Electrical engineering
glossary.

In AC coupling the battery joins the solar plant on the AC side through its own inverter; in DC coupling it connects on the inverter's DC side.

AC-coupled systems are more flexible for adding storage to an existing solar plant, since the battery and the PV array run through independent inverters. In DC-coupled systems, solar energy can flow into the battery directly as DC, which reduces conversion steps and allows energy that would be lost to inverter clipping to be stored. On the other hand, DC coupling demands tighter equipment compatibility and design. The choice weighs existing infrastructure, connection capacity, operating strategy and regulatory requirements.

A BESS stores electrical energy in batteries and returns it to the grid or a facility when needed, combining batteries, a PCS and controls.

BESS installations are used to balance renewable output, shave peak demand, support grid frequency and provide backup power. Large-scale systems today mostly use lithium iron phosphate (LFP) cells. A BESS site includes battery containers, the power conversion system (PCS), transformers, MV switchgear, an energy management system (EMS), fire detection and suppression, and HVAC. Electrical works cover DC and AC cabling, the MV connection, earthing, communications and commissioning tests.

An inverter converts direct current (DC) from PV modules or batteries into grid-compatible alternating current (AC).

String inverters and central inverters are the two most common architectures in solar plants. Using maximum power point tracking (MPPT), the inverter draws the highest available power from the modules, and it disconnects itself when grid voltage or frequency moves outside the limits set by the grid code, while riding through short disturbances within those limits (FRT). Modern inverters can provide reactive power support and send data to the monitoring system. Selection considers the DC/AC ratio, efficiency, operating temperature range and grid connection requirements.

kWp is the unit for the peak DC power PV modules deliver under standard test conditions (STC), used to express installed capacity.

Standard test conditions assume 1000 W/m² irradiance, 25 °C cell temperature and the AM 1.5 spectrum, so real output in the field is often lower. In solar plants, installed DC capacity is expressed in kWp or MWp, while AC capacity at the inverter output is given in kWac or MWac (written as kWe/MWe in Turkish regulations). The specific yield in kWh/kWp shows how much energy each kWp of installed capacity produces in a year and varies by location. These figures are used together in feasibility and payback calculations.

The payback period is the time it takes for an energy investment's savings or revenues to recover its initial cost.

For solar investments, the calculation is based on installation cost, annual yield (kWh/kWp), self-consumption rate, electricity tariffs and operation and maintenance costs. Simple payback ignores the time value of money, so a fuller assessment also uses net present value (NPV) and internal rate of return (IRR). Leaving out module degradation, inverter replacement and tariff changes can make the result look better than it is. Running a sensitivity analysis across several scenarios is therefore recommended.

Performance ratio is the energy a solar plant actually produces divided by the energy it should theoretically produce under the same irradiance.

PR captures all losses, including temperature, cabling and inverter losses, soiling, shading and faults, in a single indicator. Because it removes the effect of irradiance, it makes it easier to compare plants in different locations or across different years. A PR below expectations is an early warning of dirty modules, faulty strings or inverter issues. Accurate calculation requires reliable on-site irradiance measurement with a pyranometer.

A PCS is the bidirectional converter in an energy storage system that turns battery DC into AC and grid AC into DC for charging.

The PCS is where the BESS meets the grid: it controls charge and discharge power, reactive power support and grid protection functions. It is usually installed on site together with an MV transformer as a PCS station or power conversion skid. It operates in constant communication with the energy management system (EMS) and the battery management system (BMS). Correct DC cabling, AC connections, earthing and communication wiring are critical for smooth commissioning.

A PV string is a group of photovoltaic modules connected in series to add up voltage, feeding one inverter input.

The number of modules per string is calculated so that the open-circuit voltage in the coldest conditions stays below inverter and module voltage limits. Shading on a single module can reduce the current of the whole string, so layout and shading analysis matter. String cables, connectors and string protection devices determine the safety of the DC side. During commissioning, open-circuit voltage, short-circuit current and insulation resistance are measured for each string.

A rooftop solar PV system is installed on factory, warehouse or building roofs, with its electricity used primarily on site.

In industrial facilities, the overlap between daytime consumption and solar output makes rooftop systems economically attractive. Before installation, a structural assessment of the roof's load capacity, a shading analysis and a check of the roof covering are carried out. The system is usually connected to the facility's existing LV or MV infrastructure, and the application is made under the unlicensed generation rules. Fire safety, maintenance walkways and DC cable routing should be addressed separately in the design.

Self-consumption rate is the share of a solar system's output that is used directly on site rather than exported to the grid.

Because daytime consumption is high in industrial facilities, their self-consumption rate is usually higher than that of homes. Every self-consumed kWh replaces energy that would otherwise be bought from the grid, so the rate directly affects the payback period. It depends on system size, the load profile, shift patterns and any storage system. Calculating it from hourly consumption data in the feasibility study gives more realistic results.

A solar power plant converts sunlight directly into electricity with photovoltaic modules and feeds it to the grid or a facility through inverters.

Solar plants can be built on rooftops, on open land or as floating installations. Their main components are PV modules, mounting structures, DC cabling, inverters, LV/MV panels, a transformer and a monitoring system. The delivery process runs through feasibility, grid connection application, design, approval, installation, testing and acceptance. During operation, tracking the performance ratio, cleaning modules and carrying out thermal inspections reduce yield losses.

