Download WEEK 3 BOS version 2

SELECTED TOPIC GRID-CONNECTED PV SYSTEM DESIGN
AND SIZING
Week 3
Table of Contents
3.0 Balance of System Components (BOS) ................................................................................................... 2
3.1 Introduction ........................................................................................................................................ 2
3.2 Bypass diode ...................................................................................................................................... 3
3.3 Cables .................................................................................................................................................. 3
3.4 Over current Protection ..................................................................................................................... 8
3.5 Array junction Box (AJB)...................................................................................................................... 8
3.6 Combiner Box ...................................................................................................................................... 9
3.7 DC breaker .......................................................................................................................................... 9
3.8. Surge protection device (SPD) ......................................................................................................... 10
3.9 Inverter............................................................................................................................................. 11
3.10 AC breaker....................................................................................................................................... 12
3.11 Ground wire .................................................................................................................................... 12
Page 1 of 12
3.0 Balance of System Components (BOS)
3.1 Introduction
-
All components in the grid-connected PV system except PV modules are called
Balance of System (BOS) components.
Some of the components are: Blocking diode, bypass diode, AJB, DC breaker, DC
fuses, String cable, connectors and sockets, SPDs, AC breaker, meters and structures.
All PV grid connected systems install in Malaysia are based on Malaysian Standard
MS1837:2005
+ve
-ve
PV string over-current
protection device
PV string 1
PV string cable: min 2.5 mm2, flex-type, double insulated, UV resistant
PV array cable: flex-type, double insulated,
UV resistant, shielded
Inverter
+ve
-ve
PV kWh
meter
PV string 2
==>
+ve
-ve
PV d.c.
Main Switch
PV string n-1
+ve
-ve
L
PV a.c.
Main Switch
Surge protective device (SPD)
Surge protective device (SPD)
PV string n
E
Earthing (if required): min 6 mm2 single core Cu
(for framed PV modules and metal casing)
d.c. Electricity
a.c. Electricity
Page 2 of 12
N
3.2 Bypass diode
- A diode that is connected in parallel with a PV module
- It is usually located in a small box under the module.
- The functions of bypass diode is to provide alternative current path when shading
occurs on that particular cell. Thus prevent hotspot.
- To determine size of bypass diode based on MS1837:2005:
Current rating > 1.3 x Isc module
Voltage rating > 2 x Voc module
Bypass Diode
3.3 Cables
PV String cable
- A cable connecting the modules in a PV string, or connecting the string to a junction
box or to the d.c terminals of the inverter.
- The minimum cables sizes for PV string cable, based on current carrying capacity
(CCC), shall be based upon 2 x Isc module.
- MS1837:2005 states:
Cable sizes for PV string cables, PV sub-array cables and PV array cables shall
not be less than 2.5 mm2 and shall be determined with regard to both, the
minimum current capacity and maximum voltage drop requirements. The larger
cable size obtained from these two criteria shall be applied.
The voltage drop between the PV array and the inverter shall be less than 5%.
Page 3 of 12
DC Cable
Connectors
Array Cable
- It is a cable that carry current form AJB to inverter.
- The minimum cables sizes for PV array cable, based on current carrying capacity (CCC),
shall be based upon 1.3 x Isc module.
Page 4 of 12
- The cables in an installation must be sized correctly so that:
-
there are not excessive voltage drop (also corresponding power loss) in the cables;
there is not excessive current through the cables compared with the safe current
handling capability of the cables; and maximum voltage rating of any cable is never
exceeded
-
Array cable
INVERTER
ARRAY JUNCTION BOX
String cable
Voltage Drop in a wire is a function of three parameters :
- conductor cross sectional area ( mm² )
- length of wire; and
- current flow in the wire.
- To determine the d.c. voltage drop,(Vd_dc) the following formula is used:
Vd _ dc 
2  L
dc _ cable
 I dc   
Adc _ cable
Where
Ldc_cable = route length of dc cable in metres
( “2 x Ldc_cable” adjusts for total circuit wire length )
Idc = dc current in amperes
ρ = resistivity of the wire in  / m / mm2
Adc_cable = cross section area ( CSA ) of dc cable in mm2
The resistivity, ρ , varies dependent of the type of material. For copper the
resistivity is 1/56 (0.017857) while for aluminium it is 1/34 (0.029412).
Page 5 of 12
Or
Adc _ cable 
2  L
dc _ cable
 I dc   
Loss  Vmp _ string
Where
Loss = max voltage loss in the conductor as a % expressed as a fraction e.g.
5% = 0.05
Vmp_string = maximum power point voltage of the string/array.
Power Loss in d.c. cable
Pdc
-
2  L

