How does exciter work on alternator?

6.1.3 Exciter transient performance

Exciters must operate over a wide voltage and current range as ceiling requirements are considerably in excess of rated full-load conditions. The exciter is required to respond quickly to changes in excitation at its own rotor terminals. This requirement for a fast response characteristic is achieved by the use of a short air gap and a laminated rotor body.

Exciter transient performance is characterised by the exciter response ratio defined in BS5000 Part 2, as follows:

Exciter response ratio=The average rate of increase in excitationopen− circuit voltage(V/s)Nominal excitation voltage

Typically, exciters are required to increase output voltage from 100% to 200% in less than 0.3 seconds, corresponding to a response ratio of 3.5.

The average rate of increase of the excitation open-circuit voltage is given by the slope of AC in Fig 6.58.

Slope of AC = BC/AB but AB = 0.5 seconds. Hence AC = 2BC (average rate of increase of exciter voltage) and the nominal exciter response ratio is given by 2BC/OA.

VIII.A Exciter

The exciter supplies direct current to the field winding of the generator, at whatever voltage is required to overcome the resistance of the winding. The rating of the exciter is specified as its output power, current, and voltage corresponding to the rating (or maximum capability, if different) of the generator, recognizing the temperature of the generator's field winding. The exciter rating may have some margin over this requirement, as defined when the generator is designed.

Over the years, many types of exciters have been used, but the type most commonly used was the comutator-type dc generator. This is very rarely used for new generators today. A new turbine generator is usually supplied with one of the following types of exciter: (1) shaft-driven alternator with solid-state diode rectifiers, (2) solid-state thyristor rectifiers supplied by a transformer deriving its power from the power system or from within the generator (the latter is a form of self-excitation), or (3) shaft-driven alternator with its output winding on the rotor, its output rectified by rotating solid-state rectifiers, commonly called a “brushless exciter.”

In addition to its normal function of providing the proper level of direct current to the generator's field winding as required for the apparent power being supplied, the terminal voltage, and power factor of the generator load, the exciter must also be able to produce a ceiling voltage (which is higher than rated exciter voltage) and to operate at that condition for a specified brief period, as required by the voltage response ration which is part of the excitation system specification. The voltage response ratio is a measure of the change of exciter voltage in 0.5 sec when a change in this voltage is suddenly demanded.

When the exciter is a rotating machine driven by the generator shaft, it becomes part of the turbine generator shaft system and must be designed to accommodate axial motions due to thermal expansion of the turbine and generator rotors, and vertical motions of the generator shaft due to bearing oil film and thermal expansion of the generator bearing support.

13.4.3 Exciters

Vibration exciters or ‘shakers’ are of two main types, electrodynamic and electro-hydraulic. The former are the more common, and are used for most modal testing, except for the very largest structures. They are also used for most environmental testing, except, again, for the largest equipment items. Electrohydraulic exciters are expensive, and tend to be used only where electromagnetic units would be unsuitable, such as when very large displacements, or a combination of steady and oscillatory forces are required. They will not be discussed further here: a good introduction to electrohydraulic exciters is given in [13.4, Chapter 25]. A wealth of practical information on all forms of exciters is, of course, available from manufacturers’ literature.

Diesel generator exciter model.

The static exciter utilizes current and voltage compounding with phase compensation through current and voltage transformers supplying the field via a full wave diode bridge rectifier. A model for this type of exciter is described in the IEEE report, ‘Excitation System Models for Power System Stability Studies’, (1981).

The model parameters were estimated by comparing the output of the model with the measured data and minimising the difference by modifying the coefficients using Box's ‘complex’ method (Beveridge and Schechter, 1970).

The result of the parameter estimation gave an absolute rms error of 64 mA over a current range from 2 to 4.5A. Although this result, with a maximum relative error of less than 3.5%, is quite acceptable if measurement errors are taken into account, some systematic deviation remains. Given the intended applications, the resulting model is considered to be adequate.

28.14.3 Brushless excitation

The a.c. exciter has a rotating armature with three or more phases and a stationary field system. It usually is designed for a frequency of 0–5 times the power system frequency. The pilot exciter, when there is one, is usually a permanent magnet generator operating at around 6–8 times system frequency. The diodes and fuses are mounted on the rotor, and the rectified output is led directly to the generator field winding without need of brushes and slip-rings. The diodes are mounted on well-ventilated heat sinks, and special designs of fuse are used to withstand the centrifugal force on the fusible link.

