AS/NZS 5033:2021 Array current calculations for SMA inverters

Summary

On the 20th of May, AS/NZS 5033:2021 became mandatory. It included new formulas for calculating the maximum current expected from a PV Array. An inverter must be able to accept this current through its MPPT DC input terminals so it must be considered when selecting a suitable PV module to connect to an inverter MPPT DC input.

Calculations

The aim is to calculate the maximum array current according to AS/NZS 5033:2021 and compare it to the inverter I_SC MPPT rating to confirm the PV array design meets the standards.

Disclaimer

The information contained herein is subject to change. New information will be published as soon as it becomes available.

Last Updated: 05/31/2021

Step 1.

Calculate the maximum string current

It is typical for arrays connected to SMA inverters to be installed without DCU (DC conditioners/optimisers).
AS/NZS 5033:2021 3.3.3 (a) describes how to calculate I_{STRING MAX}where no DCU are installed.

I_{STRING MAX} = 1.25 x K_I x I_{SC MOD}

Generally, K_I has a value of 1 except where bifacial modules are used.
I_{SC MOD} is the STC short circuit current of the PV module in Amps.
It will be available from the PV module datasheet.

Step 2.

Check backfeed current of inverter MPPT DC input.

I_STRING= I_{STRING MAX} or I_{BF TOTAL} (whichever is greater)

I_{BF TOTAL} is the backfeed current from sources other than the PV Array. It can be ignored with the SMA inverters listed in Table 1. These inverters produce zero backfeed current so I_{BF TOTAL} will always be less than I_{STRING MAX}. Backfeed current is termed ‘maximum reverse current’ in SMA inverter manuals.

Step 3.

Calculate the maximum array current

I_ARRAY= S_A x I_{STRING MAX}

I_ARRAY is the maximum array current in Amps.
S_A is the number of strings in the array.
I_{STRING MAX} is the maximum current per string in Amps.

Step 4.

Confirm the maximum array current is less than the inverter MPPT DC input I_SC.

Example 1

Sunny Boy 5kW inverter (SB5.0-1AV-41) to be installed with 16 PV Modules.
Inverter MPPT DC input A has 8 series connected 425W modules, inverter MPPT DC input B has 8 series connected 425W modules.
Modules have an Isc of 11.32A.

I_{STRING MAX} = 1.25 x K_I x I_{SC MOD}

I_{STRING MAX} = 1.25 x 1 x 11.32A=14.15A

I_ARRAY= S_A x I_{STRING MAX}

I_ARRAY= 1x14.15A as there is only one string in the PV array.

If we look at the datasheet for the inverter the maximum short circuit current is 20A.

This module is therefore suitable for the inverter MPPT inputs A and B as I_ARRAY <20A.

Example 2

Sunny Tripower 50kW inverter (STP50-41) to be installed with 18, 425W PV Modules on each MPPT DC input.
Modules have an I_sc of 11.32A.
We will consider one MPPT.

If the 18 modules are connected in one string:

I_{STRING MAX} = 1.25 x K_I x I_{SC MOD}

I_{STRING MAX} = 1.25 x 1 x 11.32A = 14.15A

I_STRING = IS_{TRING MAX} = I_ARRAY14.15A as there is only one string in the PV array.

If we look at the datasheet for the inverter the maximum short circuit current is 30A.

This module arrangement is therefore suitable for the inverter MPPT DC input as I_ARRAY <30A.

If the 18 modules are connected in two strings of nine modules:

Modules have an I_sc of 11.32A

I_{STRING MAX} = 1.25 x K_I x I_{SC MOD}

I_{STRING MAX} = 1.25 x 1 x 11.32A = 14.15A

I_ARRAY= S_A x I_{STRING MAX}

I_ARRAY = 2 x 14.15A = 28.30A as there are two strings in the PV array.

This module arrangement is therefore suitable for the inverter MPPT DC input as I_ARRAY <30A.

Example 3

Sunny Boy Smart Energy inverters (SBSE5.0-50) to be installed with 425W PV non-Bifacial Modules on each MPPT DC input. Modules have an Isc of 10A.

