QDTB® TransformerQuality & Dependability, Trust Builder

All figures are configurator-generated feasibility-level estimates; drawings are schematic deliverables pending refinement by a licensed design institute, not construction documents.

500 kW Utility PV Plant: 910×550 Wp Modules + 2×250 kW String Inverters + 630 kVA Box Substation Deep-Read

Q: Where do 910 modules come from?

500 kWp ÷ 550 Wp = 909.1 → 910 modules; 34 strings × 27 modules/string (temp-corrected 6–27).

Q: What is the payback?

9 yr at $0.08/kWh PPA, 10.45% IRR, 178% 25-yr ROI, on 507,600 kWh/yr (P50).

Q: Why is an SVG needed?

PV fluctuation drives grid-point reactive/voltage limits; the ±1 Mvar SVG gives continuous dynamic reactive (PF ±0.95) + voltage support.

Q: How much energy per year?

507,600 kWh (P50), P90 481,570 kWh; estimated at 1150 full-load-hours × PR 0.8.

Q: What does $390,830 cover?

Modules $150,000 + inverters $63,705 + mounting $75,000 + box $29,500 + SVG $50,000 + HV switchgear $16,750 + RMU $5,875 (FOB).

Q: What is a string-level box substation?

Each string-inverter group steps up to a box substation (string-level box); it is the core asset distinguishing a utility base from distributed PV.

910×550 Wp · 2×250 kW string inverters · 630 kVA string-level box substation (10 kV) · ±1 Mvar SVG · 10 kV export

1 · TL;DR Conclusion

Conclusion: this 500 kW utility PV plant uses 910×550 Wp modules (500 kWp) + 2×250 kW string inverters → a 630 kVA string-level box substation (YB-630/10) → ±1 Mvar SVG → 10 kV export — configurator total ≈ $390,830 (FOB), EPC ≈ $315,850, 9 yr payback, 10.45% IRR, 178% 25-yr ROI (feasibility-level).

•910×550 Wp modules (500 kWp), 34 strings (6–27 modules/string), cold Voc 55.4 V / hot Vmp 36.6 V
•2×250 kW string inverters (MPPT 1500 V) in parallel; string-level box substation 630 kVA (YB-630/10)
•±1 Mvar SVG (PF ±0.95 continuous) + HV switchgear KYN28A-12 + RMU-SF6; 10 kV export
•Generation 507,600 kWh/yr (P50), P90 481,570 kWh, revenue $40,608/yr @ $0.08/kWh PPA
•Economics: $315,850 EPC, 9 yr payback, 10.45% IRR, 178% 25-yr ROI

2 · Solution Overview

A utility PV mega-base is a 100% generation-export case: modules feed string inverters (string-level, vs central inverters) that step up to a box substation, then export via SVG and HV switchgear. At 500 kW the engine selects 10 kV collection/export; at ≥2500 kVA (multi-MW) it steps to 35 kV collection — the standard utility-base topology.

910 × 550 Wp (500 kWp)

Modules

34 (6–27 modules/string)

Strings

2 × 250 kW string

Inverters

630 kVA (YB-630/10)

String-level box sub

±1 Mvar

SVG

10 kV

Export voltage

500 kW (100% renewable)

Installed capacity

$390,830 (FOB)

Equipment total

3 · Design Process & Rationale

1. Requirement identification

500 kW utility PV mega-base, 100% export; the explicit generationExport flag triggers generation-export mode.

2. Module selection

550 Wp mono PERC: 500 kWp ÷ 550 W = 910 modules.

3. String design

Temp-corrected string range 6–27 modules/string, taken at 27 → 34 strings; cold Voc 55.4 V, hot Vmp 36.6 V.

4. Inverter selection

String-level (vs central) 2×250 kW in parallel, MPPT 1500 V, 500 kW total.

5. String-level box substation

500 kW ÷ 0.8 PF = 625 kVA → 630 kVA tier (YB-630/10); the string-level box substation is the core asset of a utility base.

6. Reactive & export

±1 Mvar SVG (PF ±0.95) + HV switchgear KYN28A-12 + RMU-SF6; 500 kW exports at 10 kV.

7. Collector voltage upgrade

At step-up ≥2500 kVA (multi-MW) the engine auto-switches to 35 kV collection — the standard utility-base topology.

Pain Points → Solution → Evidence

Pain

Choosing central vs string inverters wrong causes MPPT mismatch loss — shading/faults drag down whole-site output.