On units small enough to require only one diode and fuse per arm, failure of one diode or fuse leaves the exciter with one phase unloaded; exciters are usually designed to supply full-load excitation in this condition without damage, so that the generator can remain in service until the fault can be repaired conveniently. However, experience shows that the failure rate of diodes is extremely low and that more often fuse links fail mechanically. Hence some makers supply salient-pole generators, up to say 25 MW, with no fuses at all, but use generously rated diodes to provide a large margin. These generators would use up to three diodes in parallel per bridge arm. For turbogenerators up to about 70 MW, some designs use two diodes in series—each of full duty, with one, two or more series pairs in parallel per bridge arm—and no fuses. On large units, redundant parallel paths, individually fused, are provided as in static equipments.

For units that use fuses, the striker-pin indicator type can still be used, the pin being observed by causing it to interrupt a light beam falling on to a photoelectric cell, or it can be observed visually with a stroboscope. Alternatively a neon lamp is connected across the fuse and glows when the fuse blows.

When diodes are in parallel, whether fused or not, if one becomes open circuit the system will continue to function apparently normally unless the remaining diodes are overloaded and eventually fail also. If an unfused diode fails by short circuiting, the short-circuit current in the exciter armature induces fundamental (exciter) frequency current in the exciter field winding. This can be detected and used to trip the set before serious damage is done. Another method of detection is to use stationary pick-up coils to see whether the diode connections are carrying current as they should as they pass the coils.

More elaborate indication, perhaps coupled with measurements of current and voltage and indication of earth fault, can be arranged by telemetry, but the telemetry may be less reliable than the diodes. Frequently instrument slip-rings are used, with solenoid-operated brushes that make contact only when readings are required.

The diodes, and fuses too if they are used, must be rated for the normal duty, including field forcing, and to withstand the abnormal conditions noted in Section 28.14.2.

40.5.5.1 Rotor-current limitation

Every exciter is capable of delivering a maximum current appreciably greater than the continuous value based on thermal considerations. This capacity for overexcitation is necessary to provide the additional reactive power demanded during selective clearance of faults and to maintain a synchronous torque even when the voltage level has fallen.

Although it safeguards the winding insulation against damage from prolonged overexcitation, rotor protection gear (such as overcurrent and overtemperature relays) disconnects the generator even when disconnection is not desired. Use of a controller to limit the excitation current to an acceptable value during operation would substantially improve the availability of the generator. For safety reasons, however, a separate protective device must be retained to safeguard the winding against overloading.

The demands to be met by the rotor-current limiter are best explained with reference to the voltage and excitation current when a short-line fault occurs. The voltage regulator reacts to the drop in voltage with surge excitation. The controller is not intended to impede this, but merely to determine the ceiling current Ifm. Although the clearance normally takes place in a few hundred milliseconds, the longest duration (back-up protection) of 2–3 s will be assumed. When, after the fault has been cleared, nominal voltage is attained with a permissible continuous excitation current, the thermal controller will not intervene. However, when because of, for example, breaker failure the fault is not cleared, or when generated reactive power no longer suffices to maintain the voltage level, overexcitation will continue to be present. The limit controller now aims to reduce the excitation current before any of the protection gear is tripped. A further important aspect is that this also causes the short-circuit power of the system to be reduced, which, in turn, will diminish the extent of the damage resulting from failure of a breaker or other protective gear.

The condition in which the maximum continuous excitation permissible is present might persist for a long time, and experience has shown that further short circuits appear relatively frequently during this time. When these secondary disturbances occur, it becomes necessary to deliver the maximum reactive current to the system once more, especially to retain system stability. This means that limitation has to be cancelled again for a short time. The characteristic which trips the reset is the steep drop in voltage, dV/dt.

A further requirement results form the fact that today most of the large excitation equipment is fed from the generator terminals. The necessary ceiling excitation current is normally already available at 90% of the rated voltage. This means that at 120% of the rated voltage, a ceiling current which is 33% higher than the necessary value will flow when an instantaneously acting excitation limiter is not provided.

The limitation signal can be introduced as a reference-value variation of the voltage regulator. However, two facts suggest applying the signal to the voltage regulation output for use as dominant parallel limitation. These are: (i) the limitation is absolute and thus fully effective even when the actual value for the voltage regulator is lost (voltage-transformer fuses blown); and (ii) the limit controller can be adapted to the dynamic conditions of the field-current controller and become independent of voltage-regulator response.

