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101 renewable - fuses selection in photovoltaic systems

Article Details

Last Updated
29th of September, 2018

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Source of  Information:~

This white paper  is referencing  the dimensioning of the PV overcurrent protection through PV fuses and it is mainly a technical interpretation of the SIBA 's "Just four steps to getting the PV Fuse that suits your needs"  by Heinz-Ulrich Haas, R & D SIBA GmbH & Co. KG. Additional to that we will make direct references to  "Theoretical Considerations of the Use of Fuses in Photovoltaic Systems"  by Norbert Henze,  Peter Funtan  from Fraunhofer Institute for Wind Energy and Energy System Technology IWES. These two articles have been widely publicized and made available without any royalties attached.  The white paper we present here has a larger application as PV over current protection on the DC side of the installation, which boils down to DC combiner fuse protection, DC disconnect fuse protection, internal Inverter fuse protection.

General Considerations:~

To determine the proper  fuse for a PV installation on the DC side (PV generator side) we need information beyond IN (Nominal Current)  of the solar panels of the string  we are looking to protect against over currents.  In general, for relative small PV generators (small arrays) the installers are looking for (fuse) rated values of 12A, 16A respective 20A . They are very general values and they refer to the solar panel fuses  not to the combiner box fuses or  DC disconnect fuses. Combiner and DC disconnect fuses are the most important because they do protect the installation on the inverter side and they must  totally disconnect the DC power generation from the rest of installation. This fuses must take in consideration other current values like the inverter feedback current in case of the ground fault and the reverse current. The reverse current it is solar cells generated current based on the difference of temperature and irradiation values along large strings with different environment exposures like different levels of irradiation or temperature, inclusive snow deposit  on the solar panels and technical miss happen.

Before selecting the proper fuse we have to give importance to following constraints (write them down as reference):

Irated < „maximum series fuse rating”

The current value for which the selected fuse is rated must be less as "maximum series fuse rating"  for the strings' solar panel.

Irated > 1,5 *Isc (Module Panel short circuit current).

The size of the rated current for the fuse must follow the Isc  (short circuit current) value recommended by the manufacturer for the specific solar panel.  We consider STC (standard test conditions) for the irradiation and that comes down to an irradiance of 1000 W/m2, a spectral distribution AM (air mass) of 1.5 and a temperature of 25 °C.

We consider during this calculation Spain as being our reference location.  Spain has the biggest open PV plant installed in the word,  or by the level of 2008 that counts to approx. 95% of the installed PV power of 3.317 GW set up in form of open field installations. Considering our plant located in Spain it will allow us to implement all the conditions we may encounter for reverse currents. Differing irradiation intensities, large temperature differences, snow coverage.  Beside the geo-specific characteristics we have technical mishaps or reverse current characteristic dependent on technology : Case of failure: Defect bypass diodes; Case of failure: Earth fault at panel level; Case of failure: Installation error at one of the panel level; The reverse current will mark down the MPPT current value for different temperature coefficients at panel level. In our case, we consider those increase with a ratio of 10C.

reverse current curve

The first evaluation of manufacturer’s data regarding the information on the reverse current capacity has shown that the double short circuit current under  STC conditions can assume to be the typical reverse current load (green line in the graph) at fault at the panel level. That will correspond to S3 on our graph.  The reverse current will modify the string output voltage following the IV curve. Here it can be a problem for the string to carry the reverse current if the string output voltage falls under a certain value.  It can be assumed that a string which is voltage-reduced by approx. 10% is just in a position to durable carry the resulting reverse current. That it means at panel level we will be able to carry the fault without the string to be disconnected by the overcurrent protection.

We conclude the reverse current value can reach following value at STC conditions and at panel level: 

I reverse = 2 x Isc; open circuit @ STC conditions or I reverse = 1.5 x Isc; panel connected to the string @ STC conditions.

Now we are ready to start selecting the fuse value for our application. But before to proceed we  have to collect  the information we need for such a scope:

Table 1: Data of the used PV Module~

Voltage at PMAX U MPP MOD = 29,2 V
Open circuit voltage U OC MOD2 = 36,4 V
MPP current I MPP = 7,9 A
Short circuit current I SC MOD = 8,7 A
Temp. coeff. of UOC 0,36 %/°C
Temp. coeff. of ISC 0,065 %/°C
Max. cell temp. 70 °C

Table 2: Installation related data~

Number of strings N = 8
Number of modules/string M = 22
Array voltage U MPP MOD x M U ARRAY = 642.4 V
String open circuit voltage
U OC ARRAY  = U OC MOD x M or 36.4V x 22 = 800 V
U OC ARRAY = 800 V
Temperature in junction box 60 °C
Lowest ambient air temperature – 25 °C
Irradiance 1200 W/m² (Spain)

Step 1 Determination of the Fuse Rated Voltage - UN:~

We start considering the string open circuit voltage and based on this value we look for the closest value of a fuse rated voltage or Urated. This is the voltage we took as a basis for future determinations.  For the fuse rated value, we had found we must  consider the Up or the fuse test value. As a thumb rule for most of the PV fuses  the Up = 1.1 Urated.

