voltage instability (Quaschning, 2005, p.54). On the other hand, most of the interfacing inverters used for grid integrated PV systems normally have a narrow voltage operating window outside which the inverter either stop their operation or their output power is significantly reduced. In order to overcome these challenges, a number of grid technologies including active and reactive power control in grid-connected solar photovoltaic systems currently present viable solutions that can be adopted to optimize power transmission of the grid connected PV systems by improving voltage stability. Operation Mechanism of the Proposed Active and Reactive Power Control In most grid connected PV systems, connected inverters are normally used to convert the DC power output voltages produced by the solar modules into AC voltage before it is fed into the grid system. On the other hand, reactive power usually occurs during the transmission of the energy via AC. An important aspect of grid connected solar photovoltaic systems is that they are able to effectively operate the double functions of active power generator as well as the reactive power compensator. The proper factor is, however, normally selected based on the active power and the reactive power demands of the utility grid (Preiser, 2003, p.109). As a potential solution to the problem, the proposed active and reactive power control for a grid connected photovoltaic system works by synchronizing the sinusoidal current output of the system with a voltage grid. For example, the active and reactive power control sequentially controls power using the load angle and the output voltage magnitude of the DC-AC inverter. This consequently allows the controller to maintain voltage stability of the system by feeding maximum active power into the utility grid at unity power factor while at the same time allowing for the adjustment of the reactive power than has been injected into the mains electricity grid. The DC-AC inverter then injects the sinusoidal current to the grid and the power factor is effectively controlled to ensure voltage stability of the system. Duffie and Beckman (2000, p.94) suggests that the use of the active power and reactive power particularly allows grid connected solar photovoltaic systems can also supply reactive power to the grid even when there is little or absolutely no solar radiation. This is critically important for compensating reactive power particularly at the peak hours when the mains grid requires relatively high amounts of reactive power than the normal consumption levels. Although the photovoltaic modules connected to the grid may not be able to generate active power during such periods, the control will allow them to supply sufficient reactive power to the maximum. The proposed active and reactive power control strategy is able to control active power of the system while at the same time dynamically changing the magnitude of the reactive power than is injected into the utility grid from the photovoltaic solar modules. For example, the active power is controlled by the load angle while the reactive power is controlled by the output voltage magnitude of the inverter. Fig 1: Diagrammatic Representation of the Proposed Active and Reactive Power Control in Grid-Connected PV System Regulation of Active power and Reactive Power According to Manuel (2006, p. 1008), reactive power normally occurs every time that energy is transferred in the form of alternating current. This is particularly attributed to the fact that unlike in the DC where electric power is simply a product of the current and voltage, the case of AC is slightly different as the intensity as well as the direction of the current and voltage alternates regularly having a sinusoidal trajectory with a frequency of either 50 or 60 Hz. Reactive power is particularly needed in the system during the AC transmission in order to support the transfer of the real power throughout the utility grid network. Generally, in most alternating current circuits, energy is normally temporarily stored in capacitive and inductive elements which often result in periodic reversal of the energy flow direction. Capacitors are AC devices that are often used to store energy in the form of electric field. Current driven through a capacitor normally takes a certain period of time before for the charge to gradually build up and produce the full voltage difference. Generally, on an alternating current grid network, the voltage across any given capacitor is often constantly changing. However, a capacitor will always attempt to oppose this change thereby resulting in a voltage lag behind the current being passed. In this regard, the potion of the power flow that remains after having been averaged over a complete alternating current waveform is considered to be the real power. When both the current and the voltage are at the same rhythm (In phase), the product will be a pure active power as shown in figure 2 below: Fig 2: Pure Active Power Produced when the current and Voltage are “In Phase” On the other hand, the segment of power flow that is often temporarily stored in the form of electric or magnetic fields due to capacitive and inductive element networks is the reactive power. The AC connected devices that normally store energy in the form of magnetic fields include inductors consisting of large coil of wires (Vanek and Albright, 2008, p. 69). For example, when a voltage is placed across the coil, a magnetic field gradually builds up and the current reaches full value after a certain period of time. This consequently causes the current to begin lagging behind the voltage thereby absorbing reactive power. Immediately after the sinusoidal trajectories of the voltage and the current begin to shift against each other, they produces an output with an alternating negative and positive sign which completely neutralize each other thereby resulting in a pure reactive power as shown in figure 3 below. Fig 3: Alternating negative and positive output resulting in Pure Reactive Power Implementation of the Control The grid-connected solar photovoltaic system comprises of the photovoltaic modules, converter with a maximum power point as well as an active and reactive power control. The implementation of the proposed active and reactive power control involves installing a control circuit into the system to help solve the problem of voltage instability due to the intermittency of the sunlight intensity. According to Ventre (2008, p.34) the control circuit should comprise of two main parts with the first part