Productive. Reliable. Smart. Safe. Martin Fornage, CTO Microinverters: an introduction May 2013 2008-2013 Enphase Energy
Microinverter: what is it? Part of a PV grid-tied system Microinverter Gateway Web server Receives energy from a photovoltaic source A single module (~250W) Safely injects the harvested and conditioned energy on the electric grid as a sinusoidal current Monitors the module and itself It will *not* work if the grid is gone 2008-2012 Enphase Energy 2
Microinverter: why? Productive Harvests more energy Reliable No single point of failure, high reliability electronics Smart Allows for full system monitoring and analysis No string calculation, regular AC wiring Lowers installation time Safe No high voltage DC: DC faults cannot lead to fires No lethal power source present when AC is shutdown 2008-2012 Enphase Energy 3
Microinverter: photovoltaic source PV module (not panel ) Translates photons (light) into electrons (electricity) Multiple technologies Silicon based (from wafers, deposited) Other materials (CdTe, CdInGaS, GaAs) passive active i.e. with concentration 2008-2012 Enphase Energy 4
Microinverter: photovoltaic Si module Cell based: a cell is typically 125mm (5 ) or 156 mm (6 ) square Cells are arranged in series (48, 50, 54, 60, 66, 72, 96) Cells provide about 0.5V at full irradiance Typical module efficiencies are in the 12 to 20% range Output power depends on irradiance (i.e. the power density) from the sun in W/m 2 (1000 is a good number on a good day) Angle of the rays, shading, soiling, matching all affect production Output power depends as well on cell temperature Typical testing is at 25ºC, but cells get hot from irradiance the power goes down about 0.4%/ºC Question: if a module STC rating is 250W, what power will be delivered at 75ºC cell temp and 1000W/m 2? 2008-2012 Enphase Energy 5
Microinverter: PV Si module specifications Number of cells and size e.g. 60 cells 156mm Power rating (STC), and Voltage/currents characterics Pmp=240W, Vmp=30V, Imp=8A, Voc=36V Size WxLxH 1000mm*1600mm*40mm Temperature coefficients Power -0.4%/ºC Current 0.04%/ºC Connectors Multicontact type IV 2008-2012 Enphase Energy 6
I P Microinverter: PV module I/V curve PV characteristics Irradiance 1000W/m2 Power vs Voltage Irradiance 1000W/m2 Irradiance 200W/m2 Irradiance 200W/m2 10 300 9 8 250 7 200 6 5 150 4 3 100 2 50 1 0-1 0 10 20 30 40 V 0-50 0 10 20 30 40 V 2008-2012 Enphase Energy 7
Microinverter: The public grid The grid always uses Alternating current (AC) The grid is essentially a voltage source operating at a 400 fixed frequency It can be multi-phase 200 150 100 50 0-50 -100-150 -200 0 10 300 200 100 0-100 -200-300 -400 Split Phase Line 1 Voltage Line 2 Voltage 0 10 20-100 -150-200 General rules for transmission/distribution: the higher the voltage, the less the losses 200 150 100 50 0-50 Single Phase Line Voltage 0 5 10 15 Three Phase Line 1 Voltage Line 2 Voltage Line 3 Voltage 2008-2012 Enphase Energy 8
Microinverter: The public grid Typical specs in USA Residential LV: 120V/240V 60Hz single/split phase Commercial LV: 120V/208V 60Hz 3 phase Industrial LV: 277/480V 60Hz 3 phase MV distribution: 12.5kV 60Hz 3 phase Typical specs in europe Residential LV: 230V 50Hz single phase Commercial/Industrial LV: 230V/400V 3 phase 2008-2012 Enphase Energy 9
Microinverter: US Grid Standards compliance Safety standards compliance is required National Electric Code LV: UL1741 Interconnection standards are required LV: IEEE1547 MV: FERC661 ElectroMagnetic emission standard LV: CFR 47 Part 15 Surge testing standard LV: ANSI C62.41 2008-2012 Enphase Energy 10
Microinverter: The public grid Is nasty Voltage surges (1,000s of V!) from indirect lightning strikes Grid events Distortion Surviving it everyday and in all cases is very, very difficult 250 200 150 100 50 0-5-50 5 15 25-100 -150-200 -250 Line 1 Voltage Line 2 voltage 150 125 100 75 50 25 0-25 -50-75 -100-125 -150 0 10 20 30 Line 1 voltage Line 2 voltage 2008-2012 Enphase Energy 11
Microinverter: Between PV and grid DC port geared for a module type e.g. 60 cell Si Crystalline module (M215) Specs AC port geared for a specific grid interface e.g.277v 60Hz Rack mount or PV module mount 2008-2012 Enphase Energy 12
