Microinverters: an introduction May 2013

Transcription

1 Productive. Reliable. Smart. Safe. Martin Fornage, CTO Microinverters: an introduction May Enphase Energy

2 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 Enphase Energy 2

3 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 Enphase Energy 3

4 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 Enphase Energy 4

5 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? Enphase Energy 5

6 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 Enphase Energy 6

7 I P Microinverter: PV module I/V curve PV characteristics Irradiance 1000W/m2 Power vs Voltage Irradiance 1000W/m2 Irradiance 200W/m2 Irradiance 200W/m V V Enphase Energy 7

8 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 Split Phase Line 1 Voltage Line 2 Voltage General rules for transmission/distribution: the higher the voltage, the less the losses Single Phase Line Voltage Three Phase Line 1 Voltage Line 2 Voltage Line 3 Voltage Enphase Energy 8

9 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 Enphase Energy 9

10 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 C Enphase Energy 10

11 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 Line 1 Voltage Line 2 voltage Line 1 voltage Line 2 voltage Enphase Energy 11

12 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 Enphase Energy 12

13 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 Enphase Energy 13

14 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 Enphase Energy 14

15 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 DC power AC power Enphase Energy 15

16 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 Enphase Energy 16

17 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) Enphase Energy 17

18 An open unit Enphase Energy 18

19 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 Enphase Energy 19

20 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 #: M LL-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) Wtd Vmin Vnom Vmax CEC Efficiency = 96.0% Vdc 29 Vdc 36 Vdc 93 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% % of Rated Output Power Enphase Energy 20

21 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 Enphase Energy 21

22 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 Enphase Energy 22

23 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! Enphase Energy 23