Solution

String inverters (2×250 kW) give string-level MPPT; a single-string fault only affects that string.

Evidence

INV-250kW ×2, MPPT 1500 V, string-level box substation.

Pain

PV output fluctuation drives grid-point reactive/voltage limits out — risk of grid penalty or tripping.

Solution

±1 Mvar SVG continuous dynamic reactive (PF ±0.95) + grid-connection protection.

Evidence

BOM includes SVG ±1 Mvar; aligned with GB/T 19964.

Pain

Wrong step-up box tier overloads or over-invests.

Solution

500 kW ÷ 0.8 = 625 kVA → 630 kVA standard tier.

Evidence

630 kVA (YB-630/10) string-level box substation.

4 · Key Parameter Deep-Read

Parameter	Value	Basis	Impact & Boundary
Module count	910 × 550 Wp	500 kWp ÷ 550 W = 910 modules	500 kWp DC capacity Module wattage changes require recalculation
String design	34 strings · 27 modules/string	Temp-corrected 6–27 modules/string	Cold Voc 55.4 V / hot Vmp 36.6 V Constrained by inverter MPPT voltage window
Inverters	2 × 250 kW string	≤250 kW tier paralleled	MPPT 1500 V, string-level tracking Continue paralleling above 250 kW
String-level box substation	630 kVA (YB-630/10)	500 kW ÷ 0.8 PF = 625 → 630 kVA	10 kV step-up export ≥2500 kVA switches to 35 kV collection
SVG rating	±1 Mvar	25% of 500 kW = 125 kvar → 1 Mvar	PF ±0.95, voltage support ≥1 Mvar engine floor
Annual P50	507,600 kWh	500 kW × 1150 h × PR 0.8	$40,608/yr @ $0.08/kWh P90 = 481,570 kWh (CV 0.04)

5 · Drawing Deep-Read

Dwg 3/13Generation Plant Single-line Diagram — Modules → string inverters → string-level box substation → SVG → 10 kV export

Generation Plant Single-line Diagram · QDTB Transformer · QDTB-DL1-E-03 · Dwg 3 / 13Rev. A · Not to scale · 2026-10-01 · Designed ______ · Checked ______ · Approved ______

Why: The generation-export skeleton — the full string-inverter + box-substation + SVG export path.

Dwg 12/13PV protection configuration — DC fusing + AC breakers + step-up protection

PV protection configuration · QDTB Transformer · QDTB-DL1-E-12 · Dwg 12 / 13Rev. A · Not to scale · 2026-10-01 · Designed ______ · Checked ______ · Approved ______

Why: DC fusing and AC breaker staged protection — a single-string fault does not propagate.

Dwg 13/13PV calculation book — Array capacity / string design / P50-P90 generation

PV calculation book · QDTB Transformer · QDTB-DL1-E-13 · Dwg 13 / 13Rev. A · Not to scale · 2026-10-01 · Designed ______ · Checked ______ · Approved ______

CALCULATION BOOK

Solar plant · 500 kWp · iec · illustrative

1. PV array

Standard: IEC 62548

Parameter	Formula	Value
Array capacity	Ppv	500 kWp
Modules	N = Ppv / Wmodule	910 pcs
Strings	S = N / ns	34 strings

2. String design (temp-corrected)

Standard: IEC 60364-5-52

Parameter	Formula	Value
Modules/string range	nMin - nMax	6-27
Cold Voc	Voc x ns @ Tmin	55.4 V
Hot Vmp	Vmp x ns @ Tmax	36.6 V

3. Performance & generation

Standard: IEC 61724

Parameter	Formula	Value
Performance ratio	PR	0.846
Annual yield (P50)	Ppv x H x 365 x PR	507,600 kWh/yr
P90 (bankable)	P50 x (1 - 1.282CV)	481,570 kWh/yr

This calculation book is illustrative a licensed engineer must verify and seal final design documents.

Why: The quantified basis of string design (temp-corrected) and P50/P90 generation.

Dwg 4/13Short-circuit calculation — POI fault levels → breaker sizing

Short-circuit calculation · QDTB Transformer · QDTB-DL1-E-04 · Dwg 4 / 13Rev. A · Not to scale · 2026-10-01 · Designed ______ · Checked ______ · Approved ______

Why: POI and box-substation fault levels size the breaker interrupting rating.

