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CHAPTER 5 GRID CONNECTED PV SYSTEM (1)

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Page 1 of 20
CHAPTER 5
GRID CONNECTED PV SYSTEM
Table of Contents
1.0 Introduction ...........................................................................................................................2
2.0 Components of a Grid-connected PV system ........................................................................2
3.0 Grid Connected Inverter ........................................................................................................3
4.0 Design procedures ................................................................................................................4
5.0 Sizing inverter with PV array .................................................................................................5
6.0 Sizing BOS components .......................................................................................................7
7.0 Power loss in the cable and voltage drop ..............................................................................9
8.0 Power Loss in a grid connected PV system.........................................................................10
9.0 System evaluation ...............................................................................................................10
10.0 Case study .......................................................................... Error! Bookmark not defined.
Tutorials ...................................................................................... Error! Bookmark not defined.
Page 2 of 20
1.0 Introduction







A grid connected PV system is a PV system where it is connected to a large
independent grid (typically the public electricity grid) and feeds power into the grid.
Grid connected systems vary in size from residential (2-10kWp) to solar power stations
(up to megawatt).
In the case of residential or building mounted grid connected PV systems, the electricity
demand of the building is met by the PV system. Only the excess is fed into the grid
when there is an excess.
The feeding of electricity into the grid requires the transformation of DC into AC by a
special, grid-controlled inverter.
In Malaysia, SEDA is an organisation that managed Feed-in tariff for GCPV power
system.
In Feed-in tariff system, the energy consumed and sold to TNB will be charged at
different rate. For further detail, visit SEDA website.
Statement from SEDA regarding GCPV power system in Malaysia:
(Source SEDA 2012)

Visit the following link to see the latest development of renewable in Malaysia managed by
SEDA: http://www.seda.gov.my
2. Components of a Grid-connected PV system

A grid connected PV system is a simple PV power system that consists of the following
components:
o Photovoltaic array
o String cable
o Array Junction Box (AJB)
o DC fuse
o Array cable
Page 3 of 20
o
o
o
o
o
o
DC SPD
DC breaker
Grid inverter
AC SPD
AC breaker
Energy export meter
+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
(Source: MS1837)
There is no energy storage such as battery in a GCPV power system.
All the power from the PV array can be transferred to the grid. However in some
countries only excess power is allowed to be transferred to the grid via distribution
board.
3. Grid Connected Inverter



The grid-connected inverter is a special type of inverter. It only works with the present of
grid supply and cannot directly connected to the load without grid line.
The grid inverter turn on when:
o String DC voltage from the PV array is within the inverter’s window voltage
o AC voltage present and its value is within the operating range of the inverter
o Frequency of the grid is within the operating limit
The inverter might damage if:
o The input voltage from PV array is higher than the maximum input voltage of the
inverter
N
Page 4 of 20

o Voltage surges from DC side or AC side i.e. due to lightning
The grid inverter might turn off during day time if:
o Voltage from the PV array is outside the inverter’s window voltage
o Grid voltage is not present or outside the operating range
o Frequency of the grid is not stable or outside the operating range
4. Design procedures

The following steps are used as a guideline to design a complete grid connected PV power
system and sizing process. All the design formulas are based on MS1837 standard.
Step 1: Get clear objective why the customer wants to install GCPV power system at his/her
dwelling. Extra income? Fill up roof top? Business advertisement? Reduce monthly
electricity bill? Environmental conscious?
 If possible ask the owner: Prefer type of PV module
 Prefer type of grid inverter
 Cable route
 Location of inverter, breakers and Array Junction Box
 Type of mounting i.e. building integrated of retrofitted
 Location to install PV module i.e. which roof side.
 Type of interconnection i.e. direct or indirect connection to the grid.
Step 2: Gather detail information about the site.
 During site visit record all the following information:
o Roof condition.
o Logistic i.e. transportation
o Safety. Surrounding areas. Workers and components e.g. storage
o PV orientation.
o Potential shading.
o Tilt angle.
o Cable route distance: Inverter to energy meter
 Inverter to Array Junction Box (AJB)
 Longest route from PV array terminal to AJB
Step 3: Design PV array and grid inverter matching.
 Design the system for maximum output energy generated to the grid.
o Select type of PV module (if not yet specified)
 As a guidelines:
 Monocrystalline PV module:
o Highest efficiency.
o Most expensive.
o Temperature coefficient is better than polycrystalline
module.
 Polycystalline PV module:
o Lower efficiency than monocrystalline.
Page 5 of 20
Cheaper than monocrystalline module.
Temperature coefficienct is higher than monocrystalline
module.
Thin Film module:
o The lowest efficiency
o The cheapest.
o The lowest temperature coefficient.
o
o

