A Study on Grid Connected PV system

Transcription

1 A Study on Grid Connected PV system J Sreedevi Joint Director Power System Division Central Power Research nstitute Bengaluru, ndia. sreedevi@cpri.in Ashwin N Senior Research Fellow Power Systems Division Central Power Research nstitute Bengaluru, ndia. ashwin-srf@cpri.in M Naini Raju Senior Research Fellow Power Systems Division Central Power Research nstitute Bengaluru, ndia. naini-srf@cpri.in Abstract Photovoltaic (PV) energy has a fast growing annual rate and is quickly becoming an important part of the energy balance in most regions and power systems. This paper aims to study the effects of connecting a PV system to the grid through simulation of the system in RSCSD software in real time on the Real Time Digital Simulator (RTDS). Effect of variation of power factor of loads, variation of PV penetration, introduction of harmonics into the system by the PV inverter and anti-islanding effect of the PV system are studied. Finally, the Performance Ratio (PR) of a typical grid connected PV system is evaluated to determine the reliability and grid connectivity of the PV system. Keywords Grid connected PV, Harmonics, Anti-islanding, Performance Ratio (PR), RSCAD, RTDS.. NTRODUCTON An important source of renewable energy is solar energy. n ndia, the average annual solar energy incident on land area alone, is about 5000 trillion kilowatt-hours, because ndia gets about 300 clear sunny days in a year. The solar energy output received in a year exceeds the possible energy output of all the fossil fuel reserves in ndia. From 10 MW of installed capacity in 2010 and MW in 2011, the installed grid connected solar power capacity, as of 31 st March 2016 in ndia is MW and an additional 10,000 MW by 2017 and total of 100,000 MW by 2022, is expected to be installed [1,2]. Some renewable energy projects are large scale, but renewable technologies are also suited to rural and remote areas in developing countries, where energy is crucial to human development Grid connected PV systems in the world account for about 99% of the installed capacity compared to stand alone systems, which use batteries. Battery-less grid connected PV are cost effective and require less maintenance. Batteries are not needed for grid connected PV, as the power generated is uploaded to the grid for direct transmission, distribution and consumption. This eases the burden on other sources supplying power to the grid. n this paper, the impacts of connecting PV system to grid are studied. Further, the Performance Ratio of a typical grid connected PV system in ndia is evaluated.. MPACTS OF CONNECTNG PV SYSTEM TO THE GRD f the PV penetration is really high Photovoltaic systems can subject the grid to several negative impacts. They are i) Reverse power flow, ii) Overvoltage along Distribution feeders, iii) Voltage control difficulty, iv) Phase unbalance, v) Power Quality problems, vi) ncreased Reactive power and vii) slanding detection difficulty. This paper considers the following three impacts. A. Power quality problems/harmonics The inverter forms the core of the grid connected PV system and is responsible for the quality of power injected into the grid. nverters also introduce harmonics into the system in the presence of non-linear loads, during DC to AC conversion. Harmonic currents introduce voltage drop and result in distortion of supply voltage. Harmonics can also cause resonance in the supply system, resulting in malfunction, reduction in lifetime or permanent damage of electrical equipment [3]. B. ncreased Reactive Power Photovoltaic inverters usually operate at unity power factor. The owners of small residential PV systems in an incentive based program are levied based on their kilowatt-hour yield and not on their kilovolt-ampere hour yield. Hence they prefer to operate PV inverters at unity power factor, maximizing the active power generation, and accordingly their returns. As a result the reactive power demand met by the PV system is minimal. Hence, the grid is responsible for supplying majority of reactive power, and it makes the distribution transformer operate at a low power factor [3]. C. slanding Detection The condition when the solar system continues to supply to the load even though grid power from the utility is not present is called islanding. slanding can be dangerous to utility workers, who may not realize that a circuit is still energized while working on repairs or maintenance. Hence, the solar inverter must detect islanding and disconnect the PV system when the grid is down. This function of the PV system is known as anti-islanding [4]. These impacts are dependent on the size and location of the PV system. According to the Solar America Board for Codes and Standards (Solar ABCs) PV systems are classified into three categories, based on the ratings of the system. Small-scale systems are rated at 10kW or less; Medium-scale systems are /14/$ EEE