A wind power plant converts the kinetic energy of wind into electricity through turbines and delivers it to the grid.

Each turbine is connected to medium voltage through its own generator and a step-up transformer, usually located at the tower base or in the nacelle. Turbines are linked to the plant substation by the MV collector system, from which power is delivered to the transmission or distribution grid. Site electrical works cover MV cables, the fiber optic communication line, turbine earthing and substation installation. Wind measurement, turbine layout and grid connection capacity are the main factors that determine project feasibility.

An active harmonic filter measures load harmonic currents and injects opposing currents to cancel them before they reach the grid.

In facilities with a variable and complex harmonic spectrum, it offers a more flexible solution than passive filters. It can filter several harmonic orders at once, and most models can also provide reactive power compensation and phase balancing. Current transformers must be positioned correctly, and filter capacity should be sized from measured harmonic currents. Because the upfront cost can be higher than passive solutions, a measurement-based assessment is recommended.

Apparent power combines active and reactive power; it equals V×I in single-phase and √3×V×I in three-phase systems and is expressed in kVA.

Transformers, generators and UPS units are rated in kVA because the current that heats the equipment is affected by both the active and reactive components. Active power (kW) is the part converted into useful work, while reactive power (kVAr) is the part that oscillates between grid and load to sustain magnetic and electric fields. A facility with a low power factor draws more kVA for the same kW and may need a larger transformer or cables. This relationship explains why compensation also frees up capacity. In the three-phase formula, V is the line-to-line voltage and I the line current; with harmonics present, apparent power also includes a distortion component and the simple power triangle no longer holds.

A detuning reactor is wired in series with a compensation capacitor to shift resonance below dominant harmonics and protect the capacitor.

In facilities with harmonics, capacitors without reactors can draw excessive current and resonate with the grid, amplifying harmonics. In a detuned design, the reactor is matched with the capacitor to a specific tuning frequency, which in common practice is kept below the 5th harmonic. This arrangement does not eliminate harmonics, but it keeps the compensation system operating safely and can reduce part of the harmonic current. The reactor and capacitor must be selected as a matched pair, with the capacitor's rated voltage chosen to allow for the voltage rise caused by the reactor.

Harmonics are components at integer multiples of the fundamental frequency (50 Hz) that distort current and voltage waveforms.

Non-linear loads such as variable frequency drives, UPS units, rectifiers, LED drivers and inverters generate harmonic currents. Harmonics can cause extra heating in cables and transformers, neutral conductor overload, capacitor failures and malfunction of sensitive equipment. Resonance between compensation capacitors and the grid can amplify their effect. The IEC 61000 series and IEEE 519 are common references for measurement and assessment.

Power factor is the ratio of active power to apparent power; the closer it is to 1, the less current is needed for the same active power.

Under sinusoidal conditions, power factor equals the cosine of the angle between current and voltage (cos φ). In systems with harmonics, the true power factor can be lower than cos φ, a distinction that matters in compensation design. A low power factor means higher current, increased losses, voltage drop and reactive energy charges. Compensation panels are used to bring the power factor close to its target value.

A power factor controller measures a facility's power factor and automatically switches capacitor steps in a compensation panel in or out.

The controller calculates the instantaneous reactive power demand from the current signal of a current transformer and the measured voltage. Target cos φ, step sizes, switching delays and the CT ratio must be set correctly during commissioning. A CT installed in the wrong place or with reversed polarity is a frequent cause of controller malfunction and reactive penalties. Advanced models also offer harmonic measurement, alarms and remote communication.

A reactive energy penalty is the extra charge billed when a consumer's reactive-to-active energy ratio exceeds regulatory limits.

Limits apply separately to inductive and capacitive reactive energy and are calculated from meter data for the billing period. Failed capacitors, a poorly set controller, growing loads or overcompensation during low-load hours are the most common causes. Regularly checking bills and meter data lets the problem be caught before it builds up over several periods. The lasting solution is a compensation system sized to the facility's load profile and properly maintained.

Reactive power oscillates between the grid and inductive or capacitive loads, does no useful work and is measured in kVAr.

Motors, transformers and ballasts draw inductive reactive power, while long cables, overcompensation and some electronic loads create capacitive reactive power. Reactive power causes extra current, losses and voltage drop in the network. For this reason, consumers connected to the distribution grid must keep their reactive energy, relative to active energy, within limits set by regulation. Exceeding those limits leads to reactive energy charges, which compensation systems are designed to prevent.

Reactive power compensation supplies a facility's reactive power locally with capacitors or reactors, cutting reactive energy drawn from the grid.

Capacitor banks are used where inductive loads dominate, and shunt reactors where a facility tends toward capacitive behavior. In stepped compensation panels, a power factor controller switches capacitor steps in and out as the load changes. Where harmonic loads are significant, detuned designs with filter reactors are preferred to keep capacitors out of resonance. Rapidly changing loads may require thyristor-switched or active compensation solutions.

THD expresses the combined effect of all harmonic components in current or voltage as a percentage of the fundamental component.

Voltage THD (THDv) reflects the quality of the supply voltage, while current THD (THDi) shows how much harmonic current the loads inject into the network. Measurements should be taken with a power quality analyzer over a period that covers the facility's different load conditions. Limits depend on the point of connection and the standard applied, with IEEE 519 and the IEC 61000 series as common references. High THD signals a possible need for harmonic filtering and a review of the compensation system.

