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TOPIC-05 · Engineering guide

PV String Voltage at Entered Temperatures

Cold string Voc is module Voc times the linear temperature factor times the series count. The teaching inputs produce 431.7058 V at −8 °C, 342.583 V at 70 °C, 325 V at the reference Vmp, 20 modules and 8000 Wdc.

Formula, variables and conditions

coldFactor = 1 + coefficient × (Tmin − Treference)
warmFactor = 1 + coefficient × (Tmax − Treference)
cold string Voc = module Voc × coldFactor × modulesInSeries
warm string Voc = module Voc × warmFactor × modulesInSeries
reference string Vmp = module Vmp × modulesInSeries
module count = modulesInSeries × stringsInParallel
nameplate Wdc = module count × module power

Variables and units

  • Module Voc, Vmp and power: entered reference-condition values. Vmp is not temperature-adjusted in this estimate.
  • Coefficient: signed relative change per °C, between −0.02 and 0.02.
  • Treference, Tmin and Tmax: module or cell temperatures in °C, from −100 to 150, with Tmin not above Tmax. They are not ambient-air temperatures.
  • Series and parallel counts: positive integers.
  • Inverter DC maximum and MPPT window: entered limits used only for an arithmetic comparison.

Conditions

  • The default call does not add the optional GB 50797 series check. That check runs only when a caller supplies it, and the lower bound stays not-evaluated until Kv′ is entered. This example omits that argument.
  • Sources SRC-028, SRC-029 and SRC-030 are stored as review-required. They are not upgraded here.
  • Parallel strings change module count and nameplate power only. Current and per-MPPT limits are not evaluated.

Worked example

Teaching inputs from the BACKLOG-015 unit test. Coefficient −0.0029 /°C, reference temperature 25 °C.

Module
Voc 39.4 V, Vmp 32.5 V, 400 W
Temperatures
minimum −8 °C, maximum 70 °C, reference 25 °C
Array
10 in series, 2 strings
Cold / warm Voc
431.7058 V / 342.583 V
Reference Vmp / nameplate
325 V / 8000 Wdc
Entered inverter limits
600 V DC maximum, MPPT 250 V to 500 V

Cold Voc = 39.4 × (1 + (−0.0029) × (−8 − 25)) × 10 = 431.7058 V. Warm Voc uses 70 °C and is 342.583 V. Both the cold-Voc comparison with 600 V and the 325 V comparison with the MPPT window pass as arithmetic against the entered limits. Equal MPPT endpoints are rejected.

Boundaries and common mistakes

Boundaries

  • Not PV system design approval or a complete inverter match.
  • Irradiance, manufacturer tolerance, fuse and current limits are outside this estimate.
  • A pass against a typed voltage limit is not a code or warranty determination.

Common mistakes

  • Adjusting Vmp with the Voc coefficient. This model leaves Vmp at the reference value.
  • Using ambient air temperature in place of the module temperature the formula expects.
  • Treating the optional GB 50797 check as part of this fixture. It was not run.

Sources