Up  = 1.1 Urated

On the fuse characteristic curve, we will find Upmin as a value plotted by Up previous determined and the lowest ambient air temperature – 25 °C from our installation-related data.

Upmin =  f(Up, – 25 °C)

Temperature correction factor  Δ ϑ  it is the value affecting our string open circuit voltage and is the temperature difference between the lowest ambient temperature and STC temperature value.

Δ ϑ = ϑstc  - ϑmin = 50°C

Now we have to correct the string open circuit value against the temp correction factor and  make sure this value it is verified against the Upmin.

UP MIN ≥ U OC ARRAY x (1+ (Δ ϑ x temp. coeff. of UOC ARRAY))

That in values translates to:

UP MIN ≥ 800 V x (1+ (50 x 0,0036)) = 945 V

We come down to a fuse selected on the new determinate set  values of 

Uvoc = 945V and I MMP= 7.9A.

We choose SIBA URZ type of fuse with main characteristics:

UP = 1000 VDC ;  UN = 900 VDC and the dimension of 10 x 38 mm

Step 2 Determination of the Fuse Rated Current - IN

Fuses rated current must resemble some of the STC  I MPP value for the solar panel we are working with.  We learned that we start looking for rated values from the STC point of view and as next step, we alter this values to match the real conditions where the fuses are installed. That process is called derating the STC values. That is what we will do next  in order to come to a real condition  value for fuse IN value. What are the derating factors ? Ambient air temperature; Fuses operation under alternating  load; Number of fuses placed  together;  This are the derating factors we look at.

Ambient air temperature of 60 °C KTH= 0,84

The ambient temp factor correction it is indicated on the selected  fuse derating temp curve for the ambient temperature value. For 60C (max temperature in the combiner  box from installation-related data ) is 0.84; KTH= 0,84;

Ambient Temperature

Alternating load factor for full range fuse (PV fuse):~

A2 = 0,9 The alternating load factor for full range fuses in PV applications is 0.9. As the fuse holders are placed in groups  of three fuses each that is the considered value Derating by high numbers of closed fuse-holders KZS = 1 (because of groups of three fuses each)KZS = 1; because of groups of three fuses each to another.

On the basis of the IMMP  current rating value which is  the STC value and the derating factors, we have considered we can calculate the lowest fuse rated current IN MIN which is the minimum nominal value for the fuse we have to consider in order to sustain the.

IN MIN = I MPP / KTH / A2 / KZS which comes to IN MIN = 7,9 A / 0,84 / 0,9 / 1 = 10,5 A

From the range of possible current ratings of the fuse type, we will choose the next higher rated current to 10.5 A  and that will be the value of 12A.

IN = 12 A (as the 1st iteration step).

Step 3 est and Iteration Steps determining the final fuse IN minimum rated value.

We do make clear we dimension this fuses for the combiner box or for the DC disconnect box. We started with the IMPP of the solar module and we had to derate the value to get the minimum IN capable of breaking in case of a fault initiated  before the solar panels and keep the solar panels fuses intact.  This is a very general situation. We need more derating  to obtain  final fuse IN value capable of handling the solar module short current, respective the string short current value amended by the temperature increase in the system ( under conditions deviating by Δ ϑ = 45 °C from the STC ). That it means the 12A minimum rating value for the fuse determined at Step2 will generate an IN RED value which becomes the new field working value for the string. This new value must be checked against the string short current derating value  at the field conditions.

IN RED = IN x KTH x A2 x KZS = 12 A x 0,84 x 0,9 x 1 = 9,1 A

This is the derating value related to the IN value selected at Step 2. Basic is a correction based on temperature and the way we group the fuses together  inside the combiner or disconnect box.  The correction  was necessary because the final value chooses at Step 2 was an STC value, not a real field value ( we talk about 12A selected from fuse rating values at STC conditions).

The next at step3 is to correct the string ISC value (short circuit value). We will consider  the 70 °C from Table 1.

Calculation of I SC at 70 °C  thus under conditions deviating by Δ
ϑ = 45 °C from the STC is:
I SC = I SC MOD x (1+ (Δ ϑ x temp. coeff. of ISC STRING))
I SC = I SC MOD x (1+ (45 x 0,00065))
Which brings us to a corrected value:  I SC = 9 A

Allowance for max. irradiance to be assumed  is  I SC  at  1200 W/m² or 1.2 correction factor for the level of irradiance  1200 W/m² from the Table 2.

