An LCI transformer is a power transformer engineered specifically to serve load-commutated inverter (LCI) drive systems and other large thyristor-based converters. This article explains what an LCI transformer does, why its design differs from a standard power transformer, how to select one, installation and testing considerations, and real-world application examples.

Definition and role of an LCI transformer

An LCI transformer provides the AC voltage source and electrical interface between the supply network and a load-commutated inverter. It isolates the converter from the grid, adapts voltages to the converter requirement, supplies the required phase relationships (including optional phase-shifting), and provides controlled leakage reactance to help with commutation and harmonic behavior.

How LCI drives use the transformer

Load-commutated inverters are line-commutated thyristor converters that rely on the AC system's voltage waveform for commutation. The LCI transformer supports the converter by:

• Providing correct phase-to-phase voltages and multiple secondary windings for separate converter bridges or auxiliary supplies.
• Introducing designed leakage reactance that limits short-circuit currents and improves commutation margins for the thyristors.
• Creating phase shifts (via delta/star or dedicated phase-shift windings) to reduce specific harmonic components injected into the grid.
• Providing galvanic isolation and grounding arrangements required for safe converter operation and protective systems.

Key design features of an LCI transformer

LCI transformers differ from general-purpose distribution transformers in several important ways; these differences are driven by the converter’s electrical demands and the need to control harmonics, inrush, and fault currents.

Winding configuration and phase shifting

Typical configurations include Y/Δ, Δ/Y or multi-winding arrangements that allow the converter to be supplied by one or more secondary voltages. Phase-shifted windings (e.g., ±15°, ±30°) are often implemented to enable harmonic cancellation between parallel bridges or to reduce the magnitude of certain triplen and non-triplen harmonics.

Leakage reactance and impedance control

Manufacturers engineer specific short-circuit impedances (expressed as % impedance or reactance) to limit fault currents and provide the necessary commutation impedance for thyristor gates. Too low impedance risks excessive fault currents and poor commutation; too high impedance increases voltage drop and heating.

Cooling, insulation and thermal design

Because converters produce significant harmonics and non-sinusoidal currents, LCI transformers often face elevated eddy and stray losses. Designs therefore use enhanced cooling systems (ONAN, ONAF, or forced oil/air), higher-grade core laminations, and insulation systems rated for higher dV/dt and peak stresses.

Selecting an LCI transformer: practical checklist

Use the following checklist to ensure the transformer you specify fits the converter and site requirements.

• Rated power (kVA) with margin for converter overloads and reactive losses.
• Primary and secondary nominal voltages and required tap-changer range for voltage regulation.
• Short-circuit impedance specified to optimize commutation and limit fault currents.
• Winding configuration and phase shift options for harmonic mitigation between converter bridges.
• Cooling method (ONAN/ONAF/OFWF) matched to expected continuous losses and ambient conditions.
• Insulation class, surge withstand capability, and dV/dt tolerance appropriate for thyristor switching transients.
• Auxiliary windings for control circuits, DC supply (if needed) and earthing/grounding schemes.
• Harmonic filters or reactor integration if required to meet grid code limits.

Typical technical specification table

Installation and pre-commissioning checks

Proper installation and commissioning minimize early failures. Key steps include oil and insulation tests, winding resistance and ratio tests, ground and neutral checks, verification of tap-changer operation, and coordination checks with converter protection.

• Transformer turns-ratio and polarity test to confirm correct phase relationships for the converter.
• Dielectric tests (power-factor, insulation resistance, and applied-voltage tests) to validate insulation integrity.
• Short-circuit impedance measurement and verification against design values.
• Mechanical checks for proper mounting, cooling fans/pumps and conduit routing for converter cabling.

Common maintenance and troubleshooting issues

Because converters inject harmonics and can create DC offsets, LCI transformers show a distinct set of issues. Regular monitoring and preventive maintenance reduce unscheduled downtime.

• Overheating due to harmonic-induced stray losses — monitor oil temperature and top-oil rise.
• Accelerated insulation aging from repetitive dV/dt and high peak voltages — track dissolved gas analysis (DGA) and insulation resistance.
• Vibration and mechanical noise from magnetostriction at harmonic frequencies — check core clamping and magnetic joints.
• Grounding or neutral shifts when converter control or blocking devices malfunction — verify earthing connections and neutral reactors if fitted.

LCI transformer vs. PWM converter transformers

Pulse-width modulation (PWM) converters have different harmonic spectra and switching characteristics compared to LCIs. Transformers used with PWM inverters tend to emphasize dV/dt immunity, lower leakage reactance for dynamic response, and different filter integration. LCI transformers, by contrast, prioritize controlled commutation impedance and phase-shift windings for harmonic cancellation.

Typical applications and case examples

LCI transformer systems are widely used where robust, high-power thyristor converters are chosen for large synchronous motor drives or HVDC light/medium links. Common industries:

• Steel rolling mills and heavy-metal processing where large synchronous motors require steady torque and simple, rugged converters.
• Ship propulsion systems and large marine drives where reliability and robustness are paramount.
• Large pumps and compressors in water treatment and oil & gas where high-power, line-commutated drives are cost effective.

Final recommendations for engineers and specifiers

When specifying an LCI transformer, coordinate early with the converter vendor to obtain converter current waveforms, harmonics, expected DC offsets, and protection settings. Request manufacturer data for stray-loss estimates and thermal ratings under non-sinusoidal loading. Include harmonic filtering and neutral/reactor arrangements in the electrical scope to ensure compliance with grid codes and long-term transformer reliability.

If you want, provide the converter rated current, nominal voltages, desired phase-shift scheme, and the ambient conditions — with those details a detailed transformer specification (kVA, impedance, cooling, winding arrangement and insulation class) can be drafted.