My Home Photovoltaic PV Power System 光伏.docx

1、My Home Photovoltaic PV Power System 光伏My Home Photovoltaic (PV) Power SystemLatest addition: A day in the life of my PV system Ive long been fascinated by electric power systems, especially ones that individuals can own and operate. During the annual Field Day ham radio contest that stresses indepe

2、ndent sources of electric power, I always seemed to have more fun playing with the generators than actually operating the radios. So when a friend and colleague, Mike Brock (WB6HHV), started a photovoltaic (PV) power system on his house, I got interested in doing one on mine. It helps that Californi

3、a has abundant sunlight and some of the most PV-friendly laws and regulations in the country. These include a net-metering law that requires the electric company to buy my surplus electricity at the full retail price, and a state-funded buy-down program that subsidizes part of the cost of such a gri

4、d tied PV system. A grid tied PV system operates in conjunction with a conventional electric utility feed. If the PV system produces more power than needed to operate local loads, the excess is sold to the utility, running the electric meter backwards and generating a billing credit. When the local

5、loads are greater than the power generated by the PV system, e.g., at night or when large loads are being operated, the meter runs forward. PV technology is maturing rapidly, though the cost is still not low enough to compete directly with utility electric rates, even with the government subsidies.

6、So I maintain no illusions that Im doing this to save money. Nor am I one of those loony Y2K survivalists. Im doing it because of a long-standing interest in the technology. I also get some side benefits, such as a UPS (uninterruptable power supply) that can run my computers for hours in the event o

7、f a power failure. Solar PanelsHere you can see the twelve Astropower AP1206 solar panels on my roof. They were installed by Carlsons Solar of Hemet, California. They are configured as three strings of four panels each. Each panel consists of 36 cells producing a nominal 12VDC under load, so each st

8、ring produces a nominal 48V (the no-load voltage is considerably higher, about 80V). The roof on which the panels are mounted slopes to the south-southwest at about 20 degrees. Ideally they would face due south, but that would have required more complex mounting brackets. Besides, it is frequently c

9、loudy in the early mornings here in San Diego, with the clouds burning off by late morning. A more westerly orientation favors the afternoon when it is more likely to be sunny. It also favors production when electric rates are at their highest (more about this later). The panel cabling is 10-gauge 2

10、-conductor Type TC cable, moisture and sunlight resistant. The cables penetrate the roof in a conventional weatherhead used for utility service entrances. The cabling goes in 1 flex conduit to a Trace TCB10 PV Combiner Box in the attic. Three #6 wires (DC +, DC- and ground) run from the combiner box

11、 in 1 flex to the garage. System ElectronicsThe array power enters the back wall of my garage, where I installed a Trace SW4048 sinewave inverter, a Trace DC disconnect, a 48 volt 220 amp-hour battery bank using Trojan T-105 commodity golf cart batteries, and a Crusing Equipment Company E-meter batt

12、ery monitor. A conventional AC subpanel on the inverter output feeds branch circuits around the house with various protected loads, such as my computer and networking gear and my VCR (so I wont lose my programs in the event of a power failure!) Here you can see the back wall of my garage. The condui

13、t from the arrays is at the right side of the photo; it enters the DC breaker panel (the upright rectangular white box with a horizontal green stripe). After passing through circuit breakers, the array power flows to a Trace Photovoltaic Ground Fault Protector , an overpriced and largely useless dev

14、ice that is nonetheless required by section 690-5 of the National Electrical Code. The PVGFP is mounted in the grey box at the lower right. After passing through the PVGFP, the array power flows through a relay in the PVGFP mounting box. This relay opens whenever the battery voltage exceeds a progra

15、mmed level. Under normal operation, the relay is always closed as any excess power from the arrays is automatically sold back to the utility. This keeps the battery voltage from ever reaching the point that would open the relay. But this relay is important to protect my batteries in the event the gr

16、id is not available as a diversion load. E.g., the inverter could fail, a circuit breaker could open, or the grid could go down. The relay does lack two features of a solid state controller: a multi-stage battery charging program, and a nighttime cutout. Because my system is grid tied, the multistag

17、e battery charger program is not very useful to me. My batteries are normally fully charged, and if they arent (e.g., after a power failure) the Trace inverter already has a multistage battery charger. The lack of a nighttime dropout means that unless I open the array breaker at sundown, Ill get abo

