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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 with a nominal power input theforcing is approximately 140 percent of the maximum continuous voltage rating for theregulator. If the power input is from the generator line and decreases due to a block loadbeing applied to the generator, the amount of available forcing also decreases. Figure 7shows an excitation system where the power input is from a separate source.

NOTE:CONSTANTPOWER INPUTTO REGULATORFROMSEPARATESOURCE.

Figure 7: Power Input from Separate Source

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With this constant power input, the voltage regulator has maximum rated forcing to assistin the transient response of the generator's line voltage during load changes. Figure 8demonstrates the case where the power input for the voltage regulator is from the genera-tor line. Notice the slower response in line voltage recovery for the same amount of loadbeing applied to the system.

NOTE:POWER INPUTTOREGULATORFROMGENERATORLINE.

Figure 8: Power Input from Generator Line

If the available forcing voltage is low, it is possible that the generator line voltage will notrecover for the system. In this case some type of excitation support system is required toassist the voltage regulator in maintaining field forcing for the varying load demandsplaced on the system. These support devices are described in a Regulator Accessoriespaper. If the generator is paralleled to an infinite voltage source, the line voltage does notchange with varying loads placed on the system. In this case, full forcing is available fromthe voltage regulator, but is not as critical since the voltage regulator is now controlling thevars of the generating system and not the generator line voltage.

SELECTING THE VOLTAGE REGULATOR

Voltage regulators come in varying models with different characteristics and output capa-bilities. This enables one regulator to give improved performance over another for a par-ticular application. The selection of a voltage regulator for an application can be brokendown into four basic steps. From these four steps, the field of selection for the voltageregulator required is narrowed, and specific system requirements can then be reviewed toselect the proper regulator for the application.

STEP 1: DETERMINING GENERATOR TYPE

When selecting a voltage regulator for synchronous generator applications, a prime con-sideration is the type of generator into which the regulator will work. For voltage regulatorselection, the synchronous generator falls into one of three categories. These are genera-

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tors that have the regulator supplying power directly into their main field, called static orslip ring excitation, and regulators supplying power into either a brush or brushless typerotary exciter. These three applications are listed below.

A. Static excitation (slip ring)B. Rotary brush type excitationC. Rotary brushless excitation

A rotary exciter, if used, is really a small generator that contains a stationary field andrevolving stator. Since this system is in line to the main generator field, the time constantsof this device are taken into consideration with the other factors when sizing a voltageregulator.

The type of excitation for a specific generating system is of prime importance for voltageregulator selection, since a voltage regulator requires selection with a proper stabilitynetwork for the application. When reviewing the stability network of the voltage regulatorand its selection, other considerations become important and are discussed more fully inthe paper titled "Voltage Regulator and Generator Instability". These are as follows:

1. Rated generator kilowatts2. Rated generator power factor3. RPM of the prime mover4. Generator line voltage5. Field requirements

STATIC EXCITATION

Figure 9: Slip Ring Generator

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Figure 9 shows a static excitation application. In this case the excitation system worksdirectly into the main field of the synchronous generator. Typically all present day voltageregulators are considered to be static devices. This implies that solid state components areused to enable the voltage regulator to perform its function, and moving parts are gener-ally not involved. This term should not be confused with the term "static excitation" used inthis paragraph. Static excitation in this case is where the voltage regulator supplies powerdirectly into the main field of the generator and not the field of a rotary exciter. With thistype of application, the power supplied by the voltage regulator feeds directly into therotating field of the generator through the use of brushes and slip rings.

This type of system has positive aspects, since the transient response for generator linevoltage recovery is quicker than if rotary type excitation is used. The quicker voltage recov-ery is induced since the generating system is not seeing the additional transient responsedelay of the generator line voltage by the time constants of the rotary exciter. Anotheradvantage of shunt static excitation over the use of a rotary exciter is the lower systemefficiency of the generating system. Static applications bypass the use of the rotary exciter,creating a more efficient system. A disadvantage of static excitation, however, is the brushand slip ring maintenance required to deliver VDC to the rotating main field.