using load angle δ to control the active power injected into the utility grid while the second part utilizes the inverter output voltage magnitude E to control reactive power. The controller will be expected to compense the reactive power that is injected into the utility grid (Qg) thereby producing a reactive power error. The error then passes through an PI controller before it is eventually added to the grid voltage amplitude and the inverter output voltage. On the other hand, the controller will also regulate the active power generated by the inverter and compares it with a reference signal in order to generate an active power error. In this regard, the generated error then passes through another PI controller that produces a reference load angle which is then added to the utility grid voltage phase angle thereby generating inverter output voltage amplitude. Generally, only active power is usable in the grid as it can be used to power lights, machines and operate other electrical devices. On the other hand, reactive power can not be consumed and is simply required to act as an additional load. When the active power is reduced, the control adjusts to increase its reactive power supplying capacity. As a result, the values of inverter current and the apparent power that is injected into the utility grid remains at the rate values even when there is low solar radiation due to the occasional intermittency of the sunlight. For example, the control system compensates the moments of little generation of active power from the PVC systems by releasing reactive power and adjusting to a new reference of good performance active power when there is sunstroke variation. Project Approach The methodology and research approach to be used in the proposed project will include carrying out a literature review as well as conducting a series of simulations using MATLAB program and Pulse Width Bandwidth to investigate the feasibility of active and reactive power control in grid-connected solar photovoltaic systems. The project will begin with a comprehensive review of published literatures on active and reactive power control and its potential application in grid-connected solar photovoltaic systems. On the hand, the other part of the project will involve the use of Matlab program to carry out simulation modeling of the active and reactive power control in grid-connected solar photovoltaic systems in order to study the performance of the equipment and its behavior when connected to the utility power grid. The simulation of the performance of the active and reactive power control in the system will particularly involve modeling a system of PV arrays, DC-AC converters, MPPT and the active and reactive power controller. The control is expected to synchronize sinusoidal current output with a voltage grid. In this regard, the control of the active power will be achieved using load angle while the reactive power will be controlled using electromotive force. In this regard, the simulation and the tests will be particularly intended to prove that the proposed control system has the ability to successfully change the load angle as well as the inverter output voltage amplitude in order to control active and reactive power that is fed into the utility grid with good performance (Mostofi, 2011, p.78). Additionally, the optimal power of the solar photovoltaic arrays in use will be determined using a simulated maximum power point tracker (MPPT). The controller will then feed maximum active power to the utility grid at unity power factor while at the same time allowing for the adjustment of reactive power that is being feed into the grid. The results from the experiments and simulation modeling showing the performance of the controller will then be analyzed and compared with the findings from the literature review to determine the feasibility of the control of active power. Some of the important performance aspects that will be analyzed include changing irradiance, tracking of the current fed into the grid as well as power factor correction. Consequently a good performance of the simulated model will mean that the problem associated with grid-connected solar photovoltaic systems such as voltage stability which are due to the intermittency of the sunlight as well as the dynamics of interfacing inverters and PV cells can effectively be mitigated with active and reactive power control regardless of the availability of solar radiation. Tentative Project Timeline Conclusion In conclusion, one of the major advantages of the proposed active and reactive power control in grid-connected solar photovoltaic systems is attributed to its simplicity with regard to hardware implementation and computational requirements. The proposed active and reactive power control in grid-connected solar photovoltaic systems is neither expensive to realize nor complex to implement. A good performance of the simulated model will mean that the problem associated with grid-connected solar photovoltaic systems such as voltage stability which are due to the intermittency of the sunlight as well as the dynamics of interfacing inverters and PV cells can effectively be mitigated with active and reactive power control. References Darul, I., Marko, S. 2007. Large scale integration of renewable electricity production into the grids. Journal of Electrical Engineering 58 (1), pp. 58–60. Duffie J.A & Beckman W.A. 2000.  Solar Engineering Technologies. Toronto: John Wiley and Sons Inc. Manuel J. C. 2006. Power-Electronic Systems for the Grid Integration of Renewable Energy Sources. Industrial Electronics 53(4) pp. 1002- 1016. Mostofi, A. H. 2011. Institutional-Scale Operational Symbiosis of Photovoltaic and Cogeneration Energy Systems. International Journal of Environmental Science and Technology 8(1), pp. 31–44. Ventre, J.A. 2008. Photovoltaic systems engineering. New York, NY: CRC press. International Energy Agency. 2007. World Energy Outlook 2007. Paris, France: OECD/IEA Publications. Preiser, K. 2003. Photovoltaic systems: Handbook of Photovoltaic Science and Engineering. West Sussex: John Wiley &Sons Ltd. Quaschning,V. 2005. Understanding Renewable Energy Systems. London: Earthscan Publications. Solar Server. 2010. Solar Electricity: Grid-Connected Photovoltaic Systems. SolarServer Online Energy Portal to Solar Energy. Available at Vanek, F. M & Albright L. D. 2008. Energy Systems Engineering; Evaluation and Implementation. New York: McGraw-Hill Inc. World Energy Council (2004). Renewable Energy Projects Handbook. London, UK: World Energy Council. . Read More