A voltage sag (dip) is a drop in RMS supply voltage below 90% of the declared value, lasting from half a cycle (10 ms) to one minute.

EN 50160 sets the dip threshold at 90% of the declared voltage, while IEEE 1159 uses a range of 0.1–0.9 pu; if the voltage falls close to zero, the event counts as an interruption. Short-circuit faults in the network, starting of large motors or transformer energization can all cause voltage sags. Although brief, a sag can trip variable frequency drives, contactors and PLCs and bring production lines to a halt. Long-term measurements with a power quality analyzer reveal how often sags occur and how deep they are. Solutions include UPS units, dynamic voltage restorers, hardening of control circuit supplies and adjusting equipment settings.

An automatic transfer switch (ATS) moves loads to a backup source such as a generator when mains fails, and back to the grid when power returns.

The transfer panel continuously monitors mains voltage; when it detects an outage, it starts the generator and transfers the load once the voltage is stable. Mechanical and electrical interlocking that prevents the grid and generator from feeding the load at the same time is essential for safety. ATS systems are common in hospitals, data centers and facilities where outages mean lost production. Transfer time and the need for a break-free changeover may require the system to be designed together with a UPS.

An energy monitoring system collects consumption data from meters and analyzers, making a facility's energy use visible and manageable.

The system consists of measuring devices, a communication network and software that visualizes the data. Tracking consumption by line, machine or department makes it easier to spot waste and efficiency opportunities. Reactive energy and demand can be tracked alongside power quality parameters such as harmonics and voltage unbalance. It is the main data source for measurement and verification in energy management systems such as ISO 50001.

IEC 61850 is the international standard defining how substation protection, control and monitoring devices communicate using a common data model.

The standard makes it easier for relays and control devices from different manufacturers to work together in the same system. With GOOSE messaging, fast interlocking and trip signals between relays can travel over the network instead of hardwired cabling. It is increasingly common in digital substation applications and modernization projects. A successful implementation requires careful planning of the network architecture, configuration files and commissioning tests.

Modbus is a widely used open communication protocol, with serial (RTU) and Ethernet (TCP) variants, for meters, analyzers, inverters and PLCs.

Its simplicity and broad support make Modbus a frequent choice for collecting device data in energy monitoring systems and solar plants. Modbus RTU usually runs over an RS-485 line, while Modbus TCP runs over Ethernet. Because register maps differ from one manufacturer to another, accurate documentation is needed for integration. The protocol itself has no built-in security features, so network segmentation and access control are important.

An RTU (remote terminal unit) gathers breaker positions, measurements and alarms in the field, sends them to the SCADA master and executes commands.

RTUs are used mainly in geographically dispersed facilities such as substations and distribution centers. They have digital and analog inputs and outputs and can communicate with intelligent devices such as protection relays and meters. They may support protocols such as IEC 60870-5-101/104, DNP3, Modbus and IEC 61850. Ruggedness for harsh environments and time-stamped event recording are the main features that set RTUs apart from general-purpose PLCs.

SCADA (supervisory control and data acquisition) collects data from distributed field equipment for central monitoring and remote control.

SCADA is used in substations, solar and wind plants, organized industrial zone networks and factories. A typical architecture includes RTUs or PLCs in the field, a communication network, servers and operator screens (HMI). Alarm management, event logging, trend analysis and reporting enable fast fault response and efficient operation. Because these systems connect to corporate networks or the internet, cybersecurity measures must be an integral part of the design.

A smart grid uses sensors, communications and automation to monitor and manage generation, transmission, distribution and consumption in real time.

Smart meters, remote-controlled switching, fault location and automatic service restoration are typical smart grid components. With the growth of distributed generation such as solar and wind and of energy storage, the grid's ability to manage bidirectional power flow has become essential. For organized industrial zones and large industrial areas, SCADA and distribution automation investments are practical steps toward a smart grid. This transition also requires attention to cybersecurity, data management and standard protocols.

A synchronization panel matches voltage, frequency and phase angle to run multiple generators, or a generator and the grid, in parallel.

Parallel operation allows total power to be shared among several units as demand requires and provides redundancy. The controllers automatically manage active and reactive load sharing between generators. Taking some units offline when load drops saves fuel and extends engine life. Parallel operation with the grid must be designed to meet the distribution company's connection and protection requirements.

A busbar trunking system carries copper or aluminum bars in a metal enclosure as a prefabricated alternative to cables for power distribution.

In factories, data centers and high-rise buildings, busbar trunking is used as an alternative to parallel cable runs for carrying high currents. Plug-in tap-off boxes allow new outlets to be added without dismantling the line when machine layouts change. Selection takes into account rated current, short-circuit withstand, degree of protection and voltage drop. Joint torque values and alignment are critical during installation, and thermal imaging after commissioning is a common verification step.

A cable joint connects two power cables electrically and mechanically while restoring insulation and screen continuity.

Cable joints are used on long cable routes, where drum lengths are not sufficient, and in fault repairs. In MV joints, the conductor connection, stress control, insulation and metallic screen continuity must all be rebuilt with the same care as the original cable. Because joints are among the most common locations of underground cable faults, installation quality directly affects network reliability. Insulation testing before energizing a jointed circuit is good practice.