dc _ cable
 I dc2  

Loss  Vmp _ string
Determining single-phase cable Size:-
Aac _ cable 
2  L
ac _ cable
 I ac    cos  
Loss  Vac
Where
Loss = max voltage loss in the conductor as a % expressed as a fraction
e.g. 1% = 0.01
Vac = a.c. voltage of the grid
L N
Single-phase
ac cable
Array cable
INVERTER
-
Page 6 of 12
-
Voltage drop in a three- Phase a.c cable:-
Vdrop_ ac 

3  Lac _ cable  I ac    cos 

Aac _ cable
Which is transposed to:
Aac _ cable 

Loss Vac
Power Loss in a single-phase cable:-
Pac 
-
3  Lac _ cable  I ac    cos 
2  L
ac _ cable
 I ac2    cos

Aac _ cable
Power loss in a three-phase cable:-
Pac


3  Lac _ cable  I ac2    cos

Aac _ cable
R
Y B
Three-phase
ac cable
Array cable
INVERTER
-

Page 7 of 12
3.4 Over current Protection
- All PV string shall be protected with an over-current protection device.
- The protection devices shall be installed in both active conductors.
- It is installed when the number of parallel string is more than three. The reason being
most manufacturers guarantee their module can stand up to 2 x Isc.
- It is also a requirement to install breaker at each string cable. The breaker with
incorporated fuse is widely used in practice (fuse combination unit).
- Fuses shall be d.c rated.
- To determine the rating of over current protection devices based on MS1837:2005:
Voltage rating > 1.2 x Voc string
Current rating: 1.3 x Isc module <= Itrip <= 2 x Isc module
Fuse combination unit
3.5 Array junction Box (AJB)
- AJB is a box where all the string cables are combined.
Page 8 of 12
AJB
String cable
+ve
DC fuses
-ve
DC Breaker
Array cable
3.6 Combiner Box
- It is quite common in practice to put all components in one box which is known as
combiner box.
DC fuse
L N
SPD
AC breaker
DC breaker
DC fuse
SPD
-VE
DC fuse
+VE
DC fuse
-VE
DC breaker
SPD
DC breaker
DC fuse
+VE
To export
energy meter
From string
2
DC breaker
From string
1
E-bar
To terminal
string #1 of inverter
To terminal
string #2 of inverter
From a.c terminal of inverter
3.7 DC breaker
- DC breaker is a switch to on/off array current especially during maintenance.
- Shall be rated for d.c use.
- Device rating:- Voltage rating >= 1.2 x Voc array
- This device must be rated to break at 1.3 time the short circuit current of the
array.
Page 9 of 12
- interrupt all poles.
- A switch must be located that can disconnect the full array under load in both
active conductors.
- This switch is known as the PV d.c. main switch.
PV d.c main switch
3.8. Surge protection device (SPD)
- Recommends the installation of surge protection devices (SPD) on:
- Between each of the array d.c. cables and earth
- Between the two d.c. cables
- Between each of the inverter output a.c. cables and earth
- Between the two a.c.cables.
- Earthing of the array frame and modules is also recommended.
- SPD Rating:
o Maximum continuous operating voltage Uc > 1.3 x Voc_stc_array
o Maximum continuous operating current Ic > 1.3 x Isc_stc_array
o Maximum discharge current Imax >= 15kA
o Voltage protection level Up; 1.3 x Voc_stc < Up < 1.1 kV
+ve
-ve INVERTER
SPDs
To TNB
Array cable
SPDs
Page 10 of 12
SPD on DC side
SPD on ac side
3.9 Inverter
-
To converter DC power from PV modules to AC power and transfer to the grid.
Inverter
Page 11 of 12
3.10 AC breaker
-
AC breaker is a switch to on/off ac current.
A PV a.c. main switch shall be located on the grid side of the PV kWh meter.
It must interrupt both the active and neutral conductors.
It will be known as the PV a.c. Main Switch
AC breaker
3.11 Ground wire
- MS1837:2005 states that the minimum size of ground wire is 6mm2. The 6 mm2
cable should be able to withstand 20kA for 1ms.
Page 12 of 12