As has been mentioned, limitation to the permissible continuous excitation current should be delayed in order to first give the voltage the maximum support possible. The permissible continuous current in the excitation circuit is determined by the thermal stressing of the field winding, the supply transformer, or the thyristors or diodes. The overload capacity of these elements can usually be represented by integration of the current. A slight overrun of the set limit for the continuous current is then tolerated correspondingly longer. It can be argued, however, that the ceiling excitation will be required only as long as fault clearance is still taking place, in which case a fixed timing element is provided. In both cases, it is usually desired that ceiling excitation should be available again in the event of secondary disturbance.

Based on previous experience, improved rotor-current limit controllers were developed to cater for the many varied applications met with in practice; these controllers can be easily adapted to the particular protection method used.

A maximum-current limit controller, instantaneous acting and always available, ensures that for terminal-fed excitation the desired ceiling current is not exceeded over the entire operating voltage range.

In the case of the simple integrator mode, the speed at which reduction to the limit value takes place is relative to the magnitude of the overcurrent. The integrator is reset only when the limit for the continuous rotor current is underrun.

With the switching mode, reduction to the limit begins after a set delay Tv of several seconds. Every time there is a sharp drop in voltage, maximum-current limitation begins anew and the timing element is reset.

A method frequently used in the combined mode: here, integration of the overcurrent is combined with resetting of the integrator by each succeeding voltage drop.

The actual current can usually be measured with a current transformer in the alternating-current (a.c.) power supply prior to rectification by the thyristors or diodes. A d.c./d.c. converter makes connection to a shunt also possible.

When partial failure of components in the excitation circuit causes the permissible continuous current to be reduced, change-over to a pre-set second current limit set-point is possible.

23.6 Exciter and Voltage Regulator

The function of an exciter is to increase the excitation current for voltage drop and decrease the same for voltage rise. The voltage change is defined as

ΔVΔ__(V t−Vref)

where Vt is the terminal voltage and Vref is the reference voltage.

Exciter Ceiling Voltage: It is defined as the maximum voltage that may be attained by an exciter with specified conditions of load.

Exciter Response: It is the rate of increase or decrease of the exciter voltage when a change in this voltage is demanded. As an example consider the response curve shown in Fig. 23.12.

Response=100 V0.4s=250V/s

Exciter Buildup: The exciter buildup depends upon the field resistance and the changing of its value by cutting or adding. The greatest possible control effort is the complete shorting of the field rheostat when maximum current value is reached in the field circuit. This can be done by closing the contact to C shown in Fig. 23.13.

When the exciter is operated at a rated speed at no-load, the record of voltage as function of time with a step change that drives the exciter to its ceiling voltage is called the exciter buildup curve. Such a response curve is shown in Fig. 23.14.

Line ac represents the excitation system voltage response (Table 23.2).

Table 23.2. Typical Ceiling Voltages

(23.7)Responseratio=Cd0a(0.5)p.u.V/s

In general the present day practice is to use 125 V excitation up to 10 MVA units and 250 V systems up to 100 MVA units. Units generating power beyond 100 MVA have excitation system voltages variedly. Some use 350 and 375 V system while some go up to 500-V excitation system.

3.4.2 Model of a beam with two exciters

We consider a beam of length L actuated by two exciters. Index 1 is for the first exciter, and 2 indicates the second. The two exciters are denoted E1 and E2, and they are placed at the positions x 1 and x2 on the beam as detailed in Fig. 3.13. As presented in Chap. 2, the deformation of the beam is the sum of the contributions of an infinite number of modes, and we write

(3.47)w(x ,t)=∑Φn(x)wn(t),

where w_ is the deformation all along the beam, w_n is the displacement of the mode n and Φn(x) is the deformation mode shape of the mode n. In this model, we consider only two modes, denoted by the modes a and b, which are the two successive modes energized to obtain the traveling wave. Therefore, Eq. (3.47) simplifies to

(3.48)w(x,t)=Φa(x)wa(t)+Φb(x)wb(t).

Chapter 2 gives the dynamics of each mode. They follow a second-order type equation, and we write:

(3.49)Ma w¨a+Daw˙ a+Kaw=fa,Mbw¨b+Dbw˙b+Kbw=fb,

where Mi, Di, Ki, i∈{a,b}, are the modal mass, damping and stiffness for the modes a and b, and Fa and Fb are the forces exerted by the exciters on the beam for each mode.

The two exciters contribute to the forces Fa and Fb but not equally. Indeed, the contributions depend on their relative position on the beam. To be concise, and without loss of generality, the two forces Fa and Fb can be written as:

(3.50)fa=N(Φa(x1)v1+Φa(x2) v2),fb=N(Φb(x1)v1+Φb (x2)v2),

where N is the electromechanical force factor. If we introduce the matrix Φ12 given by

(3.51)Φ12=(Φa(x1)Φa(x 2)Φb(x1)Φb(x2)),

How can we represent a traveling wave in a textbook?

If we consider a pure traveling wave such as w(x,t)=Wsin⁡(ωt−kx) then we can draw the deformation of the beam at different time steps, as shown in Fig. 3.14. In this figure, we also represented the vibration nodes for each time step. The positions of the vibration nodes on the beam are not fixed, they vary with time, which is typical of a traveling wave.

For sake of clarity, only few time steps can be represented, but usually, it's enough to have a qualitative evaluation of the traveling wave. For a more advanced analysis, we use a 2D plot, in a complex plane. Indeed, we can also write:

(3.52)w(x,t)=Wsin⁡(ωt)cos⁡(kx)−Wcos⁡(ωt)sin⁡(kx)=WRsin⁡(ωt)−WIsin⁡(ωt)

where WR=Wcos⁡(kx) and WI=Wsin⁡(kx). In a real-imaginary frame, the points M(W R;WI) are along a circle, as depicted Fig. 3.15.

This representation is more abstract, but has the main advantage to be concise: a perfect traveling wave will always be represented as a circle, and all the representations not showing a perfect circle are typical of a not perfect traveling wave.

then Eq. (3.50) can be written in the matrix form, using fp1=N v1 and fp2=Nv2, the two forces produced by the exciters, as

(3.53) (fafb)=Φ12(f1pf 2p).

In the same way, from Eq. (3.48), it is obvious that the matrix form can also be used for the displacement and velocity amplitude of each mode and leads to

(3.54)( u1u2)=Φ12 (uaub)

where u1 and u2 are the vibration velocities at positions x1 and x2; ua and ub are the modal vibration velocities.

It is now possible to model the system in the rotating reference frame. Following the notations of Section 3.3.2, we write

(3.55)2MiU˙id+DUid=Fid, 2MiU˙iq+DU id=Fiq,

as well as

(3.56)Fi d=Fipd+2Miδωi UiqandFiq=Fi pq−2MiδωiUid

with i∈{a,b}. Moreover, Eqs. (3.53) and (3.56) yield:

(3.57)Fipd=Φi(x1)F1d+Φi(x2)F2d ,Fipq=Φi(x 1)F1q+Φi(x2 )F2q.

The energetic macroscopic representation in the rotating reference is then depicted in Fig. 3.16. It is composed of two second-order systems which operate independently, but which are actually coupled at the level of the excitation forces. It should be emphasized here that the coupling between the axes d and q for mode a depends on δωa which represents the difference between the resonance pulsation of the mode a and the pulsation of the supply voltages. Therefore, this coupling differs from the coupling for mode b since ω0a≠ω0b. In this representation, the external forces have been represented with external mechanical sources (MS), even if they are considered null in this part.

(a) Rotary Amplifiers

An ordinary d.c. exciter may be regarded as a power amplifier since the power required to excite its field fully is considerably less (by a factor of, say, 100) than the power delivered by the armature. An example of this application is the Ward-Leonard control used in deck machinery and in some steering systems.

One of the best examples of the two-stage cross-flux excited exciter is the amplidyne. Thousands of such machines are in service and the normal working power amplification of the amplidyne is about 2500 to 1 and it has a very good rate of response.

The drawback of such rotating amplifiers is, of course, that they contain rotating parts which necessitate bearings and commutators together with associated brush gear, all of which require maintenance.

Exciter filters

The correct selection of the exciter filters is perhaps the most important single factor in fluorescence microscopy. These also may be either liquid or glass, but the advantages of the latter are such that they are rapidly becoming the automatic choice. The filter, or combination of exciter filters used, depends upon a number of factors, including the light source, the specimen under examination, the fluorochrome used as the staining reagent and the barrier filter. Instructions for the use of exciter filter combination are supplied by the manufacturers and the student should follow their directions. In addition, the original papers in which the staining procedure in use was first described should be consulted, in order to establish the filter combination originally recommended. A table of equivalent filters is given above (see Table 2.1).

Table 2.1. EQUIVALENT FILTER COMBINATIONS USED IN FLUORESCENCE MICROSCOPY