A distribution center (MV switching station) splits incoming medium-voltage power into several outgoing feeders, mainly without transforming it.

Distribution centers act as nodes that feed several transformer substations in organized industrial zones, large campuses and urban networks. They house incoming, outgoing, bus-coupler and metering cells, protection relays and often SCADA/RTU equipment for remote monitoring. A well-designed distribution center makes it easier to isolate only the faulted section while the rest of the network stays energized. In some installations the same building also contains a transformer for auxiliary supply or local consumption.

A generator set combines an internal combustion engine and an alternator to supply power during grid outages or where no grid exists.

In industrial facilities, generators are mostly used as standby power and are brought online through an automatic transfer panel when the grid fails. When several generators must run in parallel, a synchronization panel is used. Sizing should account for motor starting currents, harmonic-producing loads and future capacity growth. Regular load tests and fuel and battery checks are needed to make sure the generator actually starts when it is needed.

High voltage refers to transmission-level voltages above 36 kV; Turkey's transmission grid mainly operates at 154 kV and 380 kV.

In Turkey, the transmission grid is operated by TEİAŞ and power is carried mainly over 154 kV and 380 kV lines. Large power plants and very high-demand facilities may connect directly at transmission level, which requires an HV/MV step-down substation for consumers or an MV/HV step-up substation for power plants. Because some regulations and standards call any voltage above 1 kV "high voltage", the intended meaning should always be read from context. HV projects need dedicated planning for insulation coordination, protection schemes and the connection process with the transmission operator.

Load flow analysis calculates voltages, currents, power flows and losses across an electrical network under defined loading conditions.

When designing a new factory, solar plant or substation, load flow analysis checks whether equipment is overloaded and whether voltages remain within acceptable limits. It also helps verify reactive compensation needs, transformer tap settings and cable sizes. The study is usually run in dedicated software for several scenarios, such as full load, light load and backup supply. Together with short-circuit, selectivity and harmonic studies, it is one of the core components of power system analysis.

Low voltage is the AC voltage level up to 1000 V, at which electricity is distributed to end users in buildings and industrial plants.

In Turkey, LV networks typically operate at 400 V phase-to-phase and 230 V phase-to-neutral. The LV system starts at the transformer secondary and runs through the main distribution board, sub-distribution boards, busbar trunking and cables to motors, lighting and socket outlets. Sound LV design in a factory relies on cable sizing, voltage drop, short-circuit withstand and protection selectivity calculations. Residual current protection, earthing and periodic inspection form the basis of personnel safety in LV installations.

Medium voltage is the level above 1 kV up to and including 36 kV, used in distribution networks; 34.5 kV is the most common MV level in Turkey.

The most common MV level in Turkish distribution networks is 34.5 kV, although other levels such as 15.8 kV or 31.5 kV can still be found in some regions and older installations. Factories, organized industrial zones, solar and wind plants and large commercial buildings usually connect to the grid at MV and step the voltage down to LV in their own transformer substations. MV installations consist of specialist equipment such as switchgear, power transformers, MV cables, cable terminations and protection relays. For this reason, design, installation, testing and acceptance of MV work follow the regulations and the requirements of the distribution company.

An MV cable termination is the kit that safely ends a medium-voltage cable for connection to switchgear, a transformer or an overhead line.

Where the insulation screen is cut back at the cable end, the electric field concentrates; the termination controls this field and reduces the risk of partial discharge and breakdown. Indoor, outdoor and plug-in (separable connector) terminations are available, with heat-shrink and cold-shrink as the most common technologies. Small errors in screen cut-back length, semiconductive layer cleaning or moisture control can turn into failures months later. Installation is therefore carried out by trained crews following the manufacturer's instructions, followed by cable testing.

An MV collector system is the internal cable network that gathers power from wind turbines or solar inverter stations and carries it to the plant substation.

In wind farms, each turbine transformer is linked by MV cables to the other turbines and to the plant's collector or switching substation; in solar plants, inverter-transformer stations form a similar arrangement. Design considers cable size, losses, routing, burial depth and which sections can stay in service during a fault. A fiber optic cable for communications and an earthing conductor are often laid in the same trench. The system goes through cable testing and protection checks before it is energized.

An overhead line carries electricity through bare or covered conductors suspended on poles or towers by insulators.

Overhead lines are used in rural areas, over long distances and in transmission networks, usually at a lower initial cost than underground cables. Design work covers conductor size, pole or tower type and span, sag, wind and ice loads, and safety clearances. Because they are exposed to weather, vegetation contact and lightning, periodic maintenance and proper protection coordination are important. In cities and industrial areas, underground cable networks are often preferred for visual, safety and space reasons.

A power distribution board distributes LV power to motors, machines and sub-boards through protective devices.

The main distribution board is usually fed from the transformer's LV output and supplies the facility's sub-distribution boards. It contains air circuit breakers, molded-case circuit breakers, fuses, metering devices and a busbar system. Design considers rated current, short-circuit withstand, heat dissipation and maintenance access, with the IEC 61439 series as the main reference for LV switchgear assemblies. Loose connections are a frequent cause of panel failures, so thermal imaging is part of periodic maintenance.

Shore power lets berthed ships switch off their onboard generators and draw electricity from a grid connection on the quay.