2 rated between 10kW and 500kW; and large-scale systems are rated above 500 kw. This paper considers large-scale PV system of 4MW for simulation. SYSTEM DESCRPTON A typical grid-connected PV system is considered for simulation, to study the impacts of connecting PV to the grid. The single line diagram of the system simulated in RSCAD software for study purpose is shown in Fig.1. The network consists of a PV array, which generates peak of 4MW in a day. A DC/DC converter, which is also used as a power optimizer, is equipped with control functions such as Maximum Power Point Tracking (MPPT). The PV system is integrated to the grid by means of a DC/AC inverter, a step up transformer and an evacuating line of 33 kv. The point of common coupling is Bus1. The power is fed from PV system and the utility to the distribution network consisting of eight transformers. The PV array consists of modules. To arrive at a power output of 4 MW, the number of modules considered in series and parallel are 1500 and 30. The detailed data considered for PV array in simulation is given in Table. conditions considered for the simulation is 80% of the transformer ratings. TABLE. PV MODULE DETALS AT STC Cells per module 72 Number of modules in series (N S) 1500 Number of modules in parallel (N P) 30 Open Circuit Voltage (V OC) Short circuit Current ( SC) Maximum Power (P max) Voltage at P max Current at P max 35.5 V 8.52 A 235 W V 7.93 A deality factor of PV diode 1.5 Temperature dependency factor 3 Temperature co-efficient of SC Reference Temperature (T ref) 0.065% / 0 C 25 0 C Reference solar intensity (ns ref) 1000 W/m 2 TR1 TABLE. 33KV AND 11kV CABLE DATA PV array DC/DC converter DC/AC inverter BUSPV P Section Utility (33 kv) Parameters 33kV 11kV Name 3C x 25 mm 2 3C x 25 mm 2 R Ω/km/ph Ω/km/ph BUS1 X Ω/km/ph Ω/km/ph B e-05 Ʊ/km/ph 3.189e-05 Ʊ/km/ph TR2 TR3 P Section Where R 1, X 1, B 1 positive sequence resistance, positive sequence reactance and positive sequence susceptance (B/2) respectively Load1 Load2 TR4 TR5 TABLE. TRANSFORMER DETALS Transformer name Rating (MVA) Voltage rating (kv) TR1 3 4/33 P Section TR2 5 33/11 TR3, TR4, TR /11 TR6, TR7, TR /0.433 TR6 TR7 TR8 Fig 1: SLD of Test System Load3 V. SMULATON The system described in section is simulated in RSCAD software of RTDS simulator. Steady state simulation of the system is checked for bus voltages and power flow convergence. The two impacts of grid connectivity discussed in section are studied. The data of 33 kv and 11kV cables is given in Table. The distribution transformers data is given in Table. The percentage impedance considered for all transformers is 7.15%. The loading

3 A. Harmonics The amount of harmonics generated by the PV system depends on the type of solar inverter used. R 1 1 R 2 E g1 A multi-level inverter is capable of providing desired alternating output voltage using multiple lower level DC voltages as input. ncreasing the number of levels, increases the voltage steps of the output waveform, making the waveform more sinusoidal. Hence, as the number of levels increases, the harmonic distortion of the output waveform decreases. There are three types of multi-level inverters: i) Diode-clamped, ii) Flying Capacitor and iii) cascaded H-bridge inverter. A cascaded multilevel H- bridge inverter is the most commonly and practically used inverter, especially for integration of renewables into the grid. X 1 Eg1 2 X 2 E g2 (a) VT 1 2 (b) E g2 V T n this paper, a standard 3-phase 2-level DC/AC inverter is used which produces two voltage levels in the output wave form. This represents a simulation of worst case scenario of the system in terms of harmonics. For the operating condition considered of 4MW peak output from the PV array and 80% loading of the distribution system at 0.8 pf, the harmonic current at the output of the inverter is tabulated in Table V. R cable Xcable EPV PV R 1 1 X 1 Eg VT 1 PV E g E PV V T TABLE V. CURRENT HARMONCS AT PONT OF COMMON COUPLNG (c) (d) Order RMS Value (ka) % of Fundamental THD From Table V, it is seen that the content of 5 th order harmonic is maximum in the output waveform. The maximum current distortion limits in % of L and Sc/ L are given in EEE standard [5]. For the system considered the Sc/ L ratio at the point of common coupling is For this particular ratio, the odd and even harmonic current distortion in the Table V are within the limits of <4% and <1% respectively. The Total Demand Distortion (TDD) obtained is 1.170%, which is also within the limits 5%. Voltage distortion at PCC is 2.495%. B. Reactive power support The reactive power support required for the electrical system when renewable PV system is integrated with the grid is studied in this section with variation in load power factor and varying PV penetration levels. Fig 2: Equivalent circuit and vector diagram, (a) and (b) Two generators sharing the load; (c) and (d) PV system and utility source sharing the load. E PV is the voltage at BUSPV and Eg is the voltage at BUS1. The parallel operation of two conventional synchronous generators supplying a common load is illustrated in Fig. 2a. The corresponding vector diagram is shown in Fig. 2b. The conditions to be met for connecting generators in parallel are: (i) the generated voltage of the incoming generator connected in parallel to a bus-bar should be equal to the bus-bar voltage; (ii) phase sequence of the voltage of the incoming generator must be same as that of the bus-bar; (iii) frequency of generated voltage of incoming generator should be same as the bus-bar frequency [6]. The system chosen is the parallel operation of one renewable energy source & conventional grid represented as constant voltage source behind short circuit impedance. The equivalent circuit of the system is shown in Fig. 2c, with PV generation, grid supply and rest of the network as lumped load. The corresponding vector diagram is shown in Fig. 2d. The only difference between the two systems is the PV system does not supply the reactive part of the load, as it generates mostly real power. Hence, E PV is almost in phase with V T, and most of the reactive power is supplied by the utility. Hence, the utility operating power factor decreases. 1) Varying load power factor: Simulations are carried out by varying the power factor of the load from 0.7 pf to 0.9 pf. The power supply from PV system and grid is tabulated in Table V.