BRUSH TYPE ROTARY EXCITER

Figure 10: Rotary Excited, Brush-Type Generator

Figure 10 shows the voltage regulator supplying its power output into the field of a rotarybrush type exciter. This type of excitation is termed "brush type" since the voltage regulatoris supplying power into the rotary exciter's field and the ac produced by the rotary exciter ismechanically rectified through the use of a commutator and brushes. The dc is then sup-plied to the main field through the slip rings and brushes of the synchronous generator.The voltage regulator thus performs its regulation function through the rotary exciter. Thistype of system is deficient in several ways. The first was previously covered and is theslower response in generator line voltage due to the added inductance supplied to thesystem by the rotary exciter. The other deficiencies are energy losses causing a less

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efficient generating system and the need for increased system maintenance. The brushesrequire periodic checking for tension and wear, along with replacement and reseating. Thesurface of the commutator must also be maintained. Due to the increased maintenance, ifa rotary exciter is specified for an application today, a brushless type rotary exciter is typi-cally used.

BRUSHLESS ROTARY EXCITER

Figure 11: Brushless Rotary Exciter

A brushless rotary exciter as shown in Figure 11 is similar to the brush type rotary exciter,but instead of using a commutator and brushes to convert the ac voltage from the rotatingstator to dc, the brushless type rotary exciter uses a diode bridge network. These semicon-ductors rotate with the stator and are used to convert the ac voltage to dc and apply thisvoltage directly to the rotating field of the main generator. This is accomplished throughcabling run along the coupling shaft for the two systems. Thus the term "brushless excita-tion". Since brushes are not used, the maintenance of this type of system dramaticallydecreases. However, the deficiency of a slower responding system due to the addedsystem inductance, along with decreased system efficiency, is still present.

The next step in voltage regulator selection is ensuring that the voltage regulator's outputcapability is matched to the power requirement of the field into which it is working.

STEP 2: SELECTING A PROPERLY SIZED REGULATOR

A voltage regulator must be sized for the power requirements of the field to which it issupplying power. As an example, if the regulator is used to supply power into the maingenerator field, the data of the main field is of interest for voltage regulator sizing. If theregulator is supplying power to a rotary exciter, the field data of the exciter is now used.This data for regulator selection consists of the field resistance at ambient room tempera-ture and the voltage and current requirement with the generator at rated load, rated power

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factor, and nominal generator line voltage. These three requirements are listed below. If therequirements are satisfied, the regulator's power stage will be rated above the powerrequired by the field to maintain the nominal generator line voltage at full load and theregulator will function without damage by being undersized.

Generator Rating Regulator Rating

Field Voltage Less than Continuous voltage

Field Current Less than Continuous current

Field Resistance Greater than Minimum resistance

Table 1

The field data for regulators can be found by several methods. The easiest is to use thegenerator's data or, when given, the rotary exciter's nameplate data. However, caution mustbe employed when using the brush or brushless rotary exciter nameplate, since the name-plate often gives the output rating of the rotary exciter and not the power requirement of itsfield. In this case, the generator manufacturer is another source of information. If the fielddata is not available from the above two sources, testing must be accomplished to findthese values. Testing is required since no real relationship exists between generator sizeand field requirements to precisely select a voltage regulator. An important tool for thistesting is Ohm's law. The voltage across the field is equal to the current through the fieldmultiplied by the field resistance. If any two of the values for the equation are known, thethird can be readily found. Figure 12 shows the relationship among the three parameters.

Figure 12: Ohm's Law

If the field voltage and current are known with the regulator at rated load and power factor,the field resistance can be found by dividing the voltage by the current. It should be noted,however, that during startup the field will be at its minimum resistance value. The field willincrease in resistance as its temperature increases. For this reason it is best to determinethe field resistance with the field at room temperature. To obtain the field resistance, an

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ohmmeter can be used to measure the field across the F1 and F2 terminals. This is shownin Figure 13. Figure 14 shows the use of a manual voltage control to determine the fieldvoltage requirement by manualy controlling the VDC to the field and inducing a nominalgenerator line voltage at rated load. By using a dc voltmeter across the field, the voltagelevel can be obtained. By determining the field resistance and dividing this resistance intothe dc field voltage required, the maximum continuous current can be found. Table 1 canthen be used to select a regulator that will support the power demand of the field. If adifferent combination of unknowns is found, Ohm's law can be used to find the third.