Shore power is spreading in port areas to cut the emissions, noise and vibration caused by ships' auxiliary generators. Because a vessel's frequency or voltage may differ from the grid, the system can include frequency converters, transformers and dedicated connection boxes. Large ships are usually connected at medium voltage, while smaller vessels connect at low voltage. Quayside conditions make cable management, earthing and protection coordination critical design topics.

Short-circuit current is the fault current, far above normal load current, that flows when conductors at different potentials touch through low impedance.

Short-circuit calculations are used to set the breaking capacity of circuit breakers and fuses and the thermal and dynamic withstand of busbars and cables. The calculated value depends on the network's short-circuit power, the transformer rating and impedance voltage, cable lengths and any connected generators or motors. Equipment with insufficient rating may fail to interrupt a fault safely, causing serious damage and fire risk. Protection relay settings and selectivity studies are also based on short-circuit analysis.

A single-line diagram is the key drawing that shows a power system's transformers, switchgear, panels, cables and protection using one line for three phases.

A single-line diagram shows at a glance how power flows from the source to the loads and where each protective device sits. Design approval, short-circuit and selectivity studies, and operation and maintenance work all rely on it. Recording every site modification on the diagram is essential for safe switching and fault response. Keeping an up-to-date single-line diagram displayed in a substation is good operating practice.

An uninterruptible power supply (UPS) is a power electronics system that keeps critical loads running on battery energy when mains power fails or degrades.

Loads that cannot tolerate interruption, such as data centers, hospitals, control rooms and process automation, are supplied through a UPS. Online (double-conversion) UPS units feed the load continuously through the inverter, also filtering voltage disturbances. A UPS usually works together with a generator: the UPS bridges the first moments, and the generator provides long-term supply once it is running. Sizing depends on load power, required backup time, battery technology and the level of redundancy, such as N+1.

An XLPE cable is a power cable insulated with cross-linked polyethylene, widely used in MV and LV underground networks.

Cross-linked polyethylene (XLPE) insulation has become standard in modern MV cables thanks to its good dielectric properties and high operating temperature rating. An MV XLPE cable is built from the conductor, conductor screen, insulation, insulation screen, metallic screen and outer sheath. Cable life depends largely on correct laying, respecting the bending radius and, above all, the quality of terminations and joints. After installation, tests such as VLF, tan delta and partial discharge measurement confirm the condition of the insulation.

A call letter is issued by the distribution company to an approved unlicensed-generation applicant, opening the way to a connection agreement.

A call letter confirms that the technical evaluation of the application was positive and that a connection point has been assigned. The investor must complete the required documents and apply for a connection agreement within the period set by regulation. Missing these deadlines can cause the application to lapse. After this stage, the process continues with design approval, installation and acceptance.

A distribution system operator (DSO, called EDAŞ in Turkey) is the licensed operator of a region's MV and LV grid, handling connections and network services.

Turkey is divided into 21 distribution regions, each served by one DSO. MV or LV connections for new facilities, unlicensed solar applications, and connection and system use agreements are all handled through the DSO. The DSO is also responsible for fault and outage management, metering and network investment. The incumbent retail companies that sell electricity are separate legal entities from the DSOs.

Design approval is the review and sign-off of an electrical installation's detailed design by the competent authority for regulatory compliance.

Designs for substations, MV connections, solar plants and building installations go through approval before construction begins. The design package includes the single-line diagram, short-circuit and voltage drop calculations, protection and earthing design, and equipment lists. Installations built without an approved design may face problems at acceptance or may not be energized at all. Depending on the facility type, the approving body may be the distribution company, TEDAŞ, an organized industrial zone or another public authority.

EMRA (EPDK in Turkish) is Turkey's independent regulator for the electricity, natural gas, petroleum and LPG markets.

EMRA holds the authority to issue licenses, approve tariffs and issue market regulations in the electricity sector. Core rules on unlicensed generation, grid connection and system use, organized industrial zone distribution licenses and consumer services are set by this authority. Day-to-day applications are mostly handled through distribution companies and TEİAŞ, but the rules come from EMRA regulations. Because the regulations change frequently, current rules should be checked before starting a project.

Offsetting (net metering) nets the energy an unlicensed plant produces against the energy drawn from the grid over defined time intervals.

When generation and consumption are matched within the same interval, self-consumed energy is deducted from the bill and surplus generation is treated according to regulatory conditions. The offsetting period, for example monthly or hourly, directly affects project economics; with hourly offsetting, night-time consumption cannot be netted against daytime generation. For this reason, how well the load profile matches the generation profile is an important part of the feasibility study. The applicable rules can vary by facility type and current regulations.

An OIZ distribution license is the EMRA authorization that lets an organized industrial zone run electricity distribution within its boundaries.

A licensed organized industrial zone (OIZ) operates the MV and LV network within the zone, manages tenant connections and plans network investments. This model lets the OIZ develop its energy infrastructure around industrial users' needs, while also bringing technical and administrative obligations. The licensing process requires regulatory compliance in areas such as network inventory, technical staff and the operation and maintenance organization. For OIZs without a license, how distribution service is provided depends on the regulations and local arrangements.

Provisional acceptance is when a committee checks a completed installation against its design and regulations and approves it for operation.

