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DESIGNING AN EXCITATION SYSTEM

When generator applications are in the design and proposal stages, what needs to bedone to put excitation system theory into practice? How do the application and specifica-tion dictate the design of a generator system? The purpose of this paper is to workthrough some examples to illustrate the way these questions are answered.

Example #1 - A Simple Standby Generator

A common generator set application provides back-up power to a building or load when acommercial power outage occurs. A simple standby system (for this discussion) is definedas a standby generator that is specified to require only a minimum amount of equipment toperform its backup role. A sample specification for such a generator might read as follows:

“A diesel engine driven generator shall be provided to supply building loads during acommercial power failure. The generator shall be rated for:

277/480 Vac, 4 wire, grounded neutral60 Hertz60 kW @ 0.8 PF1800 RPMBrushless ExcitedFull Load Excitation: 63 Vdc @ 4 Adc

The generator set and all auxiliary equipment shall be provided to make up an unattendedstandby power system.”

Before we can begin to specify the excitation system we must first review the types ofloads the generator will be supplying, any economic factors that may exist, and what“auxiliary equipment” is required to make up the most functional/affordable design.

The building under consideration probably has a variety of loads, typical in most residen-tial or industrial facilities. Several areas of concern could appear. If the generator is ex-pected to pick up large block loads of kW and the engine is near its limit on availabletorque from no-load, a frequency compensated regulator should be selected. An adjust-able underfrequency circuit can tune the regulator/generator/engine to obtain optimumperformance. This allows for faster load pick-up with reduced speed variations.

The main feature of any underfrequency circuit is to protect the field windings from exces-sive current and to protect the regulator from overload. When a generator is operated atreduced frequency for extended periods of time, there is a very real danger that an exces-sive current will be supplied by the voltage regulator to the exciter field winding. Thisexcessive current can be above the continuous rating of the voltage regulator and, if oper-ated for some time, could cause the regulator to fail. The second danger is with the regula-tor delivering excessive current to the exciter field winding. In an attempt to have the gen-erator deliver 100% voltage during this underspeed condition, it causes excessive currentto flow in the main rotor (field) of the generator. This excessive current is flowing at a time

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when the cooling air moving past the rotor by the generator fan is decreased. Therefore, ata time when the rotor needs cooling air most, the cooling air is reduced and, in turn, couldcause the rotor or field windings of the main generator to fail.

Figure 1: Typical Underfrequency Curve

During a power outage, a standby generator is usually started and brought up to ratedspeed immediately, with no warm-up period. If the engine is going to be warmed up whentesting the generator system or cooled down before shutdown, the generator and excita-tion system will be operated at idle speed for extended periods of time. During such opera-tion, the excitation to the generator should be shut down, with or without a frequencycompensated regulator. A speed switch operating above the idle speed and below normalspeed can be used to turn the excitation on and off. See Figure 2 below. The adjustment ofthe speed switch pull-in and dropout points should be selected for positive shutoff ofexcitation above any cool-down or warm-up speed and below 80% of nominal speed.

Figure 2: Typical Speed Switch Connection

If the building loads have a large content of induction motors or if the generator will berequired to provide fault current to operate load breakers, the regulation system must be

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designed to provide for these needs. When a motor of significant size is started across thegenerator terminals, the voltage across the generator terminals can drop severely. If this hap-pens and the regulator is powered from the generator terminals, the regulator may not havesufficient input power to maintain generator output and the system voltage could collapsecompletely. It may be necessary to add a current boost system to the voltage regulator toensure system performance during motor starting and line fault circumstances.

This boost system can be in one of two forms. It can be either a CBS (Current Boost System)or a SBO (Series Boost System). The CBS system applies dc current directly to the field anddoes not rely on the regulator during those times when extra boosting of the excitation isrequired. The series boost system uses similar design theories, except it powers the regulatorwith a constant power supply and allows the regulator to stay in control of the generator at alltimes.

To examine the effects of a boost system, let’s look at the excitation system shown in Figure 3.

Figure 3: Shunt-Fed Voltage Regulator

As load increases, the regulator is called upon to increase power to the exciter field. If a largeblock load were to be applied to the generator at one time, a voltage drop will occur. As thishappens, the available power to the exciter is decreased and the ability of the system to re-cover is reduced. If this load is large enough, the system voltage will collapse and the genera-tor will be unable to pick up the load, as shown in Figure 4.

