Islanding control in grid connected photovoltaic system

Transcription

1 Volume 119 No , ISSN: (on-line version) url: Islanding control in grid connected photovoltaic system 1 Mr. R.Ashokkumar, 2 Dr.B.Indurani 1 Assistant Professor Level II,Department of Electrical and Electronics Engineering, Bannari Amman Institute of Technology, Erode,Tamilnadu,India 2 Professor,Department of Electrical and Electronics Engineering, Bannari Amman Institute of Technology, Erode, Tamilnadu,India 1 ashokkumarr@bitsathy.ac.in, 2 induranib@bitsathy.ac.in Abstract this paper discusses about the power quality issue termed voltage sag and its generation in a two stage photo voltaic inverter. Hence the injection of reactive power control is necessary to maintain power system stability in generation stations. In a grid connected PV system, inverter must regulate the reactive power to overcome the asso ciated problems generated during synchronization of photovoltaic inverter to the grid. In this paper, 1 kw PV system is carried also basic control strategy such as control of firing pulses for grid connected inverter is analyzed. Also in this study power quality disturbances are studied and islanding condition is detected. In this method the monitoring of instantaneous voltage is not required to detect islanding problem. Hence, an islanding detection using loop controller is implemented. During sudden changes in load or any other transients in power system causes malfunctioning of the system. Therefore, voltage control algorithms are proposed to overcome the islanding problem which detects the saturation of current in the outer voltage control loop. The entire 1kW photo voltaic system is simulated and analyzed using MATLAB Simulink software. Keywords: LVRT, MPPT, PWM, Reactive power control, Three Phase Inverter I. INTRODUCTION In recent days, in addition to generating power and distributing a quality power, need for green power has also grown. Carbon emission being high in various countries, environmental safety is equally important in power generation. This made the mankind to move towards renewable energy sources like solar, wind and the like. Though the efficiency is low in solar, it has its own advantages as it deals with free solar energy from the ultimate energy source. To get maximum power from the solar PV system, various control techniques are used which are discussed in this paper. PV panels are ins talled as PV arrays and connected to a DC-DC converter to regulate the DC power output from the panel. DC-DC converter is employed with Maximum Power Point Tracking algorithm to extract more power. Then the regulated DC is converted to AC and supplied to t he grid using AC-DC inverter. Block diagram for the system is given in Fig.1. In this paper, the inverter part with LVRT is discussed in detail. 2461

2 Fig.1. Block diagram of typical grid connected PV system 2. PV SYSTEM MODELLING The study is done for 1kW system. PV modelling in MATLAB can be done by deriving the function from the equivalent circuit of the PV cell. PV equivalent circuit is shown in Fig.2. Fig.2. Equivalent Circuit of Photovoltaic cell The relation between the terminal current I t and voltage V t is given by the equation (1)... (1) Where, I o, I ph, I and V are saturation current, photo current, current and voltage of the module respectively. V th = nn s kt/q is thermal voltage of the module; n, Ns, K, T and q are ideality factor, number of cells in series, man constant and electron charge respectively. Expressing in terms of Short circuit Current and Voltage, the equation will be as shown below (2). Where, M ss and N ss are number of shunt and series connected modules respectively. (2). Fig.3. I-V and P-V Characteristics of 1kW PV Panel

3 2.1 DC-DC BOOST CONVERTER DC-DC boost converters are used in PV systems, to boost the unregulated, constantly varying, and PV output to a required voltage level. MPPT control technique is commonly used in all DC-DC converters. Maximum Power Point Tracking algorithm can be derived from the expression that relates generated power and voltage, which is given below in equation (3)... (3) As far as DC-DC converter is concerned, it may be any of the maximum power point tracking (MPPT) algorithms, which extracts maximum power from the PV array. MPPT controller can be used to generate switching pulse for DC-DC converter and a constant terminal voltage can be maintained up to certain level of fluctuation in input. It is necessary to maintain a constant output voltage irrespective of input irradiance and temperature. MPPT techniques have been discussed in [1]-[5] in detail. In this paper, the simulations followed Perturb and Observe (P&O) Algorithm [6]. Flow chart of P&O algorithm is given in Fig.3. Fig.3. Perturb & Observe algorithm 2.2 GRID INVERTER In this study, a universal bridge type three-phase voltage source inverter (VSI) is used for the DC to AC conversion. The VSI maintains a constant DC link voltage of 2kV. This inverter is responsible for providing active and reactive power to the grid. PWM techniques are adopted in inverters to maintain constant output voltage irrespective of load. PWM is the process of modifying the width of pulse with respect to carrier in order to reduce harmonic content of the inverter output, various types of PWM is tested and adopted [7] SINUSOIDAL PULSE WIDTH MODULATION When voltage source inverter is switched in either 120 degree or 180 degree mode of operation, the output waveform obtained will be a square wave. Hence, to acquire a sine wave, sinusoidal PWM is simple and efficient method. In this method, a sine wave and triangular wave which acts as carrier are compared with comparator. The resultant pulse generated is the required PWM signal. Basic principle of SPWM is shown in Fig