Figure 13: Ohmmeter

Figure 14: Manual Voltage Control

With the newer Basler Electric regulators, the regulator's model number incorporates themaximum continuous output voltage and current rating for the regulator. As an example,the SSR63-12 has a maximum continuous voltage of 63 volts dc and a maximum

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continuous current of 12 amps. The minimum field resistance required for the regulatorcan be determined by dividing the 63 volts dc by the 12 amps. See Figure 11. This will givea minimum resistance requirement of 5.25 ohms. This information can also be found in thedocumentation for any Basler Electric voltage regulator. If the field that the regulator isworking into is below 5.25 ohms, a series field resistor is necessary to make the totalresistance equal to or above the 5.25 ohms required. Figure 15 shows an example of howto select this series resistance value by subtracting the low field resistance from the mini-mum field requirement and finding the difference. This difference can be added in serieswith the field to satisfy the total resistance required on the output of the regulator so thatthe maximum continuous current rating is not exceeded. If exceeded, regulator damagewill occur. Another consideration when selecting a series resistor is the resistor's wattagerating. By taking the current flow through this resistor with the regulator at forcing, theresistor wattage requirement can be found by using the formula in Figure 15. Caution mustbe used when selecting a series resistor due to the effect on forcing. The greater the seriesresistance value becomes, the less amount of forcing voltage will be available to the field.The limiting of forcing will decrease the transient response performance of the generatorline voltage and, if great enough, will induce generator line voltage collapse during loadon. If the voltage does recover, it is possible that the field voltage will be limited so thatnominal generator line voltage cannot be obtained at certain loads. This will be seen aspoor system regulation.

For these reasons, if a series field resistor is required, it must be of a value that will causethe total resistance across F+ and F- of the regulator not to be less than the minimumresistance requirement of the regulator, but not so large that forcing is limited at a level thatwill cause improper operation of the generating system.

5.25 OHMS - 3 OHMS = 2.25 OHMSR = 2.25 OHMS(1.4 x 63 VDC) / 5.25 OHMS = 16.8 AMPS RESISTOR MAX. WATTS = (16.8)(16.8) x 2.25 = 635 W

Figure 15: Selecting a Series Resistance Value

With the power output requirement of the regulator determined, the final two steps inregulator selection can be made.

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STEP 3: POWER INPUT

The power supply into the voltage regulator plays an important role in regulator selection.This input can come from several sources, such as the generator line as shown in Figure16, or from a separate source such as a permanent magnet generator or isolated bus.These inputs must match the power input requirement of the regulator's voltage magnitudeand frequency range. Also consideration is given to the number of phases used to supplypower to the voltage regulator. As an example, a PMG can supply either a single or threephase power input depending on design. A regulator with either single or three phasecapability for the power input voltage will be selected dependent on the rating of the PMG.If a voltage regulator has the capability of a single phase power input and a three phasePMG is used, the PMG will require reconnection for single phase and derated. This derat-ing is typically 33% or more. If the voltage level for the power input is above the operatingrange of the voltage regulator, a power stepdown transformer will be required.

When the frequency of the power input voltage is above or below the operation range ofthe regulator, the voltage regulator will require modification for the higher frequency, orselection of a compatible voltage regulator.

When the power input for a voltage regulator is from a separate bus, caution should betaken to ensure that this input is removed during the stop sequence of the prime mover.This can be accomplished by use of speed contacts that change state during the primemover's start and stop sequence. The contacts will apply and remove power to the regula-tor respectively; see Figure 17. If speed contacts are not used and the prime mover ceasesits rotation, the voltage regulator will sense a low voltage condition and, with the availablepower input, will supply maximum power to the field. This condition will induce forcingvoltage from the regulator and cause damage to the regulator and the field into which theregulator is supplying power. If the power input is coming from a PMG, the PMG will have aVAC output proportional to its speed, with the regulator receiving zero power from thePMG, when the PMG is at rest. Thus, with a PMG, speed contacts are normally not re-quired.