Select type of inverter (if not yet specified)
 As a guidelines:
 String inverter
o Power less than 20 kWp.
o Single-phase AC output.
 Central inverter
o Power more than 20 kWp.
o Three-phase AC output.
o Total number of PV modules required.
o Number of modules in series in each string.
o Number of parallel string.
o Array configuration.
Step 4: Design BOS components
 Voltage and current rating of the following components:
o DC fuse
o DC SPD
o DC breaker
o AC breaker
 Size of the following components
o DC cables
o AC cable size
Step 5: Final Schematic diagram
 Draw a complete schematic diagram
Step 6: System evaluation
 Predict the performance of the following items:
o Total energy yield from the system per annum
o Specific Yield (SY)
o Performance Ratio (PR)
o
5. Sizing inverter with PV array

Step 3 in section 4 will be elaborated further as follows:
o The word “design” means to determine the number of PV modules per string and
number of parallel string that work safely with the inverter. The inverter shall not
turn off during normal daytime condition.
o The word “array configuration” means the arrangement of PV module in parallel
and series. A proper way to write array configuration is parallel by series. For
Page 6 of 20
o
example 2 by 12 or simply 2 x 12 means 2 parallel strings and 12 PV modules
per string.
The following steps can be used as a guideline to determine array configuration:
For simplicity let’s assume, the inverter is given or known and we will determine the PV
array configuration.
Step 1: Determine the range of total number of PV module that matches with the
specific inverter.
N total_module  round up (
Pnominal_inverter
K 2  Pmodule_stc
) to round down (
Pnominal_inverter
K 1  Pmodule_stc
where
N total_module
= total number of PV modules
Pnominal_inverter = nominal power of inverter
Pmodule_stc
= maximum power of PV modules (Pmp_stc)
For crystalline PV module:
= safety factor (1.0)
K1
= safety factor (0.9)
K2
Step 2: Determine Vmax_oc, Vmax_mp and Vmin_mp.
The minimum and maximum cell temperatures for Malaysia condition.
% voc
)  (Teff_min  25)}]
100
% vmp
 Vmp_stc  [1  {(
)  (Teff_max  25)}]
100
Vmax_oc  Voc_stc  [1  {(
Vmin_mp
Note:
Use worst case percentage for all temperature coefficients.
For Malaysia condition:
Teff_min = 200C and Teff_max = 750C.
Step 3: Determine the maximum number of PV modules in series per string
Ns max  round down (
Vmax_invert er  S.F1
Vmax_oc
)
Safety factor (S.F1) of 0.95 is usually used in design.
)
Page 7 of 20
Step 4: Determine the minimum number of module in series per string
Nsmin  round up (
Vmin_window _inverter  S.F2
Vmin_mp  VD p.u
)
Where safety factor ( S.F2 ) = 1.1 and voltage drop ( VD p.u ) = 0.95 are usually
applied.
Step 5: Determine the maximum number of parallel string.
Npmax  round down (
I dc_inverter
I sc_stc  S.F3
)
Where safety factor ( S.F3 ) = 1.25
Step 6: Determine optimum array configuration
In summary: Nsmin .. Nsmin
Npmax
Ntotal module
Finally to find the optimum array configuration, you need to find a combination of
parallel x series such that the answer is in the range of Ntotal module .
So, the optimum array configuration is N s  N p  N total
6. Sizing BOS components

Step 4 in section 4 will be elaborated further as follows:

String fuse
1.5  I sc_mod_stc  Itrip  2  I sc_mod_stc
Where
I trip
tripping current of over current protection (A)
I sc_mod_stc
short circuit current of module at stc(A)

DC cable
Page 8 of 20
Amin_dc_cable 
2  Ldc_cable  I dc  ρ
loss  Vmin_mp  N s
Where
Amin_dc_cab le
is minimum CSA of the cable (mm2)
Loss
Vmin_mp
is maximum voltage loss in conductor (decimal)
is minimum MPP voltage. In Malaysia climate, at highest solar irradiance, the
I dc
module temperature becomes the highest.
is very close to I mp _ stc since the solar irradiance could reach 1,000 W/m-2.
Ns
is number of modules in series per string.
Ldc _ cable
is route length of DC cable in metres (”2” adjusts for total circuit wire length) (m)

