Adding Solar to an Operating Wind Farm in Türkiye: How Is the Electrical Infrastructure of a Hybrid Plant Planned?

An auxiliary-source hybrid plant is a licensed wind farm to which a second renewable source, typically solar PV, is added behind the same grid connection point and within the same connection capacity. In Türkiye this became possible through the March 2020 amendment to the Electricity Market Licensing Regulation, which introduced multi-source generation facilities built around a main source and one or more auxiliary sources; a plant that pairs wind with solar falls under the combined renewable generation facility type. In a wind-solar hybrid, the wind farm is the main source and the solar plant is the auxiliary source; no new grid connection is created for the solar part.

For an operating plant, the route runs through a license amendment with EPDK, Türkiye's Energy Market Regulatory Authority, which includes a technical evaluation of the solar addition by the Ministry of Energy and Natural Resources (MENR). The license's total electrical installed capacity, connection point and voltage level stay unchanged. A connection opinion is obtained from TEİAŞ or the regional distribution company, depending on the wind farm's connection voltage. It must also be confirmed early whether the solar area lies within the licensed plant site; if it does not, a site amendment is required. Land rights and environmental permits for the solar area are settled at the same stage. Our grid connection article for solar, wind and BESS projects covers the agreement sequence.

Electrically, the key point is that you are not building a new power plant but inserting a second generation block into an existing one. Design therefore starts from the wind farm's single-line diagram, short-circuit levels, protection settings and SCADA architecture rather than from a blank solar layout. Where as-built records are incomplete, the installed configuration is verified on site first. Adding battery storage (BESS) is a separate architectural decision; this article covers only wind and solar running together.

A solar add-on shares the export capacity written into the wind farm's connection agreement, so the combined output of both sources is capped at the connection point at the agreed capacity. The business case rests on wind and solar rarely peaking at the same time: capacity that sits unused when the wind is light is filled with solar generation. In sunny, windy hours, however, combined output goes over the cap and the surplus is curtailed.

Solar capacity should therefore follow from a curtailment study, not from the amount of free land. At least one year of 10-minute wind farm output from the SCADA historian is aligned with site irradiance data on the same time base, and combined output is compared with the export cap interval by interval. The result is annual curtailed energy for a range of solar capacities, which drives the choice of inverter rating and DC/AC ratio.

On site, the power plant controller (PPC) enforces the cap: it measures active and reactive power at the connection point and dispatches set points to the solar inverters and the turbine control system. Which source is curtailed first is a commercial decision; technically, inverters respond much faster than turbines. The control logic must also define a fail-safe output for inverters if PPC communication is lost, and how the reactive power obligation at the connection point is split between turbines and inverters. The export cap indirectly bounds main transformer loading, but MVA loading including reactive flows still needs checking.

Where the solar block is of meaningful size, a dedicated feeder to the substation's MV busbar is usually the sounder choice; tapping into an existing wind farm collector feeder makes sense only when the cable rating and the protection scheme can absorb it. Either way, the solar inverter-transformer stations are first collected on their own MV ring or radial circuit, and the real decision is where that circuit joins the wind farm.

Tapping into an existing feeder is attractive because the cable run is short. But collector cables were sized for the turbines on that feeder alone, taking burial depth, cable grouping and soil thermal resistivity into account. Add solar current and the cable can overheat when wind and sun peak together, so the IEC 60287 rating calculation must be redone with the combined load profile. If the turbine's MV switchgear has no spare bay, a new ring main unit is needed. Unless the tee-off has its own circuit-breaker protection graded against the feeder breaker, a solar-side fault can trip every turbine on that feeder, and a fault on the shared feeder cable takes out both sources together.

A dedicated feeder needs a spare substation bay or an extension panel compatible with the existing switchgear, plus checks on busbar rated current, short-circuit withstand and main transformer capacity. Solar transformers are specified at the same MV level as the collector system, most often 34.5 kV on Turkish wind farms. Their vector group and neutral arrangement must match the grounding method of the wind farm's MV network; otherwise ground fault current levels shift and the existing ground fault settings may no longer be valid.

