PV String Sizing: Voc at Minimum Temperature

What is checked when sizing a string

A string is a group of modules connected in series. It is sized with three independent checks, each using a different value from the module data sheet and from the inverter. You can verify the result with our free PV system sizing tool. The checks are:

1. Maximum number of modules: the open-circuit voltage, at the lowest temperature, cannot exceed what the inverter, the module and the wiring allow.
2. Minimum number of modules: the maximum-power voltage, at the highest cell temperature, must remain inside the MPPT tracking window.
3. Current: the string current must fit the inverter input and determine whether overcurrent protection is needed.

The maximum: Voc at minimum temperature

IEC 62548 takes the open-circuit voltage of the array, corrected to the lowest expected operating temperature, as the maximum array voltage. NFPA 70 (NEC) Section 690 asks for the same: add up the Voc of the modules in series, corrected to the lowest expected ambient temperature, using the manufacturer’s coefficient when it is given. That voltage must be less than or equal to the rating of every component: the module (its maximum system voltage), the inverter input, the fuses, the disconnect switches and the cables.

The data sheet values are corrected with a linear relationship from the 25 °C of STC:

Voc(T) = Voc(STC) × [1 + c/100 × (T − 25 °C)]

where c is the temperature coefficient of Voc in %/°C, which is negative. With T below 25 °C, the factor is greater than one.

Example with an assumed module

Take an example module with assumed values that do not correspond to any product: Voc of 50.0 V at STC and a coefficient of -0.25 %/°C, in a 1500 V system.

The temperatures in the table come from the weather stations at El Dorado and Ernesto Cortissoz airports in Colombia: a high-altitude site and a coastal site. There are two values: the lowest monthly mean minimum from the 1991-2020 climate normals of IDEAM (Colombia’s meteorological institute), and the lowest daily value in the historical series of the NOAA GHCN-Daily database. That second figure is a single record from a series with gaps, not a statistic, so it is shown as a reference and not as a rule.

The difference between the two cities is one or two modules per string with the same equipment. On the high plateau it is the cold at dawn that lowers the maximum, in areas prone to low temperatures and frost from about 2400 m according to IDEAM; altitude does not change Voc by itself. The design temperature of your project is a decision that is documented in the design report. It must come from the weather station nearest to the site and from an explicit criterion, not from a generic table.

The minimum: the MPPT window in hot weather

At the other extreme, the cell runs well above ambient temperature. To estimate it, the nominal operating cell temperature (NOCT) relationship given in the data sheets is used: cell temperature is the ambient temperature plus the difference between NOCT and 20 °C, scaled by the irradiance over 800 W/m². The model is not exact, but it errs on the safe side for this calculation.

At that temperature, the maximum-power voltage is lower. If the data sheet does not give the Vmp coefficient, it is estimated from the Pmax coefficient with a conservative criterion. The sum of the module Vmp values must stay above the lower limit of the inverter’s MPPT window; otherwise the inverter loses tracking and output falls during the hours of most sun and heat.

String current and protection

The inverter declares a maximum input current per MPPT. The design current is calculated from the data sheet Isc with a safety factor: NFPA 70, Section 690.8(A)(1), uses 125 % of Isc, and IEC 62548 requires the conversion equipment to accept a current of the same order. If the inverter limits the current, the excess power is cut off; this is the effect known as clipping.

When several strings are connected in parallel, a fault in one of them can be fed by the current of the others. The IEC 62548 criterion compares the possible reverse current, which is (N - 1) times Isc, with the maximum series fuse rating on the module data sheet. If the reverse current exceeds it, each string needs protection; see gPV fuses and DC disconnect switches. The local code usually requires that reverse currents do not exceed what the modules withstand and that, if they do, they be limited with blocking diodes or overcurrent protection.

Voltage drop in the DC cables is checked separately. The architecture of the inverter that receives the strings is compared in central or string inverters in solar plants.

Common mistakes

• Calculating Voc at 25 °C with no temperature correction.
• Forgetting the lowest limit among inverter, module, fuses, disconnect switches and cable.
• Mixing modules or orientations on the same MPPT input. Good practice, and many local codes, call for not combining module models in an array, nor different orientations in a string tied to the same MPPT, except with DC-DC converters.
• Leaving the design temperature and its source undocumented.

What we do at Tiza Group

Sizing the DC field is the responsibility of your designer. When the project moves on to grid connection, our primary engineering team designs, remotely and with the information from your project, the power single-line diagram, the grounding and the lightning shielding of the medium- and high-voltage part. On site, in Colombia or elsewhere on request, we commission solar plants: transformer stations, collector network, substation and interconnection protection.

Related guides

• Central or string inverters in solar plants: architecture
• Huawei SUN2000 inverter and SmartLogger 3000

References

• IEC 62548-1:2023, Photovoltaic (PV) arrays, Part 1: Design requirements (Annex F, determination of maximum voltage and current; clause 6.5, overcurrent protection). The maximum voltage and reverse current criteria were verified in the text of the 2016 edition.
• NFPA 70-2017, Sections 690.7 and 690.8.
• IDEAM (Colombia), Standard climate normals 1991-2020; NOAA NCEI, GHCN-Daily.

Frequently asked questions