Dimensioning and Protection of PV Plants with examples of design Session 2 Dr. Francesco De Lia - ENEA C.R. ENEA Casaccia

Dimensioning and Protection of PV Plants with examples

of design

Session 2

Dr. Francesco De Lia - ENEA

C.R. ENEA Casaccia

Ing. Francesco De Lia - ENEA

• Module Datasheets

• Electrical mismatch in PV array

• Electrical conduits sizing on DC and AC sides

• PV Plant protection against over currents on DC side

• Thermal sizing of electrical switchboards

• Lightning protection of PV plants

• Description of PV Plant design

Outlook


Calculation of Current Carrying Capacity of the

cablesIZ


Current carrying capacity IZ is defined as the maximum current which can be carried continuously by a conductor under specified conditions without its steady-state temperature exceeding a specified value (Tmax).

the maximum temperatures allowed are: Tmax=70 °C for cables with PVC insulation Tmax=90 °C for cables with EPR insulation

the Current carrying capacity IZ is obtained by using the formula (installation not buried in the Ground):

IZ=IZ0 * K1 * K2where:

IZ0 is the current carrying capacity of the single conductor at 30 °C reference ambient temperaturek1 is the correction factor if the ambient temperature is other than 30 °Ck2 is the correction factor for cables installed bunched or in layers or for cables installed in a layer on several supports.

Dimensioning of electrical conduitsCurrent carrying capacity (Iz)


Table 1 - Current carrying capacity (IZ0) at Tamb=30 °C

Dimensioning of electrical conduitsNot buried in the ground

• The table1 provides Izo for installation method indicated in the picture.

• There are a lot of other tables that provide Izo, depending on the specific installation method

For more information:http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/ae2141fea4bfa9d748257a700024a579/$file/1SDC010002D0206.pdf


Dimensioning of electrical conduitsNot buried in the ground

Correction factor K1 for air temperature other than 30 °C



Dimensioning of electrical conduitsCurrent carrying capacity: the factor K2



The current carrying capacity Iz of a cable buried

in the ground, is calculated by using this formula:

IZ=IZ0 * K1 * K2 * K3where:

IZ0 is the current carrying capacity of the single conductor for installation in the ground at 20°C reference temperature

K1 is the correction factor if the temperature of the ground is other than 20°C;

K2 is the correction factor for adjacent cables

K3 = is the correction factor if the soil thermal resistivity is different from the reference value 2.5 K·m/W

Dimensioning of electrical conduitsInstallation buried in the ground



Table 1 - Current carrying capacity (IZ0) at Tground=20 °C




Correction factors k1




Correction factors k2



PV Plant protection against overcurrents on DC side


Protection against overcurrents on DC sideFault on string cable

Stringboxes

• Hypothesis: Short Circuit in A• from

• B to A flows an electric current equal: (m-1)*Isc

• C to B flows an electric current equal: (m-n)*Isc

• These currents could damage the cables

• ALL STRINGS supply their short circuit current to the A point.

• Single inverter: the parallel of the strings can be indifferently made in String Boxes or in the Inverter switchboard.


Stringboxes

• Hypothesis: Short Circuit in A• from

• B to A flows an electric current equal: n*Isc

• C to A flows an electric current equal: (m-n)*Isc

• The current from C to A could damage the cable.

• The current from B to A could NOT damage the cable.

• ALL STRINGS supply their short circuit current to the A point.

• Single inverter: the parallel of the strings can be indifferently made in String Boxes or in the Inverter switchboard.

Protection against overcurrents on DC sideFault on the cable that connects the StringBox to the inverter


• SC in A is sensed by string/inverter fuses

• SC in B are NOT sensed by string/inverter fuses. However, this event has low-probability to occur because it would occur inside the switchboard

Stringboxes

Protection against overcurrents on DC sideUse of fuses

Inverter switchboard

Fuse for cable string protection

Fuse for protection the cable that connects String box with the inverter.


Protection Devices against overcurrenton DC side


On the DC side of PV-Plants we can found several protecion devices:

fuses

circuit breakers

block diodes

……..