Figure 4: Generator System Without Boost Figure 5: Generator with Multiple Loads

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If this were to occur on a system with multiple loads, for example, a hospital, high-risehousing complex or a shipboard application, the entire system could be jeopardized by ashort circuit or heavy load on only one line. Looking at a simple example, Figure 5, it isseen that if the system voltage should collapse due to a fault on the line feeding Load #1,we would most likely be forced to open all three feeder breakers to the main generatorbreaker before restoring generator voltage. This is because the other loads on the systemwould make it impossible to restart from residual. If, however, an external power sourcewere provided for the regulator, as shown in Figure 6, the regulator would continue toprovide excitation to the exciter’s field. This would allow the generator to continue to pro-vide fault current although the generator terminal voltage is low or near zero. This meansthat, as soon as the fault is cleared, the voltage can return to normal as shown in Figure 7,thus restoring system integrity.

Figure 6: Regulator Powered by a PMG Figure 7: Generator System with Boost

Another type of load condition that may cause a temporary overloading of the generator isthe locked rotor condition encountered when starting a large motor. During the initial surgecaused by starting a large motor on your system (large is relative to generator size), themotor winding may look to the generator much like a short circuit. This apparent shortcircuit can cause the same effect as a fault on your own system. However, if an externalpower source were connected to the field to apply additional excitation to the field duringthis time, you would have the same effect as providing a constant power source to theregulator. This is shown in Figure 8.

Figure 8: Excitation System with Current Boost

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The above example is the basic principle used for the SBO and CBS systems. The onlymajor difference is that we let the generator itself provide the power for the support sys-tem. This is done by use of current transformers in the generator’s output lines. During afault or motor starting condition, a high current is drawn from the generator. Power currenttransformers can be used to convert this high current to a lower level and then power theinput of a regulator, or it can be rectified and injected directly into the exciter field.

The first method to be discussed is the CBS (Current Boost System) feeding directly intothe exciter field.

In this example, the power provided to the exciter field will always be channeled throughthe boost system. If the regulator senses an incorrect low voltage condition, the boostoption is turned on and the power provided by the current transformer is rectified anddirectly injected into the field. This is an additive type of action and may provide a higherexcitation level than either the boost option or regulator could provide alone. This may bevery beneficial when motor starting is a major consideration. With this type of system, theregulator is in control of the boost option at all times. A typical interconnect for this systemcan be seen in Figure 9.

Figure 9: APR 63-5 with CBS 305

If the regulator senses an underfrequency condition and reduces its excitation, therebyreducing the generator output voltage, you will not erroneously energize the boost optiondefeating the purpose of the underfrequency circuit in the regulator.

There is another method of providing forcing level excitation to the field of the exciter. Thisis to provide the regulator with a constant power source. This can be accomplished byusing an SBO (Series Boost Option). This requires both current transformers and a reser-voir assembly based on a ferroresonant circuit. As shown in Figure 10, there are two inputsto the SBO. One is from the generator terminal voltage and one from the power currenttransformer(s).

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Figure 10: Series Boost Option Schematic

The ferroresonant circuit, consisting of Inductive, Resistive and Capacitive components,takes the voltage output from the generator and adds the power from the CTs to provide arelatively constant output voltage to the regulator input. Figure 11 shows the two CT con-nections for the SBO and an SRA type regulator.

Figure 11: SBO Interconnect - Two CTs

The output of the SBO is a constant voltage square wave. This square wave is an accept-able input to the SCR firing circuit of a voltage regulator. It also has the advantage, due tothe L.C. filtering circuit, of not being noticeably affected by the notching usually introducedfrom UPS systems and the SCR loads placed on your generator. This has made the SBO avery useful tool when applied with this type of load.

One CT is required, as shown in Figure 12, if two phases of the generator may be safelypassed through the available window. Two CTs may be used if safety dictates separatingthe two phases.

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Figure 12: SBO Interconnect - One CT

All of the systems discussed have a current transformer in two of the three power leadsfrom the generator. If proper connections of the equipment are made, excitation may bemaintained for any single phase fault by providing regulator power (or SBO input voltage)from the same phase used by the CTs for current. This will allow the regulator to maintainfield power for any fault that may be applied to the generator output.

A short involving any one or two of the lines with a CT will provide power through thatsource. In the event of a short of the other lead to neutral, the two remaining power leadsshould provide adequate excitation power for the system.

