Organized Industrial Zone (OIZ) Electrical Infrastructure in Türkiye: Connection Process and Requirements

Building electrical infrastructure in an organized industrial zone (OIZ, known as OSB in Turkish) involves determining the facility's power demand, applying to the OIZ administration for a connection, signing a medium-voltage (MV) connection agreement, designing and building the facility's own transformer substation and MV switchgear, and finally completing provisional acceptance and energization, with final acceptance following later. Because most OIZs in Türkiye hold their own distribution license, tenant facilities receive power from the OIZ administration rather than directly from the regional distribution company, which makes both the process and the counterparty different from a standard utility connection.

In practice, the flow is: (1) power demand and subscriber group assessment, (2) application to the OIZ and capacity allocation, (3) the MV connection agreement, (4) preparation and approval of the internal installation design (transformer substation, MV switchgear, LV distribution), (5) site construction and installation, (6) testing, commissioning and acceptance, and (7) metering installation and energization. Each step requires documentation that complies with IEC standards and the technical specifications of TEDAŞ (the Turkish Electricity Distribution Company).

In this article, we walk through each stage, the required documents and the most common technical requirements from an engineering perspective, so that investors and EPC contractors can see the full path to energizing a factory inside an OIZ.

Inside a licensed OIZ, the OIZ legal entity, not the regional distribution company, supplies power to the industrial tenants. A standard subscriber receives power from the regional electricity distribution company. Under Türkiye's electricity market legislation, however, organized industrial zones that meet the licensing requirements can obtain an OIZ distribution license from EMRA (the Energy Market Regulatory Authority, EPDK in Turkish) and carry out distribution within their own boundaries. In that case, the OIZ is the party distributing power to the industrial tenants, and the OIZ administration is the technical and administrative counterparty for the connection.

The topology usually looks like this: the OIZ's main step-down substation is fed from the transmission grid operated by TEİAŞ (the Turkish Electricity Transmission Corporation), typically at 154 kV, or, in smaller zones, from the distribution company's MV network. From there, the OIZ's own MV distribution network, typically at 31.5 kV or 34.5 kV, extends to the blocks and parcels. Each industrial parcel takes its MV supply from the connection point at its boundary and builds its own transformer substation within the parcel to step the voltage down to low voltage (LV).

This structure makes the facility owner's responsibility boundary clear: everything up to the connection point is OIZ infrastructure, and everything after it is the tenant's internal installation (transformer substation, MV switchgear, LV panels, reactive power compensation). Design and construction planning start with defining this boundary correctly; otherwise, responsibilities for ownership, maintenance and protection coordination become blurred.

The process runs from load calculation through capacity allocation, the MV connection agreement, design approval, construction and testing to energization, and it starts with determining the facility's installed capacity and coincidence factor. This calculation sets the subscriber group and therefore whether the connection will be at MV or LV; for industrial facilities, demand is usually at MV level. Once the power demand is settled, a connection application is submitted to the OIZ administration, and the capacity allocation and connection point are determined.

After allocation, the MV connection agreement is signed. This agreement defines the allocated capacity, the connection voltage level, the metering point and the protection coordination obligations. Next comes the internal installation design: the single-line diagram, substation layout, MV switchgear configuration, protection relay settings, grounding and LV distribution design. The design is approved by the OIZ administration and, where required, against TEDAŞ technical criteria.

With the approved design, site construction begins: building the transformer substation, installing the MV switchgear, placing the power transformer, cabling, and installing the grounding grid and protection system. In the final stage, tests (high-voltage, insulation, relay coordination testing and grounding measurement) are carried out, provisional acceptance takes place, the meter is installed and the facility is energized. In a well-planned process, the goal is commissioning without delay and in full compliance with the standards.

An OIZ connection requires a set of administrative documents that is as decisive as the technical design itself. Typically required are the parcel allocation or title deed, a power demand letter with the installed capacity calculation, an approved internal electrical installation design (including the single-line diagram), design authorship by an electrical engineer registered with EMO (the Chamber of Electrical Engineers of Türkiye), type test and conformity certificates for the equipment to be used, and the undertakings requested by the OIZ administration.

Besides the connection agreement, some OIZs link electrical acceptance to the business opening and operating license process, so when planning the schedule, it is advisable to run the licensing and acceptance steps in parallel. For metering installation, the metering circuit design and the suitability of the current and voltage transformers (CTs/VTs) are checked separately.

Missing documents and design revisions are where the process most often stalls. The three factors that speed up approval the most are a single-line diagram that matches the capacity and voltage level in the connection agreement exactly, protection coordination that is consistent with the OIZ's main protection stages, and a grounding design that can be verified by measurement.

The core requirements are a correctly rated dry-type or oil-immersed transformer, an MV switchgear lineup of incoming, metering and transformer protection panels, relay settings that are selective with the OIZ's upstream protection, and a grounding design verified by measurement. The transformer substation is the heart of a facility's internal infrastructure in an OIZ, stepping the voltage down from the OIZ's MV level to the facility's LV level. Whether to choose a dry-type or oil-immersed transformer depends on the load profile, ambient conditions, fire load and maintenance strategy. On the MV side, the incoming, metering and transformer protection panels form the standard lineup; switchgear selection and protection relay settings must be coordinated to operate selectively with the OIZ's upstream protection stages.

Protection coordination ensures that a fault within the OIZ infrastructure trips only the faulty stage without affecting neighboring facilities. For this reason, overcurrent, ground-fault and, where needed, differential protection settings are expected to be based on short-circuit and load flow analysis. Grounding design is mandatory for both personnel safety and equipment protection, and it is verified by grounding measurement before commissioning.

Two topics that are often overlooked in industrial facilities are reactive power and harmonics. Reactive power drawn by inductive loads leads to reactive power penalties when the limits are exceeded; the solution is a correctly sized reactive power compensation panel. Variable frequency drives and rectifiers generate harmonic distortion (THD), which is managed where necessary with harmonic filters and power quality monitoring. These loads should be taken into account from the outset when planning the facility infrastructure.

The most common mistake is underestimating power demand. A future capacity increase may require revising the MV connection agreement and resizing the transformer, which costs both money and time. The second common problem is protection coordination set inconsistently with the OIZ's main grid, resulting in a loss of selectivity. The third is leaving grounding and power quality issues until the end of the project.

The most effective way to speed up the process is to run design and construction under the same engineering discipline. Designing the single-line diagram, switchgear selection, protection settings and panel manufacturing (IEC 61439 compliant) to be consistent with one another minimizes the need for revisions on site. Planning the tests (insulation, relay and grounding measurement) in advance allows provisional acceptance to be completed without delay.

At Miratek, we design and build the internal infrastructure of industrial facilities in OIZs end to end, from transformer substation installation and MV switchgear erection to IEC 61439 compliant panel manufacturing and power quality solutions (reactive power compensation and harmonic filters). Backed by 15+ years of experience and 120 MVA of total transformer capacity installed across Türkiye, we aim to deliver a facility connection that is fully compatible with the OIZ infrastructure, compliant with the standards and commissioned on time. You can request a site survey and power demand analysis for your connection process.