VOLTAGE REGULATOR SELECTION AND REGULATOR SELECTION AND APPLICATION ... The voltage regulator will sense the generator line voltage and vary the dc voltage ... voltage develops. (a):

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    VOLTAGE REGULATOR SELECTION AND APPLICATION

    ABSTRACT: This paper covers the function of the voltage regulator and how to select andapply the regulator for varying applications.

    INTRODUCTION

    Michael Faraday has been accredited with one of the most important discoveries of ourtime. He discovered that a voltage potential could be induced by passing a conductivematerial through a flux field. This principle is termed electromagnetic induction and is thebasis for a wide range of technology used today, including power generation.

    A synchronous generator is an electromagnetic device that uses Michael Faraday's law. Itis designed so that the three requirements of electromagnetic induction are satisfied.These are a conductor, a flux field, and relative motion between the two. For large ma-chines, the synchronous generator is made up of an armature and a rotating field. Thearmature is located in the stationary part of the generator known as the stator. This area ismade of conductive windings, and for the above analogy of electromagnetic induction, isviewed as the conductor. The rotating field is on the rotor of the generator and is the me-dium by which flux is produced. The rotor, through its rotating action, causes relativemotion of the flux field to the stationary stator windings and, through this action, an outputvoltage is induced from the generator.

    Smaller synchronous generators in the range of 50 watts to 5 kilowatts in size and rotaryexciters that will be described later in this paper have a stationary field in the generatorstator location and a revolving armature for the rotor. The field still produces flux, but therelative motion of the conductor to this field is induced by rotation of the conductor throughthe flux field. In either case, generator action occurs and voltage is produced.

    CONTROL OF GENERATOR VOLTAGE

    For a synchronous generator, the flux field is the easiest parameter to use for varying thegenerator output voltage. The rotor of the synchronous generator will revolve at an aver-age constant speed, this function being performed by a speed governing system. SeeFigure 1. Since the relative motion of the conductor to the flux field is held constant by theconstant speed of the rotor, a method of positive control of output voltage from the genera-tor other than by controlling speed is required. This is the function of the voltage regulator.The voltage regulator will sense the generator line voltage and vary the dc voltage appliedto the rotating field of the generator. By varying the dc voltage to the field, the magnitude ofthe flux developed by the field will vary, controlling the output voltage from the generator.The remainder of this paper will describe the above operation for the voltage regulator invarying applications.

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    Figure 1: Speed Governing System

    VOLTAGE REGULATOR THEORY OF OPERATION

    Figure 2 shows a typical block diagram of a Basler Electric voltage regulator. To under-stand how the regulator functions, each block of the figure will be reviewed.

    Figure 2: Voltage Regulator Block Diagram

    The voltage regulator senses the generator line voltage and regulates this voltage at thepoint being sensed (A), terminals E1 and E3. It is imperative to understand that the regula-tion point of the regulator is at E1 and E3 and not at some other point where generator linevoltage is present. This sensing input is reduced and converted to a dc signal that repre-sents the generator line voltage (B). The dc signal will then feed into an error detector andis compared to a reference signal (C). This reference signal is the regulation point of theregulator and is directly related to the nominal generator line voltage. Another input to theerror detector is the voltage adjust rheostat (VAR). This signal (D) is combined with the

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    reference signal and enables the operator to change the regulation point of the regulatorto a new level within typically 10% of nominal. From these signals, if the generator linevoltage at E1 and E3 exceeds or decreases below the reference signal, an error signal isdeveloped (E). This error signal feeds into the error amplifier and the amplified signal willthen go to the firing circuit. The firing circuit converts the amplified error signal to a train ofpulses and these pulses (F) are applied to the power control stage where the ac voltageinput into 3 and 4 is rectified and applied to the field through (G).