Whether you use a CBS or SBO system, there are two major design characteristics thatmust be observed. The first is that all of the boost systems are phase sensitive. This isbecause they use the principle of phasor addition in order to ensure that the system actu-ally provides a boosting action. Therefore, the correct phase relationships as shown in theinterconnections must be followed. If this is not done, damage may be caused to the boostsystem, or a failure to hold excitation during a fault condition may be experienced.

As discussed in the previous pages, there are two basic methods of providing excitationsupport. One is the current boost type, providing power directly to the field. The other isthe SBO type, which provides a relatively constant power source to the regulator. Eitherone of these systems can provide adequate power to the exciter field of your machine toride through the disturbance caused by either a fault condition or application of largeinductive loads as with motor starting. However, the SBO, with its large capacitors andinductor, will eliminate the adverse effects that a distorted power input waveform may haveon the regulator’s SCR controlled output stage.

The specification suggested that some “auxiliary equipment” is also required. This “auxil-iary equipment” may refer to protective relaying that protects the generator and loads frombeing damaged from abnormal operating conditions or faults on the power system. At

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least two protective functions should be considered on every generator. These are asessential to generator and loads as the overspeed, low lube oil, and high water tempera-ture are for the engine.

The first protective device is overvoltage. If the excitation system fails to control the genera-tor voltage, the voltage may go to the maximum of the generator’s capability (saturationvoltage) ranging from 40% - 80% above rated. The generator and the owner’s load willsuffer damage if this condition persists. An overvoltage sensor or relay is recommended totrip the generator circuit breaker and remove excitation.

Figure 13: Typical Protective Relay Interconnection

Finally, after knowing all the system loads and generator requirements, we can select avoltage regulator. From Table 1 below, an APR 63-5 and CBS 305 Current Boost Systemwere selected. These devices were selected because of the concern about motor starting,block load pick-up and, of course, price.

Cont. Cont. Min. Freq. Regulator Remote Non Lin. PMG CBSOutput Output Field Comp Model Adj. Loads Opt.Current Voltage Ohms Compatible

4.0 63 16 Yes VR63-4 via pot. Fair No No

4.0 63 16 Yes AVC63-4 via pot. Fair No No

5.0 63 12.6 Yes APR63-5 via pot. Good 60-90 Hz Yes

7.0 63 9 Yes AEC63-7 via pot. Good 60-400 YesHz

15.0 63 4.2 Yes DECS63-15 via cnts Excellent 50-400 NoHz

Table 1: 63 Vdc Regulator Selection Chart

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Example #2 Marine Generator Application

Marine generator applications follow the same basic engineering practices as the standbysystem with the exception of the ship’s individual load profiles, and the standard require-ments of the Coast Guard and American Bureau of Shipping. Both U.S.C.G. and A.B.S.inspectors put requirements on the equipment that must be used onboard the vessel.The generator set is no exception.

For the application, the generator to be used is rated at:

480/277 Vac, 4 Wire, Grounded Neutral60 Hz2,000 Kw3600 RPMBrushless ExcitedFull Load Exc: 63 Vdc @ 8.5 AdcP.M.G.: 260 Vac No Load/240 Vac Full Load 240 Hz

The load profile of the ship is similar to the loads that would be seen in a normal buildingapplication except for communications equipment (Radar, Radio Equipment), LargePumps and Navigational Equipment. These special loads may require special excitationequipment to provide the generator with high performance and long term reliability. Manyships are equipped with devices called bow thrusters. Bow thrusters are electric motorsthat drive propellers to help steer the ship in tight quarters while docking and maneuver-ing. These motors are controlled by large SCR drives and create harmonics/distortion onthe generator’s terminal voltage. This distortion may cause misoperation of the regulator.The regulator itself may generate EMI at levels high enough to cause misoperation of theship’s communication system. So, extreme care must be taken in selecting a regulator forthis application.

Another very common concern in shipboard applications is the ability to control the excita-tion system from a remote location. The operator is generally some distance from thegenerator and excitation system needs to control the generator’s output. It is important forthe regulator to take a contact input directly.

As previously mentioned, the USCG and ABS put requirements on the excitation system.They require a manual back-up to the automatic regulator, and a fault current supportsystem (CBS or SBO) is required for generators over 100 kW.