4 Fig.4. Sinusoidal PWM wave generation There are certain considerations to be made, before going for SPWM. Let the amplitude of sine wave to be modula ted is A m and the triangular carrier be A c (refer Fig. 1). The modulation index (A m /A c ) has a greater impact on output voltage. Very high carrier frequency has an advantage of reducing harmonic components due to the inductive nature. High frequency switching also increases the switching losses in power electronic switches used in the inverter. For this reason, the carrier frequency is chosen between 2 khz to 15 khz which is optimum to use. Similarly, for three phase inverters, it is mandatory to maintain all three waveforms symmetrical. So, the ratio of sine wave frequency f m to the carrier frequency f c is chosen in integral multiples of 3 (f m /f c = 3n, n N). For a modulation index of 1, (i.e., A m /A c = 1 or A m = A c ) width of the pulses will be low. If carrier amplitude is lower than the modulation wave, then the modulation index goes beyond 1. In this case, some triangular spikes will not intersect with s ine wave. So, very high pulse width is possible and an uneven distribution will be followed Fig.5. Over modulation error GENERATION OF FIRING PULSES SPWM techniques are characterized by constant amplitude pulses with different duty cycles for each period. The width of these pulses are modulated to obtain inverter output voltage control and to reduce its harmonic content Sinusoidal pulse width modulation is the mostly used method in motor control and inverter application. In SPWM technique three sine waves and a high frequency triangular carrier wave are used to generate PWM signal. Generally, three sinusoidal waves are used for three phase inverter. The sinusoidal waves are called reference signal and they have phase difference with each other. The frequency of these sinusoidal waves is chosen based on the required inverter output frequency (50/60 Hz). The carrier triangular wave is usually a high frequency (in several KHz) wave. The switching signal is generated by comparing the sinusoidal waves with the triangular wave [7]. The comparator gives out a pulse when sine voltage is greater than the triangular voltage and this pulse is used to trigger the respective inverter switches. In order to avoid undefined switch ing states and undefined AC output line voltages in the VSI, the switches of any leg in the inverter cannot be switched off simultaneously. The phase outputs are mutually phase shifted by angles. The ratio between the triangular wave & sine

5 wave must be an integer N, the number of voltage pulses per half-cycle, such that, 2N= f c /f s. Conventional SPWM signal generation technique for three phase voltage source inverter is shown in Fig. 5. Block diagram of SPWM generating control circuit for three phase PWM voltage source inverter Fig.6. Amplitude modulation with 2 khz After generating the carrier wave, a comparator is used to obtain desired PWM signal. The comparator compares the amplitude of carrier and voltage of each phase. Wherever the carrier amplitude is high, the output from comparator is high [8]. The resultant waveform will be as shown in Fig.7. Fig.7. Gate Pulses for grid inverter 2465

6 3. ISLANDING DETECTION In the event of grid no avail-ability due to an upstream fault in the power system, it is likely that the utility-side breaker trips under the action of grid-side control. Under such a scenario, the inverter continues to pump power into the local load, resulting in islanding operation, sometimes islanding is desirable, but other times it is not. Islanding can create safety problems for workers and equip-ments that are not under direct control of the utility [12]. Therefore, islanding condition should be detected, and the inverter should be tripped. The inverter side breaker also needs to be opened to isolate the system from the grid. This operating mode of a grid-connected (GC) inverter is known as anti-islanding. Fig. 8. Islanding detection algorithm. Operation in islanded condition of a GC inverter, i.e., in SA mode, can create safety problems for the workers in a plant as the inverter may continue to energize some plant load even when grid power supply has failed. Therefore, SA mode of invert er operation may not be allowed and the inverter has to trip. Let us say the system initially is in the GC mode [11]. If islanding condition occurs, the system first detects it. [13]After islanding detection, the system goes into the SA mode if the operation in the islanding mode is allowed. The system trips if the operation in the islanding mode is not allowed. When islanding has not occurred, the system continues to check for low-voltage condition of the grid. As long as there is no low-voltage situation, the system continues to operate in the GC mode. Fig. 9. Control block diagram of outer voltage control loop

7 Fig. 10. Control loop for design of inner current control loop design As al-ready discussed, the PI controller of the outer voltage control loop eventually saturates in the GC mode. After saturation, the boost converter operates with a current reference of I m p p, that comes from the MPPT controller of PV. In the GC mode, the system always operates on current control with I m p p as the current reference [9]. Now, in the power system, islanding condition would occur when the grid-side breaker gets open (may be due to some upstream fault). The controller of two-stage PV inverter does not receive any direct signal from the breaker, as that is not physically at the same location, but the propose d algorithm still detects that islanding has occurred. The proposed islanding detection algorithm requires that, due to the difference between the output power of the PV at MPPT and the output power of the inverter, the load voltage has to change to such a steady-state value that the PI controller will come out of saturation[10]. Here, we calculate the amount of power that would to be drawn by the grid prior to islanding. Fig. 11. Islanding detection 2467

