PV DC SPD selection

How to Choose PV DC SPD Voltage: Ucpv, Voc & 1000V vs 1500V

A quotation that says “1000V DC SPD, 40kA” leaves an important question unanswered: what maximum continuous PV voltage does the offered assembly support? The number in the product name may describe an application range, a nominal rating or Ucpv. Those meanings are not interchangeable.

Correct PV DC SPD voltage selection starts with the maximum open-circuit voltage of the PV string under the project's cold conditions. It then checks the SPD's declared Ucpv, protection modes and voltage protection level against the actual installation.

Figure 1. Voltage selection starts with the PV string and equipment limits, then checks the complete SPD specification.

1. Voc, Ucpv and Up: Which Voltage Are You Comparing?

Some catalogues use Uc for a PV product's continuous-voltage rating. Ask which PV rating and protection mode that field represents. Do not assume that a model suffix or a nominal voltage marked Un is the complete assembly's Ucpv.

Ucpv is also not a clamping threshold. A device with Ucpv of 1500V does not promise to limit every surge to 1500V. Continuous-voltage capability and impulse protection level answer different questions. See the parameter discussion in our IEC 61643-31 guide for PV DC SPDs.

2. Calculate Cold-Weather String Voc Before Selecting the SPD

PV open-circuit voltage rises as cell temperature falls. The inverter's operating display and the module's maximum-power voltage, Vmp, therefore do not establish the highest voltage the DC circuit can experience. Use the minimum design temperature specified by the applicable module instructions and project design method. SMA's PV generator planning guide, section 3.2, explains this temperature dependence.

For identical modules in series, a common linear calculation is:

• N: number of modules connected in series.
• Voc,STC: one module's open-circuit voltage in volts.
• βVoc: the signed Voc temperature coefficient in %/°C, typically negative.
• Tmin: the design minimum temperature in °C used by the applicable calculation method.

For a coefficient of −0.28%/°C, βVoc / 100 is −0.0028 per °C. Do not insert −0.28 as a decimal coefficient, and do not substitute the power temperature coefficient. If the manufacturer supplies V/°C instead, use the corresponding voltage-based formula. The relationship above follows the series-voltage method illustrated in the JinkoSolar installation manual.

Worked example: 900V at STC becomes 1013.4V in the cold

Assume 18 modules per string, each with Voc,STC = 50.0V, βVoc = −0.28%/°C, and a design minimum temperature of −20°C.

1. String Voc at STC: 18 × 50.0 = 900V.
2. Temperature difference: −20 − 25 = −45°C.
3. Correction: 1 + (−0.0028 × −45) = 1.126.
4. Cold-weather string Voc: 900 × 1.126 = 1013.4V.

A complete SPD assembly with a declared Ucpv of 1000V for the relevant path fails this voltage screen. But moving to a higher-Ucpv SPD does not resolve the whole design: if the inverter's maximum DC input is 1000V, the string already exceeds that limit.

The string design must change, or all affected equipment must be suitable for the revised voltage. SMA's PV input requirements explicitly require the coldest-day array open-circuit voltage to remain within the inverter's maximum input voltage.

These are illustrative values, not specifications for a particular module. Before final selection, include the applicable Voc tolerance, environmental assumptions and design factors. Preserve unrounded values for limit checks: 1003V cannot be rounded down into a 1000V rating.

Figure 2. With the example module data, cold-weather string Voc exceeds 1000V even though string Voc at STC is only 900V.

3. 1000V vs 1500V PV DC SPD: A Practical Comparison

The following comparison uses the same hypothetical 50.0V module, −0.28%/°C coefficient and −20°C temperature. Here, “1000V Ucpv” and “1500V Ucpv” mean the actual declared continuous-voltage rating of the relevant assembly and protection path, not a catalogue name.

“Candidate” does not mean approved. A manufacturer's permitted array Voc at STC, project factors, protection modes or equipment coordination may impose a stricter condition. Do not select a 1500V device merely because it passes a voltage inequality: review its Up and application instructions as well.

Figure 3. The voltage screen uses declared Ucpv, not a catalogue name. Passing this screen alone does not approve an SPD.

4. Should You Always Multiply PV Voltage by 1.2?

First identify the voltage to which the factor applies. STC string Voc, cold-corrected Voc, nominal system voltage and inverter maximum input voltage are different quantities.