There are two basic types of manual voltage controllers (MVC). The first is the variabletransformer (Variac) type. This type of controller offers manual control of the excitation forsystem trouble shooting and for back-up to the automatic regulator. This type of MVCoffers operation in manual while completely isolating the regulator for maintenance pur-poses. This model of MVC is MVC 104 and is shown in Figure 14.

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Figure 14: Manual Voltage Control (MVC 104) Interconnection

A lower cost alternative to the model MVC 104 is an MVC 300. This device does not in-clude total isolation of the automatic regulator but, because it is solid state, it does offer amore constant output voltage than the variable transformer type.

Figure 15: Manual Voltage Control (MVC 300) Interconnection

On this shipboard application there is also a requirement for fault current support. Wecould fulfill this requirement with a CBS or SBO as with the simple standby generator inExample #1, but since this generator is equipped with a permanent magnet generator(PMG), we will not need to add another current boost system. The PMG will provide inputpower to the regulator during fault conditions or motor starting events.

The PMG is a separately mounted generator connected to the shaft of the main generatorfor the sole purpose of providing a power input to the voltage regulator. The only consider-ations to be made when using a PMG are the PMG’s output ratings and the regulator’scompatibility. PMGs typically have more poles than the main generator and, therefore,have a higher frequency output. The regulator must be capable of accepting this higherfrequency for proper regulator operation. Care must also be taken in the event the PMG is

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a three phase device when the regulator is a single phase device. If the regulator onlyloads two of the three lines of the PMG, the kVA rating of the PMG has to be derated by asmuch as 1/3.

For this example, a DECS 63-15 was selected for several reasons. The DECS unit is micro-processor based and will take contacts directly for remote control. This helps to meet theeconomic goals of this project, because separate motor operated potentiometers forremote control do not need to be bought. Secondly, the DECS can also accept a PMGinput, single or three phase at any frequency from 50 to 400 Hz. This means that compat-ibility with the PMG is not a concern. Thirdly, DECS has a pulse width modulated (PWM)power stage that is excellent for use with non-linear loads like the bow thrusters. Finally,DECS offers high field forcing for improved motor starting characteristics.

Also, an MVC must be added to meet the requirements of the USCG and ABS. A solidstate device for low cost and output regulation was selected. Care must be taken with asolid state MVC so that it is compatible with the PMG. Alternatively, power for the MVC maybe taken from generator output voltage, since manual control is an emergency mode ofoperation.

Prime Power Applications

Prime power applications are unique in several ways. Because the generators are gener-ally larger in kW rating, they typically have medium or high voltage outputs. This highervoltage makes step down transformers for the sensing and power inputs necessary. Primepower applications also have a concern about system integrity and redundancy for criticalgeneration sites. These applications also require direct communications because many ofthese sites are remote and unmanned.

Potential Transformers/Fuses - For generator voltages above 600 volts, fused potentialtransformers with adequate accuracy for metering are required. Single phase or threephase PTs with secondary voltage ratings of 120 or 208/240 volts may be supplied. ThePTs may be used only for voltage regulator sensing or may be combined with meters andrelays within the burden capacity of the PT. PTs are also used for “Dead Front” Switch-board design for 480 or 600 volt generators, in the same manner as for voltages above600 volts.

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Figure 16: Sensing Transformers

Power Isolation Transformers/Fuses - Excitation power for generator voltages above 600volts, and sometimes below 600 volts, is provided by a stepdown transformer rated tomatch the regulator’s input power requirements. The secondary voltage is based on theregulator specs. The maximum transformer rating is also taken from voltage regulatorspecs, but the required transformer rating depends on the exciter field resistance.

Figure 17: Low Voltage Power Isolation Figure 18: Medium Voltage PowerTransformer Isolation Transformer

For example, the AEC63-7 specifications are listed below along with some generatorspecifications. By sizing the power transformer based on field resistance, the transformersize and cost is kept low.

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Figure 19: Power Isolation Transformer

AEC63-7 Generator63Vdc 63 Vdc, exciter7 amps 15 ohms resistance1100 volt-amperes burden240 volts ac input9 ohms minimum fieldresistance

9 = .6 .6 x 1100 VA = 660 VA15

A 660 VA transformer may be used instead of the 1100 VA based on the regulator’s maxi-mum burden rating.

