Increasing a Factory's Power Capacity in Türkiye: Replace the Transformer or Add a Parallel Unit?

A factory needs a power capacity increase when measured peak demand in kVA approaches the transformer rating or the capacity in its connection agreement, or when planned machinery would push demand beyond either limit. The decision should rest on measured load, not on adding up nameplate ratings, which ignore diversity and usually overstate real demand.

Two data sources are used together. The first is the historical load profile from the utility meter: most medium-voltage customers in Türkiye are covered by the automated meter reading system (OSOS), which can provide load-profile data, typically at 15-minute intervals, that shows seasonal peaks. The second is a recording with a power quality analyzer on the LV main incomer over at least one full production week, capturing active and reactive power, power factor, total harmonic distortion (THD) and phase unbalance.

Measurement sometimes shows that capacity is not the real problem. With active power unchanged, raising power factor from 0.85 to 0.98 cuts apparent power by roughly 13 percent, headroom that a compensation upgrade can recover on the existing transformer. High harmonic content raises transformer losses and temperature, so top-oil temperature records on oil-immersed units, or winding temperatures on dry-type units, should be read alongside the load data. The new load list should then give kW, power factor, duty cycle and starting method for each large motor.

For a factory supplied from the MV grid, the application goes to the regional electricity distribution company (DSO). For a factory in an organized industrial zone (OIZ), it goes to the OIZ if the OIZ holds a distribution license for the zone, and otherwise to the regional DSO. The process typically moves through application, connection opinion, design approval, installation, acceptance and updated agreements. The DSO or OIZ handling the application first checks whether the supplying feeder and its upstream substation can carry the additional load; if not, network reinforcement or a different supply point may come into play.

After a positive opinion, the substation revision design is prepared by an authorized electrical engineer and submitted for approval. It covers the single-line diagram, transformer and switchgear selection, and the protection, short-circuit, compensation and grounding calculations. Because current transformer ratios in the metering panel change with the new capacity, the meter and metering circuit arrangement are part of the design as well. Once installation is complete, the facility goes through acceptance and the capacity in the connection and system use agreements is updated.

In an OIZ that holds its own distribution license, the process also depends on the zone's technical specification and on spare capacity in its main substation; some OIZs prescribe the MV switchgear type or protection relays above certain ratings. Obtaining that specification at the application stage avoids design revisions later. Our OIZ electrical infrastructure article covers the initial connection process.

Replacing the transformer keeps the plant on a single-transformer configuration with a simple LV side, while adding a parallel unit splits capacity across two transformers for redundancy and the option to switch one off at light load, at the cost of more switchgear, panels and space. The right choice depends on room or kiosk space, expansion plans, load criticality and the LV short-circuit level.

The most overlooked consequence of upsizing is a higher LV short-circuit current. With the same impedance voltage (uk), the short-circuit current at the transformer's LV terminals rises roughly in proportion to its rating: moving from 1600 kVA to 2500 kVA raises it by about 56 percent. The breaking capacity of the main and outgoing breakers and the short-time withstand rating of the busbar system must be checked against the new value. Room dimensions, ventilation and, for oil-immersed units, the oil containment pit must also suit the new transformer.

A parallel unit can be arranged in two ways. If the LV busbar is coupled and the transformers run permanently in parallel, short-circuit current nearly doubles and all LV switchgear must be rated for it. If the busbar is split into two sections with a normally open bus coupler, the fault level does not rise; when one transformer is out of service, the coupler closes and critical loads move to the other unit. In that arrangement the two incomers and the coupler are interlocked so that all three can never be closed at once. Our comparison of dry-type and oil-immersed transformers covers the insulation choice.

Transformers can run in parallel only if they share the same vector group, have equal voltage ratios and closely matched impedance voltages, and their rated powers should not be too far apart. The vector group, for example Dyn11 on both units, is an absolute requirement: paralleling units with different phase displacement produces a circulating current that behaves like a short circuit between the secondaries.

Even small ratio differences drive circulating current at no load, so ratios must match very closely, in practice within about half a percent, with tap changers on the same position. If impedance voltages differ, the unit with the lower uk carries a larger share of the load; keeping the difference within roughly 10 percent is the usual target. A rating ratio no greater than 1:2 keeps load sharing balanced. IEC 60076-8, the application guide for power transformers, covers these conditions in detail. The new unit's specification should quote these values from the existing transformer's nameplate and test report.

Before paralleling, the new transformer receives ratio, winding resistance and insulation resistance tests, plus a dielectric strength test on the oil for oil-immersed units. Phase sequence and phase matching are then checked between the two LV outputs: voltage is measured across corresponding phases, and the coupler is not closed until near-zero readings confirm the match. After paralleling, circulating current is measured at no load and the load split between the transformers is recorded under load.

Switchgear, panels, cables and compensation must be recalculated because a new transformer rating changes rated currents, fault currents, protection settings and reactive power demand all at once; swapping only the transformer leaves the rest of the installation unprepared. On the MV side, fuse ratings or relay settings in the transformer protection panel are reselected for the new rating and its inrush current, since protection left at the old settings can trip on a larger transformer's energization. Some distribution companies and OIZs require circuit-breaker and relay protection instead of a fused switch-disconnector above certain ratings.

On the LV side, the main breaker's rated current and breaking capacity, busbar cross-sections and the panel's thermal capacity are checked. Parallel cables from the transformer to the main panel are resized for the new current, or replaced with busbar trunking. Selectivity is reworked so that a fault on one outgoing circuit does not black out the whole factory. Voltage drop and the voltage dip during large motor starts are part of the cable calculation too.

Three compensation items are often missed. A larger transformer draws more no-load reactive power, so the fixed capacitor bank that covers it must be resized. Stepped compensation capacity must grow with the new loads, with detuned reactors where harmonic content is high. And when the LV main current transformer changes, the CT ratio and settings in the power factor controller must be updated; otherwise compensation misbehaves and reactive power penalties become a real risk. Finally, neutral and frame grounding of the new transformer are connected and grounding resistance is measured again.

The upgrade avoids a production stop when most of the work is completed in advance while the existing system stays live, and the changeover itself is compressed into one or a few short, planned outages. The new transformer is set in place and tested, new LV panel sections are built and installed, and cables are pulled and terminated, so that only connection and commissioning remain for the outage window.

A parallel unit has a clear advantage here: the new transformer is energized with its own LV section, and loads are moved across group by group during short outages at weekends or shift changes. A replacement needs a single, longer window to remove the old unit and connect the new one, usually timed to a public holiday or the annual maintenance shutdown, with a temporary generator for critical loads if needed. MV switching is carried out under the distribution company's or OIZ's switching program, so dates must be agreed in advance and the acceptance and energization procedure for the new transformer built into the schedule.

The critical check at changeover is phase sequence: a wrong sequence reverses motors, so it is verified before any load is transferred. During the first production week, transformer and panel connections are scanned with a thermal camera under load, and temperature and load data are logged.

Miratek Elektrik delivers transformer substations, MV switchgear, LV panels and compensation together for factories and industrial plants across Türkiye, in the 1–35 kV range. To date we have installed 120 MVA of total transformer capacity, completed 48 MV switchgear panel installations and manufactured more than 340 panels. Through our Factory Electrical Infrastructures service, we run a capacity increase from load measurement to the changeover plan as a single technical point of contact.