06-12-2012, 12:44 PM
APPLICATION OF STATIC EXCITATION SYSTEMS FOR ROTATING EXCITER REPLACEMENT
[attachment=42751]
ABSTRACT
Many power generation plants are faced with obsolescence, high maintenance and down
time due to the excitation system. DC field breakers, motorized rheostats, rotating exciter
failures, commutator deterioration, vibration and obsolescence and replacement parts for
the automatic voltage regulator (AVR) are just a few of the problems typical of these aged
power plants. The result is high overhead and potentially long down time of the generator
system.
The replacement of the rotating exciter and associated equipment for static excitation
systems provides the positive solutions to these problems. The static exciter offers the
design flexibility of easy retrofit for both small and larger rotating exciter systems. Additionally,
it eliminates the maintenance overhead common to the brush type exciter.
INTRODUCTION
This paper will discuss the static exciter system that includes the power control devices
(SCRs, also called thyristors), power transformer and automatic voltage regulator. The
elimination of the dc field breaker can offer substantial cost savings. Here, solid state fast
de-excitation circuits will be discussed and its benefits. Lastly, selection criteria and application
considerations will be reviewed regarding types of static exciters.
THE OPERATION OF THE STATIC EXCITER
A static exciter/regulator behaves functionally like a simple automatic voltage regulator
working into the exciter field. When the excitation system senses a low generator voltage,
field current increases to the field; when a high generator voltage is sensed, field current is
decreases. Functionally, a static exciter applies dc power into the main field for a slip ring
machine, while a voltage regulator applies dc power into the exciter field. The static exciter
system consists of three basic components: the control electronics (for example. Basler
Electric's DECS family of Digital Excitation Controllers), the power rectifier bridge and the
power potential transformer. Together, they provide accurate generator field control to
maintain generator output voltage. Figure 1 is used to illustrate a typical static excitation
system working directly into either the exciter field or main field.
POWER POTENTIAL TRANSFORMER
Power for the excitation system is derived by the generator via a large KVA transformer.
The transformer steps down the generator terminal voltage to be compatible with the field
requirements of the generator. The transformer will provide the excitation system’s full load
rating, plus a voltage and KVA margin for accommodating short time field forcing to handle
generator transient overload requirements.
Additionally, transformers are designed with BIL ratings (Basic Impulse Level) either in
accordance with NEMA specification ST. 20 or ANSI C57.13. A high BIL rating ensure that
the electrical insulation system of the transformer withstands any lightning-induced voltage
spikes or transients introduced by a generator short circuit.
AC FIELD SHUTDOWN CONTACTOR
The output of the power potential transformer, shown in Figure 1, connects to input contacts
of a shutdown contactor and the output contacts are connected to the rectifier bridge.
Unlike the common dc field breaker used at the field of the generator for shutdown, the ac
field contactor or ac breaker is used to interrupt the power input to the excitation system
for de-exciting the generator. When the ac contactor opens, the energy from the field flows
through the thyristor and a series discharge resistor known as a rapid de-excitation circuit.
See Figure 4.
The use of an interrupt switch at the ac input is the preferred method of shutdown over a
dc field breaker because of its availability, economy and small space requirement. Furthermore,
it stills provides electrical field isolation from the ac power source.
FIELD FLASHING THE GENERATOR
When a solid state excitation system connects directly to the generator field and the excitation
power is provided from the generator terminals, external means need to be provided
to build up generator voltage. A battery source meets this requirement of external dc.
Without it, insufficient generator residual voltage (generator voltage available at the machine
terminals when the machine is spinning and having no excitation) is available to
provide power to the thyristors for rectification to dc. Hence, the external battery source
forces a current in the field circuit, generating some ac voltage from the generator sufficient
to allow the thyristors to begin rectifying.
A diode in series with the positive battery source prevents the current from the power
thyristor charging the battery. Typically, the battery source is 125 Vdc, although 250 Vdc is
not uncommon. In special cases, an ac source that is rectified and supplied to the field as
an alternate flashing source may be used. The battery source for field flashing is usually
removed as generator voltage increases to 50-70 percent of rated voltage by the excitation
system. The voltage buildup circuit includes a timer that removes the battery source to
prevent excessive battery drain when the ac field contactor closes and the generator
voltage does not build up.
Six Thyristor System
For machines greater than 10 - 20 MVA, or above 150 amperes on the field, the 6 thyristor
system is generally preferred. Although the reaction time of the 3 thyristor system can be
very responsive, its output performance is limited to a zero to positive ceiling voltage in the
field circuit. See Figure 3. When fast generator voltage changes are required, the zero
minimum voltage on the 3 thyristor bridge limits the speed of voltage decay, while the
voltage recovery time will be related to the rate of field decay caused by the freewheeling
diode located across the field.
The 6 thyristor bridge in Figure 4 identifies a two quadrant system because the field output
voltage swings both the positive and negative directions, allowing faster generator voltage
recovery. When the 6 thyristor full wave bridge gates in the negative direction, the power
flows from the field back into the generator, via a power potential transformer. Figure 4
provides a schematic illustrating the 6 thyristor system, while Figure 5 highlights the change
in field output with different conduction angles of the power thyristors.
