MV/LV distribution transformers remain one of the most robust and essential assets in distribution networks. Their core function has changed only gradually over time, mainly through improvements in materials, efficiency and loss reduction driven by eco-design requirements. However, the operating context of distribution networks is changing rapidly. The increasing integration of distributed energy resources, photovoltaic generation, heat pumps and electric vehicles is creating more dynamic voltage profiles and more localized power-quality challenges. In this context, Smart Distribution Transformers should not be seen as a single uniform technology, but rather as a family of solutions with different maturity levels and DSO relevance. OLTC-equipped distribution transformers are already a mature and deployable option for targeted voltage management, especially in constrained LV networks or where conventional reinforcement is difficult, slow or hard to permit. By contrast, fully smart or solid-state transformer concepts remain longer-term technologies, with potential future value in areas such as phase-specific control, bidirectional power management, harmonic mitigation and enhanced power-quality support. This section reflects a pragmatic DSO perspective: where these technologies can provide near-term value, where their use remains limited by cost or complexity, and under which network conditions they may complement or defer conventional reinforcement.
Highlights
Power electronics-based smart transformers remain an emerging technology (voltage regulation, phase balancing). Conversely, OLTC-equipped transformers are mature and deployable for local voltage constraints. Regulatory relevance lies in digitalization, flexibility, and optimizing existing grid capacity.
Challenges and opportunities for DSOs
Opportunities
- Local Voltage Control: Highly effective in constrained LV grids.
- CAPEX Deferral: Increases DER hosting capacity, potentially delaying conventional grid reinforcement.
- Supply Quality: Provides faster, more granular voltage regulation compared to manual tap changes.
Challenges
- Higher Costs: Significantly more expensive than conventional transformers (around 30% up).
- Complexity & Reliability: Requires additional sensing and communication; long-term robustness of solid-state concepts remains unproven.
- Stranded Assets Risk: Business case is strictly limited to specific locations with persistent power-quality issues.
E.DSO considerations
Near-term value is highly specific: OLTC deployment serves as a non-wire alternative for voltage-constrained areas or where permitting is blocked. Fully smart/solid-state concepts are long-term system assets, not yet ready for mass roll-out.
Potential use cases
- Voltage management in LV networks with high PV or other distributed generation, especially where feeders are relatively homogeneous and local regulation can improve compliance without creating new issues elsewhere.
- Under suitable network conditions, increasing DER hosting capacity and in some cases delaying conventional LV reinforcement in locations with recurring voltage constraints.
- Providing a practical alternative to grid reinforcement in constrained areas where conventional grid reinforcement is difficult, slow or hard to permit.
- Supporting coordinated operation with smart inverters, LV monitoring and automation systems to improve voltage quality and reduce manual interventions.
In the longer term, solid-state transformer concepts could offer additional functionalities such as phase-specific control, harmonic mitigation and enhanced power-quality support in sensitive or highly dynamic networks.
Ongoing projects
- E.ON / Bayernwerk (Germany): Following extensive pilots, regulated distribution transformers (RONT/OLTC) have been integrated into standard asset management. Deployment is accelerated in areas with dense EV and PV penetration.
- TIGON EU Project (Spain Demo): Deploying Solid-State Transformers (SST) within a live hybrid AC/DC microgrid demonstration. The project monitors real-time bidirectional power flow, wide-bandgap semiconductor performance, and efficient integration of localized RES and EV charging.
- i-DE / Iberdrola (Spain): SST Focus: Launching dedicated field challenges under the PERSEO initiative to deploy SST. The project targets active harmonic filtering and bidirectional power control in areas with high industrial EV loads.
- ČEZ Distribuce (Czechia): ČEZ Distribuce is evaluating OLTC distribution transformers for specific cases where conventional reinforcement is difficult or too slow to implement. The current focus is on selective deployment, not system-wide rollout.
Link to related E.DSO success cases
| Success case number | Title | Company |
| 14.2024 | i-TRAFO | i-DE |
Impact on the energy system
- Improved local voltage management.
- Higly flexible use of existing distribution infrastructure.
Time to scale
- OLTC Transformers – 0-5 years (targeted use).
- Smart/Solid – state: +10 years (broader relevance).
Link to relevant EU policies
- Digitalisation.
- Flexibility.
- Resilience.
- Energy Efficiency.
- Integration of DERs.
Technology deployment across countries with E.DSO members
Last update: 12 September 2026