4 TABLE V. POWER SUPPLY FROM PV AND GRD WTH VARYNG LOAD POWER FACTOR PF Source (MVA) PV (MVA) Load (MVA) j j j j j j j j j4.72 Assuming the nominal power factor of the load to be 0.8 pf, the PV system is supplying 3.83 MW and absorbing 0.06 MVAR. With decrease in load power factor, the reactive power required from the grid is increasing and there is no contribution from the PV system. When the power factor of the load is improving, the PV system starts absorbing more reactive power The active power injected into the grid is limited by the PV module parameters and DC/DC converter. The PV inverter merely injects the active power into the grid. However, reactive power can be controlled by the PV inverter by controlling q-axis current. Controlling the reactive power will help in controlling the voltage at PCC and avoid injecting reactive power into PV system from the grid[7]. 2) Varying PV penetration levels: Simulations are also carried out by varying PV penetration levels. As the PV penetration increases, the real power supplied by the grid decreases, but the reactive power burden still remains wholly with the grid. The power supply from the grid is tabulated in Table V. TABLE V. POWER SUPPLY FROM GRD WTH VARYNG LEVEL OF PV PENETRATON PV Penetration (%) Grid Supply (MVA) PV system supply (MVA) j j j j j j j j j j j0.30 have to be oversized in relation to magnitude of required phase shift [4]. C. Anti-slanding: Currently, the regulations issued by the Central Electric Authority (CEA) for distributed PV systems in ndia do not allow it to operate independently of the grid [4]. The CEA mandates anti-islanding which means that the solar inverter must automatically switch-off when the grid goes down. Fig 3: Current fed by the PV for fault in the grid cleared before 125ms At high levels of PV penetration (in order of MW), the voltage rises at PCC when power is injected into the distribution system. PV inverters are capable of absorbing reactive power (as shown in Tables V and V) to mitigate voltage rise, just as they are capable of injecting reactive power in case of voltage sags. [8] ndia does not have a reactive power support regulation for distributed generation at present. Hence, most PV inverters connected at the distribution operate at unity power factor due to regulatory requirements. But further enhancements in PV systems will pave the way for increased PV penetration and utility operators will have to mandate the operating power factor of PV inverters (between 0.95 lagging and leading). n principle, all electrical components are usually designed for apparent power (kva) requirements. n order to provide reactive power without reducing active power, the PV inverters would Fig 4: Current fed by the PV for fault in the grid cleared after 125ms Simulations are carried out to see when the PV system has to get disconnected for a grid failure. n this study, a 3-phase-toground fault is simulated on the utility side for a fault duration of 500msec. Stability of the system is observed for varying instances of slanding of PV system at BUS1. t is found that the PV system has to get disconnected form the grid after 126ms of the grid fault for its safe islanding. Current fed by the PV system when fault is cleared at 125ms is depicted in Fig. 3 which represents the stable PV system operation. Similarly the current fed by the PV system when fault is cleared at 126ms is depicted