    POWER STAGE

    When viewing the power stage, Figure 3 shows the typical voltage output waveform of ahalfwave and fullwave voltage regulator that uses SCR's. This figure shows the rectificationof the sinusoidal ac input into Terminals 3 and 4 for the power input and how this waveformis controlled by the gating of the power SCR's. When the SCR's are gated either earlier orlater in the half cycle, the regulator will vary the VDC felt across the field and maintain thegenerator line voltage within the prescribed regulation band.

    Another component of the power control stage is the freewheeling or flyback diode. Theflyback diode is across the field and provides a path for field current flow during the offtime of the power SCR's. The field into which the voltage regulator is working is woundsuch that inductance is present. This inductance will oppose any change in the field cur-rent level. If the flyback diode is removed from the power stage, the field current does nothave a path for flow during the off time of the SCR's and erratic control of generator linevoltage develops.

    (a): Half-Wave (b): Full-Wave

    Figure 3: Typical Voltage Output Waveform

    STABILITY

    In the previous sections, a closed loop system was described where the regulator is theportion of the closed loop that ties the output of the generator to the generator's field. Dueto the varying system time constants involved, a stability network for the voltage regulatoris required for stable generator line voltage control.

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    Since the field of the exciter is a coil of wire wrapped on an iron core that has a very highvalue of inductance, the application of voltage to the field results in an exponential rise infield current. The generator output voltage changes in response to field current. The resultis a time lag from the time of regulator field voltage change until the generator voltage isrestored to the regulated value. Because of this time lag and the high sensitivity of theregulator, a stability circuit is included in the voltage regulator.

    Figure 4: Regulator Transient Response

    Figure 4 shows the effect of the stability circuit. At the point of load application, the genera-tor voltage drops due to internal generator reactance. Within one cycle, the regulatorrecognizes the error and reacts to put full voltage across the field. The amount of voltageavailable during this forcing period is directly proportional to generator voltage. Notice thatthe field voltage begins to decrease before the voltage is restored to rated value. Thiseffect is caused by the stability circuit. Without the circuit, the regulator would continue toforce the field until rated voltage was restored. Because the field current is lagging behindthe field voltage, the generator output voltage would overshoot the rated value. Figure 5illustrates the effect of removing the stability circuit of the regulator.

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    Figure 5: Unstable Regulator

    Generally, regulators are provided with some means of stability adjustment which allowstailoring of the stability circuit for generator response time. Figure 6 shows the type ofvoltage transient response on load rejection which might be found at various settings ofthe stability control, as in "A". The voltage is "hunting", typical of an unstable system. Byincreasing the stability signal, the hunting ceases after just one voltage undershoot andone overshoot as in "B". Further increase in the stability signal in "C" results in one under-shoot. Further increases in the stability level will slow the response of the system as in Dand E.

    When a regulator is initially installed, the stability circuit is adjusted for the desired systemresponse and normally requires no further adjustment during the life of the system. Of thefive types of response shown in Figure 6, only "A" would be incorrect. The other fourcurves are all acceptable. For many applications, the stability adjust is set for maximum,and the performance "E" is obtained. If fast voltage recovery is required by the application,the stability control may be decreased to achieve faster response. Note that the magnitudeof voltage rise on load rejection or voltage dip on load application does not change appre-ciably at different settings. The time to recover to rated voltage is affected.

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    Figure 6: Stability Adjustment

    FORCING

    Another characteristic of a voltage regulator, as mentioned in the last section, is an aspectknown as forcing. The forcing characteristic for a typical voltage regulator is shown inFigures 7 and 8. A voltage regulator has a maximum continuous dc voltage rating at whichit can operate without damage. The regulator is also rated at what is known as forcingvoltage level. This voltage is the maximum dc voltage that the voltage regulator can pro-duce on its output for one minute or less without damage. The forcing function assists ingenerator line voltage recovery during system load changes and generator line voltagebuildup upon system start. This is accomplished since the regulator is supplying maximumvoltage (forcing) to the field of the generator with an approximate 2 percent drop in theregulation point of the generator line. The regulator's forcing level is proportional to thevoltage regulator's power input into Terminals 3 and 4, and