Microinverter: DC side Functions MPPT Constantly find and operate the inverter at the maximum power point Speed and accuracy are important DC voltage and current reporting Check for arc faults 2008-2012 Enphase Energy 13
Microinverter: AC side functions Grid synchronization Voltage and Frequency (V&F) checks Constantly measure Voltage and frequency: disconnects if out of range Anti Islanding (AI) checks Constantly check for an island. An island is a DG that is connected to a load and operates without the grid being present. 2008-2012 Enphase Energy 14
Microinverter: Power conversion The DC power (i.e. 25V 8A) is transformed into AC power (i.e. 240Vrms 0.83Arms) The AC power is pulsating, the DC is not Energy must be stored and retrieved at 2x grid frequency, i.e. through an energy buffer function The output current must be injected onto the grid, normally in phase with the grid voltage 450 400 350 300 250 200 150 DC power AC power 100 50 0 0 5 10 15 2008-2012 Enphase Energy 15
Microinverter: Power conversion topologies Dual stage inverter DC/DC, buffer, DC/AC 400V 30V DC =/= =/~ 230V AC almost Single stage inverter buffer, DC/shaped DC, shaped DC/AC 30V DC =/ X 230V AC True single stage inverter buffer, DC/AC 30V DC =/~ 230V AC 2008-2012 Enphase Energy 16
Power control: our philosophy We believe in digital predictive control One opamp cost $1E-1, one gate costs $1E-6 Repeatable and secure IP Extremely fast response time Intimate knowledge of the power train is required anyways Allows for maximum efficiency Minimal cost Excellent protection systems are possible Inherently stable Power train is not as important (flyback) 2008-2012 Enphase Energy 17
An open unit 2008-2012 Enphase Energy 18
Microinverter: Power conversion Supervision Grid synchronization Voltage/current limits Reverse voltage protection Monitoring voltage and currents on both ports Monitors and manages the temperature Gathers energy production It s Ok to temporarily shutdown 2008-2012 Enphase Energy 19
Efficiency, % Microinverter: Power conversion efficiency Efficiency is a critical item affecting harvest directly All inefficiencies are converted into heat, thereby affecting reliability Efficiency is tested per the CEC document or EN50530 It is a weighted average efficiency over many operating points 95% is a market minimum Manufacturer: Enphase Energy Model #: M215-60-2LL-S2x Rated Maximum Continuous Output Power: 0.22 kw Night Tare Loss: 0.05 Vmin: 22 Vdc Vnom: 29 Vdc Vmax: 36 Vdc Power Level (%; kw) 10% 20% 30% 50% 75% 100% Input Voltage (Vdc) 0.02 0.04 0.06 0.11 0.16 0.22 Wtd Vmin 22 94.6 95.5 95.9 96.0 95.5 94.7 95.6 Vnom 29 95.5 96.0 96.1 96.1 96.0 96.0 96.0 Vmax 36 95.8 96.1 96.1 96.1 96.1 95.8 96.1 CEC Efficiency = 96.0% 98 97 96 95 94 22 Vdc 29 Vdc 36 Vdc 93 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% % of Rated Output Power 2008-2012 Enphase Energy 20
Microinverter: Environment & packaging Outdoor Below module Temperature range: -40/+65ºC ambient Surface and internal temperatures are higher Packaging: Is mounted on the rack or the module Corrosion resistant Has to cool the electronics Has to protect the electronics from water ingress Has to protect the electronics from mechanical damages Has to contain electro-magnetic signals generated within Has to be as small as possible, cost effective and easy to manufacture Has to do all this for 25 years minimum 2008-2012 Enphase Energy 21
Microinverter: Power line communication Critical to a microinverter Reports data to a gateway Allow for control of the inverter Transmission system ~140kHz carrier with frequency modulation Transmitted over the grid voltage Point to multi-point system Has to be very robust Very difficult problem due to the uncontrolled and noisy environment 2008-2012 Enphase Energy 22
Microinverter: Conclusion Very difficult product to get right At the crux of conflicting highly difficult requirements Efficiency cost reliability lifetime robustness ease of use compliance to standards communication etc. It s worth it! 2008-2012 Enphase Energy 23