1. Photovoltaic Systems Engineering Session 21
SEC598F17
Photovoltaic Systems Engineering
Session 21
Grid-Tied Residential Systems
BOS components
Battery Backup PV Systems
Design Considerations, part 1
November 01, 2017

2. Session 21 content Wrap-up of Session 20 Battery Backup PV Systems
Design Basics
Standby Loads
Inverter and Battery Selection
BOS components

3. Learning Outcomes
An examination of the design of a grid-connected battery-backup photovoltaic system leading to a recognition of the complexities in large-scale expansion of this technology

4. Grid-Tied PV Systems – The Design Process
Voltage drop and wire sizing
NEC requirement:
Total voltage drop in feeder and branch circuits less than 5% (combined) or 3% (either)
Size
18 (s)
10 (s)
4 (str)
0 (str)
Rdc (W/kft)
7.77
1.21
0.31
0.12
Imax (A) 14 40 95
170

5. Grid-Tied PV Systems – The Design Process
Branch circuits are source circuits that connect PV arrays to power conditioning
Feeder circuits are PV inverter output circuits that connect to the utility (service panel)
Rule of thumb: Don’t allow voltage drops to exceed 2% in any circuit

6. Grid-Tied PV Systems – The Design Process
Ampacity and wire sizing
NEC requirement (690.8):
Ampacity > (1.25) * (1.25) * ISC = ISC
Temperature derating from Tambient > 300C (NEC )
3 hrs of max current flow
Focusing effects of clouds
T (0C)
26-30
31-35
41-45
Correction
1.00
0.96
0.82

7. Grid-Tied PV Systems – The Design Process
M&A Example (modified)
ISC = 8.05A -> ISC = 12.6A
Case 1: Wire size 10 AWG
300C ampacity = 40A
Suppose the ambient temperature will rise to 380C
Derating factor is 0.91
Derated ampacity: 36A >> 12.6A
Case 2: Wire size 14 AWG
300C ampacity = 25A
Suppose the ambient temperature will rise to 600C
Derating factor is 0.71
Derated ampacity: 17.5A > 12.6A

8. Grid-Tied PV Systems – The Design Process
1. Solar PV Inspection Walkthrough - Introduction 2. Solar PV Inspection Walkthrough - Inspector Safety 3. Solar PV Inspection Walkthrough - The Array 4. Solar PV Inspection Walkthrough - Combiner Boxes 5. Solar PV Inspection Walkthrough - Wiring Methods 6. Solar PV Inspection Walkthrough - Inverters 7. Solar PV Inspection Walkthrough - Interconnection 8. Solar PV Inspection Walkthrough - Following Up
Penn State
Solar Center

9. Grid-Tied PV Systems – System 01 with microinverters
AC disconnect between PV modules and service panel

10. Grid-Tied PV Systems – System 01 with microinverters
AC disconnect between PV modules and service panel

11. Grid-Tied PV Systems – System 01 with microinverters
Second AC disconnect between PV modules and service panel

12. Grid-Tied PV Systems – System 01 with microinverters
Wiring of service panel

13. Grid-Tied PV Systems – System 01 with microinverters
Service panel, PUC

14. Grid-Tied PV Systems – PV system engineering and design
Service panel, PUC

15. Grid-Tied PV Systems – System 01 with microinverters
Service panel, PUC

16. Grid-Tied PV Systems – System 02 with string inverter
Typical module data sticker (nameplate)

17. Grid-Tied PV Systems – System 02 with string inverter
Modules and Junction Box

18. Grid-Tied PV Systems – System 02 with string inverter
DC input
from
PV array
AC ouput
from
inverter
Inverter and DC disconnect

19. Grid-Tied PV Systems – System 02 with string inverter
Vin = 600 V
Iin = 18 A
250<VMPPT<600
PAC = 4000 W
Inverter data label

20. Grid-Tied PV Systems – System 02 with string inverter
DC disconnect data sticker

21. Grid-Tied PV Systems – System 02 with string inverter
PV meter
To PUC
From inverter
PV meter and AC disconnect

22. Grid-Tied PV Systems – System 02 with string inverter
Main panel and Point of Utility Connection

23. Grid-Tied PV Systems – System 02 with string inverter
Main panel and Point of Utility Connection

24. Battery-Backup PV Systems – The Design Process
Design Steps in a battery backup PV system
Examination of site and estimation of performance
Securing financing
Carrying out PV system engineering and design
Standby loads
Connection to charge controllers and batteries
Altered inverter design
Securing relevant permits
Battery housing
Inverter operation
Construction
Inspection
Connection to the grid
Performance monitoring

25. Battery-Backup PV Systems – The Design Process

26. Battery-Backup PV Systems – The Design Process
Step 3: System Engineering and Design
Evaluation of solar availability, electrical consumption, essential electrical loads
PV array sizing
Inverter selection
Module selection
Charge controller selection
Battery selection
Balance of system

27. Battery-Backup PV Systems – Engineering Issues
The Battery-Backup Grid-Connected PV System resembles the Grid-Connected PV System in that the grid and the PV source work together to supply electrical power to the residence – when the grid is available
When the grid is unavailable, a grid-connected PV system is automatically disconnected from the grid – for safety considerations
But a battery-backup system continues to provide electrical power to the residence through the battery/PV portion of the system

28. Battery-Backup PV Systems – Engineering Issues
When there is a utility outage, the inverter in a conventional grid-tied system disconnects from the grid, but continues to monitor the status of the grid, and automatically reconnects when the grid is stable
The inverter for a battery-backup system is more complicated and serves several purposes:
It supplies AC power to the grid, when available
It serves as the conduit to charge the batteries with grid electricity if the PV system cannot do this
It supplies AC electricity to the “standby loads” in the residence when the grid is down

29. Battery-Backup PV Systems – Engineering Issues
Block diagram of battery-backup system (dc)