What Are On Load Tap Changers?

On load tap changers (OLTCs) are mechanical or electronic switching devices integrated into power transformers that allow the transformer's turns ratio to be adjusted while the transformer remains energized and carrying load current. By selecting from a set of tapped winding positions, an OLTC alters the voltage ratio between the primary and secondary windings in discrete steps, typically ranging from a fraction of a percent to several percent per step, enabling the output voltage to be regulated to within a specified tolerance despite variations in load current and upstream supply voltage. OLTCs are a foundational element of voltage regulation in transmission and distribution systems, drawing on power engineering, electrical machine design, and control systems. Their operation is governed by standards including IEC 60076-21 and related IEEE standards, which specify mechanical endurance, dielectric requirements, and permissible voltage step sizes.

The distinction between an on-load tap changer and an off-circuit tap changer (sometimes called a de-energized tap changer) is that OLTCs are designed to interrupt current during the tap-change operation through an internal diverter switch or vacuum interrupter, whereas de-energized tap changers require the transformer to be shut down and isolated before any position change. This capability makes OLTCs essential for continuously regulated systems where load voltage must remain within bounds at all times.

Operating Principles and Mechanical Design

A conventional mechanical OLTC consists of a selector switch, which moves between tap positions on the regulating winding, and a diverter switch, which interrupts and transfers the current between adjacent tap positions without arc damage to the selector contacts. The diverter switch operates through an energy storage mechanism, typically a spring, that drives the transition fast enough to prevent sustained arcing. During the brief commutation interval, a transition impedance, either a resistor or a reactor, is inserted in the circuit to limit the circulating current that flows momentarily between two different tap voltages. OLTCs are immersed in transformer insulating oil, which serves as both dielectric medium and arc-quenching fluid; the products of arc quenching degrade the oil over time, making dissolved-gas analysis and oil sampling key maintenance practices. The Reinhausen Group's tap changer knowledge base provides detailed technical descriptions of diverter-switch and selector-switch OLTC configurations as used in transmission-class transformers.

Electronic and Solid-State OLTCs

Electronic on-load tap changers replace the mechanical diverter switch with power semiconductor switches, typically thyristors or insulated-gate bipolar transistors (IGBTs), to perform the current commutation between tap positions. Because semiconductor switches can operate in microseconds rather than the tens of milliseconds required by mechanical devices, electronic OLTCs offer faster voltage regulation response and theoretically unlimited switching endurance, removing the mechanical wear that limits conventional OLTCs to a finite number of operations before maintenance is required. IEEE Transactions research on electronic OLTC design for automatic voltage regulation in distribution transformers describes a full implementation including the control algorithm, the semiconductor commutation circuit, and test results for dynamic voltage regulation performance. Electronic OLTCs are particularly attractive for distribution transformers in grids with high penetration of variable renewable generation, where rapid and frequent voltage fluctuations exceed the operating life of conventional mechanical designs. IEEE research on electronic OLTC transformers for DC railway power supply systems documents an application where solid-state switching provides the speed and reliability demanded by traction power infrastructure.

Automatic voltage regulator (AVR) control systems close the loop around an OLTC by measuring the secondary voltage, comparing it to a setpoint, and commanding tap position changes when the measured voltage falls outside a dead-band. Inverse time-delay logic prevents unnecessary tap operations from small, transient voltage excursions.

Applications

On load tap changers have applications in a wide range of power engineering fields, including:

• Transmission substation transformers for voltage support across high-voltage grids
• Distribution transformers managing voltage at the point of customer delivery
• Integration of variable renewable energy sources including wind and solar generation
• DC railway traction power supply systems
• Industrial plant transformers with large motor loads that cause voltage sag on startup