Before acceptance, an acceptance file is prepared with the approved design, test reports, material certificates and measurement results. The committee inspects the substation, switchgear, earthing, labeling and safety measures on site, and grants time to correct any deficiencies. After provisional acceptance, the facility's performance in operation is monitored, and final acceptance follows once the conditions set by regulation are met. Substations and distribution facilities follow Turkey's Regulation on Acceptance of Electrical Installations (Elektrik Tesisleri Kabul Yönetmeliği), while generation and storage plants such as solar, wind and BESS follow the Regulation on Acceptance of Electricity Generation and Storage Facilities (Elektrik Üretim ve Elektrik Depolama Tesisleri Kabul Yönetmeliği).

TEDAŞ is the state-owned company that owns Turkey's distribution assets, now operated by private companies, and takes part in acceptance and audits.

After distribution operations were transferred to regional private distribution companies, TEDAŞ retained ownership of the network assets and certain oversight duties. Provisional acceptance of transformer substations and distribution facilities is the process most often associated with TEDAŞ on site. Its technical specifications for materials and equipment are used as a reference for facilities connecting to the distribution grid. Which step is handled by TEDAŞ, the distribution company or another body can vary with the facility type and current regulations.

TEİAŞ is the state-owned company that operates Turkey's high-voltage transmission grid, balances the system and handles transmission connections.

Turkish legislation classifies voltage levels above 36 kV as transmission, and TEİAŞ is responsible for this transmission system, chiefly the 380 kV and 154 kV lines and the transmission substations. Through its National Load Dispatch Center, it balances generation and consumption in real time. Connection opinions and agreements for large power plants and facilities connecting at transmission level are handled with TEİAŞ. Even for distribution-level projects, assessments of transmission capacity may depend on TEİAŞ's opinion.

Unlicensed generation lets producers build plants, mostly for self-consumption, without an EMRA generation license, under conditions set by regulation.

Rooftop solar systems and ground-mounted solar plants linked to a consumption facility are the most common cases under this regime. Applications are made to the relevant distribution company; approved projects receive a call letter, followed by design approval, a connection agreement, installation and acceptance. Rules such as capacity limits, the requirement to be linked to consumption and the treatment of surplus energy can change with regulatory amendments. The current version of the regulation on unlicensed electricity generation should therefore be checked before applying.

A circuit breaker is a switching device that can make and break load current and automatically interrupt fault currents such as short circuits.

Vacuum and SF6 breakers are common in MV systems, where the breaker operates on a trip command from a protection relay. On the LV side, air circuit breakers and molded-case circuit breakers usually protect the circuit with their own built-in trip units. Selection considers rated current, breaking capacity and mechanical and electrical endurance. What sets a breaker apart from a disconnector is its ability to open safely under load and during a fault.

A dry-type transformer has windings insulated with resin, typically cast epoxy, instead of oil, and is cooled by air.

With no oil inside, dry-type transformers carry a low risk of fire and leakage, which is why they are often chosen for hospitals, shopping centers, data centers and indoor substations. Their initial cost is usually higher than an oil-immersed unit of the same rating, and they are not suited to direct outdoor installation. They need a well-ventilated transformer room and a protection system that monitors winding temperature. Because dust, moisture and pollution can affect the insulation, periodic cleaning and inspection matter.

Earthing intentionally connects conductive parts of an electrical installation to the ground to protect people and equipment.

System (neutral) earthing connects the neutral point of the supply to ground, while protective earthing connects exposed metal parts that are normally not live. Proper earthing gives fault current a safe path, lets protective devices trip in time and limits dangerous touch voltages. Substations, solar plants and wind farms typically use ring or mesh earthing systems. In Turkey, design and measurement principles are set by the Regulation on Earthing in Electrical Installations (Elektrik Tesislerinde Topraklamalar Yönetmeliği).

Equipotential bonding connects simultaneously accessible metal parts so that no dangerous voltage difference can arise between them.

The main equipotential bonding bar joins the earthing conductor with conductive systems such as water and gas pipes, structural steel and cable trays at a single point. Supplementary bonding can be added in wet locations, machine groups and data centers. Unlike earthing, the goal is not low resistance to ground but equal potential between parts that can be touched. Continuity of the bonding connections is measured and verified during periodic inspections.

A kiosk substation is a compact unit housing the transformer, MV switchgear and LV board in a factory-built concrete or metal enclosure.

Compared with building-type substations, kiosk substations need less civil work on site and can be installed faster. They are common in residential areas, solar plants, construction sites and medium-sized industrial facilities. Selection takes into account transformer rating, ventilation, internal arc classification, maintenance access and foundation preparation. Once placed on site, cable connections, earthing and testing are completed before the acceptance process begins.

A lightning protection system safely conducts lightning current to earth through air terminals, down conductors and an earthing system.

External lightning protection consists of air terminals or meshes, down conductors and the earthing system. Internal protection uses equipotential bonding and surge protective devices (SPDs) to protect sensitive equipment. The protection class is chosen according to the structure's risk level, with the IEC 62305 series as the main reference. Lightning protection is an integral part of design, especially for tall structures and for solar and wind plants in open terrain.

A load break switch can make and break normal load current but cannot interrupt short-circuit current on its own; it is often paired with fuses.

