Grid connected
- 1. Components of Grid connected Solar PV Systems S.Gomathy M.E.,M.B.A AP/EEE KEC
- 2. Components of Grid connected Solar PV Systems A grid-connected solar PV system typically consists of the following components: 1. Solar PV array 2. Array combiner box 3. DC cabling 4. DC distribution box 5. Inverter 6. AC cabling 7. AC distribution box.
- 4. Solar PV Array • A solar PV array is used to covert solar energy into DC electrical energy. • The array consists of a number of solar PV modules connected in series and/or parallel combinations. • The series connection is used to increase voltage output while a parallel connection is used to increase current output. • A number of modules are connected in series in a series string and a number of series strings are connected in parallel to make a series-parallel connected solar PV array. • A number of modules can also be connected in a parallel string
- 5. Series-connected solar PV string: Increasing DC voltage • The series connection is used to increase DC voltage output of the string. • The DC current output of a string is not increased by series connection. • Typical voltage output of a series connected string is in the range of 100 – 1000 V DC and typical current output is in the range of 5 – 10 A DC. • The solar string is made to operate at maximum power point when connected to inverter.
- 6. DC voltage and current of series-connected PV module string: • DC voltage of a number of PV modules connected in string can be estimated in the following steps: • Step 1 DC voltage of series-connected solar PV string: DC voltage of series connected solar PV string = DC voltage of individual solar PV module × number of modules connected in series • Step 2 DC current of series connected solar PV string: DC current of series connected solar PV string = DC current of individual solar PV module. • Step 3 DC power output of series connected solar PV string: DC power output of series connected solar PV string = DC voltage of series connected solar PV string × DC current of series connected solar PV string.
- 7. Problem A solar PV module is rated for Voc = 40 V, Vmp = 32 V, Isc = 8.5 A and Imp = 8 A.Design a solar PV string to produce DC voltage output of 384 V. What will be the DC current output of the string? What will be the DC power output of series connected string? Solution Step 1: • DC voltage of series-connected solar PV string = DC voltage of individual solar PV module × Number of modules connected in series. • Therefore, the number of modules in series = String voltage/ Module voltage 384/32=12. • Thus, 12 PV modules need to be connected to get string voltage of 384 V. Step 2: DC current of series-connected solar PV string = DC current of individual solar PV module. • Therefore, DC current of string = 8 A Step 3: DC power output of series connected solar PV string = DC voltage of series connected solar PV string × DC current of series connected solar PV string = 384 V × 8 A = 4608 W or 4.6 kW.
- 8. Parallel-connected solar PV strings: Increasing current • The parallel connection is used to increase DC current output of the string. • The DC voltage output of a string remains same inparallel connection. • Typical voltage output of a parallel-connected string is in the range of 20 – 30 V DC and typical current output is in the range of 50 – 200 A DC.
- 9. DC voltage and current of parallel-connected PV module string: • DC voltage of a number of PV modules connected in string can be estimated in the following three steps: Step 1 DC current of parallel connected solar PV string: DC current of parallel connected solar PV string = DC current of individual solar PV module × Number of modules connected in parallel. Step 2 DC voltage of parallel-connected solar PV string: DC voltage of parallel connected solar PV string = DC voltage of individual solar PV module. Step 3 DC power output of parallel-connected solar PV string: DC power output of parallel-connected solar PV string = DC voltage of parallel-connected solar PV string × DC current of parallel- connected solar PV string.
- 10. Problem A solar PV module is rated for Voc = 40 V, Vmp = 32 V, Isc = 5.5 A and Imp = 5 A.Design a solar PV string to produce DC current output of 200 A. What will be the DC voltage output of the string? What will be the DC power output of parallel connected string? Solution Step 1: DC current of parallel-connected solar PV string = DC current of individual solar PV module × Number of modules connected in parallel. • Therefore, the number of modules in parallel = String current/ Module current =200/5=40 Step 2: DC voltage of parallel-connected solar PV string = DC voltage of individual solar PV module. • Therefore, DC voltage of string = 32 V Step 3: DC power output of parallel-connected solar PV string = DC voltage of parallel-connected solar PV string × DC current of parallel-connected solar PV string = 32 V × 200 A = 6400 W or 6.4 kW.
- 11. Series-parallel connected solar PV array • The voltage output of array is identical to the voltage output of a PV string and current output of PV strings increases in multiple of number of parallel connected strings. • Typical voltage output of a series-parallel connected solar PV array is in the range of 100 – 1000 V DC and typical current output is in the range of 50 – 200 A DC.
- 12. • The PV module strings themselves have many modules connected in series, and therefore, strings have high DC voltage. • When we connect several PV modules strings in parallel currents of all the strings get added and in this way, we get high current as well. • Thus, both high voltage and high current can be achieved in series-parallel configurations. Step 1 • DC voltage of series-parallel connected solar PV array: DC voltage of series-parallel connected solar PV array = DC voltage of series- connected string. Step 2 • DC current of series-parallel connected solar PV array: DC current of series-parallel connected solar PV array = DC current of individual series connected string × Number of strings connected in parallel. Step 3 • DC power output of series-parallel connected solar PV array: DC power output of series-parallel connected solar PV array = DC voltage of series- parallel connected solar PV array × DC current of series-parallel connected solar PV array.