1 string to MPPT

I_{STRING MAX}= 1.25 x K_I x I_{SC MOD}

I_{STRING MAX}=1.25 x 1 x 10A = 12.5A

I_STRING = I_{STRING MAX} = I_ARRAY 12.5A

2 strings in parallel to MPPT

I_{STRING MAX}= 1.25 x K_I x I_{SC MOD}

I_{STRING MAX}=1.25 x 1 x (10A x 2) = 25A

I_STRING = I_{STRING MAX} = I_ARRAY 25A

a. We will consider 3 MPPT. 1 string per MPPT

From calculation above,

MPPT	PV Module Isc Max	Condition	Result
A (1 string)	12.5A	Isc per MPPT <30A	Yes
B (1 string)	12.5A	Isc per MPPT <30A	Yes
C (1 string)	12.5A	Isc per MPPT <30A	Yes
Total	37.5A	Total Isc <60A	Yes

This module arrangement is therefore suitable for the inverter.

b. We will consider 3 MPPT. 2 strings parallel to MPPT A, 1 string to each MPPT B & C

From calculation above,

MPPT	PV Module Isc Max	Condition	Result
A (2 strings parallel)	25A	Isc per MPPT <30A	Yes
B (1 string)	12.5A	Isc per MPPT <30A	Yes
C (1 string)	12.5A	Isc per MPPT <30A	Yes
Total	50A	Total Isc <60A	Yes

This module arrangement is therefore suitable for the inverter.

c. We will consider 3 MPPT. 2 strings parallel to each MPPT A & B, 1 string to MPPT C

From calculation above,

MPPT	PV Module Isc Max	Condition	Result
A (2 strings parallel)	25A	Isc per MPPT <30A	Yes
B (2 strings parallel)	25A	Isc per MPPT <30A	Yes
C (1 string)	12.5A	Isc per MPPT <30A	Yes
Total	62.5A	Total Isc <60A	No

This module arrangement is therefore NOT suitable for the inverter.

PV Only Model	MPPT Input (labels on inverter)	Max usable current per MPPT (Amps)	Inv Max I_SC per MPPT (Amps)	PV Module Isc Max ¹ 1 string (Amps)	PV Module Isc Max ¹ 2 strings (Amps)
SB1.5-1VL-40	A	10	18	14.4	7.2
SB2.5-1VL-40	A	10	18	14.4	7.2
SB3.0-1AV-41	A,B	15	20	16	8
SB4.0-1AV-41	A,B	15	20	16	8
SB5.0-1AV-41	A,B	15	20	16	8
SB6.0-1AV-41	A,B	15	20	16	8
STP3.0-3AV-40	A,B	12	18	14.4	7.2
STP4.0-3AV-40	A,B	12	18	14.4	7.2
STP5.0-3AV-40	A,B	12	18	14.4	7.2
STP6.0-3AV-40	A,B	12	18	14.4	7.2
STP8.0-3AV-40	A	20	30	24	12
STP8.0-3AV-40	B	12	18	14.4	7.2
STP10.0-3AV-40	A	20	30	24	12
	B	12	18	14.4	7.2
STP12-50	A,B,C	24	37.5	30	15
STP15-50	A,B,C	24	37.5	30	15
STP20-50	A,B,C	24	37.5	30	15
STP25-50	A,B,C	24	37.5	30	15
STP 50-41	A-F	20	30	24	12
STP 110-60	1-12	26	40	32	16

Table 1

Hybrid Model	MPPT Input (labels on inverter)	Max Usable input current per MPPT (Amps)	Inv Max I_SC per MPPT (Amps)	PV Module Isc Max ¹ 1 string per MPPT (Amps)
SBSE3.6-50	A,B,C	15	30*	See Example 3
SBSE4.0-50	A,B,C	15	30*	See Example 3
SBSE5.0-50	A,B,C	15	30*	See Example 3
SBSE6.0-50	A,B,C	15	30*	See Example 3

Table 2

*This is allowed if short circuit current on A+B+C ≤60A

Notes

¹I_BFTOTAL is equivalent to Maximum Reverse Current described in SMA product manuals.
K_I is 1 for this example table but may change with bifacial modules.
‘No DCU’ calculations used in example.