Sri Niwas Singh, Professor, Department of Electrical Engineering,
Indian Institute of Technology Kanpur, FIEEE
Due to increasing demand and environmental concerns, the PV power has become one of the fastest growing sources of electrical power generation and the emphasis is being given to the cost-effective utilization of this energy. The large scale, single-stage PV system is widely being considered for grid connection. For running the grid connected PV plant in a successful manner with effective performance, the proper operation and scheduling of PV power plants is required. Most of the PV array modelling are based on diode circuits and are not suitable for large PV systems. The conventional maximum power point tracking (MPPT) schemes discussed in the literature do not work properly in partial shaded conditions (PSCs).
PV Modelling
The classical model is suitable for small PV arrays. But when the size of the array increases, e.g. for large scale -grid integrated PV systems, the classical model is not suitable, because large number of electrical components will be required and thus, simulation time is substantially increased. Furthermore, for modeling the case of partial shaded condition, the flexibility for the change in parameter is limited. The results of the Piecewise Linearised Model (PWL), which is an improved diode circuit model, are compared with the mathematical model. The mathematical model gives very smooth PV curves as compared to another model. The shortcoming of the normal mathematical model is that after maximum power point, the PV voltage represented by the curve is less than the actual one. This problem is overcome by the modification in the mathematical equations of the PV module. The modified mathematical model gives almost accurate PV characteristics of PV array [1]. By utilizing the capacity of the DC-AC power converter of the grid connected PV system, the reactive power can also be fed to the grid along with the real power.
Also, the methodology to derive the P-Q capability curve has been developed so that by injecting P and Q powers, the grid-connected PV unit can be properly utilized as reactive power compensator. Such a curve describes all the possible values that can be assigned as reference for real and reactive powers, in order to make the control scheme effective. To validate the algorithm, the results of the approximate procedure have been compared with those provided by simulations performed with a detailed model of the PV system developed in the PSCAD/EMTDC environment, and a good agreement has been obtained. Furthermore, for PV system common operation, which is a system operating at maximum power point, the reactive power limit curve is also derived for changing environmental conditions i.e. for various irradiance and temperature. The P-Q capability of 375 kW PV array [1] is given in Figure 1. The dotted lines show the reactive power limits of the PV array at MPP. It is worth noting that the knowledge of the capability curve (giving P and Q limits) enables to perform classical load-flow studies on power systems with high penetration of large-size PV units in the same way as they are normally conducted in presence of standard synchronous machines.
Control Strategies
The designing process of various control schemes such as PLL control, current control, DC-link voltage control and MPPT control schemes which cooperate with each other in intermingled manner, can be done considering the plant transfer function, gain and phase margin, bode plots, controllers’ proportional and integral parameters. For improving MPPT performance, a modified Incremental Conductance (INC) method with variable voltage perturbation size has been adopted. A DC-link voltage controller has been proposed based on Feedback Linearization (FBL) technique. The proposed voltage controller cancels out the effect of nonlinearity of the PV characteristics in the control process and thus, improves the dynamic performance of the controller. The voltage response of the controller shows the same dynamic behaviour at different operating points and is not disturbed when the atmospheric condition is changed. The reactive power injection has been done successfully by changing the reactive power reference of the current controller.
The performance of the DC-link voltage controller at different operating conditions is verified using simulation results. Since FBL block is used in the DC-link voltage controller, the nonlinearity present in the PV source is eliminated and the controller just acts like a normal linear controller at all operating conditions. The effectiveness of the proposed control system has been tested by simulating the system in PSCAD/EMTDC environment. By using the modified MPPT and proposed feed-back linearization (FBL) voltage control scheme, the performance of the system is much improved in terms of dynamic and steady state responses. Figure 2 shows the controller response with and without FBL.
Partial Shedding Conditions
A modified particle swarm optimization (PSO) algorithm, which is very effective, accurate and simple to implement, is developed to track the Maximum Power Point (MPP) of the PV system. Most of the PSO based MPPT discussed in the previous works are applied for two-stage PV systems or single-stage systems with DC-DC converters, where the duty ratios are considered to be the PSO particles. Here, for the single-stage utility scale grid-connected PV system, the PV voltages are selected as PSO agents and the voltage reference to the DC-link controller is decided by computational results of the PSO MPPT controller to track the MPP in all partial shaded conditions, where the conventional hill climbing, and incremental conductance methods fail. Various simulation results demonstrate that the proposed method reduces the tracking time, and the oscillations at MPP are almost zero. The proposed method is suitable in even those cases, where other methods based on classical algorithms used for partial shaded conditions do not work properly. From the test results, it is found that the proposed modified PSO method is more accurate and efficient compared to the existing methods. Moreover, the proposed method is simple and fast. The method has been developed for high-power utility scale systems under several irradiance patterns including complex PSCs. Due to its simplicity, it can be easily implemented in a low-cost controller and computational speed would be fast. Figure 3 shows the system performance to the voltage dip at 0.04 second.
References
- VN Lal, and SN Singh, Modified PSO Based MPPT Controller for Single-Stage Utility-Scale PV System with Reactive Power Injection Capability, IET Renewable Power Generation,Vo. 10, No. 2, July 2016, pp. 899-907.
- SN Singh, Development of Control Strategies for Grid-connected PV System utilizing MPPT and Reactive Power Capability, CPRI Report 2020.
Biography
Prof S. N. Singh obtained his M. Tech. and Ph. D. in Electrical Engineering from Indian Institute of Technology Kanpur, in 1989 and 1995. Presently, he is Professor (HAG), Department of Electrical Engineering, Indian Institute of Technology Kanpur, India. He was Vice-Chancellor of Madan Mohan Malviya University of Technology Gorakhpur from April 2017 to July 2020. Dr. Singh received several awards including Young Engineer Award 2000 of Indian National Academy of Engineering (INAE), Khosla Research Award of IIT Roorkee, and Young Engineer Award of CBIP New Delhi (India), 1996. Prof Singh received the Humboldt Fellowship of Germany (2005, 2007) and Otto-monsted Fellowship of Denmark (2009-10).
Prof Singh became the first Asian to receive the 2013 IEEE Educational Activity Board Meritorious Achievement Award in Continuing Education. He is also recipient of INAE Outstanding Teacher Award 2016 and IEEE R10 region (Asia-Pacific) Outstanding Volunteer Award 2016. Dr Singh is appointed as IEEE Distinguished Lecturer of Power & Energy Society from 2019 and Industry Application Society for 2019-2021. He is also a recipient of NPSC 2020 Academic Excellence Award.
His research interests include power system restructuring, FACTS, power system optimization & control, security analysis, wind power, etc. Prof Singh has published more than 500 papers in international/national journals/conferences and supervised 31 PhD (12 PhD under progress). He has also written 23 book chapters, 6 edited books and 2 textbooks, one on Electric Power Generation, Transmission and Distribution and second is Basic Electrical Engineering, published by PHI, India. Prof Singh has completed three dozen technical projects in India and abroad. His two NPTEL (YouTube) video lectures on HVDC Transmission and Power System Operation & Control are extremely popular.
Prof Singh was Chairman, IEEE UP Section for 2013 & 2014, IEEE R10 (Asia-Pacific) Conference & Technical Seminar Coordinator 2015-18 and R10 Vice-Chair, Technical Activities (2019-2020). Presently Prof Singh is Immediate Past Chairman of IEEE, India Council. Dr Singh is Fellow of IEEE (USA), FIET (UK), FNAE, FIE(I), FIETE, AvH.