- 13. Array Combiner Box • An array combiner box is used to electrically interconnect solar PV strings to make an array. The combiner box also houses DC voltage and current protections used in a solar PV array
- 14. • The DC cables from the string are connected to the box using MC4 connectors. • The positive and negative string connections are separately terminated and combined inside the box using shorted terminal strips. • The positive and negative output cables from the array combiner box are secured to the box using sealed cable glands. • The DC surge protection devices are housed inside the box. • The optional string fuses and blocking diodes can also be housed inside the combiner box. • An optional 2 pole DC disconnect switch is also mounted inside combiner box. • The combiner box is rated for Protection Class II (double insulation) for protection against electric shock from high DC voltage output of array. • The Ground Fault Detector Interrupter (GFDI) is used for grounded PV arrays. • The box also needs to be rated for ingress protection rating of IP65 (Digit 6 stands for total protection against dust and digit 5 stands for limited protection against low pressure water jets from any direction).
- 15. DC Cabling • The PV modules used in a string are connected in series using DC cables. • The DC cables are also used to interconnect strings to make an array and connect PV array output to Inverter DC input. • The string cables are rated for a minimum of 1.25 times the string short circuit current at each location, string fuses are optional. • But, the string DC cables are typically rated for 1.5 times string short circuit current to be on the safer side. • The string blocking diodes are not required for small systems using typically less than four strings.
- 16. DC Distribution Box • DC distribution box is used to distribute DC cables to inverter. • Two-pole DC disconnect switch is used to isolate PV array from inverter. • The DC surge protection devices can be incorporated with DC distribution box. • The output of this box is connected to Inverter DC input
- 17. Grid-connected Inverter • The grid-connected inverter is used to convert PV array DC output to AC voltage and current. • The output of grid-connected inverter is tied to mains AC grid. • The AC voltage, frequency and phase of the inverter output are matched with mains AC grid voltage, frequency and phase. • The inverter injects AC current into the mains AC grid at mains AC voltage and supplements mains AC power supply to the load. • The presence of mains AC grid voltage (which means grid should be ‘on’ or powered) is essential for the operation of grid connected inverter. • The grid-connected inverter has Maximum Power Point Tracker (MPPT) controller input stage used to extract maximum DC power from solar PV array at all times. • The inverter MPP tracking voltage range needs to match MPP voltage range of the array. • The MPP voltage of the array varies with solar radiation, operating temperature and other environmental factors
- 18. • The grid-connected inverter has built-in DC and AC voltage and current protections. • The DC and AC disconnect switches are also integrated with the inverter. • Islanding is used to isolate inverter output from grid in the event of grid failure. • Islanding protection is used to protect operators working on the mains AC grid from electrical shock or electrocution when the grid power is not available. • Grid-connected inverters are available with and without transformer isolation at the output. • An external transformer is used at the output of inverter before connecting the inverter output to the mains AC grid. • The external transformer is also used to step up Inverter AC output voltage to the mains AC grid voltage.
- 19. Types of inverter Central inverter • The central inverter is typically used in a solar PV grid-connected system for large power application. • A number of series strings are connected in parallel combination to form a large array. • The central inverter can be used when all modules used in an array are identical type, make and power rating. • The module MPP current needs to match in a series string connection and the string MPP voltage needs to match when a number of series strings are connected in parallel. • The central inverter is not used when one or more strings are under partial or complete shading during the day.
- 20. • The central inverters are typically rated for 250 kW – 1 MW output power and 400 V – 1000 V DC array voltage. • These inverters are typically used in large to very large size installations ranging from 1 MW – 100 MW. • The use of central inverters in very large installations allows better management of AC cable routing. • The central inverters are typically installed in controlled environment and are used in plants having hostile ambient temperature, humidity and dust conditions.
- 21. Series string inverter • The series string inverter is used when a number of PV modules are connected in series combination to form a string. • The number of inverters used depends on the number of strings. • A set of series string inverters can be used with PV array when series strings use modules having non-identical type, make or power rating. • The module MPP current needs to match with that of other modules connected in a series string connection, however, the string MPP voltage need not match MPP voltage of other strings as each string has its own inverter. • The series string inverters are also used when one or more strings are under partial or complete shading during the day. • The use of string inverter maximizes power output of individual series string. The series string inverters are normally not suitable for use with PV array employing thin film technology due to low string currents.
- 22. • The series string inverters are typically rated for 10 kW – 100 kW output power and 200 – 600 V DC string voltage. • These inverters are typically used in medium to large size installations ranging from 100 kW to 1 MW. • The AC cable sizing for string inverter configuration becomes unmanageable beyond 1 MW plant rating. • The series string inverters are typically installed outdoors and are rated for IP54 or IP65 (Water and Dust Ingress Protection) for operation in hostile conditions.
