Utility-Scale Solar Electrical Works in Türkiye: Scope from DC Cabling to the MV Grid Connection

The electrical scope of a ground-mounted solar plant covers DC cabling and string connections, combiner boxes, inverter installation, inverter-transformer stations, the MV collection network, site grounding and lightning protection, the monitoring (SCADA) infrastructure, and testing and commissioning. These packages run in step with mechanical installation, so the electrical program must follow the civil and structure schedules.

On utility-scale projects in Türkiye, module and structure installation, civil works and electrical works are usually carried out by different crews under the EPC main contractor. The electrical balance-of-system (BOS) subcontractor's scope typically starts at the module connectors and ends at the MV switchgear at the plant's point of connection. The subcontract should spell out the interfaces: who opens and backfills trenches, who builds station foundations, which party procures the equipment and what role the inverter manufacturer plays during commissioning.

The inputs for site electrical works are the approved single-line diagram, the string plan, cable routing and sizing calculations, the grounding design and the manufacturers' installation manuals. Note that Turkish project documents call a solar power plant a GES (güneş enerji santrali), while English-language contracts often use SPP. Reviewing these documents before mobilization removes most of the redesigns and material shortages that otherwise surface mid-installation.

The most common DC-side mistakes are cross-mating connectors from different manufacturers, poorly crimped field connectors, reversed polarity, cables left strained or sagging along the structures, loose terminals in combiner boxes and unsealed cable entries. These defects rarely show on day one; they turn into arcing and fire risk after months of thermal cycling.

DC cabling uses purpose-made solar cable that withstands UV and weather. A 1500 V DC system voltage is now common on utility-scale plants, so every DC component must be rated for it. Connectors should come from the same series of the same manufacturer and be crimped with the tool the manufacturer specifies.

Run each string's positive and negative conductors together wherever possible to keep the loop area small and limit lightning-induced surges. Cables must not touch sharp structure edges, should be fixed with UV-resistant ties and must not sag to the ground or into the path of maintenance vehicles.

In combiner boxes, string fuse ratings, the DC load-break switch and the DC surge protective devices must match the design, and terminals should be torqued to the manufacturer's value and marked. Correctly sized cable glands keep water and dust out.

Every string should be labeled in the field, and string numbers must match combiner box inputs one-to-one in the as-built documents; otherwise a simple fault can take hours to locate during operation.

Site checkpoints on the DC side:

1. Connector compatibility and crimp quality

2. String polarity and open-circuit voltage

3. Cable fixing and bending radius

4. Torque marks in combiner boxes

5. Cable gland sealing

6. String and combiner box labels

The inverter architecture shifts the center of gravity of the site electrical works. With central inverters, the workload sits on the DC side: combiner boxes, long DC main cables and large inverter stations. With string inverters, the inverters are distributed across the site, DC cabling gets shorter, and LV AC cabling and AC distribution panels increase.

In a central architecture, strings are combined in combiner boxes and routed to the inverter station over large-section DC cables. Fewer but heavier items of equipment make skid foundations, crane placement and the quality of in-station connections critical. Because a single inverter fault can take a large share of the plant offline, spare parts and service arrangements should be agreed up front.

In a string architecture, inverters are usually mounted on the structures or on dedicated frames, and their AC outputs are collected in LV distribution panels before reaching the transformer. AC cable sizing, voltage drop, protection selectivity in the panels and the communication link between inverters and the monitoring system become more important. A faulty inverter affects only a small part of the plant and is easier to replace.

The choice depends on site layout, grid connection capacity, maintenance strategy and procurement conditions, and is made at design stage; the electrical contractor's program and crew structure are then built around it.

An inverter-transformer station combines the step-up transformer that raises the inverters' LV output to medium voltage, the MV switchgear and auxiliary services, delivered either as a pre-assembled skid or inside a concrete transformer kiosk. The MV collection network links these stations by underground cable to each other and to the MV distribution or metering substation at the point of connection.