Figure 16: Power Input from Generator Line

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Figure 17: Use of Speed Contacts

If the voltage regulator's output can be discontinued by the removal of the regulator'spower input, then the approved method of turning the voltage regulator on and off is by theapplication or removal of the power input. The regulator's output to the field should neverby removed by opening the field connection for this function. By causing an open in thefield, a high inductive kickback current will result, damaging the voltage regulator.

Since the regulator is safely shutdown by removal of its power input, then fuse protectionfor the excitation system can be placed in the power input line as shown in Figure 18. If afault occurs downstream of the power input for the voltage regulator, excessive current willbe drawn through the fuse. This excessive current flow can be induced by a fault in thefield or internally in the voltage regulator itself. Thus fuse protection will limit the damage tothe voltage regulator or generating system by removal of the power input upon sensing ofa fault condition.

Figure 18: Fuse Protection

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Figure 19: Fuse Protection for the Sensing Input of the Voltage Regulator

It should be noted that some specifications require fuse protection for the sensing input ofthe voltage regulator. This presents a problem in the operation of the generating system. Ifthe sensing fuse opens, the voltage regulator will sense a low voltage condition and sup-ply maximum power on its field output terminals to correct for the loss of sensing. Thismaximum voltage to the field of the generator will cause the machine to become overex-cited, damaging the voltage regulator and possibly the generator itself. For this reason thepower input of the voltage regulator is required to be removed before or at the same timeas the loss of the regulator's sensing input. To use the voltage regulator in this scheme, thepower sensing input can be connected as shown in Figure 19. Under this condition, if theinput fuse is open, the sensing and power input of the regulator is removed simulta-neously, resulting in loss of field excitation with the loss of the sensing input.

Again, special consideration is required with this type of application. If a transformer isused to step down the generator line and the sensing input is common to the power input,the percent regulation of the voltage regulator can be exceeded due to the burden de-mand on the secondary of the transformer. This burden is induced by the volt-amp de-mand on the secondary of the transformer to support the necessary excitation current tothe field of the generator. By the burden being induced on the transformer, thetransformer's secondary voltage will no longer be proportional to the voltage on the gen-erator line. This nonproportional voltage will be seen by the sensing input of the regulator,causing the regulator to maintain a greater voltage level that is out of the regulator's properregulation band. With a greater volt-amp rated transformer, or by receiving the commonpower and sensing input directly from the generator line, better regulation will be achieved.In most cases, however, poor regulation by a closely sized secondary of a power trans-former to the power input of the regulator will cause less than a two percent error in regula-tion.

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STEP 4: SENSING INPUT

The selection of a voltage regulator with a proper sensing input depends on the sensevoltage available and regulation requirement for varying loads. Basler Electric voltageregulators have different sense voltage characteristics for varying applications. For ex-ample, the APR model regulator has the capability of single phase sensing with a select-able sense voltage input of 240 or 48 VAC, 50/60Hz. The SR4A and SR8A model regula-tors can have either single or three phase sensing with a voltage input range for this sens-ing input of 120 or 600 VAC, 50/60 Hz. The regulator, however, must be factory built for thenumber of phases and magnitude of the sense voltage required. If the frequency of thesensing voltage is beyond the range of 50 and 60 Hz, the regulator will require modifica-tion. The SSR voltage regulator has the capabilities of field selectable single or threephase sensing input and voltage range of 120 to 600 VAC, with the added enhancement ofworking with a sense voltage frequency range of between 50 and 400 Hz without modifica-tion. See Figure 20. The voltage regulators also have different percent regulation capabili-ties for the type of applications that might be used. For example, if a regulation characteris-tic of ±.25 percent is required, an SR voltage regulator could be selected for an applica-tion. If only ±.5 percent regulation is required, an SRA could be used as long as the SRAsatisfies all the other requirements for the application. A wide range of Basler regulators isavailable with different percent regulation characteristics and sensing input capabilities forvarying applications.