8 4. SIMULATION RESULTS AND DISCUSSIONS A 1kW grid connected system is simulated in MATLAB SIMULINK. The simulation circuit consists of an equivalent solar module connected to the dc boost converter and wit the dc link it is connected to the inverter. The simulation off SPWM technique is used to generate firing pulses for grid inverter. PI controller is used as feedback controller to provide the control grid voltage. The grid voltage under fault condition without LVRT control is shown in Fig, 9. The grid voltage after including the LVRT control, get regulated and the output is shown in Fig.12 Fig.12. Simulation model of 1kW grid connected solar PV system Fig. 13. Three phase line voltage and current waveforms

9 Fig.14. Grid Voltage under double line to ground fault For testing the islanding detection algorithm, the system is initially allowed to operate in the GC mode for a small amount of time till the outer voltage control loop reaches saturation. After the PI controller saturation, islanding detection algorithm starts working. Fig. 16 shows the simulation result of islanding detection. For creating the islanding condition, the grid -side breaker is opened intentionally at 1 s (the outer voltage control loop is already saturated). As a consequence, at 1 s the grid current becomes zero. 5. CONCLUSION In this paper, an integrated control algorithm for islanding detection and LVRT operation of a two -stage PV inverter is proposed and verified. Islanding is detected from the status of current saturation at the output of the inverter vo ltage control loop. The advantage of this method is that it is insensitive to disturbances as it does not depend upon the instantaneous value of the grid voltage 6. REFERENCES [1] K. H. Hussein, I. Muta, T. Hoshino, and M. Osakada, Maximum photovoltaic p ower tracking: an algorithm for rapidly changing atmospheric conditions, IEE Proceedings: Generation, Transmission and Distribution, vol. 142, no. 1, pp , [2] W. Wu, N. Pongratananukul, W. Qiu, K. Rustom, T. Kasparis, and I. Batarseh, DSP -based multiple peak power tracking for expandable power system, in Proceedings of the 18th Annual IEEE Applied Power Electronics Conference and Exposition (APEC '03), pp , IEEE, Miami Beach, Fla, USA, February [3] Q. Mei, M. Shan, L. Liu, and J. M. Guerrero, A novel improved variable step-size incremental-resistance MPPT method for PV systems, IEEE Transactions on Industrial Electronics, vol. 58, no. 6, pp , [4] F. Liu, S. Duan, F. Liu, B. Liu, and Y. Kang, A variable step size INC MPPT method for PV systems, IEEE Transactions on Industrial Electronics, vol. 55, no. 7, pp , [5] A. Safari and S. Mekhilef, Simulation and hardware implementation of incremental conductance MPPT with direct control method using cuk converter, IEEE Transactions on Industrial Electronics, vol. 58, no. 4, pp , [6] Trishan Esram, and Patrick L. Chapman, "Comparison of Photovoltaic Array Maximum PowerPointTracking Techniques", IEEE Transactions on Energy Conversion, 22 (2), 2007, [7] Prof R. Kameswara Rao, P. Srinivas, M.V.Suresh Kumar, "Design and Analysis of Various Inverters using Different PWM Techniques", The International Journal Of Engineering And Science (IJES) 2469

10 [8] P. P. Das and S. Chattopadhyay, Smooth mode transition of A DC bus voltage controlled PV inverter using a novel phase locked loop method, in Proc. IEEE Appl. Power Electron. Conf. Expo., Mar. 2015, pp [9] J. Mulhausen, J. Schaefer, M. Mynam, A. Guzman, and M. Donolo, Ant i-islanding today, successful islanding in the future, in Proc. 63rd Annu. Conf. Protective Relay Eng., Mar./Apr. 2010, pp [10] T. Funabashi, K. Koyanagi, and R. Yokoyama, A review of islanding detection methods for distributed resources, in Proc. IEEE Power Tech Conf. 2003, Bologna, Italy, Jun. 2003, vol. 2, pp [11] P. P. Das and S. Chattopadhyay, Smooth mode transition of A DC bus voltage controlled PV inverter using a novel phase locked loop method, in Proc. IEEE Appl. Power Electron. Conf. Expo., Mar. 2015, pp [12]Balamurugan.E, jagadeesan.a, Geographic Routing Resilient To Location Errors, International Journal Of Innovations In Scientific And Engineering Research IJISER, Vol 5,no 3, pp.21-26,mar2018. [13]M. E. Ropp, M. Begovic, and A. Rohatgi, Analysis and performance assessment of the active frequency drift method of islandin g preven-tion, IEEE Trans. Energy Convers., vol. 14, no. 3, pp , Sep BIOGRAPHY R.Ashok Kumar received his B.E Degree in Electrical and Electronics Engineering from Anna University, Chennai and his master degeree M.E in Power Electronics and Drives from Anna University, Coimbatore. Currently he is working as Assistant professor (Sr.G) in Bannari Amman Institute of Technology, Sathyamangalam. His area of research includes renewable energy, power converters for renewable energy applications, solar photovoltaic grid connected system, power quality issues and mitigation techniques

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