For example, Schneider Electric's CL36 solutions guide, page 64, gives a selection relationship of Ucpv ≥ 1.2 × Uoc,STC for the products discussed there. That is a documented application rule using STC array voltage; it is not evidence that every “1500V system” universally requires an 1800V SPD.

For the 24-module example, 1.2 × 1200V = 1440V, while the temperature calculation gives 1351.2V. Where both checks apply, satisfy both. If another project's cold calculation exceeds the factor-based value, the smaller result cannot erase the higher voltage.

Do not add or remove a factor from an already corrected voltage without checking what the applicable instructions require. Record the method, its starting voltage and the corrections already included. This prevents both double-counting and omitting a required allowance.

5. Verify Protection Modes, Then Check Up

A PV SPD may protect DC+ to DC−, DC+ to PE, and DC− to PE. Read the declared ratings for the actual assembly and connection. Do not halve the string voltage to select a conductor-to-earth rating simply because the system has positive and negative conductors. Earthing arrangement and fault conditions affect the voltage stress on those paths.

Likewise, do not add the voltage ratings printed on separate cartridges unless the manufacturer declares that exact combination and wiring as a suitable assembly. Pole count alone does not establish its permissible PV voltage. Manufacturer data can explicitly distinguish the paths, as shown in DEHN's PV assembly ratings.

Ucpv establishes continuous-voltage suitability; Up helps assess surge protection. Compare Up at the stated protection mode and test conditions with the equipment's impulse withstand, allowing for the installation's connection effects and required coordination margin. Phoenix Contact's surge protection fundamentals describes this distinction.

A higher continuous-voltage version can have a higher Up. Read the actual values; neither a higher voltage label nor a lower Up alone makes a device suitable. Keep the intended installation point in view using our solar PV surge protection application overview.

Voltage is only one part of the specification. Confirm the required SPD type, In/Imax and Iimp where applicable, Iscpv, disconnection and backup-protection conditions. IEC 61643-31 addresses PV SPD product requirements and testing; IEC 61643-32 addresses selection and application. A voltage calculation does not decide whether Type 2 or Type 1+2 is required.

6. How to Read CRESIN's PV DC SPD Voltage Options

The published CSMS-B40 PV DC product table separates nominal voltage Un from maximum continuous operating voltage Uc. Two relevant columns are:

These are product-specific published values, not a universal conversion rule. Before ordering, obtain the exact variant's datasheet, declared PV Ucpv and protection modes, connection diagram and relevant test evidence. Confirm how the page's Uc relates to the offered PV assembly. The public table is a starting point for comparison, not sufficient evidence to approve a particular system.

Start with the PV DC SPD range, and identify the PV version explicitly in the RFQ. A shared series name does not establish that an AC or general DC variant has the same PV application suitability.

7. What to Send Before Requesting a PV DC SPD Quotation

“Please quote a 1500V SPD” asks the supplier to guess several design inputs. A useful enquiry identifies the voltage calculation and the installation the device must serve.

Request the returned quotation to identify the complete model, Ucpv by protection mode, Up with its test basis, Iscpv and installation conditions. Then compare offers using the same fields; our SPD quotation comparison guide covers the broader purchasing review.

8. Frequently Asked Questions

Can I use a 1000V SPD on a string that measures 900V?

One measurement is insufficient. Establish maximum string Voc under the design conditions, then check the actual Ucpv and application limits. In the example above, a string rated 900V at STC reaches 1013.4V at −20°C.

Can a 1500V PV SPD be used in a 1000V system?

It may be suitable if its declared ratings, protection modes, Up and installation instructions meet the project requirements. The higher label alone does not establish better protection or justify a substitution.

Does a 1500V system always need Ucpv of 1800V?

No. Determine what the system label represents and calculate the required voltage. Apply the selected manufacturer's rules and project requirements. A specific catalogue's Un/Uc pairing is not a rule for every PV SPD.

Does adding parallel strings increase the required Ucpv?

For otherwise identical strings connected to the same DC bus, parallel connection does not add their voltages. It changes available current, so review Iscpv and backup protection. Adding modules in series increases string voltage.

Does a 40kA rating compensate for a low Ucpv?

No. Impulse-current capability does not increase the permitted continuous PV voltage. The device must meet both requirements, together with protection-level and installation constraints.