Figure 20: SSR63-12 Voltage Regulator

When selecting the voltage regulator for a specific sensing application, other factors inaddition to the available sense voltage and percent regulation affect this process. Thesensing input for the voltage regulator is always from the generator line. If this voltage isabove 600 VAC, a voltage transformer will be required to step down the line voltage to theproper sensing input range for the voltage regulator. Figure 21 exhibits typical wye con-nections for stator windings of a synchronous generator. Each winding shows the relation-ship of the voltage that can be induced across each phase for a low voltage generator

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application. It is important to remember that the voltage regulator will only regulate thegenerator line voltage at the point being sensed. The regulator will not regulate the voltageat some other point monitored by a system voltmeter. A similar case happens with thewindings of a generator. If a single stator winding is used for the sensing input to the volt-age regulator, the regulator will only regulate that winding voltage. If the regulator is sens-ing several windings across two phases, then the regulator will maintain the voltage acrossthese phases. This is termed "single phase sensing", where the regulator will only senseone phase of the generator line. When the synchronous generator is supplying power tobalanced loads across all three phases of the stator, then a single phase sensing regulatorcan be used to maintain the generator line voltage within the regulator's prescribed regula-tion band, the percent regulation being controlled by the voltage regulator.

Figure 21: Typical Wye Connections for Stator Windings of Synchronous Generator

If an unbalanced load is placed on the stator windings of a synchronous generator andsingle phase sensing is used, the regulator will only regulate the voltage coming into thesingle phase sensing terminals of the voltage regulator, terminals E1 and E3. The otherphase of the stator can fluctuate from the unequal load, with the result being poor voltageregulation. When an unbalanced loaded condition is suspected for a system, a voltageregulator with three phase sensing should be used. The three phase sensing will notcompensate for the generator line voltage unbalance induced by unequal loading, but bymonitoring all three phases, the regulator will take the average of the three phases andregulate this average within the voltage regulator's regulation capability. See Figure 22.

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Figure 22: Reconnectable Generator Series - Wye Connection

OTHER SELECTION CRITERIA

The four basic steps for the selection of the voltage regulator have been identified.These are:

1) Generator type2) Exciter ratings3) Power input4) Sensing input

Additional selection criteria may now be reviewed since the selection range of the voltageregulators for a specific application has been reduced. Some added considerations forvoltage regulator selection are:

ADDITIONAL SELECTION CRITERIA

1) Overexcitation2) Underfrequency3) Temperature range4) Vibration and shock5) Regulation accuracy6) Thermal drift7) Non-linear generator loads8) Minimum voltage for build-up from residual

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2. An overexcitation relay will also monitor the excitation current in the field; however,instead of limiting the field current, this unit will close a set of contacts whenexcessive field current is sensed. These contacts may be used to trip the unit off-line or may be used to transfer control of the automatic voltage regulator to amanual voltage controller. More details concerning this particular transfer schemeare discussed in the “Designing an Excitation System” paper.

3. Figure 23 represents an overexcitation shutdown curve for a typical voltageregulator. Voltage regulators that incorporate this function will shut themselvesdown after an overexcitation condition exceeds the shutdown zone’s limit. Thistype of curve will usually lie just outside its forcing limits. Once the unit has tripped,the voltage regulator will require the input power to fall below a certain level toallow the unit to reset.

The additional considerations for the application of the voltage regulator are part of thedesign of the regulator itself. These characteristics will depend on the components used toproduce the particular regulator model and the mechanical design of the model. By re-viewing regulator data, a regulator can be selected to satisfy a specific performance crite-rion. If an enhanced performance feature is required from the voltage regulator and thischaracteristic is not a standard function for the unit, an accessory device is usually avail-able to help match the excitation system to the generating system requirements.Some of these considerations follow.

OVEREXCITATION PROTECTION

Today, most newer voltage regulators incorporate some sort of overexcitation protection toprevent damage to the regulator itself, the exciter and also the generator.

1. Excitation limiting is one form of protection. This feature can be set to limit howmuch field current may be delivered to the exciter field. Once the field currentexceeds the pickup setting, and any preprogrammed time delays have expired,then the field current will be driven to a safer predetermined level. Also, this featuremay be built into the voltage regulator or may be an additional external device thatinterfaces with the voltage regulator.

