Generator Excitation 101

by | Jul 25, 2017 | Control & Safety Systems, Industry, Power Generation | 2 comments

Emerson's Richard DennisGenerators turn mechanical energy into electrical energy by moving electrical conductors in a magnetic field. Excitation creates the electromagnetic field to make this mechanical to electrical conversion occur. Emerson’s Rich Dennis provided a basics presentation on excitation control at the 2017 Ovation User Group meeting.

Excitation control includes synchronous machine regulation, exciter, synchronous machine for the power system. The regulator is the source of control and the exciter system is the source of power. The regulator system includes voltage control, current control, power factor control, limiters & protection, power system stabilizer, field flashing control, de-excitation control, and field breaker control. Exciter systems can be rotating or static. Rotating includes brushless and brushed types and static includes compound sources and potential sources.

A generator has a prime mover like a turbine or diesel generator. The excitation system creates the electromagnetic field in the rotor. The stator has the armature winding that has the electrical energy induced.

The stronger the magnetic field created, the stronger the electrical power produced. The strength of the magnetic field is adjusted by controlling the current to the rotor. Three phase electrical energy is created by three separate wire windings in the stator.

The current to create the electromagnetic field is direct current (DC) which can range from 50amps to 9000amps and more depending on the generator’s size. Modern excitation systems are static where DC current is created by rectifying AC power using saturable current transformers (SCTs) and power potential transformers (PPTs). A source is required to create excitation before it can be self-sustained from the generator.

Subsystems for the excitation system includes the processors and I/O devices that monitor generator terminal voltage and current, field voltage and current, rotating exciter field voltage and current, control switches, breaker status and safety permissives. Outputs include annunciation, alarms, meters, and a full range of data for the distributed control system. A power bus is required to feed the exciter current to each end of the rotor coil.

Field breakers are used to protect both the AC and DC sides of the generator. Power rectifiers convert AC power to DC power. Cooling systems maintain operating temperatures required for reliable operations. Field discharge system is required to remove the energy from the rotor while the mechanical power source is slowing down. The field flashing equipment is used to generate the initial electromagnetic field until the generator creates enough voltage to self-excite and sustain the mechanical to electrical power conversion.

Rich described Ovation Excitation solutions to provide the upfront interface and equipment design, engineering all the way to installation and ongoing testing. Ovation Excitation Controller is IEEE 421 compliant and fits many custom and retrofit options to fit the application.

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