Protection devices against overcurrent on DC side


In Inf IfConventional

Time

In ≤ 63 A 1.25 In 1.6 In 1h

63 A < In ≤ 160 A 1.25 In 1.6 In 2h

160 A < In ≤ 400 A 1.25 In 1.6 In 3h

400 A < In 1.25 In 1.6 In 4h

for a fuse, we define following parameters:

In: is the rated current

If: is the current ensuring effective operation in the conventional time of the fuse (1h/2h/3h/4h).

Inf: is the current ensuring NO effective operation in the conventional time of the fuse

1h

If=3.2 AIn=2A

The fuses



The fuses dissipate power

The fuses



Also for a circuit breakers, we define following parameters:

In: is the rated current

If: is the current ensuring effective operation of the circuit breaker in the conventional time (1h/2h)

Inf: is the current ensuring NO effective operation of the fuse in the conventional timeCircuit breakers

Inf If Conventional time

adjustable protective

devices(CEI 17-5)

No adjustable protective

devices(CEI 23-3/1)

- In ≤ 63A 1.13 In 1.45 In 1h

- In > 63A 1.13 In 1.45 In 2h

Ir ≤ 63A - 1.05 Ir 1.3 Ir 1h

Ir > 63A - 1.05 Ir 1.3 Ir 2h

Circuit breakers



Dissipate Power per pole (W) 1.5…6.4 1.8 – 7.2

Also the circuit breakers dissipate power

the manufacturers provide PD,pole

Circuit breakers



Thermal sizing of DC switchboardsDevice derating

An example of circuit breaker derating


Block diodes are traditionally used in Stand Alone PV Plants

In Stand Alone PV-Plants, during the sunset, without block diodes, the current would flow into the PV-module instead of into the loads because of the presence of batteries

Diodes block are installed in series of strings

Sometimes, in grid-connected PV Plants, diodes block are installed because, in presence of shadowing phenomena's on PV array, one or more PV string could absorb current from the others PV string.

Block Diodes



Shadowing phenomenon on one string

V

I

Stringa 1000 W/m2

Stringa 200 W/m2

Parallelo delle stringhe

U

S

T

IU

IT

IT = absorbed current by the shadowed string in OPEN CIRCUIT conditions (I=0)

~=

Why Diodes block are used

NO SHADOWED string (1000 w/m2)

SHADOWED string (200 w/m2)

PARALLEL of two strings



Protection of electrical feeders


If in an DC cables can flow current greater than their current

carrying capacity (IZ ), the cable must be protected by an

proper protective device.

In order to protect the cable, must be satisfied the following formulas:

where:IB : is the current for which the circuit is designed (for a string cable: IB = 1.25*Isc @STC)

IN : is the rated current of the protective device; for adjustable protective devices, IN is the set current

If :is the current ensuring effective operation in the conventional time of the protective device

IZ : is the current carrying capacity

ZfZNB *I ; I I I I 45.1

Protection of electrical feeders


Choice of fuses

if we choose Fuses for protection of electrical feeders, they are “gG type” (that are fuses used for cable protection). Moreover:

The fuses must be compliant with DC installations

must be satisfied the formula:

Vn > 1.2* Voc

Where:

Vn : is the rated voltage of the fuse

Voc : is the PV-array voltage in open circuit condition 1.2 : is a security factor

Protection of electrical feedersHow-to choice the protective devices on DC side


• If shadowing phenomenon on the PV array can occur, must be evaluate the possibility of installing block diodes. If so, the diodes must:• Preferably be low drop voltage• Have max reverse voltage greater than 2*Voc,@STC • Have rated current (Id) greater than 1.25*Isc@STC.

• Be careful: block diode dissipate power (PD = VD*ID), consequently heat sink are required.


Choice of Block Diodes


• Be careful: in order to reduce loss power, choose the low-voltage version



Thank you for attention…..

Documents

Dimensioning and Protection of PV Plants with examples of design Session 2 Dr. Francesco De Lia - ENEA C.R. ENEA Casaccia