Text 3 Automatic Voltage Regulator (AVR)

In addition to regulating the generator voltage, the AVR circuitry includes under-speed and sensing loss protection features. Excitation power is derived directly from the generator terminals. Positive voltage build up from residual levels is ensured by the use of efficient semiconductors in the power circuitry of the AVR.

The AVR is linked with the main stator windings and the exciter field windings to provide closed loop control of the output voltage with load regulation of +/- 1.0%.

In addition to being powered from the main stator, the AVR also derives a sample voltage from the output windings for voltage control purposes. In response to this sample voltage, the AVR controls the power fed to the exciter field, and hence the main field, to maintain the machine output voltage within the specified limits, compensating for load, speed, temperature and power factor of the generator.

A frequency measuring circuit continually monitors the generator output and provides output under-speed protection of the excitation system, by reducing the output voltage proportionally with speed below a pre-settable threshold. A manual adjustment is provided for factory setting of the under frequency roll off point, (UFRO). This can easily be changed to 50 or 60 Hz in the field by push-on link selection.

 

Shore automatic voltage regulators

 

 

Marine AVRs

 

ELECTRONIC POWER APPARATUSES

Rectifiers and converters

Early Rectifiers

Rotary converters, double wound, rotating synchronous machines, had been used on 25-Hz power to generate DC since the early days. The need for DC by some industries had influenced the choice of 25 Hz for the initial generation at Niagara Falls because, in the early days of magnetic materials, it was difficult to make the rotary converters operate on 60 Hz. In later years, such operation became feasible, and rotary converter substations were scattered around the outlying regions of urban transit systems to provide DC for the trolly wires. Generally a large-diameter, narrow machine, the rotary converter was a fixture in DC power conversion for more than half a century.

Copper oxide rectifiers had been used for some years in small DC power supplies for battery chargers and similar applications. They had also been used for meter rectifiers. However, the copper oxide rectifier was not efficient enough for higher-power applications, nor was the voltage capability sufficient. Later, selenium rectifiers were developed that permitted power densities approaching 1 A/in 2 of plate and 30 V per plate. They could be operated in series and parallel combinations with no concerns about sharing either voltage or current. Although they were rather bulky and somewhat inefficient, they served a need and were popular for many years in applications from radio and television receivers to industrial plating rectifiers and welders. In high voltage stacks, they were used in electrostatic precipitators.

Other methods of rectification were vacuum tubes and mercury vapor rectifiers, both of which were suitable for use in the higher voltages. Vacuum tubes had a relatively high forward voltage drop but were suitable for the radios of the day that required several hundred volts at a hundred milliamperes or so for operation. Efficiency was of little concern in that application, but the high losses of vacuum tube rectifiers negated their use in higher-power equipment. However, they found a niche application in high-voltage CRT anode supplies for television receivers, where they were operated from a flyback transformer on the 15.75-kHz horizontal deflection system.

Mercury Vapor Rectifiers

A useful variant of the vacuum tube rectifier emerged in the form of a mercury arc rectifier. These tubes utilized a low-pressure mercury vapor in a vacuum environment. The mercury was vaporized by a heated filamentary cathode. The voltage drop was typically around 15 V, and they could be built for operation at several tens of kilovolts. As hot cathode tubes, they were widely used in communications, where they supplied DC voltages for most radio transmitters at power levels as high as 1 MW in voltages from 5 to 15 kV.

Very large rectifiers were made with evacuated glass and metal enclosures containing multiple anodes and a pool of liquid mercury. They were used with the double-wye interphase transformer circuit described later to provide high currents for electroplating and DC service buses for metal processing mills. They also supplied high-current “pot lines” for the electrolytic reduction of aluminum and other metals as well as chlorine production. A starter electrode vaporized the mercury in these units, and the tanks were maintained at a low pressure by vacuum pumps.

Later developments included a sealed glass and metal enclosed high-current rectifier with an ignitor electrode that permitted it to act as a high-current switch. Sold under the trade name Ignitron®, these units became very popular and ruled the rectifier field until the development of solid-state technology in mid century.