Solar-Plus-Storage Electrical Infrastructure: AC-Coupled or DC-Coupled BESS?

A solar-plus-storage plant is a solar power plant (SPP) whose PV generation is operated together with a battery energy storage system (BESS) on the same site, usually through the same grid connection point. In a conventional SPP, power flows in one direction: the modules generate DC, inverters convert it to AC, a step-up transformer raises the voltage, and the energy is exported to the grid. In a hybrid plant, the battery both charges and discharges, so power flows in both directions inside the plant.

That bidirectional flow affects almost every layer of the electrical infrastructure, from transformer loading and the directional settings of protection relays to the metering arrangement and the auxiliary power supply. On top of that, an energy management system (EMS) is added to control PV output and battery power together. In short, solar-BESS integration is not a matter of placing a container on site; the plant has to be re-engineered as a single electrical system.

The regulatory framework in Türkiye has also opened the door to storage. The Regulation on Storage Activities in the Electricity Market, issued by EMRA (the Energy Market Regulatory Authority, known locally as EPDK), allows unlicensed generation facilities, meaning plants exempt from a generation license, to install an integrated storage unit up to the electrical capacity of the generation facility. Implementation conditions should be verified against the regulations in force at the time of application. We cover application and connection procedures in our guide to unlicensed solar applications and the financial side in our feasibility and payback articles; this article focuses solely on the electrical architecture.

The core difference is where the battery connects to the plant: in an AC-coupled design, the battery connects to the AC side through its own bidirectional inverter, the power conversion system (PCS), while in a DC-coupled design it connects to the DC input of the PV inverter through a DC/DC converter. In an AC-coupled system, PV and BESS operate as two independent power conversion chains, and the PCS is typically tied into the plant's MV busbar through its own step-up transformer and MV switchgear cubicle. In a DC-coupled system, the PV array and the battery share the same inverter and the same transformer.

The strengths of AC coupling are flexibility and independence: it can be added without touching the existing inverters, it can charge the battery from the grid, and a fault on one side does not shut down the other. The trade-off is that energy moving from the modules into the battery goes through DC-AC-DC conversion, so round-trip losses are somewhat higher, and the additional PCS, transformer and switchgear add cost.

DC coupling offers an efficiency advantage in plants designed with a high DC/AC ratio, because energy that would otherwise be clipped at the inverter limit can be sent straight into the battery. However, total output is capped by the inverter rating, grid charging is possible only as far as the equipment and regulations allow, and retrofitting it to an existing plant requires inverter compatibility. On sites built with string inverters, this is often impractical.

To simplify the decision: if storage is being added to an operating solar plant, if the battery must also charge from the grid, or if PV and BESS will be commissioned on different dates, AC coupling usually comes out ahead. For a new-build plant with central inverters, a high DC/AC ratio and a goal of capturing clipped energy, DC coupling is a strong option. The final decision should be made together with yield simulations, the capacity in the connection agreement and the technical data of the selected equipment.

Yes. Every plant that adds a BESS needs its transformer loading, MV busbar capacity and switchgear requirements recalculated, because the plant's operating scenarios change. Export to the grid remains capped by the capacity in the connection agreement, but new power flows appear inside the plant: the battery charging while PV is at full output, the battery discharging on its own in the evening, or the battery charging from the grid.

In an AC-coupled design, a new feeder cubicle is usually added to the hybrid plant's substation; if the existing ring main unit (RMU) or modular switchgear line-up has no spare cubicle, the line-up is extended. At this stage, the rated current of the main busbar, the short-circuit withstand ratings of the cubicles and the fault-current contribution of the inverters are checked. If the BESS will share an existing transformer, the transformer's continuous loading and thermal calculations are redone for the simultaneous operating profile.

A DC-coupled design often does not require an additional transformer, but because the inverter and transformer will run close to full load for a larger part of the day, the thermal calculation still matters. In both architectures, the HVAC, ventilation and fire systems of the battery containers create a continuous auxiliary load. A dedicated auxiliary service transformer or a reliable LV supply should be planned for this load.