Because it costs less than a circuit breaker, the load break switch is widely used in distribution networks and in transformer cells of modest rating. In a switch-fuse combination, the MV fuse clears the short-circuit current while the switch handles normal switching. It is interlocked with an earthing switch so the circuit can be safely isolated and earthed before maintenance. For larger ratings or where sensitive protection is required, a circuit-breaker cell is preferred.

An MV switchgear panel (cubicle) is a metal-enclosed unit housing breakers, switches and metering to switch, protect and measure medium-voltage circuits.

In substations and distribution centers, different cell types for incoming, outgoing, metering, bus coupling and transformer protection are lined up side by side. Cells are mainly air-insulated (AIS) or gas-insulated (GIS); solid-insulated (SIS) types also exist, and modular cells and ring main units (RMUs) are common. IEC 62271-200 is the core standard for metal-enclosed MV switchgear, and its internal arc classification is important for personnel safety. Selection is driven by rated voltage and current, short-circuit withstand, interlocking scheme and the need for remote control.

An oil-immersed transformer has its core and windings submerged in insulating oil, which provides both insulation and cooling.

Oil-immersed units are the most common transformer type in distribution networks, kiosk and pole-mounted substations and outdoor applications. They handle overloads well and are usually more economical than a dry-type unit of the same rating. Fire and oil-leak risks call for an oil collection pit, suitable fire precautions and proper clearances. Oil analysis and, depending on the design, a Buchholz relay or a hermetic transformer protection device give early warning of developing problems.

A pole-mounted substation places a relatively small transformer on an overhead line pole as an economical distribution solution.

Pole-mounted substations serve low-power consumers fed from overhead lines in rural settlements, agricultural irrigation points and small businesses. The transformer sits on a platform on the pole, with devices such as a disconnector, fuses and surge arresters on the MV side and a metering and distribution panel on the LV side. Although economical to build, their capacity is limited and they are exposed to outdoor conditions. Facilities that need more power or higher reliability move to a kiosk or building-type substation.

A protection relay continuously monitors current and voltage and sends a trip command to the circuit breaker when it detects a fault.

Overcurrent, earth fault, differential, voltage and frequency protection are the most common relay functions. Modern digital (numerical) relays combine several protection functions in one device, record events and can communicate with SCADA. Replacing old electromechanical relays with digital ones is a typical step in substation modernization. For the relay to perform correctly, its settings must come from a selectivity study and be verified by testing during commissioning.

Selectivity ensures only the protective device closest to a fault trips, keeping the rest of the installation energized.

In a non-selective system, a minor fault on a single machine can trip the main breaker and stop the entire factory. Selectivity is achieved by comparing the time-current curves of relays, breakers and fuses, using current-based, time-based or energy-based methods. The study relies on short-circuit calculations and is usually carried out in power system analysis software. Whenever a new load or a new source such as a solar plant is added, coordination should be reviewed.

A residual current device (RCD) detects the difference between phase and neutral currents and trips within milliseconds when current leaks to earth.

Devices rated at 30 mA are generally used for personal protection, while higher trip ratings are used for fire protection. RCDs are a key safety element in socket circuits, wet locations and construction site panels. Loads with pulsating DC components need at least Type A, single-phase variable frequency drives need Type F, and loads that can produce smooth DC leakage, such as three-phase VFDs, need Type B. Regular use of the test button and periodic trip-time measurements confirm that the device actually works.

Step voltage arises between a person's feet and touch voltage between hand and feet during an earth fault; both can be dangerous.

When an earth fault occurs in a substation, the fault current spreads into the soil and creates potential differences across the ground surface. One of the main goals of earthing design is to keep these voltages below the limits that are safe for people. Mesh earthing grids, potential grading rings and high-resistivity surface layers such as crushed rock are used for this purpose. In large installations, step and touch voltages can also be verified by measurement.

A transformer substation combines a transformer, MV switchgear and LV panels to convert power between MV and LV, usually stepping it down for a facility.

Factories, industrial zone plots, hospitals and large commercial buildings usually connect to the grid through their own transformer substations. In solar and battery storage (BESS) plants, the substation instead steps the LV output of inverters or PCS units up to MV for export to the grid. Substations can be built as indoor (building-type), kiosk (prefabricated) or pole-mounted installations. A typical substation includes incoming, metering and transformer protection cells, one or more power transformers, an LV main distribution board, a compensation panel and an earthing system. The facility is not energized until design approval, installation, testing and acceptance are complete.

Earth resistance measurement checks the resistance of an earthing system to ground to confirm it can safely carry fault current.

The most common technique is the fall-of-potential method using auxiliary current and potential electrodes, though other methods are used for large systems. Because results vary with soil moisture and season, measurement conditions are recorded in the report. In Turkey, earthing measurements are carried out periodically under the earthing regulation and occupational health and safety legislation, and reported by authorized persons. Values above the limits indicate that the earthing system needs to be improved.

Infrared thermography images equipment surface temperatures to find loose connections and hot spots without contact.

The inspection is carried out with the installation energized and under load, so signs of failure can be caught without stopping production. Panel connections, breakers, busbar joints, cable terminations and transformer connections are the most frequently inspected points. Temperature differences are assessed against the load level at the time of measurement and by comparison with similar phases. Findings are reported by priority and corrected during planned maintenance.

An insulation resistance test applies a high DC voltage to measure, in megohms, the integrity of insulation in cables, motors, transformers or panels.

