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
3.3. 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.
Silicon Diodes—The Semiconductor Age
The now-familiar silicon diode grew out of the development of the point contact germanium transistor by Brattain and Bardeen at Bell Laboratories in 1947. Later work in many laboratories resulted in alloyed junctions, higher current capabilities, and the transition from germanium to silicon. By 1960, silicon rectifiers were widely available and coming into general use. Germanium power rectifiers were available in large sizes, but their restricted temperature capabilities limited their use.
The advantage of silicon for diodes was that it could operate at junction temperatures approaching 200°C. Their forward drop was higher than that of germanium diodes, but the higher temperature capability more than made up for it in their permissible power density. The early silicon diodes were silicon wafers soft soldered to a copper substrate that served as a mounting and one electrical pole. The other was made from a soldered wire attachment to the other side of the wafer that was brought out through insulation.
As the size of silicon diodes grew, it became apparent that temperature cycling with operation would fatigue the solder joints, so brazing techniques were developed. Next came the use of a metallic interface wafer with a low thermal index of expansion that was close to that of silicon. The silicon wafer was brazed to the metal, usually tungsten or molybdenum. Large diodes are now sometimes made with a floating silicon wafer that is compressed with an external clamp.
Rectifier Circuits—Single-Phase
The simplest rectifier is the half-wave circuit shown in Fig. 9.1. This circuit is often used for low-power rectifiers operating directly from the AC power line. It can deliver power to a resistive or capacitive load. The half-wave rectifier draws DC and even-order harmonic currents from the source, in addition to the usual odd-order harmonics that characterize most nonlinear loads. If a transformer is used, the DC component may saturate the iron, but a gapped core can allow flux reset and prevent this.
Half-wave rectifiers are often used with filter capacitors to provide a low-ripple DC output. The capacitive load makes the line current conduction angle decrease as shown in Fig. 9.1. High-frequency harmonic currents increase, and the apparent power factor, P/VA, decreases. The rectifier in this case is subjected to a peak reverse voltage equal to the capacitor voltage plus the peak line voltage. Since the capacitor usually is charged to near peak line voltage, the diode must be rated for repetitive operation at twice peak line voltage. Although the half-wave circuit is widely used in switch-mode power supplies where the poor current waveform may not cause a problem, an aggregation of such power supplies can have serious effects on the power system in, for example, a data processing center. Single-phase loads with such current distortion have a high percentage of triplen harmonics, both even and odd order—3, 6, 9, 12, 15, 18, 21, …. These triplen harmonic currents are additive in the neutral of a three-phase distribution system, and the neutral current can approach twice the line current. Oversized neutral conductors are required. Equipments that cause DC components in the supply lines of electric utilities are not permitted by IEEE 519, but the DC components of half-wave power supplies do not pass beyond the first transformer they encounter. However, they may cause the core of the transformer to saturate.
The full-wave, center-tapped rectifier shown in Fig. 9.2 was widely used in the days of radio, because the two rectifying elements could be contained in the same vacuum tube. It is still popular for low-voltage supplies, since there is only a single rectifier element in series with the load. However, many of these simple rectifiers have been superseded by switch-mode units that are much smaller and lighter because of their high-frequency transformer. These are described in Chapter 13.
The secondary sections in this circuit carry half-wave currents with an rms value of 50% of peak over a full cycle. With a primary voltage of 1.0 Vrms and a secondary voltage of 1.0 Vrms each side of center tap and a resistive load of 1.0 Ω, each secondary current is 0.707 Arms, and the primary current is 1.0 A rms. Circuit voltamperes are 1.0 VA. The transformer has a primary rating of 1.0 VA, but each secondary must be rated at 0.707 VA. Thus, the transformer must have a VA rating of (1 + 0.707 + 0.707)/2 = 1.207 VA. It must be some 20% oversized for the circuit rating because of the half-wave currents in the secondary sections.
Figure 9.3 shows the familiar bridge circuit. Here, the transformer is fully utilized, since both windings carry sinusoidal currents. The bridge circuit has twice the diode losses of the center tap circuit, because there are two diodes in series with the load. However, the transformer losses are lower because of reduced harmonic currents. Encapsulated diode assemblies are available for use in any of the above circuits.