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Figure 23: Overexcitation Shutdown Curve

UNDERFREQUENCY

On most Basler Electric voltage regulators, underfrequency provisions are standard. Forthose that do not have this provision, an external accessory item can be used. Why is theunderfrequency characteristic a desired feature for a voltage regulator? There are severalreasons.

A voltage regulator without the underfrequency provision will function as needed to main-tain its sense voltage input. During operation of the prime mover at lower than rated speed,a greater amount of current is required by the field to maintain the regulator’s nominalsense voltage level. If the speed is excessively low, the regulator can exceed its maximumcontinuous rating over the acceptable time frame. During this underfrequency condition,the generating system is running at a lower than rated speed such that cooling internal tothe generator becomes less. The field of the synchronous generator is operating at ahigher than normal current when cooling is at a minimum. Generator damage will occur.Another part of the generating system that can become damaged is the loads on theoutput of the generator. If the voltage regulator is maintaining the nominal generator linevoltage at a lower frequency, the inductive type loads receiving power from this lowerfrequency at rated line voltage can become damaged. An example of this type of load is amotor or transformer. With the underfrequency circuit, as the speed of the prime moverdecreases, the voltage regulator will sense this as a decrease in sense voltage frequency.The underfrequency circuit of the voltage regulator will sense this condition and act to

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induce a signal into the sensing circuit of the voltage regulator to maintain or evenslightly decrease the output voltage seen by the field. This control of field excitationinduces the respective underfrequency curves of the voltage regulators. See Figure 24.Figure 25 demonstrates the typical curves for the SSR voltage regulator.

Figure 24: Voltage Regulator Underfrequency Curves

Figure 25: SSR Voltage Regulator Curves

The underfrequency curve of the voltage regulator can be a volts per hertz function, orwhat is known as limited volts per hertz function. This will depend on regulator design,with a specific regulator sometimes having selectability of either function. A limited voltsper hertz function will enable the rolloff threshold of the underfrequency circuit to bebelow the rated frequency of the system. In this case, a minor instability of prime mover

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speed will not cause a voltage instability problem by the underfrequency curcuit beingactivated by speed swings. As shown in the previous figure, specific regulators alsohave the option of varying the slope of the frequency rolloff curve. If the underfrequencycircuit causes a volts per hertz rolloff, for every per unit drop of rated frequency a perunit drop in rated line voltage is experienced. With the two times volts per hertz, a twotimes per unit drop will be seen in the rated line voltage for every per unit drop in ratedfrequency. The advantage of two times volts per hertz over a volts per hertz functiondeals with system transient response of the prime mover speed during load changesand is directly related to the slope of the frequency rolloff curve. Figure 26 shows threedifferent underfrequency slope characteristics.

Figure 26: Underfrequency Slope Characteristics

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Figure 27: Generator Line Voltage Response - 3 Different Slopes

Figure 27 shows the graphic representation of system frequency with the generator linevoltage response for the three different slopes on the same application.

With the steeper slope, the drop in generator line voltage will increase with each perunit drop in system frequency. This increased drop in voltage is seen by the primemover as a proportional drop in the kilowatt load and, thus, prime mover speed recov-ers more quickly. Notice that the voltage recovery period is also quicker since thisrecovery is affected by the speed of the prime mover.

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CONCLUSION

Various considerations are involved in voltage regulator selection for specific system re-quirements and are covered in other papers. These include topics on: parallel operationbetween voltage regulators, excitation support, and backup excitation for the system. If thefour steps for voltage regulator selection are followed, the range of selection betweenregulators that might be used for a specific application can be reduced.

1) Determine generator type.2) Determine exciter field parameters.3) Select regulator that satisfies power input requirements.4) Match voltage regulator to type of sensing required.

The next considerations occur after the range of possible regulators for use is narrowed.These special requirements are:

1) Temperature range2) Vibration and shock3) Regulation accuracy4) Thermal drift5) Non-linear generator loads

After review, one can select a voltage regulator that will give long life and enhanced perfor-mance for a specific system.