Excitation of SG_Mostafa

Mostafa Ahmed Ibrahim

A DC current must be supplied to the field circuit on the rotor.Since the rotor is rotating, a special arrangement is required to get the DC power to its field windings. There are two common approaches to supplying this DC power:1. Supply the DC power from an external DC source to the rotor by means of slip rings and brushes.2. Supply the DC power from a special DC power source mounted directly on the shaft of the synchronous machine.

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The exciter is the "backbone" of the generator control system.It is the power source that supplies the dcmagnetizing current to the field windings of a synchronous generator thereby ultimately inducing ac voltage and current in the generator armature

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Ceiling current of the excitation system IpThe maximum output current of the excitation system is called the ceiling current Ip, and is required to be at least1.4 times the rated-load field current IfN for a minimum time of 5 s.

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Ceiling voltage of the excitation system UPThis is defined as the maximum output voltage the excitation system is capable of supplying with the thyristor converter at its full positive output voltage setting.

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The ceiling voltage depends on the way in which the excitation system is connected, on the states of operation of the synchronous machine and excitation power supply, and on the variation of these states of operation over time.

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The types of exciters1 - DC Exciter 2 - AC Exciter 3 Static Exciter The amount of excitation required to maintain the output voltage constant is a function of the generator load. As the generator load increases, the amount of excitation increases. Reactive lagging P.F loads require more excitation than unity P.F loads Leading P.F loads require less excitation than unity P.F loads

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Types Of Exciters

The amount of excitation required by a generator for a particular load is defined by the "generatorsaturation curve".

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Types Of Exciters

The amount of power that a generator can deliver is defined by the "generator capability curve".

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Types Of Exciters

1 - DC Exciter

The DC exciter still in existence for many SGs below 100 MVA per unit, consists of two DC commutator electric generators:1 - The main exciter (ME) 2 - The auxiliary exciter (AE). Both are placed on the SG main shaft. The ME supplies the SG field winding (Vf ), while the AE supplies the ME field winding.1 - DC ExciterPrevious

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Only the field winding of the auxiliary exciter is supplied with the voltage Vcon controlled by the AVR.The power electronics source required to supply the AE field winding is of very low power rating, as the two DC commutator generators provide a total power amplification ratio around 600/1.1 - DC ExciterPrevious

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The advantage of a low power electronics external supply required for the scope is paid for by the following:1- A rather slow time response due to the large field-winding time constants of the two excitation circuits plus the moderate time constants of the two armature windings1 - DC ExciterPrevious

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2 - Problems with brush wearing in the ME and AE3 - Transmission of all excitation power (the peak value may be 4 to 5% of rated SG power) of the SG has to be through the slip-ring brush mechanism4 - Flexibility of the exciter shafts and mechanical couplings adds at least one additional shaft torsional frequency to the turbine-generator shaft1 - DC ExciterPrevious

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2 - AC Exciter

AC exciters basically make use of inside-out synchronous generators with diode rectifiers on their rotors.As both the AC exciter and the SG use the same shaft, the full excitation power diode rectifier is connected directly to the field winding of SG .The stator-based field winding of the AC exciter is controlled from the AVR.2 - AC ExciterPrevious

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The static power converter now has a rating about 1/20(30) of the SG excitation winding power rating, as only one step of power amplification is performed through the AC exciter.The AC exciter in Figure is characterized by the following:1 - Absence of electric brushes in the exciter and in the SG2 - AC ExciterPrevious

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2 - Addition of a single machine on the main SG-turbine shaft3 - Moderate time response in Vf (SG field-winding voltage), as only one (transient) time constant (Td0) delays the response; the static power converter delay is small in comparison4 - Addition of one torsional shaft frequency due to the flexibility of the AC exciter machine shaft and mechanical coupling2 - AC ExciterPrevious

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5 - Small controlled power in the static power converter: (1/20[30] of the field-winding power rating)The brushless AC exciter (as in Figure ) is used frequently in industry, even for new SGs, because it does not need an additional sizable power source to supply the exciters field winding.

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3 - Static Exciter

Modern electric power plants are provided with emergency power groups for auxiliary services that may be used to start the former from blackout. So, an auxiliary power system is generally available.This trend gave way to static exciters, mostly in the form of controlled rectifiers directly supplying the field winding of the SG through slip-rings and brushes3 - Static ExciterPrevious

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1 - The excitation transformer is required to adapt the voltage from the auxiliary power source or from the SG terminals.2 - It is also feasible to supply the controlled rectifier from a combined voltage transformer (VT) and current transformer (CT) connected in parallel and in series with the SG stator windings.3 - Static ExciterPrevious

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Static exciter voltage fed

Static exciter voltage and current fed.

This solution provides a kind of basic AC voltage stabilization at the rectifier input terminals. This way, short-circuits or short voltage sags at SG terminals do not much influence the excitation voltage ceiling produced by the controlled rectifier.3 - Static ExciterPrevious

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In order to cope with fast SG excitation current control, the latter has to be forced by an overvoltage available to be applied to the field winding. The voltage ceiling ratio (Vfmax/Vfrated) characterizes the exciter.Power electronics (static) exciters are characterized by fast voltage response, but still the Td time constant of the SG delays the field current response. 3 - Static ExciterPrevious

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Consequently, a high-voltage ceiling is required for all exciters.To exploit with minimum losses the static exciters, two separate controlled rectifiers may be used,1- one for steady state 2 - one for field forcing There is a switch that has to be kept open unless the field-forcing (higher voltage) rectifier has to be put to work. 3 - Static ExciterPrevious

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When Vfmax/Vfrated is notably larger thantwo, such a solution may be considered.The development of IGBT pulse-width modulator (PWM) converters up to 3 MVA per unit (for electric drives) at low voltages (690 VAC, line voltage) provides for new, efficient, lower-volume static exciters.The controlled thyristor rectifiers in Figure may be replaced by diode rectifiers plus DCDC IGBT converters

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Diode-rectifier and four-quadrant DCDC converter as static exciter.

A few such four-quadrant DCDC converters may be paralleled to fulfill the power level required for the excitation of SGs in the hundreds of MVAs per unit.The transmission of all excitation power through slip-rings and brushes remains a problemThe solution is, thus, here for the rather lower power ratings of exciters (less than 3 to 4% of SG rating).3 - Static ExciterPrevious

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The four-quadrant chopper static exciter has the following features:1 - It produces fast current response with smaller ripple in the field-winding current of the SG.2 - It can handle positive and negative field currents that may occur during transients as a result of stator current transients.

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3 - The AC input currents (in front of the diode rectifier) are almost sinusoidal (with proper filtering), while the power factor is close to unity, irrespective of load (field) current.4 - The current response is even faster than that with controlled rectifiers.5 - Active front-end IGBT rectifiers may also be used for static exciters.

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ION BOLDEA, SYNCHRONOUS GENERATORS, McGraw-Hill, New York, 2006. The IEEE standard 421.5 contains IEEE Recommended Practice for Excitation System Models for Power Systems. 1992

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