I SC = 9 x 1,2 = 10,8 A

Next step will be to check the IN RED which is the "IN working value" or "IN field value" against the corrected string short circuit value, I SC

Requirement: IN RED > ISC 
9,1 > 10,8 A

That tells us that the "IN working value" or "IN field value" does not fulfill the requirement and subsequently the IN value selected at step 2 must be iterated a notch up.  This process of iteration is actually the process of selection based on  Fuse IN rated current.

We start over going back to the end of Step 2 and modify IN upwards to 16A. We follow once again with all the calculation and verification for Step 3:

IN = 16 A
IN RED = IN x KTH x A2 x KZS = 16 A x 0,84 x 0,9 x 1 = 12,1 A
I SC‘ = I SC MOD x (1+ (Δ ϑ x temp. coeff. of ISC STRING))
I SC‘ = I SC MOD x (1+ (45 x 0,00065))
I SC‘ = 9 A
I SC' = 9 x 1,2 = 10,8 A
Requirement: IN RED > ISC'
12,1 A > 10,8 A
The requirements are fulfilled by "IN working value"  and IN = 16A for STC conditions is the value to consider in our selection.

A note to be made: this value is for a short on the string caused by a ground fault or panel fault mainly  when the DC disconnect it is in close position. We may have a different value in case we consider  reverse currents for the most severe condition,  DC open circuit on the DC generator side. We talk here about large arrays or strings. In this case, the requirement will be:

Requirement: IN RED > I reverse  = 2 x ISC' by open circuit

That, in turn, will move the iteration some notches upwards and the correction will be made in such the open circuit voltage at reverse current will come down to max 90%  from Voc of the string or array at STC conditions with  I reverse = 2 x Isc  by an open circuit.

Requirement: IN RED > I reverse  = 1.5 x ISC' by close circuit

Or  I reverse is 1.5  times ISC'  the string short circuit corrected on a close DC generator string circuit.  In this case, we look at a 20A rated current value. IN = 20 A. But we have to take in consideration  a  quick - acting fuse(fast melting fuse ) to protect the solar modules, under 1sec time in which the link will melt.  Such a fuse will react very fast at Inrush Current Peak values.

Step 4 Fuses‘ melting time

The fuse melting  time is basic  the acting  time for  a fuse link  and that must  be correlated  to the rest overcurrent protection elements in a circuit. That will bring the over current selectivity protection in question.

The inrush current in the application should be measured and used  to calculate the proper fuse I2t value. I2t is the amount of heat energy, in terms of current and time, required to melt the fuse link . Heat dissipated from fuses can affect other components in close proximity and vice versa. If a fuse dissipates more heat than the fuse holder can withstand, the fuse holder can degrade very much  melting  or burning.  Time-lag fuses generally have lower power dissipation values than quick-acting fuses because they have a thicker fuse wire diameter.

In rush current peak:  Ip is the max current which can close the DC generator circuit for  fraction of seconds (milliseconds).    Inrush current peak is to be considered when we connect  the installation either through DC disconnector array combiner  or any fault happening during the operation.  The fuses must withstand  the thermal energy dissipated at that time for some fractions of a second until the fault  is cleared. Opposite the fuse link will melt. The time the fuse link will melt has to be usually  under  1s .  Therefore we have to check the melting time for the fuse of our choice. The melting time it is determined from the Time-Current Characteristics of the chosen PV Fuse. Based on  RMS Prospective Current which is the I SC STRING or String short circuit residual current.  In our case, we follow the fuse curve identified by the fuse IN  value and we extract the pre arching time which is the melting  time t(s). The usual value once again must be under 1 sec or between  10 -2 to shy over  10 0 (s).

String short circuit residual current:~

I SC STRING‘‘ = I SC MOD x (N-1) I SC STRING‘‘ = 60,9 A at STC conditions.
For this value we extract  the melting time T(s) = 1.5s for IN = 16A at STC conditions.

The value  is under STC conditions. We have to amend this value to the field real-time conditions.  I SC STRING at 70 °C (thus under conditions deviating by Δ ϑ = 45 °C from the STC)

I SC STRING‘ = I SC STRING‘‘ x (1+ (Δ ϑ x temp. coeff. of I SC STRING))
I SC STRING‘ = I SC STRING‘‘ x (1+ (45 x 0,00065))
I SC STRING‘ = 62,7 A
Allowance for max. irradiance to be assumed: I SC STRING at 1200 W/m²;
I SC STRING = 62,7 x 1,2 = 75,2 A
Read off melting time t(S) of 16 A fuse at I SC STRING   follow  the red path on the graph.
t(s) = 0.4s  <   T(s) = 1.5s

Melting Time

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