is resistivity of the wire (mm2m-1). For copper, the resistivity is 1/56 or
(0.017857) while for Aluminium, it is 1/34 or (0.029412).
Note: N s  Vmin_mp  Vmin_mp_str ing
Vdrop _ dc 
2  Ldc _ cable  I dc  
Adc _ cable
Where
Vdrop _ dc
is voltage drop of cable on DC side (V)
Ldc _ cable
is route length of DC cable in metres (”2” adjusts for total circuit
I dc
wire length) (m)
is DC current (A)

is resistivity of the wire (mm2m-1). For copper, the resistivity is
Adc _ cable
1/56 or (0.017857) while for Aluminium; it is 1/34 or (0.029412).
is cross section area (CSA) of DC cable (mm2)
Voltage rating  1.2  Voc array STC

Bypass diode
The ratings of the By-Pass Diode as specified in MS1837 are:
Vdiode _ bypass  2  Voc_module_ stc
Page 9 of 20
I diode _ bypass  1.3  I sc_module_ stc
Where
Vdiode _ bypass
is minimum voltage rating of bypass diode (V)
I diode _ bypass
is minimum current rating of bypass diode (A)

DC main switch
Vdc _ switch _ rating  1 .2  Voc_module_stc  N s
I dc _ switch _ rating  1.3  I sc_module_ stc  N p
Where
Vdc _ switch _ rating
minimum voltage rating of DC switch (V)
I dc _ switch _ rating
minimum current rating of DC switch (A)
7. Power loss in the cable and voltage drop
Pdc_cable 
2  L dc_cable  (I dc ) 2  ρ
A dc_cable
or
Pdc_cable  Vdrop  I dc
Where
Pdc_cable
is power loss in DC cable (W)
Vdrop_dc 
2  L dc_cable  I dc  ρ
% Vdrop_dc 
A dc_cable
Vdrop_dc
Vmin_mp  N s
Page 10 of 20
Where
Vdrop _ dc
voltage drop of cable on DC side (V)
Ldc _ cable
route length of DC cable in metres (”2” adjusts for total circuit wire length) (m)
I dc
DC current (A) which is similar to Imp_stc.

resistivity of the wire (mm2m-1). For copper, the resistivity is 1/56 or (0.017857)
Adc _ cable
while for Aluminium; it is 1/34 or (0.029412).
cross section area (CSA) of DC cable (mm2)
8. Power Loss in a grid connected PV system

In general, the power losses in a grid connected PV system are due to six factors:a. Inverter i.e. efficiency of an inverter is approximately 96%
b. Cable i.e. maximum allowable voltage drop by the MS1837 standard from
PV array to inverter is 5%
c. Temperature
=

d.
e.
f.
×
_
× [1 +
%
_
− 25 ]
_
Module miss-match or manufacturer power tolerance
Dirt
Aging
9.0 System evaluation

The energy generated from a grid-connected PV system could be estimated using:
=

_
×
×
×
×
×
_
×
Specific yield (SY) is defined as:
=
_
In Malaysia, for a good system, SY should be greater than 1200