Prime power application specifications almost always specify the need for “black start”capability. This is the ability of the generator to build up voltage when there is no otherpower source available, except for the station batteries. To ensure positive voltage build-up, a field flashing circuit can be added. Most modern generators do not require flashingexcept in those rare occasions when residual is lost. An automatic flashing circuit can beadded without much difficulty if a battery or other dc source is available. To determine howmuch flashing current is required, you can use a quantity equal to half of the no-load fieldcurrent. This information is typically available from the generator manufacturer, but if nodata is available, use a value equal to 10% of the regulator’s rating.

With the battery voltage rating, flashing current and field resistance, the value of a currentlimiting resistor can be calculated to limit maximum flashing current.

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Figure 20: Basic Field Flashing

Battery 24 Vdc R + 18 = 24 = 60 0.4

Regulator rating: 4 amps R = 42 ohms

Flashing current: 0.4 amps (10%) Use 50 ohms, 10 watts

Field resistance: 18 ohms

In addition to a battery and a current limiting resistor, a diode is required to prevent regula-tor output from charging the battery. Add a switch contact to start and stop the flashing tocomplete the system. The switch contact should close when the generator is rotating andshould open when generator voltage builds up. These functions are often provided by aspeed switch and an ac relay.

If a grounded battery is to be used to flash the field, a power isolation transformer is rec-ommended to prevent dc circuit grounds from destroying the regulator.

Figure 21: Auto Field Flashing

Most prime power applications are vital to the utility company to ensure there is adequategeneration to provide their customers with all the power required to meet their needs. To

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ensure that the generation system is extremely reliable, the levels of complexity in excita-tion systems are increased. This increase in excitation system complexity is not limited tothe prime power application; it may also be true of a standby system used on an extremelycritical load, for example, a hospital.

In the case of a standby system, its whole purpose is to minimize the power outage to asshort a time as possible, in most cases between 5 - 10 seconds. For some applicationseven this may be catastrophic. Many schemes exist to reduce or eliminate the poweroutage. They are commonly called uninterruptable power supply (UPS) systems. Althougha number of UPS system designs involving static and rotating equipment have been de-vised, most of them use conventional excitation system equipment.

Figure 22: Standby System Performance

There are many ways to improve the overall generator system reliability, whether the sys-tem is a standby system or a prime power system. One method is to provide for a redun-dant or back-up voltage regulation system. These systems vary greatly in complexity andfunction and are based primarily on the application. These redundant systems can be assimple as an automatic regulator with a manual back-up device or, the system can be ascomplex as a completely redundant automatic voltage regulator system with automatictransfer to the back-up channel. The systems can have auto-follower features that allowthe back-up device to follow the operation level of the primary device.

Figure 23: Uninterruptable Power Supply (UPS)

Figure 24 shows one of the most common and basic types of redundant regulation sys-

UTILITYPOWER

BATTERYCHARGER

BATTERYSTORAGE

DC - ACCONVERTER

GEN

LOAD

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tems. This type offers both an automatic voltage regulator and a manual voltage controlsystem. With this system, either the automatic voltage regulator or the manual voltagecontroller is in operation at any given time. Therefore, if a failure of one device occurs, atransfer from the failed device to the standby device is performed. The generator mayremain on-line during the transfer. This system offers many desirable features. One suchfeature is the ability of some manual voltage controllers to provide total isolation of theautomatic regulator for maintenance purposes, while continuing to generate power in themanual mode. Another feature of the scheme is the ability of the Manual Voltage Control tooperate without the need for a sensing input. This is an essential feature should the sens-ing transformer fail or should a fuse on the sensing input to the regulator open. When oneor both of these happen, the automatic regulator will turn full on because it has lost itsreference. Since the manual regulator does not need a reference to operate, its output iscontrolled manually, dependent only upon the operator, and the generator can stay on-lineproducing power even with a loss of sensing input.

Figure 24: Typical Backup System Schematic

The redundant scheme in Figure 24 consists primarily of an automatic voltage regulator,manual voltage controller, and transfer switch S1. There are also components shown thatare common to most redundant schemes. These components are the fuses in the power

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input circuits of each regulator and the diodes across the exciter field. If a power semicon-ductor short circuits in the automatic regulator, the fuses associated with that regulator willopen, removing it from control of the generator and prohibiting that failed component fromaffecting the operation of the back-up manual voltage controller. The series diode pairprovides a discharge path for the highly inductive field circuit during the transfer from oneregulator to the other. This discharge path allows for transfer from Auto to Manual andback to Auto while the generator is operating. The diodes are typically rated at 800 to 1000PIV and 1.5 to 2 times the rated field current. There is also an internal single diode acrossthe regulator’s output that does the conducting during normal SCR commutation.