Installation starts with checking foundation levels, cable entry openings and grounding stubs. Once the equipment is lifted into place, the transformer, MV switchgear and LV connections are made. Busbar and cable lug torque on the transformer, switchgear interlocks and protection relay settings, the auxiliary transformer and in-station lighting, ventilation and fire detection are each verified. The choice between dry-type and oil-immersed transformers also affects station layout and civil details such as oil containment.

In Türkiye, the MV collection network of a ground-mounted solar plant is typically built at 34.5 kV to match the distribution grid. Stations can be connected radially or in a ring; cable cross-section, voltage drop, losses and short-circuit withstand are the main design parameters.

Cables are laid on bedding material in a prepared trench and covered with protection tiles and warning tape; drums are handled with proper equipment, and pulling tension and bending radius stay within the manufacturer's limits. MV terminations and joints are among the most frequent sources of cable faults, so they should be made strictly to the manufacturer's instructions by trained jointers. A solar plant's MV collection network resembles the inter-turbine MV collection network of a wind farm; the difference is that a solar plant has far denser DC cabling and string connections.

Grounding on a solar site means bonding the metal parts, including structures, module frames, inverters and stations, into an equipotential network; the perimeter fence is either bonded to the main grid or grounded separately, depending on the design. Lightning protection is designed from a risk assessment and completed with surge protective devices on the DC and AC sides. Monitoring rests on a fiber-optic backbone, data loggers and a weather station.

The site grounding network usually consists of conductors laid along the cable trenches plus grounding grids around each station, with structure rows bonded to it at regular intervals. The design keeps touch and step voltages within safe limits during MV faults; in Türkiye, the Regulation on Grounding in Electrical Installations (Elektrik Tesislerinde Topraklamalar Yönetmeliği) is the main reference. Whether the perimeter fence joins the main grid is decided from the same calculations, weighing the risk of transferring touch voltages outside the site.

Watch for galvanic corrosion where dissimilar metals meet, make buried joints by welding or with suitable compression fittings, and photograph them before backfilling.

IEC 62305 is the common reference for lightning protection. Air terminals are positioned so they do not shade the modules, and surge protective devices of the appropriate class are fitted at inverters, combiner boxes and station inputs.

For monitoring, a fiber-optic network typically runs between stations in the same trenches as the MV cables, connecting inverters, meters, protection relays and irradiance sensors to SCADA. Since the active and reactive power control required by the grid operator also runs over this infrastructure, communication testing is an integral part of commissioning.

Before energization, the DC side is checked for polarity, open-circuit voltage, short-circuit current and insulation resistance; the AC and MV side goes through cable, transformer and protection tests; and grounding is measured across the site. Go into provisional acceptance with these test reports, as-built drawings and a closed punch list.

IEC 62446-1 is the common reference for DC testing and inspection: protective conductor continuity, string polarity, open-circuit voltage, short-circuit current and insulation resistance are recorded for every string. Where needed, I-V curve tracing and infrared thermography reveal module and connection defects.

On the MV side, cables receive VLF, tan delta and sheath tests, transformers get ratio and insulation measurements, and protection relay settings are verified by secondary injection.

Energization is staged in coordination with the distribution system operator, or TEİAŞ for transmission-connected plants: first the point of connection and the MV collection network, then the stations and inverters.

Provisional acceptance (geçici kabul in Turkish) requires grounding measurement reports, test protocols, the as-built single-line diagram and cable route drawings, equipment certificates and records showing that the punch list has been closed. EPC contracts may also require a performance test, in which case the accuracy of monitoring data becomes part of acceptance.

Miratek Elektrik delivers site electrical works for ground-mounted solar plants across Türkiye, from DC cabling to the MV connection, through its SPP Installation and Commissioning service. One team runs this scope together with our SPP Design, Lightning Protection and Grounding, and Transformer Substations services.

We work on sites across Türkiye with 50 MW of installed SPP capacity behind us, backed by 85 km of MV/LV cable and 120 MVA of total transformer capacity. Our references include solar companies such as IF Güneş Enerji and Zirve Enerji, and we apply the same site discipline to MV collection networks on wind farms.