Protection coordination in a hybrid plant must be designed from the outset on the assumption that power flows in both directions. Reverse power and directional overcurrent protection, which generation plants use to prevent import from the grid, can trip unnecessarily while the battery is charging from the grid; that is why the direction and threshold settings at the connection point are redefined for the hybrid operating profile.

Interface protection at the connection point, including voltage and frequency limits and anti-islanding functions, must be consistent with the built-in protection of the PV inverters and the PCS. Relay settings on the new BESS feeder are checked for selectivity together with the existing PV feeders and the incoming cubicle, and the setting values must not conflict with the connection conditions issued by the distribution company.

The metering system is redesigned so that the energy the battery imports from and exports to the grid can be separated. Bidirectional meters with hourly recording at the connection point are essential; depending on the architecture and the regulations in force, separate metering points may also be required for the storage unit and for auxiliary consumption. The location of meters and instrument transformers should be finalized at the design stage with the distribution company's approval.

The most significant additional measures are on the DC side: the lines between the battery and the PCS carry high DC currents, so conductor sizing is done by calculating voltage drop, thermal grouping in cable trays and short-circuit withstand together. DC and AC power cables should be separated from the fiber-optic and signal cables carrying BMS/EMS communications, either on separate routes or in segregated tray compartments. Recording tightening torques at terminations and verifying polarity are basic tasks that must be completed before commissioning.

For grounding, battery containers, PCS units and transformer stations are bonded equipotentially to the plant's existing grounding grid, and step and touch voltages are reassessed whenever the grid is extended. Battery DC systems are mostly operated ungrounded (floating) and continuously supervised by an insulation monitoring device, which makes insulation resistance measurements especially important during commissioning.

For fire safety, in-container gas detection, the suppression system, explosion venting solutions and the spacing between containers are planned according to the manufacturer's certified design and recognized standards. The IEC 62933 series, IEC 62619, UL 9540A test results and NFPA 855 are frequently referenced in this area. The site layout must also account for fire department access and site-specific conditions.

The EMS is the control layer that simultaneously monitors PV generation, the battery's state of charge and the power at the connection point, and decides when and how much energy the plant exports to the grid. Typical monitored data include inverter and PCS active and reactive power, state of charge (SOC) and state of health (SOH), cell and module temperatures, BMS alarms, HVAC and fire system status, weather station data, and voltage and frequency at the connection point.

The power plant controller (PPC) ensures that the power limit in the connection agreement is never exceeded, limits ramp rates and provides reactive power support when required. Operating strategies such as shifting energy to high-demand or high-price hours, recovering clipped energy and peak shaving are also defined in this layer.

In the communication architecture, field devices are typically connected via Modbus TCP or IEC 61850, while data is passed to the distribution company or the system operator using the required protocol, such as IEC 60870-5-104. A redundant fiber-optic backbone and network segmentation for cybersecurity are critical for reliable remote monitoring. We cover SCADA architecture in detail in our article on SCADA systems.

The most important criterion is the ability to treat PV, BESS and the MV side as a single electrical system. In projects where one company builds the solar plant, another connects the battery and a third handles MV switchgear and protection, interface responsibility becomes blurred and commissioning drags on.

When procuring a solar-plus-storage installation, ask specifically about high-current DC cabling and termination experience, MV cable terminations and switchgear works, grounding grid installation and measurement reporting, protection relay testing, and compliance with the PCS manufacturer's commissioning procedures. A documented handover with torque records, insulation test reports and as-built drawings protects the owner both in warranty processes and at acceptance.

Backed by more than 15 years of experience and 50 MW of installed solar capacity, Miratek Elektrik delivers SPP installation and design, MV contracting and transformer substation works across Türkiye as a single point of contact. Our BESS electrical works for Chint Power Systems at the Killik wind farm site, covering cable pulling, fiber optics, cable terminations and grounding, gave us first-hand insight into what storage integration really requires in the field. If you are considering adding storage to an existing plant or building a new hybrid plant, the architecture choice and infrastructure scope become clear after a site survey.