This type of transfer switching arrangement may create a severe system disturbance, orbump, during the transfer from auto to manual control. If the excitation system is operatingin automatic voltage regulation and a transfer is performed, a bump will occur unless themanual voltage controller happens to be at exactly the same dc output level. To preventthis bump, some method must be used to allow the output of the two devices to bematched prior to transfer. The most basic method of matching the outputs of the twocontrol devices is with a nullmeter (see Figure 25). This device compares the outputs ofthe two devices and indicates 0 volts or a null when the two sources are matched. Thenullmeter indicates zero at the center scale position.

Figure 25: Nullmeter

The addition of a nullmeter necessitates both auto and manual devices to be turned onsimultaneously. This simultaneous power-up creates problems for certain types of manualvoltage controllers. If the manual voltage controller has an SCR power stage, the output ofthat device will charge up to an extremely high level of output voltage. A loading resistor isapplied across the output of the manual voltage controller to diminish the high dc voltage.

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With the addition of the loading resistor, the output voltage of the manual voltage controllercan be accurately compared to that of the automatic regulator. Care must be taken in theselection of the loading resistor value so that it does not force the total parallel resistance(the resistance of the loading resistor and the field circuit) to be below the minimum re-quired by the regulator. If the total resistance is less, the regulator may see a decrease inits life because of the excessive field currents drawn due to the lower total field resistance.See Figure 26. The value of the load resistor may be obtained from the voltage regulatorand manual control supplier. Basler recommends sizing the resistor to draw 250mA dc atthe avr continuous voltage rating. Make sure this added current does not increase fieldcurrent from the avr above its rating. R1 and R2 are normally the same rating.

Figure 26: Bumpless Transfer by Nullmeter Method

Because both control channels are energized at the same time, and their F- outputs aretied together, it is recommended that an isolation transformer be added between avr andmanual control to prevent a failure in one of the two from causing a failure in the other.With the addition of the nullmeter, transformer and load resistor, the manual voltage con-troller output can now be accurately measured. The operator can now precisely match themanual voltage controller’s output prior to transferring from automatic voltage regulation tomanual voltage control. With this method of nulling or matching the outputs of the twocontrolling devices, the transfer will be effectively bumpless and no generator disturbanceswill be noted.

The above systems require some sort of manual intervention prior to transfer. If the ma-chine is operating at various load levels, varying amounts of excitation will be provided bythe automatic regulator. In order to maintain a null between the auto and manual modes,the operator must periodically readjust the manual voltage controller’s set point. If there isa requirement for automatic transfer to manual when a failed automatic regulator is de-tected, the manual mode must be adjusted as often as the load level/excitation level

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changes. For these systems, an auto-follower or auto-tracking device is used. See Figure27. This type of device automatically forces the manual mode to be continuously nullingthe two systems, a transfer can occur at any time, and the result is that the generator willbe at effectively the same load level as it was prior to the failure/transfer.

There are unique devices to perform the auto-tracking function. They compare the outputof the automatic voltage regulator to the output of the manual voltage controller and eitherincrease or decrease the output of the manual voltage controller to create the null.

This device eliminates the need for human intervention where an operator adjusts themanual voltage controller as the excitation varies. Because the RA-70M continuouslyadjusts the output of the manual voltage controller, a transfer can be initiated at any pointby some type of protective relay scheme. Care must be taken in selecting theauto-tracking device to ensure that the device has some intentional delay in the trackingresponse as a standard feature. This delay will ensure that the manual controller does nottrack the automatic regulator into a fault. An example of this could be in the case of loss ofsensing. When a loss of sensing occurs, the automatic controller will immediately turn fullon, forcing maximum output into the field. If the RA-70M were to respond instantaneously,it would follow the automatic regulator’s output to a maximum level and the manual modewould be at the same higher excitation point. Some delay, typically in the range of 2 - 5seconds, is incorporated to allow the protective relays time to transfer to manual modewhile the manual mode is still at the excitation level prior to the loss of sensing.

Figure 27: Auto Transfer to Manual

Obviously, during a fault the transfer to manual control is anything but bumpless. In theexample of loss of sensing, there is first a generator overvoltage/overexcitation conditionand then, after the transfer, a return to a normal level. The term “bumpless” applies only toa normal, non-faulted transfer to the back-up mode of operation where the generator’s

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output is effectively the same before and after the transfer between auto and manualmodes.