The test voltage is chosen according to the equipment's rated voltage, typically 500 V or 1000 V for LV installations and higher values for MV equipment. A low reading can point to moisture, contamination, mechanical damage or aged insulation. Time-based measurements such as the polarization index (PI) give a better picture of insulation condition, especially in motors and transformers. It is one of the basic tests performed before commissioning and during periodic maintenance.

Partial discharge measurement detects small electrical discharges at voids or defects in insulation, catching problems before they cause failure.

Partial discharges erode insulation over time and can eventually lead to breakdown, which makes them a valuable early warning sign. The measurement is used to find installation defects in MV cable terminations and joints and for condition monitoring of MV switchgear and transformers. It can be done offline with a test source or online during operation using sensors. Locating the discharge source allows repairs to be targeted precisely.

Periodic inspection examines an operating electrical installation, its earthing and its protection systems at set intervals and reports on their safety.

In Turkey, periodic inspections of electrical installations are carried out by authorized persons under occupational health and safety legislation. The scope includes earthing measurements, RCD tests, panel and connection checks, the lightning protection system and, where needed, thermographic inspection. Non-conformities are listed in the report, and correcting them is the operator's responsibility. Regular inspection both fulfils the legal obligation and reduces the risk of unexpected failures and accidents.

A power quality analyzer records voltage, current, power, power factor, harmonics and voltage events over time.

Portable analyzers are usually connected for a week or longer so that different shifts and load conditions are captured. The data provides the basis for compensation sizing, harmonic filter decisions, transformer loading checks and energy efficiency studies. Permanently installed analyzers in switchboards feed continuous data to energy monitoring and SCADA systems. IEC 61000-4-30 is a widely used reference for power quality measurement methods.

Relay testing injects simulated fault currents and voltages into a protection relay to verify it trips correctly and on time.

In secondary injection, the current and voltage transformer secondary circuits are isolated and test signals are applied directly to the relay inputs. The test checks trip times, setting values, logic functions and the trip circuit to the circuit breaker. Primary injection testing is used to verify the entire protection chain, including the current transformers. Relay tests during commissioning and periodic maintenance provide assurance that protection will operate in a real fault.

A tan delta test measures dielectric losses to assess the quality and ageing of insulation in cables, transformers or bushings.

In ideal insulation the current leads the voltage by 90°; as insulation deteriorates, the loss component grows and tan δ rises. On MV cables, the measurement is usually made with a VLF source at several voltage levels, and the change of the value with voltage is also assessed. It is useful for detecting common ageing problems such as water treeing. The results help set priorities for maintenance and cable replacement.

Commissioning is the process of testing, setting and energizing a completed electrical installation so it operates safely and as designed.

The process usually starts with de-energized checks, such as visual inspection, cable and insulation tests and relay tests, and continues with staged energization and on-load functional tests. In substations, solar plants and BESS sites, the equipment manufacturer, the contractor and the client's representatives often take part together. All test results are documented and become part of the acceptance file. Well-executed commissioning significantly reduces failures in the first months of operation.

A TTR test measures a transformer's primary-to-secondary turns ratio to verify nameplate values, tap positions and winding integrity.

The ratio is measured separately for each tap position and each phase, and the results are compared with nameplate values. Deviations can point to shorted turns, an incorrect vector group or tap changer problems. In pre-commissioning testing it is usually performed together with winding resistance, insulation resistance and vector group checks. Repeating it after the transformer is transported or after a fault is recommended.

A VLF test applies high AC voltage at very low frequency, typically 0.1 Hz, to MV cables to reveal insulation weaknesses.

Thanks to the low frequency, the high capacitance of long cables can be energized with a much smaller, portable test set than a power-frequency system would require. VLF testing is preferred over high-voltage DC testing for XLPE cables, because DC testing can leave space charges in the insulation and damage the cable. It is typically applied to newly laid cables, after terminations and joints are installed, and after repairs. Combined with tan delta and partial discharge measurements, it gives a fuller picture of cable condition.

A site survey and quantity takeoff inspects the site and quantifies the materials and work items in a design to estimate project cost.

During the survey, existing infrastructure, routes, access conditions and connection points are inspected on site. The quantity takeoff calculates items such as cable lengths, numbers of panels and switchgear cells, and quantities of trunking and cable trays from the design. An accurate survey and takeoff makes tender documents and bids comparable and reduces unexpected costs during the work. On public and large private projects it may be prepared using standard unit price item codes.

A technical specification defines the properties of materials and equipment, the applicable standards and the test and acceptance conditions for a project.

A well-prepared specification allows bids from different suppliers to be compared on the same technical basis. Equipment ratings, degrees of protection, documentation and certification requirements, and factory and site tests are stated clearly. Vague wording can lead to disputes and quality differences during execution. For facilities connecting to the distribution grid, the specifications of the relevant authorities must also be taken into account.

In turnkey (EPC) contracting, a single contractor handles engineering, procurement and construction and hands over a working facility.

EPC stands for Engineering, Procurement and Construction. For the client, it reduces the number of interfaces and places responsibility with a single party. On large solar, wind and BESS projects, the main EPC contractor may subcontract disciplines such as electrical works to specialists, in which case the boundaries of responsibility must be clearly defined in the contract. The scope of delivery usually includes testing, commissioning and acceptance.