1. Photovoltaic Systems Engineering Residential Scale – Part 2
SEC598F17
Photovoltaic Systems Engineering
Session 20
Grid Tied PV Systems
Residential Scale – Part 2
October 31, 2017

2. Session 17 content Grid-Connected Residential PV Systems
Wrap-up of Residential PV System Example
Balance of Systems Considerations

3. Grid-Tied PV Systems – The Design Process
Design Steps in any Residential Scale System
Examination of site and estimation of performance
Securing financing
Carrying out PV system engineering and design
Securing relevant permits
Construction
Inspection
Connection to the grid
Performance monitoring

4. Grid-Tied PV Systems – The Design Process
Step 3 - PV system engineering and design
Evaluation of solar availability and electrical consumption
PV array sizing
Inverter selection
Module selection
Balance of system

5. Step 3 - PV system engineering and design
Part 3: Inverter Selection
The PV system output is expected to be 4000W (Part 2). This is the peak DC power output
If we use a PV system design that employs one inverter, it must be able to accept DC electricity with power of 4000W.
Representative inverter characteristics
Inverter
AC power
Max array power
Max DC Vin
Vin MPPT Range
Max DC Iin
Vout
Iout 1
3500 W
3800 W
600 V
200 – 500 V
20 A
240 V
16 A 2
4400 W
500 V
200 – 400 V
24 A

6. Step 3 PV system engineering and design
Part 4: Module Selection
The PV system output is a combination of all the separate modules’ power outputs
Both the open circuit voltage (Voc) and the voltage at maximum power (Vm) vary with temperature; the ranges are shown in the following table
Representative module characteristics
Module
Voc
(nominal)
(max)
Isc Vm
(min) Im Pm 1
33 V
37 V
8 A
27 V
20 V
7 A
189 W 2
44 V
49 V
5.5 A
36 V
5 A
180 W

7. Step 3 PV system engineering and design
Parts 3 and 4: Inverter and Module Compatibility
Inverter #1 [Vin(max) = 600V]
Module #1 [Voc(max) = 37V] : 600/37 =  16 modules
Module #2 [Voc(max) = 49V] : 600/49 =  12 modules
Inverter #2 [Vin(max) = 500V]
Module #1 [Voc(max) = 37V] : 500/37 =  13 modules
Module #2 [Voc(max) = 49V] : 500/49 =  10 modules

8. Step 3 PV system engineering and design
Parts 3 and 4: Inverter and Module Compatibility, cont.
Inverter #1 [VMPPT(min) = 200V]
Module #1 [Vm(min) = 20 V] : 200/20 = 10  10 modules
Module #2 [Vm(min) = 27 V] : 200/27 = 7.4  8 modules
Inverter #2 [VMPPT(min) = 200V]

9. Step 3 PV system engineering and design
Parts 3 and 4: Inverter and Module Compatibility, cont.
Number of modules based on inverter requirements
Circuit
Number of modules
Inverter #1, Module #1
Inverter #1, Module #2
8 - 12
Inverter #2, Module #1
Inverter #2, Module #2
8 - 10

10. Step 3 PV system engineering and design
Parts 2 and 4: Required PV array power and modules
Module #1 [Pm(nominal) = 189 W]
NMod1 : 4000/189 =  22 modules
Module #2 [Pm(nominal) = 180 W]
NMod2 : 4000/180 =  23 modules

11. Step 3: PV system engineering and design
Part 2 and 4: Reconciling the module number calculations
Consider the Module #2 + Inverter #1 combination:
To meet the array power requirements, 23 modules are required
But if the 23 modules are connected in series, forming one source circuit, their combined voltage would exceed the allowed inverter voltage input:
23 x 49V = 1127V >> 600V
Another approach is to split the modules into two source circuits, each containing 12 modules in series, and then combining the two source circuits in parallel to meet the power requirement.
12 x 49V = 588V < 600V
This does, however, double the total current, and it must be verified that it does not exceed the allowed inverter current input:
2 x 5.5A = 11A < 20A
Success!

12. Step 3 PV system engineering and design
Modules
(15 in series)
Representative single source circuit

13. Step 3 PV system engineering and design
Modules
(2 parallel strings of 8 in series)
Representative two source circuit

14. Step 3 - PV System Engineering and Design
Recap of Inverter/Module selection (Parts 2, 3, and 4)
Inverter selection
Pin (DC, array)
Pout (AC)
Vin (DC, max) & Iin (DC, max)
Vin (DC, MPPT range)
Vout (AC) = 240V & Iout (AC)
Module selection
Voc (DC) & Isc (DC)
Vmp (DC) & Imp (DC)
Temperature Range -> Voc (max) & Vmp (min)
Module Range
Maximum -> Vin (max)/Voc (max)
Minimum -> Vin (MPPT,min)/Vmp (min)
Power -> Psystem/Pmodule

15. Grid-Tied PV Systems – The Design Process
Shaded max
power point
Original max
power point

16. Grid-Tied PV Systems – The Design Process
A new approach to combatting the effects of shading has emerged recently – the use of “microinverters”
A microinverter is a compact inverter installed directly on the back of a PV module
Therefore in a PV array, each module has its own inverter – the PV modules now have an AC output, not a DC output
Each module is separately operated at its own MPP – they are connected in parallel, so the current loss from shading is eliminated
The wiring and the connections are simplified, and high DC voltages are eliminated
A typical microinverter has a 25 year expected lifetime – a dramatic increase over the expected 10 – 12 year lifetime of a high power central inverter

17. Grid-Tied PV Systems – The Design Process
Block diagram for AC PV system

18. Grid-Tied PV Systems – The Design Process
Microinverters installed
on back of modules

19. Grid-Tied PV Systems – The Design Process
Module
VOC = 44V ISC = 5.3A
Vmp = 36V Imp = 4.9A
Pm = 175W
Microinverter
Vin(max) = 54V Iin(max) = 8A
MPPT voltage range -> 25-40V
Vout = 240V (AC) Iout = 750mA
Pout(max) = 175W

20. Grid-Tied PV Systems – The Design Process
Part 5: Balance of System (BOS) component selection
Module extension wires
Junction box and/or Combiner boxes
Wire and Conduit to connect JB or CB to inverter
DC and AC disconnects at inverter
Wire and Conduit from inverter to Point of Utility Connection (PUC)

21. Grid-Tied PV Systems – The Design Process
Block diagram of two source circuit PV system

22. Grid-Tied PV Systems – The Design Process
Wiring of service panel

23. Grid-Tied PV Systems – The Design Process

24. Grid-Tied PV Systems – The Design Process

25. Grid-Tied PV Systems – The Design Process
PV wiring
THHN – Thermoplastic, High Heat, Nylon Coated – 90C
THWN – Thermoplastic, High Heat, Water resistant,
Nylon Coated – 75C
USE-2 – Underground Service Entrance
PV –

26. 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

27. 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

28. 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

29. 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

30. 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

31. Grid-Tied PV Systems – PV system engineering and design
AC disconnect between PV modules and service panel

32. Grid-Tied PV Systems – PV system engineering and design
AC disconnect between PV modules and service panel

33. Grid-Tied PV Systems – PV system engineering and design
Second AC disconnect between PV modules and service panel

34. Grid-Tied PV Systems – PV system engineering and design
Wiring of service panel

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

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

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