SolarEdge Fixed String Voltage Concept of Operation

Version 11 Feb 2019 Application Note SolarEdge Fixed String Voltage Concept of Operation Version History Version 11 Feb 2019 Added note about M series power optimizers Version 10 Sept 2010 Initial release The SolarEdge system maintains a fixed string voltage regardless of string characteristics and environmental conditions This application note details the concept of operation of the SolarEdge fixed string voltage and its benefits Concept of Operation The SolarEdge power optimizer is a DCDC power optimizer integrated into each module replacing the junction box The power optimizers using an input control loop perform per module MPPT and enable performance monitoring of each module In an independent process the power optimizers enable the inverter to automatically maintain a fixed string voltage at the optimal point for DCAC conversion by the inverter regardless of string length and individual module performance The operating principles of the SolarEdge system are illustrated in the following example which examines a systems behavior under varying conditions The example system consists of 10 200W modules Each module has an integrated power optimizer essentially a DCDC buck boost1 converter with an MPPT controller The power optimizers are seriallyconnected to form a string multiple strings can be connected in parallel to the same input of the SolarEdge inverter The SolarEdge inverter is a single stage current source it continuously adapts the current it draws from the PV array in order to keep the input voltage constant The SolarEdge power optimizer is highly efficient maintaining over 98 conversion efficiency over a wide range of conditions However for calculation simplicity we assume 100 power optimizer efficiency in this example Scenario 1 Ideal Conditions Initially we assume all the modules are exposed to full irradiance each providing 200W of power The power output of each solar module is maintained at the modules maximum power point by an input control loop within the corresponding power optimizer This MPP loop dictates to the power optimizer an input current Iin and input voltage Vin that ensure the transfer of the entire 200W from the module to the DC bus We assume an MPP voltage for each module given perfectly matched modules for demonstration purposes of VMPP 32V This means the input voltage to the power optimizer is 32V and the input current is 200W32V 625A The input voltage to the inverter is controlled by a separate feedback loop For simplicity in this example the inverter requires a constant 400V Since there are ten seriallyconnected modules each providing 200W the input current to the inverter is 2000W400V 5A Thus the DC bus current flowing through each of the power optimizers must be 5A This means that each power optimizer in this example provides an output voltage of 200W5A 40V In this case the power optimizers are acting as up converters converting the 32V input voltage to the target 40V output voltage The various system currents and voltages in this case are illustrated in Figure 1 1 Applies to power optimizers from series PB OP and P M series power optimizers are buck only however operate similarly 2 Figure 1 Operation under Ideal Conditions Scenario 2 Partial Shading Next we assume module 9 is shaded and consequently produces only 40W of power The other 9 modules are not shaded and each still produces 200W of power The power optimizer of the shaded module maintains that module at its maximum power point which is now lowered due to the shading Assuming VMPP 28V the current is 40W28V 143A The total power produced by the string is now 9x200W40W 1840W Since the inverter still needs to maintain an input voltage of 400V the input current to the inverter will now be 1840W400V 46A This means that the DC bus current must be 46A Therefore the power optimizers of the 9 unshaded modules will have an output of 200W46A 435V In contrast the power optimizer attached to the shaded module will output 40W46A 87V The input to the inverter can be obtained by summing 9 modules providing 435V and 1 module providing 87V ie 9x435V87V 400V as required by the inverter In this case the 9 power optimizers producing 200W each are essentially acting as up converters converting the 32V input voltage to a 435V output voltage whereas the power optimizer of module 9 is acting as a down converter converting the 28V input voltage to an 87V output voltage The various system currents and voltages in this case are illustrated in Figure 2 Figure 2 Operation with Partial Shading 3 As demonstrated by this example each of the modules is operating at its maximum power point regardless of operating conditions A comparison of the system operation in both cases can be seen in Figure 3 Note that both up and down DCDC conversion are automatically used depending on environmental conditions Figure 3 Case Comparison Fixed String Voltage Benefits The fixed string voltage maintained by the SolarEdge power optimizers provides multiple benefits Flexible Design mismatched modules can be seriallyconnected in a string The number of modules in a single string is not dependant on module output voltage and therefore a wide string length range is permitted High Inverter Efficiency and Reliability the SolarEdge inverter components work at a fixed voltage operating under less stress The inverter always operates at a voltage that enables optimal DCAC inversion efficiency independent of string length or environmental conditions Reduced Installation Cost longer strings lower BoS element count and installation cost and labor Temperature Indifference the SolarEdge fixed string voltage completely removes the temperature constraints which strongly limit string length in traditional systems Improved Safety all power optimizers start up in safety 1V output mode until the power optimizers are connected to a functioning SolarEdge inverter Additionally in the event of a grid power shutdown the modules immediately stop producing power and revert to this mode