In Figure 27, transfer relay K1 is incorporated to perform the function of the transfer switch,because the transfer needs to be automatic and initiated by the protective relays. Thistransfer relay also could have been an 86 device or lockout type relay that required amanual reset to the automatic regulator. This manual reset mechanism assures that some-one has to make the decision to reset after first investigating the trip to manual mode.

The types of protective relays utilized are dependent upon the generator and its use. If thesystem is stand alone (not connected to any other generator or the utility grid), the relayswill probably be as simple as an overvoltage/undervoltage device.

Figure 28: Bi-Directional Transfer Method

If the generator is paralleled to a utility grid, the relay types could be loss of sensing, over/underexcitation relays, and reverse vars or loss of excitation relays. Any number of differentscenarios can be used to initiate a transfer.

An additional facet of the auto-manual regulation system is the ability to transfer from themanual mode back to the automatic regulator in a bumpless fashion. See Figure 28. Thistype of transfer is possible, but it tends to be more of an operator-assisted type of transferand is very difficult to automate. The actual transfer from manual controller to automaticregulator is very similar to the process used when manually synchronizing a generator witha synchroscope. The nullmeter replaces the synchroscope and the operator looks at volt-age difference instead of phase angle. While in the manual mode, the operator slowly

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adjusts the automatic regulator’s set point. As the regulator nears the regulation (null)point, the nullmeter will start to move off the extremes (full clockwise or counter-clockwise)of the meter back to the center/null position. Once the meter is at the null point, the opera-tor can initiate the transfer and obtain a virtually bumpless transfer. The operator’s skilllevel and the rate of adjustment of the regulator’s set point have a direct impact on themagnitude of the bump seen on the system.

As with the manual voltage controller, the output of the regulator cannot be left open inorder to get proper voltage measurements. To obtain accurate output comparison mea-surements, a loading resistor is applied to the output of the regulator. The sizing of thisloading resistor is the same as the one used on the manual device, and the same criteriaare used. It is likely that the resistor will be identical to that chosen for the manual mode.

AUTOMATIC - AUTOMATIC REGULATION SYSTEMS

Redundant regulator systems that incorporate two automatic voltage regulators are com-mon in unmanned or remote generation facilities. Incorporating two automatic regulatorshas the advantage of always being in the auto mode. In other words, no manual controllerand, therefore, no human intervention is required. If a transfer occurs, system perfor-mance, such as load on/off transients, will be handled automatically by the back-up auto-matic regulator, and system performance is identical to that of the prime regulator. Nomanual adjustment of excitation is required. After the initial regulator failure and systemdisturbance have occurred, the only noticeable indication of the back-up regulator being incontrol should be the status of the transfer relay.

Figure 29 illustrates the basic Auto/Auto system. It is similar to the basic Auto/Manualsystem in many aspects. There are two devices capable of generator control. There will besome form of transfer switching or relaying and there will be an isolation transformer be-tween the two regulators. One of the automatic voltage regulators will be considered theprimary regulator and the other back-up. The primary regulator will be in control at alltimes until it fails to perform its primary function, to regulate voltage or VARS.

As with the Auto/Manual system, the Auto/Auto system also has some standard circuitcriteria that have to be met. Refer to Figure 29. The Auto/Auto scheme utilizes the seriesdiode pair for field discharge during transfer from prime to back-up regulator. The powerisolation transformer is again installed to isolate the inputs of the two regulators, since theiroutputs are commoned at F-. Loading resistors are again used to provide for accuratemeasurement of the regulator’s output voltage.

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Figure 29: Dual AVRs with Tracking

To provide auto tracking of the two regulators’ set points, an RA-70 solid state referenceadjuster (with two separate outputs) or a motor operated/manual potentiometer with twopots on a single shaft can be used. These types of potentiometer arrangements providefor very close tracking of the two regulators’ outputs. As one potentiometer is adjusted, theother is automatically adjusted because of the mechanical tie between the two potentiom-eters. The accuracy of tracking is limited only to the linearity of each potentiometer.

The RA-70 has an advantage over the mechanical devices because it also offers two pre-positioned set points. These set points are preset and, when a contact is input to theappropriate terminal, the RA-70 instantly goes to a value of resistance that may be associ-ated to either off-line rated voltage or a value of resistance that is equivalent to rated on-line VARS out of the generator.