18、ut 200 mA (about 10 watts) of backfeed from the battery into the panels at night. I could stop this with diodes on the panel feeds, but the power loss is so small that I would probably lose more during the day in the voltage drop across the diodes than Id save at night. The relay coil itself draws a

19、nother 2-3 watts. The batteries are in the brown box at the bottom center of the photo. I made the box out of 3/4 MDF (medium density fiberboard). The top slopes up toward the back to encourage hydrogen gas produced by the batteries to flow into the 3 vent line connected to the left rear corner of t

20、he top of the box. This line carries the hydrogen to just below a turbine vent in the roof of my garage. (Personally, I believe hydrogen in a garage is a lot less dangerous than ordinary gasoline; while gasoline vapors collect and linger near the floor, hydrogen rapidly dissipates. But the inspector

21、 made me do it anyway.) The battery is connected back through the DC breaker box through a big 175A circuit breaker to the Trace inverter, the big white box just above the DC breaker panel. Because the inverter can carry substantial power, the cables here are quite heavy (2/0). The two grey boxes at

22、 the top center of the picture are conventional AC subpanels. The one on the left is an existing garage subpanel I put in when I got my EV1 and I installed the Magnecharger EV charger (on the left side of the picture). The right-hand subpanel is connected to the output of the inverter and supports t

23、he special branch circuits for the protected loads (protected against power failure, that is). The black cord hanging from the left side of the inverter is for a generator. In the event of an extended power outage, I can set up a portable generator and use it to supplement the power from the solar a

24、rrays to operate the protected loads and to recharge batteries. BatteriesHeres a view looking down into the battery box. The interconnects are 2/0 building wire from Home Depot. It was relatively cheap ($0.85/foot) but a real pain to work with; very stiff and unforgiving. The cables leading out of t

25、he box on the right side are 2/0 flexible boat cable. It is much easier to work with, but it is very expensive ($7.50/foot at Boat/US, a local marine store). The yellow cable is for a battery temperature sensor affixed to the battery on the right rear. The blue wire taps the battery at the mid-point

26、 to supply 24VDC for the e-meter; note the in-line fuseholder near the battery terminal. Although tapping a battery like this can theoretically cause the pack to become unbalanced, in practice the drain low enough that the imbalance is is quickly corrected by routine battery equalization. The bottom

27、 of the battery box consists of three 2x6s with gaps between to allow air to circulate up from ventilation holes drilled in the sides of the box. Under the box is a sheet of polyethylene to catch any acid spills. Before putting in the batteries I sprinkled a box of baking soda into the box to help n

28、eutralize any spilled acid. DC wiringHeres a closer look at the DC breaker panel: The E-meter is mounted in a 2 knockout on the upper left side. The 175A inverter breaker is in the center of the box. The array comes in on the right side through 1 flexible conduit, and the 2/0 cables to the inverter

29、leave in 2 PVC on the upper right. Note the white tape around one of the inverter cables indicating that it is a grounded neutral. It is connected to a 500A 50mV battery current shunt for the E-meter. To the right of the same taped cable is a negative bus bar. The array negative leads connect here.

30、The three array hot leads (red) go to 15A DC circuit breakers mounted at the upper right, upper left and lower left. The combined output of these three breakers passes down to the PVGFP mounted in the grey box at the lower right. PV Ground Fault ProtectorThis is the ground fault detector. It is noth

31、ing more than two mechanically ganged DC circuit breakers. The one on the right trips at 1 amp; the one on the left is actually a switch rated at 100 amps. The 1-amp breaker is shunted by a 50 kohm resistor. When closed, the 1-amp breaker connects the negative DC bus (the white wire, signifying that

32、 it is the system neutral) to earth ground (the green wire with a yellow stripe); this is the only place where the DC neutral is grounded. The 100A switch connects the combined array output to the input of the charge controller. The black device hanging off one of the red wires is a clamp-on DC ammeter measuring array output current. In the event of a fault between a positive lead (battery or array) to ground in excess of 1 amp, the 1 amp breaker will trip and open the 100A switch with it. This will interrupt the ground fault except for a small amount of current that will continue t