Performance ratio (PR) is defined as:
=
_
×
In Malaysia, for a good system, PR should be greater than 70%
ℎ
per annum.
Page 11 of 20
Tutorial
Q1
Discuss the advantages and disadvantages of micro inverter, power optimiser, string
inverter and central inverter.
Q2
A customer wants to install a grid connected PV system on his roof top. The detail system
specification is given below:
PV type: Yingli YL180P-23b PV module (refer Datasheet in I-Learn portal)
Grid inverter type: Sunny Boy 3000 (refer Datasheet in I-Learn portal)
Assume:
The average peak ambient temperature is 320C
Dirt de-rating factor 0.97
Maximum cell temperature is 750C
Minimum cell temperature is 200C
Power de-rating factor is between 0.90 to 1.0
The customer needs your help to design the system by determine:i. the maximum number of PV modules that will not damage the inverter due to
over voltage.
ii. the minimum number of PV module per string that work with the MPPT unit of
inverter.
i. The maximum number of parallel string.
ii. The optimum or best array configuration
Q3
You are given the following components:- total module is 36 units.
- maximum number of PV module per string is 12 units.
- the maximum current for each PV string is 8A.
- inverter with two MPPT; A:20A B:10A.
- Array junction box, fuses, cables, SPDs, inverter with two MPPTs, and
breakers.
- Type of connection to the grid is in-direct feed-in.
Construct a complete schematic diagram for grid connected PV system using the
components above and explain how the system works during day and night time.
Q4
As an Engineer, prepare a check list before you go the site for site survey prior to design
GCPV system
Q5
List and explain the standard features for a grid connected PV inverter.
Q6
PV modules only work during sunny day time and not generating power at all during night
time. Explain the concept of generating power using PV system that could compensate
the annual total energy consumed
Page 12 of 20
.
Q7
Q8
Explain how the PV grid connected power system gives effect on the following issues:
- maximum power demand
- power factor
- low voltage on grid side
Explain how a grid connected PV inverter could transfer only active power to the grid.
Q9
With the aid of suitable diagram, explain the working principle of a transformer based and
transformer less grid connected inverter. Explain why not all PV technologies can be used
with transformer less inverter.
Q10 Site visit is very important prior to start designing and sizing of a grid connected PV power
system. Explain what need to be done during the site visit.
Q11 Maintenance is very important in any power generating system. Explain the maintenance
aspect on the grid connected PV power system.
Q12 Explain three types of structure to install PV grid connected power system that commonly
used in Malaysia. Your explanation should include the advantages and disadvantages of
each types.
Q13 Explain why the following components shall not be used on DC side of GCPV system, AC
SPD, AC cable, AC Fuse, AC breakers.
Q14 Explain why a grid connected inverter might turn off during day time. Give three reasons.
Q15 A grid connected PV system consists of the following components:
PV type: Advent 220 (refer Datasheet in I-Learn portal)
Total number of PV modules: 24
Site location: Tawau, Sabah (refer Datasheet in I-Learn portal)
Latitude: 4.250N
Longitude: 117.880E
PV array orientation: Facing East
PV array tilt angle: 180
Grid inverter type: SMA SB 3000TL (refer Datasheet in I-Learn portal)
Assume:
The average peak ambient temperature is 320C
Dirt de-rating factor of 0.97
From the above information, predict:i) the total energy generated per annum
ii) the final yield of the system. State the acceptable value of FY for a grid
connected system installed in Malaysia.
iii) the Performance ratio of the system. State the acceptable value of PR for a
grid-connected system installed in Malaysia.
Page 13 of 20
Q16 Determine the optimum array configuration of Suntech STP260-24/Vb PV modules with
grid inverter SMA SB1200.
Q17 You are given a task to do an analysis on a grid-connected PV system with the following
specifications: Site location: Kuala Lumpur. The solar irradiation for selected module orientation
and tilt angle is given in attachment 1.
 PV orientation: Facing East
 Tilt angle: 45 deg
 Type of Module: Suntech STP280-24/Vb. Datasheet is attached in attachment 2.
 Number of modules in series per string: 8 modules
 Number of string: 2 strings
 Type of inverter: Fronius IG 40. Datasheet is attached in attachment 3.
Assume:
o PV power reduce 3% by dirt
o Cable loss 2%
o Daily average peak ambient temperature is 32 deg
Determine:
a. Total energy yield per annum
b. Specific yield
c. Performance ratio
Q18 The length of string and array cables of a BIPV system are given in the following figure.
Array configuration: 3 x 8
Module: STP 270-24/vb-1
Table 4
Standard size of PV
cables available