5 Performance Ratio (%) January February March April May June July August September October November December in Fig. 4 which represents the unstable PV system operation. So the critical islanding time of the PV system for the system considered is 125ms. V. PERFORMANCE RATO Performance ratio (PR) is one of the most important variables for evaluating the efficiency of a PV plant connected to the grid. The performance ratio is the ratio of the actual and theoretically possible energy outputs. t is largely independent of the orientation of a PV plant and the incident solar irradiation on the PV plant. Hence, the performance ratio can be used to compare PV plants at different locations all over the world [9]. The closer the PR value determined for a PV plant approaches 100 %, the more efficiently the respective PV plant is operating. n real life, a value of 100 % cannot be achieved, as unavoidable losses always arise with the operation of the PV plant. Highperformance PV plants can however reach a performance ratio of up to 80 %. PR is defined for a period of time (usually a month or a year). Measured enrgy at PCC ( kwh Monthly PR% = month ) nsolation ( kwh m 2. day ) Array area(m2 ) 30 η module (1) Where PCC is Point of common coupling. To calculate the annual PR%, replace month by year and 30 by 365 in (1). TABLE V. MONTHLY PR% AT STC Month Performance Ratio at STC (%) January February March April May June July August September October November December The Monthly Performance Ratio was evaluated for a PV system connected to grid at a typical location in ndia. The values are tabulated in Table V. Additional data of PV array considered for calculating the PR% is: Module efficiency (η module) = 14.44%; Array area = m x 0.156m; nsolation data [10]; Number of sunshine hours per day data [10] Monthly PR% Fig. 5 Monthly Performance Ratio (PR) The annual average PR% from Fig 5 is found to be 71.2%. Performance Ratio is also affected by ambient temperature. Higher the temperature, lower is the PV output, and hence lowers PR. As seen from Table V and Fig.5, the month of December has the highest PR%. This result tallies with the PR% of most PV systems across the world. The month of December receives a large number of sunshine hours and has relatively low ambient temperature. V. CONCLUSON Photovoltaic Systems have developed into a mature technology used for mainstream electricity generation. However, they introduce numerous negative impacts into the electrical networks. Studies on three such impacts has been provided. A gridconnected PV test system was considered and simulated in RSCAD software. Harmonic content introduced by 4MW PV system with a 3-phase, 2-level DC/AC inverter, at PCC was found to be within the limits. Reactive power support with regards to varying load power factor and varying PV penetration levels was studied. Anti-islanding function of the PV system was studied and found that the critical islanding time of the PV system for the system considered is 125ms. Further, the Performance Ratio of a typical grid connected system in ndia was calculated in order to compare the performance of the PV system with other systems throughout the world. The studies carried out will help PV power generators and utilities the issues to be studied for a grid connected PV system. ACKNOWLEDGMENT The authors would like to acknowledge the support of K.S.Meera and R.A Deshpande and wish to thank the authorities of CPR for permitting to publish this paper.

6 REFERENCES [1] "Physical Progress (Achievements)", Ministry of New and Renewable Energy, Govt. of ndia. 31 January Retrieved 21 February [2] "State wise installed solar power capacity" (PDF), Ministry of New and Renewable Energy, Govt. of ndia. 1 March Retrieved 24 March [3] D. M. Tobnaghi, A Review on mpacts of Grid-Connected PV System on Distribution Network, nternational Journal of Electrical, Computer, Energetic, Electronic and Communication Engg., Vol. 10, No.1, [4] Grid ntegration of Distributed Solar Photovoltaics (PV) in ndia, A Prayas (Energy Group) Report, July [5] EEE Standard Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. Revision to EEE [6] M. V. Bakshi, U. A. Bakshi, Electrical Machines, technical Publications, Jan [7] B. K. Perera, P. Ciufo, S. Perera, Point of Common Coupling (PCC) voltage control of a grid-connected solar Photovoltaic (PV) system, 39 th Annual Conference of the EEE ndustrial Electronics Society (ECON 2013), 2013, pp [8] P. Brucke, Reactive Power Control in Utility-Scale PV, SOLARPRO magazine, ssue 7.4, Jun/Jul [9] SMA Solar Technology AG, Performance Ratio, Technical nformation, Perfratio-UEN (PDF). [10] Gaisma.com, Sunrise, Sunset, dawn and dusk times around the World, 2005 [online]. Available: