fuji generators fer-14-01-01-1968[2]

8/3/2019 Fuji Generators FER-14-01-01-1968[2]

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UDC 621. 313. 126: 621. 314. 63-52: 546-28

THYRISTOR-TYPE EXCITATION EQUIPMENT FORWATER AND STEAM TURBINE GENERATORS

By Masaomi NagaeTakehito InoueDevelopment Dept.

I. PREFACEBecause of recent increase in electric power needs,

the generating capacity of both steam and waterpower stations has been s tepped up. Since generatorsare of prime importance in this case, static excitat ionequipment, noted for the high response ratio hasbeen used rather frequently in place of rotary exciters.

In accordance with the remarkable progress madelately in semiconductor techniques, the reliability ofvarious kinds of semiconductors such as thyris torsand transistors has increased so much that 'effectiveapplication of such semiconductor characteristics as :(1) Small and light weight,(2) High mechanical strength,(3) Low forward loss and high efficiency,(4) Rapid operating and short response time,(5) Small input' and gate signals,(6) Expansion of permissible operating temperaturerange.has made it possible to manufacture static excitation equipment which is small and sturdy, usefulwith small power sources, and above all equivalentin response to self compound excitation equipment.Thus static excitation equipment is now being usedin place of customary static-type self compound excitation equipment composed mainly of magneticamplifiers, saturable current transformer.

Generally speaking, the response in generator ex-citation systems can be considered in relation to twomajor factors: time lag existing in all sections ofthe closed loop, and regulator characteristics. Theopen-circuit time-constant is most important whenconsidering the former.

For instance, when it is necessary to intensify themagnetic field because of some disturbance, theamount of intensification of the magnetic field currentis directly proportional to the time integral of thevoltage that is induced in magnetic field by the actionof the excitation equipment during the disturbance.Therefore, it is absolutely necessary to intensifythe field current by inducing a large voltage in thefield as rap idly as possible during the disturbance.

The magnitude of voltage induced in the field is

generally limited to 1.5-2.0 times the nominal slipringvoltage according to the insulation level or excitationequipment capacity. The response ratio of variousexcitation equipment depends on how fast field voltage can be supplied during a disturbance.

Of course, in connection with the capaci ty tosupply the necessary reactive power to the systemand the synchronizing power, generator impedanceand short-circuit ratio play a large role.

Although static excitation equipment with magneticam plifiers and amplidynes has always been verycommon, there was a time lag in the ir control devicesand they were accordingly l imited in use as excitation equipment.Self compound excitation equipment has solvedthe problem of time lag in the control device bysupplying the necessary vol tage direct ly to the fieldvia a current transformer.

Progress in the manufacturing techniques of semiconductors, especially transistors and thyristors, hasled to both reliability and low prices. I t is nowpossible to manufacture control devices without timelag. This, in turn, has allowed for the fabricationof thyristor-type excitation equipment equivalent toself compound excitation equipment in response ratioand superior to 'it in price and size.

For generators used in atmospheres containingreactive particles, brushless excitation equipment withsimple maintenance and inspection has been developed using toyris ters for control.II. APPLICATIONS OF VARIOUS KINDS OF THYRISTOR

TYPE EXCITATION EQUIPMENTIn comparison with rotary exciters, the ' usual self

compound excitation equipment consists of a reactorwith shunt winding characteristics, a current transformer with series winding characteristics, and asemiconductor rectifier for ac-dc conversion. Theeffective blending of the former two factors has resulted in compound characteristics and a high responseratio.

Even in the thyristor-type excitation equipmenthowever, use of a transistor as detector and regulatorallows time lag factors to be reduced to dead time


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with detector filter and thyristor controls (this timelag can be kept as low as 20 msecs at maximum byuseing a detector filter with a high response ratio).I f the excitation equipment source is maintainedsatisfactorily, the equivalent charcteristics can be kepthigh in response to dis turbances such as vol tage andload fluctuations with the customary self compoundexcitation equipment.Principles in "Thyristor-Type Excitation Equipmentfor Water and Steam Turbine Generators" should beapplied in pract ice according to several basic methodswhich differ as follows:(1) Type of exciter(2) Type of thyristor connect ion (hybr id bridge connection)(3) Type of thyristor input source, and(4) Type of AVR device and other detailed specificat ions for application.

Each method has merits and demeri ts ; practicalapplications should be decided considering variouscharacteristics such as excited generator capacity,rotational frequency, short-circuit ratio; system response; requi red excitat ion capacity at line shortcircuits; excitat ion system response ratio; and thesurroundings. (Comparison of each method is referred to in Table 1.)

In order to maintain a stable excitat ion source, anac source generator for thyristors must be placedcoaxially with the main generator as is shown inTable 1 (a).

Although there is no particular problem in turb inegenerators with a high rotational frequency, thereis a problem in turbine generators with a low rotational frequency-i.e., application of source generatorswith high enough ceiling voltages to cope with disturbances would result in very large devices and costs.Because of this with water wheel generators, it isgenerally desirable to use the self-excitation type toremove the thyr is to r source from the main circuitvia a source transformer. Table 1 (b) shows thissource transformer method, which is extremely effective in the response to system disturbances undernormal operation.However, this method has one defect-loss of excitation source during an overall short-circuit makesforced excitat ion impossible. However, th is methodcan be adopted for medium and small capacitygenerators which have no particular importance in thesystem, or for generators which are to be aligned ina single system and desig,ned to s top during shortcircuit result in no l ine cuts.

That is, when a short-circuit occurs in the system,

Table r Various Types of Thyristor-Type Excitation EquipmentRegenerating ExcitationExcitation Thyristor Connection Brake with Capacity Brush Economicsystem Input Method Pure-Bridge Response during a Short - Main- ApplicationSource Connection of tenanceThyristor circuit

a) Ac source Direct couplinggenerator h source generator(direct Effective Highly Highly Necessary is more economicalcoupling or effective effective for high speed de-separate G (A) '"'- vice (ex. steaminsertion) power)Static Simpler and moreexcita- b) Source Inferior during economical than (a)tion transformer source Inferior Inferior Necessary or (c); possible withequip- disturbance medium and smallQ) ment (B) capacity generatorS0.. Profitable with'30" c) Source t:Q2:---r= water wheel gener-transformer ators. Rather ex-

and source ~ Highly Highly pensive in high,8 AVR Effective Necessarycurrent '\ ... effective effect ive speed machine withE transformer G ' (e) a small short-circuit'u ratio (ex. steamxp;,l power)Q) Ac kf Effective when Highly effective0..>. excitor input source is when the input Applied in unsatis-E-< (direct lAVRllnput (a) or (c); in- source is (a) or factory environ-\-< coupling Same as in ~ _ ~ - j J 0 u r c e ferior during (c); inferior No t ments; economical2 brush- static excitation source distur- Slightly during source necessary when used in high. equipment inferior\-< less bance when disturbance speed machines>. C ___genera- (D) the input when the input (ex. steam power)E-< tor) source is (b) source is (b)

Dc Effective when Highly effectiveexcitor L< input source is when the input(direct- Same as in , AVRllnput (a) or (c); in- source is (a) or Used for reforma-coupling static excitation e t ~ " ~ eferiol during Slightly (c) ; inferior Necessary tion of existingor equipment source distur- inferior during source excitationseparate bance when disturbance equipmentinser- (E) the input when the inputtion source is (b) source is (b)

2 Vol. 14, No.1 FUJI ELECTRIC REVIEW


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even if the line is cut by the last breaker , i t doesnot take more than 2 seconds. In the case of generators with a big Td' and small xd' and xd such aswater wheel generators, it is even possible for thefield current to main tain the necessary voltage forreestablishment of the voltage after the line is cut.In case of turbine generators, however, there is aslight disadvantage, but line cut ting in generators is

possible when th e thyristor source is not t aken fromthe power take-off side of the generators, but fromthe same bus line with the impor tant auxil iary machine in the boiler system, since the turbine must bestopped when this source fails.For generators that are important in the system,

thyristor supply by combining components is derivedfrom the supply transformer (voltage) and currenttransformer (current) as shown in Table 1 (c).Since the cur rent part is meant for field maintenance during short-circuits , economical design ispossible if the current transformer capacity can bemade as small as possible by making the inducedvoltage and time as low as possible.Therefore, the method is not economical whenapplied to generators that can be coupled directlyat low cost with high speed machines, and have abig xd value l ike turbine generators.The excitation equipment of brushless ac excitors

and de exciters with thyristors-(cf. Table 1 (b) and(c)- is slightly different in comparison with the abovementioned equipment, and is slightly inferior to themin response since it requires the help of excitors.The brushless method, in particular, aims at simpli

fying maintenance by decreasing the brushes, sliprings, and commutators. Since rotational frequencyis particularly high, it is th e most suitable excitationequipment for turbine generators in which brushwear is high.III. EXCITATION EQUIPMENT WITH SOURCE TRANS-

FORMERSA generator with a source transformer in which

the thyristor is supplied from the main circuitthrough a source transformer has been delivered tothe Sumise Power Plant of the Nippon Light MetalCo., Ltd. Presently it is operating well. Several moreare now under production to be delivered to variousplants.Excitation equipment with a source transformerin general will be discussed later, based on actualdata from the Sumise Plant.Specifications of machines and devices in the SumisePlant are as follows: (As to the circuit of this typeof excitation equipment and it s excitat ion cubicle,see Figs. 1 and 2 respectively.)Generator: 14,500 kva

6.6 kvAVR equipment : Transidyn AVRAVR source: 4000 va

TRANSIDYN automatic voltage regulator r

i ~ ~ f r E ~ ! h : ; - - - - PMGSurge absorber

Fig. 1 Thyristor-type excitation equipment wi th source transformer

Fig. 2 Excitat ion cubic le

Thyristor: 3 phase hybrid-bridge connectionCharacteristics of excitation equipment with sourcetransformers are: high response ratio to overall dis

turbances such as voltage and load fluctutations duringnormal operation, simplicity of control since a largetime lag occurs only in the generator field system,and ease of maintenance since the circuit is verysimple.Effective application of a TRANSIDYN device withsemiconductors such as transistors and diodes developed by Fuji has resulted in the production of veryhigh quality excitation equipment.

Thyristor-Type Excitation Equipment for Water and Steam Turbine Generators 3


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1. TRANSIDYN AVR DeviceTRANSIDYN AVR device consists of detectingunits, set ting units, regulating units, and pulse gener

ator units, as shown Fig. 1. Its characteristics areas follows:(1) Easy operation identification and maintenanceinspection since all parts are based on the stand

ard TRANSIDYN sizes with draw-out type construction. The regulating cock, meter, and test jackare arranged in the front of each panel (see Fig.3).

(2) High reliabili ty since all transistors, diodes, resistors, condensers, etc. have been selected carefullyfor the rat ed values with sufficient margins.

(3) High static gain since a PI regulator is used inthe regulating unit; the equipment is capable offull-range regulation with a trans isto r optimumregulation system.

(4) Low time lag, which occurs only at the detectionfilter and is limited to around 20 msecs maximumsince the detection filter has a high response ratio.

(5) Power can be supplied from an ordinary permanent-magnet generator with an electric governorsince power requirements are low.

Fig. 3 TRANSIDYN cucicle andTRANSIDYN unit

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2. Thyristor-Type ConverterThe thyristor-type converter contains a 3-phasehybrid-bridge connection of thyristor and silicon diode.The most prominent feature of this 3-phase hybridbridge connection is that it enables perfect voltage

maintenance by means of a commutating diode.There is no fear of abnormal voltages caused by openfields even when the gate pulse signal misfires sincethe commutating silicon diode short-circuits the fieldcircuit.

There is another thyristor-type converter the 3-phase pure-bridge connec tion consisting solely ofthyristor diodes. This system features rapid fieldcurrent reduction when compared with the hybridbridge connection, inverter action when the thyristorsource operates normally and quick demagnetizationduring some disturbance inside the generator. Thereis, however, a danger that the thyristor will misfire.This system is not effective in excitation equipmentwith a source t ransformer , since the thyristor sourceis always cut out when there is a short -c ircuit insidethe generator.Both the thyristor and silicon diodes are singleunits consisting of cooling fins, fuses, fuse alarmrelays, resistance for potential dividing resistors,connection condensers, and pulse converters (onlyin the case of thyristor diodes). They are arrangedin sequence in a cubicle of the draw-out type.

With this type of construction, it is easy to arrangea spontaneous commutation circuit. The circuit iscompact and interchangeable since each un it can bedrawn independent ly . Individual units can be replaced separate ly when there is some trouble in thediode or other parts. Maintenance inspection istherefore facilitated.

Fig. 4 Thyristor cub ic le andstructural unit

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The pulse converter shapes the gate pulse signalfed to the thyristor. It converts the signal in to twopulses; a hair pulse which is rapid at the beginningand wideband pulse to supply sufficient ignitionenergy. These are superimposed and sent to thethyristor gate.

Thus, it is possible to do away wi th overloadingwhich occasionally occurs because of spreading outof the s tart -up time, even when there is an increaseof registration with thyristors in series or parallel.The thyristor can be used even in high capacityexcitation equipment without worry.When choosing a diode, it is necessary to selecta rating with a sufficient margin for continuousresistance at maximum current during normal operation. A short time rating is also necessary in order

to protect against large overcurrents occasionallyinduced in the field when a 3-phase short-circuitoccurs in the main circuit.3. Connection of the Source TransformerAs shown in Fig. 5 there are two possible locationsfor the thyristor source. One is the power outputside of the generator (A), and the other is thet ransmission line (B). Each point has merits anddemerits and choice must be made after carefullyconsidering the situation in the individual powerstation of the transmission system.With self-excitation equipment, the former point(power output side of the generator) is generallyused since it is possible to utilize the mains as asupply source during short-circuits in the system.

In this case the generator voltage is maintained bythe sel f exciting source when the line is cut.However, there is one disadvantage: the fieldsystem requires a separate excitation source (ex.battery) at the beginning until the AVR equipmentbegins to operate. In this case, if the source voltageis supplied by TRANSIDYN AVR excitation equipment from a permanent magnet generator, the supply can be continued until the synchronizing signalof the phase regula tor takes affect and the 50%voltage is sufficient for safety purposes.

Fig. 5 Supply points ofsource transformer

When the thyristor source voltage is supplied fromthe transmission line, initial excitation is unnecessaryat generator start-up, since a parallel generatorbreaker is opened-i.e., it is the same as if thevoltage were supplied by a separate excitation sourcefrom the transmission line. In other words, thesystem vol tage is required before the generator willstart; only when the generator is connected inparallel with the system will the sel f excitat ioncircuit be formed.With this method, there is no need to form aspecial circuit even during drying when the generator

is shut down for a long time as long as the transmission line maintains the voltage. A test is veryeasy even when the generator is shut off. TheAVR device source voltage can be obtained from asource transformer.4. Field Circuit ProtectionGenerally speaking, semiconductor diodes have

such disadvantages as small overload capacit ies andlimited inverse voltages (almost zero during a timeexcess).For this reason, it is necessary to consider protecting the semiconductor diode against abnormal

phenomena which sometimes occur in the excitationcircuit during operation.(l ) Overvoltage protection

I t has always been considered that abnormalvoltage sometimes occurs during load interruption,short-circuits, lightning strikes, and commutationin excitat ion circuits which include ac circuits.Although during load in ter ruptions the re issometimes a decrease in field current in order

to resist excessive magnetomotive force that hasbeen reducing the armature reaction, no abnormalfield vol tage occurs since the field current cannot flow in the inverse direction. Depending onthe power factor, there is a voltage increase equalto the synchronous reactance drop in the generator caused by the load current on the maincircuit side. However, this can be consideredas negligible since it is only 20% maximum andis added in the direction of easy flow in thethyristor and silicon diode; it can also be inhibitedimmediately by AVR device with a high responseratio.With a 3-phase short-circuit, generator fluxdrops at the beginning of the short-circuit, andtherefore, field current increases in the direction

of increasing flux-i.e., the direction of easy flow.Even if the thyristor accidentally misfires at thismoment, abnormal voltage never occurs in thefield since current flows through the commutatingsilicon diode. With a single phase short-circuit ,the field current includes a double harmonicfactor, although i t never becomes strong enoughto invert the field current.During a lightning strike in the transmission

Thyristor-Type Excitation Equipment fo r Water and Steam Turbine Generators 5


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GeneratorsaturationGenerator curvePulsegenerator

AVR G- limit

(a) Observat ion point

Fig. 8 Indicial response

(b) ResultsFig. 6 Transfer vol tage o f 14,500 kva synchronous

generator as pe r surge t es t

Fig. 7 Block diagram for ind ic ia I response

p

Observation Point 1 2 I 3 4 5--------Transfer Voltage (%) 1.03 1.38 0.73 0.26 1.02--------Observation Point 6 7 8 9 10--------Transfer Voltage (%) 0.24 1.05 0.19 1.05 0,31

Detector

ing transient characteristics which affect the excitation system response ratio are as follows:(1) Initial responseInitial response was tested in the excitationsystem closed loop, by altering the establishedvoltage by 10% . A block diagram of theexcitat ion system loop is shown in Fig. 7. The

5. Excitation CharacteristicsA series of tests have been carried out

at the Sumise Plant of Nippon Light MetalCo. to verify various characteris tics ofthyristor excitation equipment with a sourcetransformer. The results of tests concern-

line, abnormal voltage could be t ransfer red tothe excitation circuit via stray-capacitance betweenthe primary and secondary coils of the sourcet ransformer or between the generator stator androtor.This can be avoided by placing a sta tic shieldbetween the primary and secondary coils of thesource transformer so as to avoid a static connection. For utmost safety, an isolation transformer should be inserted in all input and powercircuits of the TRANSIDYN. A selenium arresteris placed in the field circuit in order to absorbtransfer voltage by uti lizing the inverse character-istics of the selenium rectifier.

The magnitude of the selenium arrester voltageis fixed so that" silicon inverse voltage> seleniumarrester voltage> ceiling voltage."

In order to measure the magnitude of thet ransfer voltage fed to the excitation circuit whena lightn ing s trike or the like occurs in the maincircuit, impulses from impulse analyzing equipment are supplied to the stator outpu t terminals.As shown in Fig. 6, effects were negligible.

Abnormal voltage sometimes occurs duringthyristor commutation when controlling inductionvoltage with the thyristor. This can be avoidedby placing a surge absorber on the ac side.

(2) Overcurrent protectionProtection against short-circuits and overloads

in the excitation circuit can be divided into twocases: ac circuits and dc circuits.

When there is a short-circuit in the ac excitation circuit, it is necessary t o pro tect the sourcetransformer. The common method is to inserta no-fuse breaker or air circuit breaker, with anovercurrent tripping device, on the secondaryside of the source transformer. Another methodis to place an overcurrent protection relay onthe secondary side.

When there is a short-circuit or over-load in a dc excitat ion ci rcuit , immediateprotection is necessary, since a thyris tor !1Vsdiode with a small short -t ime capac ity -- cwill soon be destroyed. Immediate protection on such occasions is possible byattaching super-rapid fuses to each ele-ment depending on the ir overcurrentcharacteristics.

The super-rapid fuse is attached withan alarm relay, which gives a signal onfusing in order to avoid spreading ofdamage.

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Various types of thyristor-type excitationequipment have been introduced and, as apractical example, observation data concerning thyristor-type excitation equipment asource transformer, which was delivered to theSumise Plant of Nippon Light Metal Co. wasgiven. I t is hoped that the excellent responsecharacteristics of thyristor-type excitationequipment are eviden t from this ar ticle.

When using a thyristor excitation systemin a generator , choose the most suitable method considering the various conditions discussed in this article.

In conclusion, we'd like to thank the staffof the Nippon Light Metal Co. who havecooperated with us in various fields.

IV. CONCLUSION

response time of one circuit, i.e., the timerequired to reach the established value forthe first time-was only 0.218 seconds inaccordance with the TRANSIDYN optimum adjus ting theory. Therefore thehigh response ratio of the system has beenestablished (cf. Fig. 8).

(2) Load interruption testFig. 9 is a block diagram pertammg to

load inter rupt ion, with the generator damper effect, internal induced voltage (due tothe transformation effect), and armatureresistance and speed at constant value.

Load in terruption characteristics can beobserved by simultaneous interruption atthe I d and I q pointsin Fig. 9. Theoreticalfigures obtained by a ca lculator agree wellwith the results of actual observations asshown in Fig. 10.

According to these results, the maximumvoltage during load interruption is almostalways less than the small transformervoltage caused by transient generator reactance (xd'). This can be expla ined bythe high response ratio of the excitationequipment. Therefore, the maximum voltage during load interruption can be approximated by plotting the voltage characteristics of the generator in a vectordiagram, without considering the speed riseof the turbine.

Field current (IF)

Field voltage (VF)

Generator voltage (t::.Y)

50o - - - - - - l - - . . . . J 2 - - ' - - ' ~ - ( . , . - - s - e c - - - ; : - ) ' - - ' 4

Calculated value(b)

Id

100

-5

(a)AV(%),15

10

Fig. 9 Block d iagram for computation at load interruption

Fig. 10 2 /4 lo ad interruptioncharacteristics

Fig. 11 4/4 load interruption characteristics

Thyristor-Type Excitation Equipment fo r Water and Steam Turbine Generators 7

fuji generators fer-14-01-01-1968[2]

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