Dwg 11/13Main equipment technical data — Step-up box substation / inverters / array parameters

Main equipment technical data · QDTB Transformer · QDTB-DL1-E-11 · Dwg 11 / 13Rev. A · Not to scale · 2026-10-01 · Designed ______ · Checked ______ · Approved ______

Why: Rated parameters of the 630 kVA box and 2×250 kW inverters at a glance.

The following drawings are shown for reference (full set in the configurator “View design document”):

Dwg 1 · Electrical design basisDwg 2 · Symbol legendDwg 5 · Applicable codes & standardsDwg 6 · Construction & testing requirementsDwg 7 · Power quality assessmentDwg 8 · Grounding systemDwg 9 · Surge protection & SPDDwg 10 · Protection configuration

6 · Operation Demo (Deep-Read)

Energy flow

Modules 500 kWp → string inverters → 630 kVA box → SVG → 10 kV export, 100% online.

String design

910 modules in 34 strings (27/string), cold Voc 55.4 V, hot Vmp 36.6 V — the temp-corrected MPPT voltage window.

Reactive & power quality

±1 Mvar SVG continuous dynamic reactive, PF ±0.95, voltage support under PV fluctuation.

Economics

507,600 kWh/yr, $40,608/yr; $315,850 EPC, 9 yr payback, 10.45% IRR.

Other views (shown): LCC (life-cycle cost) · Power quality & harmonics · Voltage profile · Scheme features · Environmental derating

7 · Economics & Payback

Equipment FOB ≈ $390,830 (modules $150,000 + string inverters $63,705 + mounting $75,000 + box substation $29,500 + SVG $50,000 + HV switchgear $16,750 + RMU $5,875); EPC ≈ $315,850. Annual 507,600 kWh → $40,608/yr @ $0.08/kWh PPA; 9 yr payback, 10.45% IRR, 178% 25-yr ROI.

$315,850

EPC investment

9 yr

Payback

10.45%

IRR

178%

25-yr ROI

508 MWh

Annual generation

$40,608

Annual revenue

8 · FAQ

What step-up box substation for 500 kW PV?

630 kVA (YB-630/10): 500 kW ÷ 0.8 PF = 625 kVA → 630 kVA standard tier, string-level box substation.

String vs central inverters — how to choose?

String (2×250 kW here) gives string-level MPPT, so shading/faults affect only that string; central suits large flat unshaded arrays.

Why 10 kV and not 35 kV?

500 kW steps up to 630 kVA < 2500 kVA, so the engine uses 10 kV; at ≥2500 kVA (multi-MW) it auto-switches to 35 kV collection.

Where do 910 modules come from?

500 kWp ÷ 550 Wp = 909.1 → 910 modules; 34 strings × 27 modules/string (temp-corrected 6–27).

What is the payback?

9 yr at $0.08/kWh PPA, 10.45% IRR, 178% 25-yr ROI, on 507,600 kWh/yr (P50).

Why is an SVG needed?

PV fluctuation drives grid-point reactive/voltage limits; the ±1 Mvar SVG gives continuous dynamic reactive (PF ±0.95) + voltage support.

How much energy per year?

507,600 kWh (P50), P90 481,570 kWh; estimated at 1150 full-load-hours × PR 0.8.

What does $390,830 cover?

Modules $150,000 + inverters $63,705 + mounting $75,000 + box $29,500 + SVG $50,000 + HV switchgear $16,750 + RMU $5,875 (FOB).

What is a string-level box substation?

Each string-inverter group steps up to a box substation (string-level box); it is the core asset distinguishing a utility base from distributed PV.

Special Considerations

At 500 kW the engine selects 10 kV collection/export (string-level box substation YB-630/10); at step-up ≥2500 kVA (multi-MW) it auto-switches to 35 kV collection — "35 kV collection" is the utility-base standard, while this 500 kW demo scale is 10 kV.
"String-level box substation" reflects the string (vs central) inverter topology: 2×250 kW string inverters give string-level MPPT tracking, so a single-string fault affects only that string.
Receiving-side calcs (10/0.4 kV short-circuit, load flow, arc flash, N-1) are stale in generation-export mode — ignore them; plant-side data is in genPlant.
Module wattage and mounting tilt (25.9°) are engine defaults — confirm with the design institute against site irradiance and latitude.
Annual generation uses 1150 full-load-hours × PR 0.8 (P50); actual depends heavily on site irradiance and soiling.

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