[attachment=42751]
ABSTRACT
Many power generation plants are faced with obsolescence, high maintenance and down
time due to the excitation system. DC field breakers, motorized rheostats, rotating exciter
failures, commutator deterioration, vibration and obsolescence and replacement parts for
the automatic voltage regulator (AVR) are just a few of the problems typical of these aged
power plants. The result is high overhead and potentially long down time of the generator
system.
The replacement of the rotating exciter and associated equipment for static excitation
systems provides the positive solutions to these problems. The static exciter offers the
design flexibility of easy retrofit for both small and larger rotating exciter systems. Additionally,
it eliminates the maintenance overhead common to the brush type exciter.
INTRODUCTION
This paper will discuss the static exciter system that includes the power control devices
(SCRs, also called thyristors), power transformer and automatic voltage regulator. The
elimination of the dc field breaker can offer substantial cost savings. Here, solid state fast
de-excitation circuits will be discussed and its benefits. Lastly, selection criteria and application
considerations will be reviewed regarding types of static exciters.
THE OPERATION OF THE STATIC EXCITER
A static exciter/regulator behaves functionally like a simple automatic voltage regulator
working into the exciter field. When the excitation system senses a low generator voltage,
field current increases to the field; when a high generator voltage is sensed, field current is
decreases. Functionally, a static exciter applies dc power into the main field for a slip ring
machine, while a voltage regulator applies dc power into the exciter field. The static exciter
system consists of three basic components: the control electronics (for example. Basler
Electric's DECS family of Digital Excitation Controllers), the power rectifier bridge and the
power potential transformer. Together, they provide accurate generator field control to
maintain generator output voltage. Figure 1 is used to illustrate a typical static excitation
system working directly into either the exciter field or main field.
POWER POTENTIAL TRANSFORMER
Power for the excitation system is derived by the generator via a large KVA transformer.
The transformer steps down the generator terminal voltage to be compatible with the field
requirements of the generator. The transformer will provide the excitation system’s full load
rating, plus a voltage and KVA margin for accommodating short time field forcing to handle
generator transient overload requirements.
Additionally, transformers are designed with BIL ratings (Basic Impulse Level) either in
accordance with NEMA specification ST. 20 or ANSI C57.13. A high BIL rating ensure that
the electrical insulation system of the transformer withstands any lightning-induced voltage
spikes or transients introduced by a generator short circuit.
AC FIELD SHUTDOWN CONTACTOR
The output of the power potential transformer, shown in Figure 1, connects to input contacts
of a shutdown contactor and the output contacts are connected to the rectifier bridge.
Unlike the common dc field breaker used at the field of the generator for shutdown, the ac
field contactor or ac breaker is used to interrupt the power input to the excitation system
for de-exciting the generator. When the ac contactor opens, the energy from the field flows
through the thyristor and a series discharge resistor known as a rapid de-excitation circuit.
See Figure 4.
The use of an interrupt switch at the ac input is the preferred method of shutdown over a
dc field breaker because of its availability, economy and small space requirement. Furthermore,
it stills provides electrical field isolation from the ac power source.
FIELD FLASHING THE GENERATOR
When a solid state excitation system connects directly to the generator field and the excitation
power is provided from the generator terminals, external means need to be provided
to build up generator voltage. A battery source meets this requirement of external dc.
Without it, insufficient generator residual voltage (generator voltage available at the machine
terminals when the machine is spinning and having no excitation) is available to
provide power to the thyristors for rectification to dc. Hence, the external battery source
forces a current in the field circuit, generating some ac voltage from the generator sufficient
to allow the thyristors to begin rectifying.
A diode in series with the positive battery source prevents the current from the power
thyristor charging the battery. Typically, the battery source is 125 Vdc, although 250 Vdc is
not uncommon. In special cases, an ac source that is rectified and supplied to the field as
an alternate flashing source may be used. The battery source for field flashing is usually
removed as generator voltage increases to 50-70 percent of rated voltage by the excitation
system. The voltage buildup circuit includes a timer that removes the battery source to
prevent excessive battery drain when the ac field contactor closes and the generator
voltage does not build up.
Six Thyristor System
For machines greater than 10 - 20 MVA, or above 150 amperes on the field, the 6 thyristor
system is generally preferred. Although the reaction time of the 3 thyristor system can be
very responsive, its output performance is limited to a zero to positive ceiling voltage in the
field circuit. See Figure 3. When fast generator voltage changes are required, the zero
minimum voltage on the 3 thyristor bridge limits the speed of voltage decay, while the
voltage recovery time will be related to the rate of field decay caused by the freewheeling
diode located across the field.
The 6 thyristor bridge in Figure 4 identifies a two quadrant system because the field output
voltage swings both the positive and negative directions, allowing faster generator voltage
recovery. When the 6 thyristor full wave bridge gates in the negative direction, the power
flows from the field back into the generator, via a power potential transformer. Figure 4
provides a schematic illustrating the 6 thyristor system, while Figure 5 highlights the change
in field output with different conduction angles of the power thyristors.