There are Auto/Auto systems that use the dual voltage adjust potentiometer technique, butthey still set the regulator’s internal voltage adjust slightly differently. An example would beto set the prime regulator for the nominal voltage and the back-up regulator 1% lower. Thislower voltage can allow for both regulators to be powered up and in parallel simulta-neously. The regulator with the lower regulation point will be turned off because it is tryingto bring the voltage down. By being simultaneously powered up, if a problem occurs in theprime regulator, the generator’s voltage will fall off until it reaches the regulation point ofthe back-up regulator. At this point, since the back-up regulator has its power suppliesenergized, the back-up regulator will start to conduct and the transfer will be virtuallyundetectable. This scenario assumes the regulator has failed in the “full-off” mode. If theregulator fails in the “full-on” direction, the generator’s voltage or Vars will increase drasti-cally. There will inherently be a significant system disturbance until the transfer to the back-up regulator can occur. With this type of transfer arrangement, the bumpless transfer toback-up, for maintenance purposes, is difficult.

23

Because each regulator is a high gain circuit, very small variations in line voltage cancause drastic swings in the regulator’s output. This makes automatic tracking of two auto-matic voltage regulators very difficult. Some systems simply do not transfer to the back-upregulator while the generator is on-line. The only time a transfer is performed is off-line andon a periodic basis just to verify the back-up regulator is operational. Because the transferis to an automatic regulator and not a manually adjusted device, the system disturbance isshort-lived and usually only as long as the response time of the regulator. This may be onthe order of several cycles. If a transfer to the back-up automatic regulator is performedunder load during a failure, the system disturbance can not be forecasted or avoided.

Figure 30: Bumpless Auto-to-Auto Transfer

If a bumpless transfer to the back-up device is necessary, a manual transfer scheme isshown in Figure 31.

With the addition of the nullmeter and two separate voltage adjusters, a manual transferscheme can be provided. This system works identically to the operation of the Auto/Manual system when transferring from manual to automatic mode. In this case, we aretransferring from auto to the back-up auto mode. It is necessary to put load resistors onoutputs of each of the automatic devices. This will provide for accurate measurements ofthe regulator’s output voltages. This nulling method is again similar to the synchroscopemethod except that the operator waits for the nullmeter to read zero or null before a trans-fer is initiated.

In order to maintain the automatic tracking of the two regulator’s set points, two RA-70devices are used. See Figure 31. By providing simultaneous Raise and Lower signals (viaK2), the two RA-70 will raise and lower at the same rate and, therefore, track in their adjust-ment of the regulator’s set point.

800-1000 PIV

1.5-2X RATED

CURRENT

AUTOMATICVOLTAGEREGULATOR

AUTOMATICVOLTAGE

REGULATOR(BACKUP)

6

6

4

4

E3

E3

F-

F-

7

7

3

3

E1

E1

F+

F+

GEN

NULLMETER

R2

K1

K1

S1

R1 S2

24

Figure 31: Tracking of AVRs by dual RA-70s

Overall system reliability is of extreme importance to the prime power application. Commu-nications requirements with the excitation systems may be made a requirement. Thisrequirement may be as simple as providing a contact input for adjustment or a 4 - 20 masignal input for control. The communications may be as sophisticated as master/slavecommunications through computer language over telephone lines or even fiber optics.These communications will help with the addition or removal of available generation tomake the utility grid ultra reliable.

Generator systems vary greatly, and the excitation systems used on them are even morediverse. The end result of a correctly selected excitation system is to have a generatorfunction properly over a long life for every system condition possibility.

800-1000 PIV

1.5-2X RATED

CURRENT

AUTOMATICVOLTAGEREGULATOR

AUTOMATICVOLTAGE

REGULATOR(BACKUP)

6

6

4

4

E3

E3

F-

F-

7

7

3

3

E1

E1

F+

F+

GEN

NULLMETER

R2

K1

K1

S1

R1 S2

Basler Electric HeadquartersRoute 143, Box 269,Highland Illinois USA 62249Phone +1 618.654.2341Fax +1 618.654.2351

Basler Electric InternationalP.A.E. Les Pins, 67319 WasselonneCedex FRANCEPhone +33 3.88.87.1010Fax +33 3.88.87.0808

If you have any questions or needadditional information, please contact

Basler Electric Company.Our web site is located at:

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