Protection, metering and SCADA are arranged so that the solar block joins the wind farm's existing scheme as one more feeder, while the plant continues to behave as a single facility at the connection point. Short-circuit, load flow and protection coordination studies are updated with the solar plant included, and the new feeder's overcurrent and ground fault relays are graded against the incomer and main transformer protection.

New MV cables raise the capacitive ground fault current of the network. If the wind farm's MV system is resistance-grounded, the rating of the neutral grounding resistor and the ground fault settings must be rechecked against that increase. Inverter voltage and frequency protection and fault ride-through behavior are set to the Turkish grid code requirements and the connection-point protection scheme.

The settlement meters at the connection point stay in place. To track auxiliary-source generation separately, a dedicated metering arrangement with instrument transformers of the accuracy class required by the network operator is installed on the solar feeder. Inverter, station and weather-station data are integrated into the wind farm SCADA and the PPC, and the signal list exchanged with TEİAŞ or the distribution company is revised for the hybrid configuration. The solar fiber network should join the wind farm's fiber ring as a separate network segment, with single-source time synchronization so event logs from both plants line up.

Solar arrays built between turbines share the land with live buried MV cables, fiber routes, turbine grounding systems and maintenance areas, so their cabling and grounding need a plan of their own. The biggest site risk is driving mounting-structure piles into existing 34.5 kV cables or fiber. Piling should not start until the pile plan has been checked against as-built drawings, a cable-locator survey and hand-dug trial pits at critical crossings.

Crane hardstands, blade laydown areas and service roads must stay clear, because they will be needed again for major component replacement. Module rows are kept away from the zones most affected by tower shadow and ice throw. Where the solar MV cable runs parallel to an existing trench, mutual heating is included in the rating calculation, and crossings get separation distances and protective covers.

Mounting structures, inverter stations and transformer grounding are bonded to the site grounding network to maintain a common potential. When lightning strikes a turbine, however, part of the current spreads through that common network, and high voltages can be induced in DC string cables running close to turbine foundations. Surge protective devices (SPDs) of the appropriate class are therefore specified on both the DC and AC sides, DC cables are routed with clearance from turbine grounding electrodes, and step and touch voltages are recalculated for the combined grid. Post-bonding grounding resistance and continuity measurements go into the acceptance file. Turbine grounding itself is covered in our wind farm electrical works article.

Most hybrid works run while the wind farm keeps generating; full or partial shutdowns are limited to short, pre-planned windows for the substation busbar connection, any tie-in to an existing feeder and protection setting changes. The solar field, DC cabling, inverter stations and the new MV cable route are executed as a work area physically separated from the live system.

Outage work is scheduled into low-wind days using the wind forecast and agreed with the operator's switching program. Work near energized equipment does not start without a permit to work, lockout-tagout and temporary protective grounds at the point of work. VLF and tan delta tests on new MV cables, switchgear functional tests and relay secondary injection should be finished before the window opens, leaving only connection and short checks inside the outage.

At commissioning, the solar plant is energized first from its own feeder and inverters are brought online in sequence. The PPC is then tested under real conditions with a deliberately low limit, to confirm that combined output stays at the cap with the turbines running and that inverters fall back to safe mode on communication loss. Provisional acceptance for the auxiliary source follows these tests.

Miratek Elektrik carries out wind farm electrical works across Türkiye, from MV collector circuits to grounding and fiber, as well as site electrical works on utility-scale solar plants. To date, we have installed 50 MW of solar capacity, laid a total of 85 km of MV/LV cable lines and completed 48 MV switchgear panel installations. On the BESS project at the operating Killik wind farm, we acted as the electrical works subcontractor for the cable infrastructure, grounding, busbar connections and commissioning support. For investors and EPC contractors weighing a hybrid conversion, our Wind Farm Design service covers the technical review, from the existing single-line diagram to the connection option.