2.5 mm2
4 mm2
6 mm2
10 mm2
Page 14 of 20
If the maximum allowable voltage drop in string cable is 1% and array cable is 1%,
assuming all cable conductors are copper, determine:
a. The minimum size of string and array cable required. From Table 4, choose the
correct size of string and array cable. What is the minimum voltage rating of the
cables?
b. Based on the chosen cables, calculate the total power loss in the DC cables in
absolute value (W) and percentage (%).
c. Determine the minimum and maximum tripping current of string over current
protection devices.
d. Determine the minimum current and voltage rating of the bypass diode.
e. Determine the minimum current and voltage rating of the DC breaker.
Q19 Discuss how to protect grid-connected PV inverter from induced surge voltage due to
lightning.
Q20 List possible reasons why some people want to install grid connected PV (GCPV) power
system at their houses.
Q21 List the characteristics of a good site to install GCPV.
Q22 List the characteristics of PV module that best used in GCPV installation.
Q23 Explain the meaning of the following terms:
a. PV cell
b. PV module
c. PV array
d. Feed-in tariff
e. Grid parity
f. Building integrated PV system
g. Retrofitting system
h. Free standing structure
Q24 State which Malaysia standard related to GCPV power system.
Q25 Briefly discuss how to do test and commissioning of a GCPV power system for a
residential system.
Q26 List the differences and similarities between solar farm and residential GCPV power
system.
Q27 List the possible causes that the GC inverter might turn off during day time.
Page 15 of 20
Q28 List the possible causes of GCPV power system failure and discuss the requirements of
maintenance of GCPV power system
ATTACHMENT 1
Kuala Lumpur
Location
Kuala Lumpur
State
Federal Territory
Latitude (deg)
3.13 N
Longitude (deg)
101.6 E
Elevation A.S.L.
16.5
(m)
Global irradiation (kWhm-2) at various tilt angles facing North :
Azimuth 180 deg
Tilt angle
At
(deg)
latitude
00
10
20
30
40
50
60
70
80
90
Page 16 of 20
January
120.3
February
128.6
March
145.3
April
135.3
May
136.3
June
133.8
July
136.7
August
Septemb
er
137.9
October
Novemb
er
Decemb
er
132.9
Year
1572.6
Tilt angle
(deg)
January
February
March
April
May
June
July
130.8
117.5
117.2
122.
0
130.
0
145.
9
134.
8
135.
0
132.
0
135.
0
137.
0
130.
9
133.
9
119.
0
119.
0
1574
.5
115.
7
124.
5
143.
1
135.
5
138.
3
136.
9
139.
4
139.
0
129.
8
129.
9
113.
5
112.
3
1557
.8
107.
4
116.
5
137.
5
133.
6
138.
9
139.
0
140.
9
138.
2
126.
3
123.
4
105.
9
103.
7
1511
.3
97.3
106.
4
129.
2
129.
1
136.
8
138.
2
139.
6
134.
8
120.
4
114.
7
86.1
74.1
62.6
55.9
51.9 47.8
94.5
118.
6
122.
1
131.
9
134.
6
135.
4
128.
7
112.
4
104.
1
81.2
105.
9
112.
9
124.
5
128.
3
128.
6
120.
2
102.
4
67.1
53.6
47.7 44.5
91.6
101.
7
114.
8
119.
5
119.
3
109.
6
76.0
59.7 50.8
89.0
103.
0
108.
4
107.
8
75.0 60.2
97.1
83.2 68.3
90.8
77.9
64.1 49.9
91.8
78.4
64.5
54.0 50.0
96.6
86.0
74.5
62.9
54.5
50.7 46.7
93.4
1436
.5
82.1
1336
.4
70.4
1214
.7
59.6
1077
.8
54.5
942.
1
50.7 46.7
816. 700.
3
1
Global irradiation (kWhm-2) at various tilt angles facing East
: Azimuth -90 deg
At
latitude
00
10
20
30
40
50
60
122. 120. 117. 113. 107. 100.
121.9
0
9
9
3
5
6
92.8
130. 128. 125. 120. 113. 106.
129.9
0
8
4
2
8
2
97.8
145. 144. 140. 135. 127. 119. 109.
145.7
9
5
7
0
8
4
9
134. 133. 130. 124. 118. 110. 101.
134.7
8
6
1
9
3
5
8
135. 133. 130. 124. 118. 110. 101.
134.9
0
7
2
9
2
3
6
132. 130. 127. 121. 115. 107.
131.9
0
7
2
9
3
5
98.8
134.9
135. 133. 130. 124. 118. 110. 101.
70
89.6 74.9
95.3 80.8
94.4 79.6
80
90
84.5
76.0 67.4
88.8
79.6 70.5
99.8
89.6 79.3
92.5
83.0 73.5
92.2
82.7 73.1
89.5
91.8
80.2 70.9
82.3 72.7
Page 17 of 20
0
7
2
8
0
1
2
137. 135. 132. 126. 120. 112. 103.
August
136.9
0
7
2
9
1
1
2
93.8 84.1 74.5
Septemb
130. 129. 126. 121. 115. 107.
er
130.7
9
7
4
4
0
5
99.2 90.2 81.0 71.8
133. 132. 129. 124. 117. 110. 101.
October
133.8
9
7
3
2
8
1
5
92.3 83.0 73.5
Novemb
119. 117. 115. 110. 104.
er
118.9
0
9
0
6
9
98.1 90.6 82.5 74.2 65.8
Decemb
119. 117. 115. 110. 104.
er
118.9
0
9
0
5
8
98.1 90.5 82.4 74.1 65.7
1574 1560 1519 1458 1381 1290 1188 1080 969. 858.
Year
1573.1
.5
.0
.6
.7
.6
.6
.9
.3
5
6
(Source: Solar Photovoltaic Power: Irradiation data for Malaysia by Sulaiman Shaari et. al)
ATTACHMENT 2
Page 18 of 20
ATTACHMENT 3
Page 19 of 20
ATTACHMENT 4
Page 20 of 20
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