- 23. Parallel string inverter • The parallel string inverter is used when a number of PV modules are connected in parallel combination to form a string. • A set of parallel string inverters can be used with PV array when two or more parallel strings use modules having non-identical type, make or power rating. • The module MPP voltage needs to match with that of other modules connected in a parallel string connection, however, the string MPP current need has its own inverter. • The use of parallel string inverter maximizes power output of individual string not match MPP current of other strings as each string • The current output of the parallel string is the sum total of the individual module currents and the parallel string inverter input current rating can be selected to match this. • The parallel string inverters are typically installed outdoors and are rated for IP54 or IP65 (Water and Dust Ingress Protection) for operation in hostile conditions.
- 25. Module inverter • The module inverter is used with each module when a number of dissimilar PV modules (non-identical type, make or power rating) are used in an array or when the modules can be shaded differently due to non-uniform shading pattern • A set of module inverters can be connected to grid in parallel and they operate independent of each other. • Neither the module MPP voltage nor the MPP current needs to match these parameters with other modules as each module has its own inverter. • The use of module inverters maximizes power output of individual modules and, in turn, delivers maximum power output from the array. • The voltage and current input rating of the module inverter is selected to match the range of voltage and current output ratings of the modules.
- 26. • When module inverter is integrated with the PV module, it is called piggy- back inverter and the module is termed AC module as it delivers AC output directly compatible with mains AC grid voltage. • AC modules are directly interfaced with the mains AC grid. • The module inverters are installed outdoors and are rated for IP65 (Water and (Dust Ingress Protection) for operation in hostile conditions.
- 27. AC Cabling • The outputs of inverters used with the solar array are connected to AC distribution box using AC cables. • The cables are rated for a minimum of 1.25 times the AC circuit breaker current rating. • The AC cables are rated for indoor environment when the inverter is located indoors, for example, a central inverter. • The AC cables are rated for outdoor environment when the inverter is located outdoors, for example, string or module inverter.
- 28. AC Distribution Box • AC distribution box is used to distribute AC cables to transformer or load. • A two-pole AC circuit breaker is used to isolate inverter from transformer or load. • The AC circuit breaker current rating is typically 1.2 times the maximum load current. • The AC line fuses and surge protection devices can be incorporated with AC distribution box. • The output of AC distribution box is connected to transformer or load via AC energy meter used to monitor PV generated energy • The low voltage transformer can be incorporated with the inverter. • An external transformer is required for AC load circuit isolation when transformerless inverter is employed. • A three-phase low to medium voltage step-up transformer is used when the output of inverter is connected to medium voltage grid at the distribution level (11 kV – 33 kV).
- 30. AC energy metering and net metering • AC energy metering is required to determine the amount of energy produced by solar PV system. • The PV energy meter connected in series with the inverter output is used to monitor PV generated energy. • The load energy meter connected in series with the load is used to monitor energy consumed by load. • The net energy meter is used to monitor net energy import from or export to the grid. • The PV and load energy meters are uni-directional and register the flow of energy in forward direction. • The net energy meter is bi-directional and registers the flow of energy in both forward and reverse directions. • Three energy meters; (a) PV meter, (b) Load meter and (c) Net meter can be used in a PV system. • Minimum two energy meters are required to determine overall energy flow
- 31. • When the PV energy generated is less the energy consumed by the load, the net energy meter runs in forward direction (positive reading). • If PV generated energy is greater than energy consumed by load, then energy is exported to grid. • In this case, the net meter runs in reverse direction (negative reading). • Thus, net meter can run in forward as well as reverse direction. • During certain period if the more energy is consumed by the load (than produced by PV), then the net meter reading will be positive and if more energy is produced by the PV modules than consumed by the load, then net meter reading will be negative. • Negative net meter reading implies that energy is fed to grid.
- 32. Positive net meter reading: • If PV generated energy is less than energy consumed by load, energy is imported from grid. The energy flow in the Net meter is in the forward direction. • Energy consumed by load = PV generated energy + Energy drawn from grid • Net meter reading is positive when energy is drawn from grid. • Negative net meter reading: • If PV generated energy is greater than energy consumed by load, energy is exported to grid. • The energy flow in the Net meter is in the reverse direction. • PV generated energy = Energy consumed by load + Energy fed to grid
- 33. Problem • If PV meter reads 10 kWh and Load meter reads 8 kWh, what will be the Net energy meter reading? Solution • PV meter reading = 10 kWh • Load meter reading = 8 kWh • Since PV generated energy is greater than energy consumed by load, it indicates that energy is fed to the grid. PV generated energy = Energy consumed by load + Energy fed to grid • Energy fed to grid = PV generated energy – Energy consumed by load = 10 kWh – 8 kWh = 2 kWh • Net energy flow is 2 kWh into the grid and hence the net energy meter would read – 2 units. • The negative sign indicates the energy is fed to the grid.