String inverters: topology and MPPT
Solar Pv · Lesson 11 · Advanced
Purpose
Explain string-inverter conversion and MPPT without treating one tracker as independent control of every module.
Learning objectives
- Explain the architecture
- Match electrical limits
- Identify safety/protection boundaries
- Use exact product evidence
A string inverter transforms array DC into grid-synchronised AC. Its MPPT controller adjusts string operating voltage to seek a power maximum within declared input limits.
Core theory
One MPPT sees the aggregate I-V curve of connected strings. Differently oriented or shaded strings can create mismatch or multiple local maxima unless separated onto suitable trackers.
Cold Voc, hot Vmp, per-input Isc/current, total DC power, connector/polarity, insulation monitoring, DC isolation and AC/network limits are independent checks.
Anti-islanding/grid protection stops ordinary parallel operation after grid loss. It does not prove DC is dead or provide backup power.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| MPPT | Maximum power point tracking | Not applicable | Not applicable |
| Anti-islanding | Protection preventing unintended energisation of a disconnected network | Not applicable | Not applicable |
| Clipping | Power curtailed at a converter limit | Not applicable | Not applicable |
Assumptions: All numerical data are supplied fictional design inputs.
- Each: Each string power is 360×11=3.96 kW.
- Total: DC operating power is 2×3.96=7.92 kW.
- Limit: Confirm both tracker and total converter limits before accepting.
Combined stated DC operating power is 7.92 kW.
Reasonableness check: The result is checked against the stated architecture and limits, not treated as universal product approval.
- Selecting from power rating alone
- Assuming grid loss makes ordinary outputs backed up
- Mixing nominal, usable and delivered battery energy
Where this appears in practice
PV and storage design coordinates conversion equipment, protection, controls, network requirements, fire safety and user operating modes.
Knowledge check
Does switching off the grid make PV DC conductors dead?
No. Illuminated modules continue to generate DC.
Answer: No. Illuminated modules continue to generate DC.
Practical exercise
Map the power, sensing, protection and isolation paths of a supplied string-inverter diagram.
Summary
- Architecture changes fault and energy paths
- Every port has independent limits
- Backup behavior must be deliberately designed
Sources and review
- IEC 62109-3:2020: Safety of PV power converters: IEC; 2020; International
- IEC 62619:2022: Safety requirements for secondary lithium cells and batteries: IEC; 2022; International
- Amendment 4:2026 impact on BS 7671: IET; 2026; United Kingdom
- ENA generator type-test register: Energy Networks Association; Current status must be checked in Connect Direct; Great Britain / Northern Ireland
Editorial review date: 2026-08-22. Professional electrical review is pending.